Transformation of Biomass: Theory to Practice

Transformationof Biomass Theory to Practice

EditorAndreas Hornung

Transformation of Biomass

Transformation ofBiomass

Theory to Practice

Editor

ANDREAS HORNUNG

Fraunhofer UMSICHT – Institute BranchSulzbach-Rosenberg, Germany

and

Chair in BioenergySchool of Chemical Engineering

College of Engineering and Physical SciencesUniversity of Birmingham, UK

This edition first published 2014© 2014 John Wiley & Sons, Ltd

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Library of Congress Cataloging-in-Publication Data

Transformation of biomass : theory to practice / editor, Andreas Hornung.pages cm

Includes bibliographical references and index.ISBN 978-1-119-97327-0 (hardback)

1. Biomass chemicals. 2. Biomass. I. Hornung, Andreas.TP248.B55T73 2014662′.88–dc23

2014004300

A catalogue record for this book is available from the British Library.

ISBN: 9781119973270

Set in 10/12pt Times by Aptara Inc., New Delhi, India.

1 2014

http://www.wiley.com

Contents

About the Editor xiiiList of Contributors xvPreface xvii

1 Biomass, Conversion Routes and Products – An Overview 1K.K. Pant and Pravakar Mohanty

1.1 Introduction 11.2 Features of the Different Generations of Biomass 21.3 Analysis of Biomass 5

1.3.1 Proximate and Ultimate Analysis of Biomass 61.3.2 Inorganic Minerals’ Ash Content and Properties 8

1.4 Biomass Conversion Routes 91.4.1 Pyrolysis 9

1.5 Bio-Oil Characteristics and Biochar 151.6 Scope of Pyrolysis Process Control and Yield Ranges 16

1.6.1 Moisture Content 181.6.2 Feed Particle Size 181.6.3 Effect of Temperature on Product Distribution 181.6.4 Solid Residence Time 181.6.5 Gas Environment 181.6.6 Effect of Pressure on Product Distribution 19

1.7 Catalytic Bio-Oil Upgradation 191.8 Bio-Oil Reforming 221.9 Sub and Supercritical Water Hydrolysis and Gasification 23

1.9.1 Biochemical Conversion Routes 241.9.2 Microorganisms for Fermentation 251.9.3 Integrating the Bioprocess 25

Questions 25References 28

2 Anaerobic Digestion 31Lynsey Melville, Andreas Weger, Sonja Wiesgickl and Matthias Franke

2.1 Introduction 312.1.1 Microbiology of Anaerobic Digestion 312.1.2 Key Phases 322.1.3 Influence Factors on the AD 34

vi Contents

2.1.4 Sources of Biomass Utilised in AD 362.1.5 Characteristics of Biomass 392.1.6 Pre-Treatment of Biomass 412.1.7 Products of Anaerobic Digestion 452.1.8 Anaerobic Treatment Technology 48


3 Reactor Design and Its Impact on Performance and Products 61Yassir T. Makkawi

3.1 Introduction 613.2 Thermochemical Conversion Reactors 62

3.2.1 Types of Reactors 623.3 Design Considerations 63

3.3.1 Hydrodynamics 643.3.2 Residence Time 693.3.3 Distributor Plate and Cyclone 723.3.4 Heat Transfer Mechanisms 733.3.5 Biomass Conversion Efficiency 75

3.4 Reactions and their Impact on the Products 763.4.1 Devolatization and Pyrolysis 763.4.2 Gasification 77

3.5 Mass and Energy Balance 793.5.1 Mass Balance 793.5.2 Energy Balance 80

3.6 Reactor Sizing and Configuration 823.7 Reactor Performance and Products 85

3.7.1 Moving Beds 853.7.2 Fluidized Bed (FB) 87

3.8 New Reactor Design and Performance 92Nomenclature 94Greek Symbols 95Questions 95References 95

4 Pyrolysis 99Andreas Hornung

4.1 Introduction 1004.2 How Pyrolysis Reactors Differ 1014.3 Fast Pyrolysis 1024.4 Fast Pyrolysis Reactors 102

4.4.1 Bubbling Fluid Bed Reactor 1024.4.2 Circulating Fluid Bed Reactor 1024.4.3 Ablative Pyrolysis Reactor 1024.4.4 Twin Screw Reactor – Mechanical Fluidised Bed 1034.4.5 Rotating Cone 103

Contents vii

4.5 Intermediate Pyrolysis 1034.5.1 Principles 1034.5.2 Process Technology 104

4.6 Slow Pyrolysis 1054.6.1 Principles 1064.6.2 Process Technology 106

4.7 Comparison of Different Pyrolysis Techniques 1064.8 Future Directions 1074.9 Pyrolysis in Application 107

4.9.1 Haloclean Pyrolysis and Gasification of Straw 1074.10 Pyrolysis of Low Grade Biomass Using the Pyroformer Technology 109Questions 110References 110Books and Reviews 112

5 Catalysis in Biomass Transformation 113James O. Titiloye

5.1 Introduction 1135.2 Biomass, Biofuels and Catalysis 1145.3 Biomass Transformation Examples 1165.4 Hydrogen Production 1205.5 Catalytic Barriers and Challenges in Transformation 120Questions 120References 120

Appendix 5.A Catalytic Reforming of Brewers Spent Grain 125Asad Mahmood and Andreas Hornung

5.A.1 Biomass Characterisation 1255.A.2 Permanent Gas Analysis 1275.A.3 Pyrolysis and Catalytic Reforming without Steam 1275.A.4 Pyrolysis and Catalytic Reforming with Steam 130Reference 131

6 Thermochemical Conversion of Biomass 133S. Dasappa

6.1 Introduction 1336.2 The Thermochemical Conversion Process 136

6.2.1 Pyrolysis 1366.3 Combustion 1396.4 Gasification 140

6.4.1 Updraft or Counter-Current Gasifier 1416.4.2 Downdraft or Co-Current Gasifiers 142

6.5 Historical Perspective on Gasification Technology 1436.5.1 Pre-1980 1436.5.2 Post-1980 144

viii Contents

6.6 Gasification Technology 1456.6.1 Principles of Reactor Design 1456.6.2 Two Competing Designs 146

6.7 Open-Top Dual Air Entry Reaction Design – the IISc’s Invention 1496.8 Technology Package 151

6.8.1 Typical Performance of a Power Generation Package 1516.8.2 Engine and Generator Performance 155


7 Engines for Combined Heat and Power 159Miloud Ouadi, Yang Yang and Andreas Hornung

7.1 Spark-Ignited Gas Engines and Syngas 1597.2 Dual-Fuel Engines and Biofuels 1607.3 Advanced Systems: Biowaste Derived Pyrolysis Oils for Diesel Engine

Application 1617.3.1 Important Parameters to Qualify the Oil as Fuel 162

7.4 Advanced CHP Application: Dual-Fuel Engine Application for CHPUsing Pyrolysis Oil and Pyrolysis Gas from Deinking-Sludge 1667.4.1 Fuel Properties: Deinking Sludge Pyrolysis Oil, Biodiesel,

Blends and Fossil Diesel 1677.4.2 Combustion Characteristics 1697.4.3 Conclusions 170


8 Hydrothermal Liquefaction – Upgrading 175Ursel Hornung, Andrea Kruse and Gokcen Akgul

8.1 Introduction 1758.1.1 Product Properties 176

8.2 Chemistry of Hydrothermal Liquefaction 1778.3 Hydrothermal Liquefaction of Carbohydrates 1778.4 Hydrothermal Liquefaction of Lignin 1798.5 Technical Application 1828.6 Conclusion 183Questions 183References 183

9 Supercritical Conversion of Biomass 189Gokcen Akgul

9.1 Introduction 1899.2 Supercritical Water Gasification 1909.3 Supercritical Water Oxidation 1939.4 Water–Gas Shift Reaction under the Supercritical Conditions 193

Contents ix

9.5 Catalysts in the Supercritical Processes 1949.5.1 Alkali Salts in the Supercritical Water 195

9.6 The Solubilities of Gases in the Supercritical Water 1959.7 Fugacities of Gases in the Supercritical Water 1969.8 Mechanism of the Supercritical Water Gasification 1979.9 Corrosion in the Supercritical Water 1979.10 Advantages of the Supercritical Conversion of Biomass 1989.11 Conclusion 199Questions 199References 199

10 Influence of Feedstocks on Performance and Products of Processes 203Andreas Hornung

10.1 Humidity of Feedstocks 20610.2 Heteroatoms in Feedstocks 206References 207

11 Integrated Processes Including Intermediate Pyrolysis 209Andreas Hornung

11.1 Coupling of Anaerobic Digestion, Pyrolysis and Gasification 21011.2 Intermediate Pyrolysis, CHP in Combination with Combustion 21111.3 Integration of Intermediate Pyrolysis with Anaerobic

Digestion and CHP 21211.4 Pyrolysis Reforming 21211.5 The BIOBATTERY 21211.6 Pyrolysis BAF Application 21411.7 Birmingham 2026 21511.8 Conclusion 215References 216

12 Bio-Hydrogen from Biomass 217Andreas Hornung

12.1 World Hydrogen Production 21712.2 Bio-hydrogen 21712.3 Routes to Hydrogen 219

12.3.1 Steam Reforming 21912.3.2 Reforming 21912.3.3 Water Electrolysis 22312.3.4 Gasification 22312.3.5 Fermentation 223

12.4 Costs of Hydrogen 22312.5 Conclusion 224References 224Further Reading 225

x Contents

13 Analysis of Bio-Oils 227Dietrich Meier and Michael Windt

13.1 Definition 22713.2 Introduction 22713.3 General Aspects 228

13.3.1 Before Analysis 22813.3.2 Significance of Bio-Oil Analysis 22813.3.3 Post-Processing Reactions 22913.3.4 Overall Composition 229

13.4 Whole Oil Analyses 23013.4.1 Gas Chromatography 23013.4.2 NMR 23713.4.3 FTIR 23813.4.4 SEC 239

13.5 Fractionation Techniques 24113.5.1 Addition of Water 24113.5.2 Removal of Water (Lyophilization) 24313.5.3 Solid Phase Extraction (SPE) 24613.5.4 Solvent Partition 24913.5.5 Distillation 253


14 Formal Kinetic Parameters – Problems and Solutions inDeriving Proper Values 257Neeranuch Phusunti and Andreas Hornung

14.1 Introduction 25714.2 Chemical Kinetics on Thermal Decomposition of Biomass 25914.3 Kinetic Evaluation Methods 26114.4 Experimental Kinetic Analysis Techniques 26414.5 Complex Reaction 26414.6 Variation in Kinetic Parameters 267

14.6.1 Kinetic Compensation Effect 26714.6.2 Thermal Lag 26814.6.3 Influence of Experimental Conditions 26914.6.4 Computational Methods 270

14.7 Case Study: Kinetic Analysis of Lignocellulosic Derived Materialsunder Isothermal Conditions 27114.7.1 Instrument and Operating Conditions 27114.7.2 Kinetic Evaluation Procedure 27214.7.3 Formal Kinetic Parameters and Some Technical Applications 275

14.8 Conclusion 278Nomenclature 279Subscripts 280Miscellaneous 280

Contents xi


15 Numerical Simulation of the Thermal Degradation of Biomass –Approaches and Simplifications 285Istvan Marsi

15.1 Introduction 28515.2 Kinetic Schemes Applied in Complex Models 288

15.2.1 One-Step Global Models 28915.2.2 Competing Models 28915.2.3 Parallel Reaction Models 29015.2.4 The Broido–Shafizadeh Mechanism 29115.2.5 The Koufopanos Mechanism 29215.2.6 The Distributed Activation Energy Model (DAEM) 293

15.3 Thermal Aspects of Biomass Degradation Modeling 29415.3.1 Single-Particle Models 29515.3.2 Particles in Bed Models 298

15.4 Conclusion 299Questions 299Nomenclature 299Symbols 299Greek 300Indices 300References 300

16 Business Case Development 305Sudhakar Sagi

16.1 Introduction 30516.2 Biomass for Power Generation and CHP 30716.3 Business Perspective 308

16.3.1 Background 31016.4 The Role of Business Models 310

16.4.1 The Market Map Framework 31116.5 Financial Model Based on Intermediate Pyrolysis Technology 313

16.5.1 Pelletisation Process 31416.5.2 Pyrolysis Unit 315

References 318

17 Production of Biochar and Activated Carbon via IntermediatePyrolysis – Recent Studies for Non-Woody Biomass 321Andreas Hornung and Elisabeth Schroder

17.1 Biochar 32117.1.1 Introduction 32117.1.2 Biochar and its Application in the Field 322

References 325Further Reading 326

xii Contents

17.2 Activated Carbon 32717.2.1 Introduction 32717.2.2 Biomass Properties 32717.2.3 Activation of Biochar 32817.2.4 Formation of Granular Activated Carbon 334

References 337Further Reading 337

Index 339

About the Editor

Prof. Dr. rer. nat. Dipl.-Ing Andreas Hornung CEng FIChemE FRSC completed his studiesat the TU Darmstadt in Germany, where he graduated as an engineer in chemistry in 1991.He did his PhD at the TU Kaiserslautern in Germany whilst developing reactor systemsfor the pyrolysis-based recycling of plastics. He continued to work at the TU Karls-ruhe in Germany in developing reactor systems for the recycling of resins and electronicscrap, and expanded his topic to the conversion of biomass from 1996 onward. From2000 to 2002, Hornung worked for companies in Austria and Italy on the developmentof the first prototypes. Such units have been used since 2001 at the Karlsruhe Institute ofTechnology, where he worked until 2007 as head of the pyrolysis and gas treatment division.In 2007, he took over the chair in chemical engineering and applied chemistry at AstonUniversity in Birmingham, UK. In 2008, he founded the European Bioenergy ResearchInstitute EBRI which he led as director until the end of 2013. At the beginning of 2013 hebecame the director of the Institute Branch Sulzbach-Rosenberg of Fraunhofer UMSICHT.Since 2010 he has been a Fellow of the Royal Society of Chemistry (England), a Fellowof the Institution of Chemical Engineers as well as chartered engineer in Britain, and hebecame Green Leader of the West Midlands in 2012. In 2013, his technology received theBritish National Climate Week Award in the breakthrough category. He holds 18 patentsand has published more than 150 scientific publications to date. His institutes employed, in2013, about 120 staff members and are carrying out applied research in various sustainabletopics. In May 2014 he has been appointed as chair in bioenergy at the University ofBirmingham, UK.

xiv About the Editor

The main strategic topic of Hornung’s work today is the development of decentralisedpower providing units combined with pyrolysis, gasification and digestion units – calledthe Biobattery.

In a biogas scenario, a Biobattery installation seeks to use peaks in energy supplyto add to the energy output from a biogas installation and enable the thermochemicaltransformation of the more recalcitrant lignin-based components of digestion feedstocks.The use of digestate solids as feedstock for intermediate pyrolysis means that the amountof digestate for application to land is reduced to the liquid fraction. This is desirable wherethere is an oversupply of nitrogenous materials for application to land, such as in areasof intensive livestock production, since digestates can be a source of both greenhouse gasemissions and nitrogen losses to water bodies. Hence, the Biobattery not only adds to theflexibility of energy supply and storage, it also increases the energy and financial gainachieved from existing biogas infrastructure, while reducing their environmental impact.

The Biobattery concept aims to deliver local integrated system solutions, to capturepeaks in available power from solar and wind sources and convert and store this powerover periods of varying durations (minutes to days), thereby enabling the delivery of on-demand power compensation. The Biobattery concept uses a pool of renewable energytechnologies, that is high and low temperature thermal storage systems, thermochemicalbiomass processes, for example intermediate pyrolysis and gasification, thereby deliveringsolid, liquid and gaseous energy products which can be stored and used to produce eitherenergy on an on-demand basis, or sold as products for other use.

List of Contributors

Gokcen Akgul Department of Energy Systems Engineering, Recep Tayyip ErdoganUniversity, Turkey

S. Dasappa Indian Institute of Science, India

Matthias Franke Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg,Germany

Andreas Hornung Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg,Germany and Chair in Bioenergy, School of Chemical Engineering, College of Engineeringand Physical Sciences, University of Birmingham, UK

Ursel Hornung Karlsruhe Institut fur technologie – Institut fur Katalyseforschung und–Technologie, Germany

Andrea Kruse Universitat Hohenheim, Institut fur Agrartechnik, Konversionstechnolo-gie und Systembewertung nachwachsender Rohstoffe, Germany

Asad Mahmood European Bioenergy Research Institute (EBRI), Aston University, UK

Yassir T. Makkawi European Bioenergy Research Institute (EBRI), Aston University,UK

Istvan Marsi Faculty of Education, Department of Chemical Informatics, University ofSzeged, Hungary

Dietrich Meier Thunen-Institut fur Holzforschung, Germany

Lynsey Melville Centre for Low Carbon Research (CLCR), Birmingham City University,UK

Pravakar Mohanty Department of Chemical Engineering, Indian Institute of Technol-ogy Delhi, India

Miloud Ouadi European Bioenergy Research Institute (EBRI), Aston University, UK

xvi List of Contributors

K.K. Pant Department of Chemical Engineering, Indian Institute of Technology Delhi,India

Neeranuch Phusunti Department of Chemistry, Faculty of Science, Prince of SongklaUniversity, Hat Yai, Thailand

Sudhakar Sagi European Bioenergy Research Institute (EBRI), Aston University, UK

Elisabeth Schroder Karlsruher Institut fur Technologie – Institut fur Kern-und Energi-etechnik, Germany

James O. Titiloye Chemical & Environmental Engineering, College of Engineering,Swansea University, UK

Andreas Weger Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg,Germany

Sonja Wiesgickl Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg,Germany

Michael Windt Thunen Institut fur Holzforschung, Germany

Yang Yang European Bioenergy Research Institute (EBRI), Aston University, UK

Preface

Biomass is seen as a key feed material for the energy and material demands of mankindin the future. New businesses and technologies are therefore developing around biomassand its application. This textbook aims to help create an understanding of such processesrelated to the conversion of biomass into energy, heat and chemical products: processesbased on biological or thermal routes.

The education of new generations of engineers, scientists and technicians is importantto reach such goals. Therefore, this textbook intends to offer first guidelines to students aswell as people transferring from different sectors into the biomass conversion technologies.

The different chapters deal with fundamental details but also recent research and highlightthe possible problems and failures if methods are done wrong.

The textbook also carries two programmes for the evaluation of formal kinetic parametersas well as a calculation of business models.

Very often literature does not offer adequate answers to the questions arising fromresearch, for example how to describe the thermal conversion processes of biomass and theevaluation of data to characterise real reactor systems in terms of temperature and residencetime. The programmes related to this field will help the reader gain their own understanding.They can also be used to analyse data from lab work and therefore help to reach a bettergeneral understanding of the work done.

The business case model aims to enable the reader to compare different markets and theirspecific sensitivities, such as incentives and green subsidies, feed price and product priceimpact as well as general economic frame conditions.

Each chapter starts with a general motivation for the topic and at the end of each chapterthe reader will find some questions which should help in understanding the background ofthe chapter and in building up the mind of the reader to understand the material presentedin the right way.

No direct answers to the questions will be given by this textbook! The questions shouldsharpen the understanding and if the reader is unable to give an answer then the chaptershould be studied again!

The questions highlight the basics and interdependencies and will improve the ability ofthe reader to transfer skills within topics.

For a person in charge of new technologies or working at the front end of researchand development, such skills are of importance to give the right guidance or to find newpathways to better transform biomass.

The first chapter will give the reader a broad overview of biomass and its composition,conversion routes and products. The following chapters deal with specific technologies, suchas anaerobic digestion, pyrolysis and gasification, as well as hydrothermal and supercritical

xviii Preface

conversion. In addition, chapters for analysis and reactor design help to understand howprocesses are designed and how analysis helps to understand the sometimes complexcomposition of the products resulting from biomass. These chapters are very advanced andmight be best read at a later stage of the learning curve.

The same advice is given for the chapters on numerical simulation and formal kineticparameter evaluation. The related programme offers an up-to-date platform for calculations,but the reader will need an already profound understanding to apply them properly.

Finally no product will reach the market if it is not set properly in a business framework.The final chapter of this book gives you an insight into possible future products based

on the solid product from pyrolysis, such as char turned into activated carbon or biochar.The market for biochar particularly is developing all over the world.

I wish you a stimulating time while studying this book!

Best regardsProf. Dr. Andreas Hornung

Fraunhofer UMSICHTInstitute Branch Sulzbach-Rosenberg

Germanyand

Chair in BioenergySchool of Chemical Engineering

College of Engineering and Physical SciencesUniversity of Birmingham

UK

Online Supplementary MaterialPrograms for the evaluation of formal kinetic parameters, as well as the calculationof business models, can be found online. This software, and PowerPoint slides of allfigures from this book, can be found at http://booksupport.wiley.com.

http://booksupport.wiley.com

1Biomass, Conversion Routes and

Products – An Overview

K.K. Pant and Pravakar MohantyDepartment of Chemical Engineering, Indian Institute of Technology Delhi, India

1.1 Introduction

The world consumes nearly two barrels of oil for every barrel produced. The depletionof conventional resources and stricter environmental regulations, along with increasingdemand for commercial fuels and chemicals, has led to the need to find the alternatives toconventional fuel and chemical sources. Renewable plant materials are considered as one ofthe most promising alternatives for the production of fuels and chemicals. The conventionalsources for fuels and chemicals are fossil fuels, crude oil natural gas, coal and so on, whichare dwindling rapidly. With the concept of green chemistry, there is every necessity toproduce alternative sources of energy and fuels from renewable biomass. Biomass refers toall organic matter generated through photosynthesis and many other biological processes.The ultimate source of energy this renewable biomass is inexhaustible solar energy, whichis captured by plants through photosynthesis. It includes both terrestrial as well as aquaticmatter, such as wood, herbaceous plants, algae, aquatic plants; residues such as straw,husks, corncobs, cow dung, sawdust, wood shavings, sawn wood, wood based panels, pulpfor paper, paper board, and other wastes like disposable garbage, night soil, sewage solids,industrial refuse and so on [1]. Biomass can provide approximately 25% of our current

Transformation of Biomass: Theory to Practice, First Edition. Edited by Andreas Hornung.© 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.Companion website: http://booksupport.wiley.com


2 Transformation of Biomass

Table 1.1 Forest resources, area (ha), year (2010).

Land area Forest area Forest area perName of country (million ha) (million ha) % 1000 people

Africa 2965 674 23 683South America 1756 864 49 2246North and Central America 2110 705 33 1315Asia 3094 593 19 145Europe 2214 1005 45 1373Oceania 849 191 23 5478Caribbean 23 7 30 166World 13010 4033 31 597

energy demand, if properly utilized. Taking into account the production of biomass withrespect to land and forest area, there are 4033 million ha of forests worldwide, as presentedin Table 1.1.

In India 55.2 million ha of waste land is available for a wide range of short period energycrop productions [2]. Tropical and subtropical forests comprise 55% of the world’s forests,while temperate and boreal forests account for rest 45% [3]. The average area of forest andwooded land per inhabitant varies regionally. Production and use of wood fuel, industrialround wood, sawn wood, wood-based panels, pulp for paper, paper board (m3) usage andits production are presented in Table 1.2. The total carbon stored in forest biomass isapproximately 331 Giga tonnes (GT). About 27% of biomass is used directly as carbonfeedstock, for example, sawn wood, wood based panels, pulp for paper, paper and paperboard, mainly in developing countries. However, 33% is used as an industrial raw materialand the remaining 40% is used as primary or secondary process residues, suitable only forenergy production, for example, for production of upgraded biofuels [2,3]. Approximately70–77% of the global wood harvest is either used or is potentially available as a renewableenergy source.

The most efficient utilization of these resources comes when they are converted to liquidand gaseous products by appropriate technologies. Non-commercial biomass (biofuels)is the main source of energy available in the rural areas. An estimation by the Foodand Agriculture Organization (FAO) shows that the global production of wood fuel andround wood reached 3410 million m3 during 2010 [2–4]. Just over half of this was woodfuel, where 90% of that is being produced and consumed in developing countries. On theother hand, industrial round wood production, totaling around 1542 million m3 in 2010, isproduced and consumed both by North and Central America and Europe.

1.2 Features of the Different Generations of Biomass

Broadly, biomass can be categorized as first, second, third, and fourth generation. Firstgeneration biomass refers to traditional plant biomass like sugar and starch crops. Secondgeneration biofuels include bioethanol and biodiesel produced from the residual, non-foodparts of crops, and from other forms of lignocellulosic biomass, such as wood, grasses, andmunicipal solid wastes [5]. Third and fourth generation biofuels include algae-derived fuels,

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such as biodiesel from microalgae oil, bioethanol from micro algae and seaweeds, the finechemicals and H2 from green microalgae, and microbes by sub- and supercritical extractionprocesses. Further these extracted microalgae can be utilized as biomass in thermochemicalor biochemical routes of conversion [6]. “Drop in” fuels like “green gasoline,” “greendiesel,” and “green aviation fuel” produced from biomass are also considered as fourthgeneration biofuels [7]. Efforts are also underway to genetically engineer organisms toopen the secrete of these fourth generation hydrocarbon fuels. In Figure 1.1, both food andnon-food biomass have been integrated in the sequential downward stream for establishment

Crop (food)

Grain (rice, wheat), sugar cane,Potatoes, Corn

Seaweeds, algae, hyacinth

Palm, jatropha

Short rotationwoody

Herbaceous

Dedicated

Grass

Wood

Oilseed plant

Aquatic plant

Starch sugar crop

Energy crop

Biomass

Non-food Biomass

Cellulosicresources

Manure (cattle/fresh)

Industrial waste

Municipal waste

Forest waste

Agricultural waste

Sawdust,pulp waste,

thinned wood

Straw (rice, barley,wheat), bagasses,

corn stover

Food waste, yardwaste, container andproduct packaging

Black liquor frompaper industry,

waste from foodindustry

Animal manure,plant manure,

compost

Figure 1.1 Biomass feedstock distribution in term of food and non-food basis for bio-refinery.

Biomass, Conversion Routes and Products – An Overview 5

Table 1.3 Generation-wise biomass distribution with its features.

1st generation 2nd generation 3rd generation 4th generation

Feedstock Sugar, starch crops,vegetable oil,soya bean,animal fat, straw

Wood, agriculturalwaste, municipalsolid waste,animal manure,landfills, pyrooils,pulp sludge, grass

Micro algae biomass Geneticallymodifiedcrop

Product Biodiesel, sugaralcohol, cornethanol

Hydro treating oil,bio-oil, FT-oil etc.

Algae oil Biofuel

Advantage Environmentallyfriendly,economical andsocially secure

Not competing withfood

Environmentallyfriendly advancedtechnology underprocess to reducethe cost ofconversion

Availability of highvalue protein andnutrients, residualalgae for jet fuelanimal feed

Easily capturesCO2 andconversionto a carbonneutral fuel

Disadvantage Limited feedstock,blended partlywith conventionalfuel

Acidic, viscous,high oxygenates

content inpyrooils

Slow growth of algae,extraction of algaeoil is difficult andcostly

—

of the biorefinery concept towards energy surplus. Generation-wise details of the biomassdiversifications are presented in Table 1.3 [7, 8].

At present, biomass represents approximately 14–18% of the world’s total energy con-sumption [3, 4]. In order to utilize these resources properly, biomass should be convertedto energy that can meet a sizeable percentage of demands for fuel and chemicals. Efficientutilization of biomass as a potential feedstock depends on general information about thecomposition of plant species, heating value, production yields and bulk density. Organiccomponent analysis reports on the kinds and amounts of plant chemicals, including pro-teins, oils, sugars, starches, and lignocelluloses (fibers) required much attention about theirbehavior [1, 7].

1.3 Analysis of Biomass

The main components of biomass are cellulose, hemicelluloses, and lignin:

Cellulose or carbohydrate is the principal constituent of wood and other biomass and formsthe structural framework of wood cells. It is a polymer of glucose with a repeatingunit of C6H10O5 strung together by 𝛽-glycosidic linkages. The 𝛽-linkages in celluloseform linear chains that are highly stable and resistant to chemical attack because of thehigh degree of hydrogen bonding that can occur between chains of cellulose. Hydrogenbonding between cellulose chains makes the polymers more rigid, inhibiting the flexingof the molecules that must occur in the hydrolytic breaking of the glycosidic linkages.Hydrolysis can reduce cellulose to a cellobiose repeating unit, C12H22O11, and ultimately


Table 1.4 Organic components and composition of lignocelluloses biomass (dry basis).

FeedstockCellulose(wt. %)

Hemicelluloses(wt. %)

Lignin(wt. %)

Other(wt. %)

Bagasse 35 25 20 20Bamboo 55 28 17 0Corn stover 53 15 16 16Corncob 32 44 13 11Herbaceous energy crops 45 30 15 10Rice straw 38 25 12 25Short rotation woody crops 50 23 22 5Wheat straw 38 36 16 10Wheat chaff 38 36 16 11Waste paper 76 13 11 0

to glucose, C6H12O6. Heating values for cellulose may be slightly different based uponthe feedstock [8, 9].

Hemicellulose consists of short, highly branched chains of sugars. In contrast to cellulose,which is a polymer of only glucose, a hemicellulose is a polymer of five different sugars.It contains five-carbon sugars (usually D-xylose and L-arabinose), six-carbon sugars(D-galactose, D-glucose, and D-mannose) and uronic acid. The sugars are highly substi-tuted with acetic acid. The branched nature of hemicellulose renders amorphous proper-ties which is relatively easy to hydrolyze to its constituent sugars compared to cellulose.When it hydrolyzed, the hemicellulose from hardwoods releases products which high inxylose (a five-carbon sugar). The hemicellulose that contained in softwoods, by contrast,yields six more carbon sugars [7, 8].

Lignin is the major non-carbohydrate, polypenolic structural constituent of wood andother native plant materials that encrusts the cell walls and helps in cementing the cellsall together. It is a highly polymeric substance, with a complex, crosslinked, highlyaromatic structure and having the molecular weight of about 10 000 derived princi-pally from coniferyl alcohol (C10H12O3) by extensive condensation and polymerization[1, 8, 9].

For the efficient utilization of biomass, feedstock engineers are particularly evaluating thehemicellulosic component and the distribution among cellulose, hemicelluloses, and lignin.Table 1.4 gives an idea of the organic components of some of the dedicated energy crops,common sugar, and starch crops, respectively.

1.3.1 Proximate and Ultimate Analysis of Biomass

Analysis of biomass and its characteristics is generally accomplished by both proximate andultimate analysis. Proximate analysis separates the products into four groups: (i) moisture,(ii) volatile matter, consisting of gases and vapors driven off during torrefaction or pyrolysis,(iii) fixed carbon, the non-volatile fraction of biomass, and (iv) ash, the inorganic residue thatremains after combustion. The remaining fraction is a mixture of solid carbon (fixed carbon)and mineral matter (ash), which can be distinguished by further heating the sample in thepresence of oxygen; the carbon is converted to CO2 and only leaving the ash [9]. Table 1.5

Tabl

e1.

5Th

erm

oche

mic

alpr

oper

ties

ofth

ese

lect

edbi

omas

s(p

roxi

mat

ean

dul

timat

ean

alys

is).

Prox

imat

ean

alys

isU

ltim

ate

anal

ysis

(%w

t.dr

y)(%

wt.,

dry)

Bio

mas

sH

HV

(dry

)M

J/kg

Vol

atile

Ash

FCC

HO

NS

Cl

Ash

Ref

.

Bam

boo

18.7

73.5

63.

5119

.948

.62

5.90

45.1

50.

33—

——

[10]

Cor

nst

over

17.6

575

.17

5.58

19.2

543

.65

5.56

43.3

10.

610.

010.

66.

26[9

]C

orn

grai

n17

.286

.57

1.27

12.1

644

.00

6.11

47.2

41.

240.

14—

1.27

[9]

Coc

onut

shel

l19

.45

——

—47

.97

5.88

45.5

70.

30—

—0.

50[1

1]M

aize

stra

w—

——

—47

.09

5.54

39.7

90.

810.

12—

5.77

[9]

Oliv

ehu

sk—

——

—50

.90

6.30

38.6

01.

370.

032.

80[1

2]Pi

nesa

wdu

st20

.60

——

—50

.30

6.00

43.5

00.

10—

—0.

20[1

3]R

ape

seed

26.7

0—

——

58.5

18.

5723

.46

3.67

——

5.78

[15]

Ric

ehu

ll16

.14

65.4

717

.86

16.6

740

.96

4.30

35.8

60.

400.

020.

1218

.34

[9]

Saw

dust

18.0

6470

.55

0.83

16.3

545

.66

4.86

34.9

41.

380.

06[9

]R

ice

husk

15.6

855

.519

.52

14.9

938

.43

2.97

36.3

60.

490.

070

21.6

8[9

]R

ice

stra

w16

.28

69.3

313

.42

17.2

541

.78

4.63

36.5

70.

700.

080.

3415

.90

[9]

Suga

rca

neba

gass

es17

.33

73.7

811

.27

14.9

544

.80

5.35

39.5

50.

380.

010.

129.

79[9

]Sw

itch

gras

s18

.64

81.3

63.

6115

.03

47.4

55.

7542

.37

0.74

0.08

0.03

3.50

[9]

Wat

erhy

acin

th16

.02

—22

.40

—41

.10

5.29

—1.

960.

41—

—[9

]W

heat

stra

w17

.51

71.3

08.

9019

.80

43.2

05.

0039

.40

0.61

0.11

0.28

11.4

0[9

]


provides both the proximate and ultimate analysis (dry basis) for a wide range of biomassmaterials. Ultimate analysis deals with the determination of the carbon and hydrogen inthe material, are found in the gaseous products after combustion. Using these analysis, themolecular weight analysis becomes simpler. For example, cellulose and starch having thegeneric molecular formula C1H1.7O0.83, hemicelluloses can be represented by C1H1.6O0.8and wood by C1H1.7O0.83. Typical thermochemical properties of some selected biomassmaterials based on proximate and ultimate analysis are given below (Table 1.5) [9–15].

The calorific value of the char and the conversion efficiency based on calorific valueare given in Table 1.5. The higher heating value (HHV) of the biomass is calculated byimplementing the HHVs of lignocellulosic fuels, as the equation given below [16]:

HHV(MJ/Kg) = 0.335(C) + 1.423(H) − 0.154(O) (1.1)

Chaniwala and Parikh [17] have developed an empirical correlation based on elemental andproximate analysis to predict the HHV of raw biomass as stated below:

HHV(MJ/Kg)= 0.3491(C)+ 1.1783(H)− 0.10(S)− 0.0134(O)− 0.0151(N)− 0.0211(A)

(1.2)

Here C, H, S, O, N, and A refer to the weight percent of carbon, hydrogen, sulfur, oxygen,nitrogen, and ash in biomass respectively.

1.3.2 Inorganic Minerals’ Ash Content and Properties

Fuel contains various impurities in the form of incombustible components mainly known asash. Ash itself is undesirable, since it requires purification of the flue gas for particles withsubsequent ash and slag disposal as a result. The ash from wood comes primarily from soiland sand absorbed into the bark. Wood also contains salts thus having the importance to thecombustion process. They are primarily potassium (K), and partly sodium (Na), based saltsresulting in sticky ash, which may cause deposits in the boiler unit. The Na and K contentsin wood are normally so low that they will not cause problems for traditional heating tech-nologies. Typical mineral fractions in wood chips expressed as percentage of the dry matter(DM) of the wood are shown in Table 1.6. Apart from all these individual analysis processes,NREL researchers have developed a very interesting and rapid analysis method for biomass

Table 1.6 Total inorganic components ofplant biomass (dry basis).

Elements % of dry basis

Potassium (K) 0.1Sodium (Na) 0.015Phosphorus (P) 0.02Calcium (Ca) 0.2Magnesium (Mg) 0.04Chlorine (Cl) 0.2 to 2.0Silica (Si) 0.2 to 15


composition using near-infrared (NIR) spectroscopy. By applying this technique, the lightreflected off a biomass sample is analyzed to determine the sample’s composition [8, 18].

1.4 Biomass Conversion Routes

By a number of processes, biomass can be converted into solid, liquid, and gaseous fuels.The technologies include thermal, thermochemical, and biochemical conversions. Reac-tions involved during conversion are hydrolysis, dehydration, isomerization, oxidation,de-hydrogenation, and hydrogenation. The actual processes included these technologiesare combustion, pyrolysis, gasification, alcoholic fermentation, liquefaction, and so on [8].A schematic flow diagram for biomass conversion is shown in Figure 1.2. The main productsof conversion technologies are energy (thermal, steam, electricity), solid fuels (charcoal,combustibles), and synthetic fuels (methanol, methane, hydrogen gas, etc.). These can beused for different purposes such as cooking, lighting, heating, water pumping, electric-ity generation, and as industrial and transport fuels. Biomass fuels and residues can beconverted to energy via thermal, biological, chemical, and physical processes.

In a commercial process, biodiesel is produced by the reaction of vegetable oil oranimal fat with methanol in the presence of base or acid catalysts. Concerns over thedownstream processing of the homogeneous transesterification processes have motivatedintense research on the heterogeneously catalyzed transesterification process [18, 19]. Ingeneral, heterogeneous biodiesel production processes have few numbers of unit operations,with simpler separation and purification steps for products as no neutralization process isrequired. There are three types of solid catalysts: acid, base, and enzyme. Solid base cata-lysts, such as alkaline–earth metal hydroxide, oxides, and alkoxides such as Ca(OH)2, CaO,and Ca(CH3O)2 function as effective catalysts for the transesterification of triglycerides[18, 20]. The main advantage of solid acid catalysts is their ability to carry out the esteri-fication of free fatty acids and transesterification of triglycerides simultaneously [20–23].Moreover, these are reactive on esterification and transesterification reactions at relativelylow temperatures (i.e., 80 ◦C), as shown in Figure 1.3 [8].

Lipase has been shown to have a high catalytic reactivity to produce high quality biodiesel[18, 20–23]. As lipases break down natural lipids and oils into free fatty acids and glycerol,therefore this group of enzymes is widely used in the leather and detergent industries.Recent findings show that an alternative acyl acceptor, such as methyl acetate is used toreplace methanol, and it can obtain methyl ester yield up to 92%. In addition, the byproduct(glycerol) has a more expansive market, which can further be used for H2 production,acrolein, or several other chemicals [20].

In thermal conversion, combustion is already practiced widely, where as; gasificationattracts high level of interest as it offers higher efficiencies compared to combustion.Pyrolysis is interesting as it results into liquid product that offers advantages in storage, easytransport and versatility in applications, although it is still at a stage of early development[8, 23].

1.4.1 Pyrolysis

There are different types of pyrolysis carried out under various operating conditions, amongwhich fast, intermediate, flash, and slow having the substantial importance in the conversion


Ethanol, amino acid, bio hydrogen and protein based

chemicals

Ethanol, Gobar gas

Chemical conversion

Physicalconversion

Biochemical conversion

Thermo chemical

conversion

FT oil, syngas,solvents, acids

Bio-hydrogen,Conditioned gas

Direct

Indirect

Fast

Intermediate liquefaction

Flash

Slow

Vacuum

Torrefaction

Direct

Heavy oil

Bio oil, biogas, char, tar

Mechanical extraction

Briquetting

Distillation

Supercriticalconversion

Solventextraction

Hydrolysis

Leaching

Liquid liquid extraction

Acid hydrolysis

Enzymatichydrolysis

Cellulose, hemicelluloses, lignin, sugar

Primary and secondary metabolites

Cellulose, hemicelluloses,lignin

Partly microbial

Various anaerobes

Facultative group

Cyano bacteria

Klebsiella and clostridium

Batch

Fed batch

Continuous

Semi arranged continuous

flow arrangement

Enzyme

Fermentation

Anaerobic

Liquefaction

Pyrolysis catalytic/non-

catalytic

Gasification (partial air)

Combustion (excess air)

Biomass Feedstock

Photosynthesis bacteria

Figure 1.2 Different conversion routes to get end products (liquid and gases). (Adopted fromMohanty et al., 2014 [3])


H2C

O R

C

O

H3C

H2CR

C

O

OH

CH OH

H2C

OH

H3C

O R

C

O

H3C

O R

C

O

H2C

O R

C

Triglyceride Methanol

Catalyst

Methyl Ester Glycerol

O

O

+ +

C OH

C OH

C OH

HC

O R

C

Figure 1.3 Reaction scheme of transesterification reaction.

of biomass to different liquid and gaseous products

CnHmOk → (1 − n) CO + (m/2) H2 + C 180 (kJ/gmol) (1.3)

CnHmOk → (1 − n) CO + ((m − 4)/2) H2 + CH4 300 (kJ/gmol) (1.4)

1.4.1.1 Fast Pyrolysis

Currently, targeting the liquids production through fast pyrolysis is capturing the interest.The main features of fast pyrolysis are high heating rates and short vapor residence time.It generally requires a feedstock prepared with smaller particle sizes and a design thatremoves the vapors quickly from the presence of the hot solids. There are a number ofdifferent reactor configurations that can achieve this, including ablative systems, fluidizedbeds, stirred or moving beds, and vacuum pyrolysis systems.

Fast pyrolysis occurs in few seconds or less. Therefore, not only chemical reactionkinetics but also heat and mass transfer processes, as well as phase transition phenomena,play important roles. The critical issue is to bring the reacting biomass particle to anoptimum process temperature and to minimize its exposure to the intermediate (lower)temperatures that favor formation of charcoal. This can be achieved by using smallerparticles in fast pyrolysis as biomass decomposes to generate vapors, aerosols, and charcoal.After cooling and condensation, a dark brown liquid bio-oil is formed having the heatingvalue of about half that of conventional fuel oil. Fast pyrolysis is an advanced process, withcarefully controlled parameters to give higher yields of liquid. The essential features ofthe fast pyrolysis process for producing liquids are: (i) very high heating and heat transferrates at the reaction interface, (ii) which usually requires a finely ground biomass feed,a carefully controlled pyrolysis reaction temperature of around 450–600 ◦C and a vaporphase temperature of 400–450 ◦C, short vapor residence times of typically less than 2 s,and rapid cooling of the pyrolysis vapor to produce the bio-oil product. The main product(bio-oil) is obtained in yields of up to 75% wt on a dry feed basis (in case of wood), togetherwith byproduct char and gases which are used within the process so there are no wastestreams other than flue gas and ash. During pyrolysis, how different variants within themain operating parameters affect the yield and product distribution is tabulated in Table 1.7


Table 1.7 Range of the main variants with main operating parameters and characterizationfor pyrolysis methods. (Adopted from Mohanty et al., 2014 [3])

Different pyrolysis process Slow Intermediate Fast Flash

Feed Scores of feed reported

Temperature (◦C) Range 250–750 320–500 450–1050 550–1300Typical 350–400 350–450 550–750 1050–1150

Time Range min 15 min 0.5–10 <1 sTypical 2–30 min 4 min 0.5–5 <1 s

Heating rate (◦C/s) 1–50 10–100 100–500 >1000Particle size (mm) 5–50 5–50 <1.0 <0.2Yields (% wt) on dry basis

Char Range 2–60 19–73 15–35 10–35Typical 25–35 30–40 20–25 10–20

Liquid Range 0–60 25–60 20–75 20–65Typical 20–50 35–45 46–53 46–71

Gas Range 0–60 20–40 10–25 11–28Typical 20–50 20–30 11–15 15–22

[24]. Some researchers have defined this process as thermolysis, in which a material, likebiomass, is rapidly heated to high temperatures in the absence of air (specifically oxygen).

1.4.1.2 Intermediate Pyrolysis

During intermediate pyrolysis the reactor is operated at temperatures ranging between 400and 550 ◦C and the reactor consists of two coaxial conveyor screws, an inner screw and acovering screw widely known as a pyrolyzer. When the outer screw transports the biocharfrom one end to the other end of pyrolyzer, the chars act as a heat carrier with bed formation.The intermediate pyrolysis of biomass is carried out in a very reasonable way, resulting inbio-oil with low tar yields and viscosity, which is distinctive in intermediate pyrolysis incomparison to fast pyrolysis. Typically, this reactor has the flexibility to provide a moderateresidence time [25]. This is only the case for woody biomass, when it leads towards aherbaceous stream it fluctuates to larger extent leading to a liquid phase high in water,acids, and tars. In terms of other feedstock like straws, grasses, or industrial residues fromagricultural products like husks the picture is very different. The intermediate pyrolysisprevents the formation of high molecular tars with dry and brittle chars suitable for otherapplications like biofertilization and gasification. The advantage of such pyrolysis is thatthe non-milling character endures with the pellet charged to the pyroformer. The ease ofaccess for larger sized feedstock offers the opportunity to separate it easily as a char; andto enrich a tied gasifier with the low ash content of biochar from the pyroformer [25].The haloclean process was primarily developed for the thermal treatment of halogenatedpolymeric wastes. Any contaminated biomass can be handled inside a kiln heated fromoutside, with single- or double-screw rotation either clockwise or anticlockwise or both asper the equipment design and flexibility. This pyrolysis facilitates operating conditions for


preventing the formation of high molecular tar and enhancing quality, that is, the drynessand brittleness of the char which can be further utilized for the purpose of fertilization andcarbon sequestration. In this case, mechanical briquetting is not required for the processingof feedstock.

1.4.1.3 Slow Pyrolysis

Slow pyrolysis is also termed as carbonization due to similarities in its process conditions,like low temperature and more residence time. It can be divided into traditional charcoalmaking and more modern processes that are characterized by slower heating rates, relativelylong solid and vapor residence times, and usually a lower temperature than fast pyrolysis,typically 400 ± 10 ◦C. The target product is often the char, but this is accompanied byliquid and gas products, although these are not always recovered. Traditional processes,using pits, mounds, or kilns, generally involve some direct combustion of the biomass,usually wood, as a heat source in the kiln. Liquid and gas products are often not collectedbut escape as smoke, with consequent environmental issues [1, 25]. It can be characterizedby slow biomass heating rates, low temperatures, and lengthy gas and solid residence times.Depending on the system, heating rates are about 0.1 to 2 ◦C per second and prevailingtemperatures are around 500 ◦C. Gas residence time may be greater than 5 s. Duringconventional pyrolysis, the biomass is slowly devolatillized; hence tar and char are the mainproducts. This process yields a different range of products whose form and characteristicsare dependent on the temperature, oxygen level, and process time used.

1.4.1.4 Torrefaction

This is a thermochemical treatment of biomass in the temperature range of about 200 to320 ◦C, a kind of mild pyrolysis process that improves the fuel properties of biomass. It iscarried out under atmospheric conditions and in the absence of oxygen. During this process,the water contained in the biomass, as well as superfluous volatiles, are removed, whilethe biopolymers (cellulose, hemicelluloses, and lignin) partly decompose by giving offvarious types of volatiles. The final product is the remaining solid, dry, blackened materialwhich is referred to as “torrefied biomass” or “biocoal” [26, 27]. Torrefied products andvolatiles are formed, resulting in a hardened, dried, and more volatile-free solid product.The product is at much higher energy density than the raw biomass, increasing the distanceover which the biomass can be transported to plants for use or further processing, becauseof its relative lower weight and volume. Torrefied biomass is also hydrophobic, meaningit can be stored in the open space for long periods without taking up water, similar tothe infrastructures used for coal. Torrefied biomass requires less energy to crush, grind, orpulverize and the same tools as for crushing coal can be used. Therefore, a well-developedbiomass refinement method must interact and be integrated to obtain a biomass to liquid(BTL) process with high well-to-wheel efficiency.

Other developments have led to slow/intermediate pyrolysis technologies to create thatare of much attention for 3-different pyrolysis product distribution in wide ranges. Theseare generally based on a horizontal tubular kiln where the biomass is moved at a controlledrate through the kiln; these include agitated drum kilns, rotary kilns, and the screw pyrolyzer[28]. In several cases these have been adapted for biomass pyrolysis from their original uses,such as the coking of coal with production of “towns-gas” or the extraction of hydrocarbons


(Plant biomass)

Low molecular, macromolecular and

Polysaccharide substances

Extractives (organic matters)

Cellulose 30–45%

Hemicellulose 20–25%

Lignin14–32%

Ash

Non condensable

gases

Aqueous phase

Organic

(Liquid oil)

phase

Char

Ash (inorganic matter)

Cannot mix with hydrocarbon, cannot be distilled, substitute for fuel oil and diesel

As heating continues there is an 80% loss of weight and remaining cellulose is

converted to char. Prolonged heating or

exposure to higher temperature (900 K) reduces char formation to 9%

CO, CO2, CH4, H2, C2 - C5

Methanol, acetic acid and acetone

Inherent organic material contains S and Cl contains alkali material

Figure 1.4 Biomass component pyrolytic conversion for biorefineries. (Adopted fromMohanty et al., 2014 [3])

from oil shale (e.g., the Lurgi twin-screw pyrolyzer). Although some of these technologieshave well-established for commercial applications, yet and yet considerable numbers ofcommercial applications are still under development to acquire potential market value withbiomass to biochar production. The liquid fraction of the pyrolysis products consists oftwo phases: an aqueous phase containing a wide range of organo-oxygenate compoundswith low molecular weight, and a non-aqueous phase containing insoluble organics (mainlyaromatics), phenolic compounds of higher molecular weight. This non-aqueous phase iscalled bio-oil, which is a product of current interest. The ratios of acetic acid, methanol,and acetone of the aqueous phase were higher than those of the non-aqueous phase. Forchar production, one has to focus on low temperature and low heating rate; however, formaximum flue gas production a high temperature, low heating rate, and long residencetime process would be preferable [29]. Distinct involvement of components’ during thepyrolysis process is summarized in Figure 1.4.

1.4.1.5 Gasification

This is an alternative thermochemical conversion technology suitable for the treatment ofbiomass or other organic matter, including municipal solid wastes or hydrocarbons suchas coal. It involves partial combustion of biomass under a gas flow containing a controlledlevel of oxygen at relatively high temperatures (500–800 ◦C) yielding a main productof combustible producer gas/syngas with some char with low carbon percent. The mainreaction involved during the gasification process is given below.

Partial oxidation can be represented by these reaction schemes:

CnHmOk + (1/2) O2 → nCO + (m/2) H2 71 (kJ/gmol) (1.5)

CnHmOk + O2 → (1 − n) CO + CO2 + (m/2) H2 − 213 (kJ/gmol) (1.6)

CnHmOk + 2O2 → (n/2) CO + (n/2) CO2 + (m/2) H2 − 778 (kJ/gmol) (1.7)


Although designed for produce gas, under some conditions gasifiers can produce reasonableyields of producer gas, syngas, and char for an effective energy decentralization process[28]. The syngas production from biomass gasification can be reformed into a variety ofchemicals like methanol, olefins, green diesel, gasoline, and wax through Fischer Tropschroutes [30].

1.4.1.6 Hydrothermal Carbonization

This is a completely different process involving the conversion of carbohydrate componentsof biomass (from cellulose) into carbon-rich solids in water at elevated temperatures andpressures [31]. Under acidic conditions with catalysis by iron salts the reaction temperaturemay be as low as 200 ◦C. The process may be suitable to concentrate the carbon (%) andto handle the high moisture content in the waste streams that would otherwise requiredrying before pyrolysis, making it complementary to pyrolysis and a potential alternativeto anaerobic digestion.

1.4.1.7 Combustion

Combustion is the rapid oxidation of fuel to obtain energy in the form of heat. For combus-tion, biomass is used as the main feedstock and is primarily composed of carbon, hydrogen,and oxygen. Further it can produce H2, CO, CO2, and water by partial combustion [9].Combustion takes place in the presence of excess air; therefore carbon dioxide and water arethe pivotal components of gasification. At lower temperatures, formation of hydrocarbonstakes place during gasification in the fluidized bed reactors. The flame temperature cango beyond 2000 ◦C, depending on various factors like the heating value and the moisturecontent of the fuel, the amount of air used to burn the fuel, and construction of the furnace.For combustion, mainly a combustor is used as the device to convert the chemical energyof fuels into high temperature exhaust gases [15, 16].

1.5 Bio-Oil Characteristics and Biochar

Bio-oil is typically a dark brown liquid with a smoky acrid smell. It tends to have relativelyhigh water content – typically in the range of 20 to 25% [9]. The water comes from thepyrolysis conversion process, as well as from the initial water in the biomass feedstock.When the water content of the bio-oil is in the 20 to 25% range, it is entirely misciblein bio-oil (i.e., it does not separate). At higher moisture levels, the water can tend toseparate from the bio-oil. To prevent this from happening, it is desirable to have theincoming biomass feedstock dried to 10% moisture content, or less, before it is fed into thepyrolysis conversion process [1, 8, 23]. Bio-oil characteristics vary somewhat, dependingon the production technology and the type of biomass feedstock from which the bio-oilis produced. This means that bio-oil fuel specifications are likely to be fairly important.Bio-oil’s energy content is in the range of 18–23 MJ/kg. (At the higher end of this range,there will typically be greater amounts of suspended char in the bio-oil.) Conventionalheating oil has an energy content of about 42 ± 1 MJ/kg (lower heating value), thus bio-oilhas about 52 to 58% as much energy as heating oil per gallon. However, it is interesting tonote out that bio-oil weighs about 40% more per gallon than heating oil [9]. Bio-oil is a free


flowing liquid. Its viscosity tends to be slightly higher than conventional no. 2 fuel oil. Asthe water content in bio-oil increases, its viscosity decreases (as does its energy content).Bio-oil is moderately acidic, having a pH in the range of 2.5 to 3.5 (similar to the acidityof vinegar). This means that bio-oil fuel storage tanks will need to be made of a materialthat will not corrode due to acidic character of the fuel (i.e., they will need to be madeof materials such as stainless steel, plastic, fiberglass, etc.). Bio-oils are multicomponentmixtures comprised of different size molecules derived primarily from the depolymerizationand fragmentation reactions of three key biomass building blocks: cellulose, hemicellulose,and lignin. Therefore, the elemental composition of bio-oil resembles that of biomass ratherthan that of petroleum oils [32,33]. This raises a significant issue regarding the use of bio-oilin existing residential or commercial installations, since most of the existing fuel storagetanks used for heating oil are likely to be made of plain mild steel or stainless steel that isvulnerable to corrosion from bio-oil. As a result, it will generally be necessary to install anew fuel storage tank if bio-oil is to be used for an existing heating oil installation [9, 32].Bio-oil is a complex mixture of oxygenated compounds, which carries potential drawbacksas well as potential benefits: from a fuel storage perspective, bio-oil is not as stable aspetroleum fuel. However, bio-oil developers (such as Dyna-Motive, part of DynamotiveEnergy Systems Corporation) have found that bio-oil samples stored for over a year haveremained stable [34]. Producing bio-oil with a lower ash (char) content and/or a lower watercontent helps in prolong stability of bio-oil during storage [29].

Growing concerns about climate change have brought biochar into the limelight. Combus-tion and decomposition of woody biomass and agricultural residues results in the emissionof a large amount of carbon dioxide. Biochar can store this CO2 in the soil, leading to areduction in GHGs emission and enhancement of soil fertility. In addition to its potential forcarbon sequestration, biochar has many other advantages [23]. It can increase the availablenutrients for plant growth, increase water retention, and reduce the amount of fertilizer usedby preventing the leaching of nutrients out of the soil. It can reduce methane and nitrousoxide emissions from soil, thus further reducing GHGs emissions, and can be utilized inmany applications as a replacement for other biomass energy systems. Biochar can be usedas a soil amendment to increase plant growth yield. Further, the char can be used as a solidfuel in boilers and can be converted into briquettes alone or mixed with powdered biomassfor high efficiency fuel. The char could be used for the production of activated carbon.Furthermore, the possibility of using this carbon feedstock for making carbon nano tubescan be explored. However it can also be used further for the gasification process to obtainhydrogen rich gas by thermal cracking [8, 9, 23].

1.6 Scope of Pyrolysis Process Control and Yield Ranges

The primary products of lignocellulose (hemicellulose and cellulose) decomposition arecondensable vapors (yields to liquid products) and gases. Lignin decomposes to liquid, gas,and solid char products. Extractives contribute to liquid and gas products either throughsimple volatilization or decomposition. Minerals in general remain in the char, finallyconverted into ash. This distribution of components into products is shown schematicallyin Figure 1.4.


Vapors formed by primary decomposition of biomass components can be involved insecondary reactions in the gas phase, forming soot, or at hot surfaces – especially hotchar surfaces where a secondary char is formed [35]. This is particularly important inunderstanding the differences between slow, intermediate, and fast pyrolysis and the factorsaffecting oil, gas, and char yields. Minerals in biomass, particularly the alkali metals, canhave a catalytic effect on pyrolysis reactions leading to increased char yields in somecircumstances, in addition to that the effect of ash also contributing directly to char yield[35–38]. After synthesis of bio-oil, the physicochemical properties of the bio-oil can betested by using the standard method, making a comparison with conventional diesel astabulated in Table 1.8 [9, 23, 24].

Table 1.8 Summary of typical properties and characteristics of biomass derived crudebio-oil.

Property ASTM D975 (diesel) Pyrolysis oil

pH 2.5–3.5Flash point 52 ◦C min —Moisture content < 0.05 max vol.% 15–25%Elemental analysis — C = 56.6% H = 6.2% N = 0.1%

O = 37.2%Odor Smoky smellWater and sediment 0.05 max vol.% 0.01–0.04Kinematic viscosity

((mm2/S) 40 ◦C)1.3–4.1 mm2/s 25–1000

Sulfated ash — —Ash 0.01 max wt.% 0.05–0.01 wt.%Sulfur 0.05 max wt.% 0.001–0.02 wt.%Density (kg/m3) 820–845 ∼900–1200Iodine number — 90–125Aging — As viscosity increases, volatility

decreases, phase separation, slowdecomposition and deposition ofa tarry layer happens over time.

Miscibility Miscible in ethanol Miscible with polar solvents likemethanol, acetone, etc., buttotally miscible withpetroleum-derived fuels

Acid value KOH.mg.g−1 0.16Boiling temperature 260–315 ◦CAppearance Black or dark red-brown to dark

greenCetane number 40 min 48–65Aromaticity (%Vol, max) — —Carbon residue 0.35 max mass% 0.001–0.02 wt.%Distillation temperature

(90% volume recycle)275–611 ◦C max —


1.6.1 Moisture Content

This can have different effects on pyrolysis product yields depending on the conditions[35]. Fast pyrolysis processes in general require fairly dry feed, around 10% moisture, sothat the rate of temperature rise can not restricted by evaporation of water. Slow pyrolysisprocesses are more tolerant of moisture, the main issue being the effect on process energyrequirement. For charcoal making, wood moisture contents of 15–20% are typical [36,37].In all pyrolysis processes, water is also a product which collects together along with othercondensable vapors in the liquid product. Moisture in the reaction affects char properties,which in turn helps to produce activated carbons through pyrolysis of biomass.

1.6.2 Feed Particle Size

This can significantly affect the balance between char and liquid yields. Larger particlesizes tend to give more char by restricting the rate of disengagement of primary vaporproducts from the hot char particles, so increasing the scope for secondary char-formingreactions [35].

1.6.3 Effect of Temperature on Product Distribution

The temperature profile is the most important aspect of operational control for pyrolysisprocesses. Material flow rates, both solid and gas phases, together with the reactor tem-perature control, are the key parameters of heating rate, peak temperature, residence timeof solids, and contact time between solid and gas phases. These factors affect the productdistribution and the product properties. For fast pyrolysis, a rapid heating rate and a rapidrate for of cooling for vapors are required to minimize the extent of secondary reactions.These reactions not only reduce the liquid yield but also tend to reduce its quality, yieldinga more complex acidic mixture, an increased degree of polymerization and higher viscosity[8, 37]. Conversely, in slow pyrolysis there is some evidence that slow heating leads tohigher char yields, but this is not consistent [35]. Higher temperatures lead to lower charyield in all pyrolysis reactions [9, 35].

1.6.4 Solid Residence Time

This is also important but to a lesser degree than peak temperature, longer time at temper-ature leading to lower char yield [35]. The effect of temperature on liquid and gas yieldsis more complex. Liquid yields are higher with increased pyrolysis temperatures up to amaximum value, usually at 400–550 ◦C but dependent on equipment and other conditions.Above this temperature, secondary reactions causing vapor decomposition become moredominant and the condensed liquid yields are reduced [25, 39].

1.6.5 Gas Environment

Gas environment conditions in the gas phase during pyrolysis have a profound influenceon product distributions and on the thermodynamics of the reaction. Most of the effectscan be understood by considering the secondary char-forming reactions between primaryvapor products and hot-char [35]. The gas flow rate through the reactor affects the contact


time between primary vapors and hot char and also affects the degree of secondary charformation. Low flows favor char yield and are preferred for slow pyrolysis; high gasflows are used in fast pyrolysis, effectively stripping off the vapors as soon as they areformed.

1.6.6 Effect of Pressure on Product Distribution

Pressure has a similar effect. Higher pressure increases the activity of vapors within andat the surfaces of char particles, so increasing secondary char formation. The effect ismost marked at pressures up to 0.5 MPa. Conversely, pyrolysis under vacuum gives littlechar, favoring liquid products. For pyrolysis under pressure, moisture in the vapor phasecan systematically increase the yield of char, believed to be due to an autocatalytic effectof water, reducing the activation energy for pyrolysis reactions. The thermodynamics ofpyrolysis are also influenced by the gas environment. The reaction is more exothermic athigher pressures and low flow rates. This is rationalized as being due to the greater degree ofsecondary char-forming reaction occurring. Hence, higher char yields are associated withconditions where pyrolysis is exothermic; such conditions will favor the overall energybalance of processes targeting char as product [8, 23].

1.7 Catalytic Bio-Oil Upgradation

Steam reforming, partial oxidation, and autothermal reforming (ATR) can be an attractiveprocesses for the upgradation of bio-oils. ATR is a combination of steam reforming andpartial oxidation of the hydrocarbons to produce CO, CO2, and H2. Bio-oil obtained fromthe pyrolysis consists of a complex mixture of aliphatic and aromatic oxygenates andparticulates. It is a very viscous, acidic, and unstable liquid with relatively low-energydensity compared to conventional fossil oil. Such poor quality bio-oil requires costly post-treatment and makes the complete process less economically attractive. The presence ofproper catalysts during the pyrolysis process can affect the network of reactions and upgradethe bio-oil. Providing good contact between the solid catalyst and solid biomass/waste isessential to improve the efficiency of the pyrolysis process [29, 34, 39,40, 41]. The presenceof proper catalysts during the pyrolysis process can affect the network of reactions (e.g.,deoxygenation) and allows in situ upgrading of the bio-oil. This catalytic upgradation ofdifferent bio-oils produced from pyrolysis can be further deoxygenated with energy contentimprovement through different reactor configurations and reaction parameters, which istabulated in Table 1.9 [35, 36, 42–45]. Providing a good contact between the solid catalystand solid biomass is essential to improve the efficiency of the pyrolysis process [39–41].Further, a lower pyrolysis temperature is crucial for maximizing bio-oil yield and quality[42–44]. From an elemental analysis perspective, bio-oil produced from wood containsabout 56% carbon, 6% hydrogen, 37% oxygen, 0.1% nitrogen, 0.1% ash, and negligiblesulfur, which could translate into a number of benefits, (1) the high oxygen content of bio-oil could help to improve its combustion characteristics in comparison to petroleum-basedfuels, (2) it could help to reduce the amount of carbon dioxide emissions/pollution producedwhen bio-oil is burned as a fuel. The low nitrogen content of bio-oil could help to reduceNOx emissions. For example, tests of a combustion turbine showed that NOx emissions

Tabl

e1.

9C

atal

ytic

upgr

adat

ion

ofbi

o-oi

lwith

diffe

rent

reac

tor

confi

gura

tion

and

reac

tion

para

met

ers.

Bio

mas

sC

atal

yst

Rea

ctio

nte

mp.

(◦C

)R

eact

orB

io-o

ilG

asyi

eld

Yiel

dsof

H2

wt%

(kg

H2/k

gbi

omas

s)

Com

posi

tion

ofhy

drog

enin

gas

Ref

.

Oil

palm

shel

lLa

/Al 2

O3

900

Cou

nter

-cur

rent

fixed

bed

2.60

38.4

5vo

l.%[4

2,45

]

Oil

palm

shel

l𝛾-A

l 2O

390

0C

ount

er-c

urre

ntfix

edbe

d2.

6234

.63

vol.%

Ric

est

raw

Cr 2

O3

850

Pyro

lysi

sre

acto

r22

.87

49.5

wt.%

[43]

Saw

dust

Cr 2

O3

850

Pyro

lysi

sre

acto

r25

.751

.4w

t.%C

omm

erci

alw

ood

Cu/

MC

M-4

1—

0.87

9vo

l.%[4

5]

Saw

dust

Ni

700

5.3

—[4

2]O

akw

ood

Ni/F

eni

trat

e70

0H

oriz

onta

ltub

ular

reac

tor

44.5

mas

s%33

.1m

ass%

[45]

Pist

acia

khin

juk

seed

Cri

teri

on-

424/

BP3

198

700

Fixe

dbe

dre

acto

r66

.5%

/69.

2%21

%/2

5wt%

[42]

Com

mer

cial

woo

dbi

omas

sM

CM

-41

500

Ben

chsc

ale

fixed

bed

reac

tor

55.6

8w

t.%9.

87w

t%0.

02w

t%[4

5,36

]

Asp

enw

ood

Ni-

ZSM

-560

016

wt.%

[43]

Pine

chip

sQ

uart

zsa

nd45

027

.3w

t.%52

wt.%

[45,

36]

Pine

woo

dsa

wdu

stZ

SM-5

600

Sem

i-ba

tch

pyro

prob

ere

acto

r[4

5,36

]


Table 1.10 Density and volumetric energy content of various solid and liquid fuels.

Fuel Density (kg/m3) Volumetric energy content (GJ/m3)

Ethanol 790 23.5Methanol 790 17.6Biodiesel 900 35.6Bio-oil 1280 10.6Gasoline 740 35.7Diesel 850 39.1Agricultural residue 50–200 0.8–3.6Hard wood 280–480 5.3–9.1Softwood 200–340 4.0–6.8Baled straw 160–300 2.6–4.9Bagasse 160 2.8Rice hulls 130 2.1Nut shells 64 1.3Coal 600–900 11–33

using bio-oil were about half as much as when using diesel fuel [35–37, 45]. The lowsulfur content of bio-oil could also result in reduced SOx emissions compared to the use ofpetroleum-based fuel oil or diesel fuel. Bio-oil does not naturally blend with conventionalpetroleum fuel [28]. It may be possible to add a solvent or to emulsify mixtures of bio-oiland fuel oil in order to get homogeneous blends. Bio-oil manufacturers indicate that they areworking on techniques that will allow blending of bio-oil and fuel oil. They are optimisticthat workable approaches will be available in the future. But a necessary invention, whichhas been developed through the efforts of the Canadian government (Natural ResourcesCanada) that produces a stable bio-oil/diesel fuel mixture with properties similar to thoseof no. 2 fuel oil [36–38].

A broad comparison of both density and volumetric energy content of various solid andliquid fuels is tabulated in Table 1.10. The biomass feed, the char and liquid productshave energy values roughly related to their carbon contents. Release of this energy bycombustion can again be considered as renewable and is largely carbon neutral (someemissions are associated with feedstock production and transport); the carbon returned tothe atmosphere as carbon dioxide in the same way would otherwise have resulted frombiomass decomposition. If the char product is not burnt, but retained in such a way that thecarbon in it is stable, then that carbon can be equated to carbon dioxide removed from theatmosphere and sequestered [28].

The gas product is typically a mixture of carbon dioxide (9–55% by volume), carbonmonoxide (16–51%), hydrogen (2–43%), methane (4–11%), and small amounts of higherhydrocarbons [9]. The gases are usually present with nitrogen introduced to make theworking space inert; thus this can be treated as a diluent and ignored for material balancingbut will affect the heating value of the syngas. The carbon dioxide and nitrogen provideno energy value in combustion; the other gases are flammable and provide energy valuein proportion to their individual properties. Again, use of the energy in the gas can beconsidered renewable and largely carbon neutral. No special consideration of the carbon


dioxide in the pyrolysis gas is required as it is not additional to what would result frombiomass decomposition [1, 8].

1.8 Bio-Oil Reforming

The deleterious properties of high viscosity, thermal instability and corrosiveness presentmany obstacles to the substitution of fossil derived fuels by bio-oils. Steam reforming ofbio-oil or its model compounds is a simplified way to remove the oxygenated organiccompound (CnHmOk) by the following reactions [20, 29], where the enthalpy (kJ/gmol) ofeach step is given at reference temperature 27 ◦C and n = 6:

CnHmOk + H2O = nCO + mH2 310 kJ/gmol (1.8)

CnHmOk + nH2O → xCO + (n − x) CO2 + mH2 230 kJ/gmol (1.9)

The above reaction is followed by the water–gas shift reaction:

CO + H2O → CO2 + H2 (1.10)

The overall process can be represented as follows:

CnHmOk + (2n − k) H2O → nCO2 + (2n + m/2 − k) H2 64 kJ/gmol (1.11)

Upgrading bio-oil to the quality of transport liquid fuel still poses several technicalchallenges and difficulties and is not currently economically attractive. Some chemi-cals, especially those produced from the whole bio-oil (such as fertilizers) or its majorfractions (such as liquid smoke or for wood resins) offer more interesting commercialopportunities.

There are still many challenges to overcome before bio-oil finds large-scale acceptanceas a fuel, including: (i) the cost of bio-oil, this is 10 to 100% more than fossil fuel in energyterms; (ii) the availability of bio-oil for applications development remains a problem andthere are limited supplies for testing; (iii) the lack of standards for use and distribution ofbio-oil in consistent quality inhibits wider usage; (iv) considerable work is required to char-acterize and standardize these liquids and develop a wider range of energy applications; (v)the compatibility of bio-oil with conventional fuels and, therefore, the need for dedicatedfuel handling systems; (vi) users are unfamiliar with bio-oil; (vii) environmental health andsafety issues need to be completely resolved;(viii) pyrolysis as a technology does not enjoya good image; (ix) more research and development is needed in the fields of fast pyrolysisand bio-oil testing to develop large-scale applications. Figure 1.5 depicts the possibleroutes for the upgradation and conversion of bio-oil into various fine chemicals andhydrogen fuel and so on [8,9, 21, 23]. The most important issues that need to be addressedare: (i) scale-up; (ii) cost reduction; (iii) better oil quality; (iv) norms and standards forproducers and users; (v) environment health and safety issues in handling, transport, andusage; (vi) encouragement to implement processes and applications; (vii) informationdissemination [46].


Hydrocarbon

Light organic,heavy organic,

coke, tar

Light fractionof bio-oil andno. 2 diesel

Hydrogen

Transportationfuel

Syngas, producergas hydrocarbons

Ethanol

Fine chemicals,furfurals andhydrocarbon

SteamreformingHydro

cracking

Catalytichydro

treatment

Thermocatalytic effect

GasificationSub-andsupercriticalextraction

Hydro-deoxygenation

Catalytic crackingof pyro-vapours

Fermentation

Bio-oil upgradation

Emulsification Hydro treating

Figure 1.5 Integrated approach for bio-oil upgradation.

1.9 Sub and Supercritical Water Hydrolysis and Gasification

Water is an ecologically safe and abundantly available solvent in nature. Water has arelatively high critical point (374 ◦C and 22.1 MPa) because of the strong interactionbetween the molecules due to strong hydrogen bonds. Liquid water below the criticalpoint is referred to as subcritical water whereas water above the critical point is calledsupercritical water. The density and dielectric constant of the water medium play majorroles in solubilizing different compounds. Water at ambient conditions (25 ◦C and 0.1MPa) is a good solvent for electrolytes because of its high dielectric constant, whereasmost organic matters are poorly soluble at this condition [38, 47, 48]. As water is heatedup, the H-bonding begins to weaken, allowing dissociation of water into acidic hydroniumions, (H3O+ ) and basic hydroxide ions (OH−). It is important to mention that the dielectricbehavior of 200 ◦C water is similar to that of ambient methanol, 300 ◦C water is similar toambient acetone, 370 ◦C water is similar to methylene chloride, and 500 ◦C water is similarto ambient hexane [46, 49, 50]. Sub and supercritical water offers several advantagesover other biofuel production methods. Some of the major benefits are: (i) high energyand separation efficiency (since water remains in liquid phase and the phase change isavoided); (ii) high throughputs; (iii) versatility of chemistry and its mechanism (solid,liquid, and gaseous fuels); (iv) reduced mass transfer resistance in hydrothermal conditions;(v) improved selectivity for the desired energy products (methane, hydrogen, liquid fuel)or biochemical (sugars, furfural, organic acids, etc.); (vi) ability to use mixed feedstock aswell as wet waste biomass to produce biodiesel, which is considered a “carbon neutral”


fuel. After upgradation to generate high energy content bio-fuel it may become easy toreduce GHG emissions, with little or no toxicity [9, 48, 51].

The pyrolysis-derived bio-oil from different biomass origins was upgraded in suband supercritical ethanol using an appropriate catalyst. It is under intense research forthe supercritical upgrading process, as it performs better than the subcritical upgradingprocess. Mainly, acidic HZSM-5 facilitates esterification in supercritical ethanol to con-vert acids contained in crude bio-oil into various kinds of esters [20, 23]. Stronger acidicHZSM-5 (different Si/Al ratio) with a bimetallic catalyst effect can facilitate cracking ofheavy components of crude bio-oil more effectively in the supercritical upgrading pro-cess. Studies on promoter effects of alkali and other metals on cobalt are rare. Althoughthe effects of K, Zn, Cu, Mn, Ca, Al, and Zr have been used and can be studied exten-sively, the effects of these promoters on bimetallic catalysts are still not clear becausethese studies were conducted under different conditions or over different catalyst systems[38, 47, 48].

1.9.1 Biochemical Conversion Routes

Another promising approach that can be used for the production of chemicals is thebiochemical route, where bioethanol and biobutanol can be produced through hydrolysisin the presence of enzymes. In many countries like India, US ethanol plays a very criticalrole as a gasoline substituent and also as a feedstock for various chemicals. One ton of canecan produce approximately 100 liters of ethanol. Ethanol can be used for the production ofacetaldehyde, acetic anhydride, ethyl acetate, monoethylene glycol, and so on [24]. Duringbiochemical conversion, the aim is to extract cellulose, out of which one can easily extractethanol as a final product for its wide acceptance. Cellulose is protected by a sheath of ligninand hemicellulose that widely found in plant biomass. Researchers with leading roles aredeveloping pretreatment technologies to hydrolyze hemicellulosic sugars and open up thestructure of sugars to allow further enzyme hydrolysis of the cellulose to glucose [23, 29].Likewise many biomass researchers internationally have focused on a process involvingdilute acid hydrolysis of hemicellulose to a xylose and other sugars compounds [52]. Takingadvantage of conditioning and enzymatic hydrolysis, the material must be made less acidicfor enzymes and organisms to function optimally in the hydrolyzate environment. Duringthe process of pH adjustment or conditioning, the aim is to minimize sugar losses and topromote low hydrolyzate toxicity by removing toxic byproducts that inhibit enzyme andfermentation microorganism activity. The effectiveness of enzymatic hydrolysis depends ona variety of processing conditions, including different enzyme and solid loadings, mixingand conditioning methods, and pretreatment conditions. A new generation of enzymes andenzyme production technology is needed to cost-effectively hydrolyze cellulose and hemi-cellulose to free the sugars needed for fermentation. To get a high yield one has to focus ondecreasing the cost of the enzyme unit operation in the biomass saccharification process,which is a key factor for developing cost-competitive cellulosic ethanol. Starchy materialsare first cooked at 100 and 130 ◦C, and then hydrolyzed to glucose by using 𝛼-amylaseand gluco-amylase. After the development of enzymatic hydrolysis, research work is inprogress in close association with major industrial enzyme producers to apply recombinantDNA technology to bacteria and fungi to develop improved cellulose and hemicellulose


enzymes and to determine the most efficient method for producing these enzymes[9, 51, 53, 54].

1.9.2 Microorganisms for Fermentation

The fermentation process has been developed both at lab and industrial scale to evaluateand scale-up its use through biochemical plants for ethanol production. Researchers areapplying sophisticated metabolic engineering techniques to develop microorganisms thatcan more effectively ferment the variety of sugars derived from biomass. Lignocellulosicbiomass contains five-carbon sugars such as xylose (from the hemicellulose) as well as themore common six-carbon sugars, such as glucose found in grains. These make fermenta-tion and other bioprocessing processes far more challenging. Researchers are developingmicroorganisms that can co-ferment all the sugars in biomass to improve ethanol pro-duction economics. Sophisticated metabolic engineering techniques, like the applicationof Zymomonas mobilis can co-ferment both xylose and arabinose along with glucose.With industrial partners, researchers are working to develop designer strains for specificfeedstocks, feed streams, and processes and to validate the performance of these strains[46, 52, 55–57].

1.9.3 Integrating the Bioprocess

After integrating all the unit operations of biomass conversion through biochemical routeswith extensive knowledge of the individual unit operations, one can select different ligno-cellulosic biomasses for industrial application, and on demonstrating integrated processesat the mini-pilot and pilot scales one can attain the production of bioethanol for further use[24, 58].

C6H12O6 → 2C2H5OH + 2CO2 (1.12)

Ethanol fermentation can be carried out at room temperature and atmospheric pressurewhere Saccharomyces cerevisiae is used as the yeast for its excellent ethanol ability andethanol tolerance. The yeast strain produces approximately 51 g of ethanol from 100 g ofglucose according to the Equation 1.12. In this reaction, around 50% of carbon is consumedin terms of CO2 production, in fact 91% of energy contained in glucose (2.87 MJ/mol) isretained in ethanol. Whereas S. cerevisiae has the ability to ferment many sugar moleculeslike glucose, fructose, galactose, mannose, sucrose, and maltose, still many researchers aretrying to identify the potential of S. cerevisiae to explore and break pentoses like xyloseand arabinose [59,60]. Table 1.11 summarizes the various possible biobased products withtheir broad classification based on their market demand and industrial use [55].

Questions

1. What are the differences between torrefaction, pyrolysis, and gasification?2. What are the main building blocks of biomass?3. What are first, second, third, and fourth generation biofuels?

Tabl

e1.

11B

ioba

sed

prod

uctw

ithcl

assi

ficat

ions

and

mar

kets

trat

egy.

Bio

base

dpr

oduc

tC

lass

ifica

tions

Mar

keto

ppor

tuni

tyM

arke

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trim

ethy

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astic

s,Po

lym

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ins

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surp

ass

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dpo

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nete

reph

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(PET

)in

fiber

appl

icat

ions

and

poly

buty

lene

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phth

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e.Fo

rny

lon,

PET,

poly

-but

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dte

reph

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PTT

isa

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perf

orm

ance

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kabl

e“s

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chre

cove

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prop

ertie

s,an

dis

used

inap

pare

l,up

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tery

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carp

et.

Acr

ylic

acid

Adh

esiv

es,p

olym

ers

Acr

ylat

es(e

.g.,

coat

ings

,adh

esiv

es),

co-m

onom

er,s

uper

abso

rben

tpol

ymer

s,de

terg

entp

olym

ers.

For

acry

licac

idde

rivat

ives

Acr

ylon

itrile

Poly

mer

sA

cryl

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(car

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acry

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2Anaerobic Digestion

Lynsey Melville,1 Andreas Weger,2 Sonja Wiesgickl2 and Matthias Franke2

1Centre for Low Carbon Research (CLCR), Birmingham City University, UK2Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg, Germany

2.1 Introduction

Anaerobic digestion (AD) is a biochemical process that harnesses the complex metabolicreactions of a specific community of synergistic microorganisms. These microorganisms,in the absence of oxygen, are able to progressively transform organic materials into biogas(consisting of mainly carbon dioxide and methane) and an innocuous solid and liquidresidue. Whilst AD is an effective process for deriving energy from purpose-grown biomass,this process also plays an important role in recovering the energy from organic wastesand capturing and treating the greenhouse gases (GHGs) associated with their disposal inlandfill [1].

This chapter will outline the key biochemical reactions involved in the anaerobic diges-tion of biomass, typical designs and applications and how they are monitored, controlledand optimised.

2.1.1 Microbiology of Anaerobic Digestion

Anaerobic digestion is a natural phenomenon which can be observed in the anaerobicsediments of lakes and swamps and the digestive tracts of animals, in particular ruminants.By simulating an appropriate environment, the microorganisms found in these environmentscan be cultivated and controlled in closed reactors called digesters.




There are three categories of microorganisms that degrade organic material:

• strict aerobes

• facultative anaerobes

• obligate anaerobes (strict or aerotolerant).

Strict aerobes are commonly used in wastewater treatment processes (e.g. trickling filtersand activated sludge plants) and actively utilise free molecular oxygen to degrade solubleorganic compounds in the wastewater. Species such as nitrobacter sp. and nitrosomonassp. oxidise nitrite (NO2

−) to nitrate (NO3−) and ammonium (NH4

+ ) to nitrite (NO2−)

respectively. They also assist in the formation of flocs and sludge bulking. These bacteriacannot survive in anaerobic conditions.

Facultative anaerobes can survive in the presence or absence of molecular oxygen. Ifoxygen is present it can be utilised in enzymatic processes. In the absence of molecularoxygen, facultative anaerobes utilise other molecules such as nitrate (NO2−

3) to convertmolecules via a process called denitrification. This process converts organic molecules intocarbon dioxide (CO2), water (H2O), hydroxides (OH−) and nitrogen (N2). An example ofa facultative anaerobe is Escherichia coli. Obligate anaerobes can be further divided intostrict anaerobes (which cannot grow and may lose viability in the presence of oxygen) andaerotolerants which do not use oxygen to grow but can tolerate it [2].

The process of anaerobic digestion involves a number of sequential phases. Each phaseutilises a specific group of microorganisms that metabolise material in the waste, transform-ing it into smaller and simpler compounds which can be used by microorganisms in thesubsequent phase. From an engineering perspective, the design, operation and optimisationof an AD plant is determined by the nature and rate of these key biochemical reactions.

2.1.2 Key Phases

There are four key phases of anaerobic digestion, namely hydrolysis, acidogenesis, ace-togenesis and methanogenesis. Each phase utilises a distinct consortia of microorganismsthat use intracellular or extracellular enzymes to break down the compounds in the biomass.

These compounds (also termed substrates) are used for cell growth and energy supply. Theproducts of each phase are used as a substrate for the microorganisms in the subsequentphase. If the growth rates of any particular community are inhibited or the balance ofpopulations shifts within the process this can impact on the overall rate and efficiency ofdigestion. As AD is a sequential process, the slowest phase will determine the overall rateof the process. Whilst physical separation of phases can enhance the rate of digestion, theend products (the energy rich biogas) naturally separate from the aqueous phase into thegaseous phase.

2.1.2.1 Hydrolysis

Hydrolysis is the first phase of the digestion process whereby particulate and colloidalmatter of the biomass is disintegrated and solubilised by a group of microorganisms knownas hydrolytic bacteria. The first step of hydrolysis involves the disintegration of particulatecarbohydrates, proteins and lipids from the composite organic material. In the second step,extracellular enzymes secreted by bacteria convert these substrates into monosaccharides,amino acids and long chain fatty acids respectively [3].

Anaerobic Digestion 33

The mechanism by which solubilisation occurs involves either the release of enzymesinto the bulk solution or via microorganisms adhering to particles in the waste [3]. It isgenerally considered that if a biomass feedstock contains high concentrations of lignin orcellulose the process of hydrolysis and particularly solubilisation can be slower and mayrequire a pre-treatment step [4–6].

2.1.2.2 Acidogenesis

During the second phase, the acidogenesis, microorganisms break down the products ofthe hydrolysis phase to produce hydrogen, carbon dioxide, alcohols and volatile fatty acidslike propionate or butyrate [7].

Production of large quantities of volatile fatty acids can lead to a reduction in the pHof the process, which can inhibit the activity of the bacteria in the subsequent phases [8].Below are two typical acidogenic reactions which break down a simple sugar (glucose) intoacetic acid (Equation 2.1) and butyric acid (Equation 2.2) [7].

C6H12O6 + 2H2O ↔ 2CH3COOH + 2CO2 + 4H2 (2.1)

C6H12O6 ↔ 2CH3CH2CH2COOH + 2CO2 + 2H2 (2.2)

2.1.2.3 Acetogenesis

The third phase, the acetogenesis, is performed by acetogenic bacteria, which form the linkbetween acidogenesis and methanogenesis. During this phase the fermentation products ofthe acidogenesis, like volatile fatty acids (C3–C5) or long chain fatty acids, are oxidisedinto acetic acid, carbon dioxide, hydrogen and water. The breakdown of the volatile fattyacids is called β-oxidation. This step is important because higher volatile fatty acids likepropionate or butyrate cannot be directly utilised by methanogen archae. Equations 2.3 and2.4 illustrate the breakdown of propionate and butyrate to acetate under standard conditions(pH = 7.0; T = 298 K) [7].

CH3CH2COOH + 2H2O ↔ 2CH3COOH + CO2 + 3H2 ΔG0′ = +76, 1 kJ mol−1 (2.3)

CH3CH2CH2COOH + 2H2O ↔ 2CH3COOH + 2H2 ΔG0′ = +48, 1 kJ mol−1 (2.4)

The reactions are endergonic under standard conditions, and only at low hydrogen partialpressures are they exergonic. Hydrogen reduction is realised by symbiosis of the acetogenicbacteria with methanogenic archaea under energy win. From an energetic point of view,the acetogenic reactions run at the expense of the methanogenic reactions, because a partof the energy is transformed back to the acetogenic bacteria [7].

The interdependence between the hydrogen-producing acidogenic and acetogenic speciesfrom the hydrogen-consuming methanogenic population is of particularly high importancefor the process stability. This can be explained by the adverse effect of increasing hydrogenconcentrations on acetogenic populations, unable to oxidize, for example, the propionateof the acidogenic step at hydrogen partial pressures higher than 0.4 bar, on the one hand[7, 9, 10]. On the other hand, the methanogenic population is relying on a preferably highhydrogen partial pressure to enable an energy win during generation of methane and carbondioxide.


This leads to a very small thermodynamic window within which the entire process runsstably. Digester stress conditions caused by overloading the system with organic substratesthus quickly result in an increased hydrogen partial pressure, which inhibits the utilisation ofVFA. Due to the accumulation of VFA a remarkable acidification of the digester and a sub-sequent break down of the methanogenic population can occur in practice, leading to furtherincreased hydrogen partial pressure because no more utilisation of hydrogen is possible.

For this reason, Fraunhofer UMSICHT – Institute branch Sulzbach-Rosenberg developeda measuring and control system for the automated operation of digesters based on the on-line available parameter hydrogen content within the biogas. The results suggested that thehydrogen content within the biogas is capable of being a highly sensitive on-line parameterfor the automated management of anaerobic digesters near their maximum sustainableloading capacity [11].

2.1.2.4 Methanogenesis

During the final phase, a community of microorganisms known as methanogenic transformthe acetic acid, hydrogen and carbon dioxide into methane (CH4), carbon dioxide (CO2)and water (H2O). Two key groups of organisms are involved in this final phase as follows:

Acetoclastic methanogens – Convert acetate to methane and water

CH3COOH → CH4 + CO2 ΔG0′ = −30, 9 kJ mol−1 (2.5)

Hydrogenotrophic methanogens – Convert hydrogen and carbon dioxide to methaneand water

H2 + CO2 → CH4 + 2H2O ΔG0′ = −135, 4 kJ mol−1 (2.6)

The reactions show that the energy win via the acetoclastic pathway is lower than via thehydrogenotrophic pathway. According to literature, about 70% of the produced methaneresults from the degradation of acetic acid [7]. But recent findings have shown that atorganic loading rates of 3 kgVS m−3 d−1, a significantly higher number of hydrogenotrophicmicroorganisms are detected in the fermenter [12, 13]. By increasing the organic loadingrate, methane is exclusively produced through the reduction of carbon dioxide. This findingcontradicts the textbook opinion which refers to researches of the digestion of sewagesludge at low acetate concentrations and low organic loading rates [12].

2.1.3 Influence Factors on the AD

In order to maintain the biological populations and optimise the production and quality ofbiogas and digestate, the physical and biochemical conditions within the process must alsoclosely monitored and controlled. These key process variables include:

• temperature

• pH

• alkalinity

• nutrients and trace elements

• total solids and volatile solids

• organic loading rate

• h retention time.


2.1.3.1 Temperature

Anaerobic microorganisms can be grouped according to the optimum temperature at whichthey function. Psychrophilic organisms have an optimal temperature range of 12–18 ◦C,mesophilic from 25 to 40 ◦C and thermophilic from 50 to 55 ◦C [14]. The temperaturewithin the reactor can impact greatly on the growth and survival of microorganisms. A 10 ◦Cincrease in temperature can result in doubling growth rates up to the optimum temperaturerange [15].

Traditionally, full-scale anaerobic digesters have been operated in the mesophilic range,although there has been increasing interest in recent years in the use of thermophilicconditions. There are several benefits to the utilisation of thermophilic conditions, forexample increased reaction rates and thus organic breakdown as well as a greater pathogenickill. In addition, there is evidence to suggest that the thermophilic bacteria enhance theseeding for the subsequent mesophilic stage [16]. It has been cited by several authorsthat start-up of thermophilic reactors can often be problematic due to the sensitivity ofthermophilic bacteria to organic loading rate, influent characteristics and other operationalfactors [16, 17].

It is believed that the conversion from mesophilic to thermophilic requires acclimatisationof bacteria and this is often accompanied by a decrease in methane production [17].Research was carried out by Bourque et al. [17] into periodic switching from mesophilicto thermophilic conditions and its impact upon methane production. Findings suggestedthat short-term increases in temperature led to increased methane production and CODremoval. However, an increase to 55 ◦C for longer than 6 h led to a decrease in acetoclasticmethanogen activity.

2.1.3.2 pH

The pH within the process can also impact upon bacterial growth and survival. Microor-ganisms can rarely tolerate pHs of greater than 9.5 or below 4.0 – the optimum range beingbetween pH 6.5 and 7.5 [14]. During the fermentation stage, the bacteria present produceorganic acids from the breakdown of organic compounds. These acids can reduce the pHwithin the reactor. Low pHs can inhibit the methanogenic bacteria and thus the productionof methane gas. A laboratory-scale study was conducted by Taconi et al., (2008) to evaluatethe impact of low pH conditions on methane production. They found that methanogenesiscould be sustained at lower pH values (pH 4.0–5.3) as long as the bacterial population hadsufficient time to acclimatise.

2.1.3.3 Alkalinity

Alkalinity within the substrate provides a buffer to ensure that pH changes, as a result of acidformation and CO2 generation, do not have a deleterious effect on biochemical reactions.Addition of alkalinity prior to AD is not uncommon. Lime can be added to increasealkalinity, typically 1000–5000 mg/l provides good buffering capacity [15]. Alkalinitycan also be produced naturally in some waste streams. The conversion of amino acidsand proteins into ammonia, which subsequently combines with CO2 and H2O to produceNH4(HCO3), is also common in the digestion of sludges [14]. Co-digestion of substrates


with varying compositions can also be used as a method for increasing the buffering capacitywithin a process.

2.1.3.4 Nutrients and Trace Elements

Organic and inorganic nutrient deficiencies can often limit microbiological growth in ADsystems. The key nutrients required include N, S, P, Mg, Ca, Na and organic nutrient mayinclude amino acids, purines and vitamins. All of these are generally present in agriculturaland municipal wastewaters, however they may be absent or deficient in industrial wastesand therefore supplementation may be necessary.

2.1.3.5 Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS)

The total solids content of a feedstock can vary considerably. The operational considerationsregarding total solids content of a particular waste stream may include the increased energyrequired to mix a waste with a high solids content (>40%) and the increased vessel sizerequired for a waste with a low solids content but high water content. In addition theincreased heat input required may have a deleterious effect on process economics [18]. Thevolatile suspended solids in the seed sludge or reactor are often used as an indication ofbiomass concentration.

2.1.3.6 Organic Loading Rate

The rate at which the digester receives organic mass is also of great importance in terms ofoptimisation of the AD process. The organic loading rate (OLR) is described either in termsof chemical oxygen demand (COD), which is a measure of the oxygen equivalent of organiccompounds in solution, or as volatile solids (VS). The parameter COD is mostly used forliquid substrates, the VS content is mostly used for high solid biomass like maize silage.The OLR normally is expressed as kg COD m−3 d−1 or as kg VS m−3 d−1. The organic loadrate will be dependent upon the type of biomass being treated and the operational capacityof the reactor.

2.1.3.7 Solid Retention Time (SRT)/Hydraulic Retention Time (HRT)

In order to effectively break down the organic material to an acceptable level, the substratesrequire sufficient contact time with the microbiology within the digester. The rate of break-down will be dependent upon the characteristics of the feedstock, the bacterial populationand the reactor conditions. Solids retention time is the average time solids remain in thedigester, whereas hydraulic retention time is the average time the liquid sludge remains.HRT and SRT can vary considerably depending upon reactor design; however, it is gen-erally agreed that designs that allow shorter HRTs and longer SRTs offer greater processefficiency and reduce footprint and costs [19].

2.1.4 Sources of Biomass Utilised in AD

The basic substrates for anaerobic digestion are predominantly carbohydrates, proteins andlipids and will be found in varying concentrations in different sources of biomass. WhilstAD is suitable for a wide variety of organic materials, the most common sources include


sewage sludge, agricultural residues, food wastes (organic fraction of municipal solidwaste OFMSW) and purpose grown energy crops. In addition, novel sources of biomassare continually emerging through targeted research, including agro-industrial wastes, wastestreams from other organic bio-energy or biorefining activities and purpose grown aquaticcrops such as algae.

2.1.4.1 Sewage Sludge

During the process of municipal wastewater treatment, large quantities of sludge are pro-duced and the disposal or treatment of these can account for up to 50% of the overall costof waste water treatment [20]. These sludges are rich in carbon and nutrients and can beconverted to energy thermochemically, composted or applied to the land as a fertiliser.Anaerobic digestion is perhaps one of the most common applications for the treatment ofsewage sludge as it reduces final sludge volumes, limits odours and destroys pathogens.The biogas can also be used in situ to generate energy to run the process.

Sewage (municipal wastewater) is derived from domestic residences, effluents fromindustrial processes and storm water runoff. The volume and composition of the wastewaterwill vary considerably depending on the size of the population, the nature of the catchmentarea (i.e. the degree of urbanisation/industrialisation) and the nature of the industries in thearea. These industries can be diverse, ranging from dairies and abattoirs to metal finishingcompanies, and the composition of the composite wastewater arriving at the treatment workswill be influenced by the activities of these industries. During treatment of wastewater, solidsare separated at various points via primary settlement or biological treatment.

When biomass is derived from biological treatment processes such as activated sludgeplants a high proportion of the organic material is bound up inside the cells of microorgan-isms. The cell walls of these microorganisms are relatively tough and this can mean thatpre-treatment of the biomass to disintegrate the cell walls is required to aid with improvinghydrolysis [21].

2.1.4.2 Agricultural Wastes

A large volume of agricultural waste is generated every year from the processing of cropsand from livestock. These wastes have traditionally been composted, but this can often leadto adverse environmental impacts such as odours, leaching of nutrients into the groundwater(and potential eutrophication of water sources), pests and also risks to human health frompathogen exposure:

(i) Slurries and manures.Cattle manure consists of complex soluble and insoluble organic compounds andcan commonly exhibit high concentrations of ammonia nitrogen. Animal wastes aregenerally slurries; however, the housing, bedding and methods of collection can resultin the production of a biomass with a much higher solids content [22]. These solidscan often be fibrous and have a high percentage of lignocellulosic material.

(ii) Crop residues.A large volume of biomass is generated from the harvesting and processing of crops.Examples of this might include husks, stems, leaves and woody residues from typicalagricultural crops [23–25].


Sunflower,5.20%

Rapeseed,6.90%

Rice,1.30%

Maize,18.80%

Oats,3.20%

Barley,18.80%

Rye,3.60%

Wheat,42.20%

Figure 2.1 Share of crop residues across EU27. Reprinted from [27] with permission fromElsevier.

Figure 2.1 shows the share of different crop residues across the 27 EU member states. Thenature of these residues is dependent upon the type of crop, local climatic conditions, yieldand agricultural practices. Whilst a substantial volume of biomass could be derived fromagricultural residues, spatial and temporal variations in quality and availability togetherwith competing uses (e.g. animal feed, horticulture etc.) would also need to be consideredwhen planning future bio-energy provision [26].

2.1.4.3 Food Wastes

Biomass from food waste arises from preparation, processing and spoilage of food and canbe derived from a variety of domestic, commercial and industrial sources. Its compositioncan vary significantly depending upon its origin and season. It has been reported thatapproximately 50% of all food that is produced is lost, converted or wasted [27].

Generally it is composed of a combination of carbohydrates (e.g. bread, rice and vegeta-bles), proteins (meats, fish and dairy) and lipids (from oils, fats and meats) [28]. The wideradoption of AD for food wastes is being driven by the EU landfill directive targets [29],which require waste operators to reduce the biodegradable wastes going to landfill by 35%by 2020.

Biomass derived from fruit and vegetable waste is characterised by a high moisturecontent (>80%) high volatile solids (>95% of total solids) [30,31] and can be problematicfor landfill due to its high biodegradability [32]. A study by Jiang [33] showed thatvegetable waste was not suitable for use as a sole substrate for AD without the addition oftrace elements due to the rapid build-up of volatile acids, which can result in a significantdecrease in pH within the reactor which can in turn stress and inhibit the activity ofmethanogenic bacteria [32].


Food wastes with a high protein content can also result in higher nitrogen concentra-tions which in turn results in elevated concentrations of ammonia which can be toxic toacetoclastic methanogens [34]. In addition, food waste can often be lacking in sufficienttrace elements and this in turn can result in increased VFAs [33]. Most AD facilitiesblend or co-digest feedstocks to reduce ammonia content in the digester and or increasetrace elements.

2.1.4.4 Purpose Grown Energy Crops

According to the European Anaerobic Digestion Network (www.adnett.org) the use oftraditional energy crops in AD has not gained in significance in the EU. Depending on thelegal framework in the single member states, exceptions can, however, be observed. In 2012,energy crops for biogas production were cultivated on nearly 1 Mio. hectare in Germany[35]. With increasing competition for land and food, together with natural resources suchas water and nutrients, the AD sector has typically focused on deriving energy from wastesand residues [6]. However, a number of second-generation energy crops are emerging thathave shown good potential as feedstocks for bio-energy and particularly AD.

Algal biomass has been reported as a promising future feedstock for anaerobic digestion[36–38]. It has a relatively high oil content, high productivity and good photosyntheticefficiency compared to terrestrial crops. Algal biomass can be pre-processed to extract oilswhich can be converted into biofuels, it can be co-digested to enhance the biogas productionof other substrates and in addition the nutrient-rich liquid residues from anaerobic digestioncan be used as a media on which to grow the algae, reducing the requirement to addnutrients and thereby reducing costs and enhancing the environmental sustainability of theprocess. Park and Li [39] reported that methane production was significantly enhancedwhen microalgae was co-digested with fats, oils and grease waste.

2.1.5 Characteristics of Biomass

When considering anaerobic digestion as a conversion route for the transformation ofa particular source of biomass, it is important to evaluate its biochemical and physicalcharacteristics. From an engineering perspective the properties of the biomass togetherwith the variability of these properties over time and the volumes available will determinenot only its suitability as a feedstock for AD but also the most appropriate design, scaleand operation of the process.

Some of the key parameters which are used to characterise biomass include:

• Total Solids (TS) and Volatile Solids (VS)

• Nutrient composition – specifically nitrogen (N), phosphorus (P) and potassium (K)

• Biogas and methane (CH4) yield on fresh mass (FM) and volatile solids (VS) basis.

Collection and storage of biomass can impact significantly on biomass characteristics.For example, moisture content can vary seasonally or with dilution of wastes at source.

Materials such as grit, sand and straw can be introduced from animal housing andbedding [22], plastics from packaging materials are also often present in food wastes. Tracecompounds can also be present, for example antibiotics, pesticides and detergents (which

http://www.adnett.org


Table 2.1 Typical characteristics of raw biomass. Reprinted with permission from NEKGmbH.

Total Volatilesolids solids Na P2O5 K2O Biogas yield CH4 yield CH4 yield

Biomass (%) (%TS) (%TS) (%TS) (%TS) (m3/Mg FM) (m3/Mg FM) (m3/Mg VS)

ManureCattle slurry 10 80 3.5 1.7 6.3 25 14 210Pig slurry 6 80 3.6 2.5 2.4 28 17 250Cattle dung 25 80 5.6 3.2 8.8 80 44 250Poultry manure 40 75 18.4 14.3 13.5 140 90 280Horse manure

w/o straw28 75 — — — 63 35 165

Energy cropsMaize silage 33 95 2.8 1.8 4.3 200 106 340Green rye silage 25 90 — — — 150 79 324Cereal grains 87 97 12.5 7.2 5.7 620 329 389Grass silage 35 90 4.0 2.2 8.9 180 98 310Sugar beet 23 90 1.8 0.8 2.2 130 72 350Fodder beet 16 90 — — — 90 50 350Sunflower silage 25 90 — — — 120 68 298Sweet sorghum 22 91 — — — 108 58 291

Substrates from processing industrySpent grains 23 75 4.5 1.5 0.3 118 70 313Cereal vinasse 6 94 8.0 4.8 0.6 39 22 385Potato vinasse 6 85 9.0 0.7 4.0 34 18 362Fruit pomace 2.5 95 — 0.7 — 15 9 285Raw glycerolb — — — — — 250 147 —Rapeseed cake 92 87 52.4 24.8 16.4 660 317 396Potato pulp 13 90 0.8 0.2 6.6 80 47 336Pressed sugar

beet pulp24 95 — — — 68 49 218

Molasses 85 88 1.5 0.3 — 315 229 308Apple pomace 35 88 1.1 1.4 1.9 148 100 453Grape pomace 45 85 2.3 5.8 — 260 176 448

a N concentration in digestate, excluding losses in storage.b Results vary greatly in practice, depending on the method used for biodiesel production.

can cause foaming or microbiological inhibition within digesters) and inorganic compoundssuch as salts and food additives (which can lead to precipitation and sludge formation) [18].Table 2.1 provides examples of different biomass sources and characteristics.

The inorganic and organic sulfur content of a biomass feedstock can also have a deleteri-ous effect on the anaerobic digestion process and the quality of the biogas produced. Thesecompounds are reduced by bacteria into dissolved sulfides, which can lead to foul smelling,toxic and corrosive hydrogen sulfide gas [41]. Table 2.2 illustrates the concentrations of Sin various biomass feedstocks.


Table 2.2 Comparison of S content of various biomass feedstocks. Reprinted from [42] withpermission from Elsevier.

Feedstock Total S (g S/kg TS)

Urban wastewater treatment Primary sludge 8.6Biological sludge 8.4

Vegetable wastes Carrot pulp 1.1Onion pulp 3.0

Municipal wastes Canteen waste 3.6Lawn mowing waste 5.5

Animal wastes Dairy cow slurry 2.8–3.8Pig slurry 8.0

Other Red seaweed (harvested) 23.3Green seaweed (harvested) 29.6

2.1.6 Pre-Treatment of Biomass

According to Carlsson and Lagerkvist [42], performance of AD is measured by the vol-umetric methane productivity per unit of material (expressed as TS, VS, COD or wetweight), % reduction in TS or VS or methane productivity (m3CH4/m3 reactor, day). Theaim of process optimisation, according to Carlsson, is to achieve close to the actual methanepotential of the biomass at the highest rate.

Given the inherent characteristics of some biomass types and the potential for contami-nation and seasonal/ geographical variability, it is often a challenge to predict and maintainprocess performance. Digestion of more than one biomass type (termed co-digestion) isone method that can be adopted to improve the characteristics of a composite feedstock. Forexample, a biomass which is potentially deficient in nutrients or moisture or is highly vari-able can still be effectively treated anaerobically when it is co-digested with a biomass withmore desirable characteristics. For example, co-digestion of food waste with sewage sludgeor agricultural residues has been widely investigated [28, 43–45]. Addition of traditionallydifficult wastes can also enhance biogas production via AD. Castrillon and Fernandez-Nava [46] observed that the addition of crude glycerine to screened cattle manure increasedbiogas production by up to 400%. In other work, Li and Champagne [20] demonstratedthat addition of fats, oils and grease (FOG) to waste-activated sludge during AD increasedproduction of methane by up to 72%.

Pre-treatment of biomass can also be employed to improve the composition andcharacteristics of biomass. Physical, chemical or biological manipulation of biomassprior to anaerobic digestion can enhance methane yield without the need to significantlyalter AD design or operational conditions. Whilst various techniques have been widelyexamined and reported in the literature [5, 47–53], full-scale applications of pre-treatmentare relatively limited and information on the efficiency of various technologies and theirimpact on the whole AD system is often lacking. In addition, the pre-treatment techniquesgenerally reported in the literature are often energy intensive and therefore may haveimplications in terms of overall economic feasibility and environmental sustainability [54].

The aim of pre-treatment is to improve the biodegradability of biomass via reductionof particle size and increase in surface area, which allows exposure and solubilisation


Mechanical

• agitator bead mill

• high-pressure homogeniser

• Lysat-centrifuge

• impact jet system

• High-performance pulse

technology

• ultrasonics

Pre-treatment

(desintegration process)

Chemical• wet oxidation

• ozone

• acidic/alkaline hydrolysis

Biochemical• hydrolysis with enzymes

• autolysis

Thermal• < 100 °C

• > 100 °C

Figure 2.2 Overview of possible pre-treatment technologies for biomass.

of cellular material. In doing this, hydrolysis, which is often rate limiting, can often beenhanced. For example, lignocellulosic biomass, which is less amenable to AD, can bebroken down into smaller compounds of lignin, cellulose and hemicelluloses, which aremore easily accessible to the anaerobic micro consortia.

Pre-treatment options can be physical, chemical or biological in nature and often combinemore than one technique. Some of the pre-treatment techniques most commonly evaluatedin literature include thermal/ hydrothermal, mechanical/ ultrasonic and chemical. Figure 2.2shows an overview of possible pre-treatment technologies.

The technique and applications most commonly reported in the literature are thermaland ultrasonic pre-treatment of wastewater residues [42].

2.1.6.1 Thermal Pre-Treatment

Heat can be applied to biomass in order to break down the chemical bonds in cell walls,thereby exposing intracellular material. Techniques range from low temperature heating<100 ◦C, to utilisation of steam at high temperatures and pressures and often withcatalysts [53].

Thermal pre-treatment has been shown to have a significant impact on biogas production,methane production and solids reduction when applied to sewage sludges. Climent andFerrer [55] observed a 50% increase in biogas production during thermophillic AD ofsewage sludges pre-treated at 70 ◦C, Similarly, Ferrer and Ponsa [47] demonstrated a 10-fold increase in the solubilisation (measured as volatile dissolved solids) of sewage sludgesalso pre-treated at 70 ◦C and a subsequent increase in biogas of 30%. Whilst Salsabil [52]observed solubilisation and sludge reduction (measured as total suspended solids) across arange of temperatures from 40 ◦C (5% reduction), 60 ◦C (9% reduction) and 90 ◦C (16%reduction).

Higher temperatures have also been employed through the addition of steam and pressure.Bougrier and Delgenes [56] utilised an autoclave to evaluate the impact of high temperaturesand pressures on sludge destruction. Treatments at temperatures of 130, 150 and 170 ◦C allled to sludge reduction, with higher temperatures leading to a reduction in volatile solidsof up to 80% compared to controls. Menardo [6] noted increased methane yields of up to40% when applying 90 ◦C thermal pre-treatment to wheat and barley straw; however, neg-ligible improvements were noted for maize straw and rice stalks having received the same


Milling

Substrate

Waste water Cleaning Dewatering

Bioreactor

Ga

s c

lea

nin

g

Mashing-

tank

Process water

Heat cycleCHP

Electricalpower

Biogas

Solid residue

T PH®

Figure 2.3 Flow sheet of the process combination of ATZ-TPH®-Process and anaerobic diges-tion for different types of biomass.

pre-treatment – this being attributed to the high lignification of these samples. Hydrother-mal treatment of lignocellulosic material is gaining interest as a pre-treatment to AD asit does not require the addition of chemicals or catalysts and therefore does not lead toissues such as corrosion [57]. Qiao and Yan [58] studied the hydrothermal treatment ofpig manure, fruit and vegetable waste and sewage sludge at 170 ◦C. Results showed anincrease in soluble COD, VFA and Total Organic Carbon (TOC) and a subsequent increasein methane productivity of 14.6, 16.1 and 65.8% respectively. Pre-treatment of food wasteactually resulted in a decrease in methane productivity, however it had the highest methaneproductivity of all the samples tested due to its high lipid and protein content.

The Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg developed a pre-treatment concept based on the application of high temperature and pressure (ATZ-TPH®-Process). Thus the limiting step of microbial enzymatic hydrolysis of biopolymers intoshort-chain soluble products is replaced by a highly efficient physical hydrolysis.

Due to the increased temperature and pressure the process can be accelerated to less than30 min and up to 80% of the total solid can be transferred into soluble fraction.

Thus, the ATZ-TPH®-Process can be assigned to the disintegration processes whichcan be subdivided into thermal, chemical, mechanical and biological processes [59–61].Figure 2.3 shows the integration of the ATZ-TPH®-Process into a biomass treatment plant.

The biomass is milled and fed to the ATZ-TPH®-plant. Due to the given process condi-tions (temperature: up to 250 ◦C, pressure: up to 45 bar, retention time: 20 min) the requiredretention time within the fermenter can be reduced drastically and the biogas yield increasescompared to untreated biomass up to 50% depending on the treated biomass. The energydemand necessary for the ATZ-TPH®-Process can be covered completely from the wasteheat of a CHP for the energetic utilization of the biogas. The set-up of the ATZ-TPH®-Process for treatment of different types of biomass has been described in detail [62, 63].


By pre-treatment of organic wastes using the ATZ-TPH®-Process, even biologicallyhardly degradable biopolymers like lignocellulosic biomass can be cracked, for example,for production of bioethanol [64–67]. Treating animal byproducts, a complete disinfectioneven of highly infectious TSE-contaminated materials from rendering-plants is guaranteedduring application of the ATZ-TPH®-Process.

First results of the pre-treatment and subsequent anaerobic digestion of animal byproductswere achieved in a Bavarian rendering plant. The investigations showed a stable mono-fermentation of animal byproducts with biogas yields of 200–300 m3 per Mg of rawmaterial with a methane concentration of approx 70%. A more detailed description of theexperiments has been published elsewhere [62].

A further case of application of the ATZ-TPH®-Process is the pre-treatment of sewagesludge. Thus, an improved mass reduction of up to 60% and acceleration of anaerobic diges-tion is obvious due to cell disruption. Furthermore, the water removal from the remainingbiomass is easier as most of the sludge is transferred into liquid phase (hydrolysate).

2.1.6.2 Mechanical/Chemical Pre-Treatment

This technique utilises equipment which induces sheer stresses on the biomass and thisresults in the physical disintegration of particles, cell structures and intracellular materials.Techniques commonly employed for mechanical pre-treatment include ball mills and highpressure homogenisers [55, 68]. Menardo [6] demonstrated that particle size reduction bysimple mechanical pre-treatment improved the methane productivity from AD of agricul-tural residues by up to 80%. Lindmark and Leksell [49] illustrated that grinding of ley cropsilage using commercial scale technologies (Grubben deflaker and Krimer disperser) led toan increase in methane production of up to 59%.

2.1.6.2.1 Ultrasonic Pre-Treatment Ultrasound can also induce sheer stresses and beapplied as a pre-treatment for AD. When high frequency sound waves are applied to asolution, microbubbles form which expand and contract until they reach a critical pointwhere they undergo a violent collapse. At this point localised high temperatures andpressures are generated together with the formation of a number of hydroxyl radicals.

The temperatures and pressures together with the radicals can induce various physicaland chemical changes to the biomass, but the force of collapse can also cause lysing anddisintegration of cell membranes and structures. Frequencies of 20–40 kHz are commonlyreported in the literature as being effective for pre-treatment particularly of sewage sludgesand manures [46, 54, 69–71].

Specific energy input measured as kJ/kg, TS or TSS is often used to evaluate the efficacyof ultrasonic pre-treatment. Cesaro and Belgiorno [54] reported that increasing specificenergy values had a beneficial effect on soluble COD and volatile solids reduction in theorganic fraction of MSW but above 15 000 kJ/kg, TS there was little significant impact onsolubilisation. Soluble COD was increased by 9% and biogas production was increased byapproximately 16%.

Similarly studies conducted by Dhar, Nakhla [70] on waste activated sludge have shownthat specific energy inputs of 1000, 5000 and 10 000 kJ/kg TSS reduced VSS by 23%,28% and 30% respectively indicating that an increase in specific energy input from 5000to 10 000 kJ/kg, TSS did not improve VSS reduction significantly. In terms of methaneproduction, an increase of 15, 20 and 24% was observed with increasing energy inputs of1000, 5000 and 10 000 kJ/kg TSS respectively.


2.1.6.2.2 Alkaline Pre-Treatment Addition of chemicals such as sodium hydroxide(NaOH), calcium hydroxide (CaOH), potassium hydroxide (KOH) and ammonium hydrox-ide (NH4OH) induces chemical changes in the structure of organic molecules. This leads todelignification and swelling of the biomass, which increases surface area and exposes cellsto AD microorganisms [57, 72]. Some of the advantages of alkaline pre-treatment includethe simplicity of the technologies and ease of operation as well as their high efficiency[73, 74].

Studies have shown that relatively low doses of chemicals are required to effectivelypromote cell disintegration. Zhang [21] noted that the degree of sewage sludge disintegrationprogressed rapidly with NaOH doses up to 0.05 mol/l but that there was little increasebetween 0.05 and 0.06 mol/l. Similarly Li and Li [74] observed that soluble COD inprimary and biofilm sludge increased rapidly with addition of between 0.005–0.1 mol/lNaOH, however further increases were limited above 0.1 mol/l.

In terms of biogas and methane productivity, addition of alkali to lignocellulosic biomasscan have marked benefits. Chandra illustrated that pre-treatment of wheat straw with 4%(dry weight) NaOH led to an 87.5% increase in biogas and 117% increase in methaneproduction compared with untreated samples. Bruni and Jensen [68] observed that pre-treatment of dairy biofibres with CaO led to an increase in methane productivity of 66%(higher than observed with mechanical, hydrothermal and enzymatic pre-treatments).

Whilst alkaline pre-treatment is generally considered to be effective in isolation, it is veryoften combined with other pre-treatment techniques which utilise alternative but synergisticmechanisms for cell disintegration, such as ultrasound [73, 75], microwave [76], thermal[77] and high-pressure homogenisation [21].

Pre-treatment as a means of improving the characteristics of biomass prior to anaerobicdigestions are pretty well understood and documented, however their application to fullscale AD processes is relatively limited. Particle size reduction, whilst beneficial for AD,can impact on the dewaterability of subsequent digestates (leading to increased processing).

Generally, thermal pre-treatments have been noted to improve dewaterability [78], how-ever release of Na+ from alkaline pre-treatment can negatively impact on floc structuresthereby reducing dewaterability. Studies by Su and Huo [79] on alkaline pre-treatment ofwaste activated sludge illustrated that whilst NaOH addition in isolation had a deleteriousimpact, combinations of NaOH and CaOH or CaOH in isolation improved dewaterability.Ultrasonic pre-treatment is also considered to have a negative impact on dewaterability byreducing particle size. Dhar and Nakhla [70] observed that ultrasonic pre-treatment of at10 000 kJ/kg TSS led to a 36% reduction in digested sludge dewaterability.

Other negative effects of pre-treatment can include the production of recalcitrant com-pounds via mineralisation of biomass at high temperatures [42]. Higher feedstock pHfrom alkaline pre-treatment can also negatively impact on buffering processes and inhibitmicroorganisms within the AD process and this can have an impact on early stages ofdigestion and biogas production [74].

2.1.7 Products of Anaerobic Digestion

As a result of the anaerobic digestion of organic biomass an energy rich gas (biogas) andfibrous, nutrient rich material (termed digestate) are produced. There is a clear market forthe use of biogas for heat and power and an emerging market for the use of the digestate asa soil conditioner directly or following thermochemical pre-treatment [80].


2.1.7.1 Digestate

Following anaerobic digestion there will generally be a volume of undigested fibrousmaterial remaining. The potential end uses for this will be entirely dependent upon itscomposition (e.g. nutrient and trace metals content) and this in turn will be dependent uponthe composition of the feedstock used.

A number of studies have been conducted on the use of anaerobic digestate as a biofer-tiliser [81–83]. Where these products are to be applied to agricultural land it is crucialto evaluate their composition and how any subsequent release of chemical and biologicalcompounds may adversely impact on the soil, water sources and future crop cultivation.Whilst digestate composition can vary considerably, it is generally acknowledged thatnitrogen mineralisation capacity (NMC) together with dissolved organic carbon (DOC)and biochemical oxygen demand (BOD) can be used as an indicator of fertilizer qualityand biodegradability [81, 82].

For this type of application there exists a framework of regulations which specify whichraw materials can be used as a feedstock for AD and how the quality of these shouldbe monitored and controlled. This Publically Available Specification (PAS BSI 110) alsospecifies the criteria for the quality of the resulting digestate.

In England and Wales the Environment Agency and WRAP (Waste and Resources ActionProgramme) in consultation with the Department for Environment, Food and Rural affairs(DEFRA) have developed a Quality Protocol surrounding the use of digestate arising fromAD [84]. The draft document sets out the criteria for the production and use of qualityoutputs from anaerobic digestion from source segregated biodegradable wastes.

It is not a legal requirement to comply with the protocol; however, digestates which donot meet the criteria may be considered wastes and regulated as such. Whilst the qualityprotocol is not applicable in Scotland and Ireland both countries have developed positionstatements explaining how digestate from AD will be regulated.

In addition there is the potential to capture and process the residual carbon and nutrientswithin the digestate via thermochemical processes such as pyrolysis. It has been demon-strated that the pyrolysis of anaerobic digestates evolves biochars that have characteristicsthat are more desirable for soil conditioning, such as higher pH, surface area, cationexchange capacity, hydrophobicity and so on [80, 85]. In addition, studies undertaken byCao and Pawłowski [86] have shown that the anaerobic digestion of sewage sludge followedby pyrolysis could potentially achieve higher energy efficiencies overall than pyrolysis ofsewage sludge alone.

A programme of work is being carried out to demonstrate feasibility and acceleratethe wider adoption of AD and pyrolysis integration under a European strategic initiativefunded through the Interreg IVb programme. The BioenNW project led by the EuropeanBioenergy Research Group (EBRI) has the aim of supporting stakeholders in the bioenergysector through demonstration and testing, access to expertise through specialised regionalsupport centres and development of five regional bioenergy project development plans.

2.1.7.2 Biogas

The breakdown of organic compounds under anaerobic conditions results in the formationof gases, predominantly methane (50–60% by volume) and carbon dioxide (40–50% byvolume). In addition, trace gases like ammonia, hydrogen sulfide or nitrogen are producedin small quantities.


Performance of an AD process is often measured by the biogas yield. Methane yield isoften referred to as the input mass expressed as fresh mass (FM), volatile solids (VS) orchemical oxygen demand (COD) which is an indicator of total organic composition in thesubstrate. Theoretically, 1 mol of CH4 is equivalent to 64 g of COD, therefore the volume ofCH4 produced per unit COD at 35 ◦C is equal to 0.35 l [7]. The biogas yield is expressed aslN ⋅ kg−1

FM, lN ⋅ kg−1VS or lN ⋅ kg−1

COD at standard conditions (p = 1013 hPa, T = 273 K).Biogas can be burned to produce heat and power via a combined heat and power engine

(CHP) and this can be used in situ to run or optimise the plant (or can be used in localisedbuildings). Alternatively, it can be upgraded to biomethane by removal of trace gases,water and CO2 and injected into the public natural grid. The heating value of biogas isapproximately 18–25 MJ/m3 depending on the methane, respectively carbon dioxide, con-centration. This is approximately 30–40% lower than natural gas and therefore moderationis required prior to injection into the public natural grid after moderation of the heatingvalue [87] or use as a fuel for transport [88]. Methane is a valuable source of fuel and hasa heat value of 1000 Btu/ft3, this decreases significantly when mixed with CO2 [89].

2.1.7.2.1 Desulfurisation of Biogas But before any energetic utilisation of biogas, dif-ferent steps to clean it have to carried out. One important step is desulfurisation, the removalof hydrogen sulfide, which is toxic and corrosive.

Furthermore, oxidation leads in some part to sulfuric acid, which causes corrosion indigesters as well as CHP engines. Thus, the gas can negatively influence the operation of abiogas plant. This includes “all parts” in direct contact with biogas.

Especially CHPs can be damaged, even the pipe work system, the fittings and all addi-tional parts of the plant, which are directly in contact with biogas. Removal of hydrogensulfide can be accomplished with chemical-physical methods like adsorption on activatedcarbon, absorption with bases and precipitation with iron salts. It can also be realised withbiological methods. Utilising the ‘biological method’, the desulfurisation can be accom-plished ‘directly’ in the gas compartment of the fermenter, or as an external method in aseparate column. Concerning the biological desulfurisation, the hydrogen sulfide reacts inthe presence of oxygen and microorganisms to elemental sulfur and acid sulfur. FraunhoferUMSICHT – Institute Branch Sulzbach-Rosenberg developed a biological desulfurisationprocess, the BioSulfex® process, shown in Figure 2.4.

The process consists of a scrubber and a reservoir for washing liquid and to separateelemental sulfur. The microorganisms oxidise the hydrogen sulfide to elemental/sulfurand/or sulfuric acid, depending on the pH-value. The washing liquid is fed from the topof the scrubber and trickles down to the bottom of the column. The microorganisms settleon the surface area of the bulk material in the column, the fixed bed material. This bulkmaterial is used to increase the contact area between the microorganisms and the biogas.

The microorganisms need oxygen for the production of elemental sulfur. Air is addedto the biogas and delivers the necessary oxygen for the bacteria. The gas–air mix flowscounter-currently to the washing liquid. The bacteria in the biofilm oxidise the hydrogensulfide to elemental sulfur and sulfate [90]. It is important to add a stoichiometric amountof around 0.25% of air per 1000 ppm of hydrogen sulfide.

In practice, an amount of 4 to 6 vol.-percent of air is needed for hydrogen sulfideconcentrations of 2000 up to 3000 ppm in the biogas [90–92]. The elemental sulfur willbe washed from the biofilm by the washing liquid and will settle on the bottom of thecolumn, and in the reservoir. At pre-determined intervals the fluid must be renewed, to


Degradated biogas

Scrubber

Additionof air

Raw gas

Samplingraw gas

Circulation ofwashing fluid

Reservoir forwashing fluid/separation ofelemental sulfur

Bulkmaterial

Samplingclean gas

Figure 2.4 Flow sheet of the BioSulfex® process for the desulfurisation of biogas.

dispose of the formed elemental sulfur in the system. The dissipated liquid can be re-usedas sulfur-containing fertiliser and in this way it is returned to the agricultural circulation.

2.1.8 Anaerobic Treatment Technology

Anaerobic treatment technology has gained acceptance as the standard approach for treatingunique, high-strength substrates produced by a wide range of industries and agriculturaloperations.

Anaerobic digesters have been used for the treatment of waste and generation of biogasfor many years. The earliest known example was constructed in 1876 in Bombay, India,where human wastes were used to produce gas for lighting at the Matunga Leper Asylum[93]. The number and types of anaerobic treatment systems being applied to industrial andagricultural waste streams have grown quickly since the first technologies were introducedin the late 1970s and early 1980s.

Figure 2.5 shows an overview of different anaerobic treatment technologies.Modern day anaerobic digestion varies in levels of sophistication but generally has a

number of common components that allow separation of the liquid phase from the digestedsludge and the biogas (see Figure 2.6). Whilst a low rate digester allows for naturalstratification of the phases, a high rate digester incorporates mechanical mixing to improvecontact between the bacterial population and the substrate.

Technology for AD can be designed and configured in various ways to either separatethe various process phases or to enhance the biochemical reactions and reaction rates via


Biogas technology

Discontinuous processesContinuous processes

Wet fermentation

(DM < 10 %)

Dry fermentation

(DM ~ 15–30 %)

Dry fermentation

(DM > 30 %)

• mixed stirrer tank • plug flow process• percolation process

• container process

• boxes process

• tunnel fermenation

Figure 2.5 Overview of different anaerobic treatment technologies.

increasing process temperature or improving contact between the microbiological consortiaand the substrate. The various technological/system options include the following:

• mechanical mixing

• dispersed or attached growth (fixed film)

• continuous or batch flow

Supernatant

Evolution and separation of biogas

Biogas

Feedstock influent

Mixing

Digested sludge

Figure 2.6 Basic anaerobic digester design.


• single-phase or multi-phase reactors

• liquid or solid state systems.

2.1.8.1 Mechanical Mixing

There are several reported benefits to mixing the waste within the digester, these include:

• homogenisation of material

• improved contact between the microbiology and the influent substrate

• eliminates localised concentration spots or dead zones

• assists with dispersal and removal of byproducts.Taken from Biowaste and Biological Waste Treatment, Evans [15].

These benefits of mechanical mixing do need to be weighed against increased operationalcosts.

2.1.8.2 Dispersed or Attached Growth

There are two ways in which the bacterial population is in contact with the influent sub-strate. In the majority of reactors the bacteria is dispersed throughout the substrate. Thedisadvantage of dispersed bacterial growth is that the population is often lost with thedigestate, either slowly during continuous or semi-continuous operation or in one go whena batch operation is applied and this is more common in systems with a high hydraulicloading rate [14, 94].

Several AD technologies, namely Upflow Anaerobic Sludge Blanket Reactors (UASB)and Expanded Granular Sludge Bed Reactors (EGSB) utilise the ability of the bacterialpopulation to form dense sludge granules and flocs by adhering to incoming particulateand suspended matter in the feed. These dense granules prevent washout of the bacterialpopulation during digestion and allow for good settlement of the sludges following digestion[95]. Schemes of both reactors can be seen in Figure 2.7. EGSB reactors incorporate arecirculation of the supernatant (liquid digestate) and a larger height to diameter ratiodesign which enables more efficient mixing and expansion of the sludge bed, which in turnprevents dead zones within the reactor [95].

Fluidised beds, such as the Anaerobic Fluidised Bed Reactor (AFBR) or fixed filmreactor, utilise porous media which immobilise biological growth, reducing retention timesand overcoming operational problems such as clogging and pressure drops which arecommon in systems which used packed beds [94]. These types of systems also providebetter resistance to shock loading (hydraulic and organic) and variability in feedstockcharacteristics [96].

2.1.8.3 Continuous or Batch Process

There are two further operational categories of the AD process (batch and continuous feedloading). Batch processes treat the waste and then, after a sufficient residence time, themajority of the volume is removed. Typically 10–15% of the volume is retained in order toseed the subsequent batch [14]. In semi-continuous processes the feed is added regularly(daily or twice daily) and a corresponding volume is wasted whereas in continuously feddigesters there is no break in the influent–effluent cycle.


Gas Gas

SupernatantSupernatant

Sludgeblanket

Feed

(a) (b)

Feed

Recycle

Figure 2.7 Upflow Anaerobic Sludge Blanket Reactor (a) Expanded Granular Sludge BlanketReactor (b) Source: Seghezzo, Zeeman [95].

2.1.8.4 Single-Phase/Multi-Phase Reactors

It is of great importance that a bacterial population is properly acclimatised. The use ofprocess liquids to increase moisture content can also provide an inoculum of bacteria forthe incoming feedstock [14]. There are two common designs of AD plant (single and multi-phase). Single-phase systems contain all of the microbial population in one vessel whereasmulti-phase AD is carried out in several separate vessels (see Figure 2.8). A multi-phase

Gas

FeedGas

Supernatant

Digestedsludge

Sludge

Figure 2.8 Basic design of multi-phase anaerobic digestion.


operation often has the advantage of allowing optimisation of conditions for the specificbacteria required in each phase [16]. There is also evidence to suggest that multi-phasesystems which split the hydrolysis/acidogenic phase and the methanogenic phase requireless residence time than single phase systems.

2.1.8.5 Liquid/Solid State (Dry) AD

Liquid state AD (L-AD) operates with a total solids concentration of <15% and is gener-ally considered for biomass derived from animal wastes, sewage sludges and so on. For theorganic fractions of municipal solids wastes and agricultural residues with higher concen-trations of lignocellulosic material, solid state (or dry) AD may be considered. Solid stateAD is operated with solids in the range 20–40%w/w [97–99].

It is estimated that dry AD now accounts for approximately 60% of the total capacityin the EU, treating approximately 3.5 mt of waste per year [100]. These systems offera number of potential benefits including smaller, more compact digesters, higher organicloading rates, reduced requirements for pre-processing, and digestates which are easier tohandle [99, 101, 102]. Whilst this can have marked benefits in terms of process design,high-solids feedstocks can be difficult to handle (particularly in terms of mixing andpumping). Diffusion and mass transfer of solutes between the biomass and the microconsortia within the digester can be altered due to higher viscosity and this can then, inturn, impact upon the rate of reactions and accumulation of compounds within the digester[103, 104].

Several studies have noted an accumulation of VFAs during mesophilic dry AD whichcan lower pH, inhibit microorganisms and increase retention times [97, 101]. Aymerich andEsteban-Gutierrez [102], however, noted that although VFA concentrations in the dry ADof agro industrial wastes and sewage sludges were much higher than generally reported inthe literature for liquid AD, the process proceeded satisfactorily. It was proposed that dryAD systems may have a higher threshold capacity for VFAs owing to the differences inmass transfer and process dynamics.

Reactor configurations for dry AD can vary depending on feedstock characteristics. Plugflow reactors are often employed to achieve continuous processing whereby solids moveslowly through the reactor unmixed. In order to improve mass transfer and reaction ratesthe incoming feedstock is very often inoculated with liquid digestate from the previous AD[97]. In some cases it has been reported that up to 50% of the digested residue may berequired to improve start-up of dry AD processes, which is commonly a critical phase inbatch style processes [105]. Figure 2.9 illustrates an example of a batch style configurationused for dry AD [106].

One of the advantages of dry AD is that it can be used to digest a wide variety of biomasstypes and generally at higher OLRs. As a process it can also tolerate impurities and contam-inants, for example glass, plastic and grit, in a way that liquid systems cannot [105]. Studieshave shown that methane productivity from dry AD is comparable to liquid AD; however,volumetric productivity, that is volume of methane gas (Vmethane) produced per unit volumeof reactor (Vwork), can be much higher than liquid systems [107]. Some of the key processoptimisation steps for dry AD include operation at thermophillic temperatures, recirculationof digestate and co-digestion to improve C : N ratios and nutrient balance [97, 101, 108].


Biogas Leachatespreader

Gastight door

Removablesteel grate

Leachate pot

Recirculationpump

CHP

Figure 2.9 Schematic of single biocell cell reactor – Solid State AD. Reprinted from [77] withpermission from Elsevier.

2.1.8.6 Anaerobic Baffled Reactor

The Anaerobic Baffle Reactor (ABR) is designed to allow the bacterial population to riseand fall as they pass between each compartment with the flow of incoming wastewater(see Figure 2.10). The configuration allows the bacterial populations to be retained for along time with relatively shorter hydraulic retention times. It has been noted that thereare several benefits of ABRs. Construction is relatively simple, there is no requirementfor mechanical mixing and there is high stability to shock loading [109]. Furthermorethe design of the reactor allows some separation of acidogenic and methanogenic phaseslongitudinally and can thereby operate as a two-phase system [110, 111].

Feed

Gas

Supernatant

Figure 2.10 Anaerobic Baffled Reactor.


Questions

1. What are the biochemical phases of anaerobic digestion and what role do they play intransforming biomass to bio-energy?

2. What are the common biomass sources utilised for AD and what are their basic charac-teristics?

3. What are the key approaches employed for AD process optimisation?4. What are the main effects of pre-treatment on biomass?5. What are the key advantages and disadvantages of liquid state and solid (dry) state AD?

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3Reactor Design and Its Impact on

Performance and Products

Yassir T. MakkawiEuropean Bioenergy Research Institute (EBRI), Aston University, UK

3.1 Introduction

This chapter discusses engineering design and performance of various types of biomasstransformation reactors. These reactors vary in their operating principle depending on theprocessing capacity and the nature of the desired end product, that is, gas, chemicals orliquid bio-oil. The processing methods used can be mainly divided into two categories: (i)biological processing; (ii) thermal processing.

The thermal conversion reactors use heat to promote a range of chemical reactions whichconvert the solid fuel to gases, biochar, and ash. These gases can then be further treatedto produce fuel gas, chemicals, or bio-oil. The reactor performance and the quality of theend product strongly depend on the design and operating conditions, such as the gas/solidresidence time, temperature, and the size and type of the biomass material used. Most ofthe reactors developed for thermal processing are either typical or a modified version ofreactors commonly used in coal combustion, gasification, and liquefaction in power plants.

The reactors used in the biological processing work on the principle of biological degra-dation of biomass by fermentation or anaerobic digestion to produce gases or liquid fuels.This process usually takes place at a relatively low rates and temperatures/pressures closeto atmospheric in a simple reaction vessel that allows for packing or suspension of thebiomass material while undergoing breakdown with the assistance of microorganisms orenzymes. Unlike the thermal conversion reactors, there is no systematic design approach




for the biological conversion reactors; therefore, the focus of this chapter will only be onthe thermal conversion types of reactors.

3.2 Thermochemical Conversion Reactors

3.2.1 Types of Reactors

The most commonly used thermochemical reactors in biomass thermal conversion can bebroadly categorised under:

1. Moving bed (MB)2. Fluidized bed (FB).

These types of reactors have been used for many years in the chemical industry particu-larly those which involve catalytic conversion. Their application in biomass thermochemicalconversion came as a natural option due to their suitability for continuous operation withgood thermal and hydrodynamic uniformity. There are other unconventional types of reac-tors, which are relatively new and still under research and development, such as the ablative[1], pyroformer [2], free fall [3], cyclone, and rotating cone [4]. The design and performanceof some of these reactors are briefly discussed in the last section of this chapter.

3.2.1.1 Moving Bed (MB)

Moving bed (MB) reactors are the most simple and cost-effective options for biomassthermal conversion to gases. In some textbooks these are also referred to as fixed bedreactors, since the material is densely packed and the biomass material continuously travelsdownwards by gravity while it is thermally decomposing to gaseous phase, biochar, andash. In its conventional design, the MB reactor usually consists of a (i) reaction vessel,commonly constructed from ordinary carbon steel or stainless steel with refractory linings;(ii) biomass feeding system, typically a screw feeding type; (iii) grate to support the bedmaterial and allow for ash discharge; and (iv) inlet and exit for the gasifying agent andproduct gas. The gasifying agent is usually air introduced at ambient conditions. Steam andoxygen have also been reported as gasifying agents.

The most widely used types of MB reactors in biomass conversion are the downdraft,updraft, and cross-draft gasifiers. As shown in Figure 3.1, all of these reactors operate withbiomass feeding from the top, but differ in the configuration of the gasifying agent inlet andproduct gas exit. Apart from their simplicity, these reactors have the advantage of handlinginhomogeneous and various types and sizes of biomass material. However, the biomassfeed must be pelletized to avoid the critical agglomeration problem. Compared to othertypes of biomass reactors, it is applicable to a relatively small-scale operation.

3.2.1.2 Fluidized Bed (FB)

Fluidized bed reactors, also known as fluid bed reactors, are widely used in the processindustry. Historically, the first commercial use of FB reactors was during the early yearsof the twentieth century in the coal gasification process. The technology was then further

Reactor Design and Its Impact on Performance and Products 63

Biomass Biomass Biomass

Air

AirAir

Air

Gas GasGas

Productgas

Productgas

Productgas

(a) (b) (c)

Figure 3.1 Configurations of various types of moving bed reactors (a) updraft gasifier (b)downdraft gasifier (c) cross-draft gasifier.

modified and adopted in the petroleum industry for catalytic cracking of heavy oils. Themain reaction section in a conventional fluidized bed system consists of a vessel or acolumn equipped with a perforated plate, two inlets for the fluidizing medium and thebiomass material, and an exit for the products outlet.

The principle of the FB reactor operation is based on the suspension of a bed material(biomass and inert solid) by a fluidizing medium (gas or liquid) in a confined volume,usually in the shape of a cylindrical column or tank. This simple operation makes theFB particularly ideal for the thermochemical conversion of biomass due to the improvedtemperature uniformity and good mixing of the various bed materials.

The FB reactors are mainly classified into three basic types: (i) bubbling, (ii) circulating,and (iii) entrained. The principle of operation of these reactors is shown schematicallyin Figure 3.2. The key point in these reactors is that the biomass suspension allows forrapid thermochemical conversion. In the bubbling bed reactors, the gas velocity is usuallyin the range of 1.5 to 2 times the minimum fluidization velocity; while in the case ofthe circulating bed, the gas velocity is considerably higher to allow for entrainment andcirculation of the bed material between a riser, where usually the reaction takes place, anda downer, where solids are re-circulated back to the riser part of the system. The entrained-type reactor operates at co-current flow, with the solids, in most cases, falling freely undergravity or entrained under the influence of gas drag force. Further details on the design andperformance of fluidized bed reactors are given in Section 3.7.

3.3 Design Considerations

The performance of a biomass conversion reactor depends on the appropriate setting of theoperating conditions, such as the residence time, temperature, pressure, rate of heat transfer,and rate of reactions of the multi-phase flow mixture. It is this difference in the operatingconditions that makes the distinct variations in the biomass conversion routes and thenature of the end products. Despite the progress made during the last five decades, accurateprediction of the product quality and impact of the operating parameters on performance


Productgas

Productgas

Fluidizingagent Slag

Solid

+ g

as

Solid

+ g

as

Gas

Inert

solid

+

char

(c)(b)

Fluidizingagent

(a)

Steamand air

Biomass

BiomassBiomass

Ash

Productgas

Figure 3.2 Main types of fluidized bed reactors (a) bubbling (b) circulating (c) entrained.

is still the subject of continuing research. This is mainly due to the chaotic nature of theflow hydrodynamics and the poorly understood side-reactions between the various phasesinside the reactor. The multi-phase flow mixture may include all or part of the following:reactive product gases, inert gases, vapor, char, ash, tar, and inert solids. The next sectiondiscusses the most important parameters affecting the interaction of the various phases andtheir impact on the reactor performance.

3.3.1 Hydrodynamics

3.3.1.1 Flow Regimes

Biomass conversion reactors can be operated at various flow regimes, depending on thereactor orientation, flow velocities, and the physical properties of the various componentsinside the reactor. A well-mixed reactor results in a high degree of biomass conversion;however, in large scale processing, achieving this is not an easy task. For instance, reactorscan suffer from flow non-uniformity as a consequence of agglomeration or de-fluidization,which then makes it extremely difficult to control the temperature and conversion rate.

Figure 3.3 shows an example of the flow regimes in a fluidized bed reactor. If the fluidflow is high enough to cause a drag force higher than the solid bed weight (biomass andinert), then the reactor shifts from a static bed condition to suspension. In this case, the bedsolid mixture behaves like a fluid and the reactor is described as a fluidized bed reactor.The gas phase propagates through the bed material in the form of bubbles. The bubblepropagation and eruption at the bed surface creates back-mixing, thus increasing the heatand mass transfer between the fluidizing medium and the suspended biomass material.

There are quite a large number of empirical correlations for the design and predictionof the flow hydrodynamics in fluidized bed reactors. The reactor is said to be operating


Product gas

Biomass Biomass Biomass

Fluidizing agentFluidizing agentFluidizing agent

Incepient

fluidization

U ~ umf

Bubbling

fluidization

U ~ 1.5 – 2 umf

Transport

fluidization

U >>> umf

Product gas Product gas and solid

Figure 3.3 Examples of the flow regimes in a fluidized bed reactor with respect to increasinggas velocity.

at a fixed bed condition if the superficial fluid velocity is below the minimum fluidizationvelocity, that is, U < umf. An acceptable theoretical approximation of umf for fine particlescan be obtained from the Carman Kozeny equation:

umf =

(𝜑dp

)2

180

(𝜌p − 𝜌f

)g

𝜇f

(𝜀3

mf

1 − 𝜀mf

)(3.1)

where dp, 𝜌p, and 𝜑 are the particles diameter, density, and sphericity respectively, 𝜌 and 𝜇

are the density and viscosity of the fluidizing medium respectively, and 𝜀mf is the bed voidvolume fraction. Alternatively, umf for coarse particles can be obtained by equating theparticle weight per the bed cross-section with the pressure drop obtained from the Ergunequation as follows:

umf =

[(42.9𝜇f (1 − 𝜀mf)

𝜑dp𝜌f

)2

+(𝜌p − 𝜌f )g𝜑dp𝜀

3mf

1.75𝜌f

]0.5

−42.9𝜇f (1 − 𝜀mf)

𝜑dp𝜌f(3.2)

Experimentally, umf can be obtained by measuring the bed pressure drop as function ofincreasing and decreasing fluid flow velocity, as demonstrated in Figure 3.4.


Fixed bed

Bed p

ressure

dro

p

Bubbling bed

Gas velocity

Dec

reas

ing

velo

city

Incr

easi

ng

velo

city

umf

Figure 3.4 Relation between the gas velocity and pressure drop in a fluidized bed reactor.

The majority of the available correlations for umf, such as those shown in Table 3.1,were derived for a reactor operating at or near ambient conditions with the bed materialconsisting of a single solid phase. In biomass thermal conversion the reactor operation ismore complex, due to the following:

(i) The temperature is considerably high, reaching up to 1000 ◦C in the case of biomassgasification.

(ii) The reactor usually consists of various particle sizes of different physical properties(e.g., biomass, char, inert solid, ash).

Tests carried out at elevated temperatures and pressure have shown that umf decreaseswith increasing temperature [5], but is less sensitive to the pressure variation, especiallyfor particles larger than 500 μm [6]. In Equation 3.1, and similar equations, the effects ofboth temperature and pressure can be incorporated through the density and viscosity terms.Work carried out by Formisani et al. [5] has shown that the voidage increases linearly withincreasing reactor temperature and, accordingly, the following correlation relating the bedvoidage at incipient fluidization to the bed temperature was suggested:

𝜀mf−T = 𝜀amb + k(T − Tamb) (3.3)

Table 3.1 Example of correlation equations for the minimum fluidization velocity.

Equations Validity Source

umf =

(𝜇f

dp𝜌f

)(33.72 +

0.0408d3p𝜌f𝜌pg

𝜇2f

)0.5

− 33.7 0.01 ≤ Re ≤ 103 Wen et al. [7]

umf = 0.00075d2

p𝜌pg

𝜇fAr < 103 Grace, J. R. [8]

umf =(𝜌p − 𝜌f )

0.934g0.934d1.8p

1110𝜇0.87f

𝜌0.066f

dp < 103 μm Baeyens & Geldart [9]


Exp.-Umf

0.12

0.1

0.08

um

f (m

/s)

mf (–

)

0.06

0.04

0.65

0.6

0.55

0.5

0.451000800600

Temperature (°C)

4002000

Exp.-voidage Eq. 3

Eq. 3.4

ε

Figure 3.5 Effect of temperature variation on umf and 𝜀mf in comparison with the predictionsof Equations 3.3 and 3.4. Fluidizing medium: air; particle size, density and sphericity: 271 μmand 2650 kg/m3 and 0.72 respectively. The experimental data is extracted from Formisani et al.[5].

where T refer to the operating temperature, k is a constant function of the particle properties,and the subscript amb refers to the ambient condition. Applying this in the Carman Kozenyequation reduces Equation 3.1 to:

umf =

(𝜑dp

)2

180

𝜌s − 𝜌f

𝜇f

[𝜀amb + k

(T − Tamb

)]3[1 − 𝜀amb − k

(T − Tamb

)] (3.4)

Figure 3.5 shows an example of the variation of minimum fluidization velocity andvoidage as function of the reactor temperature.

To enhance the mixing and heat transfer, biomass thermochemical conversion is usuallycarried out in a mixture of solids, which may include catalysts or inert solids. The differencein size and densities of the various bed materials adds a further complexity to the design andoperation of the reactor. Rao et al. [10] carried out experiments using mixtures of two typesof sands and various types of biomass materials, such as rice husk, sawdust, and groundnutshell powder, in a fluidized bed reactor. It was observed that umf increases with increasingmass percentage of the biomass, as shown in Figure 3.6. Zhong et al. [11] proposed thefollowing equation for the prediction of minimum fluidization velocity in a binary mixtureof biomass and other solid material:

umf = 1.2 × 10−4

[d2

pe

(𝜌pe − 𝜌f

)𝜇f

(𝜌pe

𝜌f

)1.23]0.633

0 < 𝜌pe < 1000 kg∕m3 (3.5)

umf = 1.45 × 10−3

[d2

pe

(𝜌pe − 𝜌f

)𝜇f

(𝜌pe

𝜌f

)1.23]0.363

𝜌pe > 1000 kg∕m3 (3.6)


00

0.1

0.2

0.3um

f (m

/s)

0.4

0.5

0.6

0.7

2 4 6 8

Biomass (mass %)

10 12 14 16

Figure 3.6 Example of the effect of biomass weight percentage on the minimum fluidizationvelocity of sawdust–sand binary mixture. The average particle size of the sawdust is (−1000+ 800 μm). The density and average particle size of the sands are 2700 kg/m3 and (−355 +250 mm) respectively (data extracted from Rao et al. [10]).

Where dpe and 𝜌pe are the effective mixture particle size and density respectively, andthese are given by:

dpe = dp1

[(𝜌1

𝜌2

)(dp2

dp1

)]x2∕x1

(3.7)

𝜌pe = x1𝜌1 + x2𝜌2 (3.8)

In Equations 3.7 and 3.8, x1 and x2 refer to the mass fraction of particles in the binarymixtures with x1 < x2, while dp1 and dp2 refer to the particle diameters in the mixture, withdp1 the diameter of the particle that is in less mass fraction, no matter if it is the biomass orthe other solid phase in the bed.

When increasing the fluid velocity beyond umf the flow regime changes gradually fromincipient to bubbling. A further increase to higher than the particle terminal velocity, thatis, U > uter, results in solids elutriation, thus shifting to the transport regime demonstratedin Figure 3.3. In this case, the residence times of the biomass and product gases are limited,controlled by the fluid velocity and the reactor height. The terminal velocity can be obtainedfrom the simple force balance of the gravity force, drag force, and the weight of the singleisolated particle, leading to:

uter =

√4gdp

(𝜌p − 𝜌f

)3𝜌fCd

(3.9)

where Cd is the drag force coefficient, which is function of the flow of the regime (laminaror turbulent), usually expressed in terms of the particle Reynolds number (Red). To avoid


the complications related to the dependence of Equation 3.9 on Red, Haider and Levenspiel[12] proposed the following alternative correlation:

uter =⎡⎢⎢⎢⎣

𝜌2f

𝜇(𝜌p − 𝜌f

)g

⎛⎜⎜⎜⎝18(d∗p

)2+ 2.335 − 1.744𝜑(

d∗p

)0.5

⎞⎟⎟⎟⎠⎤⎥⎥⎥⎦−1

(3.10)

where

d∗p =

[𝜌f

(𝜌p − 𝜌f

)g

𝜇2

]1∕3

(3.11)

Similarly to umf, the effect of increasing temperature or pressure on uter can be consideredthrough the density and viscosity terms. Experiments have shown that uter is particularlysensitive to increasing pressure. For instance, in coarse particles, around 1000 μm, anincrease in the pressure from ambient to ∼20 bars may cause more than 200% reduction inuter. Conversely, increasing the temperature causes an increase in uter.

3.3.1.2 Phases Distribution and Segregation

The degree of biomass conversion strongly depends on the spatial distribution of thevarious phases inside the reactor. For instance, in a typical biomass pyrolysis/gasificationreactor there are two phases, solid and gas, with the possibility of a third liquid phase. Thesolid phase consists of biomass, char, and inert material, the latter is usually introducedto enhance the heat and mass transfer between the various phases as discussed earlier.The gas phase mainly consists of biomass volatile matters and other gases resulting fromvarious heterogeneous and homogenous reactions. The liquid phases, which may exist inconsiderably low quantities and only at low temperature regions, mainly consist of tar, aviscous liquid phase resulting from release of heavy hydrocarbons.

Phase segregation is one of the most critical problems affecting the reactor performance.This usually takes place due to the differences in particle properties (size and density) oragglomeration of the various bed materials. Zhang et al. [13,14] have shown that mixing andsegregation behavior in a biomass–sand fluidized bed reactor strongly depends on the fluidvelocity, as demonstrated in Figure 3.7. Such segregation behavior will cause significanttemperature non-uniformity, which in turn will affect the biomass decomposition rate,particularly at the upper region of the reactor where the temperature becomes low [15]. Sofar there is no established method to quantify the degree of segregation in the fluidized bed,though recent research indicates that increasing the biomass size may significantly increasethe degree of segregation; the biomass concentration has limited effects [13, 14]. Work byHalow et al. [16] has also shown that low density biomass particles of 1–5 mm diametermixed with 200 micron glass beads exhibit significant segregation for U < 2umf but nearlycomplete mixing for U > 2umf.

3.3.2 Residence Time

For the proper design of a biomass conversion reactor, knowledge of residence time of thevarious phases is significantly important due to its direct impact on the conversion rate and


(a) Low gas velocity

(b) High gas velocity

Increasing gas velocity

Increasing gas velocity

Localmixing

Well mixing

Localsegregation

u = ucfu > uif

u > umf(ucf)

u = 0

Localsegregation

Grobalsegregation

Grobalmixing

Figure 3.7 The mechanisms of phase separation and segregation in a bubbling fluidized bedreactor [13,14]. Reprinted from (Zhang et al., 2009), with permission from Elsevier.

the nature of the end product components. During the reaction, micro- and macro-scaletransformations take place simultaneously at different time scales. The extent of thesetransformations depends on the residence time.

The residence time of the various phases can be controlled by varying the processoperating conditions. Reported studies have shown that a short residence time, in the rangeof 1–2 seconds, at a high temperature above 700 ◦C, favors the formation of permanentgases, while at a relatively lower temperature below 550 ◦C, heavy hydrocarbons areproduced, which after condensation produce bio-oil [17]. At a much longer residencetime, in the order of minutes, and in the absence of or low oxygen within the temperaturerange of 300–700 ◦C, the process predominately results in the formation of biochar [18].A generalized map for the biomass conversion routes and products with respect to gasresidence time and temperature is shown in Figure 3.8.

A theoretical approach for estimating the required gas residence time can be basedon the rate of the determining reaction. For instance, in biomass gasification, the highlyendothermic water–carbon reaction is believed to be the slowest; and the following reactionscan be assumed as the rate controlling steps:

C + H2O → CO + H2 (3.12)


1600Combustion

5% ash95% gas

Gasification

5% liquid10% char85% gas

Pyrolysis


Carbonation


1400

1200

1000

800

600

400

200

0

0.5 1.0 2.0

Gas residence time (s)

Reacto

r te

mpera

ture

(°C

)

4.0 8.0

Figure 3.8 Effect of reactor temperature and gas residence time on the product yield (derivedfrom Bridgwater, 2003).

According to Gerber et al. [19], the rate of this reaction can be given by:

− r = −dCdt

= kCH2O (3.13)

where k is the rate constant given by the Arrhenius law. With the Arrhenius law constantsavailable, either from experiment or from the literature, and assuming a value for thecarbon conversion, Equation 3.13 can then be integrated to give a first approximation of therequired residence time. A similar approach has been used by Sun et al. [20] in estimatingthe gas residence during the gasification of charcoal in the presence of CH4 and H2O. Itwas assumed that the water–carbon reaction is the slowest; hence, the rate controlling stepwas governed by the following reactions:

C + H2O → CO + H2 (3.14)

CH4 → C + 2H2 (3.15)

The overall rate of the carbon conversion was then described by the following equation:

− r = −dxdt

= k′pnCH4

− k′′pmH2O (1 − x)2∕3 (3.16)

where k′ and k′′ are the Arrhenius law constants, n and m are the reaction orders, and pis the partial pressure. The required gas residence time can then be obtained as a functionof the various kinetic parameters and the specified value for the carbon conversion, x, asfollows:

t = −32𝜂0.33 + 3B0.5

2A1.5ln

[(A∕B)0.5

𝜂0.33 + 1

(A∕B)0.5𝜂0.33 − 1

]− 3B0.5

2A1.5ln

[(A∕B)0.5 + 1

(A∕B)0.5 − 1

]+ 3

A(3.17)


where

𝜂 = 1 − x (3.18)

A = k′′

pmH2O (3.19)

B = k′pnCH4

(3.20)

Similar to the gas residence time, the overall conversion of biomass in a thermal reactorstrongly depends on the biomass particle residence time, which in turn is a function ofthe reactor configuration, flow hydrodynamics, and heat transfer rate at the single-particlelevel. These three design parameters are interrelated, and therefore must be taken intoconsideration collectively when examining the required particle residence time.

3.3.3 Distributor Plate and Cyclone

In biomass thermal conversion, it is important to control the degree of mixing within thereactor as well as separating the solid and gas phases leaving the reactor. The first hasa critical effect on the fluidization regime, while the second determines the extent of thedownstream gas cleaning processes. In a bubbling fluidized bed reactor, good mixing iscommonly aided by a distributor or a perforated plate placed at the lower part of the reactor,while the solid–gas separation is primarily achieved by a cyclone at the top. Svensson et al.[21] reported that a multiple bubble regime can be achieved in a gas fluidized bed reactorby operating at a low gas velocity and a high pressure drop across the distributor (ΔPdis),while a single bubble regime can be achieved at low gas velocity and low pressure dropacross the distributor. The former regime is always desirable as smaller bubbles help limitgas bypass and ensure a good degree of homogeneity in the reaction region, thus enhancingthe overall mass and heat transfer. Kunii and Levenspiel [22] suggested a general “rule ofthumb” for smooth fluidization as follows:

ΔPb = (0.2 − 0.4)ΔPdis (3.21)

where ΔPb and ΔPdis are the pressure drop across the bed and distributor respectively; bothparameters can be determined theoretically or experimentally to confirm proper reactordesign.

Cyclones are commonly used in fluidized bed reactors because they are relatively cheap,compared to other hot filtration methods, and are reasonably effective for the separationof particle sizes close to 10 μm. Multi-cyclones can be used to improve the separation ofparticles up to 5 μm. Other more sophisticated devices, such as electrostatic precipitators,can be used downstream to remove very fine particles up to 1 μm. Figure 3.9 shows theefficiency of the various gas cleaning methods commonly used in downstream gas cleaningin comparison with the performance of a conventional cyclone.

For a given cyclone dimension, the recommended design equation for determining theminimum particle size captured in a cyclone, sometimes referred to as the cut size, can begiven by:

dp = 3Ai

√𝜇f

𝜋ZQ𝜌p

2rminD

(3.22)


Cyclones

Wet separators

Electrostaticprecipitators

Filters

Particle size, x (μm)

0.10

50

100G

rade e

ffic

iency,

G(x

) (%

)

1.0 10 10C

Figure 3.9 Grade efficiency for various gas cleaning devices used with biomass conversionreactors. Reprinted with permission from Rhodes, M. (Ed), [23]. Copyright © 2008, John Wileyand Sons.

where Q is the volumetric gas flow rate. Ai, D, and Z are the cyclone’s inlet cross-sectionalarea, diameter, and effective height respectively, rmin is the minimum radial distance orradius at which the cut size will be retained. At any point (rmin, Z) if the particle size is>dp, the particle will be pushed away from the central vortex towards the cyclone wall, andhence be collected at the bottom of the cyclone. The suggested values of rmin vary betweentwice the inlet diameter (2Do.) and half the cyclone diameter (0.5D).

3.3.4 Heat Transfer Mechanisms

During biomass thermal conversion, the solid material goes through a range of physical,chemical, and thermal changes, such as drying, shrinkage, devolatization, and combustion.Devolatization is associated with the release of volatile matters, which may then undergovarious reactions depending on the temperature and residence time. Both the drying andpyrolysis are of endothermic nature, thus requiring a source of heat supply to the reactor.The combustion and gasification are both exothermic, thus adding heat to the reactor.

The heat supply to the reactor can be carried out by various methods depending on thereactor configuration, these methods are:

1. Heat transfer through the reactor wall.2. Transferring a re-generable heat carrier between a heat source and the reactors.3. Preheated gases.4. Partial combustion of the biomass feed.5. Inserted hot tubes, steam heated for instance.6. Combination of one or more of the above methods.

Examples of the above described heating methods are demonstrated in various types ofreactors in Figure 3.10. At the macro- and micro-scale levels there are three characteristic


Biomass

Biomass Biomass

Steam+airFluidizing agente.g. nitrogen

Heat He

at

He

at

Combustion

Co

mb

usti

on

Productgas

Product gas

Hot heat carier

Cold heat carier

Flue gasPyrolysis gas

Air

Air

(a) (b) (c)

Figure 3.10 Examples of heat supply arrangement in various biomass thermal conversion reac-tors (a) downdraft gasifier (b) bubbling fluidized bed pyrolysis reactor (c) dual fluidized bedsteam gasifier.

heat exchange mechanisms that can take place inside the reactor: (i) exchange betweenthe wall and fluid; (ii) exchange between the wall and solid particles; and (iii) exchangebetween the fluid and solid particles. The latter is of particular interest in fluidized bedgasification/pyrolysis, where the rate of heat transfer between the suspended hot particles,such as sand and char, and the biomass material controls the rate of thermal conversion. Inpractical operation, the heating rate may reach in excess of 2000 ◦C per second, such as inthe case of fast pyrolysis, which favors the formation of condensable vapors for bio-oil.

Accurate calculation of the biomass heating rate in a thermal reactor is a difficult taskand requires complicated experiments or mathematical models; however, a simplified the-oretical approach can be used by assuming that the biomass particle is of a spherical shapeand ignoring the shrinkage and heat released during thermal conversion. The temperaturevariations inside a single biomass particle can then be calculated from the following heatconduction equation:

𝜌pCpdTdt

= kp

(𝜕2T𝜕r2

p

+ 2rp

𝜕T𝜕rp

)(3.23)

where rp, 𝜌p, kp, Cp are the particle radius, density, thermal conductivity, and specificheat capacity respectively. Equation 3.23 can be solved numerically with the appropriateboundary conditions to give the temperature profile inside the biomass particle; however, asimplified analytical solution can also be obtained for small particles (Biot number <1) byassuming a uniform temperature distribution, such that the heating rate can be given by theCarslaw and Jaeger [24] solution as follows:

dTdt

=

(24d2

p

)(k

𝜌pCp

)(Ts − T

)(3.24)


3.5

3

2.5

2

1.5

1

0.5

00 200 400 600 800 1000

Particle diameter (μm)

Re

qu

ire

d h

ea

tin

g t

ime

(s)

He

atin

g t

ime

(°C

/s)

1200 14001.E+02

1.E+03

Bi < 1 Bi > 1

1.E+04

1.E+05

1.E+06

Figure 3.11 Heating rate and required heating time as a function of the particle diametercalculated using the approximate solution of Equation 3.25. The parameters used are for woodbiomass: To = 20 ◦C, Ts= 900 ◦C, T= 800 ◦C, 𝜌p = 700 kg m−3, kp= 0.1 W m−1 K−1, and Cp=3.34 kJ kg−1 K−1 (obtained at an average temperature of 460 ◦C). The heat transfer coefficientused in calculating the Biot number is h=650 W m−2 K−1.

The time required for the particle to reach the surrounding temperature, Ts, is obtainedby integrating the above equation between t = 0 and 𝜏p and T = To and T to give:

𝜏p =

(d2

p

24𝛼

)ln(

Ts − To

Ts − T

)(3.25)

Figure 3.11 shows the approximate calculation of the heating rate and the time requiredfor raising the temperature of a biomass particle to the desired temperature for a selectedheating scenario. According to the calculated Biot number, this approximation is only validfor a particle size <1300 μm. An accurate numerical solution that takes into considerationthe internal thermal resistance, changes in particle physical properties, and heat of reactionsshould be considered for detailed design.

3.3.5 Biomass Conversion Efficiency

The biomass conversion efficiency is one of the most important parameters for the designand performance analysis of gasification and pyrolysis processes. This conversion efficiencycan be calculated as the mass ratio for the product (gas or bio-oil) to the biomass feed tothe reactor stock as follows:

𝜂eff =mfuel

mbiomass(3.26)

Where mfuel is the mass of the product fuel from one mole biomass and mbiomass is themass of biomass feed calculated from the mass percentage of cellulose, hemicellulose,lignin, and ash in one mole of biomass. For example, the energy crops consist of thefollowing in mass percentage: 22–36 cellulose, 13–27 hemicellulose, 11–25 lignin, 2–11


ash, 9–20 extractives. The last two components are not accounted for in the biomass feedas they do not participate in the conversion process.

Alternatively, the conversion efficiency can be expressed in terms of the higher heatingvalues (HHV) of the product fuel and feed biomass (sometimes referred to as the energyratio) as follows:

𝜂eff =mfuel (HHV)fuel

mbiomass (HHV)biomass(3.27)

In a wood gasifier, the biomass conversion efficiency in terms of the energy ratio is around60–70% [25]; however low efficiencies in the range of 20–40% have also been reportedin the literature [26]. Another definition for the biomass conversion efficiency, particularlyapplicable to biomass gasification, is given in terms of mass of carbon as follows:

𝜂eff =mcarbon in the product gas

mcarbon in the biomass feed(3.28)

where

mcarbon in the biomass feed = %Cbiomass × biomass feedmcarbon in the biomass feed = mcarbon in the product gas − %Cchar × char product

The percentage quantity %Cbiomass and %Cchar can be obtained from the ultimate analysisof the biomass feed and product char respectively.

3.4 Reactions and their Impact on the Products

Thermochemical conversion reactions take various routes depending on the reactor temper-ature, residence time, and the reactive medium. Understanding the fundamentals of thesereactions is essential for the reactor design. These various conversion routes and reactionsare discussed in detail below.

3.4.1 Devolatization and Pyrolysis

In biomass thermal conversion, the biomass first undergoes simultaneous drying and devola-tization. In the absence, or in a very limited amount, of oxygen this process is referred to aspyrolysis. The process is highly endothermic since heat is required to remove the moisturecontent as well as to complete the devolatization step. The reactions can be described bythe following simple step:

Carbon(C) + heat → char + Pyrolysis gases (CO, CO2, H2O, CH4, H2, Tar) (3.29)

The product composition of the devolatization or pyrolysis step strongly depends on thetemperature and residence time, as shown in Figure 3.12. In the case of low temperaturesand short residence times, approximately 80% of the biomass is converted to pyrolysis gasand the rest is char.

This reaction can be described in more detail by the Waterloo concept [27], where thebiomass conversion is represented by two stages depending on the reactor temperature andthe heating rate as shown in Figure 3.13. In stage 1, the biomass is converted to char, oil, and


Time + Heat

Char

Permanent gases(CO, CO2, H2, CH2)

Pyrolysis gases(CO, CO2, H2, CH4, H2O, Tar, char)

Biomass

Heat

Figure 3.12 Products from biomass thermochemical conversion process.

pyrolysis gases (primary reactions) at a slow heating rate and relatively low temperaturewithin the range of 350–550 ◦C. At a higher temperature, in the range of 550–600 ◦C, thetar and heavy hydrocarbons undergo cracking (secondary reactions), thus producing lessoil and more permanent gases. The secondary pyrolysis reactions are slightly exothermic,however the overall pyrolysis process is highly endothermic.

To maximize the bio-oil yield, it is essential to avoid the secondary pyrolysis reactionsby limiting the reactor temperature to <600 ◦C and limiting the gas residence time to <2 s.Experimental work carried out by Di Blasi et al. [28] has shown that the maximum liquidyield for various biomass materials can be achieved at the temperature range of 500–600 ◦C(see Figure 3.14).

3.4.2 Gasification

Thermochemical gasification of biomass is an extended operation of pyrolysis, wherethe operation at a relatively high temperature, in excess of 600 ◦C, prompts cracking ofheavy hydrocarbons and the formation of permanent gas. A range of combustion reactions,including carbon combustion, may also take place if an appreciable amount of oxygenis present. The product gas is usually referred to as the producer gas. In typical biomassgasification, the process combines drying, combustion, pyrolysis, and gasification in onesingle reaction vessel. In general, the gasification reactions are far more complicated thanpyrolysis because of the large number of heterogeneous and homogenous reactions thatfollow the pyrolysis step, as shown in Figure 3.15.

Char-1

Oil-1

Gas-1

Biomass

Primary reactionsat < 550 °C

Secondary reactionsat < 550 °C

Char-2

Oil-2

Gas-2

Figure 3.13 Biomass pyrolysis reaction steps as described by the Waterloo concept [27].Reprinted from Radlein D., Piskorz J., Scott D. S. (1991), with permission from Elsevier.


Wood

OliveHusks

Straw

Grape residues

Rice husks

Measured

650

55

45

35

25750

Liq

uid

yie

lds, w

t %

mf

850

Tb [K]

950 1050

By difference

Figure 3.14 Liquid yields from pyrolysis of wood chips and agricultural residues as functionsof temperature [28]. Reprinted with permission from Di Blasi, C., Signorelli, G., Di Russo, C.,Rea, G. (1999). Copyright © 1999, American Chemical Society.

In common practice, air, oxygen, steam, or a mixture of any of these can be used as thegasifying agent. In air gasification, following the release of the pyrolysis gases, the productgas consists of CO, CO2, CH4, H2, H2O, N2, and a limited amount of tar. In the case ofpure oxygen gasification the product gas is of higher heating value due to the eliminationof N2. The details of the most important gasification reactions in air or oxygen gasificationare shown in Table 3.2. These reactions may take place simultaneously or in a sequencedepending on the reactor design and operating conditions, particularly the feeding locationof the biomass and/or the gasifying agents.

In the case of steam gasification, the product gas heating value is higher due to theelimination of N2 and the considerable increase in H2 (via water–gas shift reactions).However, steam gasification is highly endothermic, and therefore an external source ofheating is usually required. Alternatively, a limited quantity of air pure oxygen can beintroduced to the reactor to allow partial combustion of the biomass feed, thus satisfying theheat demand. A summary of the main chemical reactions associated with steam gasificationis given in Table 3.3.

Pyrolysis products

Homogenous reations

Heterogenous reations

Gas–gas reactionsTar cracking

Char–gas reactions

CO, CO2, H2, CH4, H2O

Tar

Char

Figure 3.15 General reaction mechanisms and products in biomass gasification.


Table 3.2 Main gasification reactions in a biomass thermal conversionusing air.

Homogenous reactions Heterogeneous reactions

CO + O2 → CO2 C + O2 → CO2CH4 + O2 → CO2 + H2O2H2 + O2 → 2H2O

2C + O2 → 2COC + H2O → CO + H2

CO + H2O → CO2 + H2 C + 2H2 → CH4CO2 + CH4 → 2CO + 2H2 C + CO2 → 2COCH4 + 2H2O → CO2 + 4H2Tar (C2H6) + 5/2O2 → 2CO + 3H2O

Table 3.3 Main chemical reactions of biomass gasification in thepresence of steam.

Homogenous reactions Heterogeneous reactions

CO + H2O → CO2 + H2 C + H2O → CO + H2OCO2 + CH4 → 2CO + 2H2 C + 2H2 → CH4CO + H2O → CO2 + H2 C + CO2 → 2COCH4 + 2H2O → CO2 + 4H2CH4 + H2O → CO2 + H2OTar (C2H6) + 2H2O → 2CO + 5H2

3.5 Mass and Energy Balance

3.5.1 Mass Balance

Mass balance around a thermal conversion reactor is usually carried out to identify thedegree of conversion and obtain the amount of the various components in the product. Thetwo most common methods used in obtaining composition of the products in a biomassconversion reactor are the:

• thermodynamic equilibrium method

• kinetic method.

The thermodynamic equilibrium model is independent of the reactor design and onlyvalid for a conversion system reaching equilibrium. It is believed to be more appropriatefor overall process analysis of reactors with unclear reactions. The method is based on thenumerical solution of a range of equations that allow for minimizing the Gibbs free energywhen the system is at equilibrium. The kinetic method, on the other hand, is simpler andmainly based on the assumption of a continuous-flow stirred tank reactor model, thus thereactor is assumed to operate isothermally. This is particularly useful for a first estimate ofthe product composition for given operating conditions. However, the method is stronglydependent on the type of reactor used and requires identification of the main reactionskinetics, operating temperature, and flow hydrodynamics.


5.80

5

10

15

20

25

30

35

6.3 6.8 7.3

Time (s)

Gas s

pecie

s m

ol%

(dy b

asis

)

O2 CO2 H2

CH4 N2 CO

7.8 8.3

Figure 3.16 Example of CFD prediction of product gas composition as a function of timein a circulating fluidized bed steam/air biomass gasifier (tar excluded). Operating conditions:steam to biomass ratio = 0.24; air to biomass ratio = 0.04; reactor temperature = 900 ◦C [29].Reproduced from Hassan, M. (2013). PhD Thesis, Aston University.

For the simplest case of first-order reactions, the basic design equation in the kineticmethod can be given by:

rA = kCA = kCA0(1 − xA) (3.30)

This can be expressed in terms of the time and rate constant as follows:

xA = 1 − exp (kt) (3.31)

The reaction rate constant, k, is commonly given by the Arrhenius law as follows:

k = A exp(−Ea∕RT

)(3.32)

Values of the pre-exponential function (A) and the activation energy (E) for the variousreactions given in Table 3.3 can be found in the open literature. The kinetic model has theadvantage of being more applicable to computational fluid dynamics (CFD) where a largenumber of equations for the reactions and interactions between the various solid phases canbe solved numerically to detailed transient information on the performance of any type ofa biomass conversion reactor. Figure 3.16 shows an example of CFD model predictions ofthe product species composition in a circulating fluidized bed biomass gasifier.

3.5.2 Energy Balance

Thermal conversion of biomass is generally considered an endothermic process and isusually carried out in adiabatic reactors, thus allowing for significant heat exchange betweenthe solid biomass and the surrounding material inside the reactor. The energy balancearound the reactors is essential for determining the optimum reactor temperature and theamount of heat required to complete the overall reactions. For a typical gasification and


Energy inproducts

Energy inconversion

agents

Energy inbiomass feed

Energyloss

Externalenergy

Reactor

Figure 3.17 Energy balance around a biomass thermal conversion reactor.

pyrolysis processes, the recommended reactor temperature is within the ranges of 400–500 ◦C and 700–900 ◦C respectively. Figure 3.17 shows a typical energy flow around abiomass conversion reactor.

Assuming negligible losses, the generalized energy balance for such reactors can bewritten as:

Q =[∑

mΔHof,298,feed +

∑mHfeed(T)

]−[∑

nΔHof,298,prod +

∑nHprod(T)

](3.33)

where Q represent the heat added or removed from the reactor (kW) and the first and secondterms between brackets on the right-hand side represent the heat of formation and enthalpy(kJ/kg) in the total feed streams and product species respectively. The heat of formation ofthe biomass feed can be calculated using the following equation reported by Li et al. [30]:

ΔHof,298,feed = HHV − (327.63C + 1417.94H + 92.57S + 158.67W) (3.34)

where HHV is the fuel higher heating value (kJ/kg). The quantities C, H, S, and W are thepercentage weight of carbon, hydrogen, sulfur, and water content in the fuel respectively,all can be obtained from the ultimate analysis of the biomass feed. Figure 3.18 shows an

250200150

Biomass feed to the reactor (kg/h)

1005000

200

400

Required therm

al e

nerg

y input (k

W)

600

800

Figure 3.18 Relation between the biomass feed and the thermal energy demand for steamgasification of wood biomass in a circulating fluidized bed reactor.


example of the calculated heat demand for the conversion of wood biomass to fuel gas in asteam gasifier. Such a heat demand can be supplied by different heating methods dependingon the reactor configuration.

3.6 Reactor Sizing and Configuration

Experimental and pilot-plant testing is essential for proper reactor design. However, it iscommon practice to use correlation and valid parameter values in determining the realisticreactor dimensions and configurations. For detailed design and parametric analysis tomicroscopic levels, advanced computer simulation models are nowadays widely used.In this section these two design options will be discussed for general thermochemicalconversion reactors. The most important information required for deciding the geometricalconfiguration and sizing of a biomass conversion reactor is:

• The gas and solid residence time.

• Heating rate and intensity.

• Velocity of the biomass conversion agent.

• Physical and chemical properties of the biomass material.

• Quality and nature of the end product (fuel gas or bio-oil).

In obtaining the reactor size, the cross-sectional area is a function of the volume flowrate of the conversion medium, Q, and the desirable flow velocity at through the reactor, U,this is simply given by:

Ar =QU

(3.35)

It is critically important that U is calculated at the reactor operating temperature andpressure. In designing the reactor height, the operating conditions (such as residence timeand concentrations) and the cost of construction material are the determining factors. In abubbling fluidized bed reactor, the choice of the vessel height is firstly dependent on the bedexpansion. Numerous correlations are available for predicting this parameter. For example,the work by Llop et al. [31] has shown that the bed expansion for a reactor with particlesof Geldart group B and D type, operating at a high pressure and temperature, in the rangeof 1–15 bar and 208–600 ◦C, can be predicted by the following correlation:

Hr = Fm𝜒Hmf(U − umf

)0.8 + Hmf (3.36)

where

Fm = 1.41

g0.5

[1.638A0.4 −

(1.638A0.4 − 0.872A0.4

o

)exp

(−

0.15Hmf

Dc

)]0.5(3.37)

𝜒 =

[249.7d−1.69Ar0.385 exp

[−0.001467

Ardp

+ 279610d2p + 456200

(Ardp

)2]]

(3.38)

In Equations 3.31 and 3.32, A and Ao are the cross-sectional areas of the reactor vesseland the orifice in the distributor plate, Dc and dp are the diameters of the reactor vessel


Table 3.4 Example of correlations for TDH in bubbling fluidized bedsreactors.

Reference TDH correlation

Amitin et al. [32] 0.85U0.5 (7.33 − 1.2log10 U)Fournol et al. [33] 1000U2/gSmolders and Baeyens [34] 6[(U − Umf) db]0.6

where

db = 0.54 (U − Umf)0.4 (z + 4Ao)0.8 g− 0.2

and particle respectively, Hmf is the height of the fluidized bed at the minimum fluidizationcondition, and Ar is the Archimedes number.

The vessel height then must exceed the bed expansion to allow for entrained particlesto fall back to the reaction bed, thus limiting loss of the bed material. The length abovethe expanded bed, usually referred to as free board, can be determined from the availablecorrelation of the Transport Disengagement Height (TDH). This is defined as the heightabove the dense bed surface required for the solid to disengage and fall back (i.e., noentrainment takes place). Examples of TDH correlations are given in Table 3.4. Anotherconsideration that must be taken into account when choosing the bed height is the gasresidence time. For instance, in a bubbling fluidized bed pyrolysis reactor where liquidoil is the desired end product, it is recommended to lower the reactor height to avoid anundesirable secondary cracking reaction at the upper part of the reaction vessel. Therefore,the final reactor height is a tradeoff between design parameters: the TDH, entrainment, gasresidence time, and cost of construction material.

In a transport reactor, such as a circulating fluidized bed, the height of the reactor is inthe first instance a function of the biomass conversion rate, which in turn is a function ofthe gas/particle residence time. This can be given by:

Hr =Fb𝜏res

Ar (1 − 𝜀) 𝜌pxb(3.39)

Where Fb is biomass feed rate, 𝜏res is the biomass residence time, and xp is the biomassfraction in the bed material. For a transport reactor, values of 𝜀 can be assumed to bebetween 0.7 and 0.9. Note that the bed material in most biomass conversion reactorscontains a considerable amount of inert solids. For a high solid mass flux (>200 kg/m2s), itis reasonable to assume that the reactor is operating in a plug flow regime; in this case, thereactor height can simply be given by:

H = Us𝜏res (3.40)

where Us is the slip velocity, approximately given by(U − uter

), U and uter are the gas

superficial velocity and terminal velocity of the particles used in the bulk bed materialrespectively. It should also be noted that the minimum recommended height for a transportreactor must not be less than the TDH to ensure carryover of the bed material.


Table 3.5 Advantages, disadvantages and products of various biomass conversion reactors.

Reactor Advantages Disadvantages Products

Downdraft – Simple design and lowconstruction cost

– Low tar content in theproduct gas

– Moisture content ofbiomass feed up to30%.

– Proven technology

– Unconverted carbonup to 7%

– Limited scale-uppotential

– Very high exit gastemperature

– Limited scale-uppotentials

– Biomass feed must bepalletized

– Producer gas withheating value up to9 MJ/m3 when usingoxygen/steam as gasifyingagent

Updraft – Simple design and lowconstruction cost

– Moisture contentbiomass feed up to50%

– Proven technology

– High tar content in theproduct, up to 20 wt%

– Requires carefultemperature control toavoid ash softening.

– Limited scale-uppotential

– Biomass feed must bepalletized

– Producer gas withheating value up to13 MJ/m3 whenusing oxygen/steamas gasifying agent

Bubbling – Simple constructionand low running cost

– Can handle variety ofbiomass feed

– Uniform temperatureand high heat transfer

– Good carbonconversion

– Low tar content in caseof gasification process

– Can be connected witha CFB reactor to form adual fluidized bedsystem

– Moderate scale-uppotentials

– Requires carefultemperature control toavoid agglomeration

– High particulate in theproduct

– Possible loss ofunconverted biomasswith the ash removed

– High oxidant demandunless external heatingis provided

– Possible gas bypass inthe form of largebubbles

– Producer gas withmaximum heatingvalue of 20 MJ/m3

when usingoxygen/steam asgasifying agent

– Bio-oil withmaximum heatingvalue of 18 MJ/kg

Circulating – Same as in bubblingbed

– High throughput ofbiomass whencompared with thebubbling reactor

– Higher operating costwhen compared withthe bubbling reactor

– Higher operating costwhen compared withthe bubbling reactor

– High construction costfor small scaleprocessing

– Not suitable for highbiomass moisturecontent


when usingoxygen/steam asgasifying agent

– Bio-oil withmaximum heatingvalue of 20 MJ/kg


Table 3.5 (Continued)

Reactor Advantages Disadvantages Products

– Allows for theintroduction andcirculation of catalystsfor regeneration

– Can be connected witha BFB or a CFBreactors to form a dualfluidized bed system

– Good scale-uppotentials

– High oxidant demand,unless external heatingis provided

Entrained – Low tar content in theproduct gas due to veryhigh temperature

– Highest processingcapacity

– Highest biomassconversion rate

– Good scale-uppotentials

– High construction costfor small scaleprocessing

– Less understood forbiomass conversion

– Not suitable for coarseparticles >100 μm

– Not suitable for highbiomass moisturecontent

– The residence time forsolids is very short

– Operate at elevatedpressure


when using oxygenas the gasifyingagent

3.7 Reactor Performance and Products

Despite the recent progress in thermochemical conversion technology, reactor performanceand scale up potential are the subjects of continuing research. In terms of scale up, recentresearch suggests that the entrained flow reactor comes top with a maximum possiblethroughput of ∼1000 MW, followed by fluidized bed reactors with a maximum throughputof ∼100 MW, and finally come the updraft and downdraft reactors with a maximumthroughput of ∼10 and ∼1 MW respectively. The advantages and disadvantages of thevarious reactors discussed in this chapter are summarized in Table 3.5.

3.7.1 Moving Beds

Moving bed (MB) reactors commonly use air as the gasifying agent and the quality of thegas produced has a low heating value, mainly due to nitrogen diluting the product gas. TheCO2 level is also high, in the range of 25–50 vol% on a dry basis. Improved gas quality canbe achieved by using steam or an oxygen/air mixture. The reactors operate at atmosphericpressure but at a relatively high temperature, close to 1000 ◦C at the combustion region.Due to this high operating temperature, moving beds mainly produce permanent gases andash. Therefore, they are mainly used in biomass gasification to produce fuel gas.


C + H2O CO + H2

Gas

Biomass + heat --> C + H2O

C + heat --> H2, CO2, CO, H2O, CH4, tar, char

Char + air --> CO2, CO, N2, heat, ash

Ash

200 1200

Temperature (°C)

C + HO2 2CO

CO + H2O H2 + CO2

Air

Biomass

Drying

Pyrolysis

Gasification

Combustion

Figure 3.19 Demonstration of the reactions and temperature distribution in a downdraft air-biomass gasifier.

The MB reactors have four main process zones, for which the boundaries are gradual andoverlapping. In a typical downdraft type of a MB gasifier (see Figure 3.19), the biomass isfed from the top, where the first two process zones are located (drying and devolatizationzones). The hot gas (∼750 ◦C) moving upward from the zone below comes into contactwith the biomass, causing pre-heating and pyrolysis of the biomass. The gas then carriesthe pyrolysis product out with it, leaving at a temperature of around 500 ◦C. After that,the char moves down into the gasification zone (sometimes referred to as reduction zone),this is where the vast majority of the H2, CO, and methane is produced. The gasificationreactions are endothermic and require energy to be fed. As the char enters the final zone,in which combustion takes place, the reactor is at its highest temperature, around 1000 ◦C.Here the remaining carbon is combusted with the gasifying agent to produce CO2 and H2O,leaving behind just the ash and some un-reacted carbon. A typical gas composition in adowndraft wood gasifier is shown in Table 3.6.

Table 3.6 Typical product gas composition from wood-air gasification in a downdraftreactor operating at 930 ◦C [35].

Component H2 O2 CO CH4 CO2 C2H4 C2H6

Vol% (dry basis) 31.4 0.89 29.56 6.23 30.02 1.6 0.3


3.7.2 Fluidized Bed (FB)

Fluidized bed (FB) reactors are most commonly used for biomass thermal conversion,benefiting from long-standing application experience in coal gasification and combustion.The circulating and bubbling types of FB reactors have particularly shown great potentialin biomass conversion to bio-oil and producer gas.

3.7.2.1 Bubbling Fluidized Bed (BFB)

Bubbling fluidized bed reactors are by far the most widely researched and demonstratedat an industrial scale. This is mainly due to their lower operating and capital costs. Thebubbling fluidized bed system is simple in design and mainly consists of a reactor columnor vessel equipped with a distributor and a biomass screw feeding system. A cycloneis usually connected to the reaction vessel to remove any fine particulates from the exitgas. Figure 3.20 shows a typical arrangement and temperature distribution in a bubblingfluidized bed biomass pyrolysis reactor.

Foster Wheeler AG, a Finnish based company, was among the first in developingcommercial-scale bubbling fluidized bed (BFB) biomass gasifiers (production of fuel gas)during the years 1997–2003, with fuel inputs from 40 to 70 MW. BFB reactors have alsobeen extensively studied for pyrolysis (production of bio-oil); therefore, there is sufficientinformation for conceptual design and scale-up. However, the performance of the BF reac-tors in the thermal conversion of biomass suffers from a major operational problem relatedto particle agglomeration. Low melting alkali metals and ash, in addition to tar, tend to coatthe solid bed material with a sticky layer, which upon collision create large agglomerates.

Biomass

Gasifying agent

Temperature (°C)

0

Heig

ht

30050 550

Gas

Figure 3.20 Bubbling fluidized bed biomass pyrolysis reactor and temperature distribution.


Such a problem has a critical effect on the overall conversion of the biomass material dueto the formation of dead zones and, in extreme cases, complete de-fluidization.

Agglomeration was first observed during coal gasification in a fluidized bed as reportedby Yerushalmi et al. [36] and Gluckman et al. [37]. Since then, numerous publicationshave been published; however, very few have been specifically focused on agglomerationduring biomass thermal conversion. Table 3.7 summarizes the most promising methods tobe considered in the design and operation of a bubbling fluidized bed biomass conversionreactor.

3.7.2.2 Circulating Fluidized Bed (CFB)

Conventional circulating fluidized bed (CFB) reactors work on the principle of particlesuspension; however, unlike the BFB, this is usually carried out at a fluidization velocityhigher than the terminal velocity of the largest particle size in the bed. The system isrelatively more complex than the bubbling type and mainly consists of: (i) a reactioncolumn or vessel equipped with a gas distributor, usually referred to as the riser; (ii) oneor more cyclones connected to another column, usually referred to as the downer; and (iii)a biomass screw feeding system. The first cyclone serves to capture and return the coarseentrained particles back to the reactor and the second captures the very fine particles. Thisarrangement allows for closed loop circulation of the solid bed material, while the productgas leaves from the top of the cyclones to go for further treatment or processing. Figure 3.21shows a schematic diagram of this system.

CFBs are particularly suitable for biomass gasification, where the longer gas residencetime (>2 s) in the fluidization column favors the formation of permanent gases (producergases). In this case, the pyrolysis gases produced at the bottom part of the reactor undergovarious gas–gas reactions and tar cracking at the upper part of the reactor. The intense

Table 3.7 Recommended design/operation consideration to prevent agglomeration problemin BFB reactors.

Method Sources Comments

Lower operation temperaturebelow the melting point of thebed materials

van der Drift andOlsen [38], Bartelset al. [39]

This may result in reducedbiomass conversion andincreased tar and CO in thegas phase

Reduce gasifying agent velocityor temporal stopping ofbiomass feed

Bartels et al. [39];Ergudenler andGhaly [40]

May result in disturbing theproduct gas quality

Removal of the agglomerated bedmaterial and addition of freshmaterial

Ryabov et al. [41] Most commonly used inindustrial-scale operation.Particularly suitable for CFB

Installation of sieving in the solidrecycle system

Korbee et al. [42] Promising and robust method,but only suitable for CFB

Pre-treatment of the biomass feedto remove alkali metals

Arvelakis et al. [43] Has been demonstrated atlab-scale testing, howeverthere are cost implications.


Gasification

Pyrolysis

Combustion

Drying Biomass

Gas

Gasifyingagent

500

Heig

ht

450 850

Temperature (°C)

Figure 3.21 Circulating fluidized bed reactor and temperature distribution in a typicalbiomass gasification.

particle–particle and particle–wall collisions and shearing during rapid circulation help todisintegrate the bonded particles, thus considerably limiting the agglomeration problem,which is usually experienced in the bubbling type reactors. The CFB can be operated with anair–steam mixture as the fluidizing agent to increase the formation of hydrogen via water–gas shift reaction. Recent research indicates that a high calorific value with a hydrogencontent in excess of 70 vol% can be achieved in a CFB reactor with the recommendedair–steam ratio in the range of 0.6–1 and in the presence of a catalyst such as dolomite. Interms of overall energy efficiency, CFB reactors benefit from the intense heat provided bythe circulating solid particles; however, in most cases a supplementary source of heating isrequired to ensure sustainable operation of the reactor. In order to satisfy the heat demandand improve the overall performance of CFB reactors, recent research has led to the dualfluidized bed technology as discussed below.

3.7.2.3 Dual Fluidized Bed (DFB)

In recent years, the need for improved biomass conversion and concern over environmentalemissions has led to the development of the dual or twin fluidized bed system. This system isbased on integrating two fluidized bed units in a single closed loop, where the two reactors


Permanentgas

Pyrolysisgas

Permanentgas

Flue gas

Flue gas

Flue gas

Biomass Biomass BiomassBiomass

Inert gas

Ash Ash

Ash

Air AirSteam/airSteam/air

Steam/air

Gasific

ation

~ 8

00 °

C

Com

bustion

> 9

00 °

C

Com

bustion

> 9

00 °

C

Gasific

ation

~ 8

50 °

C

Gasific

ation

~ 8

00 °

C

Pyro

lysis

~ 5

00 °

C

(a) (b) (c)

Figure 3.22 Example of dual fluidized bed systems (a) bubbling-circulating reactors for pyrol-ysis and gasification (b) bubbling-circulating reactors for gasification and combustion (c)circulating-circulating reactors for gasification and combustion.

can be used to carry out simultaneous operations: (i) combustion and gasification; (ii)combustion and pyrolysis; (iii) gasification and pyrolysis. These options are schematicallyshown in Figure 3.22.

The most common system of a DFB, shown in Figure 3.22, consists of coupling a charcombustion reactor with a gasification or pyrolysis reactor. The system has the advantage ofbeing flexible for temperature and residence time adjustments and thus can be used for theproduction of permanent gases (gasification) or condensable gases to bio-oil (pyrolysis).In the first reactor, the biomass material is fluidized and thermally converted to gases. Theresulting char and inert solid leaving this reactor are then sent to the second reactor, wherecombustion takes place due to oxidation with the fluidizing air. In some cases of gasification,additional fuel may be required to ensure maintenance of the combustion reactor at therequired temperature. The resulting hot solids are re-circulated back to the gasification orpyrolysis reactor to provide the heat required for the reactions, thus completing the cycle.Studies on this relatively new technology have confirmed good scale-up potential withpossible throughput in excess of 100 tons/day. Table 3.8 gives a summary of the mainadvantages and disadvantages of the DFB technology for biomass thermal conversion.Clearly, a number of technical issues need to be addressed in order to attract large scalecommercial investment.

The DFB technology has recently shown great potential for in situ catalytic tar crackingand carbon dioxide elimination in biomass gasification [44, 45]. Catalysts such as nickel,calcium oxide, and dolomite have been frequently reported in the literature. The use ofcalcium and calcium-based materials for simultaneous tar cracking and carbon dioxidecapturing during biomass gasification has been particularly well tested (e.g., [44, 46]). Thisconcept is shown schematically in Figure 3.23, where CO2 sorption and tar cracking takes


Table 3.8 Advantages and limitations of dual fluidized bed reactor (DFB) system forbiomass conversion.

Advantages Disadvantages

– Separate gasification/pyrolysis andcombustion reactors, thus allowing forbetter control of emissions.

– Allows for the elimination of oxidantsource, which is a particularlyadvantages for the production ofbio-oil (pyrolysis)

– Allows for high steam to fuel ratio, thushigher hydrogen in the producer gas.

– Suitable for in situ CO2 sorption andcatalytic reforming/cracking

– The two reactors are interconnected thusdifficult to control the overall thermal balanceof the system.

– The system may require external auxiliary fuelto maintain the biomass conversion reactor atthe desired temperature.

– The overall hydrodynamics is complicated andthe solid circulation rate increasesexponentially for reactor temperature >800 ◦C

– High particle circulation rate (200 kg/m2s)required for sustainable operation.

place in the gasification reactor while the sorbent regeneration takes place in the combustionreactor. These reactions can be described by the following reactions:

Sorption in the gasifier: CaO(s) + CO2(g) ↔ CaCO3(s) − 178.3 kJ∕mol (3.41)

Desorption in the combustor: CaCO3(s) ↔ CaO(s) + CO2(g) + 178.3 kJ∕mol (3.42)

The carbonated solid and the resulting char leaving the gasification reactor can then besent to the combustion reactor operating at a temperature >900 ◦C. At this temperature,CaCO3 thermally decomposes to produce regenerated CaO and CO2 gas. In situ CO2 sorp-tion and regeneration requires carful adjustment of temperature for efficient and sustainableoperation, as sorption favors a relatively low temperature <800 ◦C [47, 48]. However, alow operating temperature in the gasifier has the risks of reduced conversion efficiencyand formation of undesirable condensable organics (tars). Another problem is related tothe deactivation of the sorbent material after cyclic calcination and carbonation processes.This is why future development and design of DFB with calcium-based material for gas

Combustion(> 900 °C)

Gasification(< 800 °C)Air/steam

Carbonated sorbent

Regenerated sorbent

AirCO2 + N2

Clean fuel gas

Biomass

Figure 3.23 Concept of biomass gasification and CO2/tar elimination from the product gasin a closed loop system.


cleaning and tar cracking should be focused on process optimization – particularly in termsof operating temperature, sorbent residence time, and cyclic life.

Assuming the particle flow through the reactors takes the form of a plug flow type, theresidence time can be estimated by:

𝜏res =mass of particles in the reactor

particles mass flow rate(3.43)

The solid residence time on any side of the DFB reactor depends on the flow regime,but generally falls broadly between a few seconds to few minutes. A minimum residencetime in the biomass conversion side should at least be sufficient to allow for releasing largeamounts of the volatiles from a biomass particle.

3.8 New Reactor Design and Performance

The moving and fluidized bed reactors are the most commonly used types of reactorsin biomass thermal conversion technology. Recent research and development have beenfocused on improved reactor efficiency and performance. The ablative is one of the relativelynew reactors first demonstrated by the National Renewable Energy Laboratory (formerlythe Solar Energy Research Institute) and Aston University during the late 1980s. The reactorworks on the principle of applying force and heat to biomass particles to produce pyrolysisvapor. This has the advantage of an intense and fast heating rate without the need for acarrier gas; however, scaling up of the process is difficult due to moving reactor parts andthe cost associated with heating a large rotating surface. The ablative reactor developed byAston University and shown in Figure 3.24 mainly consists of rotated heated plates (upto ∼450 ◦C) producing a pressure up to 100 MPa on the biomass particles. This systemconverts more than 85% of the biomass to vapor, which is then cooled and collected toform liquid oil.

The free-fall reactor, also known as the dower reactor or drop-tube reactor is anotherrecently developed biomass thermal conversion technology. The principle operation of thisreactor is that the solid biomass fuel is fed from the top and allowed to fall freely without theneed for, or with a limited quantity, of a carrier gas. At the bottom of the reactor the productchar is collected and the product gas is sent for further cleaning and processing. An exampleof a free-fall reactor is demonstrated in Figure 3.25. Despite the clear design simplicity andoperation, this type of reactor has not been well studied for biomass conversion application.The three most important design parameters in free-fall reactors are the solid residencetime, the gas residences, and the separation of char from the gas. The solid residence time isimportant to ensure heating of the biomass particle to the desired temperature and releasingof the volatiles. A first estimate of the solid residence time can be obtained from Equation3.25. The gas residence time and its separation from the char is important in the case ofbiomass pyrolysis (for liquid oil production), where it is critical to control the gas residencetime within the hot reactor zone and to limit the char–gas contact in order to avoid secondaryreactions. An example of the most recent effort to address the gas separation in a free-fallpyrolysis reactor has been recently reported by Huard et al. [49]. The method allows forseparating the pyrolysis gas from the solid at a very high efficiency above 99%.


Nitrogen

Wood chips

Variable anglerotating blades

Char pot

Product vaporsand gases toproduct collection

Cartridgeheaters

Sealed hopper andscrew feeder

Figure 3.24 Schematic representation of Aston University ablative reactor for biomasspyrolysis.

The pyroformer is a new emerging biomass conversion technology developed byresearchers at the European Bioenergy Research Institute (EBRI) at Aston University[2]. The reactor mainly consists of a double coaxial screw and a heated jacket which allowsfor intermediate pyrolysis of biomass driven by thermal energy available in the returningchar. The reactor operates at the temperature range of 400–500 ◦C and the gas residence

Char

Gas

(a) (b)

Carriergas

Biomassfeed

Gas Gas

Solid

+ g

as

Solid

+ g

as

Hea

t

Hea

t

Figure 3.25 (a) Example of a free fall reactor (b) demonstration of a novel gas–solid separationin a free-fall pyrolysis reactor (adapted from Huard 2010).


Pyrolysis vapor

Condensed bio-oil

Permanent gas

Feeding material

Bio-char

Solid deflector

(a) (b)

Figure 3.26 The pyroformer intermediate pyrolysis reactor.

time is limited to few seconds, while the solid residence time can be easily controlled bychanging the screw speed. The reactor has the advantage of processing biomass as well asmultiple waste feedstocks at a high moisture content to produce liquid oil, gas, and biochar.Figure 3.26 shows a schematic description of the pyroformer.

Nomenclature

A pre-exponent function in the Arrhenius law (s−1)A cross-sectional area of a reactor (m2)Ai cross-sectional area of a cyclone inlet (m2)Ar Archimedes number (= 𝜌fd

3p(𝜌s − 𝜌f)g∕𝜇2

f )

C concentration of species (kmol m−3)Cp specific heat capacity (Jkg−1 ◦C−1)Cd drag coefficient (-)dp particle diameter (μm)D diameter of the straight section of a cyclone (m)Ea activation energy (kJ kmol−1)g gravity constant (= 9.81 m s−2)H heat of formation or enthalpy (kJ kg−1)Hr height of a reactor (m)Hmf height of a fluidized bed at minimum fluidization (m)HHV higher heating value (kJ kg−1)k reaction rate constant (s−1)kp effective thermal conductivity (kg m−1 s−1)m mass (kg)p partial pressure (Pa)Q volumetric fluid flow (m3 s−1)r radius (m)Re Reynolds number (= 𝜌fUdp∕𝜇f )r rate of reaction (kmol m−3 s−1)


T temperatures (◦C)t time (s)U superficial velocity (m s−1)umf minimum fluidization velocity (m s−1)uter particle terminal velocity (m s−1)x species conversion (-)Z total height of a cyclone (m)

Greek Symbols

𝜌d, 𝜌f particle and fluid densities respectively (kg m−3)𝜂eff biomass thermal conversion efficiency (-)𝜏res residence time (s)𝜇f viscosity of fluid (kg m−1 s−1)𝜑 particle sphericity (-)𝜀mf volume fraction at minimum fluidization (-)

Questions

1. Why is a fluidized bed termed “fluidized”?2. What are the main homogeneous reactions in biomass gasification?3. What are the main heterogeneous reactions in biomass gasification?4. Describe the energy balance around a thermal conversion reactor.5. What are the typical reaction zones in a downdraft gasifier?

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4Pyrolysis

Andreas HornungFraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg, Germany and Chair in

Bioenergy, School of Chemical Engineering, College of Engineering and Physical Sciences,University of Birmingham, UK

Pyrolysis sounds like an easy process: just heat the material up, without air or oxygen, andget the products. And that’s it!

Pyrolysis, yes; but not its technical application. Here we find the most failing technologyin industry, especially if we look over the last 100 years and if we exclude traditionalcharcoal production as well as modern facilities to produce charcoal. The latter is a slowpyrolysis process, delivering one main product – a solid phase, the char, charcoal. All theother approaches promise to deliver alternative products suitable for combustion, combinedheat and power with internal combustion engines or turbines as well as a wide range ofproducts resulting from further biorefinery. Real applications are rare and barely exist at allif people go for other biomaterials than wood.

Medium to large scale applications have been supported by hundreds of millions of Eurosin Germany but have failed or not yet performed as desired, like the Choren (Germany)approach to combine pyrolysis with gasification to produce synthetic diesel, and the Bioliq(Germany) [1] approach to realise synthetic diesel via fast pyrolysis and gasification withdownstream synthesis. In Canada, approaches from Dynamotive and Ensyn have worked orhave been working for several years now: Dynamotive, by using wood as feed and using theproduct of pyrolysis for turbine application and Ensyn by producing liquid smoke from thepyrolysis oil. In Germany we find the Pyreg and Pytec approaches, one very much limitedin size and more a waste treatment plant with the aims to produce a biochar, the other stillunder development in trying to retrieve liquids suitable for engine application from wood.




Most of the processes have failed either due to insufficient understanding of the chemistryof pyrolysis, insufficient reactor design or selection, or the wrong combination of the sizeof a plant and the desired throughput.

Therefore, this chapter shows detailed information about new intermediate pyrolysisprocesses that are being used to pyrolyse biomass, plastics as well as electronic scrap, justto illustrate the range of opportunities, in the case of understanding underlying processesand in the case of adapted reactor design.

4.1 Introduction

Pyrolysis in general is the thermochemical conversion of feed material under the absenceof oxygen. Pyrolysis reactors exist of various designs and for various pyrolysis conditionsas fast, intermediate or slow. Traditionally, slow pyrolysis is known, for example, forcharcoal production. This chapter deals with the more advanced pyrolysis types for fastand intermediate pyrolysis. It will give an overview of reactors and discuss the principaldifferences between fast and intermediate pyrolysis.

Pyrolysis of biomass is different according to feedstock, applied reactors as well as theapplied type of pyrolysis – fast, intermediate or slow pyrolysis. The classical approach toslow pyrolysis leads to charcoal or even finally to activated carbon, starting from wood.The opposite of this is fast pyrolysis of wood, creating a maximum liquid phase for thisspecific feedstock and usually showing elevated tar levels within the liquids.

More difficult than the pyrolysis of wood is the pyrolysis of non-woody biomass and theapplication of its products. A new type of pyrolysis is intermediate pyrolysis. In betweenthe reaction conditions of slow and fast pyrolysis it offers very different product qualities ofliquids, gases and of the biochars – the residue from pyrolysis. The distribution of productphases is affected as well as the composition of the liquid phase. It is of importance that thereaction conditions for intermediate pyrolysis offer a wide range of variation for processoptimisation. This can be supported by integrated reforming by means of the biochar andreaction water from pyrolysis in the so called Pyroformer (Aston University).

Biomass can essentially be converted to energy and clean fuels via thermochemicalprocesses. In every thermochemical process, pyrolytic degradation plays a significant role.Attempts have been made to correlate the characteristics of biomass pyrolysis with those ofits components using overall kinetic models [2–4]. The agreement of these models was onlyat a specific heating rate and the reason could be related to influence of heating rate on charformation. Understanding the physical dynamics (weight loss behaviour, morphological andstructural changes with respect to time and different heating rates) may lead to improvedmethods of conversion. Among various approaches to obtain the kinetics, the simplest onesare to use empirical and employ global kinetics, by using the Arrhenius expression, and tocorrelate the rates of mass loss with temperature [5]. Chapter 14 offers a calculation tool toevaluate formal kinetic parameters from pyrolysis data.

Increasing applications of lignocellulosic materials for energy production have increasedthe need to know their detailed reaction mechanisms and thermal behaviour. From an energyeconomic point of view, for control over product generation and composition, the thermalbehaviour studies of lignin are very crucial. Lignin in general is more thermally stable,produces more char and a higher fraction of aromatics in the liquid product compared

Pyrolysis 101

to cellulose and hemicellulose under similar conditions. In this study, thermogravimetry-mass spectrometry (TG-MS) techniques are used to evaluate the thermal characteristics andformal kinetic parameters for pyrolysis of different kinds of lignin.

The release of gas and tar components in the vapour phase with simultaneous carbona-ceous residue formation is determined by an enormous number of chemical reactions. Theseinvolve different precursors and intermediates in the condensed phase. The evolution ofa specific product is strongly affected by the structures of the corresponding precursors.Tar is generally defined as volatiles that condense at room temperature. Many tar com-ponents are recognised to be directly derived from the starting repeated unit of a specificmacromolecular constituent of biomass. Volatiles are made of non-condensable gases witha high heating value, light oils suitable as fuels and feed stocks, and high boiling tars forsubsequent refining. Detailed studies on lignin thermal decomposition and the influenceof alkylic, aromatic hydroxyl groups, as well as methoxy groups, can be found in variousdetailed studies of Tiziano Faravelli at Politecnico di Milano.

4.2 How Pyrolysis Reactors Differ

The difference between pyrolysis reactor systems usually can be seen in the way heattransfer units are used.

Reactor shells are heated by resistor heaters or by hot gas, gas coming from char or gascombustion or just as off gas from other processes like engines. You also can find systemsheated by radiation internally in close proximity to the pyrolysis feed or externally close tothe reactor shell. Other processes use a fraction of the energy content of the feed to get thepyrolysis started, for example in traditional slow pyrolysis. If high treatment transfer ratesare required, additional mechanical measures are introduced to get this operational, likesand in a fluidised bed but also steel shot, ceramic materials as well as natural materialslike olivine or dolomite. Other approaches use high speed revolving heated surfaces, likedisks or cones, either to heat the feed direct or to get a heat transfer media heated upefficiently.

It is very important to know that, despite the huge variety of the reactor systems, almostnone are really running on a day-to-day basis in commercial application, because of prob-lems usually arising with feed and product handling. The literature is not very helpful inthis respect as many incomplete stories are given.

Systems that are in operation are either those of the classical slow type, like charcoalproduction from wood, or fluidised bed based units for wood pyrolysis, but usually withvery limited use of the products!

The existing ways to use products from pyrolysis are very often overestimated (use asinternal combustion engine fuel or products via biorefinery) and it turns out that most ofthe proposed uses are just not yet possible. It is also the case that for biomass other thanwood, very often assumptions are given in literature about the nature of uses based on theexperience of wood, and this does not help at all.

This chapter on pyrolysis should help the reader to understand the difference betweenpyrolysis systems and the related reactors in order to make up his own mind and to helpdecide which reactors and pyrolysis types are required for commercial application.


4.3 Fast Pyrolysis

Pyrolysis is thermal decomposition occurring in the absence of oxygen. During pyrolysisthe feed is cracked and not oxidised or partially oxidised like in gasification or combustion.During pyrolysis, a set of products always results: charcoal, liquids and gas.

By means of fast pyrolysis, thermal energy is transferred within seconds into the organicmaterial and the resulting vapours are removed from the hot reaction zone also withinseconds. Both parts of the process usually have residence times of 0.5 to 2 s and thecharacter of the products is mainly defined by the high energy flow into the sample. For fastpyrolysis a higher tar formation during pyrolysis is therefore typical. The liquids are higherin viscosity and tar content, especially during fast pyrolysis of non-woody biomasses, whencompared to those from intermediate pyrolysis.

In terms of woody biomass, the fast pyrolysis is of interest for liquid production, as theliquid is received in very high quantities, up to 75% [6].

The most important principles are:

– Very high heating rates, up to 1000 ◦C/s.– Finely ground materials, particles of several mm down to several 10 micro m.– Typical reaction temperatures from 400 up to 550 ◦C, higher or lower temperatures are

given in literature.– Short vapour residence times of 1 to 2 s.– High cooling rates for the vapours to reduce thermal post-decomposition.

4.4 Fast Pyrolysis Reactors

4.4.1 Bubbling Fluid Bed Reactor

The bubbling fluidised bed is a simple and well understood technology. The advantagesare good temperature control and efficient heat transfer due to high particle density. Pilot(approx. 250 kg/h) and technical scale (up to 200 t/d) units are available (e.g. [3–5]).

4.4.2 Circulating Fluid Bed Reactor

Circulating fluid bed reactors are more complex than bubbling fluid bed reactors. Comparedto bubbling fluid bed reactors the residence time of the char is almost the same as theresidence time of the gases and vapours. Due to higher gas velocity the char is transportedrapidly through the reactor, which usually leads to higher char content in the pyrolysis oil.A counter measure is the involvement of filters to keep back the particles. The circulatingfluid bed is very popular particularly because of its high throughput. The heat for theprocess is realised by burning the char while heating the sand in a separate reactor unit, andrecirculation of the hot sand into the circulating fluid bed (for examples of this technologysee [7–9]).

4.4.3 Ablative Pyrolysis Reactor

In ablative pyrolysis the heat transfer is optimised by having a huge heat transfer surfaceand a small sample surface pressed against it. The biomass, usually wood, is pressedmechanically against the hot surface and the hot surface is passed by. The reaction rates

Pyrolysis 103

are not limited by heat transfer. Therefore, larger wood particles can be used or a solidcompressed stick of wood chips can be pressed against a hot rotating plate (hydraulically fedwood rods) [10,11]. Within an alternative approach, fine particles are pressed at supersonicvelocities (high tangential pressure) against the inside of a cylindrical reactor [12]. Withinanother, development pressure and motion is derived mechanically within a rotating bladereactor [6].

4.4.4 Twin Screw Reactor – Mechanical Fluidised Bed

Here, fast pyrolysis is accomplished in a twin screw mixer reactor heated by a solid heatcarrier. The central parts of the fast pyrolysis system are a reactor with twin screws rotatingin the same direction, cleaning each other with intertwining flights, and a heat carrierloop which can be designed in several ways [13]. The heat carrier can consist of sand orsteel shot.

The abrasive character of rapidly moving sand or steel shot results in high amounts ofchar dust and usually makes it difficult to separate the vapours from the char after pyrolysis.Measures like cyclones and hot gas filtration have to be taken to improve the quality of thepyrolysis liquid.

4.4.5 Rotating Cone

The rotating cone reactor [14, 15] effectively operates as a transported bed reactor. Thetransport is affected by centrifugal forces in a rotating cone. The heat transfer is realisedby hot sand, generated in a bubbling bed char combuster, where the remaining char frompyrolysis is oxidised to heat up the sand [12]. The char application is integrated to heat thesand, the rotating cone improves the heat transfer but is less abrasive than the twin screwsystem described in Section 3.2.4. Finally the system has been applied successfully for theconversion of open fruit bunches from palm in Malaysia by [14].

4.5 Intermediate Pyrolysis

Intermediate pyrolysis differs from fast pyrolysis in terms of the heat transfer to the feed.The heating rates are much lower, in the range of 100 to 500 ◦C/min. This leads to less tarformation during the pyrolysis process as more controlled chemical reactions are takingplace instead of thermal cracking of the biopolymer. The vapour residence times are verymuch dependent on the reactor type, but can be, as in case of fast pyrolysis, as low as 2 s.

In terms of the pyrolysis of woody materials, intermediate pyrolysis produces muchlower liquid fractions, about 55% compared to 75%. Nevertheless, this is valid only for thepyrolysis of woody feeds and changes to a more equal scenario in the case of other biogenicmaterials.

4.5.1 Principles

The most important principles are summarised below:

– Moderate heating rates, up to 200–300 ◦C/min.– Residence time of several minutes, up to 10 min.


– Coarse, shredded, chopped or finely ground materials can be used as single feed or inmixtures, particles of several cm down to dust can be used.

– Water content of the feed up to 40 wt%– Typical reaction temperatures from 400 up to 550 ◦C, lower temperatures down to

350 ◦C are possible.– Short vapour residence times of 2 to 4 s.– High cooling rates for the vapours to reduce thermal post-decomposition.

4.5.2 Process Technology

Typically two major types of rotary kilns are used for intermediate pyrolysis processes:internally or externally heated systems. For internally heated kilns a heat exchanger basedon steam or gas powered tubes or electrical heaters is used. For externally heated systems,there are steam, gas (direct or indirect) or electrically powered systems. The followingexamples show engineering solutions for the processing of high volatile feeds or relevancein processing.

4.5.2.1 Conrad Process

The Conrad recycling process uses a horizontal auger kiln reactor that applies heat to thefeedstock [16–18]. The control of the pyrolysis process is decided by temperature andespecially auger speed and temperature.

4.5.2.2 Double Rotary Kiln Pyrolysis

Double rotary kiln pyrolysis is designed for mechanical coupling of a pyrolysis unit anda combustion unit. The coaxial system of two rotary kilns opens up the possibility ofevaporating organics from the feed material in the inner kiln while the carbonaceous residuesare transported through the external kiln to a combustion zone. The heat of combustionprocess is used to heat the inner kiln while the ashes are leaving the system [19]. Both kilnscan be equipped with lifters or spiral lifters.

4.5.2.3 Low Temperature Carburisation Process

The low temperature carburisation process (LTC) meets the need for a process that econom-ically reduces petrochemical and hydrocarbon residues into recyclable products, feedstocksor clean fuels. The indirectly fired rotary kiln system can be operated up to 850 ◦C. Thethroughput varies between 800 to 2000 kg/h.

The LTC kiln sealing system allows operation of the system under positive pressure,ensuring no leakage of air which can produce an explosive mixture.

For cleaning the kiln wall of carbonaceous materials and feed, a cyclindrical clean-ing device has been developed that lies on the bottom of the rotary kiln, equipped withhigh temperature bearings effective even at a temperature range up to 1100 ◦C, runningup to 8000 h/a without lubrication. The cylindrical unit is mounted on the infeed side

Pyrolysis 105

of the kiln and ends in the carburisation zone of the kiln. A cone segment connects tothe feed zone.

4.5.2.4 Haloclean-Gas Tight Rotary Kiln

The Haloclean rotary kiln is especially designed for the pyrolysis of high amounts ofinert and/or thermosetting containing materials. The invention is based on experiencewith a vertical reactor system for pyrolysis, the so-called Cycled-Spheres Reactor [20]. Asystem has been developed [21] that improves the heat transfer to poorly heat transferringmaterials like plastics and biomass, ensuring that decomposition at laboratory scale canalso be described with micro kinetic analysis [22]. The use of a spheres filled reactionvessel improves the heat conductivity by an order of magnitude of at least one [11]. Thesystem uses a screw cycling metal spheres. The idea of heat transferring metal spheres, anda screw which not only transports spheres but also feed material, was kept and transferredinto an industrial-like reaction system, a rotary kiln [23]. To keep the performance at lowrates of consecutive reactions of the pyrolysis gases, as well as low residence times of thepyrolysis gases in the system at all, the hollow screw shaft has been equipped with 200sintered metal plates. These plates are permanently cleaned by the material passing overtheir surface, thus keeping the inner core of the screw clean from pyrolysis products, andthey introduce the purge gas directly where the pyrolysis products are evolved.

Because of the screw the residence time control of the rotary kiln is completely differentto existing kiln types. Feed and heat transferring spheres are transported in distinct volumesthrough the kiln. By using the screw in forward and backward movements the residencetime can be shifted to very long times, even the material is permanently in motion andmixing. The system is indirectly electrically heated by an outer oven and by the screw. Upto now the system has been realised in pilot scale [24–26] and was transferred to technicalscale (20 000 t/a) in 2009.

4.5.2.5 The Pyroformer

The Pyroformer [27] combines pyrolysis with enhanced char catalysed reforming andincreases gas production. Higher gas production rates are favourable, especially if thepyrolysis is coupled to a gasification system. The reactor consists of two coaxial screwunits. The inner auger transports the biomass through the reactor and the outer screwtransports the char back to the inlet zone, where the char is mixed together with the freshbiomass. The char is therefore a heat carrier and reaction partner.

4.6 Slow Pyrolysis

Slow pyrolysis is the oldest way of treating biomass under oxygen-free conditions andthe most traditional. The production of barbeque charcoal is the best known example.Together with the production of charcoal, related products can be achieved like acetic acidor alcohols. Usually, pyrolysis vapours are used to deliver the heat for pyrolysis in a director indirect mode, which means either to heat the kiln from the outside or to heat the biomassby getting it in contact with the combustion gases from pyrolysis gases. A third way is togo for partial oxidation, but this is outside the theme of pyrolysis.


4.6.1 Principles

The material used for this type of pyrolysis is shaped from briquette size to whole logs.The feed material is traditionally wood; recent literature also reveals cashew nut shells andpalm [28–30].

The residence time of the solid phase is usually hours, up to weeks! For almost ash freematerials the residue goes down to 15%, usually around 30% is found.

The heating rates are several degrees per minute.A slow pyrolysis taking place within one to two hours and at a heating rate of approx

5 K/min will have a nearly even distribution between char, liquid and gas!

4.6.2 Process Technology

4.6.2.1 The Degusa Process: A Modern Batch Process with Recirculation of theCombustion Gases

The process is based on a large retort with a capacity of 100 m3. The retort is fed by a beltconveyer from the top. After charging, hot gases reach the feed and over 16 to 20 hoursof carbonisation takes place. The pyrolysis gases then leave the retort. The condensablesare removed in a cooler and scrubber unit, and non-condensables go to a heat exchangerwhere the gas is heated by using the remaining pyrolysis gas fraction. The temperature forcarbonisation is about 450 to 550 ◦C. The charcoal is discharged from the bottom and fallsinto air-tight bunkers for cooling. The production rate is about 24 000 tons/year of charcoalfrom beech wood in seven retorts of this type. A typical charcoal yield obtained from beechwood is 34%. In addition 500 tons/year of very pure acetic acid can be recovered from thisprocess, as well as smoke flavours [31].

4.6.2.2 The OET Calusco Process: An Example of an Indirect Continuous HeatedProcess

Wood is transported by trolleys through a horizontal tunnel. The tunnel, 45 m long, isU-shaped and divided into three chambers where the wood is firstly pre-dried, secondlycarbonised and finally cooled down. Each trolley contains 12 m3. The carbonisation processis energetically self-sufficient as long as the moisture content of the feedstock is below45–50% (dry basis). The pyrolysis gases are used to run the process. The total residencetime within the tunnel is 25–35 hours depending on moisture content and feedstock used.The typical production capacity of such a plant is about 6000 tons/year of charcoal [32]. Afurther selection of charcoal processes is given in [32].

4.7 Comparison of Different Pyrolysis Techniques

The literature generally points to fast pyrolysis as the measure to turn biomass into themaximum amount of liquid and minimum amount of gas. This is true for sure, but only forwood, and it usually leads to one phase liquids high in water, acids and tars [33].

In terms of other feedstock, like straw, grass or industrial residues from agriculturalproducts like husks, the picture is very different. Intermediate pyrolysis offers working

Pyrolysis 107

conditions preventing the formation of high molecular tars and offering dry and bridle charssuitable for different applications like fertilisation or combustion. An advantage of this typeof processing is the non-milling character and the applicability of larger sized feedstock,which offers the opportunity to separate easily the char from the vapours and to reacha coupled gasifier with a low ash feed independent of the content of the material beforepyrolysis as it no longer affects post-gasification.

4.8 Future Directions

Pyrolysis of biomass is an important process or process element to turn biomass into liquidand gaseous products. Worldwide companies are searching for solutions to liquefy woodand other biomasses to get an intermediate product that is higher in energy per volume. Thesought applications range from co-firing and firing in biomass boilers, fuel for gas enginesand dual-fuel engines, to feed for gasifiers. Furthermore, the chemicals in the liquids areof interest to biorefineries as high value products can be extracted. Finally, the char frompyrolysis is of increasing importance as it can be used to deliver so-called biochar. Biocharis suitable for fertilising agricultural land and in addition sequesters carbon instead ofcarbon dioxide. Today the most promising chars for biochar application are delivered byintermediate pyrolysis [34, 35].

4.9 Pyrolysis in Application

4.9.1 Haloclean Pyrolysis and Gasification of Straw

Pyrolysis of straw is still an issue for most existing pyrolysis units in the fields of fast andslow pyrolysis. The Haloclean process was tested in 2005 in a campaign of 5 weeks for theconversion of 15 tonnes of straw into liquids, gas and char. Later on, the liquids and thechar were successfully converted to synthesis gas in a commercial scale gasifier [36].

The Haloclean reactor has been used in a temperature range of 320 to 500 ◦C. Typicalfor this reactor are short residence times (1 to 10 minutes for the solid residues) forpyrolysis of chaffed straw, finely ground straw and straw pellets, with variable residencetimes of about 0.3 to 60 s for the gas phase and gaseous pyrolysis liquids. The Halocleanreactor consists of a rotary kiln equipped with an internally nitrogen-purged and heatedscrew. During the pyrolysis, metal spheres are transported through the rotary kiln for betterheat transfer.

A very important result of these tests is that the shape and size of the feed can be variablewithout changing the performance of the reactor.

Figure 4.1 shows that at a low temperature (325 ◦C) the coke yield is much higher thanthe yield of oil (73 : 18), while at higher temperature (375 ◦C) the ratio coke/oil is 38:37(1 : 1). At a pyrolysis temperature of 385 ◦C the ratio of coke : oil is at an optimum of 36 :41.6. Therefore, it is possible to obtain up to 5% more oil than coke with Haloclean pyrolysis.

Generally, one can see that the amount of pyrolysis gas is increasing with increasingtemperature; at 400 ◦C the amount of pyrolysis oil is decreasing due to the fact that moreand more pyrolysis oil is degraded to pyrolysis gas.


80

70

60

50

40

30

20

10

0

Oil %

Cokes %

325 °C

73

18

9

48

34

18

38.2

37.7

24.1

36.2

41.6

22.2

33.5

34.6

31.9

350 °C 375 °C 385 °C 400 °C

Gas %

Figure 4.1 Yield of pyrolysis products – powdered straw.

The dimensions of the Haloclean pyrolysis pilot plant are shown in Figure 4.2. Theexemplary dimensions of such a plant are given below:

– Diameter of the pipe: do = 273 mm, di = 253 mm,– Length of the oven: 2200 mm– Total length: 4500 mm– Diameter of the screw: 116 mm– Draft of the screw: 150 mm– Heat transfer medium: Spheres m = 70 g, d = 25 mm.

Pyrolyis char

Char cooling

Pyrolysis (250–500 °C)

Biomass Spheres circulation Pyrolysisgas / oil

Figure 4.2 Scheme of the Haloclean pyrolysis pilot plant.

Pyrolysis 109

Below, the results of pyrolysis of straw are shown as an example:

– Pyrolysis Char: Ho = 26 MJ/kg; Ho is the upper heating value

C: 63%, H: 3.7%, N: 1.1%, O: 12.9%Ash content of the Char: SiO2: 59%, Al2O3: 0.3%, Fe2O3: 0.7%, CaO: 7.3%, MgO:

2.1%, P2O5: 3.4%, Na2O: 0.4%, K2O: 23.8%, SO3: 3.1%.

– Pyrolysis Liquid: Ho = 7 MJ/kg (approx. 50% water phase) and 18% highly phenolicphase (Ho = 24 MJ/kg).

4.10 Pyrolysis of Low Grade Biomass Using the Pyroformer Technology

Pyrolysis is defined as the thermal decomposition of organic matter in the complete absenceof oxygen, and intermediate pyrolysis occurs under moderate residence times (10–15 min)and reaction temperatures (400–550 ◦C). Intermediate pyrolysis produces condensableorganic oils known as bio-oils. These oils are easier to transport and store than producergas from gasification. Once produced, bio-oil can either be introduced into a modifieddiesel engine or further upgraded to run in conventional engines. Intermediate pyrolysisalso has the potential to generate moderate amounts of permanent combustible gases, andthese can also be premixed with air before entering a modified engine. A byproduct fromintermediate pyrolysis is a relatively large quantity of char (approx. 30 to 40% dependanton the feed), and this can be used as a fuel or possibly returned to the soil (‘biochar’) forcarbon sequestration or as a fertiliser.

Figure 4.3 shows the Pyroformer located at the European Bioenergy Research Instituteat Aston University in Birmingham, UK.

This process is well suited to residue from anaerobic digestion, such as spent brewersgrain, biogas production or sewage sludge treatment, husks and straws, oil pressing cake,deinking sludge, cattle, pig or chicken manure as well as compost residues, as it can toleratefuels with low calorific values and high ash contents because it has the ability to separatethe solid and gas fractions that are formed. Therefore, the pyrolysis of deinking sludge wasselected as an ideal candidate for trials using the Pyroformer to find its merits within thisdifficult type of feed.

Figure 4.3 Schematic of the Pyroformer Reactor.


Figure 4.4 Cummins engine modified by NEK Germany, 150 KWel and a 100 kg\h Pyro-former, Aston University, European Bioenergy Research Institute. Reproduced from Aston Uni-versity, European Bioenergy Research Institute.

At full scale, it is envisaged that the inert solids formed from the pyrolysis of deinkingsludge or other char residues not suitable for biochar application would be co-fired ina combustion unit downstream. The ash product formed could then be sold to cementindustries as a cement or concrete admixture. The condensable organic vapours representa bio-oil which could be used in a CHP engine as a blend with biodiesel.

At full scale, the advantage of this type of intermediate pyrolysis over fixed bed gasifi-cation is that the feedstock does not need to be pelletised, although drying is still necessarybut only to 30%. It is also more tolerant of feedstock variability. A full-scale system basedon this technology could process up to 20 000 dry metric tons/year, and multiple units inparallel are possible for higher tonnages.

Recent testing has been performed with a unit of 100 kg\h throughput coupled to a150 KWel Cummins engine for combined heat and power (Figure 4.4).

Questions

1. What are the three main types of pyrolysis?2. How are those pyrolysis types distinguished?3. What distinguishes pyrolysis from pyroforming?

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(30) Abdullah, N. and Gerhauser, H. (2008) Bio-oil derived from empty fruit bunches. Fuel, 87,2606–2613. DOI: 10.1016/j.fuel.2008.02.011

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(32) Domac, J. and Trossero, M. (2008) Industrial Charcoal production, TCP/CRO/3101 (A) Devel-opment of a sustainable charcoal Industry, Zagreb, Croatia.

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Books and Reviews

Ahmedna, M., Marshall, W.E. and Rao, R.M. (2000) Production of granular activated carbons fromselect agricultural by-products and evaluation of their physical, chemical and adsorption properties.Bioresource Technology, 71, 113–123.

Finch, H.E. and Redlick, R. (1969) Rice hull method, apparatus and product. US Patent Office3,451,944.

Hornung, U., Schneider, D., Hornung, A. et al. (2009) Sequential pyrolysis and catalytic low temper-ature reforming of wheat straw. Journal of Analytical and Applied Pyrolysis, 85, 145–150.

Huang, S., Jing, S., Wang, J. et al. (2001) Silica white obtained from rice husk in a fluidized bed.Powder Technology, 117, 232–238.

Oh, G.H. and Park, Ch.R. (2002) Preparation and characteristics of rice-straw-based porous carbonswith high adsorption capacity. Fuel, 81, 327–336.

Sun, R., Thomkinson, J., Mao, F.C. and Sun, X.F. (2001) Physiochemical characterization of ligninsfrom rice straw by hydrogen peroxide treatment. Journal of Applied Polymer Science, 79, 719–732.

Yun, Ch.H., Park, Y.H. and Park, Ch.R. (2001) Effects of pre-carbonization on porosity developmentof activated carbons from rice straw. Carbon, 39, 559–567.

Hornung, A. (2012) Biomass pyrolysis. Encyclopedia of Sustainability Science and Technology, vol.3 (eds R.A. Meyers), Springer Verlag, pp. 1517–1531.

Hornung, A. (2013) Intermediate pyrolysis of biomass. Biomass Combustion Science, Technologyand Engineering (ed. L. Rosendahl), Woodhead Publishing, Cambridge.

5Catalysis in Biomass Transformation

James O. TitiloyeChemical & Environmental Engineering, College of Engineering,

Swansea University, UK

5.1 Introduction

Chemicals from biomass conversion can form the basic building blocks for a bio-basedeconomy. This can lead to a reduction in the use of fossil fuels and ultimately replacing ourdependence on petrochemicals. In consideration of world energy needs and supply for thefuture, biomass alongside alternatives such as wind, solar, water and nuclear options willsurely take centre stage in shaping our thinking on how to maximise the world’s resourcesfor our benefit.

In the production of biochemicals and development of its infrastructure, the supply of therequired biomass feedstock is undoubtedly a cause for concern, especially in the perceivedquantity, in addition to the maximum production rates feasible for any particular species.This is where the application of catalysts becomes crucial in the process line, as catalysisis known to be the key to rapid chemical conversion and can help to maximise the use ofavailable feedstock without compromise.

In biorefinery processes, catalytic technology has been proven to play a major role inthe production of both liquid and gaseous biofuels and thus becomes a critical componentand criteria in biomass transformations. The selection and choice of catalyst for thesetransformations is essential for optimum fuel production and product distributions. Catalystapplication in biorefinery processes therefore has the opportunity and flexibility to optimiseperformance irrespective of the type of feedstock used and is able to maximise the yield ofproduct and quality of bio-oil with precise selectivity in most cases.




5.2 Biomass, Biofuels and Catalysis

Any chemical with the ability to enhance the rate of a chemical reaction without it beingnecessarily consumed is termed a catalyst. Catalysts can be expressed in terms of homoge-neous, heterogeneous or biological/enzymatic forms depending on the nature and type ofchemical composition.

Homogeneous catalysts are commonly in the form of liquid organic and/or inorganicreaction media, mostly acids, bases and sometimes metal complexes, while heterogeneouscatalysts do form a solid phase in reaction media. Heterogeneous catalysts are typicallymetals and metal oxides including aluminosilicates, zeolites and related micro porousmaterials with the action largely restricted to the surface. Biological or enzymatic catalyststend to be very specific and applicable in biochemical and pharmaceutical systems. Theyare often classed as homogeneous catalysts due to their nature but sometimes behave likeheterogeneous catalysts due to their selectivity properties.

The main requirements for an acceptable industrial catalyst include an effective conver-sion rate for the process in question; a robust and stable structure under operating conditionsso as to minimise replacement frequently or regeneration; and a satisfactory or acceptableselectivity for the desired products.

The importance of catalytic actions in our world cannot be overemphasised. The simpletruth is the world would grind to a halt without the presence or application of catalysts.Photosynthesis as we know it happens to be the most common chemical process on ourplanet. The overall consequence of photosynthesis and various photochemical reactionsleads to the formation of tons of wet and dry biomass products every year with the help ofnatural enzyme catalysts.

Biomass transformation through catalysis comes in different forms. One of the mostcommon forms used is thermochemical processes via pyrolysis. Catalysis in pyrolysissystems – often referred to as Catalytic Pyrolysis – is the application of catalysts topyrolysis processes whereby the thermal decomposition of biomass material in the absenceof oxygen is aided by foreign chemicals to obtain a higher quantity and quality of targetedproducts. Catalytic application in this form is usually at the conversion end of the pyrolysisprocess rather than the initial handling, separation and preparation end.

Condensation of pyrolysis vapour to obtain bio-oil and evaporation of bio-oil for upgrad-ing purposes is not thermally efficient. Thus, the introduction of catalysts into the pyrolysisprocess before condensation of vapour in order to induce vapour-phase catalytic reactionsis a worthwhile and promising route.

Biofuels derived from biomass transformation contain significant amounts of oxygenatedcompounds which account for relatively low heating values, low stability, high viscosity,low volatility and low pH. A credible option for overcoming most of these problems is byintroducing catalysts into the transformation process designed in such a way as to enhancede-oxygenation, cracking and reforming reactions. Table 5.1 shows typical properties ofbiomass derived bio-oil with and without catalyst.

The mechanism involved in the de-oxygenation includes decarboxylation, decarbony-lation and dehydration, which ultimately results in eliminating unwanted oxygen in theform of carbon dioxide, carbon monoxide and water. During the cracking process, ligninoligomers from biomass can be selectively cracked to produce lower molecular weightcompounds and intermediate products undergoing further reforming reactions, such as

Catalysis in Biomass Transformation 115

Table 5.1 Typical properties of biomass-derived bio-oil with and without catalyst.

Physical property With catalyst Without catalyst

pH 2.2 2.53–3.26

Elemental analysis (wt%)

C 58.01 53.09H 9.16 7.27O (by difference) 31.67 38.59N 0.79 0.81HHV 27.75 18.97–23.10Ash 0.36 0.23Solid char 27.56 19.97–23.44Overall liq yield 51.45 58.81–64.93Overall gas yield 15.89 10.10–15.94Water content in oil 32.43 31.60Solid content in oil 4.86 3.13

oligomerisation, cyclisation and isomerisation. In addition to gases and liquid biofuelsobtained from the pyrolysis process, solid char and ash are also produced.

The formation of ash in pyrolysis products is a result of alkali metals present in thebiomass composition and constituents. Most often, potassium is the most common metalfollowed by sodium. Their presence is known to catalyse secondary reactions and crackingof pyrolysis vapour formed during reactions.

There are different ways that catalysts can be incorporated into the biomass transforma-tion process. It is feasible to mix or co-feed the biomass with catalyst before it enters thepyrolysis reactor. Depending on the nature of catalyst media employed, it can be used aspart of the fluidising medium in a fluidised-bed system. It is also possible to use an inte-gral fixed bed close-coupling with other reactor configurations. There are advantages anddrawbacks to these set ups, not least in the recovery and regeneration of the used catalysts.Different catalytic upgrading techniques have been reported by several researchers. Thehydrodeoxygenation process or hydrotreating is performed in the presence of hydrogenrich solvents which are activated by the catalysts usually in metal oxide form leading tothe removal of oxygen as water and carbon dioxide. The hydrotreating process itself is ahigh pressure and moderate temperature reaction usually employed in petroleum refineriesand the petrochemical industries. Hydrotreating of several bio-oil and model compoundshas been carried out by several researchers using a ruthenium metal catalyst [1–3]. Theidea behind this was the use of a conventional fluid catalytic cracking and hydrotreatmentmethod for biomass conversion. The major drawback to this approach is the susceptibilityof biomass polymeric constituents to being unstable, leading to formation of excessive cokeand thereby causing catalyst poisoning and deactivation. Catalytic cracking of pyrolysisvapours is another method where oxygen containing bio-oils are catalytically decomposedto hydrocarbons. Nowadays, emulsification techniques are being adopted, where bio-oilsare emulsified with the help of surfactants. Table 5.2 is a list of catalysts reportedly used indifferent upgrading processes of bio-oil.


Table 5.2 Lists of catalysts used in upgrading biomass pyrolysisbio-oil and/or in situ upgrading of model based compounds.

Catalyst Reference

CoMo-oxide/Al2O3 [4]NiMo-oxide/Al2O3 [5]ZSM-5 [6]Al-MCM-41 [7,8]ZnO [9]Mo-Ni/γ-Al2O3 [10]RuCl2(PPh3)3 [11]Pd/ZrO2 [12]Ru [13]Pd/C [14]Pt [15]Pd/C-nanotubes [16]ZSM5 [17]Al-MCM-41 [17]Al-MCM-41 [8]

Table 5.3 Lists of catalysts used in biomass transformation processes.

Biomass Catalysts Reference

Oil palm shell La/Al2O3 [18]γ-Al2O3 [18]

Rice straw Cr2O3 [19]Sawdust Cr2O3 [19]Wood Cu-MCM-41 [20]Sawdust Nickel [21]Pine sawdust Mo-Ni/γ-Al2O3 [7]Rice husks ZSM-5 [7]Wood H-ZSM-5 [22,23]

Various types of catalysts have been studied to assess their potential for bio-oil upgrad-ing. Catalysts such as zeolites have been used to break down large lignocellulosic biomassmolecules. The main bio-oil properties of interest during the upgrading process are the vis-cosity and the energy content in terms of the associated heating values. These properties arenotably dependent on types of feedstock producing the bio-oil products. Typical biomassesand the type of catalyst employed in their processing are shown in Table 5.3.

5.3 Biomass Transformation Examples

Biomass (Cassava Rhizome) was pyrolysed in a fluidised bed reactor at a temperature of550 ◦C with and without a catalyst to obtain bio-oil. Different types of catalysts were usedfor the pyrolysis experiments. A range of product yield distribution was observed depending


Table 5.4 Mass balance closure for catalytic fast pyrolysis run of Cassava Rhizomea.

Catalysts Non-catalytic Criterion-534 Biomass ash CuCr2O4 ZSM-5 Al-MCM-41 Al-MSU-F

Liquidb 64.93 46.71 59.45 64.70 49.67 51.45 59.02Solid char 19.97 26.39 19.41 17.70 29.09 27.56 22.69Gas 10.10 22.22 16.37 14.76 19.74 15.89 12.89Closure 95.00% 95.32% 95.23% 97.16% 98.50% 94.90% 94.60%

aProduct yields based on wt% on dry biomass basis.bTotal liquid include organics and reaction water [17, 24].

on the type of catalyst employed [17, 24]. The pyrolysis vapour obtained was passed overa secondary fixed bed of catalyst for upgrading before condensation into bio-oil. Selectedcatalysts used include criterion-534, biomass ash, copper chromite, ZSM-5, Al-MCM-41and Al-MSU-F [25]. The mass balance closure for these runs is given in Table 5.4 andcompared to a standard non-catalytic run.

These results indicate evidence of a thermal secondary reaction of the pyrolysis vapourover the catalysts. Analysis of the yields of individual gases revealed varying degrees in theconcentration of hydrocarbons produced by different catalysts compared to non-catalyticruns. The presence of catalysts also suggests significant changes in product distributions.The liquid bio-oil derived from these experiments reveals different compositions and vis-cosities and most crucially all are single phase. Sample photographs of derived oil areshown in Figure 5.1.

Non-catalytic

w/2nd reactor

MI-575 AI-MCN-41

Criterion-534 AshCopperChromite

ZSM-5

AI-MSU-F Criterion-534/ZSM-5

AI-MSU-F/ZSM-5

Figure 5.1 Sample photographs of bio-oil from catalytic pyrolysis [24]. Reproduced from [4].PhD Thesis (2007), Aston University.


Table 5.5 Elemental composition of bio-oil produced from catalytic pyrolysis ofCassava Rhizome.

Elementalcompa Non-catalytic Criterion-534 Biomass ash CuCr2O4 ZSM-5 Al-MCM-41 Al-MSU-F

C 53.09 71.54 56.81 56.07 56.35 58.01 56.98H 7.27 4.01 7.88 7.81 9.30 9.16 8.73N 0.81 0.87 1.02 0.63 0.57 0.79 1.06O 38.59 23.30 34.15 35.38 33.70 31.67 33.06Ash 0.23 0.28 0.13 0.10 0.09 0.36 0.17

awt% on dry basis.

The elemental analysis of the bio-oil produced with and without catalyst is shown inTable 5.5. The presence of catalysts was observed to contribute to a reduction in the oxygencontent in the bio-oil.

Based on the basic elemental analysis, the heating values of catalytic bio-oil werecalculated, showing a clear improvement and higher values from the activities of thecatalysts compared to non-catalytic bio-oil (Table 5.6).

In the production of bioethanol from biomass, ethanol is produced via the fermentationprocess using feedstocks rich in carbohydrate. Bioethanols are valuable as they can beused as fuel in automobiles either with or without gasoline blend. Lignocellulosic biomassis hydrolysed into sugar which is then fermented by bacteria or yeast to obtain ethanol.Using thermochemical processes, biomass can also be gasified to produce syngas, whichcan be converted into other alcohols including ethanol via catalytic processing. Typicalcatalysts applicable for such processes include metal oxide, such as CoO, CuO, sulfidedmolybdenum, ZnO and rhodium metal. Lignocellulosic biomass has provided a huge arrayof feedstock for processing via a variety of routes. The processing itself can involve theuse of the catalyst as part of co-feeding with feedstock or can be used simply as productupgrading.

In the production of biodiesel, catalytic transesterification of vegetable oil and alcohol isvery common using alkali or acid catalysts. The catalysts employed are mostly of acid/basetype that promote an increase in solubility of reactants. Examples of alkali-catalysts arehydroxides and alkoxides of sodium and potassium, such as NaOH, KOH and sodiummethoxide [26, 27], while examples of acid-catalysts include sulfuric acid, sulfonic acidand hydrochloric acid [28]. These homogeneous catalysts have operational difficulties in

Table 5.6 Calculated heating values (HHV and LHV) for catalytic bio-oil produced fromCassava Rhizome.

Non-catalytic Criterion-534

Biomassash CuCr2O4 ZSM-5 Al-MCM-41 Al-MSU-F

HHV (MJ/kg) 23.10 27.27 25.56 25.51 27.13 27.75 26.74LHV (MJ/kg) 21.51 26.39 23.85 23.40 25.10 25.76 24.83


Table 5.7 Representative lists of catalysts used in biodiesel productionfrom biomass feedstock oil.

Oil type Catalysts Reference

Soybean oil La/Zeolite Beta [29]Sunflower oil NaOH/𝛾-alumina [30,31]Cottonseed oil Mg-Al-CO3 [32]Soybean oil ETS-10 [33]Sunflower oil CaO/SBA-14 [34]Bungeanum seed oil H2SO4 [35]Cottonseed oil NaOH [36]Soybean oil NaX/KOH Zeolite [37]Waste oils Lipozyme TL 1M [38]Jatropha Curcas L. oil seed Novozyme 435 Lipase [39]Rapeseed oil Mg-Al hydrotalcite [40]Veg oil Silica/MgO [41]Soybean oil WO3/ZrO2 [42]Palm oil Mg-Al-CO3 [43,44]Soybean oil La/Beta Zeolite [45]Jatropha Curcas oil CaO [46]

their separation from the reaction and product mixtures. This has led to development ofheterogeneous solid catalysts for transesterification processes. Table 5.7 is a list of commonbiomass feedstock oils used in biodiesel production alongside the catalyst of choice for theprocess.

The potential use of algae as biomass and a biorefinery feedstock is huge. Algae ingeneral have fast growth rates, hence the biofuel produced can be investigated for variousgrowing conditions. The catalytic influence on bio-oil from algae is still in its infancy andlimited work exists in literature on this topic. However, Table 5.8 list some selected microalgae species that produce biofuel with a catalyst upgrading method incorporated into theprocess.

Table 5.8 Selected lists of catalysts used in micro algae species transformation.

Micro algae Catalysts Reference

Botryococcus Na2CO3 [47–50]Braunic Co/Mo [51]Chlorella protothecoides HCl, NaOH/MeOH [52–56]Chaetoceros muelleri H-ZSM5 [56,57]Nannochlorapsis H-ZSM-5 [47, 58]Spirulina Fe(CO)5-S [59,60]Dunaliella tertiolecta Na2CO3 [61,62]


5.4 Hydrogen Production

Although pyrolysis processes are mostly designed for biofuel production, it is feasible toproduce hydrogen directly from pyrolysis reactions provided the process is carried out athigh temperature with sufficiently high residence time. As hydrogen-rich product fuel isvaluable and potentially a clean energy carrier, with good burning characteristics for futureengine fuelling, there is the opportunity to apply catalytic processes to aid and selectivelyconvert biomass into hydrogen fuel.

5.5 Catalytic Barriers and Challenges in Transformation

The design and operation of the reactor is a particular challenge in catalytic bio-renewableand transformation processes. Biomass reactions are often carried out in multiphase reactorsincluding, and not restricted to, fixed bed, fluidised bed, entrained flow, slurry, trickle bedand bubble flow; these reactors are all design to maximise contact between solid/fluid andcatalyst material. The issue of catalytic deactivation during the process is of major concern,and longer catalyst lifetimes that are greater than 200 h are difficult to achieve due to variousside-reactions involving carbon deposition, fouling and sintering that are all liable to occurduring transformation.

Development of the specific catalyst itself to improve selectivity to desired chemicalproducts is also a challenge. Transformation into oil of quality grade with a heating valuehigher than that of conventional crude oil would be a big step forward. In achieving thesegoals, further investigation of the mechanism and evaluation of the process conditioninvolved with the kinetics of reactions is essential if suitable reactors are to be designedand become available for processing.

Questions

1. Biofuels obtained from biomass often contain considerable amounts of oxygenatedcompounds. What are the consequences of these compounds on biofuel properties?Discuss available options for removing these compounds.

2. Explain why the formation of ash in pyrolysis products can sometimes be seen asfavourable to pyrolysis reactions.

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(45) Shu, Q., Yang, B., Yuan, H. et al. (2007) Synthesis of biodiesel from soybean oil and methanolcatalyzed by zeolite beta modified with La3+ . Catalysis Communications, 8, 2159–2165.

(46) Huaping, Z., Zongbin, W., Yuanxiao, C. et al. (2006) Preparation of biodiesel catalyzed bysolid super base of calcium oxide and its refining process. Chinese Journal of Catalysis, 27(5),391–396.

(47) Banerjee, A., Sharma, R., Chisti, Y. and Banerjee, U.C. (2002) Botryococcus braunii: a renew-able source of hydrocarbons and other chemicals. Critical Reviews in Biotechnology, 22, 245–279.


(48) Tsukahara, K. and Sawayama, S. (2005) Liquid fuel production using microalgae. Journal ofthe Japan Petroleum Institute, 48, 251–259.

(49) Sawayama, S., Minowa, T. and Yokoyama, S.Y. (1999) Possibility of renewable energy produc-tion and CO2 mitigation by thermochemical liquefaction of microalgae. Biomass & Bioenergy,17, 33–39.

(50) Inoue, S., Dote, Y., Sawayama, S. et al. (1994) Analysis of oil derived from liquefaction ofBotryococcus braunii. Biomass & Bioenergy, 6, 269–274.

(51) Tran, N.H., Bartlett, J.R., Kannangara, G.S.K. et al. (2010) Catalytic upgrading of Biorefineryoil from micro-algae. Fuel, 89, 265–274.

(52) Xu, H., Miao, X. and Wu, Q. (2006) High quality biodiesel production from a microalgaChlorella protothecoides by heterotrophic growth in fermenters. Journal of Biotechnology, 126,499–507.

(53) Miao, X. and Wu, Q. (2006) Biodiesel production from heterotrophic microalgal oil. BioresourceTechnology, 97, 841–846.

(54) Li, X., Xu, H. and Wu, Q. (2007) Large-scale biodiesel production from microalga Chlorellaprotothecoides through heterotrophic cultivation in bioreactors. Biotechnology and Bioengi-neering, 98, 764–771.

(55) Xiong, W., Li, X., Xiang, J. and Wu, Q. (2008) High-density fermentation of microalga Chlorellaprotothecoides in bioreactor for microbio-diesel production. Applied Microbiology and Biotech-nology, 78, 29–36.

(56) Wu, Q. and Miao, X. (2003) A renewable energy from pyrolysis of marine and freshwater algae.Recent Advances in Marine Biotechnology, 9, 111–125.

(57) Milne, T.A., Evans, R.J. and Nagle, N. (1990) Catalytic conversion of microalgae and vegetableoils to premium gasoline with shape-selective zeolites. Biomass, 21, 219–232.

(58) de Castro Araujo, S. and Garcia, V.M.T. (2005) Growth and biochemical composition of thediatom Chaetoceros cf. Wighamii brightwell under different temperature, salinity and carbondioxide levels. I. Protein, carbohydrates and lipids. Aquaculture, 246, 405–412.

(59) Ikenaga, N.O., Ueda, C., Matsui, T. et al. (2001) Co-liquefaction of micro algae with coal usingcoal liquefaction catalysts. Energy Fuels, 15, 350–355.

(60) Matsui, T.O., Nishihara, A., Ueda, C. et al. (1997) Liquefaction of microalgae with iron catalyst.Fuel, 76, 1043–1048.

(61) Minowa, T., Yokoyama, S.Y., Kishimoto, M. and Okakura, T. (1995) Oil production from algalcells of Dunaliella tertiolecta by direct thermochemical liquefaction. Fuel, 74, 1735–1738.

(62) Wake, L.V. and Hillen, L.W. (1981) Nature and hydrocarbon content of blooms of the algaBotryococcus braunii occurring in Australian freshwater lakes. Australian Journal of Marineand Freshwater Research, 32, 353–367.

Appendix 5.ACatalytic Reforming of Brewers

Spent Grain

Asad Mahmood1 and Andreas Hornung2

1European Bioenergy Research Institute (EBRI), Aston University, UK2Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg, Germany and Chair in


Brewers Spent Grain is a widely available and rapidly rotting feedstock generated in thebrewing industry. It is either disposed of to landfill or used as cattle feed due to its highprotein content, and in some cases used in biogas units. Due to a high water fraction,the use of this material in thermal processes is limited. Usually it has to be dried; oneway is thermomechanical drying followed by co-combustion. An advantage of the watercontent is given for thermal processes containing a reforming step: the vapour acts as areactant in catalytic reforming. For example, commercial nickel-based catalysts are suitableat 500, 750 or 850 ◦C. Figure 5.A.1 shows a small bench scale apparatus for such a type ofconversion process.

The pyrolysis reactor is a cylindrical quartz tube positioned in an externally heatedelectrical furnace. The outlet of the pyrolysis reactor is connected to a catalytic reactor.The hot pyrolysis vapours leaving the pyrolysis reactor are further cracked and condensed,followed by a tar trap. The clean product gas may then be used for online or offline analysis.

5.A.1 Biomass Characterisation

Elemental analysis of brewers spent grain is presented in Table 5.A.1. Table 5.A.2 presentsthe proximate and chemical analysis for brewers spent grain.




1

2

3

4

5

67

00

T2

T1

F P-5To fume cupboard

8

9

11

1

1 11

1

Figure 5.A.1 Bench scale pyrolysis and catalytic reactor [1]. Batch fixed bed pyrolysis andcatalytic reforming reactor used in the conversion of BSG. (1) Nitrogen gas bottle, (2) N2 flowmeter, (3) pyrolysis heater controller, (4) pyrolysis heater, (5) pyrolysis reactor, (6) thermocou-ple, (7) kettle, (8) kettle heater, (9) reformer heater, (10) catalytic reformer, (11) condenser 1,(12) bio-oil pot, (13) condenser 2, (14) ice bath (15) scrubber, (16) gas sampling port, (17)flow/temp/press meter.

Table 5.A.1 Elemental composition of brewers spent grain (Mass %) [1].

Mass % dry ash free basis

Feedstock C H N S Oa

BSG 46.6 6.85 3.54 0.74 42.26

aBy difference.

Table 5.A.2 Proximate and chemical analysis of brewers spent grain (Mass %) [1].

Fixed ExtractivesFeedstock Moisture Ash carbon Volatiles (fats) Cellulose Hemicelluloses Lignin

BSG 8 4.5 9.5 78 34.82 18.98 33.59 12.61

The material is rich in nitrogen due to the high protein content.Table 5.A.3 shows the distribution of pyrolysis products of brewers spent grain. The

condensate is separated into two phases, an aqueous phase and an organic phase.Table 5.A.4 shows the elemental analysis of the organic phase which shows an increase

in C, H and S content by 1, 3 and 0.56%, respectively, and notably a reduction in N andO content by 0.1 and 12% compared to the original feedstock. Oxygen content is still,however, high.

Appendix 5.A: Catalytic Reforming of Brewers Spent Grain 127

Table 5.A.3 Yield of products (Mass %) pyrolysis finaltemperature of 450 ◦C [1].

Products Pyroformer yield (Mass %)

Char 29Total liquid 52

Water 79.15Organics 20.85

Gases (by difference) 19

Table 5.A.4 Elemental analysis of bio-oil [1].

Mass % dry ash free basis

Feedstock C H N S Oa

Bio-oil 47.6 9.9 3.4 1.3 30.7

aBy difference.

The bio-oil consists of a number of complex organic oxygenated compounds(Table 5.A.5). Many of the abundant components are aromatic hydrocarbons and alka-nes, followed by phenols.

5.A.2 Permanent Gas Analysis

The permanent gas analysis post-quench shows a hydrogen content of about 1–2 vol%.

5.A.3 Pyrolysis and Catalytic Reforming without Steam

Catalytic experiments with commercial reforming at different reforming temperatures of500, 750 and 850 ◦C show the significant influences of the reaction temperature on gasevolution, as the presence of significant water content in the pyrolysis vapours is sufficientfor the catalytic reforming reactions to proceed.

The steam reforming reaction of any oxygenated organic compound can be representedas follows:

CnH2n+2 + 2H2O ↔ Cn−1H2n + CO2 + 3H2 (5.A.1)

Other reactions that may take place are as follows:

CnHm + nH2O ↔ nCO2 +(

n + m2

)H2 (5.A.2)

CO2 + 4H2 ↔ CH4 + 2H2O (5.A.3)

CO + 3H2 ↔ CH4 + H2O (5.A.4)

CO + H2O ↔ CO2 + H2 (5.A.5)


Table 5.A.5 GC/MS tests of the (organic phase) bio-oil [1].

Retention time Chemical name Molecular formula Area %

5.271 4,6-Heptadiyn-3-one C7H6O 2.296.133 Furan, 2-methyl- C5H6O 1.698.466 2,5-Dimethylfuran C6H8O 1.21

10.375 2,4-Dimethyl-1-heptene C9H18 1.0710.938 Toluene C7H8 11.2914.582 Cyclopentanone C5H8O 0.8914.835 Pentane,2,2,3,4-tetramethyl- C9H20 1.2615.249 p-Xylene C8H10 6.2417.49 Cyclooctatetraene C8H8 5.420.031 Decane, 1-chloro- C10H21Cl 2.6320.916 2-Cyclopenten-1-one, 2-methyl- C6H8O 1.3922.146 Azetidine, 3-methyl-3-phenyl- C10H13N 1.1225.376 Heptane, 2,4-dimethyl- C9H20 1.6125.618 Benzene, (2-methylpropyl)- C10H14 1.1529.066 5-Octen-1-ol, (z)- C8H16O 1.0130.63 Phenol C6H6O 6.9530.951 Benzene, pentyl- C11H16 1.4331.676 Guaiacol C7H8O2 2.4733.17 2-Methylphenol C7H8O 1.3734.894 Phenol, 4-methyl- C7H8O 8.235.584 Phenol, 4-methyl- C7H8O 2.4436.895 2-Methoxy-4-methylphenol C8H10O2 0.9637.308 2,4-Dimethylphenol C8H10O 139.24 4-Ethylphenol C8H10O 2.5540.964 4-Ethylguaiacol C9H12O2 3.6144.746 Tridecane C13H28 5.9646.033 Phenylacetonitrile C8H7N 1.5755.586 Benzene,1,1’-(1,3-propanediyl)bis- C15H16 2.5763.483 Undecanoic acid, methyl ester C12H24O2 1.7763.736 1-Propene, 3-propoxy- C19H18N2O2 0.9965.472 Undecanenitrile C11H21N 3.71

Table 5.A.6 Composition of permanent gases.

vol% permanent gases

Gases H O CO CH4 CO2

1.6 0.45 4.6 19.74 9.43 64.18


0

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Liquid Char Permanent gas

Yie

ld o

f p

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ucts

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Pyrolysis products

Yield of products (mass %) Pyrolysis with catalysts (without steam)

BSG+10 g cat 500 °C BSG+10 g cat 750 °C BSG + 10 g cat 850 °C

Figure 5.A.2 Comparison of the yields of products from measurements with catalysts andwithout steam from pyrolysis of brewers spent grain (BSG).

These reactions are only a guide, as pyrolysis vapours contain complicated hydrocarbonchains and also oxygenated compounds.

Figure 5.A.2 shows the yield of products from pyrolysis and catalytic reforming at 500,750 and 850 ◦C without the addition of steam. The results indicate that as the reformingtemperature increases permanent gases also increase, reducing the yield of condensableliquids (7, 22 and 26% at 500, 750 and 850 ◦C, respectively). Char remained the same aspyrolysis conditions remained constant.

Figure 5.A.3 illustrates the comparison of permanent gas composition produced at thethree different reforming temperatures. As much as 43 vol% of hydrogen was produced at850 ◦C, 24 vol% at 750 ◦C and 10% at 500 ◦C, much higher values than without catalyticreforming. CO2 concentrations were 35–53 vol%, CO concentrations 15–17 vol% and CH49–14 vol%.

0%

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20%

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H2 O2 N2 CO CH4 CO2

Yie

ld o

f p

erm

an

en

t g

as v

ol%

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Yield of permanent gases (vol%) Pyrolysis with catalysts (without steam)

BSG+10 g cat 500 °C BSG+10 g cat 750 °C BSG + 10 g cat 850 °C

Figure 5.A.3 Comparison of the yields of permanent gases produced from measurements withcatalyst and without steam.


0.00

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Yie

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uc

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as

s %

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Yield of products (mass %) Pyrolysis with catalyst (with steam)

BSG + 10 g cat 500 °C steam BSG +10 g cat 750 °C steam BSG +10 g cat 850 °C steam

Figure 5.A.4 Comparison of the yields of products from measurements with catalysts andwith steam added to the pyrolysis vapours from brewers spent grain pyrolysis.

5.A.4 Pyrolysis and Catalytic Reforming with Steam

Pyrolysis combined with steam reforming at low and high reforming temperatures shows asignificant increase in the product gas yield of between 45 and 60% on a mass basis and asignificant reduction in liquid yield. Char again remains the same: see Figure 5.A.4.

Figure 5.A.5 illustrates the composition of permanent gases at the three different reform-ing temperatures with the addition of steam. As much as 57 vol% of hydrogen can be

0%

10%

20%

30%

40%

50%

60%

H2 O2 N2 CO CH4 CO2

Yie

d o

f p

erm

an

en

t g

as v

ol %

Gas component

Yield of permanent gases (vol %) Pyrolysis with catalysts (with steam)

BSG + 10 g cat 500 °C steam BSG +10 g cat 750 °C steam BSG +10 g cat 850 °C steam

Figure 5.A.5 Comparison of the yields of permanent gases produced from measurements withcatalyst and with steam added.


produced at reforming temperatures of 750 and 850 ◦C, which is an increase of about 14%in comparison to reforming at the same temperatures without steam.

Reference

(1) Mahmood, A.S.N., Brammer, J.G., Hornung, A. et al. (2013) The intermediate pyrolysis andcatalytic reforming of brewers spent grain. Journal of Analytical and Applied Pyrolysis, 103,328–342.

6Thermochemical Conversion

of Biomass

S. DasappaIndian Institute of Science, India

6.1 Introduction

Biomass, as per the dictionary meaning, is the total mass of living matter. For the discussionsin this chapter, the term biomass is restricted to plant material, vegetation, or agriculturalwaste used as a fuel or energy source. Chemically, biomass is a carbon, hydrogen, andoxygen complex resulting from photosynthesis. In the presence of sunlight, CO2 and H2Ocombine to form the C–H–O complex.

The C–H–O complex is composed of molecules of sugars resulting in cellulose andhemi-cellulose, in combination identified as holo-cellulose. Lignin is a non-carbohydrate,poly-phenolic that binds the cells together. The structural formula for cellulose, hemi-cellulose, and lignin are C(H2O)0.83, CH2O, and CH1.3O0.3 respectively. It is evident thatthe hydrogen to carbon ratio (H : C) is 1.66 for cellulose, 2 for hemi-cellulose, and 1.3 forlignin. While all these are organic fractions in biomass, the inorganic content forms ash.The ash content varies from less than 1% in wood to about 20% in rice husk. Table 6.1provides the details of biomass properties.

Wood is a complex material, mainly composed of cellulose (∼50%), hemicellulose(∼25%), and lignin (∼25%).




Table 6.1 Structural composition of various biomasses.

Species Cellulose Hemi-cellulose Lignin Extractives Ash

Eucalyptus-1 45.0 19.2 31.3 3.8 0.7Eucalyptus-2 50.0 7.6 38.8 3.6 0.6Pine 40.0 28.5 27.7 3.5 0.3Soybean 33.0 14.0 14.0 5.0 6.0Bagasse 41.3 23.8 18.3 13.7 2.9Coconut coir 47.7 26.9 17.8 6.8 0.8Coconut shell 36.3 25.0 28.7 9.3 0.7Coir pith 28.6 17.3 31.2 15.8 7.1Corn cob 40.3 26.9 16.6 15.4 0.8Corn stalks 42.7 23.6 17.5 9.8 6.4Cotton gin waste 77.8 16.0 0.0 1.1 5.1Groundnut shell 35.7 18.7 30.2 10.3 5.1Millet husk 33.3 26.9 14.0 10.8 15.0Rice husk 31.3 24.3 14.3 8.4 21.7

CelluloseCellulose is a glucan polymer. It is a linear chain formed by D-glucopyranose units linked byglucosidic bonds. Cellulose in wood is highly crystalline. It forms intra and extra-molecularhydrogen bonds and aggregates into bundles, which in turn form microfibrils. Microfibrilsconstitute the main component of the cell wall. Cellulose provides strength to the tree andis insoluble in most solvents.

HemicelluloseHemicellulose is a collection of polysaccharide polymers. They are branched polymerswithout crystalline structure. Hemicellulose has little strength and is easily hydrolyzed byacids. It is intimately associated to cellulose in the structure of the cell wall.

LigninLignins are three-dimensional, highly complex, amorphous, aromatic polymers. Lignindoes not have a single repeating unit like cellulose, but instead consists of a complexarrangement of substituted phenolic units. Lignin is an encrusting material. It fills thespaces in the cell wall between cellulose and hemicellulose. It is also the main componentof the middle lamella, the binding layer between the wood cells.

Figure 6.1 gives the distribution of C, H, and O for different biomass on an ash free basis.It is evident that the % composition of C, H, and O on weight basis is nearly the same fordifferent types of biomass. Another important fact from the above analysis is that on an ashfree basis, the energy content for all biomass is same.

The heat of combustion is the energy released as heat when the biomass undergoescomplete combustion with oxygen under standard conditions. The chemical reaction istypically a hydrocarbon reacting with oxygen to form carbon dioxide and water with theevolution of heat. It may be expressed with the quantities as kJ/mol, kJ/kg, or kJ/m3.

Thermochemical Conversion of Biomass 135

Wood

020

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100

Peach p

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ue d

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its

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s

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ut shells

Corn

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Corn

cobs

Alfa

lfa s

eed

Wheat str

aw

Barl

ey s

traw

Safflo

wer

str

aw

Bean s

traw

Figure 6.1 Ultimate analysis of different biomass.

The heat of combustion is conventionally measured with a bomb calorimeter. It may alsobe calculated as the difference between the heat of formation of the products and reactants.The heat of combustion for fuels is expressed as HHV or GHV and LHV.

Higher heating valueThe quantity known as higher heating value (HHV) (also known as gross energy, upperheating value, gross calorific value (GCV), or higher calorific value (HCV)) is determinedby bringing all the products of combustion back to the original pre-combustion temperature,and in particular condensing any vapors produced. This is the same as the thermodynamicheat of combustion since the enthalpy change for the reaction assumes a common temper-ature of the compounds before and after combustion, in which case the water produced bycombustion is liquid.

The higher heating value takes into account the latent heat of vaporization of waterin the combustion products, and is useful in calculating heating values for fuels wherecondensation of the reaction products has an effect on the overall energy balance. HHVassumes that the entire water component is in a liquid state at the end of combustion.

Lower heating valueThe quantity known as lower heating value (LHV) (also known as net calorific value (NCV),or lower calorific value (LCV)) is determined by subtracting the heat of vaporization of thewater vapor from the higher heating value. This approach assumes any H2O formed as avapor. The energy required to vaporize the water therefore is not realized as heat.

Thus, LHV calculations assume that the water component of a combustion process is ina vapor state at the end of combustion, as opposed to the higher heating value (HHV) (grosscalorific value) which assumes that all of the water in a combustion process is in a liquidstate after a combustion process. Thus, LHV assumes that the latent heat of vaporization of


water in the fuel and the reaction products is not recovered. It is useful in comparing fuelswhere condensation of the combustion products is not possible, or products of combustioncannot be used for sensible heat extraction at a temperature below 423 K.

The lower calorific value of biomass can be express as LHV = (18.0 − 2 fw)(1 − fash) forfw < 50%, where fw is the moisture content of the biomass on dry basis and fash is the ashcontent in the biomass. From the ultimate analysis the calorific value can be estimated using0.349 CC + 1.1783 CH − 0.0151CN − 0.1034 CO − 0.0211*CAsh where the subscripts C,H, N, O, and ash are the carbon, hydrogen, nitrogen, oxygen, and ash content of the biomass.

6.2 The Thermochemical Conversion Process

The thermochemical conversion process, as the name suggests, is an activity involvingboth heat and chemistry. During the thermochemical process, the fuel undergoes severalsub-processes involving degradation of the solid fuel.

DryingDrying is a physical process, during which the moisture in the biomass is removed. Typicalmoisture content in a freshly cut biomass is up to about 50%, depending upon the speciesof biomass. With an increase in the biomass temperature, the moisture is removed. Theprocesses that occur during drying process are the thermal and mass diffusion processes.With heat penetration into the particle, change of phase takes place and the water moleculesdiffuse out through the pores.

Moisture content of biomass is usually expressed as the moisture content by weight perunit weight of the dry or wet biomass. It is important to understand the significance of thetwo bases. Wet-weight basis expresses the moisture content in the biomass as a percentageof the weight of the wet biomass, whereas the dry-weight basis expresses the moisture inthe biomass as a percentage of the weight of the bone-dry biomass. Thus 50% moistureon wet-basis (w/w) signifies 50 parts by weight of water per 100 parts by weight of wetbiomass. On the other hand, biomass containing 50% moisture on a dry-weight basis willcontain 50 parts by weight of water per 100 parts by weight of bone-dry material (b/d). Theimportance of the basis is evident if the wet-basis is converted to dry-basis or vice versa.A moisture content of 100 on bone-dry basis would represent 33.3% on wet-basis. Therelationship between the two can be expressed as

Ww = Wd∕(1 + Wd) and Wd = Ww∕(1 − Ww)

where Ww is the grams of moisture per gram of wet material and Wd is the grams of moistureper gram of dry material.

6.2.1 Pyrolysis

Pyrolyis is defined as the process of thermal degradation of biomass, that is decompositionor transformation of a compound caused by heat as depicted in Figure 6.2.

The complex chemical mechanisms involved in pyrolysis are not completely understoodand the degradation pathway is a function of heating rate, temperature, gaseous environment,pre-treatment, extent of inorganic impurities, and catalysis. Based on the heating rate,pyrolysis is classified as slow and fast. As the name suggests, during slow pyrolysis the


Loss of H2O

unbound & bound

at ≤ 150 °C

Volatiles based on C–H–O

Pyrolysis at

350–450 °CHeatHeatBiomass

Figure 6.2 Pyrolysis of biomass.

biomass particles are subjected to heating rates of the order about 100 K/s while in fastpyrolysis it is in the range of 1300 K/s.

The typical products of pyrolysis are liquid, solid, and gaseous fractions of the C–H–Ocomplex. The ratio of each of these yields depends on the process. The products of slow andfast pyrolysis are significantly different. The fast pyrolysis process is adapted to achievehigher liquid fractions from biomass.

The reactions of primary pyrolysis are those that directly affect the solid biomass feed-stock, and can be classified either as fragmentation (depolymarization) or dehydrationreactions. Drying is dominant at lower temperatures in the range of 450 K. Pyrolysisinvolves the thermal degradation of the solid fuel to lower molecular weight compoundswith fractions as CnHmOp involving a large number of compounds, to products like water,carbon monoxide, and carbon dioxide along with the formation of char. Fragmentation pre-dominates at higher temperatures, greater than 550 K, and involves the depolymerizationof biomass to primary tar units whose nature depends on the type and composition of thebiomass feed and the temperature it is subjected to.

The primary products of pyrolysis may repolymerize and undergo further fragmentation(cracking and reforming) and/or react with free radicals. For those pyrolysis processes thatare intended for the production of chemical intermediates, the physical parameters such asparticle size, heating rates, and the nature of the heat transfer medium are all controllableprocess variables. Heat transfer is one of the major experimental variables exercising controlover fast pyrolysis. An understanding of the nature of heat transfer is therefore implicit inthe definition of “fast” and “slow” pyrolysis.

Extensive studies of the slow pyrolysis of massive samples of wood have been conductedduring the course of fire research (i.e., combustion mechanisms and fire retardants). Kanury[1] attempted to describe the physics of pyrolysis and subsequently superimposed thechemical transformations on the processes of heat and mass transfer. The model included aheat flux which was external to the solid and which led to the accumulation of heat withinthe solid. This accumulation of heat was the result of the net inward conduction, the outwarddiffusion of gaseous products, and the effects of local changes. The increase in thermalenergy leads to depolymerization reactions and the production of smaller molecular weightcompounds that diffuse both inwards ahead of the thermal wave and outwards through thehot char layer. Radiographs of the density of cellulose cylinders show that a decompositionwave passes radially through the cylinder. Radial temperature measurements show a thermalwave with plateaus suggestive of a chemical enthalpy change, indicating that the wood doesnot behave as an inert solid from the viewpoint of thermal conduction.

The energy conservation inside the particle defines the internal heat transfer, the chemicaldecomposition, and the external heat transfer at the surface, modeled by a global heat


transfer coefficient that describes the symmetry at the particle centre. A characteristic time𝜏 may be associated with each process:

Internal heat transfer: 𝜏internal = 𝜌cpL2/k

External heat transfer: 𝜏external = 𝜌cpLh

Chemical reaction: 𝜏reaction = Ae(−E/RT)

where 𝜌, cp L, k, and h are the particle density, specific heat, characteristic dimension,thermal conductivity, and heat transfer coefficient. The chemical reactions are typical of achemical rate expression with A, E, R, and T as the Arrhenius constant, activation energy,gas constant, and temperature respectively.

If one of the characteristic times is much greater than the others, the correspondingprocess is the limiting factor:

• 𝜏external ≫ 𝜏internal, 𝜏reactionThe external temperature is much higher than the temperature inside the particle, whichis uniform. When pyrolysis temperature is reached, pyrolysis starts and completes beforethermal equilibrium occurs at the particle surface.

• 𝜏internal ≫ 𝜏external, 𝜏reactionThe limiting process is the internal heat transfer. The surface temperature is close to theexternal temperature, while a thermal gradient occurs inside the particle. A heat waveproceeds toward the center of the particle. Pyrolysis occurs quickly inside the thermalgradient. This case typically occurs at high temperature and for large particles.

• 𝜏reaction ≫ 𝜏internal, 𝜏externalThermal equilibrium between the particle and its surroundings is reached much fasterthan pyrolysis completion. The whole pyrolysis is controlled by the devolatilization rate.This is therefore the proper condition for evaluating kinetic parameters from the particlemass loss. Physically the particle has to be very small in order to increase the ratio ofsurface to volume and the external heat transfer, while decreasing the internal thermalgradient. The relative importance of the internal heat transfer to the external heat transferis defined by the ratio of their respective characteristic times:

(𝜏internal∕𝜏external) = hL/K = Bi

This is the definition of the Biot number, a dimensionless number commonly used in thermalanalysis. Biot numbers larger than 10 characterize internal conduction limited heat transfer.

Thermogravimetric analysis identified as TGA is a procedure that can be performed onbiomass samples. The test determines changes in weight of the biomass sample in relation tochanges in ambient temperature. Such analysis depends on highly accurate measurementsof weight, temperature, and temperature change. As many weight loss curves look similar,the weight loss curve may require transformation before results may be interpreted. Aderivative weight loss curve can identify the point where weight loss is most apparent.Thermal gravimetric analysis is the act of heating a mixture to a high enough temperaturesuch that one of the components decomposes into a gas, which dissociates into the air.Figure 6.3 presents results from the TGA analysis for rice husk, which has about 20% ashcontent, and bagasse having about 5% ash. As pyrolysis is a process of release of volatilesand char, from Figure 6.3 it is evident that about 60% of the weight is lost during the TGA


Thermogram for rice husk

0

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Thermogram for bagasse

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0 100 200 300 400 500 600 700 800 900

Temperature °C

% w

eig

ht lo

ss/°C

Fra

ctio

na

l w

eig

ht

loss

Temperature °C

Figure 6.3 Thermogravimetric analysis of rice husk and bagasse.

for rice husk and about 75% for bagasse. The derivative weight loss curve suggests that thepeak volatile loss occurs in the temperature range of 550 K to 625 K for both biomasses.

6.3 Combustion

This thermochemical process converts the energy content in the fuel to sensible heat withthe aim of extracting all the chemical energy in the fuel to the product of combustion.

Combustion is an exothermic chemical reaction between the fuel, in this case biomass,and an oxidant that can be air, accompanied by the production of heat and conversion ofchemical species. The release of heat can result in the production of light in the form ofeither a glowing or luminous flame. In a complete combustion reaction n, a compoundreacts with an oxidizing element, such as oxygen, and the products are compounds of eachelement in the fuel with the oxidizing element. During the process of combustion, earlieridentified sub-processes like drying and pyrolysis also occur in the presence of the oxidizer.

For typical hydrocarbon fuel burning in air, CHn + (1 + n/4)(O2 + 79/21 N2) →CO2 + n/2 H2O + 79 n/84 N2, where the stochiometric ratio, s is given by (32 + 3.76 ×28)(1 + n/4)/(12 + n)

• for diesel/gasoline, n ≈ 1.8, s = 14.4

• for methane, n = 4, s = 17.1.

For a typical fuel with CHNO components in the fuel, the stochiometric chemical equationis given by

CHnOmNp + (1 + n/4 − m/2)(O2 + 79/21N2) →

CO2 + n/2 H2O + [3.76(1 + n/4 − m/2) + p/2)] N2


where air is used as the oxidizing medium. The stochiometric ratio is, s = [(32 + 3.76 ×28)(1 + n/4 − m/2)]/[(12 + n + 16m + 14p)].

For a typical biomass, CH1.4O.7N.002 + 1.00 (O2 + 79/21 N2) → CO2 + 0.7 H2O +3.949 N2, s = 6.0.

From the above chemical equations, one can get the stochiometric (or chemically corrector theoretical) proportions of fuel and air, that is, there is just enough oxygen for conversionof all the hydrocarbon fuel during combustion into completely oxidized products like carbondioxide and water vapor.

These values are presented as stochiometric (fuel/air) or stochiometric (air/fuel) ratiosdepending upon the notation used.

Depending upon the air to fuel ratio, the combustion process can be identified as rich orlean. Under rich conditions, the air available for combustion is less than the stochiometricair required, while in lean conditions the air available is more than the stochiometricrequirement.

Combustion as a thermochemical process is used in wide range of applications startingfrom wood stoves for cooking to high pressure boilers used in power generation.

6.4 Gasification

Gasification is sub-stoichiometric combustion of fuel with oxidant. The process is notsimply pyrolysis of biomass but involves stoichiometric combustion of pyrolysis products(oxidation) which further react with char (reduction) leading to typical products – hydrogen,carbon monoxide, methane, carbon dioxide, some higher molecular weight compounds,water vapor, and remaining nitrogen – in proportions depending on the feedstock andreactant used.

In the case of gasification, the chemical equation is

CH1.4O0.74N0.005 (Fuel) + 0.337 (O2 + 3.76 N2) (Air) ⇒

2.7(0.211 CO+ 0.18 H2 + 0.0105 CH4 + 0.1275 CO2 + 0.471 N2)+ 0.157 H2O+ 0.028 C

The air/fuel ratio is 1 : 1.8.Overall A/F tends towards fuel rich condition (less air) and the energy in biomass is

realized in the form of combustible gases (CO, CH4, and H2) as a result of gasification.Table 6.2 summarizes various processes involved in a typical gasifier.

Table 6.2 Summary of the various processes that occur in a typical gasification process.

BiomassHeating up to 425 K Drying – lose moisture

PyrolysisHeating up to 700 K Pyrolysis – lose volatiles leaving behind char

OxidationVolatiles burn with air Produce CO2 and H2O

ReductionReaction of char with CO2 and H2O Generates producer gas with CO,H2,CO2, CH4, N2


The useful end product of gasification is thermochemical energy, while it is pure thermalenergy for combustion.

Biomass gasification involves an initial pyrolysis process depending upon the temper-ature, followed by complex heterogeneous reactions where char reacts with combustionproducts of pyrolysis (CO2 and H2O) with the reaction kinetics playing an important rolein the gasification process. Typical reactions in the reduction zone are:

Oxidation:

C + O2 ⇔ CO2 + 393.8 kJ/mole

Water gas reaction:

C + H2O ⇔ H2 + CO − 131.4 kJ/mole

Boudouard reaction:

C + CO2 ⇔ 2CO − 172.6 kJ/mole

Water gas shift reaction:

CO + H2O ⇔ CO2 + H2 + 41.2 kJ/mole

Methane reaction:

C + 2H2 ⇔ CH4 + 75 kJ/mole

Based on the sequencing of the above process, two major types of gasification systems canbe identified; namely, updraft and downdraft. In the case of an updraft system, the gas iscarried from the bottom to the top of the bed, while in the downdraft the gas is carried fromthe top towards the bottom.

Various types of gasifiers have been developed depending upon the need and ease ofdevelopment. A few are explained in the following paragraphs.

6.4.1 Updraft or Counter-Current Gasifier

The oldest and simplest type of gasifier is the counter-current or updraft gasifier shownschematically in Figure 6.4.

Exit to burner

Grate

Ash pit

Combustion

zone

Volatile generation

due to heat from

burnt gases

Figure 6.4 Updraft or counter-current gasifier.


Air for gasification is drawn from the bottom of the fuel bed and producer gas leaves fromthe top. Near the grate at the bottom the combustion reactions occur, which are followed byvery few reduction reactions above the combustion zone. In the upper part of the reactor,drying, heating, and pyrolysis of the feedstock occur as a result of heat transfer by forcedconvection due to hot gas moving up and also radiation from the lower layers of the fuelbed. The tars and volatiles produced during this process will be carried in the gas stream.Ashes are removed from the bottom of the gasifier.

The major advantages of the counter-current type of gasifier are its simplicity in construc-tion, high charcoal burn-out, and internal heat exchange leading to low gas exit temperaturesand high equipment efficiency, as well as the possibility of operation with many types offeedstock (saw dust, cereal hulls, etc.).

Major drawbacks result from the possibility of “channeling” in the equipment, whichcan lead to oxygen breakthrough and dangerous, explosive situations and the necessity ofinstalling automatic moving grates, as well as from the problems associated with disposalof the tar-containing condensates that result from the gas cleaning operations. The latter isof minor importance if the gas is used for direct heat applications, in which case the tarsare simply burnt.

6.4.2 Downdraft or Co-Current Gasifiers

The issue related to the problem of tar in the gas stream is mitigated using co-current ordowndraught gasifiers, in which air is introduced at or above the oxidation zone in thegasifier with both fuel and gas moving in the same direction, as indicated in Figure 6.5.

Depending on the temperature of the hot zone and the residence time of the tarry vapors, amore or less complete breakdown of the tars is achieved. The main advantage of downdraftgasifiers lies in the possibility of producing less tar in the gas, which is a mandatoryrequirement for engine applications. In practice, however, a tar-free gas is seldom if everachieved over the whole operating range of the equipment: tar-free operating turn-downratios of factor 3 are considered standard; a factor 5–6 is considered excellent.

Storage bin

for biomass

AirGrate

Ash pit

Hot gases

Figure 6.5 Downdraft or co-current gasifier.


The major issues of downdraft gasification systems often cited in the literature are relatedto their inability to operate on a number of fuels which are fluffy, low density materialsthat give rise to flow problems and cause excessive pressure drop. Even in pelletizedor briquetted form, the design suffers from issues associated with high ash content fuels(slagging) to a larger extent than updraft gasifiers. The design issue is related to maintaininguniform high temperatures over a given cross-sectional area making the use of downdraughtgasifiers in a power range above about 350 kW (shaft power) impractical.

6.5 Historical Perspective on Gasification Technology

The development of gasification technology has taken place in spurts. The most intensiveof these was during the Second World War in order to meet the scarcity of petroleumsources for transportation both in civilian and military sectors. Some of the most insightfulstudies on wood gasifiers – basic as well as developmental – of this period have been welldocumented in the English translation of the Swedish work (SERI, 1979) [2].

6.5.1 Pre-1980

Producer gas as fuel has been known since 1785, gas generators for use with engines werereported around 1920. There were also several others designs developed and used between1940 and 1950. Further development resulted in a design named after Imbert, the mostsuccessful that went to commercial production. The shortage of petroleum fuels in Europeduring the Second World War created a new demand for gas generators in several countries;for example, Sweden converted 40% of its entire motor vehicle fleet into those runningentirely on producer gas in that period. An essential feature of all those reactors was thatthey were developed for engines of 20–200 hp, used in automobiles and other transportvehicles; smaller reactors were not built. The SERI Report (1979) summarizes the Swedishexperience with gas generators during the above period. The document also mentions thatdesign and operation of gas generators for smaller power ratings posed problems, but doesnot elaborate.

The SERI (Solar Energy Research Institute) document has in it a description of severalsystems and statements indicating the difficulty of building reliable gasification systemsat small power ratings. These relate to the quality of the gas in terms of energy andthe particulate and tar content of the gas. Though the poor energy conversion of solidfuel to gas was acceptable, the higher particulate and tar levels caused difficulties inusing the gas for engine application. A limited amount of research and development wasattempted during the early 1940s; even so, many interesting aspects have been documentedin SERI. Most of the reporting in SERI was on closed top gasification systems. Some ofthe successful designs that were tried and implemented were Imbert, Brandt, and Zeuch(SERI, 1979).

Prior to 1900, the gas generated with charcoal as a fuel was initially identified as “suctiongas” because the gas was sucked by the engine for stationary application. With the outbreakof war in 1939, vehicular operation in Sweden and also gas firing for industrial furnacesled to the suction gas being rechristened generator gas.


The pre-war period showed a steady increase in vehicles (cars, buses, trucks) usinggasifier for motive power from about 154 800 to about 248 000 between 1935 and 1939 inSweden. It is interesting to see certain policy changes were proposed for penetrating gasifier-based motive power by the formation of the Generator Gas Committee. In November 1945,there were 503 types approved, distributed among 53 different companies, manufacturers,or designers. Prior to the war in 1939, a nearly 200 000 Kroners loan was made availablefor people to use as a Generator Gas Loan, with a maximum of 1000 Kroners for a periodof 5 years at 4% interest. It is important to note that the Generator Gas Committee carriedout appropriate measures that included the purchase of 1000 gas generators for governmentvehicles and for military preparedness, better loan terms and conditions, and increasinggrants for consulting work supported by the Generator Gas Committee. Between 1940 and1945, the growth in the fleet of generator gas cars, buses, and trucks was from 9000 in thefirst year to about 600 000 in 5 years.

Some of the above facts clearly suggest that the sustainability of the technology wasvery carefully handled by the authorities by ensuring the use of local materials as the fuelto replace the unavailable fossil fuel.

Reviewing the experience during the Second World War has shown that the developmentof low power wood gasifiers posed problems of gas quality, in particular the limitation oftar, more specifically at part loads. It was first inferred that if any design were to haveany chance of success at all, it had to be of the downdraft variety. SERI analysis indicatedthat the problems at low power level were related to heat generation against heat loss rate.The heat loss through the hardware (however well designed) will be unfavorable to smallsystems. In order to ensure reliable operation with good gas quality it is imperative thatenergy conservation is ensured by providing an adiabatic thermal environment.

6.5.2 Post-1980

In the wake of the 1970s oil crisis, several groups, worldwide, continued work on theclosed top system. Several improvements in the engineering of the product and in incor-porating control systems have taken place, with limited basic research towards improvedunderstanding of the process. Most of the small-scale gasification systems available arestill based on the downdraft systems developed in the 1940s during the Second WorldWar. This technology was applied again in the early 1980s when many installations werebuilt in Europe, the USA, and some of the developing countries. Most of these demonstra-tion projects were unsuccessful due to technical, economical, and institutional problems –reflecting the sustainability of the technology package in a given environment.

For applications in the sector where heat and power is required, the combustion routewith steam is being used for 0.5–5 MWe; even though the higher end is the most economicalroute. In order to meet this combined heat and power (CHP) requirement; there have beendifferent approaches to meet the various set objectives using the engine route for powergeneration. These have been the circulating fluid bed technologies using steam or airas the medium and fixed bed and updraft technologies using catalytic tar reforming andstaged gasification technology to improve the carbon conversion. These fuel conversiondevices compared to direct combustion were identified for better handling of the gaseouscombustion.


6.6 Gasification Technology

This section briefly highlights the critical areas of scientific research that led to the state-of-the-art technology package for power generation using biomass gasification technology.The key elements presented here include the reactor design to generate engine acceptableproducer gas.

6.6.1 Principles of Reactor Design

The major consideration for the design has been to reduce the tar level in the raw gas,improve the carbon conversion in the reactor, and eliminate any channeling, which hasbeen a major issue in downdraft systems. The central part of the argument towards tarcracking is promoted by two means – uniform distribution of a high temperature acrossthe char bed and the presence of reactive char. A high temperature in the reaction zonebeing favorable for cracking of complex chemical structures to smaller ones is a well-known phenomenon. Careful measurements by Kaupp [3] have shown that the tar fractionis reduced substantially if a tar-filled gas passes through a hot bed of charcoal. The nextquestion is the residence time in the reactive zone. The effective bed thickness in whichchar and high temperatures are present adjusts itself due to the flow of air through thereactor. At low flow rates, the nominal bed temperatures attained are sufficient to crackthe tar, while the total travel distance is the same in the case of a closed top gasifier,higher bed temperatures compensate for the lower residence so that effective tar cracking ismaintained throughout the load range. Thus, bed temperature, surface area, and residencetime are critical for the thermal cracking of tar.

To avoid tar in a biomass gasifier design, the entire stream of combusting fuel–oxidantmixture should be made to pass through a sufficiently hot zone at temperatures exceeding1200 K (see SERI, 1979). In order to achieve the above, the classical design uses a throator a constriction in the flow passage which is much smaller than the chamber cross-section. The entire combustion is expected to be mostly restricted to a region around thethroat where the air is also introduced. By reducing the throat and the diameter of theair entry region, the combustion volume is made smaller, causing the air to fuel ratioto tend towards the stoichiometric ratio and the temperature of the combustion zone torise, as indicated in Figure 6.6. Higher temperatures help in the burning of the long chainmolecules, including tar, produced by the pyrolysis of the charge. The classical designsuffers from some fundamental drawbacks, like reduction in the throat life as compared toother components and potential ash fusion due to higher prevailing temperatures and lowresidence time in the reduction zone due to higher local velocities.

Morphological developments towards a new reactor design at the Indian Institute ofScience (IISc) to overcome several of the issues related to the closed top reactor with athroat are described in Dasappa et al. [4]. It is evident from the development that effort wasdirected towards increasing the residence time of the fuel as well as the gas and reducingheat loss to maintain an appropriate temperature for tar cracking by suitable choice of alength to diameter (l/d) ratio of the reactor. The combustion chamber in the open top dualair entry is designed with combustion, and the reduction zones are not separated but arecontiguous and extend right up to the ash extraction zone. The reactor wall is kept slightlytapering from the secondary air entry zone, helping to reduce low temperature zones,


Figure 6.6 The closed top classical Second World War downdraft design.

thus eliminating any possible uncracked higher molecular weight compounds escaping thereactor.

To summarize, various reactor designs have been adapted by different research groupsto reduce the contaminant in the gas, eliminate the issue related to fuel bed channeling,and attempt to gain fuel flexibility. The downdraft reactor configuration has been limited toabout 500 kWe. The updraft and the circulating fluid bed have been tested at much largercapacities and use elaborate cooling and cleaning systems. Attempts have been made byvarious groups to reduce the raw gas contamination by modifying the Second World Warclosed top configuration for small capacity systems. These have used staged gasification,dual air entry configuration, and twin fire configuration. There have been attempts to usecirculating fluid bed systems at medium capacity range using steam or air as the gasificationmedium.

6.6.2 Two Competing Designs

Figure 6.7 shows the details of the two geometries for the reactor – the closed top (SecondWorld War class) design and the recent open top design. Their various dimensions areshown in the line sketches. Though there are several variants to the reactor in Figure 6.7,they differ little in essential details. The primary dimensions of importance in the earlierreactor design are dt, the throat diameter, dr, the reactor diameter, de, the exit plane diameter,and h1, h2, h3, the relative heights. Correspondingly, the dimensions of the open top reactor(Figure 6.7b) are the lateral dimensions dr and de and the heights h1 and h2. The choice of


Open top

Closed top

Heat transfer

form gases in

the annular zone

Biomass hopper

Combustion zone

Air nozzles

Reduction zone

Hot gases

h3

h2

h2

h1

h1

de

dt de

da

dr

dr

(b) The recent design(a) The earlier design

(Second World War II class)

Figure 6.7 The two wood gas reactor designs: the earlier Second World War class design andthe modern open top design.

dr/dt, h1/dt, and h2/dt, are based on the “best” performance of some commercial designs.The size of the hopper region is decided by dr/dt and h3/dt, based on simple considerationsof the time required for a single uninterrupted run – typically 2–3 hours. For the choice ofdt a few qualitative arguments are provided. Larger values of dt imply the possibility of tarladen gases escaping the high temperature zone. Smaller values of dt would mean highervelocities through the throat as well as the reduction zone. These are expected to lead toless tar, but much larger dust content. The reduced tar is principally because the gasespass through a smaller reaction volume leading to better temperatures for tar cracking andalso for completion of all reactions. The larger dust content is due to the higher velocitygases picking up greater amounts of fine carbon dust and ash in the reduction zone. Oneof the aspects of the closed top design is that the upper region where the fuel is storedhas a thermodynamic function which it is not properly designed for. The diameter is solarge that heat transfer from the high temperature zone into the upper region generallyaffects the wood chips near the wall rather than the central region. Some designs use anouter chamber where hot gases are passed into the annular chamber to enable heat transferfrom the gas through the wall to the wood chips, and also use a heat exchanger to preheatthe incoming air before it enters the reactor. It is suggested that insulation does not helpmuch, but preheating the air is worthwhile in spite of the low heat capacity of the airand the large area requirements for heat transfer. A few other designs like the monorator


(SERI, 1979) provide an outer zone for the collection of tar. Unless regenerative heatingis done, the walls of the entire upper region become laden with tar in various forms –encrusted and hard, or liquid and sticky. The latter matter can cause bridging, particularlyduring subsequent starting, and cause problems for material movement. Also, if wood withhigh moisture content (say 20–30%) is used, problems in generating combustible gas ofreasonable quality are increased significantly. These problems, originating from improperdesign of the upper region, led sometimes to a situation in which the problem was attributedmistakenly to the moisture content in the wood chips. This led to performance which wasnot quite repeatable unless every aspect of the feedstock was attended to with care, implyingthe system was less user-friendly.

In order to reduce the tar content, the air nozzles are distributed around the peripherywith the expectation that all fuel vapor flow is intercepted and hence combusted. Duringthis process the temperature is also raised. Evidence in literature shows that with increasedvelocities the peak temperature in the combustion zone rises, and this helps reduce tar.Rules of thumb are therefore provided to choose the number of air nozzles for a given flowrate (and therefore for a given thermal and mechanical power). Qualitative arguments aremade showing the regions of influence of air distribution around the air nozzles and theregions in between the air flow zones where the flow of volatiles can escape through thelow temperature zone resulting in a gas with high tar content. It is not unlikely that earliergasifier programs in other countries have had problems because of inadequate attentionto resolution of tar problems. One of the crucial issues is that once tar escapes from thereactor, it is not easy to eliminate it by cooling/spraying systems since the vapor will alsoescape these processes to a significant extent. One of the important ways to eliminate tarwould therefore be to create a correct thermal and oxidative environment for reducing it inthe reactor itself.

The open top design, on the other hand, provides for much better homogeneity of theair flow distribution as it passes through a long porous bed. Cold flow studies indicatethe velocity distribution for a flow in a packed bed becoming homogeneous beyond a fewparticle depths. Wall heat transfer from regenerative heating enables better tar crackingas the residence time in the high temperature zone is substantially increased. Many ofthe configurations of the open top design, including the laboratory model study of, [5] donot have air nozzles as used in gasifiers of earlier classical designs. The present design,(Figure 6.7b) on the other hand, provides for an air nozzle as well as an open top. Assuch, air is shared between the nozzle and the top and this has many advantages. The airnozzle help in quick lighting with a simple wick flame. It helps stabilize the combustionzone which might move to the top because of the phenomenon of “stratification” in whichthe flame front moves in a direction opposite to the air flow. The high temperature zone,consequently, spreads more above the nozzle in comparison to what would be the case withthe closed top design in which the spread is governed by radiation, thermal conduction, andweak convection processes. In the present open top configuration forced convection heattransfer from the hot gases flowing in the annular gas passage also contributes to upwardflame propagation. These aspects enable use of wood chips with moisture contents as highas 25%. The heat pumped in by the hot gases makes the fuel chips in contact with the wallheat up and lose moisture. Generally, the air being drawn through the top is about 40–70%of the total flow taken in depending on the pressure drop conditions due to the size of woodchips and gas flow rate. The ability of the reactor to dry the wood chips within itself allows


for the possibility of reliable operation with varying conditions of moisture in the woodchips. Further, the insulation provided makes a significant difference to reliable operation,the lack of which would mean significant amounts of tar and unreliable gasifier operation.Figure 6.6 shows the temperature variation from the air nozzle region in the present opentop design. It is clear that the width of the high temperature region, 600 K and above, isabout 1 m for the open top design whereas it is constant at about 0.4 m for the earlier design.What is more is that it can be controlled by decreasing the air flow through the air nozzlein the current design.

The size of the wood chips is important for the successful operation of the reactor. TheSERI report emphasizes this but does not provide guidelines for lower power levels. Itis indicated that wood chips for power levels of 15–75 kW should be about 60–80 mmlong and 50–60 mm in diameter at the most. There are arguments about size in relationto time for gasification and hence increased bed depth for increased chip size. In view ofthe lack of a precise description of the thermodynamic requirements of the fuel storageregion (alluded to earlier) the need for a larger depth of fuel chips is unclear. Most ofthe arguments, however, seem relevant to the open top system. For this case, the majordimension of the wood chip should be about one-sixth to one-seventh the diameter of thereactor to meet the requirements of flow ability with not too-high a porosity and time forgasification. The last point is addressed with more precision by [6], who take advantageof the near one-dimensional character of the geometry and set out the design principleson a rational basis. The time for conversion is split into two parts, namely the flamingtime and the char conversion time. During the flaming period occurring with the air drawnlargely from the top, the pyrolysis process is completed and char is produced. The resultsare rightly expressed in terms of a volume-based mean diameter so that the results apply toother geometries as well. The time required for char conversion with CO2 alone is treatedwith an appropriate kinetic expression. The effect of the presence of H2O as well as theparallel reaction path of char with H2O, which is a faster reaction when compared withCO2, seems to have been ignored in this work. This is an important aspect that needs futureattention. For any assumed reactor diameter the required heights for flaming pyrolysis andchar conversion are then obtained from the given properties of the woody biomass; namely,its density, specific heat, and heats of phase change with a simple model for heat balance.The height of the reactor is then determined by requiring that the downward distancetraveled by the woody biomass must allow for the residence time equal at least to the sumof the flaming (pyrolysis) and char conversion times [5] do not provide any argument forthe choice of the diameter of the reactor. Because of the assumption of one-dimensionalitythe l/d of the reactor should be large, typically 6–8. Once the height is determined, thediameter can be obtained from the choice of a value for l/d. Qualitatively, smaller diametersare preferred in order to make the reactor compact and to permit the wall heat transfer toaffect the entire cross-section. Too small a diameter would necessitate the use of smallerwood chip size and cause higher pressure drops at reasonably high flow rates.

6.7 Open-Top Dual Air Entry Reaction Design – the IISc’s Invention

The consideration for the design has been to reduce the tar level in the raw gas, improve thecarbon conversion in the reactor, and eliminate any channeling, which has been the major


issue in the downdraft systems. The central part of the argument, tar cracking, is promotedby two means – uniform distribution of high temperature across the char bed and presenceof reactive char. That high temperatures are favorable for cracking of complex chemicalstructures into smaller ones is a well-known phenomenon. Careful measurements by Kaupp[3] have shown that the tar fraction is reduced substantially if a tar-filled gas passes througha hot bed of charcoal. The next question is the residence time in the reactive zone. Theeffective bed thickness in which char and high temperature are present adjusts itself dueto the flow of air through the reactor. At low flow rates the nominal bed temperaturesattained are sufficient to crack the tar; while the total travel distance is the same in the caseof a closed top gasifier, higher bed temperature compensates for the lower residence, sothat effective tar cracking is maintained throughout the load range. Thus, bed temperature,surface area, and residence time are critical for the thermal cracking of tar.

The evolution of throat-less designs occurred independently to overcome the problems inthe field associated with the classical designs. Figure 6.8 provides the details of the first opentop dual air entry reactor configuration with other elements for gas cooling and cleaning.Air for sub-stochiometric combustion inside the reactor is drawn from two sources: (i) anair inlet nozzle, and (ii) the open top through the bed of fuel chips. About 30–35% of theair comes from the air nozzle and the rest from the top [5] did not explore this alternativeof having an air inlet nozzle in the lower zone. With all the air drawn from the open top [5],the average combustion temperatures were lower (1300 K or lower) and this could haveresulted in poor quality gas at lower rates of gas generation.

Induction of air from the top causes what is termed stratification of the fuel charged. Thevolatiles are released at some stage in the downward path of wood chips. Mixing with airfrom the top initiates exothermic reactions and a steep temperature gradient is establishedin this zone. The transfer of heat to the upper zone in the packed bed above the air nozzle iseither by heat transfer from the bed by conduction or by radiation. The other phenomenoncould be due to preheating of the wood chips due the double walled chamber. Both of these

B

B

A

A

Air

Air (~50–70%)

Hot gases

(700–800 °C)

Biomass

Stratification (upward

propagation of flame)

1200–1400 °C

Broader than in

closed-top

Char+CO2+H2O+N2+O2

CO, H2, CH4, CO2, N2

Figure 6.8 Open top dual air entry configuration.


assist the earlier initiation of release of volatiles. This would mean that an isotherm, say at700 K, slowly creeps upwards and the gasifier never attains a steady state during the fewhours of its operation in terms of the thermal profile both in the reactor and in the annularspace, as long as air is drawn from the top. Figure 6.8 shows the typical open top downdraftreactor with air nozzle.

Revisiting the crucial requirement stated earlier arising from the various studies on thereactor operation towards ensuring a sustainable technology package, the following issueshave been addressed at IISc with a scientific rigor and translated into technology package:

• Gas quality in terms of generating gas with extremely low or negligible amount of highermolecular weight compounds collectively called “tar.”

• Consistency in the gas generation over a range of load “turn-down ratio.”

• Multi fuel capability, currently being limited to charcoal and wood chips – “fuel flexi-bility.”

• Capacities less than 500 kW – “capacity range.”

6.8 Technology Package

A typical gasifier system configuration is shown in Figure 6.9. The open top downdraftreactor design is made of a ceramic lined cylindrical vessel for improved life in the highlycorrosive thermal environment inside the reactor along with a bottom screw for ash extrac-tion. In brief, the reactor has air nozzles and an open top for air to be drawn into the systemto help in improving the residence time of the gas and to enable cracking of higher molec-ular weight compounds. The novelty in the design arises from the dual air entry – air beingdrawn from the top of reactor as well as through the nozzles – permits establishment of aflame front moving towards the top of the reactor, thus ensuring a large thermal bed insidethe reactor to improve the gas residence time. The details of the gasification technologyare discussed in Dasappa et al. [6]. A unique screw-based ash extraction system allowsfor extracting the residue at a predetermined rate. The gas is cooled and cleaned by directcontact with water sprays in the cooler and scrubber. During this process, water is contam-inated with both dust and some organic compounds like phenols, aldehydes, and so on, andis treated in a water treatment plant. The total gas conditioning system involves a cyclone,scrubbers, and a fabric filter. The gas is then de-humidified or dried using the principle ofcondensate nucleation, to reduce the amount of moisture and fine contaminants. A blowerprovides necessary the suction for meeting the engine requirements.

6.8.1 Typical Performance of a Power Generation Package

This section presents the performance of a 100 kW grid connected engine-gasificationsystem. The gasification system capable of operating on a range of biomass includingbiomass briquettes was operated using woody biomass. Biomass used for the gasificationwas sourced from the project-supported dedicated multi-species plantation. Fuel was sizedusing a multi-blade cutter in the range from 25 × 25 × 25 mm to about 50 × 50 ×50 mm depending on the feedstock. Table 6.3 provides fuel characteristics used in thegasifier. The fuel was loaded to a batch drier using diluted engine exhaust for drying thefuel. Typical residence time of the wood was in the range of 2–4 hours depending upon the

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9Sc

hem

atic

ofa

gasi

ficat

ion

syst

em.


Table 6.3 Fuel properties.

Parameter Description

Size 25 mm to 50 mm in diameter and lengthMoisture 10–15%Ash ∼ 3–5%Density 550–650 kg/m3

Bulk density 350–400 kg/m3

initial moisture content in the fuel. Fuel from the drier was loaded to the gasifier using aconveyor at a rate depending upon the fuel consumption.

6.8.1.1 Gasifier performance

The gasification system was initially tested for establishing the performance in the blowermode. Gas was generated and flared at the burner towards ensuring all the process param-eters, like pressure drop across various system elements, fuel loading, ash extraction, gascooling, water quality, and the gas quality were as per the design. It was found that thegasification system could be operated at the rated condition of 135 kg/hr continuously. Afterestablishing the gasifier performance, the engine was operated in the grid connected mode.During this period, all the relevant parameters indicated above along with the electricalparameters, like voltage, current and frequency, were recorded. The following sections willhighlight the operational performance of the total package.

6.8.1.2 Pressure drop across reactor

This is an important parameter related to the operation of the gasification system. The reactorpressure drop provides information related to the packed bed dynamics occurring insidethe reactor where thermochemical conversion processes take place. As can be seen fromFigure 6.10, the resistance posed by the bed for the gas flow is about 1500 ± 500 Pa. Otherderived information from the data is regarding the capability of the reactor to continuouslyproduce gas without building up resistance that could result in reducing the gas flow rateand the electrical load, a critical issue of any fixed bed system. This parameter decides theoverall health of the system. Under these conditions, the propagation rate within the fixedbed is sufficient to establish the thermal profile above the air nozzles to provide adequateresidence time both for the solid and the gaseous species to ensure conversion processesare nearly complete [7].

Figure 6.11 highlights the biomass consumed during the first thousand hours of operation.Based on this data, the average consumption rate was in the range of 110 ± 10 kg/hr, andthe residue removal was about 5 ± 0.5 kg/hr. The residue extracted depends on the ashcontent in the biomass, which in the present case was about 3.5–4.5%.

6.8.1.3 Gas quality

Gas composition was measured using SICK Maihak online gas analyzer. CO, H2, CH4,CO2, and O2 were recorded on a data acquisition system for certain durations during


0

0–1000

–2000

Reacto

r pre

ssure

dro

p (

Pa)

–3000

–4000

200 400 600 800

Time in hours

1000 1200

Figure 6.10 Reactor pressure drop at the rated condition of the gasifier system.

1000 hours of operation. Tar and particulate in the gas were measured using the wet method[8]. The gas composition measurement was restricted to part of the duration due to theportability of the equipment between the laboratory and the project site. A typical gascomposition trace is presented in Figure 6.12. The gas composition was measured over aperiod of about five hours during the plant operation. Measured compositions show CO

20000

40000

60000

80000

Cu

mm

ula

tive

bio

ma

s lo

ad

ed

an

d r

esid

ue

extr

acte

d (

kgs)

100000

120000

00 200 400 600

Time in hours

800 1000 1200

Figure 6.11 Biomass loading and residue extraction with time for 1000 hours operation. Fullline – biomass consumption; dotted line – ash extraction.


CO

CO2

H2

CH4

00

5

10

15

20

Gas c

om

positio

n in v

olu

me %

25

50 100 150

Time in minutes

200 250 300

Figure 6.12 Producer gas compostion at a load of 90 kWe.

and H2 in the range of 18 ± 1%, CH4 1.8 ± 0.4%, CO2 9 ± 1%, and the remainder N2.The composition would result in a gas calorific value of about 4.5 ± 0.3 MJ/kg.

The tar and particulate emission measurements were conducted at the exit of the gasfilter using methoxy benzene as the solvent. The results from four tests showed that theaverage particulate content in the gas was in the range of 19 ± 2 mg/m3 of gas while thetar was in range of 10 ± 2 mg/m3 [7].

6.8.2 Engine and Generator Performance

Biomass consumption was logged by monitoring each charge being loaded and similarlythe char removed at regular intervals was weighed using a balance. Electricity generatedwas measured using a kWh meter on the control panel and cross-checked with the voltageand ammeter recordings. The power factor was found to be around 0.92.

It is important to recognize that the entire power package has been able to generate anearly constant load. Some of the lower loads recorded are due to grid failure and reloadingthe system. During grid failure the entire system was operated on the internal load withoutstopping either the engine or the gasification system. There were about 10 grid failuresduring this operation, amounting to about 70 hours of in-house load operation withoutexporting electricity to the grid. The engine exhaust emissions were measured using aQuintox make online analyzer. The averages of several readings are that CO is 10 410 mg/m3

and NOx is 126 mg/m3. The guidelines for emissions from biomass gasification plants forgas engine applications (Anon, 2009) suggest standards used in Denmark and Germany [9].Denmark uses 3000 mg/m3 and 550 mg/m3 respectively for CO and NOx at 5% exhaustoxygen in the exhaust, whereas in Germany, the limit values are 650 mg/m3 for CO and500 mg/m3 for NOx. Compared to the standards, the site measurements indicate a slightlyhigher CO level, reflecting on the in-cylinder combustion process with producer gas as fuelhaving incomplete combustion with CO. A catalytic converter may probably be required atthe engine exhaust to meet the emission standard. In the present case, the engine exhaust


2

1.8

1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

00 20 40

Number of days

60 80

Specific

bio

mass c

onsum

ption,

(kg/k

Wh)

100

Figure 6.13 Specific fuel consumption over 2000 hours of operation.

gas is diluted with air to reduce the temperature from nearly 750 K to about 375 K fordrying the moist wood chips, thus the concentration of CO and NOx would be lower byabout 20%.

Figure 6.13 presents specific fuel consumption over the period of operation. It can beseen that the average fuel consumption is in the range of about 1.3 kg/kWh, while the dailybest has been in the range of 1.1± 0.1 kg/kWh.

Questions

1. Define moisture content in biomass on a wet and dry basis. How much moisture (in kg)is present in:(a) 1 kg biomass with 30% moisture on wet-basis.(b) 1 kg biomass with 30% moisture on dry-basis.

2. Pine wood has thermal conductivity k = 0.12 W/(m.K) and heat transfer coefficient,h = 35 W/(m2K). For the given spherical specimens of 2 mm and 40 mm, state thepyrolysis condition and temperature (compared to ambient) on the surface and insidethe core of the sphere after 2 seconds when the wood spheres are subjected to ambientcondition of 1000 K. Justify the answer with respect to the characteristic time “𝜏.”

3. What is the difference between the combustion and gasification process?4. List the different reduction reactions in gasification process. Comparing updraft and

downdraft gasifier geometry, which reactions will be more relevant in the reductionzone in each of the geometries? Compare the temperature of the reduction zone in bothreactor configurations and justify.

5. What is the mode of heat transfer (convection, conduction, or radiation) for pyrolysis inan updraft and downdraft gasifier?

6. What are the impacts of geometry of an updraft and downdraft gasifier on energyefficiency, tar content, and end use of the product gas? Which gasifier would you preferfor a) producing steam and b) running an IC engine? Justify your choice.


7. Describe the reason and use of the throat area in gasifier. What are the pros and cons ofa throat area in gasifier design?

8. Describe the essential parameters that influence low tar production during gasification.

References

(1) Kanury, A.M. (1972) Combustion and. Flame 18, 75.(2) SERI. (1979) Generator Gas – The Swedish experience from 1938–1945 (translation), Solar

Energy Research Institute, Colorado, NTIS/S 33-140.(3) Kaupp, A. and Goss, J.R. (1984) Small scale gas producer engine systems, A publication of

GATE.(4) Dasappa, S., Shrinivasa, U., Baliga, B.N. and Mukunda, H.S. (1989) Five-kilowatt wood gasifier

technology: Evolution and field experience, Sadhana, Indian Academy of Sciences, Proceedingsin Engineering Sciences, pp. 187–212.

(5) Reed, T. and Markson, M. (1983) A predictive model for stratified down-draft gasificationof biomass. Proceedings of the Fifteenth Biomass Thermo chemical Conversion ContractorsMeeting, pp. 217–254, Atlanta, GA.

(6) Dasappa, S., Paul, P.J., Mukunda, H.S. et al. (2004) Biomass gasification technology – a route tomeet energy needs. Current Science, 87(7), 908–916.

(7) Dasappa, S., Subbukrishna, D.N., Suresh, K.C. et al. (2011) Operational experience on a gridconnected 100 kWe biomass gasification power plant in Karnataka. Energy for SustainableDevelopment, 15, 231–239.

(8) Mukunda, H.S., Paul, P.J., Dasappa, S. et al. (1994) Results of an Indo-Swiss programme forqualification and testing of a 300-kW IISc-Dasag gasifier. Energy for Sustainable Development,1(4), 46–49.

(9) Anon. (2009). Guideline for Safe and Eco-friendly Biomass Gasification. Intelligent energypublication – Europe (http://www.gasification-guide.eu/gsg_uploads/documenten/D10_Final-Guideline.pdf)

http://www.gasification-guide.eu/gsg_uploads/documenten/D10_Final-Guideline.pdf

http://www.gasification-guide.eu/gsg_uploads/documenten/D10_Final-Guideline.pdf

7Engines for Combined Heat

and Power

Miloud Ouadi,1 Yang Yang1 and Andreas Hornung2

1European Bioenergy Research Institute (EBRI), Aston University, UK2Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg, Germany and Chair in


For the use of syngas or biofuels in combustion engines, to power a generator, for example,two established technologies are available. Depending on available fuel, plant size, avail-ability requirements, operating costs and returns, the spark-ignited (SI) gas engine and thedual-fuel engine have different advantages and disadvantages.

7.1 Spark-Ignited Gas Engines and Syngas

Spark-ignited (SI) gas engines are for the use of fuel gas only and are limited to theflammable range of the gas; this means the range of a concentration of a gas or vapor thatwill burn after ignition. Below the explosive or flammable range the mixture is too lean toburn and above the upper explosive or flammable limit the mixture is too rich to burn. Thelimits are commonly called the ‘Lower Explosive or Flammable Limit’ (LEL/LFL) and the‘Upper Explosive or Flammable Limit’ (UEL/UFL). The terms ‘flammability limits’ and‘explosive limits’ are used interchangeably.

Fuel gas like syngas from gasification or biogas must meet high quality and chemicalpurity standards for combustion in SI gas engines. A relatively high heating value and




suitable gas composition is needed for a certain ignition and a stable combustion. Withincrease of H2% in syngas the efficiency rises respectively. On the other hand, the efficiencydecreases for all combinations of syngas at low loads due to poor combustion of gaseousfuels [1]. Dust and particles or chemical components containing sulfur or chlorine candamage engine components and harm the environment. Also, tar within the syngas willlead to engine damage and improper emissions. Therefore an effective gas pre-treatment isoften recommended.

Caused by the operating principle of the valve-opening overlap, which means a par-tially opened inlet valve and not a finally closed outlet valve, fresh syngas can directlystream into the exhaust. This unburned syngas increases the emissions of carbon monox-ide and unburned hydrocarbons like benzene. To reduce the emissions, oxidation catalyticconverters are widely used.

The efficiency of SI gas engines is lower than dual-fuel engines based on the thermo-dynamic diesel cycle. On the other hand, SI gas engines do not need an ignition fuel likediesel which lead to lower operation and maintenance expenses.

7.2 Dual-Fuel Engines and Biofuels

Typical dual-fuel engines operate on natural gas and diesel fuel simultaneously; the majorityof fuel burned being natural gas. Diesel fuel acts essentially as an ‘ignition spark’ as it auto-ignites under compression and then ignites the gas. The use of diesel fuel allows theretention of the diesel compression ratio and its efficiency while the natural gas contributesto economy and is responsible for lowering emissions. The gas fraction yields between 50(some engine providers down to 20%, most engines run on pure liquid fuel as well) to 90%of the combustion energy. Compared to a gas engine the fuel gas can be very lean and is notlinked to the flammable limits, because the diesel combustion forces the fuel gas to reactand combust.

Natural gas and/or diesel fuel can be substituted by biofuels. For dual-fuel enginesthe most common biofuels are biogas, syngas, biodiesel, and vegetable oils. A recentdevelopment is the use of oil and gas produced by solid biomass pyrolysis [2].

Fuel gas for dual-fuel engines must meet similar requirements as for SI gas engines,except for the flammable limits. Additionally, the fuel oil for the injection should meet therequirements (e.g., viscosity, free of particles, cetane number, acid number, water content)of at least biodiesel to prevent serious fuel and injection system damage. Also, fatty acidswithin the biofuel can damage engine components like plain bearings or the injectionsystem. Bio-oils are more likely to ingress into the lubricant and reduce the lubricationof the engine than diesel. This results from a usually higher boiling point. Therefore,manufacturers recommend a shorter oil change interval, which leads to a higher maintenancecost. Depending on engine size and operation the dual-fuel engine compensates the highermaintenance costs by its efficiency.

Based on the diesel compression and the higher combustion temperatures than in theSI gas engine, a higher amount of nitrogen oxide will be emitted. Therefore oxidation-reduction catalytic converters are used for the exhaust gas treatment to reduce carbonmonoxide, unburned hydrocarbons, and nitrogen oxides.

Engines for Combined Heat and Power 161

7.3 Advanced Systems: Biowaste Derived Pyrolysis Oils for DieselEngine Application

Considerable effort has been devoted to fast pyrolysis (high heating rates, low solid res-idence time) over the years and some of the product oils have been tested in diesel ordual-fuel engines since 1993 [3–9]. Intermediate pyrolysis (slow heating rates, intermedi-ate solid residence times) is considered to be at the early stages of research and development;however, recent studies have shown that biomass and waste derived intermediate pyrolysisoils can give a product oil with improved properties which are to a certain degree morecomparable to biodiesel and diesel fuels [10–13].

Overall, the composition and physical-chemical properties of pyrolysis oils vary andlargely depend on the feedstock used and processing technology employed. Pyrolysis oilsdescribed in previous research have exhibited properties unfavourable for their use asengine fuels, such as high water content, low heating value and strong acidity [4, 5, 7–9].Particularly in conventional diesel engines, difficulties have been observed in the formationof a high quality injection spray and subsequent ignition. The characteristics of these oilsdirectly relate to their behaviour in fuel systems and to engine performance. Therefore, it isessential to carry out characterisation and evaluation of intermediate pyrolysis oils beforetheir use as an engine fuel.

For diesel engine injection systems, a large number of fuel oil properties need to be takeninto consideration.

The fuel injection characteristics – such as injection timing, injection pressure andinjection duration – largely depend on the oil density, viscosity and surface tension becauseof their influence on oil atomisation effects during injection [14, 15]. Research on oilphysical properties has shown that a relatively low density of the oil retarded injectiontiming, while a relatively low viscosity resulted in an advanced timing because of lessfriction produced by the oil travelling through the nozzle [15,16]. Surface tension of the oilalso affects the injection characteristics; a high fuel surface tension opposes the formationof the spray droplets and can generally decrease the initial spray velocity while wideningthe cone angle due to an increase in friction [17, 18].

The heating value of a fuel oil has a major effect on engine system thermal efficiencyand power output. A reduction in the heating value of a fuel oil will reduce the enginepower output for the same fuel consumption rate both because of reduced thermal inputand reduced efficiency.

The distillation (volatility) characteristic of a fuel oil critically influences performanceas well as safety during storage and transport. It describes the proportion of light, mediumand heavy distillation compounds in a fuel oil and normally is presented as a plot ofweight loss due to boiling versus temperature. A high amount of light volatiles indicatesa tendency to generate potentially explosive vapours, while a high heavy fraction is themajor determinant of solid combustion deposits [15,16]. The distillation curve corrected tothe standard temperature and atmosphere pressure is also important to determine the cetaneindex of a fuel oil, which is a substitute for the cetane number when a pre-combustionchamber type compression ignition test engine is not available [19]. The cetane numberdescribes the auto-ignition tendency of the fuel oil under the compression ignitionstroke.


The effect of the high boiling point fraction of fuel oil can be observed by the ‘carbonresidue’ test, which gives the amount of carbonaceous residue left over after the evaporationand pyrolysis of the oil under controlled conditions. The carbon residue can indicate thepropensity for injector nozzle clogging and coke formation in the combustion chamber [20].However, it is argued that some of the ash-forming materials of the fuel oil can affect theresult of carbon residue tests. Hence the ash content of the oil, which may exist in the formof abrasive solids and soluble metallic soaps, is also crucial. The ash forming materials alsocontribute to the piston and fuel system wear and result in additional combustion depositsand filter clogging [21].

Water contained in the oil, which is another unavoidable impurity from the feedstock,must be strictly controlled. Excessive water in the oil reduces the heating value and mayresult in inhomogeneity and further phase separation. In applications, water may lead torust and corrosion of the metal parts and cause emulsion formation in the fuel system.Furthermore, the relatively high heat of vaporisation and specific heat capacity of waterreduces the local temperature of the combustion chamber and fuel evaporation rate duringthe combustion, and these cause ignition delay and deteriorated emission [22].

The flash point is the lowest temperature at which a vaporising fuel oil forms an ignitablemixture with air. Although it does not directly affect the fuel oil performance, it determinesthe safety class of fuel oils in storage and transportation. It must be always specified in thefire precaution regulations.

The oil corrosiveness is another key characteristic to evaluate the quality of the fuel oil,since it determines the fuel system life and engine durability. In addition to the systemwear caused by mechanical friction due to the lack of lubricity and the presence of solidabrasive particulates [23], the electrochemical corrosion induced by the oil acidity playsan important role. Oil acidity may result from the acidity reagent present in the productionprocess or from fuel oil ageing. So far there is no general correlation known between acidnumber and corrosiveness. However, fuel oils with high acid number have been proven tobe associated with fuel system deposits and increased storage risks [24, 25].

7.3.1 Important Parameters to Qualify the Oil as Fuel

7.3.1.1 Cetane Index

The cetane index of the oils can be calculated in accordance with ASTM D4737 by usingthe oil densities at 15 ◦C and the temperatures for 10, 50 and 90% distillation recovery ofthe fuel oils. The calculated cetane index (CCI) is given by:

CCI = 45.2 + (0.0892)(T10N ) + [0.131 + (0.901)(B)[T50N] + [0.0523 − (0.420)(B)][T90N]

+ (0.00049)[(T10N)2 + (T90N)2 + (107)(B) + (60)(B)2

where:

B = [e(−3.5) (DN)] − 1,DN = D − 0.85,D = Density at 15 ◦C, g/ml determined by ASTM Test Methods D1298 or D4052,T10 = 10% recovery temperature, ◦C, determined by Test Method D86 and corrected to

standard barometric pressure,


T10N = T10 – 215,T50 = 50% recovery temperature, ◦C, determined by Test Method D86 and corrected to

standard barometric pressure,T50N = T50 − 260,T90 = 90% recovery temperature, ◦C, determined by Test Method D86 and corrected to

standard barometric pressure,T90N = T90 − 310.

7.3.1.2 Acid Number

The total acid number (TAN) of the oils can be measured with a Mettler Toledo V20 Compacttitrator using the potentiometric titration method in accordance with ASTM D664. The oilsample is dissolved in 50/49.5% toluene and isopropanol solution with 0.5% water andtitrated potentiometrically with 0.1N alcoholic potassium hydroxide using a combinationelectrode. Readings are automatically plotted against the volume of titrating KOH solutionused until the titration end-point is achieved.

7.3.1.3 Corrosiveness

The oil corrosiveness test can be carried using a pressure vessel and an oil heating bath inaccordance with ASTM D130. Polished copper strips are immersed in the test oil samples,which are then placed in a 40 ◦C oil heating bath. The copper strips are compared to theASTM corrosion standard board after periods between 6 and 24 hours.

7.3.1.4 Lubricity

Oil lubricity can be determined using a PCS High Frequency Reciprocating Rig (FHRR) inaccordance with ASTM D6079. A vibrating arm holding a non-rotatable ball specimen andloaded with a 200 g mass is lowered to contact a test disk specimen. These are submergedin the testing oil sample and the oil temperature is set to 60 ◦C. The ball is made to rubagainst the disk with 1 mm stroke at a frequency of 50 Hz for 75 min. The ball is removedfrom the vibrating arm and cleaned. The dimensions of the wear scar at the major and minoraxes are measured by 100x magnifiers and recorded, and the arithmetic average taken.

7.3.1.5 Carbon Residue

The Conradson Carbon Residue test can be performed in accordance with ASTM D189 bya manual method. A weighed sample is placed in a crucible and undergoes strong heatingby a Meeker burner. The carbonaceous residue remaining after the cracking and cokingreactions is cooled to room temperature and weighed. The Conradson Carbon Residue isthen the carbonaceous residue expressed as a mass percentage of the original oil sample.

7.3.1.6 Ash Content

The ash content of the oil can be determined in accordance with ASTM D482. The car-bonaceous solid samples produced from the Carbon Residue test is combusted in a muffle


Table 7.1 Proximate and ultimate analysis of sewage sludge and deinkingsludge feedstock.

Sewage sludge Deinking sludge

Ultimate analysisa UnitCarbon wt% 24.0 21.7Hydrogen wt% 3.5 2.8Oxygen wt% 35.7 29.8Nitrogen wt% 2.9 2.1Sulfur wt% 1.3 <0.1Proximate analysisa

Moisture wt% 4.7 1.3Volatiles wt% 63.7 55.1Ash content wt% 32.6 43.6Fixed carbon wt% <0.1 <0.1HHV MJ/kg 15.3 7.0

aAnalysis based on pre-treated feedstock, ultimate analysis is on dry basis.

furnace at 775 ◦C. The remaining ash is cooled at room temperature and weighed, and thenexpressed as a mass percentage of the original oil sample.

7.3.1.7 Complex Composition of Pyrolysis Oils

The major chemical components of intermediate pyrolysis oil obtained from industrywastes, namely sewage sludge and de-inking sludge are shown in Tables 7.1, 7.2 and 7.3respectively; although more than 200 peaks corresponding to different organic compoundsare detected in each mass spectrum.

It was found that both pyrolysis oils are complex organic mixtures consisting of carbonchains ranging from C7–C17 for sewage sludge derived pyrolysis oils and C5–C15 for deink-ing sludge derived pyrolysis oils. Similar to fossil diesel fuel, they mainly contain paraffins,naphthenes and aromatics. Aromatic hydrocarbons are the most abundant component in thepyrolysis oils, accounting for 31% of sewage sludge derived materials and 48% of deinkingsludge derived materials.

Phenols, the other major aromatic compound found in the oils, are also present insignificant quantity, over 22% in sewage sludge and 15% in deinking sludge derived oils.This is the cause of the high acidity of both oils.

The high aromatics content also gives rise to the low cetane index of the oils, as aromaticshave poorer combustibility compared with paraffins and naphthenes [26].

7.3.1.8 Water Content

Water is always produced with pyrolysis oil during the intermediate pyrolysis process.There are two main sources for water content in the oil: feedstock water and reactionwater. Sewage sludge produced approximately 60 vol% of water in the overall liquidproduct and deinking sludge produced 15 vol%. However, intermediate pyrolysis oils have


Table 7.2 Composition of sewage sludge intermediate pyrolysis oil.

# Retention time Chemical name Formula Area%

1 10.42 Unknown – 2.152 10.97 Benzene, methyl- C7H8 17.513 14.87 Unknown – 3.124 15.26 Benzene, ethyl- C8H10 8.385 15.67 Benzene, 1,3-dimethyl (p-xylene) C8H10 1.866 17.49 Cyclooctatetraene C8H8 6.97 20.06 Decane C10H22 4.338 22.15 Benzene, 1-methyethenyl- C9H10 1.539 25.42 Undecane C11H24 3.89

10 30.64 Phenol C6H6O 9.1711 33.18 Phenol, 2-methyl- C7H8O 1.612 34.9 Phenol, 4-methyl- C7H8O 8.7313 35.61 Tridecane C13H28 3.1614 39.24 Phenol, 4-ethyl- C8H10O 1.5715 40.96 Phenol, 4-methyl, 2-methoxy- C9H12O2 1.3716 44.75 Pentadecane C15H32 3.5317 45.98 Phenylacetonitrile C8H7N 4.8618 48.94 Hexadecane C16H34 1.819 49.36 1H-lindole, 5-methy- C9H9N 2.320 52.92 Heptadecane C17H36 2.1921 55.57 Benzene, 1,1’-(1,3-propanediyl) bis C15H16 1.9122 65.44 Hexadecanenitrile C16H31N 5.4723 71.97 Heptadecanenitrile C17H33N 2.68

the advantage of being largely immiscible with water, and most of the water content willbecome separated by gravity as a highly aqueous phase in a few hours, at which point itis easily removed. The water content of the remaining oil phase was measured as 4.37wt% in sewage sludge pyrolysis oil (SSPO) and 2.97 wt% in deinking sludge pyrolysisoil (DSPO). This could be considered as reasonable since it does not greatly reduce the oilheating values and does not lead to a further significant phase separation problem duringoil storage. Furthermore, modest water content in the oils can reduce the combustiontemperature in the cylinders and thereby reduce NOx emissions. However, the watercontent may result in metal electrochemical reaction in the long term, and could decreasethe oil stability by allowing the growth of microorganisms if there is any further phaseseparation [27].

7.3.1.9 Acidity

Results from acid number titration indicate that SSPO has a total acid number of 19.90and DSPO up to 33.03. Both of these are greatly above the biodiesel limit of 0.8, andthere is therefore a significant danger of corrosion and consequent damage to fuel systemcomponents.


Table 7.3 Composition of deinking sludge intermediate pyrolysis oil.

# Retention time Chemical name Mol formula Area%

1 10.99 Benzene, methyl- C7H8 10.142 14.55 Cyclopentanone C5H8O 1.673 15.29 Benzene, ethyl- C8H10 17.914 17.51 Cyclooctatetraene C8H8 16.515 18.41 Benzene, propyl- C6H8 3.396 20.07 Benzene, 1-methyl, 2-ethyl- C9H12 3.087 20.87 2-Cyclopenten-1-one, 2-methyl- C6H8O 3.478 22.16 Benzene, 1-methylethenyl- C9H10 5.649 25.96 2-Cyclopentenone, 3-methyl- C6H8O 2.19

10 29.03 1-Cyclopenten-1-one, 2,3-dimethyl- C7H10O 3.5511 30.13 Acetophenone C8H8O 1.8812 30.62 Phenol C6H6O 3.9913 31.65 Phenol, 2-methoxy- C7H8O2 2.5114 33.15 Phenol, 2-methyl- C7H8O 1.8415 34.88 Phanol, 3-methyl- C7H8O 2.3116 35.61 Tridecane C13H28 1.5717 36.86 Phenol, 2-methoxy-4-methyl- C8H10O2 2.1418 40.32 Tetradecane C14H30 2.5819 40.95 Phenol, 4-methyl, 2-methoxy- C9H12O2 2.5120 44.74 Pebtadecane C15H32 2.2321 55.57 Benzene,1, 1’-(1,3-propanediyl) bis C15H16 6.7822 65.44 Pentadecanenitrile C15H29N 2.13

7.3.1.10 Carbon Residue and Ash

The carbonaceous residual deposit from destructive distillation of both intermediate pyroly-sis oils is very high compared with the limit of 0.05 wt% specified in the biodiesel standard,being 2.38 wt% for SSPO and 5.26 wt% for DSPO. These very high values are likely tocorrelate with fuel injector nozzle clogging and combustion chamber deposits, which canaffect combustion and overall engine performance. Ash content of the oils was measuredas 0.23 wt% for SSPO and 0.16 wt% for DSPO. This may be acceptable for furnace orboiler, but is above the biodiesel standard maximum of 0.1 wt% maximum [28]. Upgradingor dilution of the oil is needed for optimisation. Such oils have been used by diluting themwith approximately 70% of biodiesel to reach the required performance of the oils.

7.4 Advanced CHP Application: Dual-Fuel Engine Applicationfor CHP Using Pyrolysis Oil and Pyrolysis Gas fromDeinking-Sludge

The applications of engines for heat and power are very well developed for biogas con-version and the use of synthesis gas from wood driven gasifiers. For biogas conversion,usually dual-fuel engines are used, while for synthesis gas the use of gas engines is morecommon. The trials using wood derived pyrolysis oil from fast pyrolysis have not yet been


Table 7.4 Elemental analysis of intermediate pyrolysis oils and biodiesel.

Elementalanalysisa

(wt%) SSPO DSPO Biodiesel Diesel

C 74.21 76.58 78.86 85.60H 9.96 8.38 12.63 13.37N 5.14 1.86 <0.10 <0.10S 1.96 0.58 0.74 <0.10O 8.73 11.27 8.36 1.01

aSamples were analysed as received.

a success story and these approaches have usually failed due to the high corrosivity ofthe pyrolysis oils (even acid numbers above 100 are reported in literature) [29]. Pyrolysisproduces liquids, solids and gases from biomass and waste materials by processing them atmoderate temperatures, typically between 250−550 ◦C, in the absence of oxygen. All theseproducts have potential as fuels; in particular liquid fuels may power internal combustion(IC) engines [30, 31]. Today, pyrolysis is the subject of much research and development[32]. Among the techniques of pyrolysis, fast pyrolysis produces the maximum quantity ofpyrolysis oil from biomass feedstocks such as wood or agricultural wastes. Slow pyrolysisis a centuries-long traditional technique to produce charcoal. Intermediate pyrolysis is arelatively new concept which can be used to produce better quality tar-free oils from a widevariety of biomass or waste feedstocks [33, 34].

Ouadi et al. [10,11,35] recently demonstrated that pyrolysis oils with a low water contentof 3−4% and a higher heating value (HHV) of around 36−37 MJ/kg could be producedthrough intermediate pyrolysis of deinking sludge obtained from both a secondary fibretissue and newsprint mill.

As an example, the test runs with deinking sludge in a mixture with biodiesel aregiven. Further positive test runs have been performed with pyrolysis oils from miscanthus,residue from anaerobic digestion, wood, residues from secondary fuel production, fibresfrom cattle manure, dried pig manure, meat and bone meal, always mixed with biodiesel(Table 7.4–7.5) shows the physical and chemical properties of intermediate pyrolysis oilsproduced from sewage sludge and de-inking sludge in comparison to fossil diesel andbiodiesel. (Table 7.6) shows the specification of the engine used to test these oils as blendswith biodiesel.

7.4.1 Fuel Properties: Deinking Sludge Pyrolysis Oil, Biodiesel, Blends andFossil Diesel

The moisture content and HHV of dried deinking sludge (DS) were approximately 3%(wt.) and 6.4 MJ/kg respectively. Table 7.7 shows physical and chemical properties ofdeinking sludge-pyrolysis oil (DSPO), biodiesel (BD), diesel (FD) and DSPO–BD blends.The viscosity and flash point temperature of the DSPO were approximately 4 times and2.5 times higher than that of fossil diesel. On the other hand, the viscosity of BD wasabout 2.7 times higher than that of fossil diesel. Flash point temperatures of the BD and


Table 7.5 Characterisation of intermediate pyrolysis oils and the ASTM Standards for dieseland biodiesel fuel oils.

Property Unit SSPO DSPOASTM D975

for dieselASTM D6751for biodiesel

HHV MJ/kg 39.38 36.54 45.36 39.65Cetane index (min) 14 40 47Density @ 20 ◦C g/ml 0.9536 0.9819 0.8246a 0.8846a

Kinematic viscosity @ 40 ◦C cSt 38.75 9.60 1.9–4.1 1.9–6.0Surface tension @ 40 ◦C mN/m 29.3 27.9 25.7a 31.7a

Flash point ◦C (min) 150 160 52 130Moisture wt% (max) 4.37% 2.97% 0.05% 0.05%TAN mgKOH/g 19.90 33.03 <0.01 0.8Lubricity (size of wear scar) mm (max) 267 215 520 202a

Copper corrosion 24 h/40 ◦C 4b 3b 1a 1bConradson carbon residue wt% (max) 2.38% 5.26% <0.01a 0.05%Ash content wt% (max) 0.23% 0.16% 0.01% 0.1%

aNot required in the standard, but tested from the fuel oil samples.

DSPO were almost the same. With regard to LHV, Table 7.7 shows only small differencesbetween DSPO and BD. In contrast, LHV of DSPO was lower by about 17% than for fossildiesel. Density, acid number and carbon residue values of DSPO were considerably higherthan those of FD and BD; in the case of DSPO−BD blends these values were decreasedsignificantly (Table 7.7). The carbon content in FD was 7–9% higher than in DSPO and inBD. Nitrogen and sulfur content in the DSPO was much higher than in the BD and in FD(Table 7.7). On the other hand, sulfur content was at trace levels in the DSPO–BD blends.

Table 7.6 Specification of the experimental engine.

Manufacturer Lister Petter (UK)

Model/type LPWS Bio3 water cooledNo. of cylinders 3Bore/stroke (mm) 86/80Rated speed (rpm) 1500Continuous power at rated speed (kW) 9.9Overload power at rated speed (kW) 10.9Type of fuel injection Indirect injection. Self-vent fuel system

with individual fuel-injection pumpsFuel pump injection timing 20◦ BTDCCylinder capacity (litre) 1.395Compression ratio 1:22Minimum full load speed (rpm) 1500Continuous power fuel consumption at 1500 rpm 3.19 litres/h (fossil diesel)Glow plug Combustion-chamber glow plugsExhaust gas flow 41.4 litres/s at full loads at 1500 rpmJacket water flow at full load 33 litres/min (at 1500 rpm)Maximum engine jacket water temperature (◦C) 99–102


Table 7.7 Measured properties of DSPO, BD, DSPO−BD blends and FD.

BD 20% DSPO 30% DSPOPhysical and chemical properties DSPO (B100%) +80% BD + 70% BD FD

Kinematic viscosity (cSt) at 40 ◦C 12.3 8.2 8.91 9.35 3.01Flash point temperature (◦C) 168 170 105 118 68pH value @ 22 ◦C 4.8 7.75 5.91 5.73 7.01Acid number (mg KOH/g) 26.0 0.489 6.74 7.66 0.023Density (kg/m3) @ 22 ◦C 980 890 906 920 832Higher heating value (MJ/kg) 37.04 39.29 38.79 38.58 44.67Lower heating value (MJ/kg) 34.91 36.49 36.22 36.08 41.87Water content (% wt.) 4.00 0.37 0.94 1.70 0.06Carbon residue (% wt.) 3.89 <0.01 0.316 0.518 0.059Ash content (% wt.) <0.02 <0.01 <0.01 <0.01 <0.01Carbon (% wt.) 78.71 77.20 77.15 77.34 84.73Hydrogen (% wt.) 10.08 13.21 12.11 11.80 13.20Nitrogen (% wt.) 1.02 0.10 <0.10 <0.10 <0.10Oxygen (% wt.) 10.08 9.39 10.54 10.66 1.40Sulfur (% wt.) 0.55 <0.10 <0.10 <0.10 <0.10

In the DSPO, hydrogen content was lower by approximately 31% but oxygen content washigher by about 7 times that of FD. Ash content was at trace levels in all fuels. The majorcomponents from the GC-MS analysis of the DSPO were found to be: toluene 4%, ethylbenzene 12%, styrene 28%, phenol 3%, methylethyl/methylethenyl benzene 12% andesters 12%.

The engine test revealed a lower combustion temperature when DSPO-BD blends wereused as a fuel, which caused lower NOx emissions. For 20 and 30% blends and at full load,NOx emissions were decreased by about 12 and 6% respectively compared to FD. In addi-tion, these observations were consistent with the higher density of 30% DSPO–BD blendas well as the presence of water within the fuel (Table 7.7); the higher the density the moreNOx emitted and an increased water content in the fuel lowers combustion temperature,thus reducing NOx. Exhaust temperature is important for poly-generation applications (e.g.CHP, tri-generation). Little difference was observed in exhaust gas temperatures among thefour fuels tested. Smoke levels were similar at low load conditions for all four fuels; but athigher loads, the smoke opacity values of DSPO–BD blends were slightly lower than forFD and BD.

7.4.2 Combustion Characteristics

Smooth engine operation was observed with the 20% DSPO blend; however, the engineexperienced minor knocking when operated on the 30% DSPO blend. The low cetanenumber of the 30% DSPO blend caused this behaviour. The typical cetane number ofpyrolysis oil (PO) has been reported as 5.6; whereas cetane numbers of FD and typicalBD are 47 and 45 respectively [36–38]. Peak cylinder pressures of 30% DSPO blend wereabout 6−13% and 5−6% higher respectively when compared to BD and FD operation.The cylinder pressure profiles for 20% DSPO blend, BD and FD were similar, and onlyminor peak pressure variations were observed. At low loads, integral heat release from


combustion was almost the same for all fuels; and at higher load conditions, the integralheat released by DSPO–BD blends was decreased but the peak burn rates were higher. Atfull load, the peak burn rate of the 30% DSPO blend was about 26 and 12% higher than withFD and BD respectively. Higher peak burn rates in the case of DSPO blends may have beencaused by long ignition delay and short combustion periods. Total combustion duration isdefined as the duration of the crank angle between 5 and 90% combustion. Ignition delayis related to the ignition quality (i.e. cetane number) of the fuel. Compression ratio, enginespeed, cylinder gas pressure, temperature of the air intake and quality of fuel spray affectthe ignition delay period [39]. In the case of DSPO–BD blends the start of combustion wasdelayed compared to FD at most load conditions. Total combustion duration increased withengine load for all fuels. In all load conditions, the combustion duration was shorter for the30% DSPO blend than for FD and BD. For the 20% DSPO blend, the duration was shorterthan for FD operation only at higher load conditions; no significant trends were observedbetween 20% DSPO blend and BD in this respect. At full load, combustion duration ofthe 30% DSPO blend operation was almost 12% lower than for FD. In the case of enginetesting with other types of pyrolysis oils [40,41], long ignition delays and short combustionperiods were also reported. Higher cylinder pressure and high heat release rates of pyrolysisoil combustion are also reported in the literature [42–44]. The fuel injection pressure washigher in the case of DSPO blends; at full load, it was higher by approximately 17% than FD.

7.4.3 Conclusions

A three-cylinder indirect-injection CI engine, with nominal output 9.9 kW, has been testedwith 20 and 30% deinking sludge pyrolysis oil blended with biodiesel. Performance, emis-sions and combustion characteristics were compared against FD and BD (B100) operation.The physical and chemical properties of all four fuels were measured. With DSPO blends,full engine power was achieved. Between the 20 and 30% blends, there were few differencesin the results from the engine tests. However, when compared to FD and BD there were anumber of small but significant differences when using DSPO–BD blends:

(i) At full load, the BSFC was increased by 6% on a volume basis and 14−18% on aweight basis when compared with FD; whereas BSFC was only 4−8% higher on aweight basis when compared to BD operation.

(ii) At full load, brake thermal efficiencies were about 3−6% lower than BD but weresimilar to FD.

(iii) Compared to FD, CO2 and NOx emissions were increased by 4% and decreased by6−12% respectively. At full load, CO emission of 30% DSPO blends was almost 10times lower than FD operation.

(iv) Compared to FD, peak cylinder pressures were about 5–6% higher for 30% blend andwere almost the same for the 20% DSPO blend.

(v) In the case of DSPO–BD blends, the start of combustion was delayed but the burn ratewas high compared to FD. At full load, the peak burn rates of 30% DSPO blend were26 and 12% higher than the FD and BD operation respectively.

(vi) Total combustion duration was decreased for both blends; for 30% blend, at full loadthe duration was decreased by 12% when compared to FD.


The cylinder gas pressure diagrams indicated stable engine operation with 20% DSPOblend, but the engine experienced some minor knocking in the case of 30% DSPO blend.This study concludes that up to 20% DSPO blended with biodiesel can be used successfullywithout adding any ignition additives or surfactants. After three hours of operation no dete-rioration in engine condition was observed. Areas of future work include: (i) long-term teststo assess engine durability; (ii) production of better quality pyrolysis oils by optimisationof the pyrolysis parameters; (iii) engine components and fuel supply modification; and (iv)engine testing using higher DSPO blends mixed with ignition additives. As the propertiesof PO change with temperature, cooling of the inlet fuel may also be investigated for betterengine performance and to preserve the engine life.

Questions

1. What is the working principle of a spark-ignited engine?2. What is the working principal of a dual-fuel engine?3. The total acid number is very relevant to keep the fuel compatible – how is this number

defined?

References

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(2) Yang, Y., Brammer, J. and Hornung, A. (2012) The Characteristics of Intermediate PyrolysisOil Derived from Sewage Sludge in Blends with Biodiesel for Use in Diesel Engines. 19thSymposium on Analytical and Applied Pyrolysis, Pyrolysis Conference, Linz 2012, PyrolysisConference Linz.

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(7) Shihadeh, A.L. (1998) Rural electrification from local resources: Biomass pyrolysis oil com-bustion in a direct injection diesel engine. D. S. Thesis, Massachusetts Institute of Technology.

(8) Ormrod, D. and Webster, A. (2000) Progress in utilization of bio-oil in diesel engines. PyNeNewsletter, 10, 15.

(9) Chiaramonti, D., Oasmaa, A. and Solantausta, Y. Power generation using fast pyrolysis liquidsfrom biomass. Renewable and Sustainable Energy Reviews [Article in Press].

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(12) Samanya, J., Hornung, A., Jones, M. and Vale, P. (2011) The use of intermediate pyrolysisprocess to increase the energy recovery from sewage sludge. 19th European Biomass Conferenceand Exhibition, Berlin, Germany. June 2011.


(13) Ouadi, M., Brammer, G.J. and Hornung, A. (2010) Sustainable energy from paper industrywastes, in Proceedings of the Bioten Conference on Biomass Bioenergy and Biofuels (ed. A.V.Bridgwater), CPL Press, Berkshire, pp. 267–278.

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(15) Torres-Jimenez, E., Dorado, M.P. and Kegl, B. (2011) Experimental investigation on injectioncharacteristics of bioethanol–diesel fuel and bioethanol–biodiesel blends. Fuel, 90(5), 1968–1979.

(16) Torres-Jimenez, E., Svoljsak-Jerman, M., Gregorc, A. et al. (2010) Physical and chemicalproperties of ethanol-diesel fuel blends. Energy Fuels, 24(3), 2002–2009.

(17) Pogorevc, P., Kegl, B. and Skerget, L. (2008) Diesel and biodiesel fuel spray simulations. EnergyFuels, 22(2), 1266–1274.

(18) Ejim, C.E., Fleck, B.A. and Amirfazli, A. (2007) Analytical study for atomization of biodieselsand their blends in a typical injector: Surface tension and viscosity effects. Fuel, 86(10–11),1534–1544.

(19) Bezaire, N., Wadumesthrige, K., Simon Ng, K.Y. and Salley, S.O. (2010) Limitations of the useof cetane index for alternative compression ignition engine fuels. Fuel, 89(12), 3807–3813.

(20) ASTM Standard D189 - 06(2010)e1, “Standard Test Method for Conradson Carbon Residueof Petroleum Products,” ASTM International, West Conshohocken, PA, 2003, DOI: 10.1520/C0033-03A, www.astm.org.

(21) ASTM Standard D86, 2011a, “Standard Test Method for Distillation of Petroleum Products atAtmospheric Pressure,” ASTM International, West Conshohocken, PA, 2003. DOI: 10.1520/C0033-03A, www.astm.org.

(22) Shihadeh, A. (2000) Diesel engine combustion of biomass pyrolysis oils. Energy& Fuels, 14(2),260–274.

(23) ASTM D975, 2011, “Standard Specification for Diesel Fuel Oils,” ASTM International, WestConshohocken, PA, 2003, DOI: 10.1520/C0033-03A, www.astm.org.

(24) Shaine Tuyson, K. Biodiesel handling and use guidelines, NREL/TP-580-30004, Sep2001 http://www.angelfire.com/ks3/go_diesel/files042803/biodiesel_handling.pdf (Accessed08.10.11)

(25) Precision Polymer Engineering Ltd. (2010) Acidity of bio-fuels could pose a storage risk.Sealing Technology, 2010(1), 3.

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(27) Schleicher, T., Werkmeister, R., Russ, W. and Meyer-Pittroff, R. (2009) Microbiological stabilityof biodiesel–diesel-mixtures. Bioresource Technology, 100(2), 724–730.

(28) Oasmaa, A. and Czernik, S. (1999) Fuel oil quality of biomass pyrolysis oils-state of the art forthe end users. Energy Fuels, 13(4), 914–921.

(29) Pollard, A.S., Rover, M.R. and Brown, R.C. (2012) Characterisation of bio-oil recovered asstage fractions with unique chemical and physical properties. Journal of Analytical and AppliedPyrolysis, 93, 129–138.

(30) Barth, T. and Kleinert, M. (2008) Motor fuels from biomass pyrolysis. Chemical Engineering& Technology, 31(5), 773–781.

(31) Jones, S.B., Holladay, J.E., Valkenburg, C. et al. (2009) Production of gasoline and diesel frombiomass via fast pyrolysis, hydrotreating and hydrocracking: a design case. US Dept of Energy,Pacific Northwest National Laboratory.

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(33) Hornung, A., Apfelbacher, A. and Sagi, S. (2011) Intermediate pyrolysis: A sustainable biomass-to-energy concept- Biothermal valorisation of biomass (BtVB) process. Journal of Scientific &Industrial Research, 70, 664–667.

http://www.astm.org

http://www.astm.org

http://www.astm.org

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http://www.chevron.com/products/prodserv/fuels/documents/Diesel_Fuel_Tech_Review.pdf

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8Hydrothermal Liquefaction –

Upgrading

Ursel Hornung,1 Andrea Kruse2 and Gokcen Akgul31Karlsruhe Institut fur technologie – Institut fur Katalyseforschung und–Technologie,

Germany2Universitat Hohenheim, Institut fur Agrartechnik, Konversionstechnologie und

Systembewertung nachwachsender Rohstoffe, Germany3Department of Energy Systems Engineering, Recep Tayyip Erdogan University, Turkey

8.1 Introduction

The majority of biomass feedstock available is ‘wet’, with water content of up to 95%.Hydrothermal processes open up the opportunity to convert biomass with a naturally highwater content. Hydrothermal means conversion in water, usually at increased temperatureand pressure. The water included in the biomass becomes the reaction medium duringthe degradation of the polymeric structure of the biomass. Additionally, biomass is veryreactive in water as the reaction medium. The polar bonds are strongly attacked by the polarwater molecules, which are extraordinary aggressive at increased temperature and pressure.Therefore, the hemicellulose and cellulose are hydrolysed quickly. Additionally the degra-dation products of biomass are soluble in hot compressed water. This avoids in many casesthe formation of an additional liquid phase, usually droplets. In droplets, reactive moleculesare close and can easily react with each other. Therefore, polymerisation is suppressed byhigh solubility as in hydrothermal conversions. This is especially true compared with ‘dry’biomass conversion, in which no solvents are present. In conclusion, the consequence




of the activity of water as reactant, forcing hydrolysis, and solvent is that hydrothermalliquefaction is possible without the formation of any solid, such as char or coke.

The best-known method for hydrothermal liquefaction is the so-called ‘hydrothermalupgrading’ or HTU® process originally developed by shell. At 300–350 ◦C and 15–20 MPa, biomass is converted into gaseous products and oil within 5–15 min. Wood chips,organic waste or sewage sludge are converted into about 50% (g/g) black and tarry oil(so-called ‘biocrude’), 30% (g/g) gas (of this >90% CO2), 15% (g/g) water, as well as 5%(g/g) dissolved organic substances. (The word ‘oil’ is common here but may be misleading:usually this organic material starts flowing above 80 ◦C.) Thermal efficiency of this processranges between 70 and 90%. The exergy efficiency is estimated to amount to more than50% [1].

8.1.1 Product Properties

The oil produced contains about 10–15% (g/g) oxygen only, while the biomass appliedtypically has an oxygen content of about 40–50% (g/g). Accordingly, the heating value isrelatively high (higher heating value: 30–35 MJ/kg, Table 8.1). The lower oxygen contentis a consequence of the different composition of hydrothermal biocrude compared withpyrolysis oils (see Figure 8.1).

The comparison in Figure 8.1 shows that the composition analysed varies a great deal.In spite of this it can be stated that in HTU-biocrude the content of polar compounds likeacids and sugars is always lower than in pyrolysis oils and the phenols dominate more.

One reason is that the special properties of hot compressed water support the eliminationof water and carbon dioxide. These two oxygen-loss reactions lead to compounds of loweroxygen content (see e.g. [7–9]). The differences concerning the composition are not onlythe consequence of different chemical reactions but also of the solubility of higher polarcompounds like acids in water. Of the carbon originally fed into the system, in the rangeof 5–20% [10–12] (depending on the reaction conditions) is converted to soluble organiccompounds.

The low content of high polar compounds in the oil is the reason for the low water contentof hydrothermal oil and other differences to pyrolysis oils (Table 8.1).

Table 8.1 Properties of pyrolysisa and hydrothermal ‘oil’ from woody biomass (taken from[6]; ranges are given, because of various reaction conditions and feedstock) [6].

Fast pyrolysis oila Hydrothermal biocrude

Water content / %(g/g) 15–30 [13,14] <0.2 [15]Density / (kg/m3) ∼ 1200–1300 [13]

1140 [16]∼ 820–845 [15]

Viscosity / Pas 0.02–0.1 [13]0.035–0.055 [16]

0.75–1b# [17]

Higher heating value / (MJ/kg) 16–19 [14] 30–36 [12]

a Some pyrolysis plants deliver two liquid products: an organic phase and a water phase. In this case the water content islower and the heating value higher.bFeedstock here is cellulose.

Hydrothermal Liquefaction – Upgrading 177

0PyA PyB HTA HTB

Organic Acids

Esters

Alcohols

Ketons

Aldehydes

Phenols

Sugars

Furans

Others

10

20Po

rtio

n (

Are

a %

)

30

40

50

60

Figure 8.1 Compound classes found in pyrolysis oils and HTU biocrude. (Data sources: PyA:pyrolysis oil average data from [2], p. 140, PyB: pyrolysis oil data from [3], HTA: hydrothermalbiocrude data from [4], HTB: hydrothermal biocrude data from [5], Diagram taken from [6].)

8.2 Chemistry of Hydrothermal Liquefaction

As mentioned above, the chemistry of hydrothermal biomass conversion is strongly con-nected with the special properties of hot compressed water [7]. Water as a polar compound,as well as the ions formed by water particularly, attack polar bonds rather than the less-polar.Therefore the degradation of the polar carbohydrates (starch, cellulose, hemicellulose etc.)is more strongly influenced and therefore faster than the degradation of the less-polar lignin.

8.3 Hydrothermal Liquefaction of Carbohydrates

The first step in the reaction of biomass in hot water appears to be the rather fast hydrol-ysis of cellulose to glucose [18–20] (for a more detailed explanation [21, 22]), and thisconsequently seems to be the most important difference between hydrothermal and ‘dry’biomass conversion [23]. In biomass conversion without water as a solvent and reactant thecarbohydrates ‘depolymerise’, which requires higher temperatures. Glucose, as a buildingblock of cellulose, is often used as a model component for biomass, in particular becausethe splitting of cellulose into glucose is well known. Glucose partially converts to fructose,while levoglucosan (from glucose [24]) or hydroxymethylfurfural (HMF – from fructose[25–27]) is produced by water elimination (Figure 8.2). On first view, it is surprising thatthe elimination of water is a supported reaction in water. The reason is the larger ther-modynamic stability of double bonds at increased temperature and the acceleration of thereaction rate by the increased content of H+ ions catalysing this reaction [8].

HMF further reacts to levulinic acid and formic acid [28]. This is a typical acid catalysedreaction with a rather high rate in hot compressed water also in the absence of an addedacid. Formic acid decomposes into H2 and CO2 mainly.


Glycolaldehyde

O C CH2

– H2O

COOHH3C

CO2 CH4+

O

CH2OH

OH

OHHO

HO

CHO

CH2OH+

CHO

CH2OH

CHO

CH2OH+

O CH2OH

OH

HOH2C

OHHO

O CHOHOH2C

O

OHHO

HO

CH2

O

CHO

OHH

CH2OH

CH2OH

O

CH2OH

COOH

O

O

OHHO

HO

O

CH3

O

H

+ HCOOH

CO2

+

H2

OH

OH

OH

– 3H2O

– H2O

– H2O

COOH

OH

CH3

H

+H2O

C2H5OH

+

CO2

H2O–

FructoseGlucose

Levoglucosan

Erythrose

Acetic acid

HMF

Levolinic acid

Formic acid

Dihydroxy-acetone

Pyruv-aldehyde

Lactic acid

Figure 8.2 Selection of reaction pathways starting with glucose, formed by the hydrothermalhydrolysis of cellulose (modified from [29]). Reprinted with permission from [29], Copyright ©2008 Society of Chemical Industry and John Wiley & Sons, Ltd.

Through aldol-splitting, smaller compounds are produced from glucose and fructose[9, 30–34]. Aldol-reactions usually need a strong base to proceed. At conditions likethose in hydrothermal liquefaction, water alone is sufficient as a catalyst. Glucose formserythrose and glycolaldehyde and fructose forms two compounds with three carbon atoms,glycerolaldehyde and dihydroxyacetone. Water elimination of these two isomers yieldspyruvaldehyde, and after addition of water lactic acid is formed. The lactic acid yield isincreased by catalysts [35–40]. Erythrose splits to glycolaldehyde by aldol-splitting. Theformation pathway of acetic acid is not clear. One possibility is via a ketene formed fromglycolaldehyde via water elimination, as shown in Figure 8.2. Acetic acid splits to methaneand CO2 [41, 42]. Phenols are the main degradation product of lignin in the hydrothermalconversion of biomass [43]. Phenols can also be formed from carbohydrates, for exampleglucose, by rearrangement from HMF [44], as shown in Figure 8.2, or by reaction and ringformation by the Diels–Alder reaction of two smaller compounds [8, 45, 46]. Diels–Alderreactions are supported by pressure and have been studied in hot compressed water bydifferent research groups [8].


The reactions compiled in Figure 8.2 are reactions in the liquid phase. Near the criticalpoint, however, free radical reaction pathways also occur to a larger extent [47,48]. Reactionproducts from proteins and lignin, and also possible biomass ingredients, lead to the forma-tion of free-radical scavengers. This means that the reaction rate of gasification is reducedby the formation of free radicals of lower reactivity. In other words, these ingredients forcethe liquefaction by suppressing the gasification at low reaction times [49–51].

8.4 Hydrothermal Liquefaction of Lignin

Lignocellulosic biomass is one of the most abundant renewable organic materials in theworld. Lignin, a major compound of lignocellulosic biomass, is mostly available as wastematerial. The paper industry worldwide produces more than 50 million tons of dry ligninevery year [52]. It is mainly burned to recover its energetic value. Lignin has a structuresimilar to brown coal, being an aromatic heteropolymer. The three basic building blocks,p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, are interlinked by C–C or etherbonds (see Figure 8.3). As lignin is relatively resistant to chemical or enzymatic degradation,harsh reaction conditions are required to break down the polymer. Aromatic monomers areformed by cleavage of the ether bonds.

Thus, lignin provides high potential to serve as a renewable source for phenol or benzene.Phenol is extremely interesting as a building block of synthetic polymers, resins or epoxy-or polyurethane. It is, however, a challenge to get a high-value product from a chemicallycomplicated and inhomogeneous component such as lignin. Char formation, for instanceby repolymerisation of (di-) hydroxybenzene as it occurs during liquefaction of coal [53],is one of these challenges. A broad product spectrum is observed in many experimentalapproaches to liquefaction of lignin, which requires cost-intensive separation. There havebeen attempts to avoid char formation through the addition of phenol and phenol relatedcompounds, such as para-cresol [54, 55].

Fang et al. [56] proposed a mechanism for the reaction path of lignin under hydrother-mal conditions as shown in Figure 8.4. Dissolved products (e.g. lignin, oligomers andmonomers) and non-dissolved lignin had different reaction mechanisms in four phases[56], which were:

• Oil phase (phenolics, polycyclic aromatics hydrocarbons (HCs) and heavy HCs).

• Aqueous phase (acids, aldehydes, alcohols, catechol and phenols).

OH

OH

(A)

OH

OHO O

(B)

OH

OHO

(C)

Figure 8.3 The three characteristic monolignols which constitute the characteristicmonomeric units of lignin: A: para-coumaryl alcohol, B: sinapyl alcohol, and C: coniferylalcohol.


Single- andMultiring Phenolics

Decomposed LigninAromaticNetwork Polymer

Water-soluble products(Methanol, Acids, Aldehydes)

Phenolic char Polyaromatic char

Residue

Dissolution

Repolymer

Phenolics andAldehydes

C–C-bondedOligomers Dealkylation

Dealkylation

CleavageAqueos phase

Acids, Aldehydes, Alcohols,Catechol, Phenols

Methanol, HCs, AcidsAldehydes and Catechol

Phenols andAromatics

Oil phase

Phenolics, PAHs,heavy HCs

Gas

Dissolution

Dissolution

Dissolution

DissolutionHydrolysis

Hydrolysis HydrolysisMonomers, Ligninand Oligomers

Syringols, Guaiacolsand Catechols

AromaticsPyrolysis

Heterogeneous

Homogenous

HCs and Gases

(H2, CO, CO2, HCs)

CO2, CO, H2C1–C2 HCs

Lignin

Figure 8.4 Reaction path of lignin decomposition promoted in homogenous and heteroge-nous environments in supercritical water. Reprinted from [56] with kind permission from Else-vier.

• Gas phase (CO2, CO, H2 and C1-2 HCs).

• Solid residue phase.

The dissolved samples with ether linkages were relatively quickly homogeneouslyhydrolysed to single ring phenolic compounds (syringols, guaiacols, catechols and phe-nols). Syringols and guaiacols in the oily phase were further hydrolysed and dealkylatedinto aqueous products [57]. Catechols were decomposed to phenols and aromatics. In con-currence with the fast hydrolysation, cleavage of the more stable C–C bonds in lignin andoligomers takes place to form single and multi-ring phenolics in more severe reaction con-ditions, such as higher temperatures or longer reaction times. Dealkylation reactions lead tothe formation of gas, hydrocarbons and solved products (alcohols, aldehydes and acids). Athigher temperatures and long reaction times, phenolics (monomers and oligomers) repoly-merise with aldehydes to form heavier crosslinked phenolic char [58], which precipitatesas a residue.

The non-dissolved samples probably underwent pyrolysis to yield gas, HCs, a mixture ofphenolics, water-soluble products (methanol, acids and aldehydes) and decomposed ligninvia free-radical and concerted mechanisms or acid-catalysed decomposition [58].

For maximising selectivity and yield of valuable phenolic products by optimising thereaction parameters, it is indispensable to have a good understanding of reaction mecha-nisms and the kinetics of the most important reactions occurring within the hydrothermaldepolymerisation of lignin.


Reaction kinetic studies under hydrothermal conditions have been performed both basedon lignin [59], and lignin derived model compounds, that is, guaiacol [60] and catechol [61].Yong et al. [62] studied the reaction kinetics of lignin depolymerisation in supercriticalwater (SCW) in a continuous reactor considering both global bulk components and singlephenolic components. The reported loadings were rather low (0.1 wt%). Some of theconsidered reactions did not conform to Arrhenius characteristics. Furthermore, Gassonet al. [63] developed a formal kinetic model of the lignin degradation in an ethanol/formicacid approach in a batch reactor and could validate that model on experiments by means ofa continuous-mode stirred-tank reactor operating at different temperatures [64]. Forchheimet al. [65] adjusted that model to hydrothermal conditions and excluded all reactionsconcerning the solvent ethanol for analogous experimental conditions in the same batchapproach. The focus of this model was on the description of the formation and degradationof the singe aromatic compounds as key products within the hydrothermal degradation oflignin (Figure 8.5), but describes the gas and char formation too.

The reaction model comprised 10 defined (lumped, i.e. summarised) components and15 reaction pathways. For each reaction pathway, a rate coefficient is defined. The reactionorder for all reactions is set to one. The rate coefficients are determined by searching theminimum of deviation between the measured yields and the calculated ones by a simplexsearch method via Matlab V7 [65]. Table 8.2 gives the apparent kinetic parameters of thatwork. The optimisation considers the formal rate coefficients of the formation of char (k5)and LD2 (k12) from catechols to be negligable. For all other reaction pathways, sensiblevalues of activation energy and frequency factor are calculated. The dominating primarypathway of lignin degradation is the formation of LD1, since the resulting formal ratecoefficient k7 is about 100 times larger than the formal rate coefficient of the formation ofmethoxyphenols k1 for all considered temperatures.

LigninPhOH

Char

Phenols

Reactiveintermediates

Solid residue,

H2O-insoluble

Stable interme-

diates, partially

H2O-soluble, not

detectable via

GC-FID

Methoxy-PhOH

Catechols

Char

PhOH

k1

k3

k4

k6

k5

k2

k10

LD1

H2CH4

H2, H2OH2, H2O

CO, H2

CO2, CH4

H2O

H2CO

k8 k9

k11

k15

k14 k13

k12

LD2

LD2

LD1

CO2, C

CO2CO

H4

k7

Figure 8.5 Scheme of the main reaction participants and pathways of the hydrothermaldepolymerisation of SEKAB-lignin [65]; gaseous components take part in reaction 2–6 andin 13. This is indicated by curved arrows.


Table 8.2 Apparent activation energies EA and Arrhenius factors for the 15 reactions definedin the model (see Figure 8.3) and rate coefficients at different temperatures.

EA log(A/A′) k(593 k) k(613 k) k(633 k) k(653 k)kJ/mol A′ = 1 min−1 min−1 min−1 min−1 min−1

58 1.4 1.91E-04 2.81E-04 4.02E-04 5.63E-04101 6.4 2.64E-03 5.16E-03 9.68E-03 1.75E-02

94 5.0 5.15E-04 9.61E-04 1.72E-03 2.98E-0355 1.0 1.34E-04 1.93E-04 2.72E-04 3.76E-04

3037 3.2 >10E-06 >10E-06 >10E-04 >10E-0666 1.0 1.84E-05 2.85E-05 4.27E-05 6.26E-0532 1.2 2.38E-02 2.94E-02 3.58E-02 4.31E-0281 4.6 3.52E-03 6.01E-03 9.90E-03 1.58E-0258 2.6 2.89E-03 4.25E-03 6.09E-03 8.53E-0372 2.5 1.55E-04 2.49E-04 3.89E-04 5.90E-0475 3.8 1.45E-03 2.39E-03 3.82E-03 5.92E-03

296 5.0 9.72E-22 6.88E-21 4.30E-20 2.40E-1948 1.6 1.92E-03 2.65E-03 3.58E-03 4.75E-0342 2.2 2.71E-02 3.59E-02 4.68E-02 5.99E-0269 2.8 4.75E-04 7.52E-04 1.16E-03 1.73E-03

In order to compare the total lignin conversion of the presented reaction model (ref.Figure 8.5) with the literature, it is necessary to sum up the two reaction pathways of thelignin decomposition (k1 and k7) which is given with k1,7 = 3.6 10−2min−1 for temperaturesof 593 to 653 K. This value, compared with other values from literature for a solovolyticdecomposition of lignin, such as those from Takami [66] and Gasson [64] of 3.0 10−2min−1

and 4.8 10−2min−1 for temperatures of 623 to 693 K, looks reliable.In terms of the decomposition of the methoxyphenols, the sum of the pathways k2 and

k3 is required (ref. Figure 8.5) and a total reaction rate of 1.1 10−2 min−1 (Ea = 85 kJ/mol,log(A/A′) = 5) is very comparable with the reaction rate of the solvolytic degradation ofmethoxyphenol (guaiacol) of 1.8 10−2 min−1 in [60].

Finally, the decomposition of catechols with the sum of the pathways k4, k5 and k12 isrequired (ref. Figure 8.5) with a total reaction rate of 2.9 10−4 min−1 (Ea = 55 kJ/mol,log(A/A′) = 1) is very comparable with the reaction rate of the solvolytic degradation ofcatechols of 2.4 10−4 min−1 (Ea = 51 kJ/mol, log(A/A′) = 0.6) in [61].

The good agreement of one of the calculated rate coefficients with those found inliterature for similar reactions show that the model is not only a mathematical method forthe simulation of experimentally determined values, but is also sensible with respect to thechemistry of lignin degradation. Therefore, the model can be used as a tool for processdesign of hydrothermal lignin liquefaction.

8.5 Technical Application

The high amount of water contaminated with organic compounds is a challenge ofhydrothermal liquefaction process development. Many studies focus on the goal of finding


catalysts to increase the oil yield and to decrease the amount of organic compounds dis-solved in the water. In most cases, alkali hydroxides, hydrogen carbonates and carbonatesare used as catalysts (e.g. [4]). Here, not only the oil yield is increased but also the com-position of the organic phase is changed by increasing the amount of phenols formed. Inaddition, heterogeneous catalysts like Co3O4 are found to increase the oil yield [67].

The process was also demonstrated on a larger scale: a hydrothermal upgrading pilotplant of 120 kg/h capacity has been successfully operated by TNO-MEP in Apeldoorn, theNetherlands [10].

The more narrow composition range of HTU biocrude (Figure 8.1), the absence ofstrongly polar compounds (Figure 8.1), and connected with this the lower water content(Table 8.1), makes it very interesting for upgrading processes like hydrogenation anddeoxygenation to produce fuel. In the Netherlands, a large project has started to producesuch a fuel, so-called HTU-diesel [12].

A similar process, called STORS (Sludge to Oil Reactor System), is used for convertingsewage sludge into oil and coke. Typically, about 44% of the initial carbon exists in the oil,20% in the coke, 16% in the gas and 20% in the aqueous solution [68].

8.6 Conclusion

Hydrothermal liquefaction is a well-demonstrated technology that has not been used forproduction up to now. The main hurdle is the high amount of aqueous product formed. Thisprocess water includes a certain content of organic compounds, like phenols. Treatmentin a sewage plant is relatively expensive leading to the lower financial attractiveness ofhydrothermal liquefaction. Here, new solutions to use the organic content in the water arenecessary. A possibility would be to convert the solution by aqueous phase reforming [69]or other processes [70, 71] for example, to hydrogen. This hydrogen can then be used forthe production of HTU-Diesel.

Questions

1. Why are polar molecules influenced more strongly from water at hydrothermal condi-tions than less-polar molecules?

2. Which model compounds are used to reveal the hydrothermal behaviour of variousbiomasses?

3. What are the mechanisms of model compounds in hydrothermal water?4. How can the complex reaction network be solved if there are numerous consecutive and

parallel reactions during hydrothermal liquefaction of biomass?5. Is hydrothermal liquefaction technically applicable?

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9Supercritical Conversion of Biomass

Gokcen AkgulDepartment of Energy Systems Engineering, Recep Tayyip Erdogan University, Turkey

The focus of this chapter is biomass conversion in the supercritical water. The two mainprocesses of the supercritical conversion of biomass are the supercritical water gasificationand the supercritical water oxidation. Some issues related to the supercritical water gasi-fication e.g. the water-gas shift reaction under the supercritical conditions, catalysts in thesupercritical processes and corrosion of the supercritical water, are also included.

9.1 Introduction

Biomass can be converted to high value-added fuel products through different technologies.However, gasification under hydrothermal conditions has a special importance in convertingwet biomass without the need for drying. While the high water content of biomass is mostlyundesirable for dry biomass processes such as pyrolysis, it is beneficial for hydrothermalgasification.

The term ‘hydrothermal’ is used to describe water at higher temperatures and pressures.Specifically, the sub- and supercritical water refer to the state of water below and aboveits critical temperature and pressure (Tc = 374 ◦C, Pc = 22 MPa). The thermophysicalproperties of water are so different at higher temperatures and pressures from those atambient conditions that water can be almost considered a different fluid. For example, thehydrogen bonds per water molecule gradually decrease as the process advances towardssupercritical conditions. Modelling and experimental results show that 1.8–2.2 are possibleaverage numbers of the hydrogen bond per water molecule at 380 ◦C and 0.73 g/cm3 density[1] (this number is 4.0 at ambient conditions). The hydrogen bonding is not completely




solid and liquidproducts

%

H2

CO, CH4, ...

CO2

tar

organic acids

gas products%

Scheme 9.1 Products of supercritical water processes.

destroyed but becomes weaker and less persistent under supercritical conditions. Waterloses its liquid characteristic properties and becomes a good solvent for organic molecules[2, 3]. Fewer mass-transfer limitations and higher temperatures in the supercritical watermedium advance the organic reactions along faster and more efficient pathways.

The principle of the supercritical water processes is to heat up wet biomass over criticalpoints of water. Once wet biomass is subject to the supercritical water processes, thecompounds in the biomass mostly move into the gas phase. The resulting gas containsmostly hydrogen, carbon monoxide, carbon dioxide and methane. Gasification is a morefavourable pathway than liquefaction under the supercritical conditions (Scheme 9.1).

Kruse et al. [4] gasified a chopped mixture of carrots and potatoes at 500 ◦C and atpressures between 30 and 50 MPa with a dry matter content of 1.8–5.4 wt% (the rest iswater). The biomass is converted mainly to gaseous and liquid products. The gaseous phasecontains hydrogen as the major component while the aqueous phase consists of variousorganic molecules, such as aldehydes, carboxylic acids, alcohols and a black precipitatethat consists of furfurals and phenols.

Currently, development in the supercritical gasification technologies is directed towardsthe production of hydrogen. Although there are different methods of hydrogen produc-tion from biomass (thermal gasification, microbial conversion) [5, 6], hydrogen from thesupercritical water conversion of biomass could be one viable solution to demand for theproduction of clean energy.

Supercritical water processes can be divided into two main processes: (i) the supercriticalwater gasification, and (ii) the supercritical water oxidation (Scheme 9.2).

9.2 Supercritical Water Gasification

Supercritical water gasification is an innovative hydrothermal process to convert aqueousorganic wastes or wet biomass directly into valuable gaseous products such as hydrogenand methane. The gaseous products are suitable for energy generation purposes.

Supercritical Conversion of Biomass 191

Sub-critical water

processes

Tc (374 °C)

Pc (22 MPa)Supercritical water

processes

Supercritical water

gasification

Supercritical water

oxidation

Hydrothermal

processes

Scheme 9.2 Hydrothermal processes.

Fossil fuel reserves are depleting and clean and renewable energy sources have becomeimportant topics of research over the last decades. Biomass is one such alternative energysource which may have the potential to contribute to future energy supplies.

With regard to clean sources of hydrocarbons for future generations, the production ofhydrogen from biomass has considerable potential [7].

Azadia et al. [8] gasified several lignocellulosic materials in the supercritical water andobtained hydrogen-rich gas as a product. Mobius et al. [9] converted aqueous pyroligneousacid – a liquid byproduct of charcoal production – into hydrogen and C-containing gasproducts such as carbon monoxide and methane.

Biomass consists of many different compounds (cellulose, hemicellulose, lignin, protein,lipid etc.) and the chemistry of biomass degradation is rather complex since all compoundsreact in different ways under hydrothermal conditions. For understanding the mechanismof the supercritical water gasification, basic model compounds are gasified. Williams andOnwudilli [10] reported sub- and the supercritical water gasification of cellulose, starch andglucose as representative biomass model compounds. The main product of the supercriticalwater treatment of cellulose is a gas composed of hydrogen, carbon monoxide, carbondioxide, methane and C2–C4 hydrocarbons. It was shown that an oily liquid product, awater-soluble product and char were formed by subcritical gasification of glucose, whilethe gaseous products ratio increased under the supercritical conditions.

Cellulose is the main constituent of many biomass types and glucose is the modelcompound for cellulose. Hao et al. [11] reported that glucose is gasified to hydrogen-richgas in the supercritical water according to the following reaction without formation of anychar or tar:

C6H12O6 + 6H2O → 6CO2 + 12H2

The pathway of the supercritical water conversion of cellulose is summarised in [12].Cellulose first hydrolyses to monomer sugars, which convert to furfurals, phenols, aldehy-des, ketones or other small organic molecules (organic acids such as acetic acid) by various


complex reactions. While many of these organic molecules gasify further to gases, sometrace amounts convert to char, tar or coke as final products. The composition of solid, liquidand gaseous products from the supercritical water gasification of cellulosic materials orglucose is studied in detail in [13].

Gasification of xylose as the principal sugar in hemicellulose, and phenol as a modelcompound for elucidating the chemistry of lignin, are investigated in [10]. The gas phaseconsists of mostly hydrogen and carbon monoxide. Further, hydrogen and methane can beobtained by water–gas shift and methanation reactions, respectively.

Protein is another ingredient of biomass. The model substances for proteins investigatedin this study are phenol and glycine. These molecules are rather resistant to the supercriticalconditions. Dielo et al. [14] showed that carbon monoxide is the most abundant gas of thesupercritical water gasification of phenol and glycine. Glycine was much more resistantto gasification than phenol. It is concluded that gasification of these molecules as modelsubstances of protein’ requires more severe conditions.

Kruse et al. [15] investigated the alanin effect on hydrothermal gasification of a mixtureof glucose and alkali salts as a model system for biomass. A lower gas yield was obtainedin the presence of alanine, showing that the presence of protein in biomass obstructs thesupercritical water gasification.

Supercritical water gasification of lipids and proteins starts with hydrolysis to fattyacids, glycerol and amino acids. Youssef et al. [16] studied the gasification of oleic acid asa model substance for fatty acids. Oleic acid decomposes theoretically to gaseous productsaccording to the following reaction:

CH3(CH2)7CH = CH(CH2)7COOH + 16H2O → 18CO + 33H2

The gaseous phase includes methane and carbon dioxide additional to carbon monoxideand hydrogen. It is commented that the generated carbon monoxide reacts further withwater or hydrogen and produces more hydrogen or methane, respectively. Furthermore, thegas yield increases with increasing temperature and use of catalysts. The components in theresidual liquid fraction comprise carboxylic acids and saturated fatty acids, fatty acid esters,alkenes, fatty alcohols, n-alkanols, and some cyclo-compounds. At higher temperatures,the distribution of liquid products shifts towards aromatic and cyclo-compounds, such asisomers of xylene and cyclododecanol.

It was shown that glycerol from hydrolysis of lipids can be completely gasified to hydro-gen, carbon dioxide, methane and small amounts of carbon monoxide by the supercriticalwater gasification [17]. At lower temperatures, water-soluble products such as acetaldehyde,acetic acid, hydroxyacetone and acrolein are obtained [18]. Organic complex molecules inliquid phase are the result of complex reactions, such as water eliminaton, aldol splitting,Cannizaro type reactions or condensation reactions [19].

The gas content changes with the temperature and pressure employed in the supercriticalwater gasification processes. When the temperature and pressure are lower than moderateconditions, water has a higher density and methane is the preferred product. The molefraction of hydrogen increases when water density decreases [20]. The temperature shouldbe higher and pressure should be lower to obtain a high hydrogen yield.

In summary, the supercritical water conversion of biomass starts with hydrolysis andcontinues with complex reactions to the final gaseous products (Scheme 9.3).


Wet biomass Hydrolysis

Complexreactions

• Decarboxylation

• Aldol splitting

• Diels–Alder cyclo-addition

• Water–gas shift

• Reforming

• ...

Final products

• H2

• CO

• CO2

• CH4

• ...

Scheme 9.3 Flow of supercritical water gasification.

9.3 Supercritical Water Oxidation

Complete supercritical water gasification of some organic materials such as lignin, proteinand phenol is still difficult under the supercritical conditions. Such rigid molecules can bebetter gasified by the supercritical water oxidation [21] where oxygen is added into thereaction medium. In the presence of oxygen, partial or complete oxidation reactions occurrapidly, facilitated by high reaction temperatures and fewer diffusion restrictions.

Under the supercritical conditions, organic compounds, oxygen and water form a singlehomogenous phase. The miscibility with oxygen allows organic compounds to be easilyoxidised and degraded. For example, ammonia and methanol are relatively stable com-pounds in the supercritical water and require high temperatures, longer residence timesor catalysts for better gasification. Supercritical water oxidation has been shown to be aneffective technology for destroying such molecules and promotes faster reaction rates, rapidand complete oxidation [22, 23].

Supercritical water oxidation is used particularly for degradation of harmful and toxicmaterials. For instance, ion exchange resins (IEFs) used in water treatment systems innuclear power plants for radionuclide removal are destroyed by this process [24].

Unlike conventional thermal incineration, the formation of additional pollutants, such asSOx or NOx gases, is avoided by the supercritical water oxidation.

9.4 Water–Gas Shift Reaction under the Supercritical Conditions

Supercritical water gasification of biomass to gaseous products consists of a series ofcomplex reactions. The water–gas shift reaction (WGSR) is one of them.

The development of the hydrothermal gasification of biomass increases the interest inthe WGSR. It is reported that the hydrogen yield is mostly the result of the water–gas shiftreaction; the reaction of carbon monoxide and water [25].

CO + H2O → CO2 + H2

The WGSR was reported first in 1780 by Felice Fontana but its technical importance wasonly recognised by coal gasification and the Haber-Bosch process. In the first ammoniaplant, carbon monoxide in the water–gas was removed by scrubbing the gas in a hot causticsoda solution. However, the carbon monoxide removal was not suitable for use on a large


scale. Thus, a catalytic process was developed to convert carbon monoxide to carbon dioxidewith WGSR. However, carbon monoxide poisons the catalysts very quickly.

High temperatures are required for efficient conversion of carbon monoxide by theWGSR but the WGSR is an exothermic reaction and the equilibrium constant decreaseswith increasing temperature [26].

CO + H2O → CO2 + H2 + 41 kJ ⋅ mol−1

However, enhancement of the hydrogen yield by the WGSR is possible at higher tem-peratures in homogeneous reaction mediums [27, 28]. Supercritical water in particular hassignificant potential to improve the WGSR [29].

Carbon monoxide is of special interest in the supercritical water oxidation process.Oxidation of carbon monoxide in supercritical water was investigated by Holgate et al. [30,31]. They showed that carbon monoxide was oxidised more slowly than other carbon com-pounds such as methane or methanol. The carbon monoxide conversion limits the overallconversion of organic molecules to carbon dioxide. The WGSR is still the key reaction inthe presence of an oxidising agent in the reaction medium under supercritical conditions.

There are still disagreements and contradictions over the reaction mechanism and kineticsof the WGSR under hydrothermal conditions. The WGSR does not proceed in a single stepbut via intermediate reactions. Akgul et al. [27, 28, 32] reported that formic acid forms byreaction of carbon monoxide with water in the first step of the WGSR:

CO + H2O → HCOOH

However, formic acid is an intermediate product and starts to decompose with increasingtemperature. The decomposition of formic acid consists of two parallel pathways, whichare dehydration and decarboxylation:

HCOOH → CO + H2O, Dehydration

HCOOH → CO2 + H2, Decarboxylation

It is proposed that formic acid is converted completely to carbon dioxide under supercrit-ical conditions by decarboxylation [33]. The decarboxylation reaction rate is much fasterunder supercritical conditions than subcritical ones.

9.5 Catalysts in the Supercritical Processes

The studies on catalysts for the WGSR and supercritical water gasification processes havefocused primarily on heterogeneous catalysts [34, 35, 36]. However, recent studies havedeal with homogeneous catalysts, for example potassium carbonate and sodium carbonate,to secure high yields of hydrogen and enhance the gasification efficiency. It was shownthat the addition of alkali metal salts to the reaction medium enhances the supercriticalgasification efficiency and suppresses the formation of tar. Minowa et al. [37] reported oncellulose decomposition in hot compressed water (200–350 ◦C, 3 MPa) in the presenceof sodium carbonate. The cellulose decomposition starts at lower temperatures comparedwith the decomposition temperatures when no catalysts are used. In other studies, it wasdetermined that hydrothermal gasification of glucose results in greater gas yields and lesschar when dilute glucose solutions contain alkali salts [38, 39].


The WGSR also proceeds more effectively in the presence of alkali salts. The reason forthe enhancement of the WGSR in the presence of alkali salts has not been well clarifiedthus far. However, some reasons have been given:

• the hydrolysis of salts causes higher pH values in the medium which promotes the carbonmonoxide conversion [40];

• the salts influence the dielectric constant of water and change the properties of water[41];

• the ionic strength of the solute could be different in the presence of salts;

• the fugacity coefficients of gases change in the presence of salts.

Research on alkali salts and their effects on gasification is meaningful because biomasshas inherent inorganic compounds, such as Na, K, P, S, Cl, Ca, Mg, Fe [42], remainingas salts in the biomass composition. These salts could have a catalytic action during thesupercritical conversion and on the WGSR.

9.5.1 Alkali Salts in the Supercritical Water

The most important question about alkali salts as homogeneous catalysts used in the super-critical water gasification of biomass is their solubility. At sufficiently high temperatures,the dielectric constant of water decreases. Thus the ions of salts will tend to accumulateand drop out of the solution. This is a problem for homogeneity in the reaction mediumand corrosion in reactors [43, 44]. Moreover, the dropped salts can cause plugging in thereactors.

The solubility of salts increases with temperature at first but at sufficiently high tempera-tures, as the degree of hydrogen bonding and the dielectric constant of water decrease, waterbehaves more like a non-polar solvent and salts drop out of the solution. Ho and Palmer [45]calculated the ion association constant of the aqueous sodium hydroxide solutions by mea-suring the electrical conductance at 300 MPa and the temperature between 100 and 600 ◦C.The conductance first reaches a maximum value with increasing temperature, but above300 ◦C it becomes independent of temperature. At constant temperatures, electrical con-ductivity increases with increasing pressure because the density of water becomes higherat higher pressures; the ions are strongly hydrated and the solubility of salts increases.

The solubility of sodium chloride was found to be 1.5 ppm at 450 ◦C and 10 MPa, but63.6 ppm at the higher pressure of 20 MPa with the same temperature [46].

According to the miscibility of salt with water under hydrothermal conditions, salts areclassified into two types; Type 1 and Type 2 (Scheme 9.4) [44, 46, 47]. The electrolytesalts in Type 1 are soluble in the vicinity of the critical point. However, Type 1 salts canprecipitate if the water density decreases. In the Type 2 group, the salts do not dissolvereadily in high temperature high pressure water.

9.6 The Solubilities of Gases in the Supercritical Water

The solubility of gases in water at ambient conditions decreases with increasing temperature.In a closed vessel, the solubility of gases increases after reaching a minimum point at highertemperatures and the gases become completely miscible with water under the supercriticalconditions.


Saltwater binary system

Very low salt solubility in the vicinity ofcritical temperature of water

High salt solubility in the vicinity ofcritical temperature of water.

KF, RbF, CsFLiCI, NaCI, KCI, RbCI, CsCILiBr, NaBr, KBr, RbBr, CsBrCaCI2, CaBr2, CaI2BaCI2, BaBr2K2CO3, Rb2CO3

K3PO4

Rb2SO4

Type 1 Type 2

LiF, NaFCaF2

BaF2

Li2CO3, Na2CO3

Li3PO4, Na3PO4

Li2SO4, Na2SO4, K2SO4

MgSO4, CaSO4

Scheme 9.4 Solubility of some electrolyte salts in the supercritical water.

Sabirzyanov et al. [48] describe the solubility behaviour of carbon dioxide with increasingtemperature and pressure (Figure 9.1). The solubility decreases with increasing temperatureat first, but carbon dioxide starts to become more soluble at higher temperatures, especiallyat higher pressures.

9.7 Fugacities of Gases in the Supercritical Water

The fugacity of a gas is used to evaluate the tendency of gas to escape from another phaseunder pressure. For the ideal gases, the fugacity is equal to the partial pressure of the gas.

20 40 60 80 100 120 140 160

0,01

0,02

0,03

0,04

0,05

0,06

10 MPa 20 MPa 40 MPa 60 MPa 80 MPa

mo

le f

ractio

n o

f C

O2

T/°C

Figure 9.1 Solubility of carbon dioxide [48].


The activity of gases for a component in a mixture is defined by the equation:

𝜇i = 𝜇0i (T) + RT ln fi

where 𝜇i and 𝜇0i are the chemical potentials at reaction and under standard conditions,

respectively. R is the gas constant, T is the absolute temperature and f is the fugacity.The relationship between the fugacity and the pressure can be written as

fi = 𝜙i × pi

where 𝜙i is the fugacity coefficient and pi is the pressure of the component.Evaluation of the fugacity is usually done using the fugacity coefficient. In general, as

p→0, 𝜙→1, therefore, the fugacity coefficient can be set to 1 at low pressures. However, itdiffers from unity when the pressure increases [48].

The fugacity coefficient may depend on the pressure, temperature and composition ofthe mixture and the change is shown in the equation as:

RT ln𝜙i =

p

∫

0

[vi −

RTp

]dp

where vi is the partial molar volume. When the fugacity becomes different from the partialpressure of the component, the equilibrium constants Kp and Kc for the components becomedifferent from the ideal gas values.

Most liquids and solids have fairly constant specific volume over wide ranges of pressurevariations. Thus, fugacity for liquids and solids is equal to their saturated fugacity orpressure.

9.8 Mechanism of the Supercritical Water Gasification

Kinetic studies and chemical reaction mechanisms are particularly important for assessmentand implementation of control strategies for the supercritical water processes.

Buhler et al. [49] investigated the reaction pathways of glycerol pyrolysis in the near-critical and supercritical water. It was shown that a relatively high density of the subcriticalwater favours ionic or molecular reactions, while a radical mechanism is more favourableat the supercritical conditions. Free radical degradation is the dominant pathway in thesupercritical water (Scheme 9.5).

As there are a great number of consecutive, complex and parallel reactions occuring dur-ing the supercritical water gasification of biomass, modelling studies for model substanceswould be very helpful in understanding such complex reaction networks.

9.9 Corrosion in the Supercritical Water

Corrosion is the main technical problem of biomass gasification processes under hydrother-mal conditions. Water becomes aggressive at high temperatures and pressures because ofthe properties of higher ionisation constants.


ionic-molecular

mechanism

subcritical waterradical mechanism

supercritical water

Critical point of water

Scheme 9.5 Mechanisms in the sub- and supercritical water.

When the biomass is fed into the reactor and heated to the supercritical conditions, solidscan precipitate, agglomerate and stick to reactor wall. Furthermore, sulfur, halogene andphosphorous in biomass feed are converted into inorganic acids such as H2SO4, HCl orH3PO4. The dissociation constants of such acids become higher, which makes these acidsextremely corrosive at high temperatures [50, 51]. The corrosion challenges increase inparticular in the presence of oxidising agents [52].

To protect against corrosion in the supercritical water, corrosion-resistant nickel-basedalloys, such as Hastelloy C-276, Inconel-625 or stainless steel 316, have been widelyused as reactor materials which are also excellent materials for the construction of high-pressure, high-temperature reactors. Nevertheless, under the supercritical conditions, eventhese materials corrode [53].

Complete elimination of corrosion is very difficult but its effect can be reduced by meansof the arrangement of hydrothermal parameters, reactor types and materials according tobiomass feedstock. The transpiring wall reactor (TWR) is an example of a reactor typewhich is resistant to corrosion and salt deposition under the supercritical conditions [54,55]. In TWRs, the reaction chamber consists of a highly porous wall where water flowsthrough the pores continuously and creates a thin layer on the reactor wall. Reactions occurin the reactor without contacting the reactor surface.

Another type of reactor used against corrosion is the double tube reactor [56]. The innertube is made from ceramic which is relatively corrosion-resistant material.

9.10 Advantages of the Supercritical Conversion of Biomass

Increasing energy demand is triggering research and development on alternative energyresources and technologies. Supercritical conversion of wet biomass is an important anddeveloping technology targeted on the production of viable alternative clean fuels suchas hydrogen. The main advantages of the supercritical water gasification are summarisedbelow:

• Although the high water content of biomass causes inefficient conversions and is amajor challenge for conventional thermal treatments, it is preferable in the supercriticalconversion. Drying of biomass is unnecessary.

• Complete gasification of biomass to gaseous fuel is possible. Moreover, homogeneityand faster reaction rates are achievable.


• The unwanted products of tar or char are effectively minimised and more gaseous fuelsare obtained by this technology.

• The gasification of biomass under the supercritical conditions is a developing technologyand is important especially for hydrogen production as a clean energy source. Thehydrogen yield achieved can be as high as 70% vol [25].

• Supercritical water conversion of wet biomass can provide significant environmentaladvantages with little or no sulfur-containing feedstocks and products compared tofossil fuels such as coal and oil.

• It is not necessary to carbonise the feedstocks for gasification.

On the other hand, reactor-related issues are the main obstacles to be overcome in theapplication of the supercritical gasification. Specialised, expensive materials are requiredto withstand reaction conditions and to safeguard against the effects of corrosion.

9.11 Conclusion

Supercritical conversion under high temperature and pressure conditions is a suitable treat-ment for gasification of especially wet biomass to gaseous fuel. This technology holdsconsiderable potential for future development, particularly with regard to hydrogen pro-duction.

However, there are some important scientific issues associated with the technology, suchas corrosion and reactor type, which require solution if future implementation on industrialscales is to be feasible. Further research into these issues and possible solutions is required.

Questions

1. What are the advantages of the supercritical water gasification of biomass over the otherbiomass conversion processes?

2. How does biomass transform to gaseous products in the supercritical water?3. What is one of the important reactions for hydrogen production in the supercritical water

gasification of biomass? What is the mechanism of WGSR?4. What is fugacity?5. Why is fugacity important for the supercritical gasification of biomass?

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(36) Resende, F.L.P. and Savage, P.E. (2010) Effect of metals on supercritical water gasification ofcellulose and lignin. Industrial & Engineering Chemistry Research, 49, 2694–2700.

(37) Minowa, T., Zhen, F. and Ogi, T. (1998) Cellulose decomposition in hot compressed water withalkali or nickel catalyst. Journal of Supercritical Fluids, 13, 253–259.

(38) Schmieder, H., Abeln, J., Boukis, N. et al. (2000) Hydrothermal gasification of biomass andorganic wastes. Journal of Supercritical Fluids, 17, 145–153.

(39) Sinag, A., Kruse, A. and Schwarzkopf, V. (2003) Key compounds of the hydropyrolysis ofglucose in supercritical water in the presence of K2CO3. Industrial & Engineering ChemistryResearch, 42, 3516–3521.

(40) Elliott, D.C. and Sealock, L.J. (1983) Aqueous catalyst systems for the water-gas shift reaction.1. Comparative catalyst studies. Industrial & Engineering Chemistry Product Research andDevelopment, 22, 426–431.

(41) Torry, L.A., Kaminsky, R., Klein, M.T. and Klotz, M.R. (1992) The effect of salts on hydrolysisin supercritical and near-critical water: reactivity and availability. Journal of SupercriticalFluids, 5, 163–168.

(42) Jenkins, B.M., Baxter, L.L., Miles, T.R. Jr. and Miles, T. R. (1998) Combustion properties ofbiomass. Fuel Processing Technology, 54, 17–46.

(43) Ho, P.C. and Palmer, D.A. (1996) Ion association of dilute aqueous sodium hydroxide solutionsto 600 ◦C and 300 MPa by conductance measurements. Journal of Solution Chemistry, 25(8),711–729.

(44) Hodes, M., Marrone, P.A., Hong, G.T. et al. (2004) Salt precipitation and scale control insupercritical water oxidation-Part A: fundamentals and research. Journal of Supercritical Fluids,29, 265–288.

(45) Ho, P.C. and Palmer, D.A. (1996) Ion association of dilute aqueous sodium hydroxide solutionsto 600 ◦C and 300 MPa by conductance measurements. Journal of Solution Chemistry, 25(8),711–729.

(46) Armellini, F.J. and Tester, J.W. (1993) Solubility of sodium chloride and sulfate in sub- andsupercritical water vapor from 450–550 ◦C and 100–2 50 bar. Fluid Phase Equilibria, 84,123–142.

(47) Weingartner, H. and Franck, E.U. (2005) Supercritical water as a solvent. Angewandte ChemieInternational Edition, 44, 2672–2692.

(48) Mizan, T.I., Savage, P.E. and Ziff, R. (1997) Fugacity coefficients for free radicals in densefluids: HO2 in supercritical water. AIChE Journal, 43(5), 1287–1299.

(49) Buhler, W., Dinjus, E., Ederer, H.J. et al. (2002) Ionic reactions and pyrolysis of glycerol ascompeting reaction pathways in near- and supercritical water. Journal of Supercritical Fluids,22, 37–53.


(50) Kritz, P. (2004) Corrosion in high-temperature and supercritical water and aqueous solutions: areview. Journal of Supercritical Fluids, 29, 1–29.

(51) McDonald, D.D. (2004) Effect of pressure on the rate of corrosion of metals in high sub-criticaland supercritical aqueous systems. Journal of Supercritical Fluids, 30, 375–382.

(52) Wellig, B. (2003) Transpiring wall reactor for supercritical water oxidation. Ph. D. Thesis, Swissfederal institute of technology, Zurich.

(53) Miksa, D. and Brill, T.B. (2001) Spectroscopy of hydrothermal reactions 17. Kinetics of thesurface–catalyzed water–gas shift reaction with inadvertent formation of Ni(CO)4. Industrial &Engineering Chemistry Research, 40, 3098–3103.

(54) Prıkopski, K., Wellig, B. and Rudolf von Rohr, Ph. (2007) SCWO of salt containing artificialwastewater using a transpiring-wall reactor: Experimental results. Journal of SupercriticalFluids, 40, 246–257.

(55) Bermejo, M.D. and Cocero, M.J. (2006) Destruction of an industrial wastewater by supercriticalwater oxidation in a transpiring wall reactor. Journal of Hazardous Materials B, 37, 965–971.

(56) Casal, V. and Schmidt, H. (1998) SUWOX–a Facility for the destruction of chlorinated hydro-carbons. Journal of Supercritical Fluids, 13, 269–276.

10Influence of Feedstocks onPerformance and Products

of Processes



When we are talking about biomass and its use in thermochemical conversion processeswe have to be careful not to create negative impacts on the technical processes used due tothe chemical composition of the biomass used. Even though biomasses usually consist ofcellulose, hemicellulose, lignin as well as starch and sugars in different compositions, theyalso differ in their ash content as well in their composition, which is the most significantconstituent in affecting thermochemical treatment processes.

Fouling processes in combustion facilities as well as gasifiers are well known if feedsother than wood or coal are used. The content of nitrogen, chlorine and sulfur might alsodiffer from those given in wood or coal and then affect the off gas quality!

In literature, most processes dedicated to biomass are used in the context of wood.Sometimes this is not obvious to the reader, and sometimes generalised assumptions aremade, that are valid for wood but not for other biomasses. As the ash content of otherbiomasses like energy crops, herbaceous crops, oil seed pressing cakes from rape, olive,soy, coconut, waste food, digestate from biogas production or sewage treatment is verydifferent, it is important to look into this in detail.




Table 10.1 Overview of main building blocks of lignocellulosic crops on a dry basis [1].

Feedstock (wt-%)Cellulose

(wt-%)Hemicellulose

(wt-%)Lignin(wt-%)

Other(wt-%)

Bagasse 35 25 20 20Corn stover 53 15 16 16Corn cobs 32 44 13 11Wheat straw 38 36 16 10Wheat chaff 38 36 16 11Short rotation woody crops 50 23 22 5Herbaceous energy crops 45 30 15 10Waste paper for comparison 76 13 11 0

Table 10.2 Alkali content of biomass [1].

Heating value Ash content Alkali in ashBiomass (MJ/kg) (wt-%) (wt-%)

Hybrid poplar 19 1.9 19.8Pine chips 19.9 0.7 3.0Tree trimmings 18.9 3.6 16.5Urban wood waste 19 6 6.2White oak 19 0.4 31.8Almond shells 17.6 3.5 21.1Bagasse, washed 19.1 1.7 12.3Rice straw 15.1 18.7 13.3Switch grass 18 10.1 15.1Wheat straw 18.5 5.1 31.5

The following tables (Tables 10.1–10.4) give an overview of the characteristic composi-tions of various biomasses.

Table 10.5 shows the temperatures for different feedstocks when ash melting starts tohappen. In the case of residue materials from wheat or corn production, this temperature isso low that combustion or gasification processes will suffer from fouling. Possibilites forgetting rid of such problems are to mix the feed with calcium carbonate which will increasethe ash melting temperature, the extraction [3] of ash via pyrolysis before gasification of

Table 10.3 Organic components of starch and sugar crops on dry basis [1].

FeedstockProtein(wt-%)

Oil(wt-%)

Starch(wt-%)

Sugar(wt-%)

Fibre(wt-%)

Corn grain 10 5 72 <1 13Wheat grain 14 <1 80 <1 5Sugar cane <1 <1 <1 50 50Sweet sorghum <1 <1 <1 50 50

Tabl

e10

.4Pr

oxim

ate

and

ultim

ate

anal

ysis

ofdi

ffere

ntfe

edst

ocks

and

high

erhe

atin

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V.

Prox

imat

ean

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is,

wt-

%,d

ryU

ltim

ate

anal

ysis

,wt-

%,d

ryba

sis

Bio

mas

sH

HV,

dry

MJ/k

gV

olat

ileA

shFi

xed

CC

HO

NS

Cl

Ash

Alfa

lfast

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18.4

572

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19.3

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21.7

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5.97

42.2

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18.7

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3618

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85.

8745

.46

0.47

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0.21

1.4

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17.6

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5.58

19.2

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Sorg

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Table 10.5 Temperature for the start of ash melting for differentfeedstocks [2].

Ash content(wt-%)

Ash meltingpoint (◦C)

Wood 1.3 1225Clover 8.3 1020Wheat straw 7.1 880Wheat 1.6 695Corn stover 3.9 920Miscanthus 3.9 1160

combustion of the vapours, and finally extracting the potassium and sodium containing saltsfrom char via water extraction as a pre-conditioning stage for combustion or pyrolysis.

What we can see is that the heating value for the dry biomasses varies by just 10%while the ash content differs up to a factor of 100; and the ash melting temperatures rangefrom 700 to 1200 ◦C. Everything below 900 ◦C becomes difficult to handle for thermaltreatment processes, either because the ash starts to melt and blocks the ash container interms of combustion, or starts to coat the heat carrier materials in fluidised bed combustorsor gasifiers, or simply creates fouling in the downstream units post-thermal treatment.

10.1 Humidity of Feedstocks

The humidity of the feedstock is also a very relevant parameter. While combustion andgasification processes can deal with feeds with a moisture content higher than 20% (likefresh wood) it is almost impossible to run a fast pyrolysis process with such high moisturecontent. Due to the short residence time in fast pyrolysis reactors, feedstocks with highmoisture content would just start to be dried or torrefied, but not pyrolysed. Usually feedmaterials for fast pyrolysis range between 1 to 6% moisture content.

This is different to intermediate pyrolysis reactors; they can cope with moisture contentsof up to 40% and more without losing their pyrolysis performance. Using feedstocks witha higher moisture content than 40% makes no sense from an energetic standpoint, as theenergy content of the feed would have to be consumed to vaporise and condense the waterfraction of the feedstock.

10.2 Heteroatoms in Feedstocks

Taking feedstocks other than wood can create problems with NOx, methyl chloride, HClor SO2 formation during combustion in engines or the formation of ammonia and H2Sin gasification processes. Such components have either to be treated after the processesby scrubbing or catalysts, or scavenged during gasification by adding dolomite to formcalciumsulfit or calciumchloride.

Influence of Feedstocks on Performance and Products of Processes 207

References

(1) Brown, R.C. (2003) Biorenewable Resources – Engineering New Products from Agriculture,BlackwellPublishing.

(2) Tetzlaff, K.H. (2008) Wasserstoff fur alle – wie wir der Ol-, Klima- und Kostenfalle entkommen,Books on Demand, Norderstedt, p. 360.

(3) Kebelmann, K. and Hornung, A. (2010) The effect of the particle size of Chlorella VulgarisBeeinerinck (CCAP211/11B) biochar derived from intermediate pyrolysis on the elution ofnutrients during Soxhlet extraction. Bioten Conference Birmingham, September 2010. ISBN978-1-872691-54-1.

11Integrated Processes Including

Intermediate Pyrolysis

Andreas HornungFraunhofer UMSICHT – Institute branch Sulzbach-Rosenberg, Germany and Chair in


Pyrolysis of biomass is an important process or process element to turn biomass into liquid,solid and gaseous products. Worldwide, companies are searching for solutions to liquefywood and other biomasses to get a product higher in energy per volume than biomass.

The sought applications range from co-firing and firing in biomass boilers, fuel for gasengines and dual-fuel engines, to feed of gasifiers. Furthermore, the chemical componentsin the liquids are of interest to biorefineries as high value products can be extracted.

Finally, the char from pyrolysis is of increasing importance as it can be used to deliver so-called biochar. Biochar is suitable for fertilising agricultural land and in addition sequesterscarbon instead of carbon dioxide. Today, the most promising chars for biochar applica-tion in combination with the production of heat and power are delivered by intermediatepyrolysis.

The bioenergy market is growing, driven by a low carbon energy policy, and by a wastepolicy diverting waste materials from landfill. There is an enormous untapped biomassresource for energy conversion going unnecessarily into landfill or counterproductivelyto incineration, and in addition to the environmental impact this loads additional costsonto regional businesses and public sector organisations. The emerging technologies inintermediate pyrolysis will offer very exciting potential to deliver substantial savings andenergy capacity along with options to net ‘carbon negative’ outcomes.




11.1 Coupling of Anaerobic Digestion, Pyrolysis and Gasification

Many companies are currently in the development stage of exploring methods of producingfuels via pyrolysis or synthetic diesel via gasification by using the Fischer–Tropsch syn-thesis. Usually these processes stick to distinct dry or very dry feedstock and/or definedconditioned and sized feeds. More flexible systems tend to need very big scales to remaineconomical. However, in 2008 Aston University applied for a new patent for the BtVBprocess – Biothermal Valorisation of (ash rich) Biomass – which is able to process all kindsof biomasses and biogenic residues, and highly flexible for different feed sizes and moisturecontents. The BtVB process couples residues and effluents of a biogas unit to a thermal lineconsisting of a pyrolyser (Pyroformer) and gasifier (fluidised bed). Finally, the mineralsstick to the charcoal from pyrolysis, used for carbon sequestration and refertilisation ofsoils (Figure 11.1).

The core of the process is ‘intermediate pyrolysis’. The pyrolysis vapours (about 60–75%of the energy) pass directly to a gasifier for gasification, no filtration or condensation of thevapours is required. The gasified gases are used to power a gas or dual-fuel engine. The ashremains in the char, which can be used as such for fertilisation or even combustion.

If the remaining pyrolysis char is used to fertilise fields or is used as soil amendmentit produces something commonly known as ‘Black Earth’, and high added value based onthe fertiliser content and sequestrated carbon can be achieved. This utilisation of char is theonly way to convert CO2 via plants to stable carbon and, furthermore, to remove it from theatmosphere. Alternatively the char can even be used for combustion in biomass power plantsor just stored in old coal mines. If the char is not used to amend soil or used as fertiliser itshould be water extracted prior to combustion or storage. The water extracts some of thefertiliser components and can be used as liquid fertiliser. Furthermore, such components

Anaerobicdigestion

Composting

IntermediatePyrolysis

GasificationHeat and

power

Hydrogen

Synthesisproduct

Methane

DME

FT-Diesel

CHP

Dryfermentation

Sludges fromalcohol

production

Figure 11.1 Coupling of anaerobic digestion, pyrolysis and gasification.

Integrated Processes Including Intermediate Pyrolysis 211

with low ash melting points are extracted and combustion properties are enhanced. Biocharfrom wood is low in fertiliser components, char from other biomasses like straw is richin fertilising components. Today, Europe is in the process of accepting biochar as a soilamendment or fertiliser, but only for virgin wood derived materials (classical charcoal) incountries like Austria, Germany and Switzerland.

Beside anaerobic digestion, other feeds can also be used, like fractions from compostproduction before the final conversion, as well as intermediate residues from dry fermenta-tion. Such residues are normally finally composted, but can be used instead for pyrolysis.Furthermore, other residues from anaerobic processes are suitable, such as sewage sludgeor spent brewers grain.

It is important to note that the feed to intermediate pyrolysis can be high in moisturecontent, up to 40%. Through intermediate pyrolysis such feeds are made suitable for agasification process. The organic vapours and steam are transferred to the gasifier throughpyrolysis. Ash-rich biochar stays away from the gasifier.

11.2 Intermediate Pyrolysis, CHP in Combination with Combustion

The use of pyrolysis oil (Figure 11.2) mixed with diesel or biodiesel and the pyroly-sis gas in dual-fuel engines leaves biochar as a byproduct. The biochar can be burned

IntermediatePyrolysis

SewageSludge

Anaerobicdigestion

dry residue

Agriculturalresidues

Dry manure

Wood andenergycrops

Compostingresidues

CHP

Biochar Combustion

Carbonstorage

Hydrogen

Fertilisers

Heat andpower

Figure 11.2 Coupling of intermediate pyrolysis and CHP in combination with combustion ofsolids.


together with coal or fired as such in a rotating grate furnace or together as mixed pelletwith biomass.

The ash is still suitable as a fertiliser, at least if virgin biomass has been used. An alterna-tive use for the char is to store it in old coal mines as a measure of carbon sequestration: analternative application to sequestration of carbon dioxide. Finally the char can be reformedto produce syngas or hydrogen and carbon dioxide.

11.3 Integration of Intermediate Pyrolysis with AnaerobicDigestion and CHP

New approaches are looking to enhance the yield of biogas from anaerobic digestion byusing fractions from pyrolysis (water phase from pyrolysis, organic phase or pyrolysis char)to be added to the anaerobic digestion process [1]. A maximum of a small percentage ofpyrolysis water phase is added to the digestion process to increase the gas yield, as wellas the methane concentration, based on the organic content in the water phase but also viastimulation of the bacterial cultures. Large European projects like Interreg BioenNW aredealing with this issue [1].

Alternatively, such processes can also be stimulated by the addition of small amounts ofpyrolysis oil and/or biochar as described by Interreg BioenNW [2].

In terms of municipal solid waste, a combination of pyrolysis as a first step and anaerobicdigestion as second step can be very advantageous, as an inhomogeneous material istransformed into a simpler to handle material for anaerobic digestion. Therefore, asidefrom the increase of gas yield, the ability to process materials is also very important.

11.4 Pyrolysis Reforming

A new type of combined pyrolysis and reforming is given with the so-called pyroformer(Figure 11.3). The pyroformer uses the char from pyrolysis to reform the gases evolved frompyrolysis. Due to the long residence time in the reaction system of 10 minutes, compared tofast pyrolysis (around 1 second), several processes can be combined in one reactor. Drying,torrefaction, pyrolysis, reforming and char conditioning take place. The pyroformer is agood example of the integration of process elements in one reactor unit.

11.5 The BIOBATTERY

Some integrated concepts even involve the management of fluctuating power productionor the transformation of power into other products if too much power is produced. Suchconcepts go along with the conditioning of low grade feedstocks to prevent, that thereare limitations in feed supply. Furthermore, to keep costs down and to prevent to get incompetition with land use for food production or food itself.

The German ‘Energiewende’ [4], the transformation from nuclear power to renewablepower, as well as the changing energy supply all over Europe have set special challenges


Figure 11.3 The pyroformer [3] combines pyrolysis and reforming. The reforming is realisedvia char formed during pyrolysis.

to every single member state. Today, renewable energy sources already cover 10% ofthe primary energy demand of the EU27. However, transforming energy from fluctuatingsources such as sun and wind into power needs to be balanced out. Solutions are neededthat guarantee a temporal separation of generation and use.

In this context, biomass and especially biogas play an important role in the future energysupply. The anaerobic degradation process of the biomass is limited to easily bio-availablesubstrates, whereas lignin-derived compounds remain in the digestate and do not contributeto the biogas yield. In addition, digestate leads to emissions of greenhouse gases togetherwith the risk of over-fertilisation, especially in regions with a high amount of cattle-breeding.

Against this background, the idea of the BIOBATTERY (Figure 11.4) from the Fraun-hofer Institute for Environmental Safety and Energy Technology UMSICHT branch inSulzbach-Rosenberg is to allow energy to be stored over different time periods. TheBIOBATTERY stands for a pool of several environmentally friendly technologies suchas biogas plants, thermal storage systems, gasifiers, pyrolysis systems and motors forenergy conversion and/or power generation.

Excess electricity from renewable energy, biogenous residual materials and other organicwaste serve as the input. Through a combination of pyrolysis and a reforming stage, residualbiomass and excess electricity are transformed into the products oil, gas and biochar.

The many biogas plants in Germany as well as in other European member states presentgreat potential for the BIOBATTERY. Through pyrolysis in conjunction with combinedgasification, the residue from anaerobic fermentation can be transformed thermochemicallyand converted into gaseous products like syngas and biochar. In terms of pyrolysis or thesubsequent combination of pyrolysis and reforming oil, gas and char are formed. Thereformer oil which is produced and treated, as well as the gas, is used to generate electricity


Pyrolysis

CHP

Biochar

Carbon

storage

Hydrogen

Fertilisers

Heat and

power

Sewagesludge

Compostresidues

Agriculturalresidues

Manure

Combustion

Chemicals

Synthetic

fuels

Reforming

Gasification

Grid

power

Thermal

storage

Power, gas and heat to grid

Feedconditioning

Power fromphoto

voltaics

Anaerobicdigestionresidues

Powerfrom wind

Figure 11.4 The BIOBATTERY – a concept to support the German Energiewende [4] devel-oped by Fraunhofer Umsicht [5].

and heat in a generator in a combined heat and power (CHP) plant. Alternatively, thereformer gas produced by pyrolysis as well as the reformer oil can be used for gasificationor combustion processes. The solid residue can be used as a fertiliser or soil improver butalso for power generation or carbon storage. Alternatively, the biochar, together with thereformer water or pyrolysis water, can be used for hydrogen production.

Optimal energy output is at the heart of the work surrounding the BIOBATTERY. Theefficiency of the concept can also be increased by use of latent heat storage systems. Inthis way, the low temperature heat that is not needed on site is transported by mobile heatstorage systems to where it is needed and used there, increasing the efficiency of the systemas a whole. Fluctuating power and excess power can be used via high temperature thermalstorage to power the thermal conversion processes.

A particular advantage of the BIOBATTERY is the possibility of individual adjustmentof the technological components to the local conditions.

11.6 Pyrolysis BAF Application

BAF stands for bio-activated fuel (Figure 11.5). This is realised by the treatment of moltenpolymers with vapours from biomass pyrolysis. Such vapours start a cracking reaction ofthe polymer at low temperatures, compared to thermal cracking, which leads to favourableproducts with respect to using such materials as fuels [6]. In terms of the use of moltenpolyethylene, the temperature can be reduced by 100 ◦C to approx 350 ◦C.


Biomass

Pyrolysis Unit

BAF Reactor

IBC

Oil

Cooling II

Filters

TarDieselWater

Separator

Cooling I

ElectrostaticPrecipitator

CHPcombined heatand power unit

Biochar

Pyrolysis Gas

PE/PP

Figure 11.5 The Pyrolysis BAF process delivers a mixture of biogenic liquid and polymer basedliquid, suitable for internal combustion engines.

11.7 Birmingham 2026

Through the Birmingham 2026 initiative [7], Birmingham plans to be the UK’s first sus-tainable global city with a low-carbon energy infrastructure and is well prepared for theimpact of climate change. Key objectives include reducing CO2 by 60% and increasingefforts to reduce, reuse and recycle waste. Aston University has developed a new pro-cess for the thermal conversion of biomass. This process, BtVB (Biothermal Valorisationof Biomass), combines a new kind of pyrolysis (intermediate) with gasification into anintegrated technology capable of treating a wide variety of biomass feedstock [8].

Biochar, one of the byproducts of the process, can be used in many ways, such as tofertilise agricultural lands, to recover brown fields, to be stored as such in mines to be usedin the future or to be co-combusted along with coal in power plants. The scheme providesan innovative solution to integrating the countryside with the city centre by creating aso-called ‘thermal ring’ or ‘thermal belt’ in and around Birmingham.

The thermal belt of Birmingham intends to create heat and power sufficient for the citycouncil’s own buildings, based on residues such as sewage sludge and green residues fromparks generated by the city itself. Usable products from combined pyrolysis gasificationcould be syngas itself, but also hydrogen as well as synthetic natural gas or liquid fuels viasynthesis routes.

11.8 Conclusion

The introduction of new integrated processes shows that it is possible to produce clean fuelgases and sequestered carbon products (biochar) as well as heat and power and chemicals.At the heart of the process integration are new ‘intermediate pyrolysis’ processes whichallow the use of a very broad range of fuels of varying quality including sewage sludge,


wood waste, algae residue, general municipal waste, residue from composting, dry manures,energy grasses and ground service materials even with high moisture content. The overallprocesses can even be carbon negative (up to 25% carbon can be saved as biochar andsequestered and return carbon (as well as potassium, phosphates, nitrogen) to the soil inthe form of fertilisers). Products other than heat and power can be hydrogen gas, syntheticnatural gas SNG and FT-diesel. Full integration could easily lead to new hydrogen bio-economy approaches. Residue biomass from cities, industry and agriculture is convertedinto new energy-carrying intermediates, like gases and liquids as well as biochar. Biocharis not a residue material, it can be converted to additional hydrogen, via reforming orgasification. Connection to grids for power, hydrogen or methane provision delivers thebiofuels and power to everybody, while heat can be used in decentralised systems forbuildings, greenhouses or even to cool buildings and food. Alternatively, the syngas can beconverted to liquid products like synthetic diesel, ethanol or methanol.

References

(1) Torri, C. and Fabbri, D. (2012) Centro Interdipartimentale di Ricerca Industriale Energia eAmbiente, UO Biomasse, Universita di Bologna. Biological upgrading of pyrolysis oil and gasto methane and hydrogen by means of adapted anaerobic bacteria consortium. 20th EuropeanBiomass Conference and Exhibition, 6, pp. 18–22.

(2) Interreg BioenNW http://bioenergy-nw.eu/ (last access May 2, 2014).(3) Hornung, A. and Apfelbacher, A. (2009) “The thermal treatment of biomass” Great Britain Patent

Application Number: GB 0808739.7.(4) German Energiewende http://bundestag.de/dokumente/textarchiv/2011/34867973 kw26 sp

energiewende/index.html (last access May 2, 2014).(5) Rottenbacher, C. Fraunhofer UMSICHT BIOBATTERIE http://www.mittelbayerische.de/

region/amberg/amberg/artikel/forschen-fuer-eine-biobatterie/911027/forschen-fuer-eine-biobatterie.html2013 (last access May 2, 2014).

(6) Ulrich Wirtz, Andreas Hornung WO 2012/136955 Methods and apparatus for the production offuels 8.4.2011

(7) http://www.birmingham.gov.uk/2026(8) Hornung, A. and Apfelbacher, A. (2009) “Biomass Processing”, Great Britain Patent Application

Number: GB2460154: Examination requested: December 7, 2009. World Patent Applied for(WO 2009/138746; November 19, 2009).

http://bioenergy-nw.eu/

http://bioenergy-nw.eu/

http://bundestag.de/dokumente/textarchiv/2011/34867973_kw26_sp_energiewende/index.html




http://www.mittelbayerische.de/region/amberg/amberg/artikel/forschen-fuer-eine-biobatterie/911027/forschen-fuer-eine-biobatterie.html2013

http://www.birmingham.gov.uk/2026



12Bio-Hydrogen from Biomass



12.1 World Hydrogen Production

In 2010, world hydrogen production reached approx 600 billion Nm3/a, so hydrogen itself isnothing new, the new approach is to realise it from biomass and to offer it to private end users.

Today hydrogen is mainly produced from fossil fuels.Approximately 48% is produced from natural gas, 18% from coal, 30% from oil and just

4% via electrolysis.Hydrogen is mainly used for the production of fertilisers, for refinery processes, methanol

production as well as steel production.

• 60% NH3 production

• 23% in refineries

• 9% methanol production

• 8% steel production

See Table 12.1 for more information.

12.2 Bio-hydrogen

The production of bio-hydrogen from biomass is of interest for several reasons: it deliversgreen hydrogen from a renewable source for storage, hydrogen for mixing with natural




Table 12.1 Hydrogen properties in comparison to methane.

Hydrogen Methane

Density at 20 ◦C 0.084 g/l 0.71 g/lBoiling temperature –252.8 ◦C –161 ◦CLower heating value 120 MJ/kg

(33.3 kWh/kg)10.8 MJ/Nm3

50 MJ/kg(13.9 kWh/kg)35.9 MJ/Nm3

Higher heating value 141.86 MJ/kg(39.41 kWh/kg)

55.5 MJ/kg(15.42 kWh/kg)

Ignition in air 4–75 vol% 5.3–15 vol%Explosion in air 18.3–59 vol% 6.3–13.5 vol%Diffusion coefficient in air 0.61 cm2/s 0.15 cm2/s

or biogas, and hydrogen for synthesis processes. Hydrogen of high purity is required torun fuel cells for heating or mobility purposes. Today, green or bio-hydrogen is usuallyrealised by using wind power or photovoltaic power to run water electrolysis. This is quitean expensive way and prices up to $30/kg are possible. Alternative routes are, for example,possible via gasification of biomass or reforming of biochars, like the Winkler process. Viasyngas, the hydrogen has to be separated from the other syngas components such as carbonmonoxide or carbon dioxide. This can be realised via membrane separation at differenttemperature levels between 800 ◦C and ambient temperature, as well as the use of differentmembrane materials, from metals at high temperatures to polymers at low temperatures.Alternatives to the given routes are the microbial conversion of biomass into hydrogen andbiogas. It is important that the new processes open the access of low-cost residue biomassto hydrogen production processes and therefore lower the costs.

The following figure (Figure 12.1) shows a range of ways to keep flexibility in terms ofthe amount of hydrogen used and the amount of gases used for heat and power, from low

Fuel celllabs

Fuel cellcars

Estates

Winklergenerator

Fermenter AD unit

Food wasteGroundservicematerial

Residuewood

Intermediatepyrolysis

Gasification CHP

Greenhouses

GridNatural gas

grid

Figure 12.1 The new bio-hydrogen bio-economy.

Bio-Hydrogen from Biomass 219

grade feed to existing uses via fermentation with combined anaerobic digestion, pyrolysis,gasification and Winkler generation, always in combination with CHP.

12.3 Routes to Hydrogen

12.3.1 Steam Reforming

Steam reforming processes are usually based on methane gas. Hydrogen and carbon monox-ide are formed via an endothermic reaction of methane and steam. This gas is called synthe-sis gas. Using an excess of steam, the water–gas shift reaction drives the carbon monoxideto carbon dioxide and further hydrogen results.

But solid biomass or biochar can also be used for such conversion processes, for examplevia the Winkler process, in which the solid feed is fluidised in a fluidised bed reactor andconverted with steam to syngas.

12.3.2 Reforming

An alternative to the use of steam is the use of oxygen, usually in air, or the use of carbondioxide to turn methane into carbon monoxide and hydrogen.

12.3.2.1 Low Temperature Catalytic Reforming of Pyrolysis Vapours

Reforming processes usually take place in the temperature range of 700 to 900 ◦C. Tocouple reforming with pyrolysis it is favourable to bring this temperature down, such thatthe pyrolysis gases can stay almost at the temperature level at which they are, between 400to 500 ◦C. This is certainly very low, so 500 to 600 ◦C seems realistic.

The following study (Figure 12.2–12.7) [1] shows an application of Ni based catalystsat 450 ◦C, already realising 14% of hydrogen in the gas phase; compared to atmosphericdriven gasifiers this is already quite high, as they usually only reach about 20% hydrogen,due to dilution with nitrogen from air.

The implementation of the catalytic low temperature reforming downstream to the Halo-clean pyrolyser significantly increases the heating value of the gas (Figures 12.3 and 12.4).The heating value of the pyrolysis gas can be increased by a factor of 1.64. Hydrogenconcentrations are increased up to 14% and the pyrolysis gas volume flow increases byabout 58% (Figures 12.5 and 12.6).

12.3.2.2 Challenges of Low Temperature Reforming

Methyl chloride is obviously a huge problem for such processes as this drives chlorine intogas phase and is no longer just given in the char. Chlorine in wheat straw was expected tobe bound inorganically and, therefore, to be transferred to the char (Figure 12.7). However,about 1 ppb remains in the hot pyrolysis gases. A total of 2081 mg chlorine is in thefeed of 1600 g wheat straw pellets. The amount of chlorine in the pyrolysis gases aftercondensation is about 200 mg, that is, 9.6%. Under the influence of catalysis, the methylchloride seems to be converted to hydrochloric acid and, therefore, to be found in the waterphase of the condensate at 8.1%, whereas the chlorine content in the gas remains about1%. Due to the conversion of methyl chloride into hydrochloric acid, the catalyst seems


Char container

feed

Haloclean kiln

reforming unit

Heating value

sample forGC

condensationunit

condensationunit

P

P

T T T

Coodingwater

Coodingwater

Oxygen

CO

CO2

H2

N2 flow

H2O-steam

Figure 12.2 System for comparative studies, based on the Haloclean pyrolysis unit. Thevapours are either streamed over a nickel-based catalyst, or go directly to the condensationtrain. Reprinted from U. Hornung, (2009), with permission from Elsevier.

to be poisoned and loses activity. In case of a technical application, gas purification of thepyrolysis gases by means of chlorine sorption prior to the reformer is necessary. For thispurpose the formed methyl chloride needs to be converted to HCl in a first step and thencaptured with sorption on materials as dolomite. As can be seen in the chapter on integratedprocesses, such a dolomite application takes place in the fluidised bed gasifier and thereforekeeps chlorine-containing matter away from the combustion engines.

Amount oforganic phase

without catalysis

60

50

40

30

Yie

ld/w

t%

20

10

gas/wt% condensates/wt% char/wt%

0

with catalysis

22.5%

13.9%

35.6%

28%

39.4%9.6%

23% 28%

Figure 12.3 Distribution of liquid, gaseous and solid products from pyrolysis, with and with-out low temperature reforming at 450 ◦C.


–5–500

0

500

1000

1500

2000

0 5

gain in heating value kJ/m3

y = 173.61 + 97.087x R = 0.95174

10

reduction of condensate/%

gain

in

heati

ng

valu

e k

J/m

3

15 20

Figure 12.4 The reforming of the vapours from pyrolysis increases the calorific value of thegases after reforming.

14

12

10

8

6

4

2

00 5 10 15 20

reducation of condensate/%

H2 V

ol%

Figure 12.5 Due to reforming, the hydrogen content of the gases increases up to 14% whichis already very comparable to the hydrogen content of atmospheric-driven gasifiers at about20%.


95000

Peak height/arb. unit

Pyrolysis gas without catalysis

Retention time of identified compounds

4.96 propane/propene

methyl chloride

acetaldehyde

2-methylpropen

acetone

acetic acid methyl ester

acetic acid ethyl ester

2-butanone

2-methyl furan

toluene

5.22

5.37

5.58

6.72

7.66

9.04

9.36

9.90

15.24

Pyrolysis gas with catalysis90000

85000

80000

75000

70000

65000

60000

55000

50000

45000

40000

35000

30000

25000

20000

15000

10000

5.00 6.00 7.00 8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00

5.37

6.72

7.66

9.03 9.36

9.99 15.24

4.96

5.22

5.58

Figure 12.6 The reforming is not delivering more hydrogen within the gas, but is changingthe composition of the main constituents of the pyrolysis vapours.

Biomass : wheat straw

1600 g wheat straw contains 2081 mg Chloride

The pyrolysis forms 480 g wheat coke (28 %).but contains 1805 g Chloride (86.7 %)

pyrolysis gas< 450° C

condensablefluid > 30° C permanent

gas < 30° C

24.5 % gas contains200 mg Chloride (9.6 %)

30 % gas contains30 mg Chloride (1.4 %)

12.5 % is oily phase (200 g) with lessthen 50 mg Chloride (less 2.4 %)

35 % is aqueous phase (575 g)contains 23 mg Chloride (1.1 %)

28 % is aqueous phase (575 g)contains 169 mg Chloride (8.1 %)

14 % is oily phase (218 g)with 77 mg Chloride (3.7 %)

Nickel-Catalyst forms

Figure 12.7 Distribution of contaminants resulting from wheat straw pyrolysis.


12.3.3 Water Electrolysis

Water electrolysis was first demonstrated by Faraday in 1820. Since 1890, water electrolysishas been widely used to generate hydrogen. If hydrogen of very high purity is required,electrolysis is usually applied. Today, the market share in hydrogen production is about5%. 30 wt% of NaOH or KOH is used in the electrolyte.

12.3.4 Gasification

The production of hydrogen from syngas via gasification of wood, biomass or residuefractions from the oil industry is becoming more and more popular nowadays. Gasificationtakes place at about 900 ◦C, but due to the feed materials H2S and NH3 are formed as well.

12.3.5 Fermentation

Non-thermal routes to hydrogen are those using microorganisms that produce hydrogenbeside carbon dioxide while digesting the biomass-based substrate. Today, several groupsare testing such processes as a first step before typical biogas production [2, 3].

12.4 Costs of Hydrogen

Today, hydrogen can be realised with efficient electrolysis processes, in which case the costof power significantly contributes to the cost of hydrogen per kg. The European Union islooking for new processes realising bio-hydrogen for less than 9 Euro/kg. Hydrogen forindustrial application can be realised at the market in the range of 6 to 10 Euro/kg, but not ifproduced as biomass based. Currently, green hydrogen realised via power from anaerobicdigestion is realising a value of up 30 Euro/kg. This is a pure market value, because thedemand is there and only a few companies can deliver. If we set the costs of power fromPV in the near future to 10 ct/kWh, efficiency of the electrolysis systems to about 75% andthe costs for electrolysis to about 4 ct/kWh, this drives the cost of one kWh of hydrogento 16.5 ct/kWh, which is equal to 6.5 Euro/kg (only power based); so close to technicalhydrogen, but green.

One kg of hydrogen can move a simple combustion engine car forward by 100 km, soit is already comparable to standard fuelled cars, but a fuel cell driven car can beat thiswith up to 300 km. What role can biomass play? Could biomass-based hydrogen be evencheaper?

Today the following numbers (Table 12.2) are discussed for production of hydrogen fromdifferent sources by large scale application.

For the American market, we find a whole set of slightly different costs per kWhof hydrogen in literature. It ranges from 1 to 12 $/kg [4] depending on the method ofproduction, the lower end via inexpensive natural gas and the higher end based on smallscale electrolysers. Nevertheless, it ranges within the set of numbers given in Table 12.1.

As an alternative, the use of biochar from pyrolysis can be seen. Based on biochar, withcosts of 100 Euro/t as a green fuel equivalent to substitute coal, hydrogen can be realisedvia the Winkler process at approx 3 to 6 ct per kWh (1.2–2.4 Euro/kg, low pressure),and not necessarily at large scale. Such units are equivalent to 1 to 5 MWel installations


Table 12.2 Hydrogen production costs.

Source Cost for 1 kWh of hydrogen Cost of 1 kg of hydrogen

Wind power 8–13 ct 3.2–5.2€Solar power 20 ct, incl. transp., electrolysis 8€Water power 7–10 ct 2.8–4€Biomass as seen today 13 ct 5.2€Natural gas 2.5 ct 1€Coal 5–12 ct (incl. sequestration) 2–4.8€Solar thermal 8–18 ct 3.2–7.2€

and are flexible in the delivery of hydrogen on demand (refer to the chapter on Integratedprocesses).

Altogether this is a change in paradigm, we do not need mega installations for hydrogen,as shown in a strategic way forward given in Ref. [4]. The calculated scenarios have shownthat under the given technology, a unit of 500 MW (producing 3 TWh of hydrogen andbeing one of 332 stressed in this scenario for Germany) could deliver the hydrogen for2.5 ct/kWh. Nevertheless, we reached the same value, but today scaled down by a factor ofapprox 100!

As given in Ref. [4] approximately another 1 ct has to be added for distribution and taxes.So even using this hydrogen for home heating would be much cheaper than the natural gascosts for private households in Germany in 2006 of approx 6 ct per kWh (based on upperheating value), more or less half the cost!

Based on the costs of hydrogen of 3 ct/kWh, households could realise their power supplyvia fuel cells at approximately 4 ct/kWh. If used in a gas CHP system available today thecosts would increase to 7 ct/kWh, but would also include the provision of hot water.

12.5 Conclusion

Obviously we can envisage solutions to produce bio-hydrogen at comparable costs to fossilfuels for end users or even cheaper. The real advantage of bio-hydrogen is the decentralisedprovision compared to the production based on natural gas as discussed in literature [4].Small scale conversion units can be located close to villages and cities and supply hydro-gen on a demand basis in combination with heat and power production as alternatives.This will drastically lower the costs of infrastructural measures for the new hydrogeneconomy.

References

(1) Hornung, U., Schneider, D., Hornung, A. et al. (2009) Sequential pyrolysis and catalytic lowtemperature reforming of wheat straw. Journal of Analytical and Applied Pyrolysis, 85, 145–150.

(2) Kyazze, G., Dinsdale, R., Hawkes, F.R. et al. (2008) Direct fermentation of fodder maize, chicoryfructans and perennial ryegrass to hydrogen using mixed microflora. Bioresource Technology,99(18), 8833–8839.


(3) Redwood, M.D., Paterson-Beedle, M., and Macaskie, L.E. (2008) Integrating dark and lightbiohydrogen production strategies: towards the hydrogen economy. Reviews in EnvironmentalScience and Biotechnology, 8(2), 149–185.

(4) Tetzlaff, K.H. (2008) Wasserstoff fuer alle – Wie wir aus der Oel-, Klima- und Kostenfalleentkommen, Books on Demand GmbH, Norderstedt.

Further Reading

Soerensen, B. (2012) Hydrogen and Fuel Cells, Emerging Technologies and Applications, AcademicPress, Elesevier.

Mahmood, A.S.N., Brammer, J.G., Hornung, A. et al. (2013) The intermediate pyrolysis and catalyticsteam reforming of brewers spent grain. Journal of Analytical and Applied Pyrolysis, 103, N/A,p. 328–342.

Hornung, A., Apfelbacher, A., Richter, F. et al. (2007) 6th International Congress Valorisation andRecycling of Industrial Waste (Varirei), L’Aquila, I, June 27–29, 2007.

Turner, J. et al., International Journal of Renewable Energy Research, online Wiley InterScience.Dahmen, N., Dinjus, E., and Henrich, E. (2007) Erdol -Erdgas-Kohle 123 (Nr. 3), OG31.Hornung, A. and Seifert, H. (2006) Rotary kiln pyrolysis of polymers containing heteroatoms, in

Feedstock Recycling and Pyrolysis of Waste Plastics: ConvertingWaste Plastics into Diesel andOther Fuels, John Wiley & Sons Ltd, Chichester, S.549–S.567.

Leible, L., Kalber, S., Kappler, G. et al. (2007) Technikfolgenabschatzung - Theorie und Praxis 16,94.

13Analysis of Bio-Oils

Dietrich Meier and Michael WindtThunen-Institut fur Holzforschung, Germany

13.1 Definition

“Bio-oil” is a term for liquids that are obtained by fast or intermediate pyrolysis of lig-nocellulosic biomass through condensation of the hot vapors from the thermochemicalconversion process. Some other terms exist, such as: fast pyrolysis liquid, biocrude, andbiocrude oil (BCO).

13.2 Introduction

Bio-oils are derived from the thermal scission of polymeric cell wall constituents of theplant material, namely cellulose, hemicelluloses, and lignin. Cellulose and hemicellulosesare chain molecules made of sugar units connected via glucosidic bonds. They are denselypacked and cellulose has even crystalline structures. Details can be found elsewhere [1]. Onthe other hand, lignin is a three-dimensional aromatic polymer built from mainly phenyl-propanoic moieties connected by ether and carbon–carbon bonds. The thermal cracking ofthese natural polymers is a very unspecific process as many different bonding energies arepresent, leading to hundreds of single individual components that make up the bio-oil. Itschemical analysis therefore creates a great challenge for the chemist not only because ofthe numerous components but also due to phase separation and instability during storage.Phase separation is the result of a too high water content in the bio-oil. Generally, wateris the main single component in bio-oils and is introduced both by the moisture content of




the processed feedstock and in addition as reaction water derived from the scissions of theglucosidic bonds between the sugar units of the carbohydrates. The pyrolysis of a bone-drybiomass still leads to a water content of approximately 15 wt.% in the bio-oil.

13.3 General Aspects

Analysis of bio-oils requires skilled persons, well-proven methods, and carefully maintainedinstruments. Unfortunately, the term “bio-oil” is often misunderstood as the term “oil” isrelated to oily substances, which are lighter than water, immiscible with it, have low oxygencontent and thus low polarity. However, nothing compares with “bio-oil” as it is misciblewith water to a great extent until phase separation occurs, giving an aqueous top phaseand a tarry bottom phase. Crude bio-oils contain a lot of oxygen (40–45 wt.%) and arecompletely soluble in polar organic solvents such as methanol, ethanol, acetone, and so on.

13.3.1 Before Analysis

Due to its micro-emulsion character (see Figure 13.1) and phase separation tendency, correctsampling is of paramount importance before any analysis. Make sure that the sample isproperly homogenized, for example through vigorous shaking.

Correct storage is also an important aspect. Based on various round robins organized bythe Pyrolysis Network Europe (PyNE) and the International Energy Agency (IEA) Task 34“Pyrolysis” it became clear that keeping samples in a refrigerator is most suited to delayingageing processes [2–5].

13.3.2 Significance of Bio-Oil Analysis

Knowledge of the chemical composition of bio-oils together with their physical properties isimportant, as they are a result of feedstock properties and process conditions (temperature,quenching, residence time, char removal, etc.). Moreover, the composition may determine

Figure 13.1 Pictures of one phase bio-oil from spruce (left) and phase separated oil frommiscanthus (right).

Analysis of Bio-Oils 229

further process steps, such as upgrading or direct use in boilers, diesel engines, and gasturbines.

13.3.3 Post-Processing Reactions

In fast pyrolysis, residence time of hot vapors is in the range of 1–5 seconds until theyare quenched to room temperature. Hence, little time remains for further recombinationreactions so the obtained bio-oil resembles a mixture of highly reactive components.

Most of the numerous monomeric species are unsaturated components having at leastone reactive functional chemical group. They can be summarized as follows:

• Aliphatic hydroxyl

• Phenolic hydroxyl

• Carboxyl

• Carbonyl

• Alkenyl

• Vinyl.

The presence of these functional groups may easily lead to the following reactions:

• Condensation (phenols + formaldehyde)

• Polymerization (Aryl-vinyl)n, (Aryl–C–C=C)n

• Acetal formation (R–C=O + alcohol).

These reactions can be further catalyzed by solid particles in bio-oil, such as ash and microcarbon.

13.3.4 Overall Composition

The following Figure 13.2 demonstrates the overall composition of bio-oil.The pie chart shows four sections with their typical percentage yields. Water is the main

single component. It stems from both unavoidable reaction water and adjustable moisturecontent in the feedstock. The main portion of bio-oil resembles monomeric components,which can be separated and analyzed by gas chromatography (GC) and high performanceliquid chromatography (HPLC). Another important fraction is represented by oligomericcomponents. They are summarized as “pyrolytic lignin” as they are insoluble in water andoriginate from lignin. This fraction cannot be vaporized and is thus amenable to GC.

24%

24%

14%

38%

GC-detectable

polar (HPLC)

oligomers(pyrolytic lignin)

water

Figure 13.2 Overall composition of fast pyrolysis liquids.


13.4 Whole Oil Analyses

There are numerous methods available for chemical analysis of bio-oils. The first decisionto be made is whether the sample should be fractionated or not. The answer depends on thequestion and objective. However in most cases whole oil analysis by gas chromatographygives sufficient answers to the problem.

13.4.1 Gas Chromatography

Gas chromatography is a well established method for separation of complex mixtures intotheir individual components. In principle, a sample is dissolved in an organic solvent andinjected with a syringe into a special injector. The injector may be cold or pre-heated. In anycase, the samples must be heated so that the components are vaporized to be transportableby a carrier gas through a column, which currently is made of quartz capillaries. The innerwall of the column is coated with a thin film made of polysiloxanes or other high-boilingsubstances (e.g., polyethylene glycols, PEG), which effect the separation due to interactionwith the molecules of the sample. After passing through the column, which is at least30 m long and preferably 60 m, the components are measured by a detector. The mostcommon one is a flame ionization detector (FID), in which the components are burnt ina hydrogen flame, ionized and the resulting signal amplified, digitized, and stored on acomputer awaiting further data analysis with dedicated software packages.

Today, many GCs are also equipped with a mass selective detector (MSD), which allowsthe obtainment of a mass spectrum of every eluted compound. The mass spectra can beregarded as a fingerprint, such that in many cases an unambiguous identification is possible.Nowadays, commercial databases are available for spectra comparison.

13.4.1.1 Dilute and Shoot

The most effective and simple method for bio-oil analysis is the “dilute and shoot” method.It requires little or no pre-treatment of the sample. Bio-oil is simply diluted with an organicsolvent and measured, preferably with a GC-FID/MS system.

Sample preparation starts by vigorous shaking to ensure proper mixing of the sample.The next step is the determination of the water content in order to determine the organicmass of the fraction to be dissolved. Depending on the chromatographic conditions, suchas split ratio and injection volume, the organic portion should be adopted.

In the following, typical conditions for sample preparation of a single phase bio-oil arepresented. Weigh an amount of bio-oil in the range of 60 μg, that gives an amount of45 μg of organics, assuming typical water content of 25 wt.%. For quantitative analyses,the internal standard is added to the sample. The internal standard should be a componentthat does not co-elute with a sample peak and is absent from the sample. We have chosenfluoranthene. It is very stable, elutes at the end of the complex chromatogram, and isnot present in bio-oil. An internal standard stock solution should be prepared with aconcentration of circa 200 μg/ml acetone. From this stock solution, exactly 1 ml is usedand added to the bio-oil sample to be analyzed. Bio-oil is readily dissolved in acetone andthe exact amount of internal standard is now known. The sample is now ready for analysisby gas chromatography. Figure 13.3 illustrates the FID gas chromatogram of a whole oildemonstrating the complexity of the sample. The major peaks are named. Interestingly, with


10.00

FID

-Sig

nal

Aceta

ldehyd

e, hyd

roxy-

Aceto

l

Pro

panoic

acid

Buta

none, 1-

hyd

roxy-

2-

Pro

pio

nald

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Buta

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Cyc

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Pyra

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)-

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3-m

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-meth

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Guaia

col, 4

-rth

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col, 4

-pro

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-meth

yl

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ngol, 4

-eth

yl-

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-ally

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-(1-

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trans

Syri

ngald

ehyd

e

Sin

apald

ehyd

e

Syri

ngyl aceto

ne

Sin

apyl alc

ohol, Isom

er

of

Dih

ydro

sin

apyl alc

ohol

Flu

ora

nth

ene (

IS)

Aceto

syri

ngoneLevo

glu

cosan

Vanill

in

Dia

nhyd

ro-a

lpha-D

-glu

copyra

nose, 1,4

:3,6

-

Anhyd

rosugar

unknow

n

Acetic a

cid

20.00 30.00 40.00 50.00 60.00 R. T.

Figure 13.3 GC chromatogram of a whole bio-oil from beech wood.

the GC method mentioned it is possible to separate roughly carbohydrate and lignin derivedcomponents, with some exceptions such as levoglucosan and other minor anhydrosugars.This is demonstrated in Figure 13.4 for carbohydrate derived components and in Figure 13.5for lignin derived components.

The GC-conditions are presented in Table 13.1.

13.4.1.2 Headspace

Headspace analysis of bio-oils gives additional information on very volatile organic com-pounds (VVOCs) such as methanol, acetone, formaldehyde, acetic acid methyl ester, formicacid, and so on, which normally co-elute with the solvent of bio-oil used for direct analysisby GC, in this case acetone. Headspace analysis is basically a technique whereby the vaporsin the gas zone above, and in equilibrium with, a solid or liquid is sampled. The advantageof this approach is that “dirty” liquid and solid samples can be analyzed, providing four tofive orders of magnitude greater sensitivity. A typical application is the determination ofblood alcohol, where blood fills the vial and is heated until an equilibrium exists betweenthe liquid phase and the gas phase in the headspace of the sample vial. Normally, the sampleis conditioned at a certain temperature to ensure phase equilibrium of the analytes. Theprinciple of the method is illustrated in Figure 13.6.

For complex samples like bio-oil, headspace sampling is the fastest and cleanest methodfor analyzing VOCs. Generally, the headspace sample is prepared in a vial containing thesample, the dilution solvent, a matrix modifier, and the headspace (see Figure 13.1). Volatile


FID

-Sig

na

l

10.00 15.00 20.00 25.00 R. T.

Bu

tan

dio

ne,

2,3

- (D

iace

tyl)

Bu

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on

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, 3

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no

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3-

Fu

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2-

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pa

n-2

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1-(a

ce

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an

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ranyl)-

Cyc

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-dih

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)-

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-

Cyc

lop

en

ten

e-1

-on

e,

2-h

ydro

xy-

2-

Fu

ran

-2-o

ne,

3-m

eth

yl-2

(5H

)-Is

om

er

of

Cyc

lop

en

ten

-1-o

ne,

3-e

thyl-2

-hyd

roxy-

Ph

en

ol

Bu

tan

ed

ial

Ace

tald

ehyd

e,

hyd

roxy-

Ace

tol

Ace

tic a

cid

To

lue

ne

Figure 13.4 GC chromatogram section of carbohydrate-derived products.

35.00 40.00 45.00 50.00 55.00 60.00 65.00 70.00 R. T.

Gu

aia

co

l

FID

-Sig

na

l

Cre

so

l, o

-

Cre

so

l, m

-

Gu

aia

co

l, 4

-me

thyl

Gu

aia

co

l, 4

-eth

yl

Gu

aia

co

l, 4

-vin

yl-

Dia

nhyd

ro-a

lph

a-D

-glu

co

pyra

no

se,

1,4

:3,6

-

Gu

aia

co

l, 4

-ally

l-

Syri

ng

ol

Syri

ng

ol, 4

-me

thyl

Syri

ng

ol, 4

-eth

yl-

Syri

ng

ol, 4

-vin

yl

Syri

ng

ol, 4

-ally

l-

Syri

ng

ol, 4

-(1-

pro

pe

nyl)-,

cis

Syri

ng

ol, 4

-(1-

pro

pe

nyl)-

tra

ns

An

hyd

ro-b

eta

-D-g

luco

pyra

no

se

(L

evo

glu

co

sa

n)

Syri

nga

lde

hyd

e

Ho

mo

syri

nga

lde

hyd

eA

ce

tosy

rin

go

ne

Syri

ng

yl a

ce

ton

e

Pro

pio

syri

ng

on

eS

ina

pyl a

lco

ho

l, I

so

me

r

Dih

ydro

sin

apyl a

lco

ho

l

Flu

ora

nth

en

e (

IS)

Sin

ap

ald

ehyd

e

Ph

enyle

tha

no

ne,

4-h

ydro

xy-

3-m

eth

oxy; (A

ce

tog

ua

iaco

ne

)

Va

nill

inH

ydro

qu

ino

ne

Gu

aia

co

l, 4

-pro

pe

nyl-; (I

so

eu

ge

no

l) c

is

Gu

aia

co

l, 4

-pro

pe

nyl-; (I

so

eu

ge

no

l) t

ran

s

Figure 13.5 GC chromatogram section of lignin-derived products.


Table 13.1 Conditions for GC/MS/FID analysis of bio-oils.

System Description

Gas chromatograph Hewlett Packard HP 6890, with microflow splitterMass selective detector Hewlett Packard HP 5972Autosampler CTC Analytics (Combi Pal)Carrier gas Helium, constant flow 1.3 ml/minColumn (medium polar) Varian DB 1701, 60 m x 0.25 mm

Film thickness: 0.25 μmCoating: 14%-Cyanopropylphenyl-86%dimethylsiloxan

CopolymerInjection volume 1 μlSplit ratio 1 : 15Injector 250 ◦CFlame ionization detector 280 ◦CIon source 140 ◦CIonization energy 70 eVOven program 45 ◦C, 4 min. hold

3 ◦C/min to 280 ◦C, 20 min holdData evaluation MassFinder4®

components from bio-oil can be extracted from non-volatile fractions and isolated in theheadspace or vapor portion of a sample vial. An aliquot of the vapor in the headspace isthen delivered to a GC system for separation of all of the volatile components. For moreinformation on headspace analysis, check out the textbook by Kolb and Ettre [6].

13.4.1.2.1 Static Headspace Standard headspace analysis of bio-oil samples is astraightforward method. For comparative and quantitative work, the sample volume shouldbe constant as this parameter influences the sensitivity of the method. As a general rule the

SVolatileanalytes

Sample, dillution solventand matrix modifier

Step 1Sample reaches

equilibrium

Step 2Sample is extractedfrom the headspace

Step 3Sample is injected

G

Figure 13.6 Principle of headspace analysis.


method is more sensitive when the ratio of sample volume to vial volume is high, or whenthe headspace is relatively small. However, this depends also on the individual partitioncoefficients of each analyte [6].

A typical preparation of headspace analysis of bio-oil is as follows:A 20 ml headspace vial is filled with 500 mg of bio-oil. In order to compensate for different

water contents and to exclude any matrix effect of bio-oil (e.g., amount of pyrolytic lignin),7 ml of a saturated sodium chloride are added plus a small amount (tip of a spatula) of solidsodium chloride (NaCl). This ensures permanent saturation of the solution. The salt additionchanges the intermolecular interaction between solute and solvent. By changing the samplematrix it is possible to increase the partition coefficient and thus the headspace sensitivity.In the case of aqueous samples like bio-oil, the addition of salt makes the polar compoundsin the polar matrix leave the liquid phase, so that they move to the headspace at a giventemperature. For quantitative work, the standard addition method is applied, viz. knownamounts of target compounds are added to the sample solution at various concentrationsto establish a calibration curve. The increase in peak area is then attributed to the amountof added standard. An example of the addition of acetone is illustrated in Figure 13.7. Thepicture also shows another option for headspace analysis, namely “full evaporation FET.”This technique is described in the following paragraph.

Figure 13.7 Determination of acetone content by addition of a known amount into the orig-inal bio-oil sample.


2

4

6

7

53

1

without aqueous matrix (full evaporation)

with aqueous matrix

R. T.

1 Formaldehyde

2 Acetaldehyde

3 Methylformate

4 Propanal

5 Acetone

6 Acetic acid methyl ester

7 Methyl alcohol

FID

-Sig

nal

Figure 13.8 Comparison of headspace analysis with and without matrix.

As discussed before, bio-oil is a difficult matrix for headspace due to its complexcomposition consisting mainly of water, water-solubles, and water-insolubles (pyrolyticlignin). To eliminate any disturbing influence, the “Full Evaporation Technique” (FET) canbe applied, so that the matrix can no longer influence the phase distribution and all analytescan be transferred from the sample into the gas phase. In practice, this is accomplished byselecting a large vial (20 ml) filled with a tiny amount of sample. Under proper heatingconditions exhaustive extraction can be achieved [7]. An example is given in Figure 13.8.The upper chromatogram shows the result of the standard headspace technique and thelower chromatogram depicts the result of using the full evaporation technique with thesame bio-oil sample. It can be seen that the chromatograms are very similar with respectto the very volatile components. FET even shows in addition a formaldehyde peak. Somedifferences can be observed further on in the process, but they are irrelevant because thesecomponents can be detected by the dilute and shoot method discussed earlier.

13.4.1.2.2 Solid Phase Micro-Extraction (SPME) Solid phase micro-extraction(SPME) is one of several important sample preparation techniques. It integrates sampling,extraction, concentration, and sample introduction into a single step. It enables solventlessextraction via a fused silica or stainless steel fiber coated with a thin film polymer, whichacts as the solvent during the extraction of compounds. It is comparable with a GC columnturned inside out. Several fiber coatings with different polarities are available. The fiberis mounted on syringe-like device for extraction of analytes from various matrices andintroduction to a chromatographic system (see Figure 13.9).

The extraction principle can be described as an equilibrium process in which the analytepartitions between the fiber and the aqueous phase. Either immersing the fibers in a liquidsample or exposing them to the headspace of a solid, liquid, or gas sample may extractvolatile or semi-volatile organic compounds. The analytes partition or adsorb to the polymer


Fiber

Bio-oil

Figure 13.9 Headspace vial with introduced syringe with fiber tip.

coating on the fibers. Hence, the equilibrium in the gas phase is changed as the adsorbedmolecules do not create any vapor pressure, consequently more analytes can be released outof the liquid phase into the headspace. The absorbed analytes may be thermally desorbedin the injector of a gas chromatograph (GC) for separation and further quantification.

Aside from the obvious advantages of a solventless sample preparation technique, SPMEis fast, amiable to automation, and easy to use. Like other sample preparation techniques,however, users still need to understand the nature of the target analytes and the complexityof the bio-oil matrix. As Figure 13.10 shows, the peaks eluting in the first 15 minutes arebroad. This is because of the relative long desorption time from the SPME fiber. Analyteshave no exact starting point at the beginning of the column. Improvement is possible by

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00 55.00 min.

Figure 13.10 Chromatogram of whole bio-oil after SPME-GC/FID.


using cryofocusing using a pre-cooled injector (e.g. −20 ◦C). Furthermore, the peaks in thechromatogram mainly show phenols due to the selectivity of the fiber.

13.4.2 NMR

Spectroscopic techniques like Nuclear Magnetic Resonance (NMR) can give insights intofunctional groups present in the bio-oil. Some authors have used 1H and 13C-NMR as ameans of obtaining approximate ratios of the chemical environments of protons and carbonatoms and have determined the approximate aromatic : aliphatic ratios [8, 9], but little isknown about the results of in-depth analysis of bio-oils using NMR that includes the relativesubstructure compositions [8].

Figure 13.11 gives an example of 13C-NMR analysis of three different pyrolysis liquidsmade from mixed hardwood, poplar, and beech. Acetone-d6 can be used as solvent. Therough assignments are also given. In a recent paper, Mullen et al. describe the applicationof NMR techniques (both proton and carbon NMR) in a more detailed manner. They alsocarried out DEPT experiments to get a more detailed picture of the neighboring of carbon

Aromatic carbons Carbons from propylside chain

Saturated alipahticcarbons

–OCH3

Solvent

Mixed softwood

Poplar

Beech

150 130 110 90 70 50 30 ppm

Figure 13.11 13C-NMR spectra of different bio-oils.


atoms, allowing further classification of the number of attached protons. A drawback of themethod is the long acquisition time. NMR experiments are dependent on the instrumentsused, so no general information can be given. For experimental details, please refer torecent papers [8, 10].

13.4.3 FTIR

FTIR is a technique for analyzing functional groups like hydroxyl, carbonyl, aromatics, orstructures with heteroatoms (N, Cl, O, S, P). The application of FTIR for bio-oils is verylimited [11, 12]. This can be attributed both to the high water content and the extremelyhigh complexity of its composition. Table 13.2 gives the assignments for general spectrafrom wood as a basis for spectra interpretation of bio-oils.

Wulzinger [17] has established a system for comparison of bio-oils by dividing thefinger-print region of the whole spectrum into seven sections (see Figure 13.12). The areaof each section can be calculated and used for semi-quantitative comparisons of bio-oils.He also established a table (Table 13.3) with band assignments derived from Table 13.2.

As bio-oils contain a lot of water, care must be taken during sample preparation forFTIR analysis. If an absorption technique needs to be used, the liquid sample can be placedbetween two Irtran II windows (zinc selenide, ZnSe). Film thickness should be around20 μm.

Table 13.2 Assignment of bio-oil IR bands in the fingerprint region.

Group frequency(cm−1) Functional group/assignment

1724 Unconjugated carbonyl groups1657 Conjugated carbonyl and carboxyl groups1610 Skeletal vibrations of aromatics1518 Skeletal vibrations of aromatic structures1462 –CH deformation, asymmetric in –CH3 and –CH21427 Skeletal vibrations of aromatic structures with –CH-deformation1370 Symmetric –CH3 deformation and phenolic hydroxyl groups1331 Skeletal vibrations of the syringyl ring in conjunction with carbonyl

groups [13]; condensed guaiacyl units [14]1273 Uncondensed guaiacyl rings and C=O stretch; asymmetric C–O

stretch of phenolic ethers [15]1240 C–C, C–O and C=O stretch, methoxyl groups especially in syringyl

moieties [16]1221 C–O in O=CO groups, phenolic OH [15]1197 Ester (H–CO–O–R)1182 Ester (R–CO–O–R)1115 Vibrations of guaiacyl ring, overlay with stretch of syringyl ring1081 C–O deformation in secondary alcohols and aliph. Ethers1051 Furans; uncondensed guaiacyl moieties1028 C–O deformation in primary alcohols; Ether (C–O–C)953 Asymmetric HC=CH vibrations


2.0Absorption

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

–0.21900 1800 1700

1818.91554.6

1496.8

1685.8

II

I

III

IV

V

VI

VII

1047.3

1311.61180.4

941.3

1600 1500 1400 1300

wavenumber

1200 1100 1000 900 800 700

Figure 13.12 Example IR-spectrum of bio-oil from beech with the sections for integration [17].From P. Wulzinger, Untersuchungen zum Einsatz von Katalysatoren bei der Flash- Pyrolyse vonHolz, Department of Biology, Hamburg University, Hamburg, 1999.

13.4.4 SEC

Owing to the complex nature of pyrolysis oil and its intrinsic instability, chemical charac-terization of fast-pyrolysis oil is challenging. Physico-chemical analytical techniques (e.g.,viscosity, water content, and elemental composition) are available, but provide limitedinformation to characterize the oil [18]. Analyses at the molecular level (e.g., NMR, FTIR,LC, GCMS/FID and several fractionation schemes) are still under development [19]. An

Table 13.3 Assignment of area section from IR-spectra of bio-oil [17]. From P. Wulzinger,Untersuchungen zum Einsatz von Katalysatoren bei der Flash- Pyrolyse von Holz,Department of Biology, Hamburg University, Hamburg, 1999.

Section Main assignment Wave number (cm−1)

I Unconjugated aldehydes, carbonyls withoutunconjugated aldehydes

1818.9–1685.8

II Carbonyl groups 1818.9–1554.6III Aromatics 1554.6–1496.8IV Aromatics, phenolic OH 1496.8–1311.6V C–O; phenolic esters 1311.6–1180.4VI Unspecific degradation products 1180.4–941.3VII Alcohols and furans 1180.4–1047.3


1 2

Mixture beforeentering the

column

Mixture uponhead of the

column

Sizeseparation

begins

Completeresolution

3 4

Figure 13.13 Separation mechanism of size exclusion chromatography.

important technique to analyze pyrolysis oil is size exclusion chromatography (SEC), alsoknown as gel permeation chromatography (GPC). With this technique an indication of themolecular mass distribution of a sample can be obtained [20–23].

The separation mechanism of SEC is based on volume exclusion (see Figure 13.13): forsmaller molecules more pore volume is accessible, thus these molecules elute later thanmolecules with a larger size [24].

In practice, secondary separation effects, for example hydrogen bonding with the mobilephase and/or adsorption with the column packing, may affect the basic separation mecha-nism. Hydrogen bond formation between the solvent (often THF) and the sample can lead toa lower elution time [25] whereas interactions between the column packing and solute canlead to a longer retention time. The interactions between the column packing and solute arepartly responsible for the lower than expected molecular weights as observed, for example,for aromatics. Garcia-Perez et al. [20] and Greinke and O’Conner point out [26] a relationbetween the solvent polarity and the degree of interaction between the column packing andsolute. Since the column used for pyrolysis oil analyses is usually a co-polymer of styreneand di-vinyl benzene [27], the affinity of non-polar compounds to the column packing isexpected to increase as solvent polarity increases. A critical view on the use of THF wasreported recently by Hoekstra et al. [28]. In general, polystyrene standards with a suitablemolecular mass range between 162 and 29 510 g/mol can be used for calibration. In thisrange and for these types of columns a linear relation exists between the elution time andthe logarithm of the molecular mass. It is important to point out that the SEC column isdesigned to separate molecules up to 30 000 g/mol while molecular weights up to approx.2000 g/mol are observed in pyrolysis oil. The SEC system was operated at the lower endof the molecular weight range.

A proper combination of solvent/mobile phase and column material should limit the sec-ondary separation mechanisms as much as possible. In a recent paper, Ringena et al. favoredthe use of DMSO as solvent with addition of lithium chloride to suppress adsorption effects[29]. This approach was developed during a round robin on lignins [27]. This approach hasbeen successfully applied to pyrolytic lignins and could also be applied to bio-oils [30].


Table 13.4 Conditions for SEC of pyrolytic lignins and bio-oils.

System Agilent 1100er Series

Flow 0.8 ml/minDetectors UV-Photometer, 𝜆 = 254 nm

Refractive Index detector (RI) @ 40 ◦CColumns 2 PolarGel-L-columns in series (Varian), 300 x 7,5 mmOven 60 ◦CInjection volume 100 μlSolvent Dimethylsulfoxide (DMSO) + 1% Lithiumbromide (LiBr)Calibrations 10 Polyethylenglycol standards

In the mass range between 106 and 21 030 g/molSoftware HP Chemstation GPC add-on (PSS)

Table 13.4 gives the actual SEC conditions for pyrolytic lignins and bio-oils: 1 to 2 mgbio-oil is added to 1 ml Dimethylsulfoxide (DMSO) followed by continuous mixing in aroller mixer for 24 hours. Prior to analysis samples are filtered through a 0.2 μm filter.

13.5 Fractionation Techniques

Its heterogeneous composition makes bio-oil impossible to analyze completely with onemethod. All results from the analytical techniques described in the preceding sections havealso their specified limitations. While whole analysis methods are relatively easy to apply,they suffer from giving always a fragmented picture of the overall composition of bio-oil.For example, in the case of gas chromatography, the non-volatiles like pyrolytic ligninare stuck in the injection system. Headspace, for example, shows only the very volatilemolecules.

Nevertheless, analysis of bio-oil can be facilitated through fractionation and subsequentstudy of the fractions. This gives deeper insights into the composition on the one hand andmethods can be better adapted to the character of the fraction on the other hand. Fractionationmay also lead to improved qualitative determination of compounds because, for example,co-elution of components is reduced or because application of derivatization techniques ispossible, leading to detection of formerly undetectable components. The following sectionhighlights established and newly developed fractionation methods, which add to the morecomplete characterization of bio-oils.

13.5.1 Addition of Water

Water is the most prominent single component in bio-oil (see Figure 13.2). The typicalwater content ranges from 20–30 wt.%. Mixing with water is possible up to about 45 wt.%when phase separation occurs and a tar-like product separates as a bottom phase and anaqueous phase as a top phase. Radlein et al. [31] were the first to describe the tarry phaseas “pyrolytic lignin.”


pyrolysis oil

filtration

turrax

water

water

ice

vacuum dryingpyrolytic lignin

as fine powder@ 20 °Cwashing

Figure 13.14 Schematic diagram of pyrolytic lignin recovery.

13.5.1.1 Precipitation of Pyrolytic Lignin

Pyrolytic lignin (PL) is a frequently used term for the high molecular water-insoluble partsof bio-oil. Scholze and Meier [32] for the first time reported on the isolation of a powder-likesolid material from bio-oil by adding small amounts of oil to an excess of water (weightratio > 1/200). This approach is suitable for quantitative precipitation of hydrophobic ligninderived fragments, which results after drying as a brown powdery material.

The production of pyrolytic lignin is performed as follows: 60 ml pyrolysis oil is addeddrop wise to 1 liter of ice-cooled water while stirring at 6000 rpm with an ultra turrax (oilto water ratio must be a least 1 : 100). A schematic diagram of the experimental setup isdepicted in Figure 13.14. After slowly adding pyrolysis oil to water the precipitation ofpyrolytic lignin begins. Subsequently, the pyrolytic lignin is filtered over a Buchner funnel,re-suspended in water and slowly stirred for about 4 h in order to remove further watersoluble material. Finally, the suspension is filtered again and the wet pyrolytic lignin isdried carefully under vacuum at room temperature.

Pyrolytic lignin precipitation is material- and time-consuming and in the majority ofcases simply done to obtain the content of PL. Therefore, simpler methods are needed toget satisfactory reproducibility and to make the process more flexible towards the amount ofavailable sample material. Hence, for PL quantification the following modified precipitationmethods can also be applied:

(A) A ultra turrax® mixer (22 000 rpm) filled with distilled water (500 ml) is used toprecipitate PL through drop-wise feeding 1 ml of bio oil. The obtained oil-in-watersuspension is filtered under slight vacuum (500 mbar) using Blauband filter paper.Sticky residues in the mixer are dissolved in ethanol, which is removed under vacuum(78 ◦C; 200 mbar) with a rotary evaporator.

(B) A rapid application can also be carried out by a careful introduction of a 100 μl bio-oilsample in 30 ml water using a micro Ultra Turrax (20 000 rpm) and a 60 ml plasticsyringe. The syringe is first coupled to a disposable syringe filter (1 μm) connectedvia a Luer lock connection and then mounted on a vacuum manifold normally usedfor solid phase extraction (SPE). After filtering the suspension, the PL residue on thefilter is rinsed with another 30 ml of water, dried under vacuum and weighed to get thePL-content by weight difference.


Table 13.5 Characteristics of pyrolytic lignins from different feedstocks [33]. From R.Bayerbach, Uber die Struktur der oligomeren Bestandteile von Flash-Pyrolyseolen ausBiomasse, Department Biologie der Fakultat Mathematik, Informatik undNaturwissenschaften, Universitat Hamburg, Hamburg, 2006.

Pyrolytic WaterExperiments lignin content Ash C H 0 N

BCOfeed (No) (%) (%) (%) (%) (%) (%) (%)

Beech 6 15.55 (±3.15) 8.9 0.73 46.7 6.4 45.9 0.2HCl-Beecha 2 8.60 (±0.20) 9 0.2 48.4 6.3 44.5 0.7Oak (sup) 2 13.5 (±1.40) 8.1 0.36 46.3 6.3 46.8 0.3Oak (heart) 2 8.40 (±0.20) 8 0.16 44.9 6.2 48.6 0.2Robinia (heart) 4 15.05 (±1.45) 6.9 0.21 47.7 6.3 45.6 0.2Pine 4 17.20 (±2.20) 10 0.23 49.1 6.3 43.7 0.7Spruce (sup) 4 17.25 (±0.65) 8.8 0.2 48.9 6.4 43.9 0.6Spruce (heart) 3 18.60 (±1.30) 8.4 0.21 48.9 6.6 44.2 0.1Juniper (heart) 3 18.90 (±2.10) 8.8 0.18 50.9 6.1 42.8 0.1Bamboo 4 17.00 (±4.10) 8.3 0.54 46.9 6.3 46 0.3Wheat straw 2 11.50 (±0.90) 9.4 5.12 44.1 6.2 44.1 0.4Maize silage 1 18.30 (n.a.)b 8.4 3.47 44.6 6.4 44.6 :Rape silage 1 12.70 (n.a.)b 4.9 5.78 48.3 7.1 37.4 1.4

aHGI treated beech: 400 g beech sawdust washed over 24 h in 0.1 molar HGI.bNot analyzed.

The method from Scholze and Meier is also suitable for preparing larger amounts ofpyrolytic lignin as presented in Table 13.5 [33]. This summary table provides basic dataabout yield, water content (ISO 760:1978-12), ash content (ASTM D2584), and elementalcomposition. Displayed are the average values only for the PL yield. Standard deviation isgiven, which is high compared to the small-scale methods.

With the help of numerous analytical methods, Bayerbach and Meier [34, 35] proposeda structural formula for pyrolytic lignin presented in Figure 13.15.

13.5.1.2 Size Exclusion Chromatography of Pyrolytic Lignin

Apart from its chemical composition, the most interesting data on bio-oil is its averagemolecular weight determination and weight distribution by size exclusion chromatography(SEC), as these parameters reflect to a great extent the quality of bio-oil. The lower themolecular weight and the narrower the distribution, the easier is combustion (smallermolecules combust faster), the lower is the viscosity, and the lesser the tendency to phaseseparation. BCO (beech) and its respective Py-lignin fraction are compared in Figure 13.16.

13.5.2 Removal of Water (Lyophilization)

The converse of water addition is its total removal via freeze-drying, resulting in a highmolecular weight residue, which resembles not only the lignin fragments but also carbo-hydrate derived molecules. Lyophilization offers further analytical possibilities for charac-terization of non-volatile components of BCO.


Tetramers

Structures of:

O

O

OH

OH

O

OH

OH

OH

OH

OH OH

HOHO

OH

OH

OH

HO

HO O OO

O

O

O

O

OO

O

O

O

O

O

O

O

O

stilbene & phenyl coumaran structures

Pentamers

Structures of: stilbene, phenyl coumaran, biphenyl &

diphenylether

Hexamers

Structures of: stilbene, phenyl coumaran, biphenyl,

diphenylether & carboxy groups

Further structures:

Heptameres, Oktameres...

Figure 13.15 Proposed structures for pyrolytic lignins in bio-oils [34]. Reprinted from R. Bayer-bach and D. Meier, 2008, with permission from Elsevier.

The procedure is simple: BCO is mixed with a dedicated amount of water, a reasonableratio sample : water of 1 vol : 10 vol. Depending on the initial water content of the bio-oilsample the water amount can be reduced. Prior to lyophilization the aqueous sample isfrozen at minus 20 ◦C and then freeze-dried.

The visual appearances of different freeze-dried samples are illustrated in Figure 13.17.The crude bio-oil (a) exhibits heterogeneous agglomerates of convolutedly interwovenpowdery, tarry, and syrupy looking fractions. Picture (b) illustrates some FDRs of aqueous

2.0

1.5

1.0

0.5

0.040 400 4000

Crude beech oil Pyrolytic lignin

40 000 Mol weight (Da)

Figure 13.16 SEC characterization of BCO and related Py-lignin.


BCO(beech)

A B

D′

C′

B′

A′

Figure 13.17 Pictures of freeze-dried fractions of (a) biocrude oil and (b) HDO aqueousphases.

fractions obtained from hydro-deoxygenated biocrude oil (HDO). In comparison to thefreeze-dried whole oil (a) their images look more homogeneous because of the HDOpre-treatment. Nevertheless, their individual consistencies might be a function of differentproduction conditions. The appearance of the FDRs (b) varies between a black powder(A′), tar (B′ and C′), and a brown syrup (D′). Hence, freeze-dried residues (FDRs) areinteresting materials for demonstrative studies.

Water content and FDRs from organic fractions, aqueous fractions, and a bio-oil arecompared in Figure 13.18. On the one hand the data allow for a draft control of previousanalytical results (e.g., GC/MS). On the other hand – with exception of the non-aqueous

100

90

80

70

60

50

[wt. %

]

40

30

20

10

0

A B

Organic fractions

C D A′ B′

Aqueous fractions BCOs (beech)

C′ D′ BCO 1

Lyophilisate residues Water content

BCO 2

Figure 13.18 Comparison of water contents and freeze-drying residues from several biomassconversion products.


0

0.1

0.2

0.3

0.4

800180028003800

Ad

so

rpti

on

Wave number [cm–1]

A′ B′ C′

R-C-OR′

D′

R-O-R′

CH2/H3

HO

-

H3 C

O

H3 C

O

HO

Figure 13.19 Comparison of FTIR spectra from various freeze-dried aqueous phases.

volatiles – nearly the complete sample is monitored. Hence, if the total is around 100% theFDR then represents the complete non-aqueous part.

As mentioned before, functional groups can be analyzed with FTIR measurements.Lyophilization offers the opportunity to also analyze aqueous fractions, since water-freesamples are produced. This avoids undesirable overlapping from OH-groups and evenallows the analysis of OH-groups from the organic part of bio-oil. The FDRs describedbefore (A′–D′) are used as exemplars to present the possibility of FTIR measurementsand respective spectra (see Figure 13.19). The FTIR spectra confirm the heterogeneouscharacter for HDO aqueous phases since significant variations in functional groups can beobserved.

13.5.3 Solid Phase Extraction (SPE)

Solid phase extraction (SPE) features a set of special separation functions to be explainedin this chapter. In broad terms, functionality in BCO can be divided into two categories:polar cellulose derivatives and non-polar remaining structures from lignin. The affinityof dissolved analytes for a solid phase through which the sample is passed addressesthe general principle of effect; whereby compounds can be categorized according to theirspecific properties (see Figure 13.20). Solid phase extraction has been developed to separatechemical compounds or groups from a wide variety of matrices. Especially in the area ofpharmacy (e.g. purification) and sport medicine (e.g. doping tests) this method is usedsuccessfully, but in the area of bio-oil only limited information about the utilization of SPEis available [36–38].

Numerous types of SPE-cartridges are now available, and new types are continually beingintroduced. Analogous to Figure 13.20 the systematic procedure required commences withselection of an appropriate type. In that respect, a comprehensive summary of applicableSPE-materials was compiled by Brodzinski [37, 39], who devoted her thesis to the analysisof BCO with SPE. One outcome is the recommendation to use reversed phase C18 columns.


= Matrix

Key to Processes

= Impurity

= Compound of interest

= Solvent A

= Solvent B

= Solvent C

Step 5

Elute furtherset of targetcompounds

Step 4

Wash packingor elute firstset of targetcompounds

Step 3

Addsample

Step 2

Conditionthe SPEtube

Step 1

Select aproperSPE tube

Figure 13.20 Application scheme for utilization of solid phase extraction (SPE).

The columns are conditioned first with methanol followed by water. The amount of usedsample material is around 10 wt.% of the column’s bed material.

The succeeding pattern is the assigned starting procedure; a certain amount of BCOis introduced (see step 3 in Figure 13.20) and distributed on top of the column throughapplication of vacuum, as the samples are often viscous. This helps to distribute themevenly. In the subsequent steps 4 and 5 dedicated solvents are used to elute correspondingfunctional groups.

This approach can serve two different objectives: (i) clean-up to obtain a desired targetfraction and (ii) fractionation into distinctive groups. Brodzinski [39] studied the latter casetaking whole bio-oil. Separations with water and increasing amounts of methanol led to thefollowing Table 13.6. However, the results also revealed incomplete separation and so farthe use of the method is restricted.

However, the clean-up approach using SPE as an enrichment system seems to be promis-ing for the analysis of the sugars in bio-oil. In this case the aforementioned pyrolytic ligninprecipitation method B is the starting procedure.

After filtration of the pyrolytic lignin in the syringe filter, the aqueous filtrate contains allwater solubles in a very dilute form. Lyophilization can now be applied to get at least thenon-volatile portion of the filtrate, which typically enfolds approximately 40 wt.% of thetotal BCO amount. The essential difference to the previous fractionating approach is thatsample quantity is calculated on the basis of the dry organic content.


Table 13.6 Elution of chemical groups with C18 cartridge.

Solvent Groups

100% Water Anhydrosugars, acids, furanes15% Methanol Furan- and pyran derivatives35% Methanol Aromatic ketones and aldehydes50% Methanol Alkyl-derivatives from syringol100% Methanol Alkene-derivatives from von guaiacol and syringol

The filtrate can be passed through a C-18 SPE cartridge for separation of the most polarcompounds comprising mainly sugars. Less-polar lignin derivatives are retained and canbe completely eluted with organic solvents like acetone.

All water-soluble BCO components are now separated into only two solvents: water andacetone. The water phase can be freeze-dried and the acetone phase can be analyzed byGC/MS without injecting dissolved pyrolytic lignin as was the case with the whole oilapproach.

Using lyophilization, volatile components will get lost, but this is not a big problembecause they are easily analyzed with the aforementioned analytical methods, like GC/MSor the headspace technique. The SPE-derived fractions offer now the application of fur-ther analytical opportunities. An interesting option is the possibility to derivatize (e.g.,acetylation) the sample for GC-analysis of sugars.

13.5.3.1 Acetylation for Sugar Characterization

The freeze-dried polar fraction of the SPE separation can be subjected to analysis of sugars,as they are now concentrated in the sample. Whole oil analysis by direct gas chromato-graphic separation gives a good signal for levoglucosan and some other anhydrosugars, buta better approach is acetylation. The derivatization leads to improved vaporization in theinjector and improved peak shape in the chromatogram [40–45].

The analytes are dissolved in dried pyridine and acetic anhydride (500 μl each). Theacetylation proceeds over 24 hours at room temperature. Preliminary investigations demon-strate that acetylation is complete under these conditions. Reference materials (compareTable 13.7) like hexoses, pentoses, anhydrosugars, and several other BCO components (e.g.,glycolaldehyde) were individually acetylated to obtain their specific information on massspectra, retention times, and detector response. Some components (e.g., glycolaldehyde orglyceraldehyde) dissociate during acetylation and show multiple peaks.

A typical chromatogram is presented in Figure 13.21. Notwithstanding their marginalresponse, a remarkable number of “sugars” are detected, with levoglucosan being the majorcomponent. Assignment of unknown components to the group of “sugars” is done with thehelp of their mass spectra, as unambiguous identification is not always possible due to lackof reference components.

Apart from sugars, various dihydroxybenzenes are detected. They are also better ana-lyzable in their derivatized form. Although several new sugars could be identified, theiramount is rather low.


Table 13.7 Characterization of various acetylated “sugars” from beech BCO.

RT Acetate from: FID Area RRF wt. %

36.4 Dianhydro-a-D-mannopyranose, 1,4:3,6- 619 877 2 0.0937.6 Catechol, Benzenediol, 1,2- 3 044 410 1 0.2339.1 Anhydro-ß-D-arabino-furanose, 1,5- 130 248 2 0.0239.7 Anhydro-ß-D-xylofuranose, 1,5- 440 569 2 0.0740.8 Unknown sugar 1 006 746 1 0.1142.2 Hydrochinon, Benezenediol, 1,4- 501 736 1 0.0443.0 Benzenediol, methyl- 533 909 1.5 0.0644.4 Unknown sugar 155 694 1 0.0245.0 Benzenediol, ethyl, 883 871 1 0.0749.4 Anhydro-xylitol, 1,5- 600 234 2 0.0954.2 1,4-Benzenediol, 2-methoxy-, 417 074 1 0.0358.2 D-Lyxofuranose 4 270 707 1 0.3261.7 Unknown sugar 925 800 1 0.1064.0 Xylose, 2-O-Methyl-D- 248 023 1.3 0.0265.2 Levoglucosan 22 088 912 1.8 3.0465.6 Xylose, a-D- 1 306 462 1.3 0.1367.2 Anhydro-ß-D-galactopyranose1,6- 476 968 2 0.0767.5 Alpha-D-Ribopyranose 1 211 481 1.3 0.1267.8 Anhydro-ß-D-mannopyranose, 1,6- 669 475 2 0.1068.5 Xylopyranose, ß-D- 596 820 1.3 0.0673.2 Anhydro-D-glucitol, 1,5- 524 293 1 0.0479.3 Fluoranthene (internal standard) 8 833 215 1 0.6681.0 Glucose, α-D- 180 080 1.3 0.0281.1 Glucose, ß-D- 158 683 1.3 0.0289.0 Cellobiose 288 464 1 0.0391.0 Glucose methylglycoside 370 012 1.3 0.04

It should be noted that the recovery rate for the analytes after SPE is unclear. Blanktests with pure levoglucosan in water gave a recovery rate of 97%. On the other hand, onlyapproximately 85% of the levoglucosan yield as determined by direct GC analysis couldbe determined.

Effects of reusing SPE cartridges by multiple extractions are shown in Figure 13.22.The same stock solution was used and the weight of FDRs determined. After using a newcartridge 24 wt.% FRD is determined. In the subsequent repetitions the yields increase byapproximately 4 wt.%. This could be attributed to saturation of active groups on the C18material after the first extraction, resulting in losses and lower yields of FDR.

13.5.4 Solvent Partition

A further approach to separate bio-oil is through solvent addition. Similar to the SPE pro-cedure, this method allows formation of dedicated BCO fractions for subsequent analysis.However, the main difference between SPE and solvent addition is the required time andsample amount. SPE is more effective; it is completed within a few minutes and needs just afew sample amounts: 100–200. On the other hand, solvent extractions (SEs) are appropriate


unknow

n S

ugar

Dia

nhyd

ro-a

-D-m

annopyra

nose, 1,4

:3,6

-B

enzenedio

l, 1

,2-

Anhyd

ro-β

-D-a

rabin

o-f

ura

nose, 1,5

-A

nhyd

ro-β

-D-x

ylo

fura

nose, 1,5

-

Anhyd

ro-x

ylit

ol, 1

,5-

1,4

-Benzenedio

l, 2

-meth

oxy-

d-L

yxo

fura

nose

unknow

n S

ugar

Xylo

se, 2-O

-Meth

yl-D

-

Xylo

se,

α-D

-

α-D

-Rib

opyra

nose

seve

ral C

ate

chols

Anhyd

ro-β

-D-g

ala

cto

pyra

nose, 1,6

-

Levo

glu

san

Anhyd

ro-β

-D-m

annopyra

nose, 1,6

-

Xylo

pyra

nose β

-D-

Glu

cose-α

/β-D

-

unspecifie

d C

ellu

lose F

ragm

ents

Cello

bio

se

Glu

cose m

eth

ylg

lycosid

e

Anhyd

ro-D

-glu

cito

l, 1

,5-

Gly

cera

ldehyd

e (

Acety

lation P

roducts

)

Flu

ora

nth

ene (

inte

rnal sta

ndard

)

40

1.0e5

2.0e5

3.0e5

cts.

50 60 70 80 90 min

Figure 13.21 Selected GC-MSD detectable compounds in SPE sugar phase (acetylated).

0 1 2

Times of use

43 5 620%

21%

22%

23%

24%

25%

26%

27%

FD

R b

ased

on

to

tal sam

ple

weig

ht

28%

29%

30%

Figure 13.22 Yields of FDR after replicate extractions using the same SPE cartridge.


ETHER-INSOLUBLES

DIETHYLETHEREXTRACTION (1:1)

WATER-SOLUBLES WATER-INSOLUBLES

WATER FRACTIONATION (1:10)

PYROLYSIS OIL

pH, water, solids,ash, CCR, CHN, S*, CI*,density, viscosity, pour point,HHV, flash point, GC/MS

volatile acids,alcohols, pH*,TOC/TC*

yield, water, CHN, HHV*, ash*,IR*, Py-GC/MS*

ETHER-SOLUBLES

yield (evap. residue),GC/MS

yield (evap. residue), CHN,GC/MS*, Py-GC/MS*

Figure 13.23 Fractionation scheme for bio-oil based on water and diethylether with basic andadditional analyses for pyrolysis oils. Additional analyses are marked with ∗.

when larger quantities of fractions are required, which is evidently only possible if adequateamounts of sample material are available. Three processes are presently known and will beintroduced:

1. Sipila et al. introduced an analytical scheme for the separation of biomass-based flashpyrolysis oils (see Figure 13.23) [46]. Bio-oil is first mixed with water to separatethe pyrolytic lignin (water insoluble) by dropping BCO under stirring into distilledwater (1 : 10 weight ratio). The water-insoluble fraction (WIS) is removed by filtration(<0.1 μm), washed with distilled water and weighed after vacuum-drying (<40 ◦C).The water-soluble fraction is further extracted with diethylether (1/1). Ether is removedby evaporation and the residues are vacuum-dried and weighed. It should be noted thatevaporation leads to losses of volatile compounds. The diethylether-insoluble fraction isalso vacuum-dried. A temperature below 45 ◦C is used to avoid possible degradation ofcarbohydrates. For further GC/MS analysis this fraction is derivatized by trimethylsily-lation (TMS).

2. Figure 13.24 shows a further development of the scheme depicted in Figure 13.23. Itis extended by introduction of a further separation of the water insoluble (WIS) intodichloromethane (DCM) solubles (low-molecular mass lignin) and DCM insolubles(high-molecular mass lignin) [47–49].


WATER BYKF

PYROLYSIS LIQUID

WATER SOLUBLES WATER INSOLUBLES

DCM-INSOLUBLES

ETHER-INSOLUBLESETHER SOLUBLES

DCM-SOLUBLES

WATER EXTRACTION

SOLIDS AS METHANOL/DCM INSOLUBLESEXTRACTIVES AS HEXANE-SOLUBLES

ETHER EXTRACTIONDICHLOROMETHANE EXTRACTION

O

HO

O

HO

ACIDS BY CE, ALCOHOLS BY GC/FID

ALDEHYDES, KETONES OH O

CHH2C

PHENOLS “SUGARS”

FURANS

C CH3

C CH3 C CH3

H3C OH

OH O

O

H2CCH2

OCH3OHOH

CHOO

HO

O

O

OH

OHOH

Figure 13.24 Solvent fractionation scheme for fast-pyrolysis liquid [47]. Reprinted with per-mission from [37]. Copyright © 2010, American Chemical Society.

A different fractionation approach, developed by Garcia-Perez et al. [20], is presentedin Figure 13.25. In contrast to previous methods the non-polar fraction is the startingpoint. Doubts about the optimal fractionation strategy were the main motivation for thisnew approach.

3. Toluene extraction is done with 7 g of bio-oil dissolved in 200 ml toluene. Woodyextractives, which might interfere with other HMM-compounds, are removed by thisstep. The remaining materials in the toluene insoluble fraction are filtered and solubilized

PYROLYSIS OIL

Toluene extraction

Toluene solublesToluene insolubles

MeOH extraction

Water extraction

Water insolublesWater solubles

Ether solubles Ether insolubles

MeOH insolubles

CH2CI2 extraction

CH2CI2 extractionDiethylether extraction

CH2CI2 insolubles

MeOH solubles

FRACTION 1

FRACTION 2

FRACTION 3 FRACTION 4 FRACTION 5 FRACTION 6

Figure 13.25 Fractionation scheme for bio-oil chemical characterization [20]. Reprinted from[18], with permission from Elsevier.


in methanol (200 ml). A successive filtration step is necessary in order to remove thechar, non-polar waxy materials, and other very heavy oligomeric compounds. A rotaryevaporator is used to remove the solvent and the residue is dried and weighed. It is againdissolved in methanol (0.2 g/ml) and 10 g of this mixture is introduced drop wise in300 ml ice-cooled, distilled water. The water-insoluble fraction is removed by filtrationand washed with distilled water and further extracted with DCM until the filtrate iscolorless.

The solids remaining in the filter are dried at 105 ◦C overnight. The water-soluble fractionis further extracted with 300 ml of diethylether in a separation funnel. Diethylether-solubleand DCM-soluble fractions are evaporated in a rotary evaporator at 40 ◦C. Low molarmass compounds are also removed during the solvent removal. These losses are reportedas volatile compounds.

13.5.5 Distillation

A common method to separate discrete liquids from a heterogeneous sample is distillation.However, bio-oils are complex mixtures of reactive components and are difficult to distillas heating of the oils beyond 80 ◦C leads to coking of the sample. Only a light fractioncan be obtained. Because of this drawback, only some works were identified describingvacuum distillation of BCO from a wood feedstock [50–55].

For the preparation of special fractions like phenols, steam distillation has been studied[56].

Pyrolysis oil from a mixture of birch wood and birch bark was steam distilled and therecovery of phenols at various steam pyrolysis oil ratios was studied. A 14.9 wt.% volatilefraction (based on the total feed oil) was recovered at a steam : oil ratio of 27. The distillatewas analyzed by GC/MS after acetylation and showed 21.3% by wt. of phenolic compoundson pyrolysis oil basis. The distillate was further distilled under a total pressure of 0.7 kPato recover 16 sub-fractions. The steam-distilled fractions were found to be chemically andthermally stable when subjected to further purification processes. The 2,6-dimethoxyphenol(syringol)-rich fraction was separated and further purified. Syringol with a purity of 92.3%could be obtained.

To further assess the comparability of bio-oil with current transport fuels, simulateddistillation was carried out [57] with the following yields presented in Table 13.8.

The results were partly comparable with fractions of fossil crude oil, like kerosene ordiesel.

Table 13.8 Results of a simulated distillation ofBCO from hazelnut shells [57]. Reprinted from [42],with permission from Elsevier.

Boiling point (◦C) Yield (%)

<140 32.4140–240 30.1240–350 22.5>350 15.0


Questions

1. What is the rough composition of bio-oil?2. How would you analyze acetone in bio-oils?3. Describe the origin of water-insoluble bio-oil components and their determination?4. Which is the most useful analysis technique for monomeric components in bio-oils?5. Why is distillation of bio-oils inappropriate compared to fossil crude oils?

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(55) Zheng, J.L. and Wei, Q. (2011) Improving the quality of fast pyrolysis bio-oil by reducedpressure distillation. Biomass & Bioenergy, 35, 1804–1810.

(56) Murwanashyaka, J.N., Pakdel, H. and Roy, C. (2001) Separation of syringol from birch wood-derived vacuum pyrolysis oil. Separation and Purification Technology, 24, 155–165.

(57) Putun, A.E., Ozcan, A. and Putun, E. (1999) Pyrolysis of hazelnut shells in a fixed-bed tubularreactor: yields and structural analysis of bio-oil. Journal of Analytical and Applied Pyrolysis,52, 33–49.

14Formal Kinetic Parameters –

Problems and Solutions in DerivingProper Values

Neeranuch Phusunti1 and Andreas Hornung2

1Department of Chemistry, Faculty of Science, Prince of Songkla University,Hat Yai, Thailand

2Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg, Germany and Chair inBioenergy, School of Chemical Engineering, College of Engineering and Physical Sciences,

University of Birmingham, UK

14.1 Introduction

Chemical kinetics is the measurement and interpretation of chemical reaction rates. Thereare two main reasons to study the rate of a chemical reaction. First, kinetics provides uswith a powerful tool to determine the values of the rate equation as a function of statevariables (i.e. temperature, pressure and volume). Although kinetic study has been carriedout as empirical measurements in certain conditions, consequent functional forms havebeen developed to predict the rate of reaction for any set of conditions that is difficult tostudy experimentally [1]. Second, kinetics may not be the most efficient tool to investigateany reaction mechanism; however, together with complementary evidence from other tech-niques, kinetic analysis is useful for the understanding of the mechanism during a chemicalreaction [1, 2]. For practical purposes, chemical kinetics also provides critical data forchemical engineers to design, operate, control and optimise the reactors in the chemicalindustry. The chemical reactor is the most important component in any chemical process.




Time

A

B

C

t1 t2 t3

Mass

Figure 14.1 The conversion of solid A to products B and C in the reaction aA(s) → bB(g) +cC(s).

Its performance controls other upstream and downstream units and has a large impact onthe cost of the process.

An example of how important kinetic information on technological processes is, ispresented here in the case of thermal decomposition of solid materials. The production ofgas ‘B’ from the decomposition of solid ‘A’ presents as aA(s) → bB(g) + cC(s).

This process occurs quickly at the beginning and then turns to slow conversion. If theproduction is stopped when the conversion completes or there is no solid ‘A’ remaining inthe reactor, one might consider waiting until t3 is reached. However, from the kinetic infor-mation in Figure 14.1, the most profitable producing time can be t2 when the process reachesabout 90% conversion. This is because time in industry is money. The time consumptionfrom t2 to t3 with low productivity can cost a lot with a small product yield. Furthermore,the reaction rate depends on the reaction conditions, such as temperature. Hence, when theproduction process is operated under different conditions, the new kinetic data for thoseconditions needs to be analysed. In the case that the new conditions are difficult to studyexperimentally, their kinetic parameters can be predicted from the existing kinetic data.When the reaction rates control the productivity, the cost of the production and the profit ofthe plant, the kinetic study of the main reactions is vital for every chemical plant to controland design chemical reactors to achieve the specified throughput and conversion.

In addition, the accuracy of the calculated kinetic parameters is crucial. Figure 14.2illustrates the large error which is caused by the small activation energy deviation of10 kJ/mol. A measured data set (∙) with the activation energy (Ea) of 162 kJ/mol and pre-exponential factor (A) of 1.0 × 1010 min−1 is evaluated for its kinetic parameters. Then,its determined parameters are Ea = 160 ± 10 kJ/mol and A = 1.0 × 1010 min−1. Althoughthe model plot which is simulated from Ea = 160 kJ/mol and A = 1.0 × 1010 min−1 inFigure 14.2 cannot absolutely represent the measured data, the error between the measuredand model data (Ea = 160 kJ/mol) is considered small and it is possible to use this modelfor reactor design and process control. However, under the deviation range of activationenergy at ± 10 kJ/mol, the possible upper Ea value becomes 170 kJ/mol and the possible

Formal Kinetic Parameters – Problems and Solutions in Deriving Proper Values 259

00

0.2

0.4

0.6

Degre

e o

f conve

rsio

n (

)α 0.8

1

10 20 30

Time (min)

40 50 60

Measured data

Ea = 150 kJ/mol

Ea = 160 kJ/mol

Ea = 170 kJ/mol

70

Figure 14.2 The measured data (Ea = 162 kJ/mol, A = 1.0 × 1010 min−1) and models(Ea = 150, 160 and 170 kJ/mol and A = 1.0 × 1010 min−1) showing the possible error rangefrom ±10 kJ/mol.

lower Ea value is 150 kJ/mol. At the same pre-exponential factor value, the plots of thekinetic model at Ea = 150 and 170 kJ/mol (Figure 14.2) show obvious divergence fromthe measured data. If these kinetic parameters were used for reactor design and operatingcontrol, they could cause a huge fault in the production process. Therefore, even if thedeviation from the kinetic evaluation is considered to be small, this small deviation canlead to a significant effect on technical uses.

14.2 Chemical Kinetics on Thermal Decomposition of Biomass

In a general homogeneous reaction,

A + B → C

the rate of reaction is measured from the decrease of reactant concentration or the increaseof product concentration at a constant temperature. For this reaction,

Rate = −d[A]∕dt = −d[B]∕dt = d[C]∕dt

The minus sign is used to make the rate of reaction positive. The functional relationshipbetween the rate of concentration change of one species and the concentrations of allkinetically active species or a rate expression, for example, can be written as:

d[C]∕dt = k[A]a[B]b (14.1)

where constant k is called the rate constant and a, b are the reaction order with respect tocomponents A and B, respectively.

For heterogeneous reactions, such as thermal decomposition of biomass, the reactioninvolves at least one initially solid reactant. Thus, the concept of the change in concentration


of reactants or products has no significance in a heterogeneous reaction [3]. Usually, theextent of reaction (𝛼) is defined either in terms of the mass fraction of solid that hasdecomposed (𝛼 = (m0 − m)∕(m0 − mf ) where m is the mass at any time, m0 is the initialmass and mf is the final mass of the sample when reaction is complete) [4], or as the amountof gaseous products evolved.

The kinetics of biomass thermal decomposition based on a single reaction can beexpressed at a constant temperature (isothermal conditions) by the following equation:

d𝛼∕dt = k(T)f (𝛼) (14.2)

or it can be written in the integral form when g(𝛼) =𝛼

∫

0(1∕f (𝛼))d𝛼 as

g(𝛼) = k(T)(t − t0) (14.3)

where t represents time and t0 is initial time, T is the absolute temperature in Kelvin, k(T) isthe temperature-dependent rate constant, and f (𝛼) is a function called the reaction model.

The temperature dependence of the reaction rate is widely expressed by the Arrheniusequation [5]:

k(T) = A exp(−Ea∕RT) (14.4)

where Ea is the activation energy of the reaction, A is the frequency factor or pre-exponentialfactor and R is the gas constant.

By placing the rate constant of the Arrhenius equation into Equation 14.2, the kineticequation under isothermal conditions is:

d𝛼∕dt = A exp(−Ea∕RT)f (𝛼) (14.5)

Kinetic parameters for thermal decomposition of solid samples have been evaluated byisothermal or static methods, where the experiments are carried out at constant temperatures.The changes in the mechanism are relatively easy to detect because decomposition ratesare obtained for several single temperatures and therefore a change in the order of reactioncan be determined [6]. In addition, the reaction rate is possible to calculate analyticallywith the well-defined homogeneous sample temperature [7]. However, isothermal analysisrequires several measurements at varied temperatures. Hence, it needs a large amount ofsample and a long whole experimental time. The isothermal experiment is studied eitherunder very slow or very fast heating rates. In the former case, the temperature gradient dueto non-stationary heat conduction is minimized throughout the sample particles. However,there are some transformations that happen during the initial heating period to reach theconstant temperature. Owing to the fact that a typical solid state process has its maximumrate at the beginning of the process under isothermal conditions, the significant informationat heating stage may be missing for the kinetic analysis [8]. On the other hand, the high-heating rate isothermal experiment suffers from the heat transfer effect [8]. The missinginformation at the initial heating step and the time-consuming work are the reasons for thegradual decrease in interest in isothermal methods [9].

Conversely, with the development of thermal analysis techniques, numerous kineticstudies under non-isothermal conditions have recently been reported. The non-isothermal


or dynamic method is when samples are heated with time according to an assigned heatingrate (𝛽 = dT∕dt). The generalised kinetic expression for the non-isothermal method is

d𝛼∕dT = (A∕𝛽) exp(−Ea∕RT)f (𝛼) (14.6)

When Equation 14.6 is integrated between 𝛼 = 0, T = T0 and 𝛼 = 𝛼 and T = T,

𝛼

∫

0

(1∕f (𝛼))d𝛼 = g(𝛼) =

T

∫

T0

(A∕𝛽) exp(−Ea∕RT)dT (14.7)

If T0 is selected before the initiation of the reaction, then

g(𝛼) =

T

∫

0

(A∕𝛽) exp(−Ea∕RT)dT = (A∕𝛽)

T

∫

0

exp(−Ea∕RT)dT (14.8)

The termT

∫

0exp(−Ea∕RT)dT is called the temperature integral. By replacing x = Ea∕RT ,

Equation 14.8 is simplified as:

g(𝛼) = (AEa∕𝛽R)

∞

∫

0

e−x

x2dx = (AEa∕𝛽R)p(x) (14.9)

where∞∫

0

e−x

x2 dx = p(x).

Moreover, the logarithm form of the generalized non-isothermal kinetic expression(Equation 14.6) is:

ln[(d𝛼∕dt)∕f (𝛼)] = ln(A∕𝛽) − Ea∕RT

If the accurate form of f(𝛼) is known, the Ea and A values can be obtained from this linearrelationship.

The possibility of investigating the reaction rate for a wide range of temperatures withless effort than the isothermal method drives the interest in non-isothermal measurements.However, non-stationary heat conduction causes the temperature gradient in the sample[10]. Also, the difficulties in determining the real sample temperature in non-isothermalmeasurement influence the accuracy in formal kinetic parameter evaluation [7]. Moreover,the heating rates studied in non-isothermal experiments are much lower than the heatingthat is achieved in pyrolysis or a gasification reactor [8] or most of pyrolysis reactors areoperated at constant temperature [11]. A summary of advantages and disadvantages ofisothermal and non-isothermal kinetic analysis is presented in Table 14.1.

14.3 Kinetic Evaluation Methods

When the kinetic evaluation methods are classified based on the form of experimentaldata, they are grouped into ‘differential methods’ and ‘integral methods’. Differential


Table 14.1 The advantages and disadvantages of isothermal and non-isothermal kineticanalysis.

Conditions Advantages Disadvantages

Isothermal 1. Detectable changes onmechanism

2. Exactly defined sampletemperature

3. Analytical solving of formalkinetic parameters

1. Several measurements at varioustemperatures

2. Large sample amountrequirement

3. Extent of reactions duringheating stage

Non-isothermal 1. An entire, continuouslymeasured, temperaturerange

2. Rapid analysis method3. Removing sample-to-sample

errors

1. The presence of a temperaturegradient due to non-stationaryheat conduction

2. Difficulty of identifying sampletemperature

3. Difficulty of evaluating kineticsanalytically

methods under non-isothermal analysis require the derivative of the measured weight losscurve which is easily affected by bad signal to noise ratios. Hence, smoothing is oftennecessary but leads to inaccuracy on the kinetic evaluation [12], whereas the integralmethods use the measured weight loss data without differentiation need. However, someapproximation approaches are required to evaluate the temperature integral for integralmethods (Equation 14.9). For example, Doyle’s approximation [13] is:

log p(x) ≈ −2.315 − 0.4567(Ea∕RT) (14.10)

There are several approaches to solve the temperature integral. Nevertheless, the need forapproximations has been diminished by the availability of supporting computer softwarenowadays [14]. Moreover, a very low or very high degree of conversion cannot be appliedin the integral method [15].

Apart from differential and integral methods, another kinetic evaluation method called the‘direct least square method’ uses the original thermo-gravimetric data without algebraicoperations. In addition, this method does not need the approximations concerning theintegration of the rate expression while computing kinetic parameters [12, 16]. The directevaluation is used to find the optimum variables when the sum of the difference betweenthe actual value (𝛼(meas)) and the predicted value (𝛼(calc)) is a minimum. For thermaldecomposition by thermo-gravimetry, the actual values refer to weight loss data and thepredicted values refer to the integrated rate expression. When the number of data is equalN, the related equation for direct fitting is:

SSQ = 1N

N∑i=1

[𝛼(meas)i − 𝛼(calc)i]2 (14.11)

If the classification is based on the calculation methods of the kinetic parameters, they aregrouped into the ‘model-fitting methods’ and ‘model-free methods’. Model-fitting methodsare an attempt to fit the reaction models to measured data, while the model-free methods


Table 14.2 Some common reaction models of solid-state reactions for kinetic analysis.

Reaction model g(𝛼) f(𝛼)

Reaction order modelsZero-order (F0) 𝛼 1First-order (F1) − ln(1 − 𝛼) (1 − 𝛼)Second-order (F2) (1 − 𝛼)−1 − 1 (1 − 𝛼)2

nth-order (Fn) (n − 1)−1(1 − 𝛼)(1−n) (1 − 𝛼)n

Geometrical modelsContracting area (R2) 1 − (1 − 𝛼)1∕2 2(1 − 𝛼)1∕2

Contracting volume (R3) 1 − (1 − 𝛼)1∕3 3(1 − 𝛼)2∕3

Diffusional models1-Dimension (D1) 𝛼2 1∕(2𝛼)2-Dimension (D2) [(1 − 𝛼) ln(1 − 𝛼)] + 𝛼 [− ln(1 − 𝛼)]−1

3-Dimension (Jander) (D3) [1 − (1 − 𝛼)1∕3]2 3(1 − 𝛼)2∕3∕2[1 − (1 − 𝛼)1∕3]3-Dimension (Ginstling–

Brounshtein) (D4)1 − (2∕3)𝛼 − (1 − 𝛼)2∕3 3∕[2((1 − 𝛼)−1∕3 − 1)]

Nucleation and nucleigrowth models

Avrami–Erofeev (A2) [− ln(1 − 𝛼)]1∕2 2(1 − 𝛼)[− ln(1 − 𝛼)]1∕2



Power law (P2) 𝛼1∕2 2𝛼1∕2

Power law (P3) 𝛼1∕3 3𝛼2∕3

Prout–Tompkins (B1) ln[𝛼∕(1 − 𝛼)] 𝛼(1 − 𝛼)

eliminate the necessity of reaction model [17] during evaluation by applying multi-heatingrates. Some common reaction models of solid-state reaction are shown in Table 14.2 (seealso [18–20]). Under isothermal conditions, model-fitting methods are mainly employedto get formal kinetic parameters. For non-isothermal conditions, some kinetic analysesare investigated based on only one heating rate measurement, which has been a criticismabout the strong kinetic compensation effect. This is because the thermo-gravimetric curveat a single heating rate can be represented by several sets of formal kinetic parameterswith different kinetic models [21, 22]. Due to the attempt to solve the rate equation, whichhas several unknown variables, one determined variable can compensate for the others. Inaddition, the dependence on temperature (T) and on the extent of conversion (𝛼) is variedsimultaneously in a non-isothermal experiment. Thus, the model-fitting methods of a singleheating data cannot separate these two dependences (the form of k(T) and f(𝛼)). Therefore,the measurements under non-isothermal conditions are recommended to be carried out atseveral heating rates, which can minimize the compensation effect and produce a morereliable mathematical description of the reaction kinetics [23].

Furthermore, comparison of the measurements at different heating rates leads to thetemporary elimination of the reaction model from the kinetic evaluation. ‘Isoconversionalmethods’ or model-free methods determine the activation energy as a function of the extentof conversion [5]. These methods can be applied to either isothermal or non-isothermaldata [20]. The stand-out benefit from isoconversional methods is the possibility of revealing


complexities of reaction [24]. More information on the model-fitting and model-free meth-ods or isoconversional methods is available for further reading in several articles [24–31].

14.4 Experimental Kinetic Analysis Techniques

Kinetic data from solid-state reactions were traditionally studied using isothermal mea-surements before instruments of non-isothermal analysis were first developed for kineticevaluation. Since the late 1950s, the interest in non-isothermal kinetics has increased dueto the commercially availability of thermal analysis instruments [32].

Thermo-gravimetry (TG) is the most common technique for thermal decomposition ofsolid phase samples and biomass [33–35]. The TG technique measures the decrease in massof a sample by the release of volatiles as a function of time and/or temperature. A specifiedheating or cooling rate is applied to the mass of a substance. The other common methodsof thermal analysis utilised in kinetic studies are differential thermal analysis (DTA) [36]and differential scanning calorimetry (DSC) [37, 38]. In DTA, the temperature differencebetween a sample and an adjacent inert reference material subjected to an identical heat fluxis measured. Similarly, DSC measures the difference of heat flux into or out of a sample andan inert reference material when they are simultaneously heated or cooled at a constant rate.

Although the thermo-gravimetric analyser is usually employed for thermal decompo-sition of biomass under non-isothermal conditions, occasionally they may study it underisothermal temperatures [15]. A sample is heated to a desired temperature and then thetemperature is kept constant until the end of the measurement is reached. The temperatureovershoot needs to be considered for using TG for isothermal measurements [11, 39, 40],especially at high heating-up rates. Moreover, typical thermo-gravimetric analysers do notsupport measurements at rapid heating rates due to the limitations of the instrument.

Thermal analysis is an excellent tool for heterogeneous reaction but these techniques donot provide any information on the nature of the reaction. To obtain sufficient evidenceto draw mechanistic conclusions on the thermal decomposition process of biomass, othercomplementary techniques need to be employed to analyse the changes in the chemicalcomposition and/or structure of material. Evolved gas analysis (EGA) is one effectivetechnique for quantitative and qualitative analysis of released volatile products. The generalEGA techniques which are combined with thermal analysis are Fourier transform infrared(FTIR) spectroscopy, mass spectrometry (MS) and gas chromatography (GC). Due to thecomplexity of biomass thermal conversion processes, the simultaneous volatile detection inrelationship with temperature (non-isothermal) or time (isothermal) leads to the possibilityof predicting the formation rate of individual products and providing the supporting datafor understanding the mechanism. Examples of the evolved gas profiles obtained froman isothermal measurement by pyrolysis-mass spectrometry (Py-MS) and non-isothermalmeasurement by thermo-gravimetry-mass spectrometry (TG-MS) are shown in Figure 14.3.

14.5 Complex Reaction

Thermal analysis only measures the overall reaction rate, but thermal decomposition ofbiomass is a complex process including numerous reactions. Generally, it can be seen inthermo-gravimetric data that at lower heating rates, the difference in thermal behaviour of


3.5E+04

m/z = 60

m/z = 73

Ion inte

nsity (

A)

Ion c

urr

ent (A

)

m/z = 44

m/z = 31

m/z = 18

2.8E+04

2.1E+04

1.4E+04

7.0E+03

0.0E+00

3.0E-08

2.5E-08

2.0E-08

1.0E-08

1.5E-08

5.0E-09

0.0E-000 2 4 6 8

Time (min)

10 12 14 16 18 100 200 300

Temperature (°C)

400 500 600

m/z = 17

m/z = 28

m/z = 44

m/z = 16

(a) (b)

Figure 14.3 (a) Evolved gas profiles of cellulose decomposition under isothermal measure-ment at 350 ◦C by Py-MS, (b) evolved gas profiles of micro algae decomposition under non-isothermal conditions at 20 ◦C/min by TG-MS.

fragments in the initial solid sample appears in several zones. For example, the thermo-gravimetric curve (TG curve) and its negative first derivative curve (DTG curve) of woodpyrolysis at a heating rate of 25 ◦C/min in Figure 14.4 show the complex reaction withmulti-peaks. When the heating rate increases, the different peaks tend to merge together andbecome a single decomposition region at significantly higher heating rates. Any processinvolves several reactions with different kinetic characteristics, the overall rate distributingfrom these steps depends on the extent of conversion and temperature. Thus, a single setof Arrhenius parameters and a reaction model is considered inadequate to describe thecomplex process [41].

200 300 400 500 6000

20

40

60

80

100

Norm

alis

ed w

eig

ht (%

)

Temperature (°C)

TG

0.000

0.002

0.004

0.006

0.008

0.010–d(m

/m0)/

dt

(1/m

in)

DTG

Figure 14.4 Thermo-gravimetric (TG) curve and its negative first derivative (DTG) curve ofwood pyrolysis under inert atmosphere at a heating rate of 25 ◦C/min.


0.0011–4

–3

–2

–1

– I

n k

0

1

2

0.0016

1/Temperature (K)

0.0021 2501

1.1

1.2

1.3

Ap

pa

ren

t o

rde

r o

f re

actio

n (

n)

1.4

1.5

1.6

350 450

Temperature (°C)

550

Figure 14.5 (a) Arrhenius plot and (b) apparent order of reaction of a complex thermaldecomposition process.

Under isothermal experiments, the complexity of the solid-state reaction is presentedas the change of reaction orders that are dependent on temperature. Also, the curved ormulti-linear relationships in Arrhenius plots show the occurrence of complex reactions.The examples of an Arrhenius plot and an apparent order of reaction plot at varioustemperatures, showing the presence of process complexity, are presented in Figure 14.5aand Figure 14.5b, respectively. These data were obtained from a pyrolysis process of microalgae under isothermal conditions between temperatures 250–600 ◦C [42].

In non-isothermal experiments, the complexity of biomass thermal decomposition hasbeen revealed successfully by the model-free isoconversional methods. In the case thatseveral steps of a process have different activation energies, the change of Ea value on theextent of reaction (𝛼) can be observed. However, when there are no significant differencesof activation energies of any steps of a process, the complexity is not detected in thedependence of Ea on 𝛼 [43].

Since biomass comprises several main components with various contributions, the pri-mary decomposition characteristic is considered to involve the thermal behaviour of eachcontribution. Some publications [44–47] report the devolatilisation model of biomass as thesuperposition of three primary components’ (cellulose, hemicellulose and lignin) kineticbehaviour. In the case of the independent parallel reactions mechanism, the overall rate iscalculated from the linear summation of all individual reactions regarding the fraction ofeach pseudo-component contributing to the total mass loss or conversion rate.

𝛼 =∑

j

cj𝛼j (14.12)

The coefficient cj is the contribution of the individual process (j) to the overall process.𝛼 is the overall reacted fraction and 𝛼j is the reacted fraction of pseudo-component (j). Forthe conversion rate, the linear summation can be written as [44]:

d𝛼dt

=∑

j

cj

d𝛼j

dt(14.13)


Cellulose

Hemicellulose

Lignin

2.5E-02

2.0E-02

1.5E-02

1.0E-02

5.0E-03

0.0E+00200 300 400

Temperature (°C)

–d

(m/m

0)/

dt(

1/m

in)

500 600

Figure 14.6 The assumed decomposition regions of cellulose, hemicellulose and lignin com-pared to wood decomposition.

Generally, the pyrolytic behaviours of lignocellulosic components are reported with asimilar conclusion [44, 48–50] that hemicellulose decomposes at low temperatures andpresents in the DTG curve of wood as the shoulder of the dominant peak of cellulosedecomposition at higher temperatures (Figure 14.6). On the other hand, lignin decomposesslowly and covers a broad temperature range, such that one can sometimes barely see thechanges from the measurement result. Moreover, in a few cases, the total devolatilisation rateincludes more than three basic steps, when the decomposition of each pseudo-componentis suggested as a multi-step reaction [23] or minor components are included [51]. Althoughthe main decomposition zones in lignocellulosic materials can be identified as cellulose,hemicellulose and lignin, the characteristics of each single component cannot be directlyapplied to biomass due to the interaction among components, the presence of mineralcontent and the chemical and physical change from separation processes [23, 52].

14.6 Variation in Kinetic Parameters

Numerous kinetic data have been reported on the thermal decomposition of biomass mate-rials under different operating conditions. Even for the same sample materials, however,the presence of variation among the reported kinetic data is considered as a result ofseveral factors.

14.6.1 Kinetic Compensation Effect

From the least square fitting (Figure 14.7) between the measured data and the predictedkinetic data based on Equation 14.11 for a given reaction order (n), a small valley indicatesthe presence of the ‘kinetic compensation effect’. That means that there are several possiblesets of Arrhenius parameters (Activation energy, Ea and Pre-exponential factor, A) whichgive small deviation values (SSQ) between the experimental and the calculated data.


Ea (×104 J/mol)Ea (×104 J/mol)

SS

Qln A (min–1)

(a) (b)

40

0.5

1

68

1012

14 20

15

10

5

5 6 7 8 9 10 11 12 13 14

20

15

10

5

4

ln A (min–1)

Figure 14.7 The presence of kinetic compensation effect from least square fitting (a) three-dimensional (b) two-dimensional surface.

The possibility of different sets of activation energy and pre-exponential factors todescribe the single experimental data defines the existence of a ‘kinetic compensationeffect’. The linear correlation between the variables lnA and Ea can be described by

lnA = aEa + b (14.14)

This relationship is derived from the Arrhenius equation at the isokinetic temperature orcompensation temperature (Ti) at which all rate coefficients (ki) are the same.

lnA = Ea∕RTi + ln ki (14.15)

The reactions which present the compensation effect may contain common chemicalcharacteristics and exhibit an isokinetic behaviour [53–57]. At a measured rate constant,the change in value of either A or Ea from the alteration in experimental conditions demandsa compensatory change in the other.

For instance, from a range of literature for thermal decomposition of cellulose, the linearrelationship between apparent activation energy and the log of pre-exponential factors canbe observed (Figure 14.8).

Even if an identical sample is investigated under the same conditions, different mathe-matical method and errors of experimental procedures can result in the presence of kineticcompensation effect [58]. Inaccurate temperature measurements due to the large tempera-ture gradients within the sample and the thermal lag, as well as differences in the reactants(i.e. particle size), are also commonly cited as reasons for the compensation behaviour[59–61].

14.6.2 Thermal Lag

The difference between the measured sample temperature (Te) and the true sample temper-ature (Ts) can be indicated as ‘Thermal lag (ΔTTL)’.

ΔTTL = Te − Ts (14.16)


30

25

20

15

10

5

050 100

log A

(m

in–1)

150

[15]: Cellulose powder; isothermal

[54]: Cellulose, 280–350 °C

[54]: Cellulose, 350–400 °C

[55]: Cellulose(Whatman CF-11), 9 mg, 65 K/min

[55]: Cellulose(Whatman CF-11), 0.3 mg, 65 K/min

[55]: Cellulose(Whatman CF-11), 0.3 mg, 1 K/min

[56]: Microcrystalline cellulose, dynamic

[57]: Cellulose, TG 10 K/min

Ea (kJ/mol)

200 250 300

Figure 14.8 The relationship of log A and Ea obtained from a range of literature for pyrolysisof cellulose.

An accurate real sample temperature reading is crucial for any kinetic evaluation. In anydesign of furnaces or thermal analysis analysers, the thermocouple is located closed to thesample to reduce the thermal lag. However, the external thermocouple to the sample detectsonly the estimated sample temperature.

During thermal decomposition of biomass, there is competition between the heat ofreaction and the heat transfer coefficient from the external heating source governed bythermal radiation [62]. When the heat of reaction increases, the thermal lag increasessignificantly. On the other hand, the increase of the heat transfer coefficient or slow externalheating rate leads to the reduction of thermal lag [59]. Moreover, the size of the sample,the composition of the carrier gas and the endothermicity of the reaction also influence thethermal lag problem [63]. In addition, thermal lag has been related to the compensationeffect and variation of the Arrhenius parameters [64].

14.6.3 Influence of Experimental Conditions

Along with heat and mass transfer limitations, the experimental conditions, such as tempera-ture, heating rate, particle size, atmosphere and the presence of inorganic content, can affectthe kinetic parameters [65–68]. During the thermal decomposition process, there are severalphenomena related to the external and internal heat and mass transfer. When a particle of


solid sample is exposed to high temperature operation, external heat is transmitted to theparticle surface by radiation and/or convection and/or conduction, and then heat conductiontakes place in the biomass particle from the surface to the inside of the particle [69–71]. Thethree combined mechanisms of heat transmission inside the pyrolysing solid are the conduc-tion through the solid particle, the radiation from the pore walls and the heat transmissionthrough the gas phase (volatile and gaseous products) inside the particle pores [9, 23, 65].

Generally, a uniform temperature is assumed for solid-state kinetic evaluation, but theheat changes during the reactions cause the temperature profile or temperature gradientinside the sample particles. This profile depends on the chemical reaction heat, sample size,heat conductivity and heating rate [7, 71]. The poor heat conduction of biomass leads tothe temperature difference between the surface and inside of the particle. Moreover, theself-heating and self-cooling phenomena also affect the reaction rate evaluation [72, 73].

The size of the sample particle and the heating rate are implicated as influences on the tem-perature gradient. As the heating rate and sample particle size increase, the temperature gra-dient within the biomass particle increases [9]. The small size of sample in analytical exper-iments is recommended for kinetically controlled conditions. However, thermal decompo-sition of large particle size biomass needs to be studied for practical purposes because, inthe real processes, biomasses with large particle sizes are often employed [74–76].

14.6.4 Computational Methods

Despite the improvement of experimental data quality and instrumentation, variationsof kinetic parameters of solid-state decomposition is still present [77]. The unequivocalkinetic characteristics of the non-isothermal kinetic analysis by thermal analysis have beenthe subject of numerous criticisms [78–80]. Therefore, not only the experimental set-up,but also the computational aspects, should be emphasised in the development of kineticanalysis of solid-state reactions [81].

The single heating rate measurement under non-isothermal experiments has been avoidedowing to the non-uniqueness of the kinetic parameters [82]. For model-fitting methods,although the accuracy of non-isothermal kinetic analysis can be improved by applying multi-heating rates, the goodness-of-fit of experimental data to the limited sets of kinetic reactionmodels is considered an insufficient condition to describe the solid-state reaction [81]. Thisis because thermal decomposition of a solid sample is a complex process involving severalsteps with different kinetic characteristics. The rate expression of non-isothermal kineticsintegrates the simultaneous changes in both temperature and concentration. Moreover,the overall rate is also influenced by physical processes such as melting, diffusion andadsorption [25]. In the case that each step of a process has significantly different activationenergy, model-free isoconversional methods reveal the complexity by the dependence ofactivation energy on the extent of conversion [83, 84]. The necessity of the goodness-of-fitto the reaction model is temporarily eliminated for model-free methods. However, the rateequation of solid-state decomposition needs to be described by at least three parametersor a kinetic triplet (Arrhenius parameters Ea and A, and reaction model) which allowreconstruction of the conversion curves to compare with measured data [82].

Furthermore, there is a disagreement between isothermal and non-isothermal data.Maciejewski [82] commented on this issue that two reasons for the inconsistency of kinetictriplets from isothermal and non-isothermal measurements are the different temperature


Mass

spectrometer

Heated line

Capillary

Pyrolyser

Figure 14.9 Simple diagram of pyrolysis–mass spectrometry.

range and the limitation of isothermal analysis at very low and very high degrees of conver-sion. In addition, Vyazovkin and Wight [25] gave the reason that apart from the differenttemperature regions of isothermal and non-isothermal experiments, the model-fitting meth-ods cannot separate the dependence on temperature and on the reaction model. Undernon-isothermal conditions, the temperature and the extent of reaction vary simultaneouslyin the reaction rate expression. Thus, it is possible that more than one reaction model canfit to the measured data by compensating with the Arrhenius parameters.

14.7 Case Study: Kinetic Analysis of Lignocellulosic Derived Materialsunder Isothermal Conditions

This section presents a case study of isothermal kinetic analysis of biomass derived mate-rials (cellulose, hemicellulose and lignin) by a pyrolysis microreactor connected to onlinemass spectrometry. The three materials – cellulose, hemicellulose and lignin – are wellknown as the basic components of lignocellulosic biomass. There are numerous studieson the mechanisms and kinetic behaviours [85–88] of these materials for thermochemicalconversion processes. Most kinetic studies have been carried out under non-isothermalconditions by using the thermo-gravimetry technique.

For practical purposes for the pyrolysis process at constant temperatures, this kinetic anal-ysis has been investigated under isothermal conditions. The analytical instrument and oper-ating conditions were optimised to eliminate experimental influences on kinetic evaluation.This section is separated into three parts: (i) instrument and operating conditions; (ii) kineticevaluation procedure; (iii) formal kinetic parameters and some technical applications.

14.7.1 Instrument and Operating Conditions

The selected technique for these isothermal measurements was pyrolysis mass spectrometry,which is the combination of pyrolysis reactor with an evolved gas analysis technique.This instrument (Figure 14.9) consists of three components: (i) a pyrolysis microreactor;(ii) a transfer line; and (iii) a mass spectrometer. The design of the microreactor wasdeveloped to make it suitable for thermal decomposition of biomass under isothermalconditions without the temperature overshooting during the heating period. A very smallinitial amount of sample of less than 200 μg per measurement was required, which reducesthe heat transfer limitation. The transfer line was kept at a high temperature to minimise thecondensation of volatile products. Moreover, the system was purged with inert gas to removethe devolatilisation products from the reaction zone and eliminate the secondary reactions.To monitor the progress of reaction, rapid evolved gas analysis by mass spectrometerdetected the released gaseous products in relation to time.


The operating temperature range for each sample material was recommended to beinitially considered from the non-isothermal decomposition temperature range, which isidentified from the mass loss and its derivative curves by using thermo-gravimetry. Thedata obtained from this pyrolysis–mass spectrometry was plotted by the total ion current(the evolved gas intensity) against time. Since the progress of reaction was monitored bymeans of the evolved gas profile, the kinetic analysis presented here was focused only onthe reactions involving the formation of total volatile products.

14.7.2 Kinetic Evaluation Procedure

The overall thermal decomposition rate of solid samples can often be described by degres-sive kinetics [89] and assuming the nth order reaction model: f (𝛼) = (1 − 𝛼)n.

d𝛼∕dt = k(T)(1 − 𝛼)n (14.17)

The degree of conversion for isothermal degradation obtained by integration of Equa-tion 14.17 for n = 1 is:

𝛼(t) = 1 − e−k(T)t (14.18)

The degree of conversion is given by n ≠ 1:

𝛼(t) = 1 − (k(T)t(n − 1) + 1)1∕(1∕n) (14.19)

where k(T) is the rate coefficient and n is the apparent order of the overall decompositionreaction.

The measured data or evolved gas profiles obtained from the mass spectrometric detectorcan be derived to the form of degree of conversion by being based on the concept of an idealreactor. From the concept of ideal flow reactors (Continuous Flow Stirred-Tank Reactor andContinuous Tubular Reactor), the conversion of solid-state thermal decomposition reactionis derived from the mass flow balance.

In a continuous stirred-tank reactor, the mass balance around the reaction volume (VR) is:

m(in) + m(generation) = m(out) + m(accumulation) (14.20)

For the thermal decomposition, the inlet flow is only the inert gas which is not involved inthe chemical reactions. Hence, the total produced gaseous components up to a certain timerelate to the products that left the reactor and the products that remain in the reactor. Dueto the thorough mixing, the composition in the reactor is ideally equal to that of the outlet.

Therefore, in the case of online gas analysis data by means of mass spectrometry forkinetic study, the ion currents in relation to time I(t) are detected to follow the progressionof thermal decomposition of solid or liquid samples. The degree of conversion is equivalentto the mass having already left the reactor (∫ t

0 I(t)M(t)Vdt) plus the mass of products in

the gas phase remaining in the reactor (I(t)M(t)VR) normalized by the total evolved massof products.

𝛼exp(t) =(∫

t

0I(t)M(t)Vdt + I(t)M(t)VR

)∕∫

∞

0I(t)M(t)Vdt (14.21)

where M(t) is the mean molecular mass at time t (calculated from the single ion currents),V is the flow rate and VR is the reactor volume.


0im , V

V

V V

im ,

VR , TL

im , mi ,0

S

VR = S × L

Continuous Flow Stirred-Tank Reactor

(a) (b)

Continuous Tubular Reactor

Figure 14.10 Continuous Stirred-Tank Reactor (CSTR) (a) and Continuous Tubular Reactor(CTR) (b) with reacting volume VR and flow rate V at temperature T. m0

i is the mass flow ofcomponent i fed to the reactor and mi is the mass flow of component i leaving the reactor.

For the case of a continuous tubular reactor, the reactor is a cylindrical pipe of constantcross-section (S). When a section of reactor of length L is considered, the reacting volumeis calculated as VR = S× L. If the inert flow rate is relatively high and the reacting volumeis considerably small, term I(t)M(t)VR can be ignored from Equation 14.21.

From the experimental results, the fraction of total evolved gas products, which weredetected by the mass spectrometer, is presented in relation to measured time (Figure 14.11).The evolution rates at high temperatures are faster than those at lower temperatures, whichcan be seen from the higher maximum ion current at shorter measuring time. It is noted herethat most illustrations in this section are presented only on the measurements of cellulosedecomposition.

6.0E+05

5.0E+05

4.0E+05

3.0E+05

Ion c

urr

ent (A

)

2.0E+05

1.0E+05

0.0E+000 1 2 3

Time (min)

4 5 6

350 °C

360 °C

370 °C

380 °C

390 °C

400 °C

Figure 14.11 Experimental evolution curves for the isothermal decomposition of cellulose atthe different operating temperatures.


3

2

1

00

0.2

0.4

0.6D

eg

ree

of

co

nve

rsio

n (

)

0.8

1

2 4 6

Time (min)

8 10 12

α

Figure 14.12 A conversion curve obtained under an isothermal measurement.

Next, the evolution curves (i.e. in Figure 14.11) were used to simulate the conversioncurves based on Equation 14.21. For instance, the conversion curve in Figure 14.12 froman isothermal measurement shows basically three different sections of thermal decompo-sition behaviour. The first section (1) is an initial induction period with a low velocity ofdecomposition, which can be attributed to the heating of the sample particles to reach aset temperature. The induction time reflecting heat transfer effect depends on the reactiontemperature and particle size; when the set temperature increases or particle size decreases,induction time is shortened. Next, section (2) is the period with rapid weight loss of thesample or rapid volatiles production. At this stage, several reactions take place in parallel orin series producing complex products, until we reach section (3). Here, there is a relativelylong period of time with a reduced weight loss or volatile formation rate before moving tothe constant final weight of solid residue or no more volatile evolution.

When several conversion curves at various temperatures are plotted together (Fig-ure 14.13), they show the corresponding pattern that the gap between adjacent curvesreduces proportionally to high temperatures. The induction period and reaction timedecrease as temperature increases. The major devolatilisation occurs at the initial stageof the measurements, while small weight changes over a quite long period of time occur ata high degree of conversion.

The induction period was eliminated by linear extrapolation by extending the maindevolatilisation region linearly to reach 𝛼 = 0. From the conversion curves, to evaluatethe rate coefficient (k(T)) and apparent order of reaction (n), the degree of conversiongiven by Equation 14.19 was fitted by means of least square fitting (Equation 14.11) to theexperimental degree of conversion (Equation 14.21) 𝛼exp(t) by variation of k(T) and n. Theoptimal values of k(T) and n are the set (k(T), n) that gives the global minimal derivative(S) of the fitting.

When the optimal values of k(T) and n were obtained for every temperature, the apparentactivation energy (Ea) and the pre-exponential factor (A) could be calculated from the slope


0

0.2

0.4

0.6

Degre

e o

f conve

rsio

n (

) 0.8

1

20 4 6

Time (min)

8 10 12

360 °C

370 °C

380 °C

390 °C

400 °C

350 °C

α

Figure 14.13 Conversion curves for thermal decomposition of cellulose at varioustemperatures.

and the intercept at the Y-axis of linear regression of –ln k(T) versus 1/T based on the linearform of the Arrhenius equation as

ln k(T) = lnA − Ea∕RT (14.22)

Although the evaluation procedure demonstrated above is only for cellulose thermaldecomposition, the Arrhenius parameters and apparent reaction order for the cases ofhemicellulose and lignin can be evaluated following the same procedure. The Arrheniusplots and apparent order of reaction plots for cellulose, hemicellulose and lignin are sum-marily shown in Figure 14.14. The Arrhenius plots of these three materials present asthe straight lines, which suggests no change of rate determining reaction within the stud-ied temperature ranges. The apparent reaction order at the narrow operating temperatureranges of cellulose and hemicellulose decomposition were considered to be constant aroundn = 1.1. This suggests no mechanism change during processes within those temperatureranges. The operating temperatures of lignin decomposition covered a wide temperaturerange and the order of reaction was observed around n = 1.8. Generally, in isothermalmeasurements, the complex nature of a process which may show the presence of changesin reaction order or curved and/or multi-linear relationship in Arrhenius plot can be clearlydetected if broader temperature ranges are investigated.

14.7.3 Formal Kinetic Parameters and Some Technical Applications

Formal kinetic parameters from this case study are reported in the sets of apparent activationenergy (Ea), pre-exponential factor (A) and apparent order of reaction (n). For cellulosepyrolysis, its formal kinetic parameters are Ea = 131 ± 6 kJ/mol; log A= 10.9 ± 1.2 min−1;n = 1.1. For hemicellulose, its formal kinetic parameters are Ea = 125 ± 5 kJ/mol; log A =11.5 ± 0.9 min−1; n= 1.1. For lignin, its formal kinetic parameters are Ea = 72 ± 4 kJ/mol;log A = 6.5 ± 0.7 min−1; n = 1.8. Due to the available kinetic triplet, these parameters


0.00120–4

–3.5

–3

–2.5

–2

–1.5

–In

k–1

–0.5

0

0.5

(a)

(b)

1

0.00130 0.00140 0.00150 0.00160

1/Temperature (K)

Temperature (°C)

0.00170 0.00180 0.00190

Cellulose

Hemicellulose

Lignin

Cellulose

Hemicellulose

Lignin

280 330 380 430 4800.8

1

1.2

1.4

Appare

nt ord

er

of re

action (

n)

1.6

1.8

2

0.00200

Figure 14.14 (a) Arrhenius plots and (b) apparent order of reaction plots for the thermaldecomposition of cellulose, hemicelluloses and lignin.

can be used for reconstructing the predicted models to compare with the measured data(Figure 14.15).

The importance and accuracy of kinetic data for reactor design and operating control wasindicated previously in Section 14.1. For the pyrolysis process, there are several reactortypes supporting the pyrolysis conditions. To obtain the optimal yield and specified productproperties, the operating conditions of any reactor need to be controlled based on kineticinformation. The reactor size determination is also based on the kinetic model. Therefore,the quality of fitting of the calculated and measured data reflects effective design. Forexample, the agreement between the model (lines) and the experimental data (dots) inFigure 14.15 shows the ability to accurately use these models on reactor design. This isbecause the models can well represent the measured data. However, at a high degree ofconversion of more than 95% in Figure 14.15c, there is deviation between the model and


350 °C360 °C370 °C380 °C390 °C400 °C

290 °C280 °C

300 °C310 °C320 °C330 °C340 °C

340 °C320 °C

360 °C380 °C400 °C420 °C440 °C460 °C480 °C500 °C

00

0.2

0.4

0.6

Degre

e o

f conve

rsio

n (

)

0.8

(a) 1

0 3 6 9 12 150

0.2

0.4

0.6

Degre

e o

f conve

rsio

n (

)

0.8

(b) 1

0 1 2 3

Time (min)

4 50

0.2

0.4

0.6

Degre

e o

f conve

rsio

n (

)

0.8

(c) 1

2 4 6 8 10 12

αα

α

Figure 14.15 The comparison between the experimental (dots) and calculated (lines) con-version curves for the pyrolysis of (a) cellulose, (b) hemicellulose and (c) lignin.


experimental data due to the limitations of the analytical instrument employed in this casestudy and also the nature of isothermal measurement. The difficulty of determining suchsmall weight changes over a long period of time is the result of the limited stability of thebaseline of the mass spectrometric signal.

Moreover, in most practical continuous reactors, the temperature is kept constant andthe residence time is set suitably for the biomass material and desired process. FromFigure 14.15a, at temperatures higher than 400 ◦C, cellulose completely decomposes tototal volatiles in a reaction time of less than 3 min. Whereas hemicellulose decomposes(Figure 14.15b) at lower temperatures and is completely processed in less than 3 min attemperatures higher than 340 ◦C. Thermal decomposition of lignin (Figure 14.15c) hasa significantly longer reaction time compared to that of cellulose and hemicellulose. At420 ◦C, lignin decomposes almost 100% within 3 min. For example, therefore, based onthis information, lignocellulosic biomass, which mainly consists of cellulose, hemicelluloseand lignin, is simply suggested to have a suitable residence time for a pyrolysis reactorof 3 min when the reaction temperature is set at 420 ◦C. This indication is based on theassumption that there is no interaction among components and negligible influence fromminor components.

14.8 Conclusion

Kinetics is important for both chemistry and engineering studies and applications. Two mainobjectives of kinetic analysis are to help in elucidating the mechanism and to predict the rateof reaction. Generally, it is known that kinetics is not the most effective tool to determine thesolid-state mechanism. However, together with other complementary techniques, kineticscan help to draw reasonable mechanistic conclusions.

Kinetic analysis under isothermal conditions is considered to provide a physical meaningof solid-state reaction due to the separated dependences of temperature and the extent ofreaction in the rate equation. On the other hand, the kinetic analysis under non-isothermalconditions has several unknown variables in the rate equation. This leads to difficulty in thephysical interpretation of solid-state reactions, but is able to well predict the reaction rateof the practical processes that are applied heating rates. In addition, the lack of alternativesin the reaction model forces the best fit to a reaction rate from the limited reaction models.Although the true model is not one of these model choices, there will be a model that can berepresented as the best model. Moreover, the heating rates applied for pyrolysis and gasi-fication reactors are much higher than the heating rates applied for analytical experiment,and most pyrolysis reactors are operated at a constant temperature. Furthermore, the singleheating rate experiment should be avoided for kinetic analysis because of the influencefrom the compensation effect. To report the formal kinetic parameters, the complete kinetictriplet should be determined to enable the characterisation of a reaction or reconstructionof the rate equation for comparison with measured data.

The accuracy of kinetic analysis is very important not only to help us better understandthe nature of biomass decomposition, but also for technical application to reactor design andoperating control. Therefore, attempts to obtain accurate kinetic parameters should focus onboth experimental development and scrutiny of the evaluation methods. The measurementsaiming at kinetic study should be carefully controlled. Also, analytical instruments should


be developed to support kinetic analysis. Using small sample particle sizes can minimisethe heat transfer effect. However, in real processes, the particle sizes of biomass materialsare varied and often are larger than those applied in analytical experiments. Hence, aspectsof the heat and mass transfer phenomena may be unavoidable. To summarise: there are stillseveral challenges in the development of kinetics and in examining the existing conceptsfor the study of biomass decomposition reaction to obtain proper values.

Nomenclature

A Frequency factor or pre-exponential factora Stoichiometric number of component A; reaction order with respect to component

A; constantb Stoichiometric number of component B; reaction order with respect to component

B; constantc Stoichiometric number of component Ccj Contribution of the individual process to the overall processd Differential operatorEa Activation energyf(𝛼) Reaction modelg(𝛼) The integral form of reaction modelI(t) Ion current in relationship with timek Rate constant or rate coefficientk(T) Temperature-dependent rate constantki Isokinetic rate constantL Reactor lengthm Mass at anytimem0 Initial massmf Final mass of sample when reaction is completem0

i Mass flow of component i fed to the reactormi Mass flow of component i leaving the reactorm/z Mass per charge ratioM(t) Mean molecular mass at timen Apparent order of the reactionN Number of dataR Gas constantR2 Coefficient of linear regressionS Cross-section of a cylindrical pipeSSQ Sum of the difference between the actual value and the predicted valuet Timet0 Initial timeT Absolute temperature in KelvinT0 Initial temperatureTi Isokinetic temperature or compensation temperatureTe Measured sample temperatureTs True sample temperatureΔTTL Thermal lag value


V Flow rateVR Reactor volume𝛼 Extent of reaction or degree of conversion𝛼j Reacted fraction of a component𝛼(calc) Predicted value of reacted fraction𝛼(exp) Experimental degree of conversion𝛼(meas) Actual value of reacted fraction (similar to 𝛼(exp))𝛽 Heating rate

Subscripts

(s) the solid state(g) the gaseous state

Miscellaneous

[ ] Concentration of, e.g. [A]Also, used as reference citation, e.g. [1]

Σ Summation operator∫ Integral sign

Questions

1. From Figure 14.2, company A set the production time based on the model at Ea =150 kJ/mol, A = 1.0 × 1010 min−1 to control their production line, even though thereal kinetic data of this process agree with Ea = 162 kJ/mol, A = 1.0 × 1010 min−1.Therefore, they stop the reactor after 10 minutes has passed. What is the real productionyield that company A gets?

2. For the purpose of obtaining fundamental information on the physics and chemistry ofthe reaction, which method – isothermal or non-isothermal – is more suitable? Whatreasons can you give to support your answer?

3. Discuss the factors that influence features of the thermogram from thermo-gravimetricanalysis.

4. Explain why the isothermal experiments can separate the temperature-dependent andconcentration-dependent parts of a rate expression.

5. Compare the thermal behaviour of cellulose, hemicellulose and lignin under non-isothermal and isothermal measurements.

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15Numerical Simulation of the ThermalDegradation of Biomass – Approaches

and Simplifications

Istvan MarsiFaculty of Education, Department of Chemical Informatics, University of Szeged, Hungary

15.1 Introduction

From the very beginning, chemical engineering science has played a pioneering role inthe application of the tools developed and offered by mathematics and informatics. Thegeneral mathematical model of the reactions and transport phenomena occurring in chem-ical reactors has been known since the late 1930s. Mathematically they are (systems of)differential equations, the application of which was practically restricted to analyticallysolvable problems. (Concerning the present topic, see, e.g., [1].) The imposing develop-ment of effective numerical algorithms and their computer implementations have driven anaccelerating expansion of frontiers concerning both the extent of problems studied and thedepth of insight. Due to the complexity of the processes in the chemical industry, the mod-eling cannot dispose of the proper simplifications. Thus, modeling remains the concertedinterplay of more and more sophisticated computational methods and carefully selectedsimplifications. These aspects are also valid in the description of biomass pyrolysis, andwill therefore be the governing principle of the present chapter. To follow this approach,modeling has to face the multiple challenges of the complexity of biomass pyrolysis.

Native biomass is composed of cellulose, hemicellulose, and lignin being present invarying ratios. The mathematical description of the transformation of these constituents




is the primary task of modeling biomass pyrolysis. With an increase of temperature, thefollowing processes may be experienced that should also appear in the models. In the caseof cellulose, the predominant biomass constituent, the occurrence of the following reactionscan typically be expected:

– Up to 200 ◦C the moisture content of the reacting biomass evaporates and the decom-position of less stable molecules begins to lead to evolution of the first low-molecularvolatile products.

– Above 200 ◦C the formation of solid char can already be detected and aromatization canbe observed. At moderate temperatures (<280 ◦C), due to bond scission and dehydration,a lowering of the degree of polymerization takes place. Carbonyls, carboxyls, and furtheroxygen-containing products are formed accompanied by the evolution of CO and CO2.

– On further increase of temperature, between 280 and 500 ◦C, depolymerization by thecleavage of glycosidic bonds will be the main reaction. The typical product is tarrypyrolyzate consisting of primarily levoglucosan, other anhydrosugars, oligosaccharides,and some glucose decomposition products.

– Above 500 ◦C, the direct conversion of cellulose to low molecular gaseous products canbe observed. The predominant reactions are fission, disproportionation, dehydration, anddecarboxylation.

The typical reactions (bond scissions, degradation, charring, formation of low molecularproduct and tar) of cellulosic pyrolysis are more or less applicable also to the other twoconstituents, but the role and importance of the individual reactions may strongly differ.A characteristic product of hemicellulose pyrolysis is acetic acid, while lignin degradationoccurs on a wider temperature interval producing more char. In spite of certain analogies,the differences in the behavior of the three main constituents are obvious. However, the con-tribution of these reactions shows a similar diversity even in the case of cellulosic biomass.The initial degree of polymerization, crystallinity, and eventual impurities (including inor-ganics) may have strong influence on the kinetic behavior of predominantly cellulosicmaterials.

Ultimately, even this simplified qualitative picture is ample to compile directly componentbalance equations. The prevalent kinetic models do not pass over the aforementioned triad:they comprise cellulose, hemicellulose, and lignin as archetypes of biomass constituents.In terms of analytical chemistry, these species are multicomponent materials themselvesrepresenting a class of compounds; they are therefore referred to as lumped species.

The multitude of reactions has to be treated in a similar way: sequences of reactionshave to be comprised within lumped reactions. The motivation for such simplifications isthe lack of reliable information on the detailed mechanisms rather than the limitations of the

Solid Materialkg,1

Coke

Gas

Scheme 15.1

Numerical Simulation of the Thermal Degradation of Biomass 287

computational tools. The insufficiency of experimental kinetic data encumbers the identifi-cation of the individual steps of the mechanism. On the one hand, in terms of kinetic curves,sets of parallel reactions may apparently be merged to a single reaction of average rate; onthe other hand, among the consecutive steps, only the slowest can reliably be recognized.

Before the selection of the reactions between the components involved, a fundamentalquestion has to be answered: What kind of equation can properly express the rate ofthe solid state decomposition? The application of the mass action type kinetics including theArrheniusian temperature dependence of rate coefficients seems to be plausible. The changeof concentrations of gaseous reactants/products can easily be expressed, which should beexemplified here as an nth order irreversible reaction of a single gaseous reactant A:

dcA

dt= −kcn

A = −A exp(−

Ea

RgT

)cn

A (15.1)

(For symbols and abbreviations, see Nomenclature at the end of the chapter.)Beyond its relative simplicity, this form of rate equations has gained further preference

through the clear interpretation of the parameters A and Ea. (The underlying theory of thedynamics of molecular collisions is treated in detail, for example, in [2,3].) The applicationof Equation (15.1) to solid-state reactions is not as straightforward as replacing the symbolA by B (biomass). Primarily, the change of concentration is not appropriate to express theprogress of the reaction in the solid bulk or pellets. c has to be therefore replaced by thedegree of conversion, 𝛼, defined as follows:

𝛼 =mo − m

mo − mf

Hence Equation (15.1) can be written as:

d𝛼dt

= k (1 − 𝛼)n = A exp(−

Ea

RgT

)(1 − 𝛼)n (15.2)

But, beyond the c – 𝛼 interchange, the adaptation of Arrheniusian k(T) dependenceposes another question. The plausible interpretation of A and Ea (A provides a measureof the frequency at which molecular collisions occur, while Ea can be regarded as theenergy threshold that must be overcome by the molecules to react and form products)cannot directly be transferred to the solid phase. The energy distributions of the moleculestraveling in the gas phase or located in the bulk/on the surface of the biomass particles,evidently differ and strictly speaking they cannot, therefore, obey rate equations of thesame type. Depending on the nature of the solid-state system investigated, both completeadequacy and moderate inadequacy of the Arrheniusian approach could be proven. A finalconclusion could not be drawn and the uniform formalism is also missing to date. To followup the decades-long discussion on the applicability and interpretability of the Arrheniusrate expressions to solid-state reactions, some milestones of the abundant literature can bereferenced [4–7].

The efforts to develop alternative formulae for the k(T) dependence should be hereacknowledged, but due to the aims and limitations of the present chapter, we feel compelledto give priority to the Arrheniusian approach without forgetting its certain shortcomings.

As an effective way of problem solving in chemical engineering science, the advancefrom reaction engineering towards reactor engineering recalling B. A. Finlayson’s subtle


distinction can often be proposed [8]. In other words, the description of the bare chemicalreactions has to be successively adorned with details of physical transports and technicaldetails. We can commit ourselves also to the setting up of the present chapter in this way.

These aspects of the treatment will be summarized as statements on the following points:

(a) The number and nature of the constituents in the reacting biomass and products formed.The appearance of the chemical diversity in the model.

(b) Stoichiometric and rate equations of the reactions considered.(c) The influence of thermal effects and transports in the model.(d) Physical properties of the biomass processed including shape and size distribution.

Before presenting the fundamental models of biomass degradation, several general intro-ductory remarks should be made:

(i) The number of compounds occurring in systems of biological origin may range from104 to 106, while that of the reactions they may undergo is about a magnitude higher[9]. These approximate data clearly show that even the most detailed mechanismscan embrace only small shreds of complexity. To grasp the essence of the processesmodeled, in spite of the inevitable simplifications, is the fundamental challenge ofmodeling.

(ii) The chemical reactions starting in the solid biomass are referred to as primary reac-tions. Most of their products may undergo further transformations that represent thesecondary reactions. Primary reactions always take place in the solid while secondaryreactions primarily occur in the gas phase. The latter show an obvious analogy to thereactions of cracking. In common modeling of biomass pyrolysis, secondary reactionsare readily included in the equations of primary reactions, in other words intermedi-ates and intermediate reactions are often neglected. Their distinct appearance can beencountered only in more detailed models.

(iii) In the description of reactors performing pyrolytic reactions, the importance of thefluid dynamic aspect is commensurable to that of chemical reactions. However, thepresent treatment focuses on chemical reactions and tackles macroscopic transportwithin narrow bounds as this topic is assigned to another chapter of the book.

15.2 Kinetic Schemes Applied in Complex Models

The huge number of compounds occurring in biomass entails considerable diversity in thechemical transformations that cannot be exhaustively reflected even in the most detailedmodels. Consequently, a reasonable decrease of the number of compounds involved isunavoidable. Certain groups of similar substances have to be replaced by lumped species.The latter can be existing compounds characteristically representing the behavior of acertain group of substance occurring in the system, but they can also be fictive com-pounds named “pseudocomponents” that play a similar role. The selection of these(pseudo)components may be intuitive when their designation is based on empirical evi-dence. The diversity of reacting biomass is generally approximated by systems composedof cellulose–hemicellulose–lignin components. As concerns the products, the volatiles–tar–char (carbonaceous residue) partition can be viable. As the number of species considered


increases, the intuitive selection encounters more and more severe difficulties. To overcomethese problems, algorithmic procedures were developed to select lumped components.

The application of these models is generally based on the segregation principle, whichpostulates that the individual components of the reacting biomass do not influence thebehavior of the others. In reality, such interactions do exist between reactive components;however, they may be more or less expressed. These interactions were not only recognized,their systematic investigation was also reported [10, 11]. However, the errors introducedby the application of the segregation principle did not exceed the ones caused by othersimplifications and/or factors, for example, the distorting effects on kinetics by the presenceof inorganic impurities (primarily salts) [12]. In the models to be presented in this chapter,the segregation principle will therefore be accepted. In principle, a similar phenomenoncan be observed as the biomass is expediently contacted to inorganic compounds actingas catalysts. Although the catalytic conversion of biomass is of distinguished practicalimportance, [13–16], models of these processes do not need to be treated separately, as infirst approximation they can be described by quasi homogeneous kinetics.

In the subsequent points, various fundamental models will be overviewed. To do this, anapproved classification will be adapted (see [17–20]).

15.2.1 One-Step Global Models

(a) A single component balance is set up with respect to the reacting (active) biomass.Since the ratio of gas : coke (carbonaceous residue) is fixed in advance, the amount ofelevated volatiles and coke can be calculated through knowledge of conversion.

(b) A single solid material → gas reaction is considered.(c) If non-isothermal conditions prevail, the actual temperature is determined by a pre-

scribed T-profile with the assumption that thermal equilibrium is instantaneouslyachieved, resulting in homogeneous temperature distribution throughout the sample.

(d) Both inhomogeneities in physical properties and irregularities in shape are disregardedin the biomass sample.

The one-step global model will primarily be applied when the aim is the evaluation ofexperimental data obtained exclusively by thermal analysis in order to derive formal kineticparameters. This topic was exhaustively discussed in the previous chapter.

This model provides satisfactory agreement with experimental results primarily in thosecases when pure cellulose is pyrolyzed instead of virgin biomass [21] and the conversiontime has to be calculated. Due to its simplicity, its general application was reported in theinitial era of pyrolysis modeling [22]. By nature, it is not appropriate to predict the amount ofproducts. However, if the aim is the modeling of systems with extremely complex transportprocesses, such a strong simplification of the kinetic picture may be enforced [23, 24]. Inthese cases, if conservation equations for energy are also involved, the simplification in (c)can be omitted.

15.2.2 Competing Models

(a) Component balance equations are based on parallel reactions of the reacting biomass.The model is therefore appropriate to predict the amounts of the gas–tar–coke productsand the influence of reaction conditions on them.


Solid Material

kg,1

kT,1

kC,1Coke

Tar

Gases

Scheme 15.2

(b) The kinetic model consists of three irreversible competing reactions that are assumed tobe of first order. Three pre-exponential factors and three activation energies are neededto calculate the kinetic curves at different temperatures.

(c) The same as before (cf. 15.2.1 (c)).(d) The same as before (cf. 15.2.1 (d)).

The model was developed by Thurner and co-workers [25]. The authors validated it bysuccessfully describing literature data on pyrolysis of cellulose and woody materials [26].Due to the insufficiency of independent experimental data, the derivation of six Arrheniusparameters (indicated by Scheme 15.2) cannot always be reliably performed. The numberof parameters to be calculated can be decreased if the activation energies of gas evolutionand char formation are assumed to be approximately equal, which is often the case. Theweakness of the model is the deficient prediction of the final amount of carbonaceousresidue: apparently it does not depend on the reaction temperature.

The model has not only the aforementioned simplified version: an extended one was alsoreported. In the latter, the source of char and gas can be not only the solid material but alsothe tar. In other words, the model was completed by secondary reactions. The Tar → Gasreaction represents cracking, while Tar → Coke is a polymerization reaction.

15.2.3 Parallel Reaction Models

(a) In component balances, the constituents of the reacting biomass are distinguished. Theirconversion leads to a uniform set of products (usually gas–tar–coke).

(b) Four to six irreversible reactions are involved in the model. Each of them is assumedto be of first order, therefore four to six pairs of Arrhenius parameters are needed tocalculate the amount of components.


In this treatment, this is the first model considering the diverse composition of biomasswhich generally contains several components of different reactivity (apart from purifiedpre-treated samples). However, the more detailed and exact description covers only thereacting biomass; as concerns the products the same simplifications can be seen as thoseof the one-step global models. In other words, the parallel reaction model appears as thesuperposition of several one-step global models. The scheme was developed by Alves andco-workers to describe the pyrolysis of wet wood [27] (see Scheme 15.3). As the modelcan provide exactly only the amounts of gaseous products, tar and char formation can becalculated on previously fixed product ratios. A similar approach was published by Larfeldt


k1

k2

k6

S6

S2

S1 G

G

G

...

Scheme 15.3

[28], reporting a successful description of the pyrolysis of large sized wood particles thatwere postulated to consist of cellulose, two sorts of hemicellulose, and lignin.

15.2.4 The Broido–Shafizadeh Mechanism

(a) Among the components, one reactant of uniform consistency and its active form aredistinguished. The conversion of the latter results in three different types of char, theformation of which is accompanied by evolution of various volatiles.

(b) The model comprises a competitive, multi-step reaction sequence providing generallyfour reactions.


Despite all the divided opinions on the peculiarities of biomass pyrolysis, it is widelyacknowledged that pyrolysis consists of primary initiation and fragmentation reactions fol-lowed by secondary cracking reactions of volatiles. Among the schemes hitherto discussed,the Broido–Shafizadeh mechanism gives the most authentic descriptions of these details.This merit manifests itself also in more general relevance and considerable predictive power.Its original form (see Scheme 15.4) was developed by Broido and co-workers [29–31] forthe pyrolysis of cellulose at lower temperatures. The consecutive multi-step mechanismpartly eliminated the limitations of the former models that showed salient dependence ofkinetic parameters on the temperature range and further experimental conditions applied.The model gives a correct interpretation of the production of tar, char, and gas, distinguish-ing the successively formed fractions of the latter. As an obvious novelty, the mechanismincludes the transformation of cellulose to “active cellulose.” This substance is difficult tocharacterize exactly, but several statements on its behavior are widely accepted: it mustform a phase different from that of cellulose; it is water soluble, of lower degree of poly-merization, and it acts as the common source of further products. Active cellulose is formed

Tar Gas

Char + Gas

Activecellulose

Cellulosek1

k3

k2

k4

Scheme 15.4


in a relatively slow irreversible initiation reaction then it undergoes faster transformationsyielding a variety of products. An up-to-date review of the “active cellulose story” can befound in L. Jacques’ comprehensive paper [32]. Experiences are also reported that belittlethe active cellulose concept, as some experimental results could correctly be interpretedwithout the assumption on the participation of active cellulose.

Another simplified version of Scheme 15.4 was postulated by Shafizadeh and co-workers [33]. In this approach, the secondary reaction (represented by a dashed arrowin Scheme 15.4) was found to be superfluous and was discarded. The validity of varioussimplifications and extensions can be exemplified abundantly. The frontiers of the perfor-mance of the Broido–Shafizadeh model are carefully explored [34–37]. This mechanismappears also in complex models incorporating transport equations.

15.2.5 The Koufopanos Mechanism

(a) Among the components, one reactant of uniform consistency and three different types ofgaseous, tar, and char products are considered, the formation of which is accompaniedby evolution of various volatiles.

(b) The model is a three-step reaction sequence involving a gas–solid phase secondaryreaction. Extended versions of the mechanism assume partial reaction orders differentfrom one.


Models known as the “Koufopanos mechanism” have several variants. After publishinga scheme [38] similar to the one proposed by Broido and Shafizadeh [31, 33], the authorsdeveloped an extended model [39] for the pyrolysis of cellulose (Scheme 15.5). The firstpaper was a pioneering attempt to describe the degradation of complex lignocellulosicbiomass. Through the investigation of various cellulose–hemicellulose–lignin systems,three sets of Arrhenius parameters (incl. reaction orders) could be derived that proved tobe appropriate to characterize the reactions of the three constituents, showing a convincingindependence on the composition of the individual samples.

The second mechanism was integrated in a complex model providing also transportequations to represent the chemical source. The treatment must be restricted here to thechemistry (see Scheme 15.5). The specificity of the latter is the occurrence of a secondaryreaction formulated as the interaction of the volatile primary products and char resulting ina modified final product distribution.

The accompanying physical phenomena of the complex Koufopanos model will bediscussed later in the chapter.

Biomass k3

k1

k2

Gas2 +

Gases1 Volatiles2 +

Volatile1 +

Char2Char1

Scheme 15.5


In the present approach, we have to confine ourselves to the most frequently appliedmechanisms. Although their repertoire is more ample than suggested by the above selection,the rest of them failed to gain similar acceptance as those in Sections 15.3.1–15.3.5. Tothose who are interested in further details, we may recommend reviews by Babu et al. orPrakash et al. [14, 18]. In these publications, the relevant rate equations can also be found.

15.2.6 The Distributed Activation Energy Model (DAEM)

(a) On the reactant side, the diversity of the biomass constituents can eminently be consid-ered in terms of widely changing reactivity, while on the product side the total amount ofvolatiles released, or the amount of an individual volatile constituent, can be calculatedby the model.

(b) Several irreversible parallel reactions are thought to set up the mechanism. In contrastto the uniformity in reactions, the values of the kinetic parameters vary over a widerange.


The compositional and structural diversity of reacting biomass constituents results insubstantial diversity in reactivity. The key concept of the DAEM is to compress the manifolddiversity (appearing in composition, structure, reaction complexity) into a proper set ofkinetic parameters. For this aim, the mechanism is chosen to be as simple as possible,generally involving a single (or at most a few) type(s) of reactions. To compensate thisambiguous uniformity, the description gains flexibility by the inclusion of numerous specificArrhenius parameters that are adjusted to the diversity of the reacting biomass. In the moststraightforward approach, the concept of DAEM can be grasped as an extended variant ofthe parallel reaction model (Section 15.3.3). The biomass sample is assumed to contain1, 2,… , i,… n distinguishable constituents. In the pyrolysis, all of them undergo a first-order irreversible reaction. The individual reacting constituents can be assigned specificactivation energies Eai but they share the same pre-exponential factor. Accordingly, the ithfractional conversion 𝛼i can be expressed as follows

𝛼i = 1 − exp⎛⎜⎜⎝−

t

∫

0

ki (T (t)) dt⎞⎟⎟⎠

Summing up the fractional conversions, the total conversion 𝛼 can be obtained.

𝛼 =n∑

i=1

𝛼i = 1 −n∑

i=1

Φiexp⎛⎜⎜⎝−

t

∫

0

ki (T (t)) dt⎞⎟⎟⎠

where Φi is the relative frequency of Eai and that of the relating ith constituent in the sampleas well. If the differences Ea,i+ 1 – Eai are satisfactorily small, the discrete distributionΦ canbe replaced by the corresponding continuous distributions F, and the sum by an integral.

𝛼 = 1 −∞

∫

0

exp⎛⎜⎜⎝−

t

∫

0

ki (T (t)) dt⎞⎟⎟⎠F (E) dE (15.3)


The shape of the function F(E) can sensitively reflect the diversity of reactivity exhibitedby the biomass constituents. In a mathematical sense, F(E) is a common continuous density(frequency) function having a norm equal to 1.

As in many other cases, the success of pioneering works could be ascribed to theapplication of Gaussian distribution, namely:

F (E) = 1

(2𝜋)1∕2 𝜎

exp

(−(E − E0

)2

2𝜎2

)where 𝜎 and E0 are the distribution parameters. The latter is equal to the expectation ofactivation energies, that is, their mean.

Although the above treatment had to be confined to the simplest version of the DAEM,the principle could certainly be revealed. Omitting the detailed formalism, the followingcomments are devoted to presenting some further aspects of this multi-faceted tool.

The basic idea was published by Vand about 70 years ago [40]. As the calculation of thedouble integral in Equation 15.3 can only be numerically obtained, this made the methodtoo cumbersome to apply. The pervasive application of computers expedited the progress ofthe DAEM (with special reference to Pitt’s pioneering work [41]). Improved computationalfacilities have widened the field of application: the concept has permeated much of thethermal degradation modeling ranging from coal [42], to plastics [43] and biomass [44]. Inaddition, this has inspired development of more detailed and sophisticated models. A briefselection of tendencies can be given below:

(i) Beyond activation energy distributions F(E), the influence of the diversity of the react-ing biomass can be considered also in A(E) functions. In other words, the assumptionon A1 = A2 . . . = An = A was surpassed [45, 46]. Procedures were developed to deriveboth A(E) and F(E), without requiring a priori assumptions on the kinetic parametersinvolved [46]. The interdependence of A(E) and F(E) functions and parameters thereinwas also revealed [47, 48]. The DAEM was extended also to non-first-order kinetics.In the case of Gaussian F(E)s, the correlation of reaction order and 𝜎 can also bedemonstrated.

(ii) The applicability of distributions different from Gaussian was investigated. In theanalysis of real systems, the validity of Gamma [49] and Weibull distribution [50] wasproven.

(iii) Although it does not obviously appear in Equation 15.3, DAEM can be applied alsoto non-isothermal kinetic models [50].

In summary, DAEM applies a strongly simplified kinetic picture; however, owing to thestatistical finesse, it is accepted as the best method available for mathematically representingthe physical and chemical heterogeneity of solid substances in pyrolysis [51, 52].

15.3 Thermal Aspects of Biomass Degradation Modeling

Biomass pyrolysis takes place as superposition of strongly interacting chemical reactionsand transport phenomena. To focus on the essential chemical details of the models, thethermal effects were disregarded in the previous sections. The assumption of thermal


equilibrium instantaneously achieved and fast internal heat conduction can establish suchan approach that results in homogeneous temperature distribution throughout the sample.The insufficiency of this simplification becomes even more obvious as the heating rateand/or the particle size of the biomass increases [53]. The technical operation favors eventhese conditions attended by complications, we cannot therefore omit the consideration ofthermal effects.

15.3.1 Single-Particle Models

This extension of the model poses a real challenge as thermal phenomena are comparable incomplexity to chemical reactions. If the material processed is a single wet biomass particleplaced in a hot ambient gas, the following processes may be triggered by increasingtemperature:

1. Permanent heat transfer at the outer surface of the particle.2. Internal heat conduction inside the particle.3. Convective and/or diffusive transport of water through capillaries and voids combined

with evaporation and transport of vapor.4. In the dry region, thermal degradation starts.5. Volatile products move towards the outer surface then leave; the remaining solid biomass

frame is converted to char.6. The volatile products may undergo secondary cracking.7. With the progress of the above processes the region involved will be extended towards

the center of the particle.8. The transformation of the solid phase is primarily due to the biomass → char reaction

accompanied by secondary char reactions that finally result in a more compact structure –particle shrinkage can be observed.

9. The gaseous products evolved inside the particle cannot always continuously leave thesolid and their internal accumulation may lead to pressure generation, and finally suddensplitting/exploding of the particle.

Each of the above phenomena is worth some detailed investigation [54]. As expected,they may be of different importance and therefore can partly be neglected depending onthe actual operating conditions. Besides the analysis of such reasonable simplifications,successful efforts to develop exhaustively complex models integrating processes (1)–(9)are also reported, for example, in a recent paper of Park et al. [55]. Kinetic schemes andthermal models themselves represent different levels of approximation leading to numerouscomplex models. To provide an overview of the resulting complex models, a clear-cutcategorization may be helpful. Several consequent classifications were elaborated, in thepresent chapter those that are described in [1, 19, 56] will be adopted. In this approach,preference will be given to the comprehensive models that describe the degradation couplingchemical kinetic scheme with physical conservation equations for heat and mass transfer sothat a realistic insight into the details of the chemical and physical processes in the biomasssample could be obtained. This is in contrast to the simple thermal models (algebraic,differential, and integral models alike) that provide a rather formal description, due to thecritical pyrolysis temperature criterion, that neglect the kinetic aspects. In addition, singleparticle and particles in bed models will be treated here. Although modern computational


tools enable the full 3D modeling of particles of arbitrary form to illuminate the fundamentalprinciple, rather than the sophisticated formalism, pyrolysis models on regular bodies willbe presented below. By referring to Liliedahl and Sjostrom’s paper [57], the concept willbe explicated as the stepwise extension of the one-dimensional heat conduction model forideal slabs, cylinders, and spheres.

The basic assumptions are as follows:

(A) The underlying chemistry is the one-step global model.(B) Gaseous products leave the solid phase without hindrance; no condensation of tar

occurs.(C) Enthalpy flux due to species diffusion is negligible.(D) Swelling or shrinkage of particles is ignored.(E) Kinetic and potential energy effects, body forces are uniformly negligible.

This approach recalls the pioneering work of Bamford et al. from 1946 [58].In this context, the conservation equation for energy can simply be formulated. (To illu-

minate the background, we recommend [59] as an introduction to differential conservationequations from the aspect of chemical engineering.)

𝜕

𝜕t(𝜌cPT) = 𝜆

(𝜕2T𝜕r2

+ b − 1r

𝜕T𝜕r

)+ ΔHP

𝜕𝜌

𝜕t(15.4)

Equation 15.4 is a concise and useful formula as it is valid for ideal slabs, cylinders, andspheres, depending on the value of the geometric factor b, where b = 1, 2, 3, for slabs,cylinders, and spheres, respectively. r is the characteristic distance. In the case of slabs, itcan be defined as a distance from the middle plane, that is, the slab has a thickness of 2R.Both in spheres and cylinders r is the radial distance. For cylinders, both the density andtemperature changes along the axis are neglected. For symmetry reasons, it is satisfactoryto model the conditions for 0 ≤ r ≤ R. It has to be emphasized that ΔHP is an apparentreaction enthalpy in the sense that it also includes the enthalpy change of eventual phasechanges of the products evolved.

For the initial conditions, a uniform density and temperature are assumed all over thebodies modeled. If Equation 15.4 is coupled to a mass conservation equation and properboundary conditions are added, the model based on the considerations (A)–(E) is completeand can be formulated as follows:

𝜕𝜌

𝜕t= A exp

(−

Ea

RgT

)(𝜌f − 𝜌

)(15.5)

𝜌 (r) = 𝜌0 T (r) = T0 all r at t = 0 (15.6)

𝜕T𝜕r

||||r=0= 0 all t (15.7)

𝜆𝜕T𝜕r

||||r=R= h

(Ta − T (R)

)+ 𝜀𝜎S

(T4

a − T (R)4) all t (15.8)

The appearance of 𝜌 instead of 𝛼 in the rate Equation 15.5 seems to be inconsistent withEquation 15.2. However, the occurrence of a single first-order reaction and the constantvolume make possible the substitution (1 − 𝛼)n(m0 − mf )∕V =(𝜌f − 𝜌0).


Equation 15.6 defines the full homogeneity of the initial conditions while Equation 15.7refers to the symmetry of the modeled body: the temperature profile has either a (local)extremum in the center or a constant interval around that. Equation 15.8 accounts forthe heat transfer effects at the gas–solid phase boundary, namely the result of incomingradiation and external heat transfer has to be strictly equal to the conductive heat transportarriving at the surface or outgoing from there towards the center of the particle at any time.

After adding the uniqueness conditions, the model (15.4)–(15.8) can numerically besolved.

In the subsequent points, the possibility of extending the model will be briefly surveyed.In other words, it has to be questioned how the severity of the simplifications could bemoderated.

The most obvious insufficiency of the above model is the extremely simple kinetics inEquation 15.5. The inclusion of extended mechanisms does not conflict with the princi-ples of particle models. However, care must be taken with the correct formulation. If thechemistry can be described, for example, by the Koufopanos mechanism, it has to be con-sidered that individual component densities should be introduced that are often referred toas component concentrations in the biomass pyrolysis literature. In addition, if the gaseoussecondary reactions take place in voids of the solid sample, the volume defined by the voidfraction has to be applied.

Responses obtained by mathematical models are determined by both the formalism andthe parameters involved. As concerns the latter, it is generally not the individual parametervalues but rather their ratios are of distinguished importance. This perception evoked theapplication of dimensionless equations also in chemical engineering [60]. In the field ofbiomass pyrolysis, this approach was initiated by Pyle and Zaror [1]. As an illustration, letus reformulate Equation 15.4 in a dimensionless form:

𝜕𝜃

𝜕𝜏= 𝜕2𝜃

𝜕x2+ b − 1

x𝜕𝜃

𝜕x+

QR2k (𝜏) 𝜌f

𝛽𝜌rel (15.9)

where Equation 15.9 contains only normalized variables. The specificity of this balanceequation is determined by a single coefficient of 𝜚rel in the rightmost member that comprisesall of the relevant transport parameters.

As claimed in (B), the model (15.4)–(15.8) postulates the instantaneous and completeleaving of product gases. A more realistic approach takes into account their finite ratetransport. If this is a convective component transport, its driving force is the non-zeropressure gradient. The underlying full moment equations can be incorporated, as in themodel developed by Miller and Bellan [61, 62]. Satisfactory approximate results couldbe obtained by Darcy’s law as well [63]. The modeling was extended also to componentdiffusion in pores and voids [62,63], however the latter effect generally represents a smallercontribution than convection.

(D) is a pivotal point of the assumptions. It admits that the pyrolysis of biomass isaccompanied by more or less expressed swelling or shrinking, primarily by the latter. Oneof the tools to treat this phenomenon is the shrinking core mechanism. This postulates thatthe primary pyrolysis reactions start at the best heated thin outer shell of the particle. Thisshell is gradually exhausted with the progression of the reaction and the shrinking core,that is, the reactive non-pyrolyzed sphere inside the particle narrows towards the center. Inthis period the outer surface of the shrinking core represents a transient layer between the


unreacted biomass and the char that increasingly surrounds the particle. The thermal energyrequired by the endothermicity of the process is transferred through this increasing charlayer by conduction. The pyrolysis gases evolved also have to penetrate this layer outward.In this approach, the particle size is assumed to be unchanged and only the unreacted coreshows shrinkage.

There are different levels of approximation to treat this phenomenon. Shrinkage can beexpressed by a single shrinkage factor (as the ratio of macroscopic sizes taken with andwithout shrinkage), the process can be assumed to proceed in the overall biomass sample ina uniform manner [64], or along a moving shell as presented above. In this sense, shrinkageeffects can be calculated on additional assumptions where explicit approximations arederived to describe the propagation of reaction fronts [1, 57]. As an alternative to theprevious approach, sophisticated models were developed considering both the kineticsof the pyrolysis reaction and different physical properties of the virgin wood and char.Accounting for differences in their heat conduction, spatial temperature gradients alongboth the unreacted-core and char layer, and convective heat transport associated with therelease of volatile products, the shrinkage effects could reliably be estimated also by meansof comprehensive models [65, 66].

A further crucial point of biomass pyrolysis is the modeling of wet biomass. The dryingstarts first but it overlaps with the initial period of pyrolysis. A general overview of thedrying process is provided by [67] while an integrated approach of structural changes dueto both drying and pyrolytic shrinkage can be found in [66, 68].

The range of the one-dimensional particle modeling is restricted to regular bodies. How-ever, this approach may prove to be insufficient even in cases of particles of regular form.The background to the problem is the anisotropic character of wood particles. They showdifferent thermal conductivity and permeability along radial and tangential directions. Thisspeaks for the application of 2D and 3D models with parameters considering anisotropicbehavior of the particles [69].

Reaction mechanisms often show changes with the progress of the reaction. Both therelative importance of the individual reactions and the kinetic parameters involved maychange, with special respect to less detailed mechanisms consisting of non-elementaryreactions. (On this point, the changes of kinetic parameters are not meant as the evidentconsequences of temperature change expressed in the Arrhenius law.) This is pronouncedlytrue for fast pyrolysis, where higher conversions are achieved at a fraction of seconds andextreme temperature changes take place at the same time. In these cases, the validity of themodels can be improved if the temperature dependence is accounted for not only for rateequations but also for thermochemical and transport parameters [70, 71].

15.3.2 Particles in Bed Models

From a technical point of view, fixed beds, fluidized beds (including bubbling and circulatingfluidized beds) are of distinguished practical importance. For this aim, the simulations onsingle particles are inevitable; however, fluidized bed models cannot automatically bederived from them: the description of intra-particle phenomena does not directly predictreaction rates and product distributions as a function of the operating conditions prevailingin practical pyrolysis reactors. Convincing results obtained by such a direct approach canrarely be found in the literature and, if so, primarily for fixed bed reactors [72]. In this


paper, the conservation equations were set up for mass, momentum, and energy and thensolved with respect to the phase-averaged variables. The latter provided an experimentallyvalidated description for the overall behavior of the reactor.

For modeling of fluidized beds, two possibilities are available. One is the sophisticatedconcept of strict and detailed fluid mechanic treatment. (A methodological overview suitedfor biomass pyrolysis can be found in [73].) The other is a rather practical approach [74–76]performed at a level of approximation closer to macroscopic scale with special respect tothe key points of fluidized bed operations:

• Particle characterization.

• Kinetic model and eventual equilibrium assumptions.

• Fluidization of particles.

• Gas flow in the bed.

• Phases in the reactor, including their mixing and transfers between them.

• Heating rate and heat flux and the developing reactor temperature profiles.

15.4 Conclusion

Even this limited selection gives us a taste of the results of development and applicationof mathematical modeling in biomass pyrolysis. Although the detail and depth appearingin models always falls behind the exuberance of experiments and plant experiences, modelcalculations can effectively expedite even the most practice-oriented activity. Developmentof more detailed and sophisticated models to tackle the real-world problems on the one hand,and collection of more reliable and profound experimental data to validate and select the bestmodels on the other hand, may be a mutual motivation for the whole biomass community.

Questions

1. What are the fundamental reactions and resulting products of biomass pyrolysis? Arrangethem along an increasing temperature gradient.

2. How can the system of kinetic models be developed? Give an overview of the assump-tions applied in various approaches from the one-step global model to the Koufopanosmechanism.

3. Why is the DAEM approach suitable to describe the compositional and reactive diversityof biomass?

4. What are the typical thermal processes occurring inside the biomass particle and at itssurface?

5. In single-particle models, how can the above processes be formulated in the conservationequation for energy?

Nomenclature

Symbols

A pre-exponential factorEa activation energy


G gaseous productΔHP apparent reaction enthalpy (kJ/kg)Rg gas constant (8.31451 J/molK)Q heat of reaction number (m3/kg)R characteristic size: radius of spheres and cylinders, half-thickness of slabs (m)Si ith solid constituent in the biomass sampleT thermodynamic temperature (K)V volume (m3)b geometry factor (slab = 1, cylinder = 2, sphere = 3)c concentration (mol/m3)h heat transfer coefficient (W/m2K)k rate coefficients (s−1 for first-order reactions)m mass (kg)n reaction order (dimensionless)t time (s)x normalized distance (dimensionless)

Greek

𝛼 degree of conversion (dimensionless)𝛽 thermal diffusivity (m2/s)𝜀 emissivity coefficient (dimensionless)𝜃 normalized temperature (dimensionless)𝜆 thermal conductivity (W/mK)𝜚 density (kg/m3)𝜎 parameter of the Gaussian distribution (variance)𝜎S Stefan–Boltzmann constant (W/m2K4)𝜏 normalized time (dimensionless)

Indices

a ambient valueo initial valuef final valueg gasC charT tar

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(59) Jakobsen, H.A. (2008) Chemical Reactor Modeling – Multiphase Reactive Flows, Springer-Verlag, Berlin Heidelberg.

(60) Aris, R. (1999) Mathematical Modeling – A Chemical Engineer’s Perspective, Academic Press,San Diego.

(61) Miller, R.S. and Bellan, J. (1996) Analysis of reaction products and conversion time in thepyrolysis of cellulose and wood particles. Combustion Science and Technology, 119(1–6), 331–373.

(62) Miller, R.S. and Bellan, J. (1997) A generalized biomass pyrolysis model based on superimposedcellulose, hemicellulose and lignin kinetics. Combustion Science and Technology, 126(1), 97–137.

(63) Di Blasi, C. (1996) Heat, momentum, and mass transport through a shrinking biomass particleexposed to thermal radiation. Chemical Engineering Science, 51(7), 1121–1132.

(64) Bryden, K.M. and Hagge, M.J. (2003) Modeling the combined impact of moisture and charshrinkage on the pyrolysis of a biomass particle. Fuel, 82, 1633–1644.

(65) Galgano, A. and Di Blasi, C. (2003) Modeling wood degradation by the unreacted-core-shrinking approximation. Industrial & Engineering Chemistry Research, 42, 2101–2111.

(66) Galgano, A. and Di Blasi, C. (2004) Modeling the propagation of drying and decompositionfronts in wood. Combustion and Flame, 139, 16–27.

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(68) De Souza Costa, F. and de Castro, A. (2007) Temperature Evolution and Propagation of Drying,Pyrolysis and Charring Fronts Inside Wood Slabs and Cylinders. Proceedings of the 19thInternational Congress of Mechanical Engineering, November 5–9, 2007, Brasılia.

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16Business Case Development

Sudhakar SagiEuropean Bioenergy Research Institute (EBRI), Aston University, UK

16.1 Introduction

Production of bioenergy, biofuel and bioindustrial products derived from agriculture isaccelerating rapidly. These trends create new opportunities, risks and choices for farms;they fuel changes in the economy, social structures and ecology of farms, rural com-munities and landscapes, and foster dramatic changes in agricultural and woodland sys-tems. This emerging bioeconomy affects decisions ranging from private land use choicesto public infrastructure investment. Research, education and extension topics that canclarify the broad social, cultural, economic and environmental implications of interac-tions among technologies, policies, behaviours and management practices shape up thenew bioeconomy.

The recent spike in energy prices coincides with technological advances and increasedinvestment in alternatives. To date, bioenergy developments have focused primarily onliquid transportation fuel, especially corn ethanol, soy biodiesel and cellulosic conversion.Focus is now shifting to creation of bio-energy, biofuel and bioproducts from cellulosicbiomass derived from various sources, including existing and new crops and their residues,trees and forest residues, and municipal or industrial wastes.

The bio-economy is multi-faceted, with many potential benefits. Elements include cornethanol, soy biodiesel, electric co-generation with switchgrass and other wastes, fuel andco-products from cellulosic biomass, energy from wind, solar, hydro or geothermal systems,energy efficient technologies, and industrial products from diverse farm and forest sources.Strong rural economies and improved environmental quality are among the anticipated




benefits. A bio-economy, along with energy conservation and efficiency, represents anopportunity to enhance farm viability and foster rural development.

As economic drivers of the farm sector, biobased energy, fuels and value-added productsmay provide farmers with more crop rotation options, local premiums, higher crop prices,larger and more diverse domestic markets, new investment options and savings in their ownenergy expenditures [1–4]. Improvements in direct farm income and savings, coupled withhigh returns on farm and bio-industry investments, could multiply in local economies. Ruralcommunities benefit as farmers increase purchases of local goods and services. Ancillaryactivities, such as processing, packaging and transportation, can create more employmentand revenue that is locally retained and reverberates through the local economy [1–5].

Nationally, a bio-economy may reduce reliance on petroleum imports, hedge againsthigher petroleum costs and price volatility, reduce the trade deficit, improve the balanceof World Trade Organization blue and green box payments through reduced commoditysubsidies, and enhance wealth through carbon credits and green payments [6–8]. Thedecentralised nature of a farm-based bio-economy lends itself to more distributed energygeneration and reduces potential vulnerability due to congestion, disruption or attack.

Higher incomes, more local job opportunities, improved rural infrastructure and servicesand increased tax revenues can yield an overall improvement in quality of life that may helpstem rural out-migration [1, 9, 3,4, 8]. A strong bio-economy may also provide significantenvironmental benefits. These include new non-polluting products, reduced wastes, reducedgreenhouse gases, climate change mitigation, enhanced wildlife habitat, more biodiversityand better soil, water and air quality [3, 4, 10, 11].

Against the backdrop of dwindling fossil fuels and advancing climate change, we face theurgent task of making our energy supply more efficient and more environmentally sound.Energy from sustainable biomass production is thus a key resource we cannot and should notdo without. When used as a source of energy, biomass has three major advantages: it sparesfossil fuel reserves, helps mitigate the effects of climate change and fosters value creationand employment. Bio-energy now meets almost 5% of Germany’s primary energy demand.This share will increase significantly by 2020 as we implement the targets stipulated in theEU Climate and Energy Package announced in April 2009 and in the German government’sIntegrated Energy and Climate Change Programme launched in August 2007.

Mitigating the effects of climate change and securing sustainable energy and raw materialsupplies are two of the key challenges we face. Substituting finite fossil fuels with renewableenergy is thus vital, as is improving energy efficiency and reducing energy consumption.Biomass is playing an increasingly important role in all of this: it is currently the onlyrenewable energy source that can make a lasting contribution to securing our supply ofelectricity, heat and fuel. And greater use of biomass brings many new opportunities forindustry and for rural development, both here in Germany and in other countries aroundthe world.

When low cost biomass residues are used for fuels, the cost of electricity is often com-petitive with fossil fuel-based power generation [12]. The potential threat posed by climatechange, due to high emission levels of greenhouse gases – the most important being CO2 –has become a major stimulus for renewable energy sources in general. When produced bysustainable means, biomass emits roughly the same amount of carbon during conversionas is taken up during plant growth. The use of biomass therefore does not contribute toa build up of CO2 in the atmosphere and can be treated as CO2 neutral. It is possible to

Business Case Development 307

generate energy from biomass agricultural residue without affecting existing production ofcrops. A recent report released by UKERC suggests that up to one-fifth of global energycould be provided by biomass from plants without damaging food production [13].

The key drivers for the bioenergy sector growth in India are:

• Increased dependence as a nation on imported fossil fuels vis-a-vis the abundant avail-ability of biomass in India.

• Improved technologies for biomass based energy systems, including the use of biofuelsas an alternative or additive (displacing) fuel.

• Government has also set targets, for instance, in the current Five Year Plan period (2007to 2012), the government’s target for biomass power capacity is 1200 MW.

16.2 Biomass for Power Generation and CHP

ProcessesBiomass combustion is a carbon-free process because the resulting CO2 was previouslycaptured by the plants being combusted. At present, biomass co-firing in modern coal powerplants with efficiencies up to 45% is the most cost-effective biomass use for power gen-eration. Due to feedstock availability issues, dedicated biomass plants for combined heatand power (CHP), are typically of smaller size and lower electrical efficiency comparedto coal plants (30–34% using dry biomass, and around 22% for municipal solid waste).In co-generation mode the total efficiency may reach 85–90%. Biomass integrated gasi-fication in gas-turbine plants (BIG/GT) is not yet commercial, but integrated gasificationcombined cycles (IGCC) using black-liquor (a byproduct from the pulp and paper industry)are already in use. Anaerobic digestion to produce biogas is expanding in small, off-gridapplications. Biorefineries may open the door to combined, cost-effective production ofbiochemicals, electricity and biofuels. The new trend in thermal conversion of biomass isthe Intermediate Pyrolysis technology which is bridging the gap between fast and slowpyrolysis and can handle diversified feedstock range in various forms. The products arealso of diversified range comprising CHP, hydrogen production, biochar, and also furthersynthesis of bio-oils to chemicals.

Typical CostsBecause of the variety of feedstocks and processes, costs of biopower vary widely. Co-firing in coal power plants requires limited incremental investment ($50–$250/kW) andthe electricity cost may be competitive (US$ 20/MWh) if local feedstock is available atlow cost (no transportation). For biomass typical cost of $3–$3.5/GJ, the electricity costmay exceed $30–$50/MWh. Due to their small size, dedicated biomass power plants aremore expensive ($1500–$3000/kW) than coal plants. Electricity costs in co-generationmode range from $40 to $90/MWh. Electricity costs from new gasification plants is around$100–$130/MWh, but with significant reduction potential in the future.

StatusAbundant resources and favourable policies are enabling biopower to expand in NorthernEurope (mostly co-generation from wood residues), in the United States and in coun-tries producing sugar cane bagasse (e.g. Brazil). Proliferation of small projects, including


digesters for off-grid applications, is recorded in both OECD and emerging economies.Global biomass electricity capacity is in the range of 47 GW, with 2–3 GW added in 2005.Associated investment accounted for 7% of total investment in renewable energy capacityin 2005 ($38 billion excluding large hydro). The use of biomass is also of great importancein developing countries like India, which has huge potential of agri residues that can beused as feedstock for energy conversion.

Potential and BarriersIn the short term, co-firing remains the most cost-effective use of biomass for power gen-eration, along with small-scale, off-grid use. In the mid–long term, BIG/GT plants andbiorefineries could expand significantly. International Energy Agency (IEA) projectionssuggest that the biomass share in electricity production may increase from the current 1.3%to some 3–5% by 2050 depending on assumptions. This is a small contribution comparedto the estimated total biomass potential, but biomass is also used for heat generation and toproduce fuels for transport. The main barriers remain costs; conversion efficiency; trans-portation cost; feedstock availability (competition with industry and biofuels for feedstock,and with food and fibre production for arable land); lack of supply logistics; risks associatedwith intensive farming (fertilisers, chemicals, biodiversity).

Feedstock and ProcessesBiomass resources include agricultural residues; animal manure; wood wastes from forestryand industry; residues from food and paper industries; municipal green wastes; sewagesludge; dedicated energy crops such as short-rotation (3–15 years) coppice (eucalyptus,poplar, willow), grasses (Miscanthus), sugar crops (sugar cane, beet, sorghum), starchcrops (corn, wheat) and oil crops (soy, sunflower, oilseed rape, jatropha, palm oil). Organicwastes and residues have been the major biomass sources so far, but energy crops aregaining importance and market share. With re-planting, biomass combustion is a carbon-neutral process as the CO2 emitted has previously been absorbed by the plants from theatmosphere. Residues, wastes and bagasse are primarily used for heat and power generation.Sugar, starch and oil crops are primarily used for fuel production.

16.3 Business Perspective

To evaluate the full impact of bio-energy systems on rural livelihoods requires improvedunderstanding of the nature of the complete market chains, and of the different businessmodels, technologies, institutional arrangements and power dynamics at the various stagesin the chain, which can lead to very different livelihood outcomes. The bio-energy systemsare conceived as energy pathways which may be illustrated as below.

From Figure 16.1 it can be observed that the various resources are transformed finallyinto energy carriers and livelihood outputs. It is not only that the use of the energy resultsin better living standards via energy access and beneficial uses in enterprises, but each stepand sub-step in the system also represents a separate livelihoods opportunity and has its owninterlinked characteristics in terms of possible technologies, capacities required, financialimplications, governance issues, access rights, risk characteristics, environmental impactsand so on. The pillars of bio-energy research are: (i) sustainable entrepreneurship, includingsustainability innovations; (ii) the business case for sustainable energy, including its


PRODUCTION STEPS

WA

ST

ES

AG

RI

RE

SID

UE

S

BIO

FU

EL

S

PROCESSING STEPS

SO

LID

LIQ

UID

GA

SDELIVERY VECTORS

BIOENERGY SERVICES

EN

ER

GY

A

CC

ES

S

LIV

EL

IHO

OD

S

BIO-ENERGYRESOURCES

Figure 16.1 Bio-energy resources to services.

realisation as viable business; and (iii) a focus on local or regional production systems. Theresearch interest is to open the ‘black boxes’ in which sustainable entrepreneurs discoverand capitalise on business cases for sustainable energy and the theoretical and empiricalways these business cases can be realised. While the concept of distributed economies helpsto identify quality driven development strategies of local or regional production systems,sustainable entrepreneurship and the business model perspective allow for focusing on thebusiness management aspects. Therefore it is suggested to apply the distributed economiesapproach as a conceptual perspective and as a merging frame for issues concerning regionalsustainability and local or regional production systems. This perspective has to be refinedwith theories of sustainable entrepreneurship and business models.


16.3.1 Background

Focusing on Germany, actual statistic data show that the dominating oligopoly of a fewmultinational energy companies is based on and highly addicted to fossil fuels and nuclearpower [14, 15]. Often, energy production and consumption are declared to be key topicsof sustainable development [16]. The negative impacts of the given energy system arethoroughly analysed and publicly discussed [17]: natural resources are depleted whilstthe dependency on foreign resources grows, the global environment’s ability to absorbemissions and overexploitation is overstrained, energy prices are mounting and leading tosocial problems, even in developed and industrialised countries like Germany. Accordingto the Federal Ministry of Economics and Technology (BMWi) the number of companiesactive in the German energy markets is rising to date. This development is related to thecomplex process of liberalisation which started in 1998 [18] and which is still an importantand widely discussed topic on the agendas of economic and energy policy.

The German energy industry has historically and politically grown to a very complexproduction system that is aligned to a large-scale production strategy. Despite numerouspolitical efforts to change essential aspects of that system (market liberalisation, networkregulation, promotion of renewable energies and measures of energy efficiency etc.), still,from a macro-perspective, change happens incrementally and to some extent moves back-wards, as the current discussions on nuclear power show. In other words, the energy system’sconstitution as sketched above is an example of a ‘locked-in’ and ‘path-dependent’ socio-technical system [19] characterised by a centralised large-scale production strategy. Thelarge-scale production units herein can amongst others be explained by neoclassical eco-nomic drivers (economies of scale) [16, 20]. To sum up, it is a path dependent systemthat could only be changed by a multidimensional transformation process based on systeminnovations shifting from the current socio-technical system to a more sustainable one [21].The different factors of success that have to be considered when a sustainable transforma-tion of the energy system is discussed are the general economic conditions, strategies ofassertion, the complex multilevel political system, the integration of very different actorsand at least the technological determination.

16.4 The Role of Business Models

The second pillar – (ii) the business case for sustainable energy, including its realisation as aviable business – includes research related to the identification of opportunities that enablesustainable entrepreneurs to discover and capitalise on the business case for sustainableenergy. That is, to identify, explain and create windows of opportunity, for example for thedistribution of sustainability innovations such as micropower technology [22] or the reor-ganisation of consumption patterns and production structures of whole communities [23].

The business model is not a guarantee for success as it has to be implemented andmanaged. It is something other than the company’s business process model. It is importantto avoid two main misconceptions: firstly, the business model perspective, from a businessmanagement point of view, does not refer to the so-called ‘business modelling’. Whilebusiness models essentially focus on value creation and customers [24]. Secondly, applyinga business model perspective is not the same as having or developing a business strategy[25–27]. Stahler [26] discusses the relations between the concepts of business strategy and


business models. ‘A business model itself is not a strategy. Simply having a business modelis not a strategy.’

16.4.1 The Market Map Framework

Mapping the market was principally done to ensure that all aspects of the initiative wereaccounted for to analyse the later Livelihoods Analysis. The market mapping methodemployed in this study was developed by Practical Action [28] and was made using acombination of participatory, interview and research methods. The framework’s emphasisis on working with the whole market system, and seeking subtle, cost-effective and pro-poor interventions. The Market Map is a useful tool for collating and neatly summarisingcomplex information about specific market systems in which poor producers are involved(if used in a participatory way), helping different market chain actors to talk to each otherabout their concerns and needs and thus identify ways to improve the performance of thechain, encourage coordination between producers and along the market chain and providea starting point for exploiting market opportunities.

The Market Map framework aims to:

• improve the competitiveness of market chains involving poor producers;

• strengthen linkages between actors along these market chains (including provision ofembedded services);

• enhance collaboration and collective action among producers;

• improve access to critical services;

• tackle disabling features of the business environment.

The decision framework helps us look at all the features necessary for developing sucha complete model aimed at technology evaluation. Here in this work we are producingbio-oil from Intermediate Pyrolysis from unused agricultural waste biomass in the area ofPunjab, India. The alternative constraints, decision variables and objectives for the scenarioare given below.

Constraints:

• Feedstock types.

• Feedstock calorific values.

• Monthly costs and geographical availability of feedstock.

• Locations of districts providing biomass.

• Technology: intermediate pyrolysis.

• Transported by tractors.

• Costs for infrastructure and transport.

• Annual demand for bio-oil in Punjab area?

Decision variables:

• Locations and scheduling for biomass collection.

• Quantity and parcel size of biomass collections.

• Size, location, number and integration of storage–pelletiser–pyrolysis units.

• Downstream applications (grid-connected or decentralised power plant for electricity-only, co-generation or tri-generation, or sales of bio-oil for use in diesel generators).


Objective function

• Minimise levelised energy cost.

MIN = Capital cost ∗ capital recovery factor + fixed operational costs/

energy utilised + variable operating costs

Capital costs = pelletiser + storage unit + pyrolysis unit

Operational costs = cost of raw material + transport of raw material

+ transport of pellets + transport of bio-fuel, etc.

Energy utilised = electricity + heat + cooling

Decision frameworkFigure 16.2 shows us the overall parameters of the decision framework to analyse thealternative scenarios. The criteria used in this study for evaluating the biomass to energysupply chain are hereby listed:

Conversion ConsumerSupplier

Supplierselection

Collectionscheduling

Transportationlogistics

Smalllocal plants

Decentralisedapplications

Gridconnect

Mobileplant

Purchaseof

biofuels

Purchase ofelectricity,

heat and/orcooling

Electricity-only

(power)

Co-generation(power

and heat)

Tri-generation(power, heat,

cooling)

Locations

Number

Distribution

Minimiselevelised

energy cost

Centralplant

Capacity

Preprocessing

Capacities

CapacityLocation

Feedstock type

Figure 16.2 Decision framework to analyse the three alternative scenarios.


Harvesting and collecting – frequency, environmental impact of harvest equipment, geo-graphical distribution, selective harvesting, harvest equipment (stacker, raking, chopper,in-field briquetting, compact roller, ram baler, round baler, high density baler, chipping),cost of equipment, field losses, cost of on-farm haulage.

Feedstock – type (husks, stalks, straws, etc.), cost, availability, quantity, seasonal variability,characteristics (calorific value, moisture content, etc.).

Preparation – process (ensiling, torrefaction, pressing, drying, mixing, wafering, pelletis-ing, pulverising), cost, environmental impact, increase in energy content, decreased riskof deterioration.

Storage – location, number, capacity, type of facility (outdoors, outdoors with cover, polebarns, farm building, silo, tank), risk of degradation, cost, environmental and socialimpact.

Transport – vehicle (trailer, lorry, tractor, trans-stacker, pipeline, conveyor belt, ship,train), route, schedule, quantity, environmental and social impacts of transport, laws andinfrastructure, cost per kg to transport, density and weight of biomass.

Conversion – technology (gasifier, moving grate, fluidised bed, pyrolysis, AD, etc.), prod-uct (biogas, bio-oil, ethanol, methanol, etc.), application (power, heat, cooling), users(grid, decentralised, domestic, industrial), seasonal demand, number, capacity, location,incentives.Additional financial variables to consider:

Financial decision variables – levelised energy cost, capital cost, operations and mainte-nance costs, ROI, NPV, IRR, payback period.

16.5 Financial Model Based on Intermediate Pyrolysis Technology

The financial model discussed here is suitable for all countries both in the West and in thedeveloping world. Also the numbers change depending on the country of operation andthe prices of feedstock and other consumables required for the conversion. The economicanalysis for conversion of straw (agricultural residue) gives an overview of the productioncost of pellets, bio-oil (pyrolysis oil) and biochar considering various resources that go intothe production of the same. Precise determination of cost will help farmers to assess thebenefits and return from such systems. Based on the cost–benefit analysis, different businessmodels can be proposed. Table 16.1 summarises the economic data per acre basis basedon the various unit costs and an overall cost scenario using straw as residue. The analysispresented here is for the production of straw per acre of land. The economic analysis viathe pyrolysis route gives benefit to the farmer in terms of economic incentives either forselling the straw or for utilising it for energy generation purposes. The utilisation of strawfor energy purposes can create a whole bio-economy value chain.

Table 16.1 is an overall summary sheet that shows the values for the cost of pelletisation,cost of pyrolysis and the cost of running the engine. The process includes pelletisation ofthe straw, pyrolysis of the straw to produce vapours and to condense the vapours to producethe oil blend. The costs associated and the energy required are presented as tables relatingto the particular steps of process. The economic analysis is basically divided into twosections as fuel preparation and fuel utilisation. Fuel preparation is basically costs incurredfor the feedstock, pellet preparation, oil production as well as engine operation. Basic


Table 16.1 Overall economic analysis of bio-agricultural residue.

Detailed economic analysis of the conversion of straw to CHP

Bio-oil Production from straw Per Acre Land

ParticularsPer Mass

[%] [kg]

Energy

[kWh/hr]

Energy

kWh

Cost

[Rs]

Cost Contribution

Fuel Preparation

Fuel Utilization

1.0 AcreLand considered

Feed Stock

Pellets Production

Engine Operation

Engine Application (125 KVA DG Set)

Energy Surplus Factor (ERF)

Pyrolysis

Pellets

Losses

Total

Total

Total

Total

Oil

Oil

Oil

Biodiesel

Biodiesel

Gas

Char

Straw

Kg/hr

[%]

Operation

[Hrs]

considerations for the analysis are as follows. The straight line method for depreciation,electricity charges per unit as per market and the country, manpower charges also dependenton the country, operation of plant for 300 days per year (in reality the plant has to beoperated continuously to make it more attractive in terms of economics), yearly operationand maintenance (O & M) charge as 20% of capital cost, and annual escalation in thecost as 10%.

16.5.1 Pelletisation Process

The basis for calculation is depicted in Table 16.2. As a first step towards pyrolysis, straw(or any other feedstock) is converted into pellets. Basic considerations for the processparameters are as follows. Capital cost of pellet production unit has an operation life of6 years. Consumption of raw material is 1 ton per hour. Loss of material in production ofpellets, in the form of evaporation of water, loss of material while handling, and so on is3%. Number of operators required for the operation is 1.


Table 16.2 Economic analysis of pellets production (cost consideration).

Costing for Economic Analysis of Pellets Production

Major parameters

Capital cost of pelletiserModel with depreciation cost? (1 for yes 0 for no)Consumption of raw materialMaterial loss during pellet productionCapacity of pellet productionCost of raw materialEffective cost of raw materialConnected electric load for operationDuty factor for operation of the pelletiserElectricity chargesCost of manpowerLife of the plantOperation of the plant per dayOperation of plant per yearO & M charges per yearAnnual escalation

After taking the considerations mentioned in the Table 16.2, we get the actual values forthe net cost of pellet production. This is independent of the country and the process. Oncethe values are given for the data required then the net cost of pellet production is achieved.Table 16.3 represents all the values necessary to determine the net cost of pellet productionand this can be done for several years taking into account annual inflation.

16.5.2 Pyrolysis Unit

Pellets from the pellet making machine are fed to the pyrolysis unit where pyrolysispelletisation takes place. Basic considerations for the process parameters are as follows.Capital cost of a pyrolysis unit having operational life of 15 years, consumption of straw

Table 16.3 Economic analysis of pellet production (cost calculation).

Year of operation 1 2 3 4

Depreciation of the machine Rs/hrCost of raw material Rs/hrElectricity charges Rs/hrCost of manpower Rs/hrO & M charges Rs/hrTotal cost Rs/hrPellets produced Rs/hrCost of the pellets Rs/hrNet cost of pellet production Rs/kg


Table 16.4 Economic analysis of pyroformer (cost considerations).

Costing for economic analysis of pyroformer

Major parameters

Capital cost of pyroformerModel with depreciation cost? (1 for

yes 0 for no)Capacity of pellet consumptionPercentage of bio-oil yield/productionChar yieldRevenue from charConnected electric load for heatersDuty factor for operation of the heatersConnected electric load for both

screwsDuty factor for operation of the screw

motorsConnected electric load for chillerConnected electric load for circulation

pumpElectricity chargesCost of manpower (1 person for

operation)Life of the plantOperation of the plant per dayOperation of plant per yearO & M charges per yearAnnual escalation

pellets and cost of pellets are carried forward from Table 16.3. Pyrolysis oil yield is 35%of the pellet consumption, connected electric load for heaters, for motors to run the feedconveying screw as well as the char transport screw, chillers and oil circulation pumprespectively. Number of operators required for the operation is 1. Table 16.4 gives thecost consideration of pyrolysis oil production for the service life of the pyrolysis unit andalso represents the economic analysis of pyrolysis oil production. The cost of the bio-oilproduction is derived considering all the above mentioned details.

It can be inferred from Table 16.4 that the cost of the Pyroformer is the key component interms of fixed capital cost and the rest of the equipment also contributes to the overall costof the pyrolysis process. The running costs of the process are very much country specificand may vary greatly from country to country. Table 16.5 shows the input values for allprocess parameters and their power consumption including the cost of man power and otherexpenses there by resulting in the net cost of the bio-oil produced.

Once the cost of bio-oil is estimated, then comes the application part where the producedbio-oil is blended with conventional fossil fuels like diesel and is used to run normal dieselengines. The blending percentage is very much dependent on the feedstock and also theprocess parameters. At this stage, for wheat straw pyrolysis, we achieved up to 35% blend


Table 16.5 Economic analysis of pyroformer (cost calculations).

Year of operation 1 2 3 4 5

Depreciation of the machineCost of raw materialElectricity charges

for heatersfor screwsfor circulation pumpfor chiller unitTotal electricity charges

Cost of manpowerO & M chargesChar recovery costTotal costBio-oil productionCost of the bio-oil production

with the engine operating for more than 75 hrs. The cost analysis for the engine is alsodivided into the cost considerations and the benefit part in terms of power generation.Table 16.6 depicts the cost considerations. Table 16.7 shows the actual costs involved andthe net electricity produced and thereby calculates the net cost of production. This valuehas to be compared to the net cost of power generation from the diesel mode operation toobtain the net savings from switching from normal usage mode.

Table 16.6 Economic analysis of engine operation.

Costing for economic analysis of engine operation

Major parameters

Capital cost of engineModel with depreciation cost? (1 for yes

0 for no)Cost of dieselBio-oil blend consumption (for a fixed load)Bio-oil blend percentageElectric load on generator – Bio-oil

biodiesel modeElectricity chargesCost of manpower (1 person for operation)Life of the plantOperation of the plant per dayOperation of plant per yearO & M ChargesAnnual escalation


Table 16.7 Net cost of power generation and savings.

Diesel and bio-oil blend

Cost of dieselCost of bio-oilTotal cost of fuelDepreciation of the engineCost of manpowerO & M chargesTotal operating costElectricity productionNet cost of power generationSavings from switching

This model has been developed specifically for the study of economics for straw con-version. However, this model can be widely applied to different feedstocks and differentgeographic locations and the detailed analysis can be easily deduced for any given scenariowith respect to specific country and specific feedstock and the required blend of the gener-ated oils. The model has been developed in Excel and results can be obtained for variousscenarios, such as different operating hours, varied prices of feedstock, varied power tariffsand so on. The detailed program can be obtained as a CD-ROM upon request. The currentmodel has been tested for straw conversion to energy in a joint project with India. From theeconomic analysis it can be concluded that intermediate pyrolysis technology proves to bevery effective in terms of product qualities of the oil produced (for wheat straw) and alsothe return on investment is around 4 to 5 years. It can also be said that with economies ofscale and continuous operation the ROI can be further improved.

Feedstock supply resource assessments identify the geographic location, price and envi-ronmental sustainability of accessing existing and potential future feedstock resources, andprojecting future supply availability and prices as well. Strategic analysis activities utilisethese data to understand price effects of competition from various biomass utilisation tech-nologies and to assess cross-technology impacts of feedstock cost, quantity and quality.Market assessment helped the programme focus its technology development priorities in thenear-, mid- and long-term by analysing the potential cost, commercialisation time and mar-ket demand products. This analysis drew on a broad range of other analyses including futureenergy demand forecasts; infrastructure assessments; state of biomass utilisation technol-ogy development; national and local sustainability analysis; and consumer, economic andpolicy scenarios. This also helped identify current and future market attractiveness, gaps,strengths and risks that may impact producer, investor and consumer decision-making.

References

(1) Gronski, R. (2006) Developing Sustainability Criteria for Renewable Energy: A Road Mapfor a Sustainable Future. National Catholic Rural Life Conference paper presentation at theCSREES-SARE Roundtable Discussion, Washington, DC. February 13, 2006.


(2) Kleinschmit, J. and Smith, M. (2006) Biofuels or Bust!: How We Can Make the BioeconomySustainable for Farmers and the Land. Ag Matters, Spring, pp. 1–6.

(3) Miranowski, J. and Otto, D. (2005) Impacts to the Iowa Rural Economy of an ExpandingEthanol Industry. PowerPoint presentation to USDA.

(4) Morris, D. (2006) Ownership Matters: Three steps to ensure a Biofuels Industry that trulybenefits rural America. http://www.ilsr.org./ The Institute for Local Self-Reliance (last accessMay 2, 2014).

(5) English, B.C., De La Torre Ugarte, D.G., Hellwinckel, C.M. et al. (2006) Economic Impactsfrom Increased Competing Demands for Agricultural Feedstocks to Produce Bioenergy andBioproducts. PowerPoint presentation at NRI Agricultural Markets and Trade Project Directors,Meeting, June 15–16, 2006, Washington, DC.

(6) Conway, R.K. and Duncan, M.R. (2006) Bioproducts: Developing a Federal Strategy for Suc-cess. Choices: The Magazine of Food, Farm, and Resource Issues. 1st Quarter 21(1), 33–36.

(7) Morris, D. (2006) The Once and Future Carbohydrate Economy. American Prospect Online.http://www.prospect.org/web. The Institute for Local Self-Reliance (last access September 9,2012).

(8) Werner, C. (2006) Revitalizing the Farm Economy through Renewable Energy Development.Environmental and Energy Study Institute. PowerPoint presentation at the CSREES-SARERoundtable Discussion, Washington, DC. February 13, 2006.

(9) Kleinschmit, J. and Muller, M. (2005) Cultivating a New Rural Economy: Assessing the Poten-tial of Minnesota’s Bioindustrial Sector, Institute for Agriculture and Trade Policy. August,2005, http://www.environmentalobservatory.org/library.cfm?refid =76223 (last access August,2012).

(10) Conway, R.K. and Erbach, D. (Co-Chairs), (2004) Bioenergy: Pointing to the Future. Councilfor Science and Technology (CAST) Issue Paper. No. 27A, November 2004.

(11) Greene, N. (2004) Growing Energy: How Biofuels Can Help End America’s Oil Dependence.National Resources Defense Council, December, 2004.

(12) McKendry, P. (2002) Energy production from biomass (part 1): overview of biomass. Biore-source Technology, 83, 37–46.

(13) Raphael, S., Robert, S., Robert, G. and Ausilio B. (2011) Energy from biomass: the size of theglobal resource. Imperial College Centre for Energy Policy and Technology and UK EnergyResearch Centre, London.

(14) Bundesministerium fur Wirtschaft und Technologie (BMWi) (2007a) Einsatz von Energi-etragern zur Stromerzeugung. Deutschland. Online verfugbar unter http://www.bmwi.de/BMWi/Navigation/Energie/energiestatistiken,did=180896.html, zuletzt gepruft am 01.07.2008.

(15) Bundesministerium fur Wirtschaft und Technologie (BMWi) (2008) Stromerzeugungska-pazitaten und Bruttostromerzeugung nach Energietragern. Deutschland. Online verfugbar unterhttp://www.bmwi.de/BMWi/Navigation/Energie/energiestatistiken,did=180894.html, zuletztgepruft am 02.07.2008.

(16) Johansson, A., Kisch, P. and Mirata, M. (2005) Distributed economies - a new engine forinnovation. Journal of Cleaner Production, 13(10–11), 971–979.

(17) Hennicke, P., Supersberger, N. and Huncke, W. (2006) Krisenfaktor Ol. Abrusten mit neuerEnergie. Munchen: oekom Verl.

(18) Bundesministerium fur Wirtschaft und Technologie (BMWi) (2007b) Anzahl der Betriebeund Beschaftigte im Energiesektor. Deutschland. Unter Mitarbeit von. Online verfugbar unterhttp://www.bmwi.de/BMWi/Navigation/Energie/energiestatistiken,did=177840.html, zuletztgepruft am 03.07.2008.

(19) Arthur, W.B. (1989) Competing technologies, increasing returns, and lock-in by historicalevents. The Economics Journal, 99(394), 116–131.

(20) Frank, R.H. (2006) Microeconomics and Behavior, 6th edn, Internat. edn. McGraw-Hill/Irwin,Boston, Mass.

(21) Geels, F.W., Elzen, B. and Green, K. (2004) Understanding system innovations: critical literaturereview and a conceptual analysis, in System Innovation and the Transition to Sustainability.Theory, Evidence and Policy (eds B. Elzen, F.W. Geels and K. Green), Cheltenham, Elgar,pp. 19–47.

http://www.ilsr.org./

http://www.prospect.org/web

http://www.environmentalobservatory.org/library.cfm?refid=76223

http://www.bmwi.de/BMWi/Navigation/Energie/energiestatistiken,did=180896.html





(22) Wustenhagen, R. and Boehnke, J. (2007) Business models for sustainable energy, in SystemInnovation for Sustainability: Perspectives on Radical Changes to Sustainable Consumptionand Production (eds A. Tukker, M. Charter, C. Vezzoli et al.), Greenleaf Publishing, Sheffield,pp. 253–258.

(23) McCormick, K. and Kaberger, T. (2005b) Exploring a pioneering bioenergy system: the caseof enkoping in Sweden. Journal of Cleaner Production, 13(10–11), 1003–1014.

(24) Osterwalder, A., Pigneur, Y. and Tucci, C.L. (2005) Clarifying Business Models: Origins,Present and Future of the Concept. Submitted to CAIS (Communications of the Association forInformation Systems), vol. 15.

(25) Stahler, P. (2002a) Business Models as an Unit of Analysis for Strategizing. InternationalWorkshop on Business Models, Lausanne, Switzerland (1st Draft, 30. September 2002).

(26) Stahler, P. (2002b) Geschaftsmodelle in der digitalen Okonomie. Merkmale, Strategien undAuswirkungen. 2. Aufl. Lohmar: Eul (Reihe, 7).

(27) Osterwalder, A. and Pigneur, Y. (2004) An ontology for e-business models, in Value Creationfrom E-Business Models (ed. W.L. Currie), Elsevier, Oxford, pp. 65–97.

(28) Mike A. and Alison G. (2005) Mapping the Market: A framework for rural enterprise devel-opment policy and practice. http://practicalaction.org/docs/ia2/mapping_the_market.pdf (lastaccess May 2, 2014).

http://practicalaction.org/docs/ia2/mapping_the_market.pdf

17Production of Biochar and ActivatedCarbon via Intermediate Pyrolysis –

Recent Studies for Non-WoodyBiomass

Andreas Hornung1 and Elisabeth Schroder2

1Fraunhofer UMSICHT – Institute Branch Sulzbach-Rosenberg, Germany and Chair inBioenergy, School of Chemical Engineering, College of Engineering and Physical Sciences,

University of Birmingham, UK2Karlsruher Institut fur Technologie – Institut fur Kern-und Energietechnik, Germany

17.1 Biochar

17.1.1 Introduction

Pyrolysis char, a so-called black (dark) earth or biochar, is obtained today usually as abyproduct from charcoal production from wood. The dusty fraction is used as such, mixedwith compost or worm cast. The materials are usually used in Europe or the USA, whiledirect application is reported from Africa, New Zealand and Australia as well as SouthAmerica and Switzerland.

There is a well known and very ancient technique of adding carbon to soil. This naturaltechnique of carbon sequestration has recently been placed under consideration because ofthe increase in concerns about environmental degradation due to irreversible and uncon-trolled production of greenhouse gases. At the same time, the need to protect soils from anincreasingly uncertain climate makes the apparent ability of biochar to increase the capacity




Table 17.1.1 Basic biochar utility properties – required for all biochars [IBI].

Moisture Declaration% of total mass,dry basis

ASTM D1762-84 [1] (specifymeasurement date with respect totime from production)

Organiccarbon

Class 1: ≥60%Class 2: ≥30%

and <60%Class 3: ≥10%

and <30%

% of total mass,dry basis

Total C and H analysis by drycombustion–IR detection. InorganicC analysis by determination ofCO2–C content with 1N HCl, asoutlined in ASTM D4373-02.Organic C calculated as Total C –Inorganic C

H : Corg 0.7 (Maximum) Molar ratio C, H, N analysis by dry combustion(Dumas method), before (total C)and after (organic C) HCl addition

Total ash Declaration % of total mass,dry basis

ASTM D1762-84 [1]

Total nitrogen Declaration % of total mass,dry basis

Dry combustion–IR detectionfollowing the same procedure fortotal C and H above

pH Declaration pH pH analysis procedures as outlined insection 04.11 of US CompostingCouncil and US Department ofAgriculture [2], following dilutionand sample equilibration methodsfrom Rajkovich et al. [3], SeeAppendix 2.

Electricalconductivity

Declaration dS/m EC analysis procedures as outlined insection 04.10 of US CompostingCouncil and US Department ofAgriculture [2], following dilutionand sample equilibration methodsfrom Rajkovich et al. [3], SeeAppendix 2.

Liming Declaration % CaCO3 Rayment & Higginson [4]Particle size

distributionDeclaration % <420 μm

% 420–2380 μm% 2380–4760 μm% >4760 μm

Progressive dry sieving with4760 μm, 2380 μm and 420 μmsieves, as outlined in ASTMD2862-10 Method for activatedcarbon.

of soil to absorb and store water vitally important. It also appears that adding biochar tosoil may be one of the only ways by which the fundamental capacity of soils to store andsequester organic matter can be increased.

17.1.2 Biochar and its Application in the Field

To use biochar in co-combustion or for activated carbon production is a straight forwardapplication; to use biochar in the agricultural environment is a more sensitive approach.

Production of Biochar and Activated Carbon via Intermediate Pyrolysis 323

Therefore, the International Biochar Initiative (IBI) released a set of guidelines for properbiochar production and management which is summarised in Table 17.1.1–17.1.4.

In addition, biochar offers potential agronomic and environmental benefits (e.g. reducingsoil CO2, CH4 and N2O emissions as well as the adsorption of contaminants in soil,increased soil fertility and crop growth at least for low quality soils). Biochar is the onlyexisting way to reduce CO2 in the atmosphere by storing it as char.

Table 17.1.2 Biochar toxicant reporting – required for all biochars [IBI].

RequirementRangeallowed

Maximumthresholds Test method

Germinationinhibition assay

Pass/Fail OECD methodology [5] 3 test species,as described by Van Zwieten et al. [6]

Polycyclic aromatichydrocarbons(PAH)

6–20 mg/kg TM Method following US EnvironmentalProtection Agency [7]

Dioxin/Furan(PCCD/F)

9 ng/kg I-TEQ Method following US EnvironmentalProtection Agency [8]

Polycyclic biphenyls 0.2–0.5 mg/kg I-TEQ Method following US EnvironmentalProtection Agency [7]

Arsenic 12–100 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Cadmium 1.4–39 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Chromium 64–1200 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Cobalt 40–150 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Copper 63–1500 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Lead 70–500 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Molybdenum 5–20 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Mercury 1–17 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Nickel 47–600 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Selenium 1–36 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Zinc 200–7000 mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Boron Declaration mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Chlorine Declaration mg/kg US Composting Council and USDepartment of Agriculture [2] dry wt

Sodium Declaration mg/kg US Composting Council and USDepartment of Agriculture [2]


Table 17.1.3 Biochar advanced analysis and soil enhancement properties – optional for allbiochars [IBI].

Requirement Criteria Unit Test method

Mineral N (ammoniumand nitrate)

Declaration mg/kg 2M KCl extraction, followedby spectrophotometry [4]

Total phosphorus andpotassium (total K issufficiently equivalentto available K for thepurpose of thischaracterisation) (P&K)

Declaration % of total mass,dry basis

Modified dry ashingfollowed by ICP [9]

Available P Declaration mg/kg 2% formic acid followed byspectrophotometry asdescribed by Wang et al.[10] after Rajan et al. [11]and AOAC [12]

Volatile matter Declaration % of total mass,dry basis

ASTM D1762-84 [1]

Total surface area Declaration m2/g ASTM D 6556-10 [13]Standard Test Method forCarbon Black – Total andExternal Surface Area byNitrogen Adsorption

External surface area Declaration m2/g ASTM D 6556-10 [13]Standard Test Method forCarbon Black – Total andExternal Surface Area byNitrogen Adsorption

It is important to reduce the oxygen to carbon level to a value close or lower than 0.25 toachieve persistence of the char in soil of about 1000 years or more due to the inability ofmicroorganisms to digest the matter.

The following table shows an example for char straw char C\O ratios for differentpyrolysis temperatures.

Table 17.1.4 Wheat straw pyrolysed at temperatures between325 and 450 ◦C and its resulting C:O ratios for biochar.

Wheat straw pyrolysed at T\◦C C : O ratio for char

325 1.3350 2.1375 2.9400 3.4450 3.9Wheat straw non-pyrolysed 0.9


Table 17.1.5 Biomass feedstock, pyrolysis temperature and elemental composition ofbiochar samples (elemental percentages are on dry mass basis), for corn stover. The data isgiven for four different temperatures [15].

Feedstock Pyrol. T/◦C C H N O S

Hardwood sawdust 500 69.5 3.06 0.32 13.1 0.01Wood waste 550 91.4 2.89 0.38 4.6 0.00Corn stover 815 45.0 1.66 0.50 1.0 0.04Corn stover 515 51.0 0.9 1.00 0.00 0.04Corn stover 505 66.0 1.5 1.00 4.0 0.04Distillers grain 350 68.6 4.81 7.52 6.6 0.96Distillers grain 400 69.4 4.31 7.43 5.9 0.9

The Table 17.1.5 shows the content of carbon, oxygen, nitrogen, hydrogen and sulfur fora set of biomasses.

References

(1) ASTM D1762-84 (2007) Standard Test Method for Chemical Analysis of Wood Char-coal, ASTM International, http://www.astm.org/Standards/D1762.htm (accessed September2011).

(2) US Composting Council and US Department of Agriculture (2001) Test methods for the exam-ination of composting and compost. (TMECC) (ed. W.H. Thompson) http://compostingcouncil.org/tmecc/. (Accessed January 2012).

(3) Rajkovich, S., Enders, A., Hanley, K. et al. (2011) Corn growth and nitrogen nutrition afteradditions of biochars with varying properties to a temperate soil. Biology and Fertility of Soils.DOI: 10.1007/s00374-011-0624-7. Published Online.

(4) Rayment, G.E. and Higginson, F.R. (1992) Australian Laboratory Handbook of Soil and WaterChemical Methods, Reed International Books, Australia/Inkata Press, Port Melbourne.

(5) OECD Organisation for Economic Co-operation and Development (1984b) Terrestrial Plants,Growth Test no. 208. In Guideline for Testing of Chemicals. http://www.oecd.org/dataoecd/18/0/1948285.pdf. (Accessed January 2012).

(6) Van Zwieten, L., Kimber, S., Morris, S. et al. (2010) Effects of biochar from slow pyrolysisof papermill waste on agronomic performance and soil fertility. Plant and Soil, 327, 235–246.DOI: 10.1007/s11104-009-24 0050-x.

(7) US Environmental Protection Agency (1996) METHOD 8275A Semivolatile organic com-pounds (PAHs and PCBs) in soils/sludges and solid wastes using thermal extraction/gas chro-matography/mass spectrometry (TE/GC/MS). http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/8275a.pdf (accessed September 2011).

(8) US Environmental Protection Agency (2007) EPA METHOD 8290A Polychlorinated Dibenzo-P-Dioxins (PCDDs) and polychlorinat ed dibenzofurans (PCDFs) by high resolution gaschromatography/high resolution mass spectrometry (HRGC/HRMS). http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/8290a.pdf (Accessed September 2011).

(9) Enders, A. and Lehmann, J. (2012) Comparison of wet digestion and dry ashing methods fortotal elemental analysis of biochar. Communications in Soil Science and Plant Analysis, 43,1042–1052.

(10) Wang, T., Camps Arbestain, M., Hedley, M. and Bishop, P. (2012) Predicting phosphorusbioavailability from high-ash biochars. Plant and Soil. DOI: 10.1007/s11104-012-1131-9. Pub-lished Online.

http://www.astm.org/Standards/D1762.htm

http://compostingcouncil.org/tmecc/

http://www.oecd.org/dataoecd/18/0/1948285.pdf

http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/8275a.pdf


http://compostingcouncil.org/tmecc/

http://www.oecd.org/dataoecd/18/0/1948285.pdf




(11) Rajan, S.S.S., Brown, M.W., Boyes, M.K. and Upsdell, M.P. (1992) Extractable phosphorus topredict agronomic effectiveness of ground and unground phosphate rocks. Nutrient Cycling inAgroecosystems, 32(3), 291–302.

(12) AOAC (Association of Analytical Communities) International (2005) In AOAC Official Methodsof Analysis, 18th edn. (ed. G. Latimer), www.eoma.aoac.org (accessed September 2011).

(13) ASTM D6556-10 (2009) Standard Test Method for Carbon Black – Total and External SurfaceArea by Nitrogen Adsorption, ASTM International, http://www.astm.org/Standards/D6556.htm(accessed January 2012).

(14) Amlinger, F., Faroino, E. and Pollack, M. (2004) EU Heavy Metals and Organic Compoundsfrom Waste Used as Organic Fertilizers Final Report. ENV.A.2./ETU/2001/0024 REF.NR.:TEND/AML/2001/07/20. (Accessed January 2012).

(15) Fabbri, D., Torri, C. and Spokas, K. A. (2012) Analytical pyrolysis of synthetic chars derivedfrom biomass with potential agronomic application (biochar). Relationships with impactson microbial carbon dioxide production. Journal of Analytical and Applied Pyrolysis, 93,77–84.

Further Reading

ASTM D5158-98 (2005) Standard Test Method for Determination of Particle Size of Powdered Acti-vated Carbon by Air Jet Sieving, ASTM International, http://www.astm.org/Standards/D5158.htm(accessed September 2011).

Brinton, W.F. (2000) Compost quality standards and guidelines. Woods End Research Laboratory,prepared for New York State Association of Recyclers. http://compost.css.cornell.edu/Brinton.pdf(accessed September 2011).

Bureau de normalisation du Quebec (2005) National Standard of Canada, Organic Soil Condition-ers – Compost. CAN/BNQ 0413-200 (2005) ISBN: 2-551-22659-7 http://www-es.criq.qc.ca/pls/owa_es/bnqw_norme.detail_norme?p_lang=en&p_id_norm=8184&p_c22 ode_menu=NORME(accessed September 2011).

Canadian Council of Ministers of the Environment (CCME) (2002) Canadian Soil Quality Guidelinesfor the Protection of Environmental and Human Health: Polych lorinated 25 Dibenzo-p-Dioxinsand Polychlorinated Dibenzofurans (PCDD/Fs). In: Canadian environmental quality guidelines,1999, Canadian Council of Ministers of the Environment, Winnipeg Manitoba, Canada. ISBN1-896997-34-1 http://ceqg-28rcqe.ccme.ca/ (accessed January 2012).

Canadian Council of Ministers for the Environment (CCME) (2005) Guidelines for Compost Quality.PN 1340 Winnipeg Manitoba, Canada. ISBN 1-896997-60-0.

European Commission Agriculture and Rural Development (2010) Biomass Potential http://ec.europa.eu/agriculture/bioenergy/potential/index_en.htm (accessed September 2011).

European Commission COM (2006) Directive Establishing a Framework for the Protection ofSoil and Amending Directive 2004/35/EC. http://ec.europa.eu/environment/soil/pdf/com_2006_0232_en.pdf (accessed September 2011).

International Biochar Initiative (2010) IBI Guidelines for the Development and Testing of PyrolysisPlants to Produce Biochar http://www.biochar-international.org/sites/default/files/IBI-Pyrolysis-Plant-Guidelines.pdf (accessed September 2011).

ISO 17512-1:2008 (2008) Soil quality – Avoidance test for determining the quality of soils and effectsof chemicals on behaviour – Part 1: Test with earthworms (Eisenia fetida and Eisenia andrei), ISO,http://www.iso.org/iso/catalogue_detail.htm?csnumber=38402 (accessed January 2012).

Li, D., Hockaday, W.C., Masiello, C.A. and Alvarez, P.J.J. (2011) Earthworm avoidance of biocharcan be mitigated by wetting. Soil Biology & Biochemistry, 43, 1732–1737.

Milne, T.A., Brennan, A.H. and Glenn, B.H. (1990) Sourcebook of Methods of Analysis for BiomassConversion and Biomass Conversion Processes. SERI/SP-220-3548. Golden, CO: Solar EnergyResearch Institute, February 1990.

http://www.eoma.aoac.org



http://compost.css.cornell.edu/Brinton.pdf

http://www-es.criq.qc.ca/pls/owa_es/bnqw_norme.detail_norme?p_lang=en&p_id_norm=8184&p_c22ode_menu=NORME

http://ceqg-28rcqe.ccme.ca/

http://ec.europa.eu/agriculture/bioenergy/potential/index_en.htm

http://ec.europa.eu/environment/soil/pdf/com_2006__0232_en.pdf

http://www.biochar-international.org/sites/default/files/IBI-Pyrolysis-Plant-Guidelines.pdf

http://www.biochar-international.org/sites/default/files/IBI-Pyrolysis-Plant-Guidelines.pdf

http://www.iso.org/iso/catalogue_detail.htm?csnumber=38402

http://www-es.criq.qc.ca/pls/owa_es/bnqw_norme.detail_norme?p_lang=en&p_id_norm=8184&p_c22ode_menu=NORME

http://ec.europa.eu/agriculture/bioenergy/potential/index_en.htm

http://ec.europa.eu/environment/soil/pdf/com_2006__0232_en.pdf


National Institute of Health, National Cancer Institute, Dictionary of Cancer Terms, http://www.cancer.gov/dictionary?cdrid=687391 (accessed February 2012).

OECD Organisation for Economic Co-operation and Development (1984a) Earthworm acute toxicitytests no. 207. In Guideline for testing of chemicals. ISBN 9789264070042 http://browse.oecdbookshop.org/oecd/pdfs/free/9720701e.pdf (accessed January 2012).

Rayment, G.E. and Lyons, D.J. (2011) Soil Chemical Methods – Australasia, CSIRO Publishing,Collingwood, Victoria, Australia. Stockholm Convention. What are POPs? http://chm.pops.int/Convention/ThePOPs/tabid/673/Default.aspx (accessed March 2012).

Torri, C. and Fabbri, D. (2012) Centro Interdipartimentale di Ricerca Industriale Energia e Ambiente,UO Biomasse, Universita di Bologna. Biological upgrading of pyrolysis oil and gas to methane andhydrogen by means of adapted anaerobic bacteria consortium. 20th European Biomass Conferenceand Exhibition, 2012, 6, pp. 18–22.

US Environmental Protection Agency (1999) Background report on fertilizer use, contaminants andregulations. Prepared by BATELLE, Columbus OH. National Program Chemicals Division; Officeof Pollution Prevention and Toxics, Washington D.C. http://www.epa.gov/oppt/pubs/fertilizer.pdf,via http://www.epa.gov/agriculture/tfer.html (accessed February 2012).

US Geological Service. Polynuclear Aromatic Hydrocarbons (PAHs)/Polycyclic Aromatic Hydro-carbons (PAHs) http://toxics.usgs.gov/definitions/pah.html (accessed March 2012).

17.2 Activated Carbon

17.2.1 Introduction

As a result of environmental requirements in many countries and new areas of application,the demand for activated carbon is still growing. Due to the unavailability of the main basicmaterials like hard coal, wood or coconut shells in many countries, other biomasses arebeing tested for their appropriateness of activated carbon production. The preconditioningof biomass for activated carbon production can realise a so-called biochar in the first stage.Biochar itself is a product for soil conditioning or co-combustion with coal. Biomass that isnot wood usually contains many more inert materials, like fertilisers or silicates. A biocharrich in ash therefore generates an activated carbon rich in soluble salts, which is not veryfavourable. Biochar from non-woody biomass therefore needs to be conditioned. Extractionwith water is one cheap and applicable solution to realise a liquid fertiliser.

17.2.2 Biomass Properties

As an activation reactor, a tube furnace is used which can be heated to 1100 ◦C. Pre-conditioned biochar from pyrolysis is positioned within the reactor prior to the experiment.In the hot steam atmosphere the char is partially oxidised, which leads to the loss of charmass and the production of gaseous products like H2, CO and CO2. Higher amounts ofgaseous long-chain hydrocarbons are produced during the heat-up interval of the char as aresult of incomplete pyrolysis at 600 ◦C in the biochar production step. These gases maybe of interest in terms of energetic utilisation in order to raise the economy of the activatedcarbon production chain (Table 17.2.1).

As a result of partial oxidation under steam atmosphere, the surface area of the charincreases. The surface area created by the chemical reactions in the steam atmospherereaches a maximum.

http://www.cancer.gov/dictionary?cdrid=687391

http://browse.oecdbookshop.org/oecd/pdfs/free/9720701e.pdf

http://chm.pops.int/Convention/ThePOPs/tabid/673/Default.aspx

http://www.epa.gov/oppt/pubs/fertilizer.pdf

http://www.epa.gov/agriculture/tfer.html

http://toxics.usgs.gov/definitions/pah.html

http://www.cancer.gov/dictionary?cdrid=687391

http://browse.oecdbookshop.org/oecd/pdfs/free/9720701e.pdf

http://chm.pops.int/Convention/ThePOPs/tabid/673/Default.aspx


Table 17.2.1 Activated char yield from biochar as function ofactivation time for different biomasses based on the dry initialbiochar mass.

Rice straw [wt%] Olive stones [wt%]Time [min] act. temp.: 800 ◦C act. temp.: 750 ◦C

30 55 7045 50 6060 45 5090 40 30

17.2.3 Activation of Biochar

The following diagrams show the surface area as a function of the conversion rate, that is,loss of char mass resulting from steam activation. The values are based on dry initial charmass. The initial char was produced via pyrolysis through the use of various biomasses.

As shown by the diagrams, the surface area increases with an increasing conversionrate. At conversion rates of more than 80 wt% the surface area diminishes due to the lackof carbon.

Figures 17.1 and 17.2 show the influence of the conversion rate on the formation ofsurface area and the influence of activation temperature on activation time. The higher theactivation temperature, the lower the resulting activation time for the accessibility of highsurface areas. This example is given for crushed olive stones, but can be observed for allthe other investigated biomass matters. Figures 17.3–17.12 give a summary of the biomasstype investigation for the applicability of activated carbon production.

0 10 20 30 40 50 60 70 80 90 100

0

100

200

300

400

500

600

700

800

900

1000

Active

su

rfa

ce

[m

2/g

]

Conversion rate [wt%]

hard coal

beechwood

olive stones

Figure 17.1 Active surface of crashed olive stones compared with prevalent raw materials.[16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.


0 10 20 30 40 50 60 70

0

100

200

300

400

500

600

700

800

900

Active

su

rfa

ce

[m

2/g

]

Activation time [min]

Tactivation :

700 °C 750 °C 780 °C 800 °C

Figure 17.2 Influence of activation temperature on activation time in the case of crashed olivestones. [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

0 20 40 60 80 1000

100

200

300

400

500

600

700

800

900

1000 wheat straw

pyrolysis: 600 °Cactivation: 800–900 °C, 0.5 l/min steam

Active

su

rfa

ce

[m

2/g

]


Figure 17.3 Wheat straw [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.


0 20 40 60 80 1000

100

200

300

400

500

600

700

800

900

1000rice straw

pyrolysis: 600 °Cactivation: 900 °C, 0.5 l/min steam

Active

su

rfa

ce

[m

2/g

]


Figure 17.4 Washed rice straw [16]. Ash was extracted by the use of NaOH solutions.Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

20 40 60 80 100

0

200

400

600

800

1000

1200

1400

1600pistacchio shells


Active

su

rfa

ce

[m

2/g

]


Figure 17.5 Pistachio shells [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0license.


0 20 40 60 80 1000

100

200

300

400

500

600

700

800

900

1000walnut shells

pyrolysis: 450–600 °Cactivation: 800 °C

Active

su

rfa

ce

[m

2/g

]


Figure 17.6 Walnut shells [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

0 20 40 60 80 100200

400

600

800

1000

1200

1400 coconut shells


Active

su

rfa

ce

[m

2/g

]


Figure 17.7 Coconut shells [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0license.


20 40 60 800

100

200

300

400

500

600

700

800

900

1000

1100

1200sunflower shells


Active

su

rfa

ce

[m

2/g

]


Figure 17.8 Sunflower shells [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0license.

0 20 40 60 80 100

0

100

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300

400

500

600

700

800

900

1000

Active

su

rfa

ce

[m

2/g

]


coffee waste


Figure 17.9 Coffee waste [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.


0 20 40 60 80 100

0

100

200

300

400

500

600

700

800

900

1000

spent grains


Active

su

rfa

ce

[m

2/g

]


Figure 17.10 Spent grain [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

0 20 40 60 80 1000

100

200

300

400

500

600

700

800

900

1000rape seed


Active

su

rfa

ce

[m

2/g

]


Figure 17.11 Rape seed [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.


0 20 40 60 80 1000

100

200

300

400

500

600

700

800

900

1000

oak fruits


Active s

urf

ace [m

2/g

]


Figure 17.12 Oak fruit [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

From Figures 17.1 to 17.12 [16] it is shown that any kind of nut shell is appropriate foractivated carbon production. Straw materials end up with surface areas around 800 m2/gwhich is the minimum value that commercially available activated carbons provide.

17.2.4 Formation of Granular Activated Carbon

Dependent on the application of activated carbon, the material needs to be granulated forbetter handling purposes. In order to form stable pellets, the use of a binder is usuallynecessary. The use of molasses as a binder material prior to pyrolysis is state of the art [17].For economic reasons, high viscous pyrolysis tars from biomass pyrolysis can also be usedas binder materials. The scheme of pellet production is shown in Figure 17.13.

The pelletising procedure is implemented in between the pyrolysis and the activationstep. During activation, the binder is decomposed.

Table 17.2.2 gives an overview of the pelletising conditions, single chars and charmixtures.

The following pictures show the influence of the binder on the formation of surface area,Figure 17.14, and the influence of char mixing, Figure 17.15, for wheat straw carbons. The

pyrolysis char milled char

binder

mixture pressed pellets stable pellets activation

Figure 17.13 Scheme of the pelletising method [16]. Reprinted from InTech, 2011, underCC BY-NC-SA 3.0 license.


Table 17.2.2 Examples of pelletising conditions, chars and char mixtures [16]. Reprintedfrom InTech, 2011, under CC BY-NC-SA 3.0 license.

CharBinder pyrolysisoils from

Pressingconditions Temperature

Wheat strawRice strawPistachio shellsOlive stonesCoffee groundsMixtures of wheat straw and

pistachio shellsMixtures of rice straw and

pistachio shells

Coconut press cakeCoffee groundWheat straw

150–350 bar 200 ◦C afterpressing

200 ◦C whilepressing

mixing ratios are based on mass ratio. Figures 17.16 and 17.17 present the same effects ofpelletising and char mixing for use of rice straw.

In Figure 17.14 it is shown that the surface area of the pelletised char is similar to theunpelletised char, but the values are shifted to higher conversion rates due to the fact thatthe binder evaporates and/or reacts with the steam atmosphere. Figures 17.15 and 17.17demonstrate that the surface area is shifted to higher numbers when the wheat/rice strawchar is mixed with char from pistachio shells.

0 20 40 60 80 100

0

100

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300

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600

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1000

1100

1200

Active

su

rfa

ce

[m

2/g

]


wheat straw unpelletised

wheat straw pelletised

with coconut pyrolysis oil 2/1

Figure 17.14 Influence of binder on the formation of active surface during activation of wheatstraw [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.


0 20 40 60 80 1000

100

200

300

400

500

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700

800

900

1000

1100

1200

char/coco pyrolysis oil: 2/1

wheat straw char

wheat straw/pistachio char: 1/1

Active s

urf

ace [m

2/g

]


Figure 17.15 Influence of char mixing on surface formation during activation in the case ofwheat straw [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

0 20 40 60 80 1000

100

200

300

400

500

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700

800

900

1000

1100

1200

rice straw unpelletised

rice straw pelletised:

char/molasses: 1/1.4


Active s

urf

ace [m

2/g

]

Figure 17.16 Influence of binder on the formation of active surface during activation of ricestraw [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.


0 20 40 60 80 1000

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Active

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]


rice straw char/coco pyrolysis oil: 2/1

rice straw char/pistachio char: 1/1

rice straw char/molasses: 1/1.4

rice straw

Figure 17.17 Influence of char mixing on surface formation during activation in the case ofrice straw [16]. Reprinted from InTech, 2011, under CC BY-NC-SA 3.0 license.

References

(16) Schroeder, E. Thomauske, K., Oechsler, B., et al. (2011) Activated carbon from waste biomass.Progress in Biomass and Bioenergy Production, 333–356.

(17) Pendyal, B., Johns, M.M., Marshall, W.E. et al. (1999) The effect of binders and agriculturalby-products on physical and chemical properties of granular activated carbons. BioresourceTechnology, 68, 247–254.

Further Reading

Goldberg, E.D. (1985) Black Carbon in the Environment: Properties and Distribution, John Wiley &Sons Inc., New York.

Kuhlbusch, T.A.J. and Crutzen, P.J. (1995) Toward a global estimate of black carbon in residues ofvegetation fires representing a sink of atmospheric CO2 and a source of O2, Global Biogeochem.Cycles, 9, 491–501.

Lehmann, J. and Joseph, S. (2009) Biochar for Environmental Management, MPG Books, pp 1–12(Chapter 1).

Spokas, K.A., Koskinen, W.C., Bker, J.M. and Reicosky, D.C. (2009) Impacts of woodchips biocharadditions on greenhouse gas production and sorption/degradation of two herbicides in a Minnesotasoil. Chemosphere, 77, 574–581.

Sohi, S.P., Krull, E., Lopey-Capel, E. and Bol, R. (2010) A review of biochar and its use and functionin soil. Advances in Agronomy, 105, 47–82.

Sharma, R.K., Wooten, J.B., Baliga, V.L. et al. (2004) Characterization of chars from pyrolysis oflignin. Fuel, 83, 1469–1482.

Brewer, C.E., Schmidt-Rohr, K., Satrio, J.A. and Brown, R.C. (2009) Characterization of biocharfrom fast pyrolysis and gasification systems. Environmental Progress & Sustainable Energy, 28,386–396.


Keiluweit, M., Nico, P.S., Johnson, M.G. and Kleber, M. (2010) Dynamic molecular structure of plantbiomass-derived black carbon (biochar). Environmental Science & Technology, 44, 1247–1253.

Abdullah, H., Mediaswanti, K.A. and Wu, H. (2010) Biochar as a fuel 2: significant differences in fuelquality and ash porperties of biochars from various biomass components of mallee trees. EnergyFuel, 24, 1972–1979.

Lee, J.W., Kidder, M., Evans, B.R. et al. (2010) Characterization of biochars produced from corn-stovers for soil amendment. Environmental Science & Technology, 44, 7970–7974.

Joseph, S.D., Camps-Arbestain, M., Lin, Y. et al. (2010) An investigation into the reactions of biocharin soil. Australian Journal of Soil Research, 48, 501–515.

Spokas, K.A., Baker, J.M. and Reicosky, D.C. (2010) Ethylene: potential key for biochar amendmentimpacts. Plant and Soil, 333, 443–452.

Zimmermann, A.R. (2010) Abiotic and microbial oxidation of laboratory-produced black carbon(biochar). Environmental Science & Technology, 44, 1295–1301.

Spokas, K.A. (2010) Review of the stability of biochar in soils: predictability of O:C molar ratios.Carbon Management, 1, 289–303.

Clough, T.J., Bertram, J.E., Ray, J.L. et al. (2010) Unweathered wood biochar impact on nitrous oxideemissions from a bovine-urine-amended pasture soil. Soil Science Society of America Journal, 74,852–860.

Deenik, J.L., McCleillan, T., Uehara, G. et al. (2010) Charcoal volatile matter content influences plantgrowth and soil nitrogen transformation. Soil Science Society of America Journal, 74, 1295–1270.

Graber, E., Meller, Y., Kolton, M. et al. (2010) Biochar impact on development and productivity ofpepper and tomato grown in fertigated soilless media. Plant and Soil, 337, 481–496.

Kaal, J., Brodowski, S., Baldock, J.A. et al. (2008) Characterisation of aged black carbonusing pyrolysis-GC/MS, thermally assisted pyrolysis and methylation (THM), direct and cross-polarization 13C nuclear magnetic resonance (DP-CP NMR) and the benzenepolycarboxylic acid.Organic Geochemistry, 39, 1415–1426.

Kaal, J., Cortizas, A.M. and Nierop, K.G.J. (2009) Characterisation of aged charcoal using a coilprobe pyrolysis – GC/MS method optimized for black carbon. Journal of Analytical and AppliedPyrolysis, 85, 408–416.

Kaal, J. and Rumpel, C. (2009) Can pyrolysis-GC/MS be used to estimate the degree of thermalalteration of black carbon? Organic Geochemistry, 40, 1179–1187.

Nocentini, N., Certini, G., Knicker, H. et al. (2010) Nature and reactivity of charcoal produced andadded to soil during wildfire are particle-size dependent. Organic Geochemistry, 41, 682–689.

Gonzales-Vila, F.J., Tinoco, P., Almendros, G. and Martin, F. (2001) Pyrolysis-GC-MS analysis ofthe formation and degradation stages of charred residues from lignocellulosic biomass. Journal ofAgricultural and Food Chemistry, 49, 1128–1131.

Song, J. and Peng, P. (2010) Characterisation of black carbon materials by pyrolysis-gaschromatography-mass spectrometry. Journal of Analytical and Applied Pyrolysis, 87, 129–137.

de la Rosa Arranz, J.M., Gonzales-Vila, F.J., Lopez-Capel, E. et al. (2009) Structural properties ofnon-combusiton-derived refractory organic matter which interfere with BC quantification. Journalof Analytical and Applied Pyrolysis, 85, 399–407.

Fabbri, D., Torri, C. and Spokas, K.A. (2012) Analytical pyrolysis of synthetic chars derived frombiomass with potential agronomic application (biochar). Relationships with impacts on microbialcarbon dioxide production. Journal of Analytical and Applied Pyrolysis, 93, 77–84.

Index

ablative pyrolysis reactors 102–3acetogenesis 33–4acetylation for sugar characterization

248–9acid number 163acidogenesis 33acrylamide 26acrylic acid 26, 27acrylonitrile 26activated carbon see carbon, activatedaerobic baffled reactors (ABRs) 53agricultural residue

anaerobic digestion (AD) 37–8characteristics 40density 21energy content 21

air/fuel (A/F) ratio 140aldol reactions 178algae 119anaerobic digestion (AD) 31

microbiological aspects 31–2acetogenesis 33–4acidogenesis 33agricultural waste 37–8alkalinity 35–6biomass characteristics 39–41biomass pre-treatment 41–5biomass source 36–7energy crops 39factors influencing process 34–9food waste 38–9hydraulic retention time (HRT) 36

hydrolysis 32–3key phases 32–4methanogenesis 34nutrients 36organic loading rate (OLD) 36pH 35sewage sludge 37solid retention time (SRT) 36temperature 35total suspended solids (TSS) 36volatile suspended solids (VSS) 36

products 45biogas 46–8digestate 46

anaerobic fluidised bed reactor (AFBR) 50anaerobic treatment technology 48–50

anaerobic baffled reactors (ABRs) 53basic digester 49continuous versus batch processes 50dispersed versus attached bacterial

growth 50liquid/solid-state (dry) AD 52, 53mechanical mixing 50overview 49single-phase/multi-phase reactors 51–2

analysis of bio-oils 227–8fractionation techniques 241

acetylation for sugar characterization248–9

distillation 253pyrolytic lignin (PL) precipitation

242–3



340 Index

analysis of bio-oils (Continued )size exclusion chromatography

(SEC) 243solid phase extraction (SPE) 246–8solvent partition 249–53water addition 241–3water removal (lyophilization) 243–6

general aspects 228before analysis 228overall composition 229post-processing reactions 229significance 228–9

whole oil analyses 230Fourier transform infrared (FTIR)

spectroscopy 238–9fractionation techniques 241gas chromatography (GC) 230–7nuclear magnetic resonance (NMR)

spectrometry 237–8size exclusion chromatography

(SEC) 239–41Arrhenius law 71, 275

factors for lignin hydrothermalliquefaction 182

ash contentbiomass 8–9bio-oils 163–4

attached bacterial growth 50ATZ-TPH® process 43–4autothermal reforming (ATR) 19aviation fuel, green 3

bagassecomposition 6, 134, 204density 21energy content 21thermochemical properties 7thermogravimetric analysis (TGA) 139

bamboocomposition 6thermochemical properties 7

bioactive fuel (BAF) 214–15BIOBATTERY 212–14biochar 16, 209, 321–2

activation 328–34applications 322–5

properties 322soil enhancement properties 324toxicant reporting 323

biochemical oxygen demand (BOC) 46biocoal 13biocrude oil (BCO) 243–4


distillation 253solid phase extraction (SPE) 246–8solvent partition 249–53

biodieselcatalysts 119density 21energy content 21production 9

biofuels 160biogas 46–7

desulfurisation 47–8bio-hydrogen see under hydrogen

productionbiomass conversion efficiency 75–6biomass to liquid (BTL) process 13biomass

biochar 16bio-oil

catalytic upgradation 19–22characteristics 15–16, 17reforming 22–3

componentsalkali content 204ash content 8–9cellulose 5–6, 134, 204hemicellulose 6, 134, 204lignin 6, 134, 204proximate and ultimate analysis 6–8

conversion routes 9, 10pyrolysis 9–15

definition 1feedstock distribution 4, 5generations

distribution 5features 2–5

overview 1–2pyrolysis process control 16–17

feed particle size 18

Index 341

gas environment 18–19moisture content 18pressure, effect on product

distribution 19solid residence time 18temperature, effect on product

distribution 18water hydrolysis 23–4

biochemical conversion 24–5bioprocess integration 25fermentation 25

bio-oilssee also analysis of bio-oilscatalysts 116

properties with and without catalysis115

catalytic upgradation 19–22characteristics 15–16, 17density 21energy content 21parameters

acid number 163acidity 165ash content 163–4, 166carbon residue 163, 166cetane index 162–3complex composition 164corrosiveness 162, 163lubricity 163water content 164–5

reforming 22–3Biot number 74–5, 138biothermal valorisation of biomass (BtVB)

210, 215biowaste-derived pyrolysis oils for diesel

engines 161–2fuel parameters

acid number 163acidity 165ash content 163–4, 166carbon residue 163, 166cetane index 162–3complex composition 164corrosiveness 163lubricity 163water content 164–5

Birmingham 2026 initiative 215Boudouard reaction 141brewers spent grain, catalytic

transformation 125biomass characterisation 125–7gas analysis 127pyrolysis with steam 130–1pyrolysis without steam 127–30

Broido–Shafizadeh mechanism 291–2bubbling fluidized bed (BFB) reactors 63,

64advantages and disadvantages 84agglomeration prevention 88fast pyrolysis 102heat supply 74performance 87–8products 84

business case development 305–7biomass for power generation and CHP

307feedstock 308potential and barriers 308processes 307status 307–8typical costs 307

business perspective 308–9background 310

financial model 313–14pelletisation process 314–15pyrolysis unit 315–18

role of business models 310–11market map framework 311–13

butanol 27

calculated cetane index (CCI) 162–3carbohydrates, hydrothermal liquefaction

177–9carbon, activated 27, 327

activation of biochar 328–34biomass properties 327–8formulation of granules 334–7pelleting conditions 335

carbon, total stored in forests 2carbon residue test 162, 163, 166carbonization, hydrothermal 15Carman Kozeny equation 65

342 Index

Carslaw and Jaeger equation 74Cassava Rhizome pyrolysis with catalysts

116–18catalysis in biomass transformation 113

barriers and challenges 120brewers spent grain 125

biomass characterisation 125–7gas analysis 127pyrolysis with steam 130–1pyrolysis without steam 127–30

catalysts for bio-oil 116examples 116–19hydrogen production 120overview 114–16supercritical water–gas shift reaction

(WGSR) 194–5alkaline salts 195

cellobiose 5cellulose 5–6, 134, 204

content in biomass materials 6supercritical water gasification 191–2

cetane index 162–3chemical oxygen demand (COD) 36circulating fluidized bed (CFB) reactors 63,

64advantages and disadvantages 84–5fast pyrolysis 102performance 88–9products 84–5

closed-loop biomass gasification 91closed top reactors 146–8coal

density 21energy content 21

coconut coir composition 134coconut shell

composition 134thermochemical properties 7

co-current gasifier 142–3coir pith composition 134combined heat and power (CHP)

production 47, 159, 170biowaste-derived pyrolysis oils for

diesel engines 161–2fuel parameters 162–6

dual fuel engine applications 166–7

combustion characteristics 169–70deinking sludge pyrolysis oil (DSPO)

167–9dual fuel engines and biofuels 160spark-ignited (SI) gas engines and

syngas 159–60combustion of biomass 15, 139–40

alkali content 204cellulose 5–6, 134, 204hemicellulose 6, 134, 204lignin 6, 134, 204

computational fluid dynamics (CFD) 80Conrad recycling process 104continuous stirred-tank reactor (CSTR) 273continuous tunbular reactor (CTR) 273conversion efficiency 75–6conversion of biomass 9, 10

pyrolysis 9–11combustion 15fast pyrolysis 11–12gasification 14–15hydrothermal carbonization 15intermediate pyrolysis 12–13slow pyrolysis 13torrefaction 13–14

corn cob composition 6, 134, 204corn grain thermochemical properties 7corn stalk composition 134corn stover

composition 6, 204thermochemical properties 7

corrosivenessbio-oils 162, 163supercritical water 197–8

cotton gin waste composition 134cottonseed oil, catalysts for biodiesel

production 119counter-current gasifier 141–2crop residues used in anaerobic digestion

(AD) 37–8crops grown for energy in anaerobic

digestion (AD) 39cross-draft moving bed (MB) gasifiers 62,

63Cycled-Spheres Reactor 105cyclone of a reactor 72–3

Index 343

Degusa process 106deinking sludge pyrolysis oil (DSPO) 165

composition 166dual fuel engines 167–9

combustion characteristics 169–70desulfurisation of biogas 47–8devolatilization 76Diels–Alder reaction 178diesel

density 21energy content 21green 3

differential thermal analysis (DTA) 264differential kinetic evaluation methods

261–2differential scanning calorimetry (DSC)

264digestate 46digesters 31

basic anaerobic digester 49dispersed bacterial growth 50distributed activation energy model

(DAEM) 293–4distributor plate of a reactor 72double rotating kiln process 104dower reactors 92downdraft gasifier 142–3downdraft moving bed (MB) gasifiers 62,

63advantages and disadvantages 84heat supply 74products 84

drop in fuels 3drop-tube reactors 92dry matter (DM) composition 8drying biomass 136dual fluidized bed (DFB) reactors 74

advantages and limitations 91performance 89–92

dual fuel engines 160, 166–7deinking sludge pyrolysis oil (DSPO)

167–9combustion characteristics 169–70

ecalyptus composition 134electrostatic precipitators 72

elemental composition of biomass 8energy crops for anaerobic digestion (AD)

39energy production costs compared 224entrained fluidized bed (FB) reactors 63, 64

advantages and disadvantages 85products 85

Ergun equation 65Escherichia coli 32ethanol


ethyl lactate 27evolved gas analysis (EGA) 264expanded granular sludge bed (EGSB)

reactors 50, 51

fast pyrolysis 11–12, 102fast pyrolysis reactors

ablative pyrolysis reactors 102–3bubbling fluidized bed (BFB) reactors

102circulating fluidized bed (CFB) reactors

102rotating cone reactors 103twin screw reactors 103

fatty acid oils 26fatty acids, volatile (VFAs) 33, 34, 52feedstock influence on process

performance and products 203–6heteroatomic components 206water content 206

fermentation 25alkalinity 35–6biomass characteristics 39–41biomass pre-treatment 41–2

alkaline 45thermal 42–4ultrasonic 44

biomass source 36–7agricultural waste 37–8energy crops 39food waste 38–9sewage sludge 37

factors influencing process 34hydraulic retention time (HRT) 36

344 Index

fermentation (Continued )hydrogen production 223key phases 32

acetogenesis 33–4acidogenesis 33hydrolysis 32–3methanogenesis 34

microorganisms 31–2nutrients 36organic loading rate (OLD) 36pH 35products 45

biogas 46–8digestate 46

solid retention time (SRT) 36temperature 35total suspended solids (TSS) 36volatile suspended solids (VSS) 36

first generation biomass 2features 5

flash point 162flow regimes 64–9fluidised bed reactors 50fluidized bed (FB) reactors 62–3, 87–92

flow regimes 64–5gas velocity and pressure drop

relationship 66minimum fluidization velocity 66smooth fluidization equation 72terminal velocity 68

food waste used in anaerobic digestion(AD) 38–9

forest resources 2total carbon stored 2

formic acid 177fouling 203Fourier transform infrared (FTIR)

spectroscopy 238–9fourth generation biomass 2–4

features 5fractionation analysis of bio-oils 241free-fall reactors 92, 93fructose 177fugacity of gases in supercritical water

196–7full evaporation technique (FET) 234–5

gas chromatography (GC)dilute and shoot 230–1headspace 231–3

solid phase micro-extraction (SPME)235–7

static headspace 233–5gas fugacity in supercritical water 196–7gas solubility in supercritical water 195–6gasification 14–15, 140–1

closed-loop system 91historical perspective 143

pre-1980 143–4post-1980 144

hydrogen production 223mechanisms and products 77–9open-top dual air entry reactor design

149–51supercritical water gasification 190–3

corrosion 197–8mechanism 197

technology 145, 151competing designs 146–9engine and generator performance

155–6power generation performance

151–5reactor design principles 145–6

gasolinedensity 21energy content 21green 3

gel permeation chromatography (GPC)240

glucose 6, 177glycerin 27𝛽-glycosidic linkages 5green aviation fuel 3green diesel 3green gasoline 3greenhouse gasses (GHGs) and biochar 16gross calorific value (GCV) 135groundnut shell composition 134

Haloclean-gas tight rotary kiln process 12,105

straw gasification 107–9

Index 345

hard wooddensity 21energy content 21

heat conduction equation 74heat transfer 138

mechanisms 73–5hemicellulose 6, 134, 204

content in biomass materials 6herbaceous energy crops composition 6,

204heterogeneous catalysts 114higher calorific value (HCV) 135higher heating value (HHV) 8, 76, 135homogeneous catalysts 114hydraulic retention time (HRT) in

anaerobic digestion (AD) 36hydro-deoxygenated biocrude oil (HDO)

245–6hydrodynamics

flow regimes 64–9phase distribution and segregation 69,

70hydrogen production 217, 224

bio-hydrogen 217–19catalysts 120hydrogen costs 223–4hydrogen properties 218processes

fermentation 223gasification 223reforming 219–20steam reforming 219water electrolysis 223

hydrolysismicrobiological 32–3water 23–4

biochemical conversion 24–5bioprocess integration 25fermentation 25

hydrothermal carbonization 15hydrothermal liquefaction 175–6, 183

carbohydrates 177–9chemistry of process 177lignin 179–82

activation energies and Arrheniusfactors 182

product properties 176technical application 182–3

hydrothermal upgrading (HTU®) seehydrothermal liquefaction 176

hydroxymethylfurfural (HMF) 177

integral kinetic evaluation methods262–3

integrated processes 209, 215–16anaerobic digestion, pyrolysis and

gasification 210–11BIOBATTERY 212–14Birmingham 2026 initiative 215intermediate pyrolysis and CHP

combined with combustion211–12

intermediate pyrolysis, anaerobicdigestion and CHP 212

pyrolysis bioactive fuel (BAF)application 214–15

pyrolysis reforming 212intermediate pyrolysis 12–13

integrated processes 210–11combined with CHP and combustion

211–12intermediate pyrolysis reactors 103

principles 103–4process technology 104

Conrad recycling process 104double rotating kiln process 104Haloclean-gas tight rotary kiln

process 105low temperature carburisation (LTC)

process 104–5Pyroformer reactor 105

kinetic parameters 257–9, 278–9complex reactions 264–7evaluation methods 261–4

advantages and disadvantages ofisothermal versusnon-isothermal methods262

common reaction models 263experimental kinetic analysis techniques

264

346 Index

kinetic parameters (Continued )lignocellulosic materials under

isothermal conditions example271

instrument and operating conditions271–2

kinetic evaluation 272–5technical applications 275–8thermal decomposition 259–61variation 267

computational methods 270–1influence of experimental conditions

269–70kinetic compensation effect 267–8thermal lag 268–9

Koufopanos mechanism 292–3

levoglucosan 27, 177levulinic acid 177lignin 6, 134, 204

see also pyrolytic lignin (PL)content in biomass materials 6hydrothermal liquefaction 179–82

activation energies and Arrheniusfactors 182

lipase 9lipids 192liquid state anaerobic digestion (L-AD) 52low temperature carburisation (LTC)

process 104–5lower calorific value (LCV) 135lower explosive or flammable limits

(LEL/LFL) 159lower heating value (LHV) 135–6lubricity of bio-oils 163lyophilization 243–6

maize straw thermochemical properties 7manure

anaerobic digestion (AD) 37characteristics 40

market map framework 311–12decision framework 312–13

methane, properties 218methane reaction 141methanogenesis 34

methanoldensity 21energy content 21

methyl esters of fatty acids 11millet husk composition 134moisture content 136monorator reactor design 147–8moving bed (MB) reactors 62

performance 85–6multi-cyclones 72

near-infrared (NIR) spectroscopy 9net calorific value (NCV) 135nitrobacter sp. 32nitrogen mineralization capacity (NMC) 46nitrosomonas sp. 32nuclear magnetic resonance (NMR)

spectrometry 237–8numerical simulation of thermal

degradation 285–8, 299complex models 288–9

Broido–Shafizadeh mechanism291–2

competing models 289–90distributed activation energy model

(DAEM) 293–4Koufopanos mechanism 292–3one-step global models 289parallel reaction models models

290–1thermal aspects 294–5

particles in bed models 298–9single-particle models 295–8

nut shellsdensity 21energy content 21

OET Caluscoa process 106oil palm shell, catalysts for biomass

transformation 116olive husk thermochemical properties 7open top reactor design 146–7, 148–9open-top dual air entry reactor design

149–51organic fraction of municipal solid waste

(OFMSW) 37

Index 347

organic loading rate (OLD) in anaerobicdigestion (AD) 36

oxidationpartial 19supercritical water oxidation 193

palm oil, catalysts for biodiesel production119

paper, wastecomposition 6, 204

partial oxidation 19petrol see gasolinepH in anaerobic digestion (AD) 35phase distribution and segregation in

reactors 69, 70phenol 179photosynthesis 114pilot-plant testing 82pine sawdust

catalysts for biomass transformation116

composition 134thermochemical properties 7

polytrimethylene terephthalate (PTT) 26power generation costs compared 224pressure, effect on pyrolysis product

distribution 19processing waste/byproducts

characteristics 40propylene glycol 27proteins 192pseudocomponents 288Pyroformer reactor 93–4, 105, 210

low grade biomass 109–10pyrolysis 9–11, 76–7, 99–101, 136–9

applicationsHaloclean pyrolysis and straw

gasification 107–9Pyroformer pyrolysis of low grade

biomass 109–10catalysts 116combustion 15comparison of techniques 106–7fast pyrolysis 11–12, 102fast pyrolysis reactors

ablative pyrolysis reactors 102–3

bubbling fluidized bed (BFB)reactors 102

circulating fluidized bed (CFB)reactors 102

rotating cone reactors 103twin screw reactors 103

features 101future directions 107gasification 14–15hydrothermal carbonization 15intermediate pyrolysis 12–13intermediate pyrolysis reactors 103

principles 103–4process technology 104–5

process control 16–17feed particle size 18gas environment 18–19moisture content 18pressure, effect on product

distribution 19solid residence time 18temperature, effect on product

distribution 18slow pyrolysis 13, 105

principles 106process technology 106

torrefaction 13–14pyrolysis bioactive fuel (BAF) application

214–15pyrolysis reforming 212pyrolytic lignin (PL) 229, 241


distillation 253precipitation 242–3size exclusion chromatography (SEC)

243solid phase extraction (SPE) 246–8solvent partition 249–53

rapeseed, thermochemical properties 7rapeseed oil, catalysts for biodiesel

production 119reactor design 61–2

competing designs 146–9considerations 63–4

348 Index

reactor design (Continued )biomass conversion efficiency 75–6distributor plate and cyclone 72–3heat transfer mechanisms 73–5hydrodynamics 64–9residence time 69–72

energy balance 80–2mass balance 79–80new design and performance 92–4performance and products 85

fluidized bed (FB) reactors 87–92moving bed (MB) reactors 85–6

principles 145–6reactions and impact on products 76

devolatilization and pyrolysis 76–7gasification 77–9

sizing and configuration 82–3thermochemical conversion reactors

types 62–3reactors

fast pyrolysis reactorsablative pyrolysis reactors 102–3bubbling fluidized bed (BFB)

reactors 102circulating fluidized bed (CFB)

reactors 102rotating cone reactors 103twin screw reactors 103

Reynolds number 68rice hull

density 21energy content 21thermochemical properties 7

rice huskcatalysts for biomass transformation 116composition 134thermochemical properties 7thermogravimetric analysis (TGA) 139

rice strawcatalysts for biomass transformation 116composition 6thermochemical properties 7

rotating cone reactors for fast pyrolysis 103

sawdust see wood sawdustsecond generation biomass 2

features 5

Second World War class reactor design146–8

sewage sludgeanaerobic digestion (AD) 37pyrolysis oil (SSPO) composition 165water content 164

short rotation woody crops composition 6,204

shrinking core mechanism 297–8size exclusion chromatography (SEC)

239–41pyrolytic lignin (PL) 243

slow pyrolysis 13, 105principles 106process technology

Degusa process 106OET Caluscoa process 106

sludge-to-oil reactor system (STORS) 183small deviation values (SSQ) 267smooth fluidization equation 72softwood


Solar Energy Research Institute (SERI)143

solid phase extraction (SPE) for pyrolyticlignin (PL) 246–8

solid phase micro-extraction (SPME)technique 235–7

solid retention time (SRT) in anaerobicdigestion (AD) 36

solvent partition for pyrolytic lignin (PL)249–53

soybean composition 134soybean oil, catalysts for biodiesel

production 119spark-ignited (SI) gas engines 159–60steam reforming 19sugar characterization 248–9sunflower oil, catalysts for biodiesel

production 119supercritical conversion of biomass

189–90, 199advantages 198–9catalysts 194–5

alkaline salts 195gas fugacity in supercritical water 196–7

Index 349

gas solubility in supercritical water195–6

supercritical water gasification 190–3corrosion 197–8mechanism 197

supercritical water oxidation 193supercritical water–gas shift reaction

(WGSR) 193–4switch grass thermochemical properties 7syngas 26, 159–60

temperatureanaerobic digestion (AD) 35effect on pyrolysis product distribution

18thermal lag 268–9thermochemical conversion reactors

types 62fluidized bed (FB) reactors 62–3

moving bed (MB) reactors 62thermochemical conversion of biomass

133–6combustion 139–40gasification 140–1

downdraft (co-current) gasifier 142–3updraft (counter-current) gasifier

141–2gasification technology 145

competing designs 146–9reactor design principles 145–6

historical perspective 143pre-1980 143–4post-1980 144

open-top dual air entry reactor design149–51

process overview 136drying 136pyrolysis 136–9

technology 151engine and generator performance

155–6power generation performance 151–5

thermogravimetric analysis (TGA) 138–9,264

third generation biomass 2–4features 5

torrefaction 13–14

total acid number (TAN) 163total organic carbon (TOC) 43total suspended solids (TSS) in anaerobic

digestion (AD) 36transesterification reaction 11Transport Disengagement Height (TDH) 83triglycerides, transesterification of 11turn-down ratio 142twin screw reactors for fast pyrolysis 103

updraft gasifier 141–2updraft moving bed (MB) gasifiers 62, 63

advantages and disadvantages 84products 84

Upflow Anaerobic Sludge Blanket (UASB)Reactors 50, 51

upper explosive or flammable limits(UEL/UFL) 159

very volatile organic compounds (VVOC)s231

volatile fatty acids (VFAs) 33, 34, 52volatile organic compounds (VOCs) 231volatile solids (VS) 36volatile suspended solids (VSS) in

anaerobic digestion (AD) 36

waste oil, catalysts for biodiesel production119

watergas fugacity in supercritical water 196–7gas solubility in supercritical water

195–6hydrogen production via electrolysis

223supercritical water gasification 190–3

corrosion 197–8mechanism 197

supercritical water oxidation 193water contamination 162water gas reaction 141water–gas shift reaction (WGSR) 22, 141

catalysts 194–5alkaline salts 195

supercritical 193–4water hyacinth thermochemical properties

7

350 Index

water hydrolysis 23–4biochemical conversion 24–5bioprocess integration 25fermentation 25

Waterloo concept 76–7wheat chaff composition 6, 204wheat straw

composition 6, 204density 21energy content 21thermochemical properties 7

wood fuelcatalysts for biomass transformation 116global production 2, 3utilization 3

wood sawdustcatalysts for biomass transformation 116thermochemical properties 7

xylose 192

Zymomonas mobilis 25

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Transformation of Biomass: Theory to Practice