Queensland University of Technology

Cellulosic ethanol from sugarcane bagasse in Australia: exploring industry

feasibility through systems analysis, techno-economic assessment and pilot

plant development

Ian OHara BE (Chem), MBA

Principal Supervisor: Dr Les A Edye

Associate Supervisor: Dr Geoff A Kent

A thesis submitted for the degree of

Doctor of Philosophy

in the Faculty of Science and Technology

Queensland University of Technology

according to QUT requirements

2011

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Keywords

sugarcane, bagasse, lignocellulose, fibre, biofuels, biorefinery, ethanol,

pretreatment, systems analysis, uncertainty, risk, techno-economic

assessment, feasibility, plant expressed enzymes, pilot plant

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Abstract

Overcoming many of the constraints to early stage investment in biofuels

production from sugarcane bagasse in Australia requires an understanding of the

complex technical, economic and systemic challenges associated with the transition

of established sugar industry structures from single product agri-businesses to new

diversified multi-product biorefineries.

While positive investment decisions in new infrastructure requires technically

feasible solutions and the attainment of project economic investment thresholds,

many other systemic factors will influence the investment decision. These factors

include the interrelationships between feedstock availability and energy use,

competing product alternatives, technology acceptance and perceptions of project

uncertainty and risk.

This thesis explores the feasibility of a new cellulosic ethanol industry in Australia

based on the large sugarcane fibre (bagasse) resource available. The research

explores industry feasibility from multiple angles including the challenges of

integrating ethanol production into an established sugarcane processing system,

scoping the economic drivers and key variables relating to bioethanol projects and

considering the impact of emerging technologies in improving industry feasibility.

The opportunities available from pilot scale technology demonstration are also

addressed.

Systems analysis techniques are used to explore the interrelationships between the

existing sugarcane industry and the developing cellulosic biofuels industry. This

analysis has resulted in the development of a conceptual framework for a bagasse-

based cellulosic ethanol industry in Australia and uses this framework to assess the

uncertainty in key project factors and investment risk. The analysis showed that the

fundamental issue affecting investment in a cellulosic ethanol industry from

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sugarcane in Australia is the uncertainty in the future price of ethanol and

government support that reduces the risks associated with early stage investment is

likely to be necessary to promote commercialisation of this novel technology.

Comprehensive techno-economic models have been developed and used to assess

the potential quantum of ethanol production from sugarcane in Australia, to assess

the feasibility of a soda-based biorefinery at the Racecourse Sugar Mill in Mackay,

Queensland and to assess the feasibility of reducing the cost of production of

fermentable sugars from the in-planta expression of cellulases in sugarcane in

Australia. These assessments show that ethanol from sugarcane in Australia has the

potential to make a significant contribution to reducing Australias transportation

fuel requirements from fossil fuels and that economically viable projects exist

depending upon assumptions relating to product price, ethanol taxation

arrangements and greenhouse gas emission reduction incentives.

The conceptual design and development of a novel pilot scale cellulosic ethanol

research and development facility is also reported in this thesis. The establishment

of this facility enables the technical and economic feasibility of new technologies to

be assessed in a multi-partner, collaborative environment. As a key outcome of this

work, this study has delivered a facility that will enable novel cellulosic ethanol

technologies to be assessed in a low investment risk environment, reducing the

potential risks associated with early stage investment in commercial projects and

hence promoting more rapid technology uptake.

While the study has focussed on an exploration of the feasibility of a commercial

cellulosic ethanol industry from sugarcane in Australia, many of the same key issues

will be of relevance to other sugarcane industries throughout the world seeking

diversification of revenue through the implementation of novel cellulosic ethanol

technologies.

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Contents

Keywords ii Abstract iii Contents v Figures ix Tables x Authorship xi Acknowledgements xii

Chapter 1 Introduction 1 1.1 Introduction 1 1.2 Aims and objectives of the research 2 1.3 Research and communication methodology 3 1.4 Thesis outline 4 1.5 Original contributions 7 1.6 Conclusion 8

Systems analysis

Chapter 2 Introduction to biofuels and the Australian sugar industry 11 2.1 Transportation fuels in the early 21st century 11

2.1.1 The use of crude oil as a transportation fuel 11 2.1.2 The contribution of transport fuels to climate change 12 2.1.3 Peak oil and future oil price 13 2.1.4 Energy security and development 14

2.2 Bioethanol a renewable transport fuel 14 2.2.1 Ethanol as a transportation fuel 14 2.2.2 First-generation ethanol 15 2.2.3 Second-generation bioethanol 16 2.2.4 The global biomass resource 17

2.3 Sugarcane as a bio-energy resource 18 2.3.1 The global sugar industry 18 2.3.2 The sugarcane biomass resource 19 2.3.3 The Australian sugar industry 20

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2.3.4 Current uses of sugarcane bagasse in Australia 22 2.3.5 The sugarcane biorefinery 23

2.4 The composition and structure of sugarcane bagasse 24 2.4.1 Cellulose 26 2.4.2 Hemicelluloses 27 2.4.3 Lignin 28

2.5 Overview of the process for ethanol production from sugarcane bagasse 28

2.6 Conclusion 30

Chapter 3 Pretreatment technologies for ethanol production from

sugarcane bagasse 31 3.1 Introduction 31 3.2 The objectives of the pretreatment process 31 3.3 Chemical pretreatments 34

3.3.1 Concentrated acid hydrolysis 34 3.3.2 Dilute acid hydrolysis and pretreatment 34 3.3.3 Alkaline pretreatments 38 3.3.4 Oxidative pretreatments 40 3.3.5 Solvent pretreatments 41 3.3.6 Ionic liquid pretreatments 43

3.4 Physical pretreatments 43 3.4.1 Steam explosion pretreatment 43 3.4.2 Other explosive pretreatments 44 3.4.3 Liquid hot water pretreatments 45 3.4.4 Mechanical pretreatments 46 3.4.5 Ultrasonic and radiation pretreatments 47

3.5 Biological pretreatments 47 3.5.1 Microbiological degradation 47

3.6 Conclusion 49

Chapter 4 Commercialising cellulosic ethanol from sugarcane bagasse:

use of systems analysis to reduce the risk and uncertainty associated with early stage investment 51

4.1 Introduction 51 4.2 Systems analysis 52 4.3 Scoping and exploring the problem space 54 4.4 Defining the system purpose and CONOPS 58

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4.5 Scoping the solution space through techno-economic modelling 64

4.6 Manifesting the optimum solution 70 4.6.1 Ethanol price and production incentives 70 4.6.2 Bagasse price 71 4.6.3 Cellulase price 73 4.6.4 Bioethanol plant capital cost 73

4.7 Creating the solution and deep learning 73

Techno-economic assessment

Chapter 5 The potential for ethanol production from sugarcane in

Australia 77 5.1 Introduction 77 5.2 Transport fuel use in Australia 77 5.3 The capacity of the Australian sugarcane industry 78 5.4 Ethanol production from sugarcane juice and molasses 79 5.5 Ethanol production from bagasse and sugarcane trash 80 5.6 Scenario analysis 83 5.7 Discussion 86 5.8 Conclusion 89

Chapter 6 Economic feasibility of a soda-based biorefinery at

Racecourse Mill 91

Chapter 7 Feasibility assessment of in-planta cellulolytic enzyme

expression for the production of biofuels from sugarcane bagasse in Australia 93

Pilot plant development

Chapter 8

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Towards a commercial lignocellulosic ethanol industry in Australia: the Mackay Renewable Biocommodities Pilot Plant 97

8.1 Introduction 97 8.2 Pilot plants facilitating commercial development 98 8.3 MRBPP funding 98 8.4 Design and construction of the MRBPP 100 8.5 Site services 101 8.6 Plant and equipment 102 8.7 Lignin product recovery 105 8.8 Future developments 105

Discussion

Chapter 9 Discussion 109 9.1 Introduction 109 9.2 Achievement of research objectives and key findings 109 9.3 Importance of research 112 9.4 Recommendations for future work 112 Bibliography 115

Appendices

APPENDIX A Supplementary data for Chapter 6 145

APPENDIX B The Mackay Renewable Biocommodities Pilot Plant

photographic record of construction and equipment installation 147

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Figures Figure 2.1 Leading sugarcane producing countries 2006 [32] ................................. 18 Figure 2.2 Map of the Australian sugar industry [39] .............................................. 21 Figure 2.3 Australian No.1 sugar pool price 1990-91 to 2005-06 and QSL

seasonal pool price 2006-07 to 2010-11 (AU$/t) [38, 41] ..................... 22 Figure 2.4 An overview of current and potential products from sugarcane in

Australia current products shown in black and potential products shown in red ........................................................................................ 24

Figure 2.5 Simple schematic of the key processes required for the ethanol from sugarcane bagasse ...................................................................... 30

Figure 4.1 Issues impacting the commercialisation of bioethanol technologies viewed through economic, technical, sustainability and public policy lenses ........................................................................................ 53

Figure 4.2 Conceptual map of a sugarcane processing system in Australia ............. 55 Figure 4.3 Objectives tree for the sugarcane bioethanol system ............................ 60 Figure 4.4 Schematic representation of the sugarcane bioethanol system ............. 61 Figure 4.5 Techno-economic model of the sugarcane bioethanol system (the

sugarcane bioethanol model) based upon the common methodological framework [194] ......................................................... 64

Figure 4.6 Sensitivity of the key factors in bagasse based ethanol project viability (net present value) to the project assumptions....................... 68

Figure 4.7 Sensitivity of the major factors in bagasse based ethanol project viability (net present value) to the assumptions in the techno-economic model .................................................................................. 69

Figure 5.1 Schematic representation of the QUT techno-economic model of an integrated sugar factory, juice and molasses distillery and cellulosic ethanol production facility .................................................... 83

Figure 8.1 Typical biorefinery process diagram .....................................................102

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Tables Table 2.1 Typical constitutive analysis of Australian sugarcane bagasse ................. 26 Table 4.1 Summary of the key issues relating to bagasse-based bioethanol

commercialisation in the sugarcane industry in Australia..................... 57 Table 4.2 Summary purpose, concept of operations (CONOPS) and key

measures of effectiveness of the integrated sugar ethanol system ................................................................................................. 63

Table 4.3 Key variable inputs to the sugarcane bioethanol model .......................... 66 Table 4.4 Key fixed inputs to the sugarcane bioethanol model ............................... 67 Table 5.1 Consumption of petroleum products in Australia, Queensland and

NSW 2007-08 [198] .............................................................................. 78 Table 5.2 Approximate ethanol yields per tonne of product................................... 80 Table 5.3 Common input data for scenario analysis ............................................... 87 Table 5.4 Input data for the scenario analysis ........................................................ 87 Table 5.5 Results from scenario analysis ................................................................ 88

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Authorship

The work contained in this thesis has not been previously submitted to meet the requirements for an award at this or any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made. Signature

Name Ian Mark OHara

Date

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Acknowledgements I would like to thank my Supervisors Dr Les Edye and Dr Geoff Kent for their support

throughout the research program and their invaluable advice and feedback on the

various aspects of the work.

I would like to especially acknowledge receipt of scholarship

funding from the Australian Government and the Australian

Sugarcane Industry as provided by the Sugar Research and

Development Corporation.

The author of this thesis is not a partner, joint venturer, employee or agent of SRDC

and has no authority to legally bind SRDC, in any publication of substantive details

or results of this Project.

I would also like to acknowledge and thank the QUT Centre for Tropical Crops and

Biocommodities for financial support in this project.

This research program would not have been possible without the strong support of

several research partner organisations. I would like to acknowledge the support of

the partners of the Biorefinery Development Project including the Queensland

Government through the Research Industries Partnership Program (RIPP), Mackay

Sugar Ltd, Sugar Research Ltd, Veridian Chemicals Pty Ltd and Hexion Specialty

Chemicals Inc. I would also like to acknowledge the partners of the Syngenta Centre

for Sugarcane Biofuels Development including the Queensland Government

through the National and International Research Alliances Program (NIRAP),

Syngenta Biotechnology Inc, and Farmacule Bioindustries Pty Ltd.

I would like to thank funding partners of the Mackay Renewable Biocommodities

Pilot Plant for the opportunity to be involved in such an exciting and visionary

project. The funding for the design and construction of the pilot plant was provided

by the Australian Government through the National Collaborative Research

Infrastructure Strategy (NCRIS) and the Education Investment Fund (EIF), the

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Queensland Government through the Innovation Building Fund (IBF) and QUT. The

strong support of Mackay Sugar Ltd in the development of the facility has again

been invaluable.

There are many individuals who have contributed to the research program or this

thesis in many ways and your contributions are very much appreciated. In

particular, I would like to acknowledge the contributions and support of Professor

James Dale, Dr William Doherty, Dr Zhanying Zhang, Dr Heng-Ho Wong and Mr

Peter Albertson from the QUT Centre for Tropical Crops and Biocommodities and Dr

Bryan Lavarack from Mackay Sugar Ltd for your support in various aspects of the

work.

Finally I would like to thank my family and in particular my wife Penny for your on-

going patience and support.

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1

Chapter 1

Introduction

1.1 Introduction

This thesis reports the results of a research program exploring the feasibility of

ethanol production from sugarcane bagasse in Australia. The nature of the research

undertaken in this research program acknowledges that overcoming many of the

constraints to early stage investment in biofuels production from sugarcane bagasse

requires a multi-disciplinary approach to the technical, economic and systemic

challenges associated with the transition of established sugar industry structures

from single product agri-businesses to new multi-product, diversified, integrated

biorefineries. These challenges include not only the technical challenges associated

with the novel biofuel technology, but also the integration of new and existing

facilities (site integration), the requirement to produce surplus bagasse (energy

efficiency), changed imperatives for sugarcane variety selection (higher fibre) and

the need to balance agronomic and industrial value-adds (trash collection or field

retention of trash).

Some of the work reported in this thesis was undertaken within research projects at

QUT and funded by several project partners. Of particular note are:

- The work undertaken for Chapter 6 was funded by the partners of the

Biorefinery Development Project including the Queensland Government

through the Research Industries Partnership Program (RIPP), Mackay Sugar

Ltd, Sugar Research Ltd, Veridian Chemicals Pty Ltd and Hexion Specialty

Chemicals Inc.

- The work undertaken for Chapter 7 was funded by the partners of the

Syngenta Centre for Sugarcane Biofuels Development including the

Queensland Government through the National and International Research

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Alliances Program (NIRAP), Syngenta Biotechnology Inc, and Farmacule

Bioindustries Pty Ltd.

- The work undertaken for Chapter 8 was funded by the partners of the

Mackay Renewable Biocommodities Pilot Plant (MRBPP) project including

the Australian Government through the National Collaborative Research

Infrastructure Strategy (NCRIS) and Education Investment Fund (EIF), the

Queensland Government through the Innovation Building Fund (IBF),

Mackay Sugar Ltd and QUT.

- Scholarship funding for the overall PhD project was provided by the Sugar

Research and Development Corporation (SRDC).

1.2 Aims and objectives of the research

The research program aimed to answer key questions relating to the technical and

economic feasibility of ethanol production from sugarcane bagasse in Australia and

the systemic impediments to commercialisation of the technology in Australia.

The research program aimed to:

- Identify the key technical, economic and systemic factors impacting upon

investment in commercial scale facilities for the production of ethanol from

sugarcane bagasse in Australia;

- Explore leading technologies for the biochemical production of ethanol from

sugarcane bagasse to determine the conceptual feasibility of the technology;

- Conceptualise and develop a framework for assessing the interrelationships

between energy use, feedstock availability and potential cellulosic ethanol

production of integrated sugar and bagasse-based ethanol production

facilities;

- Model the use of the framework through its application to the design and

construction of a pilot scale facility for demonstration of technology for the

production of ethanol from bagasse; and

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- Communicate key outcomes to the Australian sugar industry to develop a

deeper understanding within the industry of the potential opportunities and

economic feasibility of the technology.

1.3 Research and communication methodology

The research program was based on developing a comprehensive understanding of

the issues impacting on the feasibility of ethanol production from sugarcane

bagasse in Australia. This understanding was formed through both literature

reviews and the use of systems analysis techniques to explore the complex

interrelationships between the existing sugarcane industry and the developing

cellulosic biofuels industry.

The systems analysis led to the development of new technical and economic models

of integrated sugarcane processing, sugar production and cellulosic ethanol

production facilities. These models were then used to undertake comprehensive

assessments of technology options that impact on the feasibility of the system.

These models were applied to the development of a pilot plant for research and

demonstration of ethanol production from sugarcane bagasse. Many of the

elements associated with the design and construction of the facility resulted from

the modelling framework developed in the systems analysis and techno-economic

assessments.

Information contained in two of the chapters in this thesis (Chapter 5 and Chapter

8) were presented as peer-reviewed conference papers to the Australian Society of

Sugar Cane Technologists (ASSCT) in 2009 and 2010. Two further papers have been

submitted to the ASSCT conference in 2011. The decision to address aspects of the

reporting for this research project to the ASSCT conference was made on the basis

that:

- ASSCT is the preeminent research forum of the Australian sugarcane

industry and globally recognised for leading industry-specific research;

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- ASSCT attracts many of the Australian sugar industry leaders,

researchers and industry practitioners to discuss innovation and the

future directions of the industry;

- The Australian sugar industry is actively seeking diversification options

for bagasse, however, most industry participants have only a limited

understanding of the technology and the economics of ethanol

production from bagasse;

- The papers addressed to the ASSCT conference will serve to inform and

educate participants in the Australian sugar industry on the technology

and economics of ethanol production from bagasse and, through

engaging in on-going dialogue in the ASSCT forum, promote

consideration of sugar industry investment in this technology; and

- Presenting work at the ASSCT forum was encouraged by the scholarship

provider for the research project (SRDC).

1.4 Thesis outline

This thesis explores the progress toward the feasibility of ethanol from cellulosic

biomass feedstock through three different approaches to understanding and

analysing the biofuels system.

Section 1 contains three chapters that provide an analysis of the sugarcane and

bioethanol systems. These chapters provide an introduction to the national and

global drivers impacting upon ethanol production from cellulosic biomass, describe

the literature underpinning the research and address strategies that promote

investment in the technology.

Chapter 2 is an introduction to transportation fuels, the global and national

challenges impacting upon future transportation fuel use and the drivers for

the development of biofuels from cellulosic feedstocks. In addition, this

chapter describes the sugarcane industry in Australia and the factors

5

impacting upon the production of biofuels (and in particular) ethanol from

sugarcane fibre (bagasse).

Chapter 3 provides a brief review of the leading pretreatment technologies

for ethanol production from sugarcane bagasse and the strategies for

producing a fibre that is more amenable to enzymatic hydrolysis.

Chapter 4 reports on a comprehensive analysis of the sugarcane bioethanol

system and uses complex decision making tools to analyse the risks and

uncertainties associated with early stage investment in cellulosic ethanol

production facilities. From this analysis, the chapter draws conclusions about

the relative magnitude of the key investment risks and proposes strategies

that seek to minimise risk and hence promote the likelihood of positive early

stage investment decisions in cellulosic ethanol production from bagasse.

Section 2 contains three chapters that provide techno-economic assessments of

various cellulosic ethanol systems. These assessments reflect different model

systems and focus upon increasing the understanding of the technical and

economic feasibility of each system.

Chapter 5 reports on an assessment of the potential quantum of ethanol

production from sugarcane in Australia and analyses several case studies of

integrated sugarcane processing, juice and molasses-based ethanol

production and bagasse-based ethanol production facilities. This chapter

was presented as a peer-reviewed conference paper at the Australian

Society of Sugar Cane Technologists annual conference in Bundaberg,

Queensland in May 2010.

Chapter 6 is an assessment of the conceptual feasibility of a soda-based

biorefinery at a specific site in Australia, namely the Mackay Sugar Ltd

Racecourse Mill in Mackay, Queensland. The chapter details the results of a

comprehensive techno-economic assessment of the proposed project,

reports on one and two-component sensitivity analyses and assesses several

project alternatives. This chapter was provided as a confidential research

6

report to the partners of the Queensland Government Research Industry

Partnerships Program (RIPP) and multi-partner funded Biorefinery

Development Project.

Chapter 7 is an assessment of the conceptual economic feasibility of the in-

planta expression of cellulase enzymes in the sugarcane production and

processing system, exploring several processing strategies. This chapter was

provided as a confidential research report to the partners of the Syngenta

Centre for Sugarcane Biofuels Development (SCSBD).

Section 3 reports on the development of the Mackay Renewable Biocommodities

Pilot Plant (MRBPP). The author of this thesis was responsible for the conceptual

and detailed process design of the MRBPP, was responsible for the selection and

purchasing of equipment and was the key client representative during the design,

construction and installation phases. The development of this novel facility has

provided significant capability in Australia for the development and demonstration

of innovative technologies for ethanol production from bagasse and other cellulosic

feedstocks and is one of the only flexible and publicly accessible cellulosic ethanol

pilot scale development facilities in the world.

Chapter 8 reports on the development of the MRBPP and discusses the

funding of the facility, the value of pilot plants to commercial development

and provides an overview of the sugarcane biorefinery. Information

contained in this chapter was presented as a peer-reviewed conference

paper at the opening general session of the Australian Society of Sugar Cane

Technologists annual conference in Ballina, NSW in May 2009.

Section 4 is a critical evaluation of the key themes of the thesis and highlights the

fundamental contributions and key outcomes that have resulted from the overall

research project.

Chapter 9 presents the discussion of the key themes of the thesis and draws

conclusions on the value of this work to the development of a sustainable

cellulosic ethanol industry in Australia.

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Throughout this thesis, the terms cellulosic ethanol and bioethanol have been

used to refer to ethanol produced from cellulosic feedstocks. While a purified

ethanol product from cellulosic feedstocks is indistinguishable from ethanol

produced from other feedstocks and processes, the terms are convenient ones to

imply an ethanol product manufactured from a cellulosic feedstock.

1.5 Original contributions

This thesis is the first comprehensive assessment of the integration of bagasse-

based ethanol production facilities into established sugar processing systems and

the first to take an integrated approach to systems analysis, feasibility assessment

and pilot plant development. This thesis describes the following original

contributions to the fields of sugar and biofuels research:

- A detailed analysis of the Australian sugarcane processing system with

reference to the integration of ethanol from bagasse into the system;

- The development of a new framework and comprehensive techno-

economic models for assessing the feasibility of ethanol production from

sugarcane in integrated processing facilities;

- An assessment of the economic and systemic uncertainties that will

impact upon early stage investment in cellulosic ethanol technology in

Australia and the identification of strategies for reducing investment risk.

This assessment used Monte Carlo analysis to identify the key variables

and to simulate the impact of uncertainty on the economic indicators of

investment;

- A comprehensive assessment of the technical and economic feasibility of

a soda-based biorefinery in Australia, including a one and two-

component sensitivity analysis of the key variables affecting feasibility;

- An assessment of the economic and technical impact of energy systems

integration for co-located sugar and bagasse-based ethanol production

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facilities including the impact of energy demand on feedstock

availability, electricity use and ancillary fuel requirements; and

- The conceptual design and development of a novel pilot scale facility for

demonstrating the technical and economic feasibility of processes for

the ethanol production from sugarcane bagasse.

Despite sugarcane being perhaps the best biomass feedstock for early stage

cellulosic ethanol production, such an integrated and multi-dimensional analysis for

cellulosic ethanol production from sugarcane has not previously been undertaken in

Australia, and an extensive literature review has not revealed a similar study

elsewhere in the world.

1.6 Conclusion

This chapter has reviewed the key research question, the aims and outcomes of the

research and provided an outline of the thesis. The next section of the thesis

provides a more detailed introduction to the sugarcane and biofuels systems and

analyses the key factors impacting upon early stage investment in cellulosic ethanol

technologies.

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Systems analysis

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Chapter 2

Introduction to biofuels and the Australian sugar industry

2.1 Transportation fuels in the early 21st century

2.1.1 The use of crude oil as a transportation fuel

Although some of the earliest combustion powered transportation vehicles were

fuelled with ethanol, crude oil derivatives have provided the vast majority of

transportation fuels throughout the 20th and early 21st centuries. The overwhelming

reliance on crude oil derivatives as the source of virtually all transportation fuels

throughout this period has been the result of abundant crude oil deposits that have

been inexpensive to extract, refine and distribute to the consumer. The high energy

density of crude oil and its derivatives (including automotive gasoline, diesel and

aviation fuels) has also contributed to the popularity of these products as

transportation fuels.

In 2006, global demand for petroleum and other liquid fuels was 85.0 million barrels

oil equivalent per day (Mb/d) and this is forecast to grow to 106.6 Mb/d in 2030,

with the growth in transportation fuel use being responsible for 80 % of the higher

total crude oil use [1]. Despite improvements in energy efficiency standards in many

countries and the dampened demand resulting from the global economic recession

experienced in 2008-09, global crude oil consumption continues to increase by over

1 % annually, driven primarily by the increased demand for fuel in developing

countries [2], and particularly by the growth in demand in India and China [2, 3].

The only non-fossil liquid transport fuels currently of significance on a global scale

are biofuels, including bioethanol and biodiesel. World production of biofuels

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exceeded 0.7 Mb/d in 2007, an increase of 35 % from 2006 and accounting for 1.5 %

of total road transport fuel use [4]. Biofuels production is forecast to grow by about

8.6 % annually to approximately 5.9 Mb/d in 2030, increasing to 5.5 % of total liquid

fuel consumption [2].

2.1.2 The contribution of transport fuels to climate change

The Stern Review on the Economics of Climate Change [5] concluded that the

scientific evidence on climate change is now overwhelming, a serious and urgent

issue and that the benefits of strong, early action considerably outweigh the costs

of action. Independent reviews from many sources now recognise the majority

scientific opinion that the climate is changing as a result of anthropogenic

greenhouse gas emissions [5-8] and that the energy future we are creating is

unsustainable [9]. In general, these reports conclude that it is economically

advantageous to undertake early action, and that the introduction of deep cuts in

carbon emissions in the first half of the 21st century is not only essential but

achievable and affordable. Emissions reduction actions, however, are likely to

require a high carbon price in an emissions trading scheme depending upon the

stabilisation goal and emissions target trajectory to achieve the goal [10].

Transport fuels account for 14 % (6.5 GtCO2-e) of global greenhouse gas emissions,

with the majority of these from road transport (76 %) and aviation (12 %), without

accounting for non-CO2 effects of aviation or upstream CO2 emissions from fuel

production. These percentages are expected to remain stable although the total

greenhouse gas emissions from the transport sector are projected to grow to 9

GtCO2-e by 2030 and 12 GtCO2-e by 2050 [5].

It is generally recognised that there is no single solution for the challenges that

climate change will bring through the 21st century and beyond, and that multiple

strategies are required to both reduce carbon emissions and to adapt to the climate

change effects that will inevitably occur. Cost effective greenhouse gas emissions

savings in transportation are expected to result from improvements to fuel

efficiency, behavioural change and the increased use of biofuels. A combination of

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energy efficiency measures in transport fuel use and increased biofuel use are

estimated to have the potential to result in greenhouse gas savings of 7 GtCO2-e

per annum by 2050 at a cost of $25 /tCO2-e [5, 11].

2.1.3 Peak oil and future oil price

In 1956, M. King Hubbert [12] proposed a state where the production rate of crude

oil in the USA would peak, which would be followed by rapid depletion of the

remaining reserves. He later proposed a similar global state and this point became

known as Hubberts peak. Many commentators have since attempted to estimate

the date of this peak, although some commentators doubt the existence of a near

term peak [13].

One of the difficulties in estimating the peak is whether or not to include in the

analysis non-conventional oil deposits such as oil shale and tar sand deposits. While

these deposits are significant, the cost of extraction and environmental concerns

may limit the future viability of these deposits for large scale oil production. The use

of synfuels (liquid fuels produced from coal or gas) also affects the date of the peak.

Synfuels, oil shale and tar sand based fuels have much higher carbon emissions than

conventional crude oil based fuels as a result of emissions released in the

production process [5, 9].

It appears certain, however, that increasing scarcity of economically recoverable

conventional oil deposits will lead to higher costs of crude oil and its fuel

derivatives. Estimates of the future cost of crude oil are highly variable, but it is very

likely that crude oil prices will increase as conventional crude oil deposits deplete

and become more geographically concentrated.

The US Energy Information Agency reference case in 2009 [2] shows the crude oil

price being greater than US$100 /barrel in 2013 and rising to US$130 /barrel in

2030 (2007 dollars). Uncertainty in the projections is evident from the range of

alternative oil price scenarios between US$50 /barrel and US$200 /barrel [2].

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In their 2009 study, the International Energy Agency [4] reports a reference case

import crude oil price of US$115 per barrel in 2030 (2008 dollars), and also

acknowledge considerable uncertainty in attempting to estimate future oil prices

[9].

2.1.4 Energy security and development

Conventional crude oil reserves are becoming increasingly geographically

concentrated with 62 % of known reserves in Middle Eastern and North African

countries [9]. As conventional reserves diminish, supply pressures are likely to

increase and continuing supply may become politically prejudiced.

Many nations are increasingly concerned with ensuring the security of their future

energy resource and seek to ensure that a sizable portion is able to be produced

domestically. Renewable energy technologies (including renewable transport fuels),

have been reported to have the potential to play a significant role in enhancing

energy security [14] through diversifying energy sources.

In addition to the potential environmental benefits, many developing countries

have a particular interest in developing biofuel industries with the aim of

diversifying energy sources, reducing exposure to price volatility in the international

oil market, stimulating rural development, creating jobs and saving foreign

exchange [15].

2.2 Bioethanol a renewable transport fuel

2.2.1 Ethanol as a transportation fuel

Ethanol has been used as an alternate transportation fuel since the introduction of

the very first combustion engines. Although crude oil fuel derivatives became the

primary fuel for transportation, ethanol production spikes occurred during the

1920s and 1930s (following the first world war), and during the 1970s and early

1980s as a result of high petroleum prices [16].

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Ethanol has been used in combustion engines as a standalone fuel, fuel extender in

petroleum blends and as an additive. As an additive, ethanol increases the octane

rating of the fuel, reducing or eliminating the need for toxic octane enhancing

additives such as benzene [17]. While ethanol has a volumetric energy content

about two-thirds that of petroleum, the higher efficiency of combustion of ethanol

leads to an ethanol volumetric fuel efficiency about 75 - 80 % that of petroleum

[17].

Ethanol burned as a standalone fuel, or in blends with petroleum products,

produces fewer tailpipe particulate emissions, fewer oxides of nitrogen emissions

(NOx) and fewer emissions of aromatics, although produces higher volatile organic

carbons (VOCs) [17]. A recent Australian study [18] reported significant health cost

savings in urban Australia from a move to 10 % ethanol substitution in spark-

ignition engines from both a 50 % and 100 % uptake of E10 use in these vehicles.

The majority of post-1986 vehicles operating on Australian roads are suitable for

use with ethanol in blends up to 10 % ethanol [19]. In Brazil, vehicles with an

ethanol - petroleum fuel management system, known as flex-fuel vehicles are

capable of using a wide range of ethanol fuel blends. Eighty-five percent of all new

cars sold in Brazil are flex-fuel, capable of utilising any blend of petrol and ethanol

up to ethanol concentrations of 100 % [20].

2.2.2 First-generation ethanol

First generation ethanol has been produced primarily from starch based feedstocks

(grains such as wheat and corn) or sugar based feedstocks including sugarcane juice

and molasses. Both starch and sucrose are readily hydrolysed into simple hexose

sugars that can be fermented at high efficiency using conventional fermentation

organisms [21].

Starch and sucrose based feedstocks, however, are also used for both human

consumption and for livestock feed, and as a result, the price of these feedstocks

may be impacted by their relative value as a food. The impact of the diversion of

food crops such as corn into ethanol has already been linked to higher food prices in

16

some countries including Mexico and the United States of America [22] although

other reports suggest that the increased use of biofuels accounted for only 10

30 % of the food price increase evident during 2007 and 2008 [23, 24]. Other factors

such as the effects of drought, higher oil prices and economic growth increasing

global demand for wheat, dairy and protein in Asia and Africa, along with market

speculation and trade barriers, also impacted on the price of grain [24]. As the cost

of first generation feedstocks is typically 60 80 % of the ethanol production cost,

factors that act to increase the price of feedstocks used for both ethanol and food

production will have a significant impact on first generation bioethanol viability

during these periods of high feedstock prices.

2.2.3 Second-generation bioethanol

In contrast, second generation biofuels utilise lower value lignocellulosic materials

from forestry, agricultural residues or dedicated energy crops for ethanol

production. Materials considered for second generation biofuel production are

generally low value feedstocks that are often excess to that required in the farming

system.

Lignocellulosic biomass consists principally of the biopolymers cellulose,

hemicellulose and lignin. Both the cellulose and hemicellulose can be pretreated,

hydrolysed and fermented with varying efficiencies into ethanol [21, 25].

While considerable research has been undertaken on lignocellulosic ethanol since

the early 20th century, there remain some significant challenges to the economic

commercialisation of the technology. Apart from the financial challenges of

developing a cost-effective process, one of the major issues for any biomass

processing system is developing an efficient collection and transportation system

for the high volume, low density biomass feedstock to the ethanol processing

facility [22].

17

2.2.4 The global biomass resource

Cellulose is the most abundant organic material on the earth with natural processes

producing biomass from carbon dioxide and water. As the biomass resource can be

replenished in a short timeframe, the resource is both renewable and carbon

neutral. The continental biomass resource resulting from the growth of plants is

estimated to be 117.5 billion t/y, with 62 % of this resource in tropical rainforests

and other woods [26]. Agricultural crops contribute currently about 9.1 billion t/y

[26], with biomass typically yielding an ethanol volume of 275 - 309 L/t feedstock

(dry basis) [27].

Biomass contributes about 45 EJ/y of the current 467 EJ/y (2004 data) of global

energy demand, supplying up to 10 % of the energy in developed countries and 20

30 % in developing countries. Average estimates of global biomass energy farming

potential on current agricultural land are reported typically in the range of 100 - 300

EJ/y, without jeopardising future food supply. The use of organic wastes and

residues are reported to offer the potential of an additional 40 - 170 EJ/y, making

the total potential contribution from biomass this century up to 400 EJ/y [28]. A

review of 17 previous biomass energy studies reported estimates from less than

100 EJ/y to greater than 400 EJ/y [29].

Biofuels currently contribute about 1.5 EJ/y or about 1.5 % of global transportation

fuel use [28]. Production of ethanol in 2006 was 39 billion litres, increasing 18 %

from 2005 [30]. Estimates of the long-term world liquid biofuel production potential

range from 12 - 455 EJ/y, with most studies in the range of 48 - 158 EJ/y [21],

although the economically viable production potential may be significantly lower

than the technical production potential frequently reported. In Australia, up to

140 % of existing transport fuel use could be supplied by biofuels if the industry

develops around second generation biofuel technologies [31].

18

2.3 Sugarcane as a bio-energy resource

2.3.1 The global sugar industry

Sugar is one of the major food carbohydrate energy sources in the world. It is

principally produced from two major crops sugarcane, grown in tropical and sub-

tropical regions of the world, and sugar beet grown in more temperate climates.

In 2006, 1.392 billion tonnes of sugarcane were grown globally at an average yield

of 68.3 t/ha dominated by production in Brazil and India. Sugar beet production in

2006 was 256 million tonnes at an average yield of 47.1 t/ha [32]. The leading

sugarcane producing countries are shown in Figure 2.1.

Figure 2.1 Leading sugarcane producing countries 2006 [32]

The principal use of sugarcane throughout the world is for crystal sugar production

for human consumption. In several countries including Brazil, a sizable portion of

the crop is also used for ethanol production from both sugarcane juice and

molasses. Many other countries including Australia produce lesser quantities of

ethanol from molasses.

0

100

200

300

400

500

Braz

ilInd

iaCh

ina

Mexic

o

Thail

and

Pakis

tan

Colom

bia

Austr

alia

Indon

esia

USA

Philip

pines

South

Afric

a

2006

sug

ar c

ane

prod

uctio

n (m

illio

n to

nnes

)

19

Over the past decade, global sugarcane production has increased by 8 %, driven by

a 37 % increase in sugarcane production in Brazil [32]. This increased sugarcane

production has resulted in both increased crystal sugar production and increased

ethanol production, and has had a significant impact on the world price of raw

sugar. Land use change enabling this global expansion of sugarcane production has

both direct and indirect sustainability implications and the factors relating to these

implications are diverse and complex [33-35].

2.3.2 The sugarcane biomass resource

Sugarcane is a C4 monocotyledonous perennial grass grown principally in tropical

and subtropical regions of the world. Modern sugarcane varieties cultivated in

Australia are complex hybrids derived through intensive selective breeding between

the species Saccharum officinarum and Saccharum spontaneum [36].

Globally, the 1.4 billion tonnes of sugarcane produced annually is grown on about

20.4 million hectares [32] in tropical and sub-tropical regions of the world. In

Australia, modern sugarcane varieties are capable of producing in excess of 55 t/ha

of biomass (dry weight). The development of high biomass sugarcane (often

referred to as energy cane) has the potential to significantly increase the amount

of biomass available.

Traditional sugarcane harvesting processes remove the top of the stalk (tops) and

leaf material, and only the stalk is transported into the factory for extraction and

production of sugar. Tops and leaf material remaining after harvesting are either

left in the field to decompose, acting as mulch and providing organic matter and

nutrient for the soil, or burnt depending upon farming practices. It is likely that only

a portion of this leaf material is of value in the agricultural system, and for

improving soil condition. The remainder of this extraneous matter is potentially

available as a feedstock for biomass value adding processes such as bioethanol

production. The impacts of harvesting and transporting extraneous matter on the

sugar milling process and the economics of the industry are complex and an

integrated modelling approach has been developed to analyse these effects [37].

20

2.3.3 The Australian sugar industry

Over the past decade, the Australian sugar industry has harvested approximately 28

38 million t/y of sugarcane from approximately 400,000 hectares [38] along the

eastern coast of Australia (Figure 2.2). Approximately 95 % of the sugarcane is

grown in Queensland with the remainder of the industry operating in Northern New

South Wales (NSW). Sugarcane is Queenslands highest value agricultural crop with

an annual value of approximately $1.5 - $2.5 billion [39].

Sugarcane in Australia is crushed at one of 25 sugar factories and processed into key

products including crystal sugar and molasses. Typically, 4.5 5 million tonnes of

raw sugar is produced [39] and 75 % of the sugar produced is exported. While

Australia is only the eighth largest producer of sugarcane [32], Australia is typically

the second or third largest exporter of sugar after Brazil and (in some years)

Thailand.

The average area of sugarcane harvested in Queensland has decreased over the

past decade as a result of economic challenges posed by drought and disease,

extended periods of poor sugar prices and industry restructuring programs. In

particular, low sugar prices during the early 21st century resulted in an industry

restructuring program that led to up to a quarter of the growers in Australia exiting

the industry. A survey of the financial performance of sugarcane growers in 2007-08

[40] determined that the volume of production is relatively stable with a trend

toward a smaller number of larger farms improving the viability of sugarcane

producers. In the period since 2008, higher prices have provided improved financial

conditions for sugarcane growers (Figure 2.3).

For domestic sugar consumption, raw sugar is processed into refined sugar at

refineries in Mackay and Bundaberg (Queensland), Yarraville (Victoria), and

Harwood (NSW).

The only distillery of significant capacity currently producing ethanol from

sugarcane products in Australia is the Sucrogen 60 ML/y molasses-based distillery

located on the site of the Plane Creek sugar factory in Sarina, Queensland. Small

21

quantities of ethanol are also produced in boutique distilleries in Bundaberg and

Beenleigh, Queensland, producing rum and other consumer products from

molasses.

Figure 2.2 Map of the Australian sugar industry [39]

22

Figure 2.3 Australian No.1 sugar pool price 1990-91 to 2005-06 and QSL seasonal pool price 2006-07 to 2010-11 (AU$/t) [38, 41]

2.3.4 Current uses of sugarcane bagasse in Australia

In most sugar factories, bagasse from the crushing or diffuser station is burnt in

suspension fired boilers to generate steam for electricity, mechanical power and

process heat requirements for the factory. Historically, sugar factory boilers and

factory production technologies have been designed to be energy inefficient to

ensure that the energy requirements of the factory match the availability of bagasse

from the sugarcane. This approach has ensured that the factories required little if

any supplementary fuels (such as coal or oil) for process energy, while ensuring that

the factories were not left with an expensive bagasse disposal problem. Small

quantities of surplus electricity have been sold to the electricity transmission or

distribution networks.

With increasing value in the market for energy products, sugar factories are

investing in higher efficiency boilers and more efficient process technologies to

2010-11 Estimated pool price range

200

250

300

350

400

450

500

550

Seas

onal

poo

l pri

ce

(AU

$/t)

Year

23

enable a significantly greater quantity of electrical export and hence capture

additional value from the sugarcane resource [42, 43].

Around the world, sugarcane bagasse is used for many applications including animal

feed, pulp and paper production, particle and fibre board production and furfural

production. Other potential uses of bagasse include xylitol production, speciality

building products, microcrystalline cellulose production and the production of

furfural and lignin derivatives [44, 45].

Sugarcane has some major advantages as a feedstock for lignocellulosic ethanol

production compared to other feedstocks. One of the most significant advantages is

that the sugarcane bagasse is an existing centrally located resource supported by a

harvesting and transport infrastructure that supplies the sugarcane to the sugar

factory.

2.3.5 The sugarcane biorefinery

Several studies have commented on the need to improve the economics of the

bioethanol production process through the integrated production of multiple co-

products in a biomass biorefinery [46-54]. In a biorefinery, bagasse is typically

fractionated into its components and value is added to each component through

the production of multiple high value co-products. Bioethanol is generally

considered to be a significant (but not the only) revenue stream for a biorefinery.

Products that are able to be produced in a biorefinery include ethanol, compounds

derived from lignin, specialty sugars, organic acids, fermentation products, and

other energy products including biodiesel, hydrogen and methane.

Typical products able to be produced in a sugarcane biorefinery are shown in Figure

2.4.

24

2.4 The composition and structure of sugarcane bagasse

Bagasse from the sugarcane diffusion and milling processes generally contains 44

53 % moisture, 1 2 % soluble solids, 1 5 % insoluble solids (ash) and the

remainder lignocellulosic fibre [45]. The fibre analysis of bagasse by standard sugar

factory methods [55] includes dirt and other insoluble impurities and these

impurities can vary from quite small quantities to very significant quantities

depending upon the sugarcane supply and processing technologies.

Lignocellulosic materials such as sugarcane bagasse are complex mixtures of

cellulose, hemicellulose and lignin with minor amounts of ash, proteins, lipids and

extractives. The actual composition of the lignocellulosic material depends upon the

growth conditions of the plant, the plant tissue and the age at harvesting [16].

Reports of bagasse fibre composition in the literature vary with cellulose typically

34 47 %, hemicellulose 24 29 % and lignin 18 28 % on a dry basis [27, 44, 45,

56-58].

Figure 2.4 An overview of current and potential products from sugarcane in Australia current products shown in black and potential products shown in red

Sugar cane

Renewable electricity

Crystal sugar

Ethanol, Bio-crude Chemicals

Filter mudBagasse

Export

Juice

Fertiliser

High value chemicals

Molasses

Pulp

ChemicalsBio-plastics

Ethanol

WaxesProteinsPlant made products

BiofuelsPharmaceuticalsIndustrial products

Ethanol Animal feed

25

Sugarcane is a non-homogenous material and can be thought of as consisting of

peripheral fibres (rind) enclosing a soft central pith [58]. The rind is covered by a

waxy coating. The sugarcane stalk transports water and nutrients from the soil to

the growing portion of the plant and stores sugar that has been synthesised in the

plant leaves. Vascular bundles in the stalk account for a large proportion of the stalk

fibre and the sugar is stored in parenchyma tissue surrounding the vascular bundles

[45].

In sugar extraction operations, the structural order of the fibres in the sugarcane

plant is lost [44] and the resultant bagasse is a mixture of fibre components of

varying length and composition. Pith cells are broken into fine particles generally

much less than 1 mm in length, while other fibres may retain a length of up to

25 mm. For the practical measurement of pith, all of the fibres passing through a

fine screen of approximately 1.5 mm aperture are generally considered to be pith

fibres. By this definition, pith constitutes approximately 40 % of the total bagasse

fibres by weight. Pith is chemically similar to the non-pith fibre, although the non-

pith fibre has been reported to have lower hemicellulose concentrations [59] and

higher -cellulose concentrations [45]. For bagasse fibre pulping operations, the

pith is generally removed prior to digestion as the presence of pith increases

chemical usage and adversely affects fibre drainage.

A typical constitutive analysis of Australian bagasse fibre on a dry basis is shown in

Table 2.1.

In lignocellulosic materials such as bagasse, cellulose is ordered into fibrils which are

surrounded by lignin and hemicellulose [60]. The hemicellulose provides an

interpenetrating matrix for the cellulose microfibrils with molecular interactions

including hydrogen bonds and Van der Waals forces, while lignin is incorporated

into the spaces around the fibrillar elements, forming lignin polysaccharide

complexes [61].

26

Table 2.1 Typical constitutive analysis of Australian sugarcane bagasse

Weight

percent

Cellulose 43

Hemicellulose

xylose arabinose

27

4

Lignin 23

Extractives 1

Ash 2

2.4.1 Cellulose

To describe the structure of native celluloses, it is necessary to consider three levels

of structure, including at the molecular scale of the macromolecule, the

supramolecular level of packing and ordering and the morphological architecture

[62].

At the molecular level, cellulose is a linear homopolymer of D-glucopyranose units

linked at the 1 and 4 carbon atoms by b-glycosidic bonds, with hydroxy groups at C-

2, C-3 and C-6. The hydroxy group at the C-1 end of the glucose chain has reducing

properties and the hydroxy group at C-6 is non-reducing [62]. The solubility of the

anhydroglucose polymer in water decreases above a degree of polymerisation (DP)

of 6, due to strong intermolecular hydrogen bonds. Sugarcane bagasse celluloses

typically have a molecular weight between 150,000 and 350,000 [44] which equates

to a DP between 800 and 1900.

At the supramolecular level, the chemical composition and spatial conformation of

cellulose molecules results in cellulose having the tendency to aggregate into highly

ordered structural entities through an extensive network of hydrogen bonds. This

27

structural aggregation is not uniform throughout the structure with regions of high

crystalline order and regions of relatively low crystallinity (amorphous) [62].

Native cellulose morphology is characterised by the well-ordered aggregation of

microfibrils into macrofibrils. The macrofibrils contain a non-uniform system of

pores, capillaries, voids and interstices that increase the surface area of the

cellulose fibrils [62].

2.4.2 Hemicelluloses

Hemicelluloses are heterogeneous polymers of pentoses (xylose, arabinose),

hexoses (mannose, glucose and galactose), and uronic acids [54]. Hemicelluloses

are typically branched with much lower degrees of polymerisation than cellulose

(typically 80 - 200) [63]. Hemicelluloses are not crystalline and as a result are more

readily accessible for hydrolysis than cellulose [64]. The structure of hemicelluloses

is generally considered to be rod-shaped with branches and side chains folded back

to the main chain through hydrogen bonding [65].

In cell walls, hemicellulose molecules hydrogen bond to the cellulose microfibrils.

While they act to coat the microfibrils, restricting the enzyme pathway to the

cellulose, they are also long enough to span the microfibrils and link them together

[66].

In sugarcane bagasse, the principle hemicelluloses are heteropolymers based on a

D-xylose polymer backbone with side groups containing mainly glucuronic acid and

arabinose. The average viscometric molecular weight of sugarcane bagasse

hemicelluloses is between 10,000 and 20,000 [44]. A review of previous research

has found considerable variation in the proportions of the relative constituents of

hemicellulose, with a mole ratio of xylose to arabinose of 4.0 - 52.6 and a mole ratio

of xylose to glucuronic acid of 7.4 - 100 [67].

Hemicellulose extraction from bagasse with water at temperatures between 150 oC

and 170 oC resulted in xylose yields of 60 %, with 80 % of the extracted xylose in the

oligo- or polysaccharide form [68].

28

2.4.3 Lignin

Lignin is a natural amorphous polymer composed of phenylpropane olignol units

with hydroxyl and carbonyl substitutions. There are three major phenylpropane

units, p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) which differ in the O-

methyl substitution of the aromatic ring. The structure of lignin that has been

isolated from biomass is dependent upon both the plant and the process used for

delignification [69, 70].

In a lignocellulosic material, lignin is covalently linked to both cellulose and

hemicellulose. Cross-linking possibilities exist through hydrogen bonds, ionic

interactions, ester and ether linkages and Van der Waals interactions. Lignin

carbohydrate interactions have been shown to strongly affect ruminant and

enzymatic digestibility [66].

Both the total quantity and structure of the lignin within the plant varies with cell

tissue and these have been shown to affect the recalcitrance of the tissue to

biodegradation. Warm season grasses such as sugarcane are reported to have both

lignified cell walls as well as high levels of phenolic acid esters linked to arabinose

[71]. In addition, warm season grasses contain ferulic acid esterified with

hemicelluloses and etherified with lignin while p-coumaric acid is esterified with

lignin [72]. Each of these linkages, in addition to the structure and quantity of lignin

present, has a substantial effect on digestibility for bioethanol production through

both the covalent linkages themselves and the effect they have of physically

reducing access to the carbohydrate polymers [73].

Sugarcane bagasse lignin has a higher content of p-hydroxyphenyl lignin, and as a

result, a lower methoxy content than lignin from other hardwood and softwood

lignins [69]. The importance of delignifying bagasse to produce a residue that is

readily hydrolysed by enzyme has been highlighted [74].

2.5 Overview of the process for ethanol production from sugarcane bagasse

Unlike the starch or sugar feedstocks upon which first generation bioethanol has

been based, the structural rigidity of lignocellulosic materials results in a material

29

that is extremely resistant to hydrolysis (depolymerisation). As a result, the ethanol

production process from biomass such as sugarcane bagasse requires aggressive

thermochemical or physical pretreatments, or combinations of both to generate a

material more amenable to hydrolysis. These pretreatment processes add to the

cost of bioethanol production from biomass feedstocks and, depending upon the

process used, generate significant degradation products that can detrimentally

affect the fermentation productivity and product yield [75].

Due to the formation of degradation products in the acid hydrolysis of cellulose and

hemicellulose, considerable attention is being given to the development of efficient

enzymatic hydrolysis processes for the conversion of cellulose and hemicellulose

into fermentable sugars. Significant quantities of cellulolytic and hemicellulolytic

enzymes are required for this conversion process to ensure both high yields and

rapid hydrolysis rates.

Despite significant research investment into improved enzyme efficacy, the cost of

the enzymes and the capital required to produce them in the quantities required for

commercial bioethanol facilities remain major cost impediments to the

commercialisation of the technology. In the landmark 2002 study by Aden, et al [76]

on ethanol production from corn stover, cellulase enzyme cost was assessed to be

9 % of the total cost contribution to the process, with pretreatment and

conditioning accounting for 19 % of the total cost contribution (including feedstock

and capital depreciation costs). A later study by Tao and Aden [77] showed an

enzyme cost of 7 % of total operating costs (including feedstock and capital

depreciation costs).

Effective pretreatment strategies reduce the quantity and cost of enzymes required

for hydrolysis of cellulose and hemicellulose. These strategies include hydrolysing

the hemicellulose fraction of the fibre, decreasing the lignin content of the material,

reducing the crystallinity of the cellulose fibrils or modifying the fibre architecture

to enable more rapid transport of the enzyme into the fibre.

A simple schematic of the key processes required for ethanol production from

sugarcane bagasse via a biochemical pathway is shown in Figure 2.5.

30

Figure 2.5 Simple schematic of the key processes required for the ethanol from sugarcane bagasse

2.6 Conclusion

This chapter has provided an introduction to transportation fuel use and the

challenges associated with commercialising biofuels production from cellulosic

feedstocks. An overview of the global and Australian sugar industries and the

structure of sugarcane bagasse as a bioenergy feedstock have also been provided.

Chapter 3 provides more detail on the technologies for pretreatment of fibre from

sugarcane bagasse.

31

Chapter 3

Pretreatment technologies for ethanol production from sugarcane bagasse

3.1 Introduction

Chapter 2 provided an introduction to the sugarcane system and to the drivers

affecting biofuel production from sugarcane. This chapter builds upon the

information in the previous chapter discussing in more detail the objectives of

the pretreatment processing of sugarcane bagasse and reviews the key research

work that has been reported for the pretreatment of sugarcane bagasse.

3.2 The objectives of the pretreatment process

The economic production of ethanol from lignocellulosic fibre requires a

feedstock to the hydrolysis process that is readily amenable to enzymatic attack

and subsequent fermentation at high yields. Native lignocellulosic materials are

extremely resistant to enzymatic hydrolysis and require an effective

pretreatment process prior to hydrolysis.

The pretreatment process in a lignocellulosic ethanol facility can be considered

to have the following key objectives [16, 78]:

- To improve the structure and accessibility of the carbohydrate

compounds to enable rapid and cost-effective enzymatic hydrolysis;

- To avoid the degradation of carbohydrates, ensuring maximum

fermentable sugar and ethanol yield;

- To avoid the production of degradation products inhibitory to

hydrolysis or fermentation; and

32

- To be a cost-effective process within the context of an economically

viable facility.

To achieve these objectives, the following specific attributes are desirable in a

pretreatment process [79, 80]:

- Low cost of chemicals for both the pretreatment and neutralisation or

chemical recovery stages;

- Minimal generation of wastes;

- Minimal requirement for energy-intensive biomass particle size

reduction prior to pretreatment;

- Preservation of hemicelluloses and enhancement of the accessibility

of hemicelluloses for fermentation;

- Short reaction times with non-corrosive chemicals to minimise

reactor costs;

- High fermentable sugars concentration to minimise fermentation

reactor sizes and energy costs in ethanol recovery;

- High product yields in hydrolysis and fermentation with minimal

hydrolysate conditioning (for removal of fermentation inhibitory

compounds) required;

- Hydrolysate conditioning should not form products that present

processing or waste disposal challenges;

- The pretreated cellulose and hemicellulose should require minimal

enzyme loadings to obtain greater than 90 % digestibility in less than

three days; and

- Facilitate recovery of lignin and other products for conversion to

valuable co-products.

33

Several reviews have been undertaken relating to the pretreatment processing

of lignocellulosic materials and the technology involved [16, 25, 27, 78, 79, 81-

85]. In general, most pretreatment strategies improve the digestibility of the

fibre through one or more of the following strategies:

- Reducing the lignin content or modifying or redistributing the lignin

component;

- Reducing the hemicellulose content;

- Reducing the crystallinity or degree of polymerisation of the cellulose

component; and

- Influencing the fibre particle size, porosity, cell wall thickness or fibre

surface area.

The lignin concentration of the fibre and the degree of cellulose crystallinity have

been shown to have the most significant effect on biomass digestibility by

enzyme and this has been shown to hold true for bagasse [86]. Reducing the

acetyl content has been shown to have a lesser impact on biomass digestibility

although this remains an effective strategy [86]. While effective pretreatment is

critical to bagasse digestion by enzymes, the hydrolytic effectiveness is also

dependent upon digestion conditions including pH, temperature, solids content

and enzyme loading [87].

Bagasse pretreatment technologies can be categorised as chemical, physical and

biological treatments and have been used either singly or in combinations of

treatments. The following sections review some of the key work that has been

undertaken.

34

3.3 Chemical pretreatments

3.3.1 Concentrated acid hydrolysis

Concentrated acid hydrolysis has been used commercially (during the Second

World War) for hydrolysing biomass. In the concentrated acid process, sulphuric

acid is typically used at concentrations greater than 40 % at room temperature

for periods of approximately 1 hour [25]. The use of concentrated acids for

hydrolysis at low temperatures results in high yields of both pentoses and

hexoses, with reported yields of 85 95 % of theoretical yields and with minimal

production of degradation products [25].

Commercialisation of the concentrated acid process has been hindered by the

high cost of acid, necessitating expensive acid recovery processes (such as

chromatographic techniques for separating the acid and sugars) and the

requirement for expensive alloys in plant construction [25].

3.3.2 Dilute acid hydrolysis and pretreatment

Dilute acid hydrolysis of biomass for ethanol production is favoured by many

researchers as the process is simple, rapid and requires no solvent recovery

process. In dilute acid hydrolysis, both the cellulose and the hemicellulose

fractions are substantially hydrolysed.

In general, the dilute acid hydrolysis process is a single or double stage process

using sulphuric acid in concentrations of up to 1.5 % acid, with reaction times of

several minutes and temperatures between 180 oC and 230 oC. Higher

temperatures are mostly used to ensure rapid hydrolysis rates and high glucose

yields during saccharification. The higher temperatures, however, also increase

the rate of generation of pentose degradation products, primarily furfural, and

hexose degradation products, primarily 5-hydroxymethyl furfural (HMF) [56, 79].

Furfural and HMF can further degrade to other products including furan, levulinic

acid and formic acid. Several phenolic compounds resulting from lignin

degradation can also be formed under these conditions [25].

35

Glucose yields from the dilute acid hydrolysis process have been mostly reported

between 50 % and 60 % of theoretical glucose yield, however, more recent

studies have reported glucose yields over 80 % and xylose yields above 90 % with

new reactor designs [25]. Despite the improvements in glucose and xylose

yields, significant quantities of inhibitory degradation products are formed and

low hydrolysate sugar concentrations have been achieved [25].

Acetic acid is also formed from the hydrolysis of acetyl groups in the

hemicellulose fraction and can be a further inhibitor to microbial growth in

concentrations as low as 4 g/L [88]. HMF and furfural concentrations as low as

0.5 g/L have been shown to reduce microbial growth substantially in

lignocellulosic materials [88] and in sugarcane bagasse hydrolysates at

concentrations greater than 0.9 g/L [89].

Mild acid pretreatment processes utilise lower process temperatures, shorter

reaction times and lower acid concentrations than dilute acid hydrolysis to

substantially hydrolyse the hemicellulose with a resultant 80 90 % yield of

monomer sugars. The cellulose and lignin remain in the solid residue following

pretreatment, and the cellulose can be subsequently enzymatically hydrolysed

[79].

Mild acid pretreatments on bagasse attack the lignocellulosic structure through

hydrolysing hemicellulose chains attached to the lignin, as well as degrading

some of the lignin. The degree of cellulose crystallinity of the fibre can increase

during mild acid pretreatment as a portion of the amorphous cellulose is

solubilised, resulting in a residual solid with a higher proportion of more resistant

crystalline cellulose [90].

One approach to reducing the formation of degradation products in mild acid

pretreatment processes is to utilise a two stage pretreatment process, with a

moderate temperature first stage solubilising the most readily available

hemicellulose and separating the hydrolysate from the solid residue prior to a

second stage higher temperature process. Following the second stage hydrolysis,

36

the fibre undergoes rapid decompression in a process known as steam explosion

(Section 3.4.1) to affect fibre morphology.

One of the major challenges with mild acid hydrolysis or pretreatment is the

corrosive nature of the process conditions (low pH, elevated temperature and

pressure) resulting in a requirement for pressurised reactors manufactured from

exotic and expensive alloys. Other concerns include the need for neutralisation

chemicals for hydrolysate conditioning and the disposal costs associated with the

salts formed (typically gypsum). The continuing presence of lignin in the solid

residue results in non-productive adsorption of a portion of the enzymes on the

lignin, requiring a higher enzyme usage rate [79].

Studies with sugarcane bagasse have looked at the kinetics of hydrolysis with a

range of mineral acids. A kinetic study [56] of sulphuric acid hydrolysis of bagasse

modelled xylose, glucose, acetic acid and furfural concentrations at

temperatures of 100 - 128 oC and acid concentrations of 2 6 %. Up to 90 % of

the hemicelluloses were hydrolysed under these conditions with minimal

hydrolysis of cellulose. Further detailed studies [59, 67] looked at the kinetics of

xylose, arabinose, glucose and furfural production under a large range of

temperature conditions, solid to liquid ratios and bagasse type, comparing both

sulphuric and hydrochloric acids. About 80 % of theoretical xylose yields were

achieved. Bagasse particle size was found to have a negligible effect on the rate

of hydrolysis.

Further studies with sugarcane bagasse have also investigated the kinetics of

hemicellulose hydrolysis in dilute sulphuric acid [91], hydrochloric acid [92],

phosphoric acid [93-96] and nitric acid [97]. The use of sulphur dioxide

impregnated bagasse with steam treatment has been studied and resulted in

sugar yields of 87 % [98].

A study [99] on dilute acid pretreatment of sugarcane bagasse and other biomass

sources (rice hulls, peanut shells and cassava stalks) using dilute sulphuric acid at

122 oC and times up to 1 hour showed that bagasse was the most susceptible of

these materials to hemicellulose hydrolysis, with conversion of the xylan of 73

37

81 %. Cellulose was only marginally hydrolysed (less than 10 %) under these

conditions. Minor inhibition of the fermentability of the prehydrolysate was

reported as a result of inhibitory compound formation, but the yield of glucose

from cellulose from enzymatic hydrolysis of the solid residue was only 40 %

taking into account losses from the dilute acid prehydrolysis [99].

Another study of sugarcane bagasse with sulphuric acid pretreatment has shown

that hemicellulose monomer sugar yield is most influenced by acid concentration

and that higher temperatures increase degradation product formation, favouring

the selection of reaction conditions with higher acid concentrations, longer

reaction times and lower reaction temperatures [91]. Despite the hydrolysis and

removal of hemicellulose from the residual solid, the relative increase in lignin in

the solid residue has been shown to restrict the potential gains in susceptibility

of the solid residue to enzymatic hydrolysis [100].

Reprecipitated cellulose from sugarcane bagasse pretreated with zinc chloride

and dilute hydrochloric acid was found to have a significantly greater rate and

extent of hydrolysis than untreated bagasse cellulose [101].

Acid pretreatments under very mild concentrations have also been trialled for

enhancing the digestion characteristics of bagasse feeds for ruminant animals

[102].

Strategies for minimising the impact of fermentation inhibitors on ethanol

production from acidic treatments of bagasse include control of process

conditions to minimise the production of inhibitory compounds, detoxification

prior to fermentation and the selection and adaptation of inhibitor tolerant

fermentation organisms [103]. Strategies for detoxification of hydrolysates from

bagasse include overliming [89, 103], laccase treatment [103], pH adjustment

[104], activated carbon adsorption [105] and electrodialysis [106]. Mechanisms

of inhibition and detoxification have been reviewed generally for lignocellulosic

materials [88, 107, 108].

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3.3.3 Alkaline pretreatments

Alkaline pretreatments are extensively used in the pulping industry for both

wood and non-wood feedstocks. The pulping industry principally uses the Kraft

process for pulping of wood fibres which combines the use of caustic soda and

sodium sulphite for effective delignification. Non-wood fibres such as bagasse

more readily delignify than fibres from woody plants and as a consequence, for

bagasse, caustic soda is a satisfactory delignifying agent. In the bagasse pulping

soda process, caustic soda is typically used at a concentration of 18 - 26 % NaOH

on dry fibre at temperatures up to 160 oC.

Alkaline pretreatments aim to dissolve a large proportion of the lignin from the

biomass with the rate and extent of dissolution varying with the alkali

concentration, reaction time and reaction temperature [109]. The removal of

lignin from lignocellulosic materials is a key strategy in improving cellulose

digestibility [75, 79, 100]. Pulping processes aim to delignify bagasse to a target

lignin concentration (known in the pulping industry as the Kappa number [110]).

Some dissolution of hemicellulose also occurs in alkaline pretreatments but this

is generally undesirable as this leads to a reduction in pulp yield.

Alkali pretreatments of sugarcane bagasse have been shown to remove lignin

and hemicellulose through both solubilisation and hydrolysis from the fibre,

resulting in a more open structure that is more readily accessible to cellulosic

enzymes than untreated bagasse [90]. Delignification of bagasse fibre in alkali

pretreatment is rapid to about 75 % delignification with the preferential removal

of p-hydroxyphenol lignin [111]. The major degradation products from alkali

bagasse pretreatments are formic acid, acetic acid and hydroxymonocarboxylic

acids [112], although the inhibitory impact of these on fermentation are much

less significant than the degradation products that result from acidic

pretreatments.

Due to the less corrosive environment, the cost of materials for the fabrication of

pretreatment reactors for alkaline pulping is significantly lower than the cost of

materials required for acidic pretreatments, however, it is reported that the cost

39

of chemicals is likely to be significantly higher with caustic soda being four times

as expensive as sulphuric acid. As the processes operate in aqueous

environments above 100 oC, pressure vessels are required for pretreatment

processing. Little testing of alkaline processes at pilot scale has been reported in

the literature and little information is available on the process economics [80].

Low temperature, low concentration NaOH treatment of bagasse has been

trialled with long residence times (1 - 6 days) although improved results were

obtained with bagasse pretreatment by sodium chlorite prior to NaOH

pretreatment [113].

Lime pretreatment has been studied for its effectiveness in enhancing enzymatic

digestibility of bagasse and wheat straw [114]. Short pretreatment times (1 - 3

hours) at high temperatures (85 135 oC) were effective in achieving high sugar

yields, while lower temperatures (50 65 oC) required much longer pretreatment

times (24 hours). Glucans and xylans were not removed in the pretreatment and

a maximum of only 14 % of the lignin was solubilised. Enzymatic hydrolysis of the

lime pretreated bagasse produced 75 % of theoretical sugar yield after 72 hours

[114]. A comparison of lime and alkaline hydrogen peroxide pretreatments

achieved glucose yields of up to 87.5 % for lime and 62.4 % for alkaline hydrogen

peroxide with longer reaction times, higher temperatures and higher lime

loadings all favoured in producing a higher glucose yield [115].

Aqueous ammonia has been trialled for its effectiveness as a pretreatment agent

for enzymatic hydrolysis of bagasse, corn husk and switchgrass [116]. Bagasse

was treated with aqueous ammonia at 120 oC for 20 minutes and glucan and

xylan yields of 72.9 % and 82.4 % respectively were reported. The residual

ammonia was separated from the bagasse by vacuum drying and no washing of

the biomass prior to hydrolysis was required. The enzymatic effectiveness of

various cellulase and hemicellulase preparations and mixtures have also been

studied on aqueous ammonia and ammonia freeze explosion pretreated bagasse

[117].

40

The addition of potassium hydroxide has been used to significantly improve

delignification of aqueous ammonia bagasse pulps for paper applications. The

use of aqueous ammonia and potash offers an alternative alkaline pretreatment

strategy as the black liquor from the process can be converted into a valuable

fertiliser, reducing the necessity for expensive alkali recovery processes. Eighty

percent delignification was achieved using 35 % NH4OH and 5 % KOH and minor

amounts of anthroquinone at temperatures of 165 oC for 1 hour [118].

Alkaline pretreatments have been conducted in conjunction with oxidative

pretreatments and these are discussed in the following section.

3.3.4 Oxidative pretreatments

Wet oxidation involves the reaction of a lignocellulosic material with water

(under alkaline conditions) and oxygen or air at temperatures greater than

120 oC, more typically at 170 - 200 oC and pressures of 10 - 12 bar [25]. Sodium

carbonate is often added to the process to prevent the formation of degradation

products that would occur under acidic conditions.

During wet oxidation, both a low temperature hydrolytic reaction and a high

temperature oxidative reaction occur. Wet oxidation of sugarcane bagasse under

alkali conditions has been shown to reduce the formation of toxic formaldehydes

and phenol aldehydes compared to wet oxidation alone [119-121].

Alkaline wet oxidation is reported to enhance the susceptibility of bagasse to

enzymatic hydrolysis. In the studies, alkaline wet oxidation at 195 oC for 15

minutes produced a