Synthetic natural GaS from coal Dry BiomaSS anD PoWer-to-GaS aPPlicationS
Synthetic natural GaS from coal Dry BiomaSS anD PoWer-to-GaS aPPlicationS
edited by
tilman J SchilDhauerSerGe ma BiollazPaul Scherrer Institut VilligenSwitzerland
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
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Library of Congress Cataloging‐in‐Publication Data
Names Schildhauer Tilman J editor | Biollaz Serge MA editorTitle Synthetic natural gas from coal dry biomass and power-to-gas applications
[edited by] Tilman J Schildhauer Serge MA BiollazDescription Hoboken New Jersey John Wiley amp Sons 2016 |
Includes bibliographical references and indexIdentifiers LCCN 2016006837 (print) | LCCN 2016014453 (ebook) | ISBN 9781118541814 (cloth) |
ISBN 9781119191254 (pdf) | ISBN 9781119191360 (epub)Subjects LCSH Synthesis gas | Coal gasification | Biomass conversion | Gas manufacture and worksClassification LCC TP360 S96 2016 (print) | LCC TP360 (ebook) | DDC 6602844ndashdc23LC record available at httplccnlocgov2016006837
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
List of Contributors xi
1 Introductory Remarks 1Tilman J Schildhauer
11 Why Produce Synthetic Natural Gas 112 Overview 3
2 Coal and Biomass Gasification for SNG Production 5Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 Introduction ndash Basic Requirements for Gasification in the Framework of SNG Production 5
22 Thermodynamics of Gasification 6221 Gasification Reactions 7222 Overall Gasification Process ndash Equilibrium Based Considerations 7223 Gasification ndash A Multi‐step Process Deviating from Equilibrium 11224 Heat Management of the Gasification Process 13225 Implication of Thermodynamic Considerations for Technology
Choice 1823 Gasification Technologies 18
231 Entrained Flow 19232 Fixed Bed 20233 Direct Fluidized Bed 22234 Indirect Fluidized Bed Gasification 27235 Hydrogasification and Catalytic Gasification 34
References 37
CoNteNtS
vi CONTENTS
3 Gas Cleaning 41Urs Rhyner
31 Introduction 4132 Impurities 42
321 Particulate Matter 42322 Tars 43323 Sulfur Compounds 43324 Halide Compounds 44325 Alkali Compounds 44326 Nitrogen Compounds 44327 Other Impurities 44
33 Cold Warm and Hot Gas Cleaning 45331 Example of B‐IGFC Gas Cleaning Process Chains 45
34 Gas Cleaning Technologies 47341 Particulate Matter 47342 Tars 52343 Sulfur Compounds 57344 Hydrodesulfurization 59345 Chlorine (Halides) 60346 Alkali 61347 Nitrogen‐containing Compounds 61348 Other Impurities 62
35 Reactive Hot Gas Filter 62References 65
4 Methanation for Synthetic Natural Gas Production ndash Chemical Reaction engineering Aspects 77Tilman J Schildhauer
41 Methanation ndash The Synthesis Step in the Production of Synthetic Natural Gas 77411 Feed Gas Mixtures for Methanation Reactors 79412 Thermodynamic Equilibrium 82413 Methanation Catalysts Kinetics and Reaction Mechanisms 88414 Catalyst Deactivation 97
42 Methanation Reactor Types 107421 Adiabatic Fixed Bed Reactors 109422 Cooled Reactors 117423 Comparison of Methanation Reactor Concepts 129
43 Modeling and Simulation of Methanation Reactors 132431 How to Measure (Intrinsic) Kinetics 133432 Modeling of Fixed Bed Reactors 136433 Modeling of Isothermal Fluidized Bed Reactors 139
44 Conclusions and Open Research Questions 14645 Symbol List 148References 149
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
Synthetic natural GaS from coal Dry BiomaSS anD PoWer-to-GaS aPPlicationS
Synthetic natural GaS from coal Dry BiomaSS anD PoWer-to-GaS aPPlicationS
edited by
tilman J SchilDhauerSerGe ma BiollazPaul Scherrer Institut VilligenSwitzerland
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at (800) 762‐2974 outside the United States at (317) 572‐3993 or fax (317) 572‐4002
Wiley also publishes its books in a variety of electronic formats Some content that appears in print may not be available in electronic formats For more information about Wiley products visit our web site at wwwwileycom
Library of Congress Cataloging‐in‐Publication Data
Names Schildhauer Tilman J editor | Biollaz Serge MA editorTitle Synthetic natural gas from coal dry biomass and power-to-gas applications
[edited by] Tilman J Schildhauer Serge MA BiollazDescription Hoboken New Jersey John Wiley amp Sons 2016 |
Includes bibliographical references and indexIdentifiers LCCN 2016006837 (print) | LCCN 2016014453 (ebook) | ISBN 9781118541814 (cloth) |
ISBN 9781119191254 (pdf) | ISBN 9781119191360 (epub)Subjects LCSH Synthesis gas | Coal gasification | Biomass conversion | Gas manufacture and worksClassification LCC TP360 S96 2016 (print) | LCC TP360 (ebook) | DDC 6602844ndashdc23LC record available at httplccnlocgov2016006837
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
List of Contributors xi
1 Introductory Remarks 1Tilman J Schildhauer
11 Why Produce Synthetic Natural Gas 112 Overview 3
2 Coal and Biomass Gasification for SNG Production 5Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 Introduction ndash Basic Requirements for Gasification in the Framework of SNG Production 5
22 Thermodynamics of Gasification 6221 Gasification Reactions 7222 Overall Gasification Process ndash Equilibrium Based Considerations 7223 Gasification ndash A Multi‐step Process Deviating from Equilibrium 11224 Heat Management of the Gasification Process 13225 Implication of Thermodynamic Considerations for Technology
Choice 1823 Gasification Technologies 18
231 Entrained Flow 19232 Fixed Bed 20233 Direct Fluidized Bed 22234 Indirect Fluidized Bed Gasification 27235 Hydrogasification and Catalytic Gasification 34
References 37
CoNteNtS
vi CONTENTS
3 Gas Cleaning 41Urs Rhyner
31 Introduction 4132 Impurities 42
321 Particulate Matter 42322 Tars 43323 Sulfur Compounds 43324 Halide Compounds 44325 Alkali Compounds 44326 Nitrogen Compounds 44327 Other Impurities 44
33 Cold Warm and Hot Gas Cleaning 45331 Example of B‐IGFC Gas Cleaning Process Chains 45
34 Gas Cleaning Technologies 47341 Particulate Matter 47342 Tars 52343 Sulfur Compounds 57344 Hydrodesulfurization 59345 Chlorine (Halides) 60346 Alkali 61347 Nitrogen‐containing Compounds 61348 Other Impurities 62
35 Reactive Hot Gas Filter 62References 65
4 Methanation for Synthetic Natural Gas Production ndash Chemical Reaction engineering Aspects 77Tilman J Schildhauer
41 Methanation ndash The Synthesis Step in the Production of Synthetic Natural Gas 77411 Feed Gas Mixtures for Methanation Reactors 79412 Thermodynamic Equilibrium 82413 Methanation Catalysts Kinetics and Reaction Mechanisms 88414 Catalyst Deactivation 97
42 Methanation Reactor Types 107421 Adiabatic Fixed Bed Reactors 109422 Cooled Reactors 117423 Comparison of Methanation Reactor Concepts 129
43 Modeling and Simulation of Methanation Reactors 132431 How to Measure (Intrinsic) Kinetics 133432 Modeling of Fixed Bed Reactors 136433 Modeling of Isothermal Fluidized Bed Reactors 139
44 Conclusions and Open Research Questions 14645 Symbol List 148References 149
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
Synthetic natural GaS from coal Dry BiomaSS anD PoWer-to-GaS aPPlicationS
edited by
tilman J SchilDhauerSerGe ma BiollazPaul Scherrer Institut VilligenSwitzerland
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
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Library of Congress Cataloging‐in‐Publication Data
Names Schildhauer Tilman J editor | Biollaz Serge MA editorTitle Synthetic natural gas from coal dry biomass and power-to-gas applications
[edited by] Tilman J Schildhauer Serge MA BiollazDescription Hoboken New Jersey John Wiley amp Sons 2016 |
Includes bibliographical references and indexIdentifiers LCCN 2016006837 (print) | LCCN 2016014453 (ebook) | ISBN 9781118541814 (cloth) |
ISBN 9781119191254 (pdf) | ISBN 9781119191360 (epub)Subjects LCSH Synthesis gas | Coal gasification | Biomass conversion | Gas manufacture and worksClassification LCC TP360 S96 2016 (print) | LCC TP360 (ebook) | DDC 6602844ndashdc23LC record available at httplccnlocgov2016006837
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
List of Contributors xi
1 Introductory Remarks 1Tilman J Schildhauer
11 Why Produce Synthetic Natural Gas 112 Overview 3
2 Coal and Biomass Gasification for SNG Production 5Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 Introduction ndash Basic Requirements for Gasification in the Framework of SNG Production 5
22 Thermodynamics of Gasification 6221 Gasification Reactions 7222 Overall Gasification Process ndash Equilibrium Based Considerations 7223 Gasification ndash A Multi‐step Process Deviating from Equilibrium 11224 Heat Management of the Gasification Process 13225 Implication of Thermodynamic Considerations for Technology
Choice 1823 Gasification Technologies 18
231 Entrained Flow 19232 Fixed Bed 20233 Direct Fluidized Bed 22234 Indirect Fluidized Bed Gasification 27235 Hydrogasification and Catalytic Gasification 34
References 37
CoNteNtS
vi CONTENTS
3 Gas Cleaning 41Urs Rhyner
31 Introduction 4132 Impurities 42
321 Particulate Matter 42322 Tars 43323 Sulfur Compounds 43324 Halide Compounds 44325 Alkali Compounds 44326 Nitrogen Compounds 44327 Other Impurities 44
33 Cold Warm and Hot Gas Cleaning 45331 Example of B‐IGFC Gas Cleaning Process Chains 45
34 Gas Cleaning Technologies 47341 Particulate Matter 47342 Tars 52343 Sulfur Compounds 57344 Hydrodesulfurization 59345 Chlorine (Halides) 60346 Alkali 61347 Nitrogen‐containing Compounds 61348 Other Impurities 62
35 Reactive Hot Gas Filter 62References 65
4 Methanation for Synthetic Natural Gas Production ndash Chemical Reaction engineering Aspects 77Tilman J Schildhauer
41 Methanation ndash The Synthesis Step in the Production of Synthetic Natural Gas 77411 Feed Gas Mixtures for Methanation Reactors 79412 Thermodynamic Equilibrium 82413 Methanation Catalysts Kinetics and Reaction Mechanisms 88414 Catalyst Deactivation 97
42 Methanation Reactor Types 107421 Adiabatic Fixed Bed Reactors 109422 Cooled Reactors 117423 Comparison of Methanation Reactor Concepts 129
43 Modeling and Simulation of Methanation Reactors 132431 How to Measure (Intrinsic) Kinetics 133432 Modeling of Fixed Bed Reactors 136433 Modeling of Isothermal Fluidized Bed Reactors 139
44 Conclusions and Open Research Questions 14645 Symbol List 148References 149
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
Copyright copy 2016 by John Wiley amp Sons Inc All rights reserved
Published by John Wiley amp Sons Inc Hoboken New JerseyPublished simultaneously in Canada
No part of this publication may be reproduced stored in a retrieval system or transmitted in any form or by any means electronic mechanical photocopying recording scanning or otherwise except as permitted under Section 107 or 108 of the 1976 United States Copyright Act without either the prior written permission of the Publisher or authorization through payment of the appropriate per‐copy fee to the Copyright Clearance Center Inc 222 Rosewood Drive Danvers MA 01923 (978) 750‐8400 fax (978) 750‐4470 or on the web at wwwcopyrightcom Requests to the Publisher for permission should be addressed to the Permissions Department John Wiley amp Sons Inc 111 River Street Hoboken NJ 07030 (201) 748‐6011 fax (201) 748‐6008 or online at httpwwwwileycomgopermissions
Limit of LiabilityDisclaimer of Warranty While the publisher and author have used their best efforts in preparing this book they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose No warranty may be created or extended by sales representatives or written sales materials The advice and strategies contained herein may not be suitable for your situation You should consult with a professional where appropriate Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages including but not limited to special incidental consequential or other damages
For general information on our other products and services or for technical support please contact our Customer Care Department within the United States at (800) 762‐2974 outside the United States at (317) 572‐3993 or fax (317) 572‐4002
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Library of Congress Cataloging‐in‐Publication Data
Names Schildhauer Tilman J editor | Biollaz Serge MA editorTitle Synthetic natural gas from coal dry biomass and power-to-gas applications
[edited by] Tilman J Schildhauer Serge MA BiollazDescription Hoboken New Jersey John Wiley amp Sons 2016 |
Includes bibliographical references and indexIdentifiers LCCN 2016006837 (print) | LCCN 2016014453 (ebook) | ISBN 9781118541814 (cloth) |
ISBN 9781119191254 (pdf) | ISBN 9781119191360 (epub)Subjects LCSH Synthesis gas | Coal gasification | Biomass conversion | Gas manufacture and worksClassification LCC TP360 S96 2016 (print) | LCC TP360 (ebook) | DDC 6602844ndashdc23LC record available at httplccnlocgov2016006837
Set in 1012pt Times by SPi Global Pondicherry India
Printed in the United States of America
10 9 8 7 6 5 4 3 2 1
v
List of Contributors xi
1 Introductory Remarks 1Tilman J Schildhauer
11 Why Produce Synthetic Natural Gas 112 Overview 3
2 Coal and Biomass Gasification for SNG Production 5Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 Introduction ndash Basic Requirements for Gasification in the Framework of SNG Production 5
22 Thermodynamics of Gasification 6221 Gasification Reactions 7222 Overall Gasification Process ndash Equilibrium Based Considerations 7223 Gasification ndash A Multi‐step Process Deviating from Equilibrium 11224 Heat Management of the Gasification Process 13225 Implication of Thermodynamic Considerations for Technology
Choice 1823 Gasification Technologies 18
231 Entrained Flow 19232 Fixed Bed 20233 Direct Fluidized Bed 22234 Indirect Fluidized Bed Gasification 27235 Hydrogasification and Catalytic Gasification 34
References 37
CoNteNtS
vi CONTENTS
3 Gas Cleaning 41Urs Rhyner
31 Introduction 4132 Impurities 42
321 Particulate Matter 42322 Tars 43323 Sulfur Compounds 43324 Halide Compounds 44325 Alkali Compounds 44326 Nitrogen Compounds 44327 Other Impurities 44
33 Cold Warm and Hot Gas Cleaning 45331 Example of B‐IGFC Gas Cleaning Process Chains 45
34 Gas Cleaning Technologies 47341 Particulate Matter 47342 Tars 52343 Sulfur Compounds 57344 Hydrodesulfurization 59345 Chlorine (Halides) 60346 Alkali 61347 Nitrogen‐containing Compounds 61348 Other Impurities 62
35 Reactive Hot Gas Filter 62References 65
4 Methanation for Synthetic Natural Gas Production ndash Chemical Reaction engineering Aspects 77Tilman J Schildhauer
41 Methanation ndash The Synthesis Step in the Production of Synthetic Natural Gas 77411 Feed Gas Mixtures for Methanation Reactors 79412 Thermodynamic Equilibrium 82413 Methanation Catalysts Kinetics and Reaction Mechanisms 88414 Catalyst Deactivation 97
42 Methanation Reactor Types 107421 Adiabatic Fixed Bed Reactors 109422 Cooled Reactors 117423 Comparison of Methanation Reactor Concepts 129
43 Modeling and Simulation of Methanation Reactors 132431 How to Measure (Intrinsic) Kinetics 133432 Modeling of Fixed Bed Reactors 136433 Modeling of Isothermal Fluidized Bed Reactors 139
44 Conclusions and Open Research Questions 14645 Symbol List 148References 149
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
v
List of Contributors xi
1 Introductory Remarks 1Tilman J Schildhauer
11 Why Produce Synthetic Natural Gas 112 Overview 3
2 Coal and Biomass Gasification for SNG Production 5Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 Introduction ndash Basic Requirements for Gasification in the Framework of SNG Production 5
22 Thermodynamics of Gasification 6221 Gasification Reactions 7222 Overall Gasification Process ndash Equilibrium Based Considerations 7223 Gasification ndash A Multi‐step Process Deviating from Equilibrium 11224 Heat Management of the Gasification Process 13225 Implication of Thermodynamic Considerations for Technology
Choice 1823 Gasification Technologies 18
231 Entrained Flow 19232 Fixed Bed 20233 Direct Fluidized Bed 22234 Indirect Fluidized Bed Gasification 27235 Hydrogasification and Catalytic Gasification 34
References 37
CoNteNtS
vi CONTENTS
3 Gas Cleaning 41Urs Rhyner
31 Introduction 4132 Impurities 42
321 Particulate Matter 42322 Tars 43323 Sulfur Compounds 43324 Halide Compounds 44325 Alkali Compounds 44326 Nitrogen Compounds 44327 Other Impurities 44
33 Cold Warm and Hot Gas Cleaning 45331 Example of B‐IGFC Gas Cleaning Process Chains 45
34 Gas Cleaning Technologies 47341 Particulate Matter 47342 Tars 52343 Sulfur Compounds 57344 Hydrodesulfurization 59345 Chlorine (Halides) 60346 Alkali 61347 Nitrogen‐containing Compounds 61348 Other Impurities 62
35 Reactive Hot Gas Filter 62References 65
4 Methanation for Synthetic Natural Gas Production ndash Chemical Reaction engineering Aspects 77Tilman J Schildhauer
41 Methanation ndash The Synthesis Step in the Production of Synthetic Natural Gas 77411 Feed Gas Mixtures for Methanation Reactors 79412 Thermodynamic Equilibrium 82413 Methanation Catalysts Kinetics and Reaction Mechanisms 88414 Catalyst Deactivation 97
42 Methanation Reactor Types 107421 Adiabatic Fixed Bed Reactors 109422 Cooled Reactors 117423 Comparison of Methanation Reactor Concepts 129
43 Modeling and Simulation of Methanation Reactors 132431 How to Measure (Intrinsic) Kinetics 133432 Modeling of Fixed Bed Reactors 136433 Modeling of Isothermal Fluidized Bed Reactors 139
44 Conclusions and Open Research Questions 14645 Symbol List 148References 149
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
vi CONTENTS
3 Gas Cleaning 41Urs Rhyner
31 Introduction 4132 Impurities 42
321 Particulate Matter 42322 Tars 43323 Sulfur Compounds 43324 Halide Compounds 44325 Alkali Compounds 44326 Nitrogen Compounds 44327 Other Impurities 44
33 Cold Warm and Hot Gas Cleaning 45331 Example of B‐IGFC Gas Cleaning Process Chains 45
34 Gas Cleaning Technologies 47341 Particulate Matter 47342 Tars 52343 Sulfur Compounds 57344 Hydrodesulfurization 59345 Chlorine (Halides) 60346 Alkali 61347 Nitrogen‐containing Compounds 61348 Other Impurities 62
35 Reactive Hot Gas Filter 62References 65
4 Methanation for Synthetic Natural Gas Production ndash Chemical Reaction engineering Aspects 77Tilman J Schildhauer
41 Methanation ndash The Synthesis Step in the Production of Synthetic Natural Gas 77411 Feed Gas Mixtures for Methanation Reactors 79412 Thermodynamic Equilibrium 82413 Methanation Catalysts Kinetics and Reaction Mechanisms 88414 Catalyst Deactivation 97
42 Methanation Reactor Types 107421 Adiabatic Fixed Bed Reactors 109422 Cooled Reactors 117423 Comparison of Methanation Reactor Concepts 129
43 Modeling and Simulation of Methanation Reactors 132431 How to Measure (Intrinsic) Kinetics 133432 Modeling of Fixed Bed Reactors 136433 Modeling of Isothermal Fluidized Bed Reactors 139
44 Conclusions and Open Research Questions 14645 Symbol List 148References 149
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
CONTENTS vii
5 SNG Upgrading 161Renato Baciocchi Giulia Costa and Lidia Lombardi
51 Introduction 16152 Separation Processes for SNG Upgrading 163
521 Bulk CO2CH
4 Separation 163
522 Removal of other Compounds and Impurities 16953 Techno‐Economical Comparison of Selected Separation Options 174References 176
6 SNG from Wood ndash the GoBiGas Project 181Joumlrgen Held
61 Biomethane in Sweden 18162 Conditions and Background for the GoBiGas Project in Gothenburg 18463 Technical Description 18564 Technical Issues and Lessons Learned 18865 Status 18866 Efficiency 18867 Economics 18868 Outlook 189Acknowledgements 189References 189
7 the Power to Gas Process Storage of Renewable energy in the Natural Gas Grid via Fixed Bed Methanation of Co2H2 191Michael Specht Jochen Brellochs Volkmar Frick Bernd Stuumlrmer and Ulrich Zuberbuumlhler
71 Motivation 191711 History ldquoRenewable Fuel Paths at ZSWrdquo 191712 Goal ldquoEnergiewenderdquo 192713 Goal ldquoPower Based Carbon Based Fuelsrdquo 192714 Goal ldquoP2Gregrdquo 192715 Goal ldquoMethanationrdquo 193
72 The Power to Fuel Concept Co‐utilization of (Biogenic) Carbon and Hydrogen 193
73 P2Greg Technology 196731 Methanation Characteristics for CO
2 Based Syngas 197
732 P2Greg Plant Layout of 25 kWel 250 kW
el and 6000 kW
el Plants 202
74 Experimental Results 206741 Methanation Catalysts Screening Cycle Resistance
Contamination by Sulfur Components 206742 Results with the 25 kW
el P2Greg Plant 209
743 Results with the 250 kWel P2Greg Plant 210
744 Results with the 250 kWel P2Greg Plant in Combination
with Membrane Gas Upgrade 21375 P2Greg Process Efficiency 214
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
viii CONTENTS
76 Conclusion and Outlook 217Acknowledgements 219References 219
8 Fluidized Bed Methanation for SNG Production ndash Process Development at the Paul‐Scherrer Institut 221Tilman J Schildhauer and Serge MA Biollaz
81 Introduction to Process Development 22182 Methane from Wood ndash Process Development at PSI 223References 229
9 MILeNA Indirect Gasification oLGA tar Removal and eCN Process for Methanation 231Luc PLM Rabou Bram Van der Drift Eric HAJ Van Dijk Christiaan M Van der Meijden and Berend J Vreugdenhil
91 Introduction 23192 Main Process Steps 233
921 MILENA Indirect Gasification 233922 OLGA Tar Removal 236923 HDS and Deep S Removal 237924 Reformer 238925 CO
2 Removal 239
926 Methanation and Upgrading 23993 Process Efficiency and Economy 24094 Results and Status 241
941 MILENA 241942 OLGA 242943 HDS Reformer and Methanation 243
95 Outlook 245951 Pressure 245952 Co‐production 245953 Bio Carbon Capture and Storage 246954 Power to Gas 246
Acknowledgements 246References 247
10 Hydrothermal Production of SNG from Wet Biomass 249Freacutedeacuteric Vogel
101 Introduction 249102 Historical Development 252103 Physical and Chemical Bases 253
1031 Catalysis 2541032 Phase Behavior and Salt Separation 2591033 Liquefaction of the Solid Biomass Tar and Coke Formation 263
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
CONTENTS ix
104 PSIrsquos Catalytic SNG Process 2661041 Process Description and Layout 2661042 Mass Balance 2681043 Energy Balance 2691044 Status of Process Development at PSI 2691045 Comparison to other SNG Processes 271
105 Open Questions and Outlook 273References 274
11 Agnionrsquos Small Scale SNG Concept 279Thomas Kienberger and Christian Zuber
References 291
12 Integrated Desulfurization and Methanation Concepts for SNG Production 293Christian FJ Koumlnig Maarten Nachtegaal and Tilman J Schildhauer
121 Introduction 293122 Concepts for Integrated Desulfurization and Methanation 295
1221 Sulfur‐Resistant Methanation 2951222 Regeneration of Methanation Catalysts 2971223 Discussion of the Concepts 300
123 Required Future Research 3011231 Sulfur Resistant Methanation 3011232 Periodic Regeneration 302
References 303
Index 307
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
List of Contributors
renato baciocchi University of Rome Tor Vergata Roma Italy
serge MA biollaz Paul Scherrer Institut Villigen Switzerland
Jochen brellochs Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Giulia Costa University of Rome Tor Vergata Roma Italy
Volkmar frick Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Joumlrgen Held Renewable Energy Technology International AB Lund Sweden
stefan Heyne Chalmers University of Technology Goumlteborg Sweden
thomas Kienberger Montanuniversitaumlt Leoben Leoben Austria
Christian fJ Koumlnig Paul Scherrer Institut Villigen Switzerland
Lidia Lombardi Niccolograve Cusano University Roma Italy
Maarten nachtegaal Paul Scherrer Institut Villigen Switzerland
Luc PLM rabou Energieonderzoek Centrum Nederland Petten The Netherlands
urs rhyner AGRO Energie Schwyz Schwyz Switzerland
tilman J schildhauer Paul Scherrer Institut Villigen Switzerland
Martin seemann Chalmers University of Technology Goumlteborg Sweden
xi
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
xii LIST Of CONTRIBUTORS
Michael specht Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
bernd stuumlrmer Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Eric HAJ Van Dijk Energieonderzoek Centrum Nederland Petten The Netherlands
bram Van der Drift Energieonderzoek Centrum Nederland Petten The Netherlands
Christiaan M Van der Meijden Energieonderzoek Centrum Nederland Petten The Netherlands
freacutedeacuteric Vogel Paul Scherrer Institut Villigen Switzerland
berend J Vreugdenhil Energieonderzoek Centrum Nederland Petten The Netherlands
Christian Zuber Agnion Highterm Research GesmbH Graz Austria
ulrich Zuberbuumlhler Center for Solar Energy and Hydrogen Research (ZSW) Stuttgart Germany
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
1
1Introductory remarks
Tilman J Schildhauer
11 Why produce synthetIc natural gas
The answer to this question [which may also explain why one should read a book on synthetic natural gas (SNG) production] changes with time
During the years from 1950 to the early 1980s SNG production was an important topic mainly in the United States in the United Kingdom and in Germany The interest was caused by a couple of reasons In these countries a relative abundance of coal and the expected shortage of natural gas triggered several industrial initia-tives partly funded by public authorities to develop processes from coal to SNG Due to the oil crisis during the 1970s the use of domestic coal rather than the import of oil became a second motivation A third motivation is the possibility to make domestic (low quality) energy reserves available de‐centrally as an energy carrier for which the distribution infrastructure already exists and that allows for both clean and efficient use by consumers
These boundary conditions lead in 1984 to the start‐up of the 15 GWSNG
plant in Great Plains which is run by the Dakota Gas Company and converts lignite into SNG and many other products This plant stayed the only commercial SNG production for nearly 30 years because with the drop of the oil price in the mid1980s the explora-tion of natural gas in the North Sea and the gas pipelines between Russia and Europe the interest in SNG from coal ceased
Especially in the United States the interest came back in the years after the turn of the millennium now triggered by the again rising oil price and the meanwhile established use of CO
2 (which is an inherent by‐product of coal to SNG plants) for
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
2 INTRODUCTORy REmARKS
enhanced oil recovery (EOR) Back then a dozen coal to SNG projects were started including EOR Now due to the rapidly increasing exploitation of the shale gas and the connected possibility for a significant reduction of CO
2 emission all the projects
in the United States have been stoppedHowever all the mentioned motivations for SNG production that is shortage of
domestic natural gas use of domestic coal reserves which are far away from the highly populated areas and the possibility for clean and efficient combustion still prevail in China Therefore China is now by far the most important market for the production of SNG from coal Three large plants have started operation and further plants are planned or under construction
In Europe several aspects triggered a reconsideration of SNG production about 15 years ago Due to its cleaner combustion and inherently lower CO
2 emission
using natural gas in transportation (eg for CNG cars) is supported in many coun-tries and has even been economically beneficial for the past few years due to the lower gas price With the aim of the European Commission to replace up to 20 of European fuel consumption by biofuel replacing natural gas partly with bio‐methane becomes necessary So far bio‐methane is mostly produced by up‐grading biogas from anaerobic digestion However due to the limited amount of substrate this pathway cannot be increased much more and other sources of bio‐methane are sought
Additionally many European countries wish to use their domestic biomass resources for energy production in order to decrease CO
2 emissions and the import
of energy A major part of the biomass is ligno‐cellulosic (mostly wood) and mainly used for heating for example in wood pellet heating As the heat demand is gen-erally decreasing due to better building insulation the conversion of wood to high value forms of energy that is electricity and fuels is of increasing interest Like in the case of coal conversion to fuels requires (so far) gasification as the first step As shown by process simulations and the first demonstration plants the conversion of wood to SNG can reach significantly higher efficiencies than conversion to liquid fuels
Very recently a third aspect began to gain greater importance especially in Central Europe Due to the increasing integration of stochastic renewable sources like photovoltaics and wind energy into electricity generation the demand for balancing the electricity supply and the demand over spatial and temporal distances is increasing For the future even the seasonal storage of electricity may be necessary Here the production of SNG can play an important role While the gasification of solid feedstocks is a more or less continuous process the further conversion to electricity or SNG can be flexibly adjusted to the balancing needs of the electricity grid within so‐called polygeneration schemes
moreover in times where the electricity production from renewables exceeds the actual demand in the electricity grid (a situation that today occasionally is observed in Central Europe and is expected to be more common in future) producing SNG could utilize the excess electricity instead of curtailing photovoltaics or wind tur-bines In so‐called power to gas applications hydrogen is produced from excess electricity by electrolysis of water and then converted to SNG by methanation of
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
OVERVIEW 3
carbon oxides As a source of carbon oxides biogas producer gas from (biomass) gasification flue gas from industry or even CO
2 from the atmosphere can be consid-
ered opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure
12 overvIeW
This book aims at a suitable overview over the different pathways to produce SNG (Figure 11)
The first four chapters cover the main process steps during conversion of coal and dry biomass to SNG gasification gas cleaning methanation and gas upgrading The main technology options will be highlighted and the impact of a technology choice for downstream processes and the complete process chain In these chapters espe-cially in the chapter on methanation reactors the state of the art coal to SNG processes are discussed in detail
The following chapters describe a number of novel processes for the production of SNG with their specific combination of process steps as well as the boundary con-ditions for which the respective process was developed These processes comprise those which are already in operation (eg the 20 mW
SNG bio‐SNG production in
Gothenburg Sweden or the 6 mWSNG
power to gas plant in Werlte Germany) and processes which are still under development
The gasification chapter covers the thermodynamics of gasification and presents both coal and biomass gasification technologies
Coal Dry biomass
gasification
Gas cleaning
Methanation
Methanation
SNG (CH4)
CO2 from air or industry
H2O CO2(H2)
H2 from electrolysis(power-to-gas)
Algae manure
Hydrothermalgasification
Biogas fromdigestion
Raw SNG CH4 H2O (CO2 H2)
Gas upgrading
FIgure 11 The different pathways to produce SNG
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
4 INTRODUCTORy REmARKS
The gas cleaning chapter discusses the impurities to be expected in gasification‐derived producer gas explains the state of the art gas cleaning technologies and focuses on the innovative gas cleaning steps which are developed for hot gas cleaning
The chapter on methanation reactors presents the chemical reactions proceeding inside the reactors their thermodynamic limitation and their reaction mechanisms Further an overview of the different reactor types with their advantages and chal-lenges is given covering coal to SNG biomass to SNG and power to gas processes The last section of this chapter focuses on the modeling and simulation of methana-tion reactors including the necessary experiments to determine reaction kinetics and to generate data for model validation
The chapter on gas‐upgrading discusses technologies for gas drying CO2 and
hydrogen removal based on adsorption absorption and membranes and includes a techno‐economic comparison
The chapter on the GoBiGas project (ldquoGothenburg Bio Gasrdquo) presents the boundary conditions and technologies applied in the 20 mW
SNG wood to SNG plant
in Gothenburg Sweden which was commissioned in 2014The next chapter explains the development of the power to gas process at the
Zentrum fuumlr solare Wasserstofferzeugung (ZSW) including the 6 mWSNG
plant in Werlte Germany
The chapter on fluidised bed methanation describes the process development at the Paul Scherrer Institut aiming at a flexible technology for efficiently converting wood to SNG and for hydrogen conversion within power to gas applications
The following chapter presents the technologies developed at the Energy Center of the Netherlands (ECN) for efficient SNG production from wood especially their allothermal gasification technology (mILENA) and their broad experience with gas cleaning
The chapter on hydrothermal gasification discusses the unique technology allow-ing for the simultaneous catalytic gasification and methanation of wet biomass under super‐critical conditions
The chapter on agnionrsquos small scale SNG concept focuses on two novel technol-ogies that allow for significant process simplification especially in small scale bio‐SNG plants the pressurized heatpipe reformer and the polytropic fixed bed methanation
The last chapter offers a view on the research for even more simplified SNG processes that is for methanation steps that allow for integrated desulfurization and methanation
The author of these lines wishes to express his gratitude especially to the contrib-utors of this book and to the persons at the publisher for their excellent work but also to all colleagues scientific collaborators partners friends and scientists in the community for many fruitful and interesting discussions All of you bring the field forward and made this book possible
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
Synthetic Natural Gas from Coal Dry Biomass and Power-to-Gas Applications First EditionEdited by Tilman J Schildhauer and Serge MA Biollazcopy 2016 John Wiley amp Sons Inc Published 2016 by John Wiley amp Sons Inc
5
2Coal and Biomass GasifiCation for snG ProduCtion
Stefan Heyne Martin Seemann and Tilman J Schildhauer
21 introduCtion ndash BasiC requirements for GasifiCation in the framework of snG ProduCtion
Within the production of synthetic natural gas ndash basically methane ndash from solid feed-stock such as coal or biomass the major conversion step is gasification generating a product gas containing a mixture of permanent and condensable gases as well as solid residues (eg char ash) The gasification step can be conducted in different atmospheres and using different reaction agents Figure 21 represents the basic pathway from solid fuel to methane considering the main elements carbon hydrogen and oxygen It is obvious that an increase in hydrogen content is necessary for all feedstock illustrated At the same time the oxygen content needs to be reduced in particular for biogenic feedstock that is oxygenated to a higher degree
There exist different strategies or pathways for performing the conversion from feedstock towards methane within gasification as illustrated in Figure 21 Adding steam as a gasification agent is common practice not only due to the stoichiometric effect but also for enhanced char gasification and temperature moderation within the reactor H
2 addition is used in hydrogasification leading to a higher initial methane
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
6 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
content in the product gas [3 4] Co2 removal is an intrinsic part of the SnG produc-
tion process some gasification concepts using adsorptive bed material for direct Co2
removal within the gasification reactor [5] The addition of oxygen (common practice for all direct gasification technologies) actually leads to an increased need for Co
2
removal downstream of the reactor For indirect gasification where ungasified char is combusted in a separate chamber for heat supply the composition is changed towards methane via path (c) in Figure 21 Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand
22 thermodynamiCs of GasifiCation
For an increased understanding of the role of gasification for the overall SnG process the basic thermodynamic aspects within gasification are discussed in the following The gasification process is a series of different conversions involving both homoge-neous and heterogeneous reactions The basic steps from solid fuel to product gas are drying pyrolysis or devolatilization and gasification depending on the physical size of the fuel these different steps occur in a sequential order for small particles or
40
20
0
60
80
0 20 40 60 80
0
20
40
60
80
Oxygen
Carbon
Hyd
roge
n
Feedstock
CH4
H2O
CO2
O2
H2
Carbon1 Biomass (hardwood softwood straw)2 Lignite coal3 Bituminous coal4 Anthracite coal
3
4
1
2
a removing CO2b adding H2c removing char (C)d adding steame adding O2
a
b d
e
c
fiGure 21 CHo diagram for coal and biomass Feedstock composition based on [1 2] data from Higman 2008 [1] Phyllis2 database for biomass and waste httpswwwecnnlphyllis2 Energy research Centre of the netherlands
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
THErModYnAMICS oF GASIFICATIon 7
overlap in bigger particles The detailed description of the complete process is a com-plex task and the considerations made in the following therefore focus on specific parts of the process and their implications for the overall conversion starting with some stoichiometric aspects for gasification
221 Gasification reactions
The major reactions occurring during the gasification step that commonly are consid-ered relevant are
C s o g Co g kJ mol partial oxidation0 5 1112 (21)
Co g o g Co g kJ mol carbon monoxide combustion0 5 2832 2 (22)
C s o g Co g kJ mol carbon combustion2 2 394ndash (23)
C s Co g Co g kJ mol reverse Boudouard reaction2 2 172 (24)
C s H o g Co g H g kJ mol water gas reaction2 2 131 (25)
Co g H o g H g Co g kJ mol water gas shift reaction2 2 2 41ndash (26)
CH g H o g H g Co g kJ mol steam reforming4 2 23 206 (27)
The char gasification reactions converting carbon into gaseous fuels [Equations (24) and (25)] are endothermic reactions requiring heat supply that is realized either by combusting part of the fuel in the same reactor (direct or autothermal gasification) or by indirect heat supply from an external combustion or heat source (indirect or allo-thermal gasification)
222 overall Gasification Process ndash equilibrium Based Considerations
Considering the overall process from coal or biomass to methane at the example of steam gasification the reaction stoichiometry can be expressed as
C o H o CH Cox y zH a b c2 4 2 (28)
with a xy z
bx y z
cx y z
4 2 2 8 4 2 8 4
Table 21 gives the coefficients for steam gasification to methane [Equation (28)] for different coal and biomass feedstock materials allowing the calculation of the heat
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
ta
Bl
e 2
1
Com
posi
tion
and
ove
rall
rea
ctio
n d
ata
for
stea
m G
asif
icat
ion
for
dif
fere
nt f
eeds
tock
mat
eria
ls
Feed
stoc
kM
olar
C
ompo
sitio
nl
HV
[M
Jkg
daf
]H
HV
[M
Jkg
daf
]c
rea
ctio
n C
oeff
icie
nts
for
Equ
atio
n (2
8)
ΔH
rM
etha
ne Y
ield
ab
c[M
Jkg
daf
Fe
edst
ock]
[kg
CH
4kg
daf
Fe
edst
ock]
Coa
laB
row
n co
al ndash
rhe
in
Ger
man
yC
H0
88o
029
262
273
063
20
537
046
3ndash0
19
048
9
lig
nite
ndash n
dak
ota
uSA
CH
072
o0
2526
727
70
697
052
90
471
06
050
9B
itum
inou
s ndash
typi
cal
Sout
h A
fric
aC
H0
68o
008
3435
10
792
056
70
433
12
10
654
Ant
hrac
ite ndash
ruh
r G
erm
any
CH
047
o0
0236
237
00
873
055
30
447
14
60
693
Bio
mas
sbW
illow
woo
d ndash
hard
woo
dC
H1
46o
065
185
199
031
00
520
048
0ndash0
45
035
0B
eech
woo
d ndash
hard
woo
dC
H1
47o
069
179
192
028
60
511
048
9ndash0
71
033
3Fi
r ndash
soft
woo
dC
H1
45o
065
196
210
031
30
520
048
0ndash1
58
035
0Sp
ruce
ndash s
oftw
ood
CH
142
o0
6818
419
70
304
050
80
492
ndash11
70
335
Whe
at s
traw
CH
146
o0
6818
319
60
297
051
20
488
ndash08
40
338
ric
e st
raw
CH
143
o0
6817
518
80
303
050
80
492
ndash02
30
335
a Tak
en f
rom
Hig
man
and
van
der
Bur
gt [
1]
b Tak
en f
rom
Phy
llis
[2]
ndash av
erag
e da
ta f
or m
ater
ial g
roup
c H
HV
[M
Jkg
daf
] =
lH
V [
MJ
kg d
af]
+ 2
44
middot 89
4 middot H
[w
t d
af]
100
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
THErModYnAMICS oF GASIFICATIon 9
of reaction (based on the HHV of feedstock and methane all reactants and products at 25 degC) as well as the stoichiometric methane yield per kilogram of feedstock The heat of reaction for the overall reaction corresponds to well below 10 for all feed-stock materials For coal based feedstock with low oxygen content the reaction is endothermic while for brown coal and biomass feedstock it is exothermic The methane yield per kilogram feedstock also is considerably lower for biomass based feedstock due to the high oxygen content one option to improve the methane yield would be the additional supply of hydrogen as for example considered in hydrogas-ification The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (28) is gasification under supercritical conditions so‐called hydrothermal gasification [6ndash8] This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter Common gasifica-tion technology converts the feedstock to a product gas being a mixture of Co Co
2
H2 H
2o CH
4 light and higher hydrocarbons and trace components followed by a
downstream gas cleaning and methane synthesis stepThe major operating parameters for gasification are pressure and temperature
Equilibrium calculations for steam gasification (05 kg H2okg daf feedstock) for a
generic biomass are used to illustrate the basic trends for the product gas composition and heat demand with changing pressure and temperature All feedstock is assumed to be converted to product gas As can be observed from Figure 22 methane formation is favored by lower temperatures and higher pressures Hydrogen and carbon monoxide formation increases with temperature and so does the endothermi-city of the overall reaction The theoretically exothermic reaction to CH
4 and Co
2 at
25 degC [similar to Equation (28)] turns into an endothermic reaction requiring heat supply at higher temperature
light hydrocarbons (represented by C2H
4) and tars (represented by C
10H
8) are only
formed to a very small extent according to equilibrium calculations The amount of steam added for gasification will mainly influence the H
2Co ratio via the water gas
shift reaction Equation (26) In reality the equilibrium state is usually not reached in the shown temperature range but the actual product gas composition resulting from gasification is influenced by a number of other parameters as will be discussed in subsequent sections An increase in gasification pressure favors methane formation predicted by equilibrium calculations at 800 degC the methane molar fraction increases from 0 to about 155 from 1 to 30 bar With more methane being formed the endo-thermic heat of reaction is reduced by 666 from 1 to 30 bar Again no to very little formation of light hydrocarbons and tars is predicted by the equilibrium even at higher pressures At high pressures and moderate temperatures a mixture of basically CH
4 Co
2 and H
2o ndash representing Equation (28) ndash can be obtained A process
example is hydrothermal gasification which is operating at these conditions but still is at development state [6ndash8] The above‐mentioned trends are all under the assumption of complete conversion of feedstock to product gas Many gasification concepts how-ever have a considerable amount of char remaining unconverted being removed with the ashes or in indirect gasification being converted in a separate combustion chamber for supplying the gasification heat The carbon feedstock entering the gas phase
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
10 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
during gasification will therefore be drastically changed when carbon conversion is incomplete Even minor effects on the hydrogen and oxygen balance in the gas phase can be expected as for example biomass char still contains oxygen and hydrogen [9] Figure 23 depicts the influence of pressure and temperature on carbon conversion at temperatures below 800 degC considerable amounts of solid carbon formation are pre-dicted This will in turn influence the equilibrium conditions in the gas phase as the carbon stock in gas phase is reduced The major reactions affected are the Boudouard water gas and waterndashgas shift reactions Equations (24) to (26) Incomplete carbon conversion leads to a decrease of Co concentration in the product gas as well as a decrease in H
2 compared to equilibrium at complete conversion
200 400 600 800 1000 1200
0
10
20
30minus2000
0
2000
4000
6000
ΔH
r [kJ
kg
daf
feed
]
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
300
01020304
y CH
4y H
2y H
2O
y CO
2y C
O
200 400 600800 1000 1200
0
10
20
200 400 600800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
200 400 600 800 1000 1200
0
10
20
300
01020304
300
01020304
T [degC] T [degC]
T [degC]T [degC]
T [degC] T [degC]
P [bar]
P [bar]
P [bar] P [bar]
P [bar]
P [bar]
fiGure 22 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (SB = 05 kg H
2okg daf) of a generic
biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH143
o066
) assuming complete carbon conversion calculated by ASPEn PluS
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
THErModYnAMICS oF GASIFICATIon 11
223 Gasification ndash a multi‐step Process deviating from equilibrium
Equilibrium calculations while useful for identifying trends with changing operating conditions cannot however predict the performance of technical equipment to a full extent They represent a boundary value that can be approached but never reached The gasification process on a physical level is a multi‐stage process starting with drying of the feedstock followed by pyrolysis and gasificationcombustion The kinetics of the numerous homogeneous and heterogeneous reactions occurring ndash as well as the residence time and reactor setup ndash ultimately determine the product gas composition resulting from gasification Figure 24 illustrates a simplified reaction network for the conversion from received fuel to product gas The drying and pri-mary pyrolysis (also referred to as devolatilization) steps are similar for all gasifica-tion technologies whereas the extent of gasification reactions occurring as well as the approach to equilibrium are a strong function of the gasification medium and the reactor setup during pyrolysis a considerable amount of tars a complex mixture of 1‐ to 5‐ring aromatic hydrocarbons is formed that will undergo various conversion pathways during gasification In the final product gas tar can still represent a consid-erable amount of energy for example biomass steam gasification at 800 degC can result in more than 33 g tarsnm3 corresponding to about 8 of the total product gas energy content on a lower heating value basis [10]
200400
600800
10001200
0
10
20
30
04
02
0
06
08
1
T [degC]
P [bar]
Am
ount
of
feed
stoc
k ca
rbon
conv
erte
d to
gas
pha
se
fiGure 23 Carbon conversion predicted by equilibrium calculations for steam gasifica-tion (SB = 05 kg H
2okg daf) of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt
CH143
o066
)
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
12 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
The gas composition from primary pyrolysis represents the starting point for the gasification reactions neves et al [9] conducted an extensive literature review of data on pyrolysis experiments and derived an empirical model for estimating gas species yields as well as tar and char yields and elemental composition as a function of peak pyrolysis temperature Figure 25 illustrates the pyrolysis gas composition and the total gas and char yield based on nevesrsquo model In contrast to the equilibrium calculations for gasification represented in Figure 22 the model predicts consider-able amounts of tars as well as a considerable amount of char produced from pyrol-ysis Even light hydrocarbons are present in the pyrolysis gas Generally speaking the product distribution from pyrolysis as presented in Figure 25 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 24 ndash the gasification step
The extent of conversion towards the equilibrium state is a function of a large number of parameters such as pressure and temperature reactor design and associated residence time for gas and solids as well as gasndashsolid mixing and the presence of catalytically active materials promoting specific reactions among others
Secondary PyrolysisGasication
Water
Char
Permanentgases
Tars
DehydrationPolymerizationGasication
ReformingCrackingOxidationWater-gas shift
dry fuel
Primary Pyrolysis
Water
Char
Permanentgases
Tars
Drying
As-receivedfuel particle Moisture
Dry fuel
Gasication medium(H2O H2 O2 CO2
)
Productgas
fiGure 24 Conversion process of a fuel particle during gasification (adopted and modi-fied from neves et al [9])
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
THErModYnAMICS oF GASIFICATIon 13
224 heat management of the Gasification Process
As temperature is the major influencing parameter on the kinetics of the different gasification reactions the thermal management of the gasification reactor is of particular importance The conversion steps from solid fuel to product gas as illus-trated in Figure 24 occur at different temperature levels A qualitative representation
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
Temperature (ordmC)
Tars (C10H8)
CxHy
CH4
CO
CO2
H2O
H2
Mol
ar f
ract
ion
of c
ompo
nent
i
(a)
Temperature (ordmC)
Yie
ld (
kgk
g da
f fu
el)
300 400 500 600 700 800 9000
01
02
03
04
05
06
07
08
09
1
Char yield
Total yield per kg daf fuelGases tars and pyrolytic water
(b)
fiGure 25 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on neves [9]) for a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066)
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
14 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of the temperature profile for a fuel particle over time is illustrated in Figure 26a After particle heat‐up the moisture is evaporated The dry fuel particle is further heated releasing pyrolysis gases and finally the char particle is gasified Heat for gasification needs to be supplied by the hot environment Particle combustion indicated by the dashed line occurs at a particle temperature above environ-ment due to the exothermic nature of the combustion reactions For complete conversion of a biomass fuel with initial moisture content of 20 wt the distribution of the heat demand for conversion on the different processes is illustrated in Figure 26b The gasification heat demand is dominant but even pyrolysis and drying
Dry
ing
Time
(a)
(b)
Tem
pera
ture
Dev
olat
iliza
tion
Pyro
lysi
s Char gasication
Char combustion
Surrounding temperature
a dcb
a ndash Preheating ndash 40b ndash Drying ndash 82
c ndash Pyrolysis ndash 76d ndash Gasication ndash 802
Fraction of total heat demand(6566 kJkg daf fuel)
0
100
200
300
400
500
600
700
800
900
0 1000 2000 3000 4000 5000 6000 7000
Tem
pera
ture
[ordmC
]
Heat demand [kJkg daf fuel]
Gasication temperature (850 degC)
Pyrolysistemperature
(450 degC)
fiGure 26 (a) Qualitative representation of the temperature evolution of a fuel particle during gasification (b) Steam gasification heat demand profile for full conversion of a generic biomass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) at equilibrium (SB ratio = 05
steam supply at 400 degC 20 wt initial biomass moisture)
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
THErModYnAMICS oF GASIFICATIon 15
represent a considerable share of the specific heat demand for conversion of course in reality pyrolysis and gasification rather occur over a certain temperature range instead of the fixed temperatures used for the calculation (nevesrsquo model for pyrolysis [9] and equilibrium calculations for the gasification) In addition the processes are not strictly sequential but partly occur in parallel within a gasification reactor nevertheless the gasification heat demand will be largest and above all requires the highest temperature level
All heat for the conversion process within the gasification unit needs to be supplied by combustion of part of the fuel or additional external fuel at high temperature Changing the initial moisture content of the biomass will reduce the drying heat demand [b in Figure 26b] and therefore improve the conversion efficiency External drying also allows for using a heat source at lower temperature improving the conversion process from an exergy perspective Even the pyrolysis process can be conducted in a separate reactor with a separate heat source For these considerations in relation to the thermal management of the gasification process temperaturendashheat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process Figure 27 presents such a graph as an example of an indirect gasification process modelled using nevesrsquo pyrolysis model and gasification at equilibrium It is assumed that the amount of char combusted is set to ensure that the combustion heat covers the heat demand for fuel heat‐up drying pyrolysis and gasification The supply of steam to gasification (400 degC) and hot air to combustion (400 degC) is an additional heat demand that needs to be covered The thick curve in Figure 27 represents the aggregation of all the above‐mentioned heat demands whereas the dashed curve is a representation of all heat sources namely the combustion
0
100
200
300
400
500
600
700
800
900
1000
0 2000 4000 6000 8000 10000 12000
Tem
pera
ture
[ordmC
]
Heat load [kJkg daf fuel]
Preheating biomass air and water (steam)
Moisture evaporationand steam generation
Pyrolysis andpreheating air and steam
Gasific
ation
Tpyrolysis = 450 ordmC
Tgasification = 850 ordmC
Tcombustion = 900 ordmCChar combustion
Gas
coo
ling
(pro
duct
gas
and
com
busti
on fl
ue g
as)Maximum amount
of excess heat3610 kJkg daf
fiGure 27 Temperature heat load curve for indirect steam gasification of a generic bio-mass (C ndash 50 wt H ndash 6 wt o ndash 44 wt CH
143o
066) SB ratio = 05 air and steam supply
at 25 degC and heated to 400 degC 20 wt initial biomass moisture
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal
16 CoAl And BIoMASS GASIFICATIon For SnG ProduCTIon
of char and the cooling of hot product gas and combustion flue gases to ambient tem-perature It is obvious that for ideal heat transfer the process has a considerable amount of excess heat allowing for a further increase in conversion to product gas by more efficient use of the heat and reducing the char combustion Converting more solid fuel to product gas will increase the heat demand while at the same time the heat supply will decrease as less char is burnt Even considering the overall SnG process these curves can be used for a holistic analysis of the heat integration of sinks and sources including the operations up‐ and downstream of the gasification step Heat from the methanation reaction might for example be used for biomass drying or for regeneration of an amine solution used for downstream Co
2 removal
The slope of the heat demand curves for pyrolysis and gasification also is dependent on the gas composition that actually is obtained during the different processes A deviation from equilibrium conditions for the gasification will result in considerable changes of the conversion heat demand Two parameters commonly used in thermal conversion processes are used to illustrate the influence of different parameters on the energy performance of the gasification process The first param-eter is the relative air to fuel ratio λ defining the amount of air (oxygen) actually supplied to the reaction in relation to the amount necessary for complete combustion according to stoichiometry
n
nair o actual
air o stoichiometric
2
2
(27)
The second parameter commonly used in gasification is the chemical efficiency ηch
relating the chemical energy content of the product gas to the fuel chemical energy η
ch can be defined on both a lower and a higher heating value basis but in order to
avoid confusion with respect to moisture content the higher heating value is used here
ch HHV
PG
fuel fuel
HHV
HHV
i
i i
n
n (28)
Figure 28 shows λ and ηchHHV
for the base case as illustrated in Figure 27 as well as the influence of changes in different operating parameters Increasing the feed tem-perature for both steam to gasification (Point 1 in Figure 28) and air to combustion (Point 3 in Figure 28) increases the chemical efficiency and reduces λ as less char needs to be burnt reducing the incoming moisture content of the biomass from 20 to 10 wt (Point 4 in Figure 28) results in a remarkable effect reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 32 Assuming heat losses from the gasification unit (point 6 in Figure 28) corresponding to 2 of the thermal input on a higher heating value basis on the other hand considerably increases the air to fuel ratio and leads to a relative decrease of the chemical efficiency by 3 All changes except for points 5 7 and 8 represent thermal