Synthetic Natural Gas from Coal, Applications

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3 Synthetic Natural Gas from Coal, Dry Biomass, and POWER-TO-GAS Applications

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5 Synthetic Natural Gas from Coal, Dry Biomass, and POWER-TO-GAS Applications Edited by Tilman J. Schildhauer Serge M.A. Biollaz Paul Scherrer Institut, Villigen/Switzerland

6 Copyright 216 by John Wiley & Sons, Inc. All rights reserved Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published 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 17 or 18 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 1923, (978) 75 84, fax (978) , or on the web at Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 73, (21) , fax (21) , or online at Limit of Liability/Disclaimer 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 (8) , outside the United States at (317) or fax (317) 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 Library of Congress Cataloging in Publication Data: Names: Schildhauer, Tilman J., editor. Biollaz, Serge M.A., editor. Title: Synthetic natural gas from coal, dry biomass, and power-to-gas applications / [edited by] Tilman J. Schildhauer, Serge M.A. Biollaz. Description: Hoboken, New Jersey : John Wiley & Sons, 216. Includes bibliographical references and index. Identifiers: LCCN (print) LCCN (ebook) ISBN (cloth) ISBN (pdf) ISBN (epub) Subjects: LCSH: Synthesis gas. Coal gasification. Biomass conversion. Gas manufacture and works. Classification: LCC TP36.S (print) LCC TP36 (ebook) DDC 66/.2844 dc23 LC record available at Set in 1/12pt Times by SPi Global, Pondicherry, India Printed in the United States of America

7 Contents List of Contributors 1 Introductory Remarks 1 Tilman J. Schildhauer 1.1 Why Produce Synthetic Natural Gas? Overview 3 2 Coal and Biomass Gasification for SNG Production 5 Stefan Heyne, Martin Seemann, and Tilman J. Schildhauer 2.1 Introduction Basic Requirements for Gasification in the Framework of SNG Production Thermodynamics of Gasification Gasification Reactions Overall Gasification Process Equilibrium Based Considerations Gasification A Multi step Process Deviating from Equilibrium Heat Management of the Gasification Process Implication of Thermodynamic Considerations for Technology Choice Gasification Technologies Entrained Flow Fixed Bed Direct Fluidized Bed Indirect Fluidized Bed Gasification Hydrogasification and Catalytic Gasification 34 References 37 xi v

8 vi CONTENTS 3 Gas Cleaning 41 Urs Rhyner 3.1 Introduction Impurities Particulate Matter Tars Sulfur Compounds Halide Compounds Alkali Compounds Nitrogen Compounds Other Impurities Cold, Warm and Hot Gas Cleaning Example of B IGFC Gas Cleaning Process Chains Gas Cleaning Technologies Particulate Matter Tars Sulfur Compounds Hydrodesulfurization Chlorine (Halides) Alkali Nitrogen containing Compounds Other Impurities Reactive Hot Gas Filter 62 References 65 4 Methanation for Synthetic Natural Gas Production Chemical Reaction Engineering Aspects 77 Tilman J. Schildhauer 4.1 Methanation The Synthesis Step in the Production of Synthetic Natural Gas Feed Gas Mixtures for Methanation Reactors Thermodynamic Equilibrium Methanation Catalysts: Kinetics and Reaction Mechanisms Catalyst Deactivation Methanation Reactor Types Adiabatic Fixed Bed Reactors Cooled Reactors Comparison of Methanation Reactor Concepts Modeling and Simulation of Methanation Reactors How to Measure (Intrinsic) Kinetics? Modeling of Fixed Bed Reactors Modeling of Isothermal Fluidized Bed Reactors Conclusions and Open Research Questions Symbol List 148 References 149

9 CONTENTS vii 5 SNG Upgrading 161 Renato Baciocchi, Giulia Costa, and Lidia Lombardi 5.1 Introduction Separation Processes for SNG Upgrading Bulk CO 2 /CH 4 Separation Removal of other Compounds and Impurities Techno Economical Comparison of Selected Separation Options 174 References SNG from Wood The GoBiGas Project 181 Jörgen Held 6.1 Biomethane in Sweden Conditions and Background for the GoBiGas Project in Gothenburg Technical Description Technical Issues and Lessons Learned Status Efficiency Economics Outlook 189 Acknowledgements 189 References The Power to Gas Process: Storage of Renewable Energy in the Natural Gas Grid via Fixed Bed Methanation of CO 2 /H Michael Specht, Jochen Brellochs, Volkmar Frick, Bernd Stürmer, and Ulrich Zuberbühler 7.1 Motivation History Renewable Fuel Paths at ZSW Goal Energiewende Goal Power Based, Carbon Based Fuels Goal P2G Goal Methanation The Power to Fuel Concept: Co utilization of (Biogenic) Carbon and Hydrogen P2G Technology Methanation Characteristics for CO 2 Based Syngas P2G Plant Layout of 25 kw el, 25 kw el, and 6 kw el Plants Experimental Results Methanation Catalysts: Screening, Cycle Resistance, Contamination by Sulfur Components Results with the 25 kw el P2G Plant Results with the 25 kw el P2G Plant Results with the 25 kw el P2G Plant in Combination with Membrane Gas Upgrade P2G Process Efficiency 214

10 viii CONTENTS 7.6 Conclusion and Outlook 217 Acknowledgements 219 References Fluidized Bed Methanation for SNG Production Process Development at the Paul Scherrer Institut 221 Tilman J. Schildhauer and Serge M.A. Biollaz 8.1 Introduction to Process Development Methane from Wood Process Development at PSI 223 References MILENA Indirect Gasification, OLGA Tar Removal, and ECN Process for Methanation 231 Luc P.L.M. Rabou, Bram Van der Drift, Eric H.A.J. Van Dijk, Christiaan M. Van der Meijden, and Berend J. Vreugdenhil 9.1 Introduction Main Process Steps MILENA Indirect Gasification OLGA Tar Removal HDS and Deep S Removal Reformer CO 2 Removal Methanation and Upgrading Process Efficiency and Economy Results and Status MILENA OLGA HDS, Reformer, and Methanation Outlook Pressure Co production Bio Carbon Capture and Storage Power to Gas 246 Acknowledgements 246 References Hydrothermal Production of SNG from Wet Biomass 249 Frédéric Vogel 1.1 Introduction Historical Development Physical and Chemical Bases Catalysis Phase Behavior and Salt Separation Liquefaction of the Solid Biomass, Tar, and Coke Formation 263

11 CONTENTS ix 1.4 PSI s Catalytic SNG Process Process Description and Layout Mass Balance Energy Balance Status of Process Development at PSI Comparison to other SNG Processes Open Questions and Outlook 273 References Agnion s Small Scale SNG Concept 279 Thomas Kienberger and Christian Zuber References Integrated Desulfurization and Methanation Concepts for SNG Production 293 Christian F.J. König, Maarten Nachtegaal, and Tilman J. Schildhauer 12.1 Introduction Concepts for Integrated Desulfurization and Methanation Sulfur Resistant Methanation Regeneration of Methanation Catalysts Discussion of the Concepts Required Future Research Sulfur Resistant Methanation Periodic Regeneration 32 References 33 Index 37

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13 List of Contributors Renato Baciocchi University of Rome Tor Vergata, Roma, Italy Serge M.A. 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 Jörgen Held Renewable Energy Technology International AB, Lund, Sweden Stefan Heyne Chalmers University of Technology, Göteborg, Sweden Thomas Kienberger Montanuniversität Leoben, Leoben, Austria Christian F.J. König Paul Scherrer Institut, Villigen, Switzerland Lidia Lombardi Niccolò Cusano University, Roma, Italy Maarten Nachtegaal Paul Scherrer Institut, Villigen, Switzerland Luc P.L.M. 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, Göteborg, Sweden xi

14 xii List of Contributors Michael Specht Center for Solar Energy and Hydrogen Research (ZSW), Stuttgart, Germany Bernd Stürmer Center for Solar Energy and Hydrogen Research (ZSW), Stuttgart, Germany Eric H.A.J. 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 Frédéric Vogel Paul Scherrer Institut, Villigen, Switzerland Berend J. Vreugdenhil Energieonderzoek Centrum Nederland, Petten, The Netherlands Christian Zuber Agnion Highterm Research GesmbH, Graz, Austria Ulrich Zuberbühler Center for Solar Energy and Hydrogen Research (ZSW), Stuttgart, Germany

15 1 Introductory Remarks Tilman J. Schildhauer 1.1 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 195 to the early 198s, 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 initiatives, partly funded by public authorities, to develop processes from coal to SNG. Due to the oil crisis during the 197s, 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 1.5 GW SNG 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 3 years because, with the drop of the oil price in the mid198s, the exploration 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 Synthetic Natural Gas from Coal, Dry Biomass, and Power-to-Gas Applications, First Edition. Edited by Tilman J. Schildhauer and Serge M.A. Biollaz. 216 John Wiley & Sons, Inc. Published 216 by John Wiley & Sons, Inc. 1

16 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 stopped. However, 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 (e.g., for CNG cars) is supported in many countries 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 2% 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 generally 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 turbines. 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

17 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 considered, opening a pathway to produce SNG without solid feedstock that can be stored or transported over long distances within the existing natural gas infrastructure. 1.2 Overview This book aims at a suitable overview over the different pathways to produce SNG (Figure 1.1). 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, especially 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 conditions for which the respective process was developed. These processes comprise those which are already in operation (e.g., the 2 MW SNG bio SNG production in Gothenburg, Sweden, or the 6 MW SNG 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 Algae, manure Hydrothermal gasification Biogas from digestion CO 2 from air or industry Raw SNG CH 4, H 2 O, (CO 2, H 2 ) H 2 from electrolysis (power-to-gas) Methanation Gas upgrading H 2 O, CO 2, (H 2 ) SNG (CH 4 ) Figure 1.1 The different pathways to produce SNG.

18 4 Introductory Remarks The gas cleaning chapter discusses the impurities to be expected in gasificationderived 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 challenges 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 methanation 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, CO 2 and hydrogen removal based on adsorption, absorption, and membranes and includes a techno economic comparison. The chapter on the GoBiGas project ( Gothenburg Bio Gas ) presents the boundary conditions and technologies applied in the 2 MW SNG wood to SNG plant in Gothenburg, Sweden, which was commissioned in 214. The next chapter explains the development of the power to gas process at the Zentrum für solare Wasserstofferzeugung (ZSW), including the 6 MW SNG 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 allowing for the simultaneous catalytic gasification and methanation of wet biomass under super critical conditions. The chapter on agnion s small scale SNG concept focuses on two novel technologies 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 contributors 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.

19 2 Coal and Biomass Gasification for SNG Production Stefan Heyne, Martin Seemann, and Tilman J. Schildhauer 2.1 Introduction basic requirements for gasification in the framework of SNG production Within the production of synthetic natural gas basically methane from solid feedstock 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 (e.g., char, ash). The gasification step can be conducted in different atmospheres and using different reaction agents. Figure 2.1 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 2.1. 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 Synthetic Natural Gas from Coal, Dry Biomass, and Power-to-Gas Applications, First Edition. Edited by Tilman J. Schildhauer and Serge M.A. Biollaz. 216 John Wiley & Sons, Inc. Published 216 by John Wiley & Sons, Inc. 5

20 6 Coal and Biomass Gasification for SNG Production 1 Biomass (hardwood, softwood, straw) 2 Lignite coal 3 Bituminous coal 4 Anthracite coal 2 Carbon 8 a removing CO 2 b adding H 2 c removing char (C) d adding steam e adding O 2 Hydrogen Feedstock 6 Carbon 4 CH 4 8 b a c 1 d e CO 2 2 H 2 2 H 2 O Oxygen Figure 2.1 CHO diagram for coal and biomass. Feedstock composition based on [1, 2]. Data from Higman 28 [1]; Phyllis2, database for biomass and waste, phyllis2 Energy research Centre of the Netherlands. content in the product gas [3, 4]. CO 2 removal is an intrinsic part of the SNG production process; some gasification concepts using adsorptive bed material for direct CO 2 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 2.1. Pretreatment technologies such as torrefaction decrease the oxygen content of the feedstock at the cost of increased energy demand. O 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 homogeneous 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

21 THERMODYNAMICS OF GASIFICATION 7 overlap in bigger particles. The detailed description of the complete process is a complex 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 Gasification Reactions The major reactions occurring during the gasification step that commonly are considered relevant are: C s 5. o g Co g 111kJ / mol partial oxidation (2.1) 2 Co g 5. o g Co g 283 kj/ mol carbon monoxide combustion (2.2) 2 2 C s o2 g Co2 g 394 kj / mol carbon combustion (2.3) C s Co2 g 2Co g 172 kj/ mol reverseboudouard reaction (2.4) C s H2o g Co g H2 g 131 kj/ mol water gas reaction (2.5) Co g H2og H2 g Co2 g 41kJ / mol water gasshift reaction (2.6) CH4 g H2og 3H2 g Cog 26 kj / mol steam reforming (2.7) The char gasification reactions converting carbon into gaseous fuels [Equations (2.4) and (2.5)] 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 allothermal gasification) Overall Gasification Process 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 CxHyoz ah2o bch4 cco (2.8) 2 y z x y z x y z with a x b c Table 2.1 gives the coefficients for steam gasification to methane [Equation (2.8)] for different coal and biomass feedstock materials, allowing the calculation of the heat

22 Table 2.1 Composition and Overall Reaction Data for Steam Gasification for Different Feedstock Materials. Reaction Coefficients for Equation (2.8) ΔH r Methane Yield Feedstock Molar Composition LHV [MJ/ kg daf] HHV [MJ/ kg daf] c a b c [MJ/kg daf Feedstock] [kg CH 4 /kg daf Feedstock] Coal a Brown coal Rhein, Germany CH.88 O Lignite N. Dakota, USA CH.72 O Bituminous typical, South CH.68 O Africa Anthracite Ruhr, Germany CH.47 O Biomass b Willow wood hardwood CH 1.46 O Beech wood hardwood CH 1.47 O Fir softwood CH 1.45 O Spruce softwood CH 1.42 O Wheat straw CH 1.46 O Rice straw CH 1.43 O a Taken from Higman and van der Burgt [1]. b Taken from Phyllis [2] average data for material group. c HHV [MJ/kg daf] = LHV [MJ/kg daf] H [wt% daf]/1.

23 THERMODYNAMICS OF GASIFICATION 9 of reaction (based on the HHV of feedstock and methane, all reactants and products at 25 C) as well as the stoichiometric methane yield per kilogram of feedstock. The heat of reaction for the overall reaction corresponds to well below 1% for all feedstock 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 hydrogasification. The only technical conversion process capable of converting carbonaceous feedstock directly to methane and carbon dioxide according to the reaction in Equation (2.8) is gasification under supercritical conditions, so called hydrothermal gasification [6 8]. This technology is capable of handling wet feedstock but not yet available at commercial scale and is not covered in this chapter. Common gasification technology converts the feedstock to a product gas, being a mixture of CO, CO 2, H 2, H 2 O, CH 4, light and higher hydrocarbons, and trace components, followed by a downstream gas cleaning and methane synthesis step. The major operating parameters for gasification are pressure and temperature. Equilibrium calculations for steam gasification (.5 kg H 2 O/kg 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 2.2, methane formation is favored by lower temperatures and higher pressures. Hydrogen and carbon monoxide formation increases with temperature and so does the endothermicity of the overall reaction. The theoretically exothermic reaction to CH 4 and CO 2 at 25 C [similar to Equation (2.8)] turns into an endothermic reaction requiring heat supply at higher temperature. Light hydrocarbons (represented by C 2 H 4 ) and tars (represented by C 1 H 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 2 /CO ratio via the water gas shift reaction, Equation (2.6). 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 8 C the methane molar fraction increases from to about 15.5% from 1 to 3 bar. With more methane being formed, the endothermic heat of reaction is reduced by 66.6% from 1 to 3 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 2 O representing Equation (2.8) can be obtained. A process example is hydrothermal gasification which is operating at these conditions but still is at development state [6 8]. The above mentioned trends are all under the assumption of complete conversion of feedstock to product gas. Many gasification concepts however 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

24 1 Coal and Biomass Gasification for SNG Production y CH y CO P [bar] T [ C] 1 12 P [bar] T [ C] y H y CO P [bar] T [ C] 1 12 P [bar] T [ C] 12 y H2 O ΔH r [kj/kg daf feed] P [bar] T [ C] 1 12 Figure 2.2 Pressure and temperature dependence of the molar concentration of major gas species and the heat of reaction for steam gasification (S/B =.5 kg H 2 O/kg daf) of a generic biomass (C 5 wt%, H 6 wt%, O 44 wt%: CH 1.43 O.66 ) assuming complete carbon conversion, calculated by ASPEN PLUS. P [bar] T [ C] 1 12 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 2.3 depicts the influence of pressure and temperature on carbon conversion; at temperatures below 8 C, considerable amounts of solid carbon formation are predicted. 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 water gas shift reactions, Equations (2.4) to (2.6). 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.

25 THERMODYNAMICS OF GASIFICATION 11 Amount of feedstock carbon converted to gas phase P [bar] T [ C] Figure 2.3 Carbon conversion predicted by equilibrium calculations for steam gasification (S/B =.5 kg H 2 O/kg daf) of a generic biomass (C 5 wt%, H 6 wt%, O 44 wt%: CH 1.43 O.66 ) Gasification 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 gasification/combustion. The kinetics of the numerous homogeneous and heterogeneous reactions occurring as well as the residence time and reactor setup ultimately determine the product gas composition resulting from gasification. Figure 2.4 illustrates a simplified reaction network for the conversion from received fuel to product gas. The drying and primary pyrolysis (also referred to as devolatilization) steps are similar for all gasification 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 considerable amount of energy, for example, biomass steam gasification at 8 C can result in more than 33 g tars/nm 3, corresponding to about 8% of the total product gas energy content on a lower heating value basis [1].

26 12 Coal and Biomass Gasification for SNG Production Secondary Pyrolysis/Gasification Reforming Dehydration Cracking Polymerization Oxidation Gasification Water-gas shift Primary Pyrolysis Product gas Drying Water Water As-received fuel particle Moisture Tars Tars Permanent gases Permanent gases Dry dry fuel Char Char Gasification medium (H 2 O, H 2, O 2, CO 2...) Figure 2.4 Conversion process of a fuel particle during gasification (adopted and modified from Neves et al. [9]). 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 2.5 illustrates the pyrolysis gas composition and the total gas and char yield based on Neves model. In contrast to the equilibrium calculations for gasification represented in Figure 2.2, the model predicts considerable amounts of tars as well as a considerable amount of char produced from pyrolysis. Even light hydrocarbons are present in the pyrolysis gas. Generally speaking, the product distribution from pyrolysis as presented in Figure 2.5 undergoes conversion towards the equilibrium conditions during the final conversion step in Figure 2.4 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 gas solid mixing and the presence of catalytically active materials promoting specific reactions, among others.

27 THERMODYNAMICS OF GASIFICATION 13 (a) Molar fraction of component i Tars (C 1 H 8 ) CxHy CH 4 CO CO 2 H 2 O H 2 (b) Yield (kg/kg daf fuel) Temperature (ºC) Char yield Gases, tars and pyrolytic water Total yield per kg daf fuel Temperature (ºC) Figure 2.5 Pyrolysis gas molar composition (a) and overall mass yields (b) as a function of temperature (based on Neves [9]) for a generic biomass (C 5 wt%, H 6 wt%, O 44 wt%: CH 1.43 O.66 ) 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 illustrated in Figure 2.4 occur at different temperature levels. A qualitative representation

28 14 Coal and Biomass Gasification for SNG Production of the temperature profile for a fuel particle over time is illustrated in Figure 2.6a. 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 environment due to the exothermic nature of the combustion reactions. For complete conversion of a biomass fuel with initial moisture content of 2 wt% the distribution of the heat demand for conversion on the different processes is illustrated in Figure 2.6b. The gasification heat demand is dominant but even pyrolysis and drying (a) Char combustion Surrounding temperature Temperature Drying Devolatilization/ Pyrolysis Char gasification (b) 9 8 Time Gasification temperature (85 C) Temperature [ºC] 7 6 Pyrolysis temperature 5 Fraction of total heat demand (45 C) (6566 kj/kg daf fuel) 4 a Preheating 4.% 3 b Drying 8.2% 2 1 c Pyrolysis 7.6% d Gasification 8.2% a b c d Heat demand [kj/kg daf fuel] Figure 2.6 (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 5 wt%, H 6 wt%, O 44 wt%: CH 1.43 O.66 ) at equilibrium (S/B ratio =.5, steam supply at 4 C, 2 wt% initial biomass moisture).

29 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 (Neves 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 2.6b] 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, temperature heat load graphs can be a useful tool for identifying improvements for the energy efficiency of the gasification process. Figure 2.7 presents such a graph as an example of an indirect gasification process modelled using Neves 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 (4 C) and hot air to combustion (4 C) is an additional heat demand that needs to be covered. The thick curve in Figure 2.7 represents the aggregation of all the above mentioned heat demands, whereas the dashed curve is a representation of all heat sources, namely the combustion Temperature [ºC] T combustion = 9 ºC Maximum amount of excess heat 361 kj/kg daf Moisture evaporation and steam generation Gas cooling (product gas and combustion flue gas) Char combustion Gasification 1 Preheating biomass, air and water (steam) Heat load [kj/kg daf fuel] T pyrolysis = 45 ºC Pyrolysis and preheating air and steam T gasification = 85 ºC Figure 2.7 Temperature heat load curve for indirect steam gasification of a generic biomass (C 5 wt%, H 6 wt%, O 44 wt%: CH 1.43 O.66 ). S/B ratio =.5, air and steam supply at 25 C and heated to 4 C, 2 wt% initial biomass moisture.

30 16 Coal and Biomass Gasification for SNG Production of char and the cooling of hot product gas and combustion flue gases to ambient temperature. 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 parameter 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 n air o2, actual air o2, stoichiometric (2.7) The second parameter commonly used in gasification is the chemical efficiency η ch, 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 n i, PG. HHV i i n fuel. HHV fuel (2.8) Figure 2.8 shows λ and η ch,hhv for the base case as illustrated in Figure 2.7, as well as the influence of changes in different operating parameters. Increasing the feed temperature for both steam to gasification (Point 1 in Figure 2.8) and air to combustion (Point 3 in Figure 2.8) increases the chemical efficiency and reduces λ as less char needs to be burnt. Reducing the incoming moisture content of the biomass from 2 to 1 wt% (Point 4 in Figure 2.8) results in a remarkable effect, reducing the necessary air to fuel ratio and gives a relative increase of the chemical efficiency of 3.2%. Assuming heat losses from the gasification unit (point 6 in Figure 2.8) 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