Thermochemical Processing of Biomass E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 641

Veröffentlichungsjahr: 2011

Beschreibung

Thermochemical pathways for biomass conversion offer opportunitiesfor rapid and efficient processing of diverse feedstocks intofuels, chemicals and power. Thermochemical processing has severaladvantages relative to biochemical processing, including greaterfeedstock flexibility, conversion of both carbohydrate and lignininto products, faster reaction rates, and the ability to produce adiverse selection of fuels. Thermochemical Processing of Biomass examines the largenumber of possible pathways for converting biomass into fuels,chemicals and power through the use of heat and catalysts. The bookpresents a practical overview of the latest research in thisrapidly developing field, highlighting the fundamental chemistry,technical applications and operating costs associated withthermochemical conversion strategies. Bridging the gap between research and practical application,this book is written for engineering professionals in the biofuelsindustry, as well as academic researchers working in bioenergy,bioprocessing technology and chemical engineering. Topics covered include: * Combustion * Gasification * Fast Pyrolysis * Hydrothermal Processing * Upgrading Syngas and Bio-oil * Catalytic Conversion of Sugars to Fuels * Hybrid Thermochemical/Biochemical Processing * Economics of Thermochemical Conversion For more information on the Wiley Series in Renewable Resources,visit www.wiley.com/go/rrs

Leseprobe

Contents

Cover

Wiley Series in Renewable Resources

Title Page

Copyright

Dedication

Series Preface

Acknowledgements

List of Contributors

Chapter 1: Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals, and Power

1.1 Introduction

1.2 Direct Combustion

1.3 Gasification

1.4 Fast Pyrolysis

1.5 Hydrothermal Processing

1.6 Hydrolysis to Sugars

1.7 Technoeconomic Analysis

References

Chapter 2: Biomass Combustion

Nomenclature

2.1 Introduction

2.2 Combustion Systems

2.3 Fundamentals of Biomass Combustion

2.4 Pollutant Emissions and Environmental Impacts

References

Chapter 3: Gasification

3.1 Introduction

3.2 Fundamentals of Gasification

3.3 Feed Properties

3.4 Classifying Gasifiers According to Method of Heating

3.5 Classifying Gasifiers According to Transport Processes

3.6 Pressurized Gasification

3.7 Product Composition

3.8 System Applications

References

Chapter 4: Syngas Cleanup, Conditioning, and Utilization

4.1 Introduction

4.2 Syngas Cleanup and Conditioning

4.3 Syngas Utilization

4.4 Summary and Conclusions

References

Chapter 5: Fast Pyrolysis

5.1 Introduction

5.2 Bio-oil Properties

5.3 Fast Pyrolysis Process Technologies

5.4 Bio-oil Fuel Applications

5.5 Chemicals from Bio-oil

5.6 Concluding Remarks

Acknowledgements

References

Chapter 6: Upgrading Fast Pyrolysis Liquids

6.1 Introduction to Fast Pyrolysis and Bio-oil

6.2 Liquid Characteristics and Quality

6.3 Significant Factors Affecting Characteristics

6.4 Norms and Standards

6.5 Bio-oil Upgrading

6.6 Chemical and Catalytic Upgrading of Bio-oil

6.7 Conclusions

References

Chapter 7: Hydrothermal Processing

7.1 Introduction

7.2 Background

7.3 Fundamentals

7.4 Hydrothermal Liquefaction

7.5 Hydrothermal Gasification

7.6 Pumping Biomass into Hydrothermal Processing Systems

7.7 Conclusions of Hydrothermal Processing

References

Chapter 8: Catalytic Conversion of Sugars to Fuels

8.1 Introduction

8.2 Chemistry of Sugars

8.3 Hydrogen from Sugars

8.4 Sugar to Light Alkanes

8.5 Sugars to Oxygenates

8.6 Sugars to Larger Alkanes

8.7 Sugar Conversion to Aromatics

8.8 Conclusions and Summary

Acknowledgements

References

Chapter 9: Hybrid Processing

9.1 Introduction

9.2 Syngas Fermentation

9.3 Bio-oil Fermentation

References

Chapter 10: Costs of Thermochemical Conversion of Biomass to Power and Liquid Fuels

10.1 Introduction

10.2 Electric Power Generation

10.3 Liquid Fuels via Gasification

10.4 Liquid Fuels via Fast Pyrolysis

10.5 Summary and Conclusions

References

Index

Wiley Series in Renewable Resources

Series EditorChristian V. Stevens – Faculty of Bioscience Engineering, Ghent University, Ghent, Belgium

Titles in the Series

Wood Modification – Chemical, Thermal and Other Processes Callum A. S. Hill

Renewables-Based Technology: Sustainability Assessment Jo Dewulf & Herman Van Langenhove

Introduction to Chemicals from Biomass James H. Clark & Fabien E.I. Deswarte

Biofuels Wim Soetaert & Erick Vandamme

Handbook of Natural Colorants Thomas Bechtold & Rita Mussak

Surfactants from Renewable Resources Mikael Kjellin & IngegÄrd Johansson

Industrial Applications of Natural Fibres - Structure, Properties and Technical Applications JÖrg MÜssig

Forthcoming Titles

Introduction to Wood and Natural Fibre Composites Douglas Stokke, Qinglin Wu & Guangping Han

Biorefinery Co-Products: Phytochemicals, Lipids and Proteins Danielle Julie Carrier, Shri Ramaswamy & Chantal Bergeron

Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals Charles E. Wyman

Bio-based Plastics: Materials and Applications Stephen Kabasci

Cellulosic Energy Cropping Systems David Bransby

This edition first published 2011

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Library of Congress Cataloging-in-Publication Data

Thermochemical processing of biomass: conversion into fuels, chemicals, and power / editor, Robert C. Brown.

p. cm. – (Wiley Series in Renewable Resources)

Includes bibliographical references and index.

ISBN 978-0-470-72111-7 (hardback)

1. Biomass energy. 2. Thermochemistry. 3. Energy conversion. I. Brown, Robert C. (Robert Clinton)

TP339.T476 2011

6620.88–dc22

2010050378

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

Print ISBN: 9780470721117

ePDF ISBN: 9781119990857

oBook ISBN: 9781119990840

ePub ISBN: 9781119990994

This book is dedicated to the staff and students who helped build the thermochemical processing programs of the Center for Sustainable Environmental Technologies and the Bioeconomy Institute at Iowa State University.

Series Preface

Renewable resources, their use and modification are involved in a multitude of important processes with a major influence on our everyday lives. Applications can be found in the energy sector, chemistry, pharmacy, the textile industry, paints and coatings, to name but a few.

The area interconnects several scientific disciplines (agriculture, biochemistry, chemistry, technology, environmental sciences, forestry, etc.), which makes it very difficult to have an expert view on the complicated interaction. Therefore, the idea to create a series of scientific books, focusing on specific topics concerning renewable resources, has been very opportune and can help to explain some of the underlying connections in this area.

In a very fast changing world, trends are characteristic not only for fashion and political standpoints. Even science is not free from hypes and buzzwords. The use of renewable resources is again more important nowadays; however, it is not part of a hype or a fashion. As the lively discussions among scientists continue about how many years we will still be able to use fossil fuels, with opinions ranging from 50 to 500 years, they do agree that the reserve is limited and that it is essential not only to search for new energy carriers but also for new material sources.

In this respect, renewable resources are a crucial area in the search for alternatives for fossil-based raw materials and energy. In the field of energy supply, biomass and renewable resources will be part of the solution alongside other alternatives such as solar energy, wind energy, hydraulic power, hydrogen technology and nuclear energy.

In the field of material sciences, the impact of renewable resources will probably be even bigger. Integral utilization of crops and the use of waste streams in certain industries will grow in importance, leading to a more sustainable way of producing materials.

Although our society was much more (almost exclusively) based on renewable resources centuries ago, this disappeared in the Western world in the 19th century. Now it is time to focus again on this field of research. However, it should not mean a retour à la nature, but it should be a multidisciplinary effort on a highly technological level to perform research towards new opportunities, to develop new crops and products from renewable resources. This will be essential to guarantee a level of comfort for a growing number of people living on our planet. It is ‘the’ challenge for the coming generations of scientists to develop more sustainable ways to create prosperity and to fight poverty and hunger in the world. A global approach is certainly favoured.

This challenge can only be dealt with if scientists are attracted to this area and are recognized for their efforts in this interdisciplinary field. It is therefore also essential that consumers recognize the fate of renewable resources in a number of products.

Furthermore, scientists do need to communicate and discuss the relevance of their work. The use and modification of renewable resources may not follow the path of the genetic engineering concept in view of consumer acceptance in Europe. Related to this aspect, the series will certainly help to increase the awareness of the importance of renewable resources.

Being convinced of the value of the renewables approach for the industrial world, as well as for developing countries, I was delighted to collaborate on this series of books focusing on different aspects of renewable resources. I hope that readers become aware of the complexity, the interaction and interconnections, and the challenges of this field and that they will help to communicate the importance of renewable resources.

I certainly want to thank the people from the Chichester office of Wiley, especially David Hughes, Jenny Cossham and Lyn Roberts, for seeing the need for such a series of books on renewable resources, for initiating and supporting it and for helping to carry the project to the end.

Last but not least, I want to thank my family, especially my wife Hilde and children Paulien and Pieter-Jan, for their patience and for giving me the time to work on the series when other activities seemed to be more inviting.

Christian V. Stevens Faculty of Bioscience Engineering Ghent University, Belgium Series Editor Renewable Resources June 2005

Acknowledgements

The genesis of this book was an invitation by Christian Stevens to describe the ‘thermochemical option’ for biofuels production at the Third International Conference on Renewable Resources and Biorefineries at Ghent University in 2007. At that time, many people working in the biofuels community viewed thermochemical processing as little more than an anachronism in the age of biotechnology. I was very appreciative of Chris' interest in exploring alternative pathways. After the conference, he followed up with an invitation to submit a book proposal on thermochemical processing to the Wiley Series in Renewable Resources, for which he serves as Series Editor. At the time I was busy with other responsibilities and declined his invitation. A year later Chris repeated his offer and I agreed to edit a volume on thermochemical production of biofuels, biobased chemicals, and biopower. I am very grateful that several prominent colleagues in the field agreed to contribute chapters: Bryan Jenkins, Richard Bain, David Dayton, Wolter Prins, Tony Bridgwater, Douglas Elliott, George Huber, Mark Wright, and DongWon Choi. The project editors at Wiley were extremely helpful and patient during the 2 years that my colleagues and I struggled to find time to write on a subject that was rapidly moving from obscurity to prominence and was presenting us with a variety of distractions. These steadfast project editors include Richard Davies, Jon Peacock, and Sarah Hall. I am also indebted to several people who helped me with administrative and management responsibilities at the Bioeconomy Institute (BEI) and the Center for Sustainable Environmental Technologies (CSET) at Iowa State University while this book was being prepared: Jill Euken, deputy director of the BEI; Ryan Smith, deputy director of CSET; Becky Staedtler, business manager of the BEI and CSET; and Diane Meyer, manager of the BEI proposal office. Finally, I wish to acknowledge my wife, Carolyn, who has been the most steadfast of all during the preparation of this book.

List of Contributors

Richard L. Bain, National Renewable Energy Laboratory, Colorado, USA

Larry L. Baxter, Brigham Young University, Utah, USA

Anthony V. Bridgwater, Bioenergy Research Group, Aston University, UK

Karl Broer, Department of Mechanical Engineering, Iowa State University, USA

Robert C. Brown, Department of Mechanical Engineering, Iowa State University, USA

David C. Chipman, Center for Sustainable Environmental Technologies and Department of Mechanical Engineering, Iowa State University, USA

DongWon Choi, Department of Biological and Environmental Sciences, Texas A&M University – Commerce, Commerce, TX 75429, USA

David C. Dayton, Center for Energy Technology, RTI International, North Carolina, USA

Alan A. DiSpirito, Department of Biochemistry, Biophysics and Molecular Biology, Iowa State University, USA

Douglas C. Elliott, Pacific Northwest National Laboratory, Washington, USA

Raghubir Gupta, Center for Energy Technology, RTI International, North Carolina, USA

George W. Huber, Department of Chemical Engineering, University of Massachusetts--Amherst, USA

Bryan M. Jenkins, University of California, Davis, California, USA

Jaap Koppejan, Procede Biomass BV, The Netherlands

Ning Li, Department of Chemical Engineering, University of Massachusetts--Amherst, USA

Wolter Prins, Faculty of Bioscience Engineering, Ghent University, Belgium

Geoffrey A. Tompsett, Department of Chemical Engineering, University of Massachusetts--Amherst, USA

Brian Turk, Center for Energy Technology, RTI International, North Carolina, USA

Robbie H. Venderbosch, Biomass Technology Group B.V., The Netherlands

Mark M. Wright, Department of Mechanical Engineering, Iowa State University, USA

Chapter 1

Introduction to Thermochemical Processing of Biomass into Fuels, Chemicals, and Power

Robert C. Brown

Iowa State University, Department of Mechanical Engineering, Ames, IA 50011, USA

1.1 Introduction

Thermochemical processing of biomass uses heat and catalysts to transform plant polymers into fuels, chemicals, or electric power. This contrasts with biochemical processing of biomass, which uses enzymes and microorganisms for the same purpose. Although biochemical processing is often touted as a fundamentally new approach to converting plant materials into useful products and thermochemical processing is often described as “mature” technology with little scope for improvement, in fact both have been employed by humankind for millennia. Fire for warmth, cooking, and production of charcoal were the first thermal transformations of biomass controlled by humans, while fermentation of fruits, honey, grains, and vegetables was practiced before recorded time. Despite their long records of development, neither is mature, as the application of biotechnology to improving biochemical processes for industrial purposes has revealed [1]. The petroleum and petrochemical industries have accomplished similar wonders in thermochemical processing of hydrocarbon feedstocks, although the more complicated chemistries of plant molecules have not been fully explored.

Ironically, the domination of thermochemical processing in commercial production of fuels, chemicals, and power from fossil resources for well over a century may explain why it is sometimes overlooked as a viable approach to biobased products. Smokestacks belching pollutants from thermochemical processing of fossil fuels is an indelible icon from the twentieth century that no one wishes to replicate with biomass. However, as described in a report released by the US Department of Energy in 2008 [2], thermal and catalytic sciences also offer opportunities for dramatic advances in biomass processing. Thermochemical processing has several advantages relative to biochemical processing. As detailed in Table 1.1, these include the ability to produce a diversity of oxygenated and hydrocarbon fuels, reaction times that are several orders of magnitude shorter than biological processing, lower cost of catalysts, the ability to recycle catalysts, and the fact that thermal systems do not require the sterilization procedures demanded for biological processing. The data in Table 1.1 also suggest that thermochemical processing can be done with much smaller plants than is possible for biological processing of cellulosic biomass. Although this may be true for some thermochemical options (such as fast pyrolysis), other thermochemical options (such as gasification-to-fuels) are likely to be built at larger scales than biologically based cellulosic ethanol plants when the plants are optimized for minimum fuel production cost [3].

Table 1.1 Comparison of biochemical and thermochemical processing.

Adapted from NSF,2008, Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels: Next Generation Hydrocarbon Biorefineries, Ed. George W. Huber, University of Massachusetts Amherst. Courtesy: National Science Foundation

Biochemical processingThermochemical processingProductsPrimarily alcoholsRange of fuelsReaction conditionsLess than 70°C, 1 atm100–1200°C, 1–250 atmResidence time2–5 days0.2 s–1 hSelectivityCan be made very selectiveDepends upon reactionCatalyst/biocatalyst cost$0.50/gallon ethanol$0.01/gallon gasolineSterilizationSterilize all feedsNo sterilization neededRecyclabilityDifficultPossible with solid catalystsSize of plant (biomass input)2000–8000 tons/day5–200 tons/day (fast pyrolysis)

The first-generation biofuels industry, launched in the late 1970s, was based on biochemically processing sugar or starch crops (mostly sugar cane and maize respectively) into ethanol fuel and oil seed crops into biodiesel. These industries grew tremendously in the first decade of the twenty-first century, with worldwide annual production reaching almost 19 billion gallons (∼72 × 109 L) of ethanol and 4.4 billion gallons (∼16.7 × 109 L) of biodiesel in 2008 [4]. This has not been achieved without controversy, including criticism of crop and biofuel subsidies, concerns about using food crops for fuel production, and debate over the environmental impact of biofuels agriculture, including uncertainties about the role of biofuels in reducing greenhouse gas emissions [5]. Many of these concerns would be mitigated by developing advanced biofuels that utilize high-yielding nonfood crops that can be grown on marginal or waste lands. These alternative crops are of two types: lipids from alternative crops and cellulosic biomass.

Lipids are a large group of hydrophobic, fat-soluble compounds produced by plants and animals for high-density energy storage. Triglycerides, commonly known as vegetable oils, are among the most familiar form of lipids and have been widely used in recent years for the production of biodiesel. As illustrated in Figure 1.1, triglycerides consist of three long-chain fatty acids attached to a backbone of glycerol. It is relatively easy to hydrotreat triglycerides to yield liquid alkanes suitable as transportation fuels and propane gas. The hydrogenation of vegetable oils has already been proven technically feasible using conventional distillate hydrotreaters at petroleum refineries [6], although the high price of traditional vegetable oils has discouraged companies from producing transportation fuels in this manner. Commercial deployment will require alternatives to traditional seed crops, which only yield 50–130 gal/acre (467.5–1215.5 L/ha) [7]. Suggestions have included jatropha [8] (200–400 gal/acre (1870–3740 L/ha)) and palm oil [9] (up to 600 gal/acre (5610 L/ha)), but the most promising alternative is microalgae, which can be highly productive in natural ecosystems with oil yields as high as 2000 gal/acre (18 700 L/ha) in field trials and 15 000 gal/acre (140 250 L/ha) in laboratory trials [10]. This promise requires considerable engineering development to reduce capital costs, which are estimated to be $100 000 to $1 million per acre ($250 000 to $2.5 million per hectare), and production costs, which exceed $10–$50 per gallon (about $2.60–$13 per liter) [10, 11]. Thus, the challenge of lipid-based biofuels is producing large quantities of inexpensive lipids rather than upgrading them.

Figure 1.1 Simplified representation of hydrogenation of triglyceride during hydrotreating

Cellulose, on the other hand, is the most abundant form of biomass on the planet. In the form of lignocellulose, a composite of cellulose, hemicellulose, and lignin [12], it dominates most natural ecosystems and is widely managed as sources of timber and animal forage. As illustrated in Figure 1.2, cellulose is a structural polysaccharide consisting of a long chain of glucose molecules linked by glycosidic bonds. Breaking these bonds releases the glucose and makes it available for either food or fuel production. A variety of microorganisms secrete enzymes that hydrolyze the glycosidic bonds of cellulose (and hemicellulose). Many animals, like cattle and other ruminants, have developed symbiotic relationships with these microorganisms to allow them to digest cellulose.

Figure 1.2 Cellulose is a long chain of glucose units connected by glycosidic bonds

However, cellulose is usually found in nature as lignocellulose, a composite of cellulose fibers in a matrix of hemicellulose and lignin. The lignin, which few microorganisms are able to digest, protects the carbohydrate against biological attack. Thus, even ruminant animals that have evolved on diets of lignocellulosic biomass, such as grasses and forbs, can only extract 50–80% of the energy content of this plant material because some of the polysaccharides and all of the lignin pass through the gut undigested. Biochemical processing has many similarities to the digestive system of ruminant animals. Physical and chemical pretreatments release cellulose fibers from the composite matrix, making them more susceptible to enzymatic hydrolysis, which releases simple sugars that can be fermented or otherwise metabolized [13]. Biochemical processes occur at only a few tens of degrees Celsius above ambient temperature, with the result that they can take hours or even days to complete even in the presence of biocatalysts.

Thermochemical processing occurs at temperatures that are at least several hundred degrees Celsius and sometimes over 1000°C above ambient conditions. At these temperatures, thermochemical processes occur very rapidly whether catalysts are present or not. Although thermochemical processing might be characterized as voracious in the pace of reaction and the variety of materials it can consume (not only carbohydrate, but lignin, lipids, proteins, and other plant compounds), its selectivity is not necessarily as indiscriminate as is sometimes attributed to it. Thermal depolymerization of cellulose in the absence of alkali or alkaline earth metals produces predominately levoglucosan, an anhydrosugar of the monosaccharide glucose [14]. Under certain conditions, it appears that lignin depolymerizes to monomeric phenolic compounds [15]. Under conditions of high-temperature combustion and gasification, chemical equilibrium among products is attained. Thus, thermochemical processing offers opportunities for rapid processing of diverse feedstocks, including recalcitrant materials and unique intermediate feedstocks, for production of fuels, chemicals, and power.

As shown in Figure 1.3, thermochemical routes can be categorized as combustion, gasification, fast pyrolysis, hydrothermal processing, and hydrolysis to sugars. Direct combustion of biomass produces moderate- to high-temperature thermal energy (800–1600°C) suitable for electric power generation. Gasification generates both moderate-temperature thermal energy (700–1000°C) and a flammable gas mixture known commonly as producer gas or syngas, which can be used to generate either electric power or to synthesize fuels or other chemicals using catalysts or even microorganisms (syngas fermentation) [16]. Fast pyrolysis occurs at moderate temperatures (450–550°C) in the absence of oxygen to produce mostly condensable vapors and aerosols that are recovered as an energy-rich liquid known as bio-oil. Fast pyrolysis also produces smaller amounts of flammable gas (syngas) and solid charcoal, known as char or sometimes biochar [17]. Bio-oil can be burned for electric power generation or processed into hydrogen via steam reforming or into liquid hydrocarbons via hydroprocessing. Whereas fast pyrolysis requires relatively dry feedstocks (around 10 wt% moisture), hydrothermal processing is ideal for wet feedstocks that can be handled as slurries with solids loadings in the range of 5–20 wt%. Hydrothermal processing occurs at pressures of 50–250 atm (∼5–25 MPa) to prevent boiling of the water in the slurry and at temperatures ranging from 200 to 500°C, depending upon whether the desired products are fractionated plant polymers [18], a partially deoxygenated liquid product known as biocrude [19], or syngas [20]. Finally, hydrolysis of plant polysaccharides yields simple sugars that can be catalytically or biocatalytically converted into fuels. Concentrated acid or the combined action of dilute acid and heat are well known to hydrolyze polysaccharides to monosaccharides. The biotechnology revolution has encouraged the use of enzymes to more efficiently hydrolyze sugars from biomass, but the high cost of enzymes has slowed commercial introduction of so-called cellulosic biofuels by this biochemical route [21]. Although acid hydrolysis qualifies as thermochemical processing, more direct thermal interventions can also yield sugars from biomass. Hydrothermal processing at modest temperatures fractionates biomass into cellulose fibers, hemicellulose dehydration products, and lignin [18]. Further hydrothermal processing of the cellulose can produce glucose solutions. Fast pyrolysis also yields significant quantities of sugars and anhydrosugars under suitable processing conditions [22]. These “thermolytic sugars” can either be fermented or catalytically upgraded to fuel molecules.

Figure 1.3 Thermochemical options for production of fuels, chemicals, and power

1.2 Direct Combustion

Much of the focus on bioenergy in the USA has been production of liquid transportation fuels in an effort to displace imported petroleum. Recently, it has been argued that a better use of biomass would be to burn it for the generation of electricity to power battery electric vehicles (BEVs) [23]. Well-to-wheels analyses indicate that BEVs are superior to biofuels-powered internal combustion engine vehicles in terms of primary energy consumed, greenhouse gas emissions, lifecycle water usage, and cost when evaluated on the basis of kilometers driven [24].

Combustion is the rapid reaction of fuel and oxygen to obtain thermal energy and flue gas, consisting primarily of carbon dioxide and water. Depending on the heating value and moisture content of the fuel, the amount of air used to burn the fuel, and the construction of the furnace, flame temperatures can exceed 1650°C. Direct combustion has the advantage that it employs commercially well-developed technology. It is the foundation of much of the electric power generation around the world. In principle, existing power plants could be quickly and inexpensively retrofitted to burn biomass, compared with greenfield construction of advanced biorefineries, which would be based upon largely unproven technologies. Plug-in hybrid electric vehicles will soon be widely available to utilize this biopower. In the long term, high efficiency combined-cycle power plants based on gasified or pyrolyzed biomass will provide power for long-range electric vehicles based on advanced battery technology [25].

However, combustion is burdened by three prominent disadvantages. These include penalties associated with burning high-moisture fuels, agglomeration and ash fouling due to alkali compounds in biomass, and difficulty of providing and safeguarding sufficient supplies of bulky biomass to modern electric power plants. Chapter 2 is devoted to a description of biomass combustion as a thermochemical technology.

1.3 Gasification

Thermal gasification is the conversion of carbonaceous solids at elevated temperatures and under oxygen-starved conditions into syngas, a flammable gas mixture of carbon monoxide, hydrogen, methane, nitrogen, carbon dioxide, and smaller quantities of hydrocarbons [26]. Gasification has been under development for almost 200 years, beginning with the gasification of coal to produce so-called “manufactured gas” or “town gas” for heating and lighting. Coal gasification has also been used for large-scale production of liquid transportation fuels, first in Germany during World War II and then later in South Africa during a period of worldwide embargo as a result of that country's apartheid policies.

Gasification can be used to convert any carbonaceous solid or liquid to low molecular weight gas mixtures. In fact, the high volatile matter content of biomass allows it to be gasified more readily than coal. Biomass gasification has found commercial application where waste wood was plentiful or fossil resources were scarce. An example of the former was Henry Ford's gasification of wood waste derived from shipping crates at his early automotive plants [27]. An example of the latter was the employment of portable wood gasifiers in Europe during World War II to power automobiles. With a few exceptions, gasification in all its forms gradually declined over the twentieth century due to the emergence of electric lighting, the development of the natural gas industry, and the success of the petroleum industry in continually expanding proven reserves of petroleum. In the twenty-first century, as natural gas and petroleum become more expensive, gasification of both coal and biomass is likely to be increasingly employed.

As illustrated in Figure 1.4, one of the most attractive features of gasification is its flexibility of application, including thermal power generation, hydrogen production, and synthesis of fuels and chemicals. This offers the prospect of gasification-based energy refineries, producing a mix of energy and chemical products or allowing the staged introduction of technologies as they reach commercial viability.

Figure 1.4 Gasification offers several options for processing biomass into power, chemicals, and fuels

The simplest application of gasification is production of heat for kilns or boilers. Often the syngas can be used with minimal clean-up because tars or other undesirable compounds are consumed when the gas is burned and process heaters are relatively robust to dirty gas streams. The syngas can be used in internal combustion engines if tar loadings are not too high and after removal of the greater part of particulate matter entrained in the gas leaving the gasifier. Gas turbines offer prospects for high-efficiency integrated gasification–combined-cycle power, but they require more stringent gas cleaning [28]. As the name implies, syngas can also be used to synthesize a wide variety of chemicals, including organic acids, alcohols, esters, and hydrocarbon fuels, but the catalysts for this synthesis are even more sensitive to contaminants than are gas turbines.

Chapter 3 describes gasification technologies, Chapter 4 covers gas stream clean-up and catalytic upgrading to fuels and chemicals, and Chapter 9, which covers hybrid thermochemical–biochemical processing, includes a description of syngas fermentation [16].

1.4 Fast Pyrolysis

Fast pyrolysis is the rapid thermal decomposition of organic compounds in the absence of oxygen to produce liquids, gases, and char [17]. The distribution of products depends on the biomass composition and rate and duration of heating. Liquid yields as high as 72% are possible for relatively short residence times (0.5–2 s), moderate temperatures (400–600°C), and rapid quenching at the end of the process. The resulting bio-oil is a complex mixture of oxygenated organic compounds, including carboxylic acids, alcohols, aldehydes, esters, saccharides, phenolic compounds, and lignin oligomers. It has been used as fuel for both boilers and gas turbine engines, although its cost, corrosiveness, and instability during storage have impeded its commercial deployment.

Its great virtues are the simplicity of generating bio-oil and the attractiveness of a liquid feedstock compared with either gasified or unprocessed biomass. Bio-oil can be upgraded to transportation fuels through a combination of steam reforming [29] of light oxygenates in the bio-oil to provide hydrogen and hydrocracking lignin oligomers and carbohydrate to synthetic diesel fuel or gasoline [30, 31]. Recent technoeconomic analysis [32] indicating that bio-oil could be upgraded to synthetic gasoline and diesel for $2–$3 per gallon (about $0.53–$0.79 per liter) gasoline equivalent has spurred interest in fast pyrolysis and bio-oil upgrading.

Hydroprocessing bio-oil into hydrocarbons suitable as transportation fuel is similar to the process for refining petroleum. Hydroprocessing was originally developed to convert petroleum into motor fuels by reacting it with hydrogen at high pressures in the presence of catalysts. Hydroprocessing includes two distinct processes. Hydrotreating is designed to remove sulfur, nitrogen, oxygen, and other contaminants from petroleum. When adapted to bio-oil, the main contaminant to be removed is oxygen. Thus, hydrotreating bio-oil is primarily a process of deoxygenation, although nitrogen can be significant in some bio-oils. Hydrocracking is the reaction of hydrogen with organic compounds to break long-chain molecules into lower molecular weight compounds. Although fast pyrolysis attempts to depolymerize plant molecules, a number of carbohydrate and lignin oligomers are found in bio-oil, which hydrocracking can convert into more desirable paraffin or naphthene molecules. Some researchers are attempting to add catalysis to the pyrolysis reactor to yield hydrocarbons directly. Similar to the process of fluidized catalytic cracking used in the petroleum industry, the process occurs at atmospheric pressure over acidic zeolites. A yield of 17% of C5–C10 hydrocarbons has been reported in a study of upgrading of pyrolytic liquids from poplar wood [33]. Although superior to conventional bio-oil, this product still needs refining to gasoline or diesel fuel. Fast pyrolysis of biomass to bio-oil is described in Chapter 5. Upgrading of bio-oil to transportation fuels is discussed in Chapter 6.

1.5 Hydrothermal Processing

Hydrothermal processing describes the thermal treatment of wet biomass at elevated pressures to produce carbohydrate, liquid hydrocarbons, or gaseous products depending upon the reaction conditions. As illustrated in Figure 1.5, processing temperature must be increased as reaction temperature increases to prevent boiling of water in the wet biomass. At temperatures around 100°C, extraction of high-value plant chemicals such as resins, fats, phenolics, and phytosterols is possible. At 200°C and 20 atm (∼2 MPa), fibrous biomass undergoes a fractionation process to yield cellulose, lignin, and hemicelluloses degradation products such as furfural. Further hydrothermal processing can hydrolyze the cellulose to glucose. At 300–350°C and 120–180 atm (∼12.2–18.2 MPa), biomass undergoes more extensive chemical reactions, yielding a hydrocarbon-rich liquid known as biocrude. Although superficially resembling bio-oil, it has lower oxygen content and is less miscible in water, making it more amenable to hydrotreating. At 600–650°C and 300 atm (30.4 MPa) the primary reaction product is gas, including a significant fraction of methane.

Figure 1.5 Temperature/pressure regimes of hydrothermal processing

Continuous feeding of biomass slurries into high-pressure reactors and efficient energy integration represent engineering challenges that must be overcome before hydrothermal processing results in a commercially viable technology. Chapter 7 is devoted to hydrothermal processing of biomass.

1.6 Hydrolysis to Sugars

Although biochemical processing is sometimes referred to as the “sugar platform,” it is possible to thermally depolymerize biomass into monosaccharides and catalytically synthesize fuel molecules from these carbohydrate building blocks. Thus, the so-called sugar platform can be a pure play in biochemical processing (enzymatic hydrolysis of plant carbohydrates to sugar followed by fermentation), a hybrid thermochemical–biochemical process (thermally or chemically induced hydrolysis followed by fermentation of the released sugar), a hybrid biochemical–thermochemical process (enzymatic hydrolysis followed by catalytic synthesis of the sugar to hydrocarbons), or a pure play in thermochemical processing (thermal depolymerization followed by catalytic upgrading of the sugar to fuel molecules).

As described in Chapter 9, fast pyrolysis can produce both anhydrosugars and fermentable sugar from biomass, the yield of which is significantly enhanced if the biomass is washed or otherwise treated to eliminate the catalytic activity of naturally occurring alkali and alkaline earth metals [22]. Limited technoeconomic analysis of the process suggests that fermentation of sugar extracted from bio-oil could yield ethanol at costs competitive with cellulosic ethanol derived from either acid or enzymatic hydrolysis [34]. Similarly, hydrothermal processing under mild conditions can produce aqueous solutions of fermentable sugar [18].

These sugars can also be catalytically converted to fuels. Sugars that exist as five-member rings, like the five-carbon sugar xylose or the six-carbon sugar fructose, are readily dehydrated to the five-member rings of furan compounds [35], some examples of which are illustrated in Figure 1.6. Furans are colorless, water-insoluble flammable liquids with volatility comparable to hydrocarbons of similar molecular weight. Some kinds of furans have heating values and octane numbers comparable to gasoline, making them potential transportation fuel [36]. Catalysts can improve yields by making furan-producing pathways more selective among the large number of competing reactions that can occur during pyrolysis of biomass. 2,5-Dimethyl furan in particular has received recent interest because new catalytic synthesis routes from sugars have been developed [37, 38]. Neither the fuel properties nor the toxicity of these compounds have been much studied, raising questions as to their ultimate practicality as transportation fuel.

Figure 1.6 Furans relevant to the production of transportation fuels by thermochemical processing of sugars. Source: Ref. [36]

A more promising approach known as aqueous-phase processing reacts monomeric sugar or sugar-derived compounds in the presence of heterogeneous catalysts at 200–260°C and 10–50 bar (1–5 MPa) to produce alkanes, the same hydrocarbons found in gasoline [39, 40]. Catalytic conversion of sugars would have several advantages over fermentation, including higher throughputs, ready conversion of a wide range of sugars, and the immiscible hydrocarbon products could be recovered without the expensive distillations required in ethanol plants. Chapter 8 explores the possibilities of catalytically converting sugars to fuel molecules.

1.7 Technoeconomic Analysis

Of the several technologies explored in this book, only a few are in commercial operation. Although a number of thermochemical technologies have been demonstrated with biomass feedstocks, very limited information on economic performance based on actual construction or operating costs is available in the literature. In the absence of such information, technoeconomic analyses are useful in estimating capital and operating costs for commercial-scale facilities, despite the well-known limitations of such analysis. Although by no means comprehensive, Chapter 10 provides cost estimates for a wide range of thermochemical processes, ranging from electric power generation to the production of biopolymers and hydrogen via syngas fermentation. Although differences in basis years, feedstock costs, financing options, and granularity of the analyses make it difficult to make comparisons among the various technology options, these analyses provide a useful starting point for exploring the feasibility of different approaches to thermochemical processing.

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Chapter 2

Biomass Combustion

Bryan M. Jenkins,1 Larry L. Baxter2 and Jaap Koppejan3

1University of California, Davis, CA, USA

2Brigham Young University, UT, USA

3Procede Biomass BV, Enschede, Netherlands

Nomenclature

ParameterUnitsDescriptiona, bconstant coefficients of linearized enthalpy functionsCpJ kg−1 K−1, J mol−1 K−1mass or molar specific heatekg kg−1excess oxidant or air in reactantsfi,jkg kg−1mass fraction of fuel constituent i in product jHkJ kmol−1, kJenthalpy, total enthalpyhkJ kmol−1molar enthalpyhfgkJ kg−1enthalpy of vaporizationMdbkg kg−1dry basis moisture content of feedstockmp,jkgmass of the product species jmr,ikgmass of the reactant species imwkgmass of water as moisture in feedstockMwbkg kg−1wet basis moisture content of feedstockQlkJheat transfer to reaction system (negative for heat loss)QphkJ kg−1higher heating value at constant pressureQvhkJ kg−1higher heating value at constant volumeTKabsolute temperatureufgkJ kg−1internal energy of vaporizationWikg kmol−1molar mass of species iϕfuel/air equivalence ratioλair/fuel equivalence ratio, air factorνstoichiometric coefficient

2.1 Introduction

Since humans first learned to manage fire a quarter of a million years ago or more [1], the burning of fuels has served as a defining phenomenon for the development of societies. Releasing the energy needed for large-scale land clearing and agricultural expansion, combustion also provided the means for industrial growth, rapid transportation, the increase and concentration of populations, the waging of world wars, and the globalization of trade and culture. As the world population continues to expand, the environmental impacts of current fuel burning practices cannot be sustained into the future. Continuing evolution of heat and power generation is likely to see dramatic transformations toward low- and zero-emission alternatives, and the future design of combustion systems will be heavily challenged to adapt to more stringent regulations affecting environmental performance while maintaining economic competitiveness.

Biomass resources of wood and straw supported early industrialization efforts until largely supplanted by fossil energy resources – coal, petroleum, and natural gas – and hydroelectric and nuclear power. Ancient uses of fire are still employed by a large fraction of the world's population that is without access to more expensive fuels or electricity. Firewood gathering constitutes a significant burden of work and environmental harm, and uncontrolled emissions are responsible for high levels of respiratory and other diseases mostly among women and children [2]. Firewood use in fireplaces and woodstoves for heating purposes is a major demand sector for biomass. These uses of biomass are typically associated with low conversion efficiency and high pollutant emissions. Although the sustainability of biomass production and conversion to fuels and power has recently seen increasing scrutiny due to indirect land use change and other effects associated with global food and energy markets [3], as a well-managed renewable resource biomass has the potential to contribute more substantially to the development of a sustainable economy. The combined processes of plant photosynthesis and respiration produce in biomass a chemically complex resource supporting a wide range of uses. Emulating these processes in manufacturing fuels and chemicals from sunlight but without the need of life processes is now viewed as one of the scientific grand challenges [4]. The energy storage in biomass also enables its use as a renewable resource for baseload power generation, an integral component in managing electricity distribution systems as generating capacity increases among more intermittent solar and wind energy resources.

Historically, and still so today, the most widely applied conversion method for biomass is combustion. The chemical energy of the fuel is converted via combustion into heat which is useful in and of itself, and which may be transformed by heat engines of various types into mechanical and, hence, electrical energy. Direct conversion of biomass to electricity by magnetohydrodynamic energy conversion has been investigated, but the technology is still speculative at this time. Burning of wood and agricultural materials in open fires and simple stoves for cooking and space heating is common around the world and a vital source of heat, although less desirable than advanced conversion techniques from the perspective of atmospheric pollution and undue health impacts from incomplete combustion.

Electricity generation using biomass fuels expanded rapidly in the USA following legislation changing utility regulatory policy in 1978, but stalled for economic and environmental reasons after the mid 1990s. US generating capacity at present is approximately 10 GWe of electricity, with global capacity about five times that amount [5]. Combustion plays a major role in waste disposal, complementing other waste management practices. Incentives for future expansion exist in the form of renewable portfolio standards, such as that enacted in California in 2002 calling for 20% renewable electricity by 2010 and 33% by 2020 [6]. Integration of power and heat generation in biorefinery operations will also lead to capacity expansions for biomass combustion and related systems. Meeting environmental and economic performance requirements into the future will prove challenging, however, and there continues to be the need for targeted research in advanced system design.

This chapter outlines technologies and performance issues in biomass combustion, summarizing system designs, feedstock properties, and environmental impacts. Combustion fundamentals are also briefly reviewed, including combustion stoichiometry, equilibrium, and kinetics. As highlighted by the simple burning of logwood, combustion is a complex process involving multiple simultaneous phenomena. More detailed predictive capability facilitating analysis, design, operation, control, and regulation remains a goal for further research and development.

2.2 Combustion Systems

2.2.1 Fuels

Combustor design and selection are dictated both by fuel type and end use. Within the class of biomass fuels are solids, gases, and liquids, the latter two being derived by physical, chemical, or biological conversion of the parent feedstock. Comparative properties of selected fuel types are listed in Table 2.1.

Table 2.1 Properties of selected fuels.

2.2.1.1 Solids

Solids constitute the primary class of biomass fuels, including woody and herbaceous materials such as wood and bark, lumber mill residues, grasses, cereal straws and stovers, other agricultural and forest residues, and energy crops such as switchgrass, Miscanthus, poplar, willow, and numerous others. Manures and other animal products include a fraction of solids that are also used as fuels. Municipal solid waste (MSW) is used in waste-to-energy (WTE) systems to provide volume reduction along with useful heat and electricity. Depending on location and local policies, WTE units may employ mass burning of unseparated wastes or combustion of separated wastes in which recyclables and other constituents have first been sorted from the waste stream. Properties of biomass feedstocks are reviewed in Section 2.3.1.

Other solids derived from biomass include torrefied materials and charcoal. Torrefaction is a light pyrolysis of the feedstock and results in a partially carbonized fuel with a lower moisture and volatile content than the original feedstock. Charcoal production is an ancient technology in which a large fraction of the volatile matter in biomass is first driven off by heating and pyrolysis. Charcoal yields from traditional processes are often below 10% of the biomass dry matter, with industrial charcoal making in the range up to about 30%, although more modern techniques can increase this substantially [7]. Charcoal is widely used throughout the world as a “smokeless” cooking and heating fuel, although pollutant emissions are still high in most applications using open fires and simple stoves. Traditional charcoal making as practiced in many countries is a heavily polluting process due to uncontrolled venting of volatiles to the atmosphere. In some applications, charcoal has advantages over crude biomass in terms of handling, storage, gasification, and combustion, but unless the manufacturing process includes energy recovery, a large fraction of the energy in biomass goes unutilized.

2.2.1.2 Gases

Gaseous fuels can be produced from biomass by anaerobic digestion, pyrolysis, gasification, and various fuel synthesis pathways using intermediates from these processes. The biological conversion of biomass through anaerobic digestion generates a biogas consisting primarily of methane (CH4) and carbon dioxide (CO2) with much smaller amounts of hydrogen sulfide (H2S), ammonia, and other products. The CH4 concentration typically ranges from 40 to 70% by volume, depending on the types of feedstock and reactor. Anaerobic digesters are employed for conversion of animal manures, MSWs, food wastes, and many other feedstocks, and have long been used in waste-water treatment operations. Incentives such as feed-in tariffs for renewable power have stimulated wider use of digesters for grain, energy crop, and other agricultural biomass in addition to wastes, especially in Europe. The anaerobic conditions in landfills also result in the production of a similar biogas. Biogas or landfill gas can be burned directly or treated to remove contaminants such as H2S to improve fuel value for reciprocating engines, microturbines, fuel cells, boilers, and other devices. Sulfur removal is important to avoid catalyst deactivation where stringent nitrogen oxides (NOx) emission limits must be met and post-combustion catalysts employed, a common problem for reciprocating engines used for power generation. Scrubbing of the biogas to remove CO2 and contaminants generates biomethane (or renewable natural gas), which in some cases is suitable for injection into utility natural gas pipelines.

Pyrolysis and gasification produce fuel gases, although pyrolysis is more generally optimized for solids or liquids production. Gasifiers generate fuel gases of variable composition depending on the type of feedstock and oxidant used and the reactor design. Air-blown units make a producer gas consisting of carbon monoxide (CO), H2, CO2, H2O along with hydrocarbons (HCs) and a large fraction of N2. Oxygen-blown units incur the cost of oxygen separation but eliminate nitrogen dilution in the gas to produce a synthesis quality gas, or syngas, useful for burning as well as chemical synthesis or electrochemical conversion via fuel cells after reforming to hydrogen (some fuel cells are internally reforming). Steam gasifiers also produce low nitrogen syngas, and several dual reactor designs have been developed to provide heat demand and energy for steam raising through residual char combustion. Syngas can be used to make substitute natural gas (SNG), another type of biomethane, and reformed to produce hydrogen. Details on gasification processes are described elsewhere.

2.2.1.3 Liquids

Liquid fuels from biomass include bio-oils produced by thermochemical processes, particularly pyrolysis; HCs, alcohols, and other fuels produced by chemical synthesis (e.g. Fischer–Tropsch) using syngas from gasification; ethanol, butanol, and other alcohols produced by fermentation of sugars derived from biomass; and lipids extracted from oil seeds, algae, and other oil-containing species. The latter can be refined to produce biodiesels through transesterification or enzyme-mediated reactions, or through hydrotreating to make HCs similar to petroleum-based fuels with higher heating value than the oxygenated biodiesels. Black liquor from chemical pulping is commonly burned in recovery boilers for chemical recycling and supply of heat and power to paper mills.

2.2.2 Types of Combustor

Biomass combustion involves a range of technologies from primitive open fires and traditional cooking stoves to highly controlled furnaces used for power generation and combined heat and power (CHP) applications. These span a wide range of scales, from kilowatt-size stoves to multi-megawatt furnaces and boilers. Current estimates of the energy in biomass used annually for traditional and modern combustion applications are 33.5 EJ and 16.6 EJ respectively [8]. The largest use of biomass by combustion is still in traditional cooking, heating, and lighting applications, mostly in developing nations. Pollutant emissions from these systems are a major health concern [2, 9] and contribute to greenhouse gas emissions. More modern uses for power generation and CHP are roughly equally deployed around the world among developed and developing nations. Cofiring of biomass with coal and other fuels is also expanding the industrial use of biomass for power and heat.

2.2.2.1 Small-scale Systems

Considerable effort is focused on the development of clean and efficient wood burning and other biomass combustion appliances for heating and cooking, both to reduce fuel demand and emissions. Developments in stove design for these types of application are the subject of active discussion and debate around the world [10]. More sophisticated stoves have been developed for residential and small commercial and industrial heating applications. These often involve automatic control and the use of preprocessed fuels, such as pellets, to maintain good control over the combustion and reduce emissions. Despite many improvements in combustor design, biomass remains one of the most difficult heating fuels to burn cleanly [11]. Small biomass systems typically emit considerable amounts of CO, particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), and other products of incomplete combustion. These emissions are exacerbated by heat control schemes that limit air supply to reduce the rate of heat output and the frequency of manually stoking new fuel to the stove. The ability to automatically fire more uniform fuels such as pellets provides substantially greater control over heat output rates while maintaining adequate air supply with reduced emissions compared with stick- or log-wood-fueled furnaces. The inclusion of a catalytic combustor in some designs improves emissions performance by continuing to react combustion products to lower temperatures (around 260°C) than would occur otherwise outside the primary firebox. Reductions in emissions have accompanied improvements in stove design, test standards, flue gas cleaning systems, system installation, and better education of users on stove operations [9]. Average emissions of CO, for example, have been reduced by half over the last decade. PM emissions from advanced pellet stoves now range from 15 to 25 mg MJ−1 compared with log-wood boilers and stoves that commonly exceed 300 mg MJ−1. Electrostatic precipitators and cloth baghouses are now being deployed for emission control on small systems in addition to their more conventional use on large-scale biomass combustors. The International Energy Agency (IEA) coordinates research and outreach on these and other combustion technologies through its Task 32, Biomass combustion and cofiring [9, 11, 12].

2.2.2.2 Large-scale Systems for Power and Heat Generation

Total installed capacity in biomass power generation around the world is approaching 50 000 MWe including large-scale solid fuel combustion as well as smaller scale digester and landfill gas applications [12]. In many regions of the world, Asia being an exception, biomass utilization is below the sustainable resource capacity and potential exists to increase uses for fuels, heat, and power [13].

The most common type of biomass-fueled power plant today utilizes the conventional Rankine or steam cycle (Figure 2.1). The fuel is burned in a boiler, which consists of a combustor with one or more heat exchangers used to make steam. Typical medium-efficiency units designed for biomass fuels utilize steam temperatures and pressures of up to 540°C and 6–10 MPa, although installed systems include pressures up to 17 MPa [14]. The steam is expanded through one or more turbines (or multistage turbines) that drive an electrical generator. In smaller systems, reciprocating and screw-type steam engines are sometimes used in place of the steam turbine. The steam from the turbine exhaust is condensed and the water recirculated to the boiler through feedwater pumps. Combustion products exit the combustor, are cleaned, and vented to the atmosphere. Typical cleaning devices include wet or dry scrubbers for control of sulfur and chlorine compounds, especially with WTE units burning MSW, cyclones (or other inertial separation devices), baghouses (high-temperature cloth filters), and/or electrostatic precipitators for PM removal. Selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) of NOx may also be included. Low CO and HC emissions are generally maintained by proper control of air/fuel ratio in the furnace and boiler. Organic fluids can also be used with the Rankine cycle instead of water, in which case the system is referred to as an organic Rankine cycle (ORC). These are typically applied to lower temperature operations such as waste heat recovery or solar thermal systems.

Figure 2.1 Schematic Rankine cycle

Fireside fouling of steam superheaters and other heat exchange equipment in boilers by ash is a particular concern with biomass fuels (Figure 2.2), and larger boiler designs frequently incorporate soot-blowing capacity for intermittent cleaning. Severe fouling may require an outage (shutdown) of the plant to remove deposits more aggressively. Such outages reduce operating time and thus increase the cost of delivered energy. Corrosion is also an issue with many biomass types, especially those containing higher concentrations of chlorine. Feedstock pretreatment to remove chlorine prior to firing has distinct advantages in reducing corrosion and fouling, but also increases cost.

Figure 2.2 Ash fouling on superheaters in biomass-fueled boilers. Left: flame impingement on a superheater pendant and incipient ash deposition during cofiring of energy cane biomass and coal. Center: ash deposits on a superheater in a wood-fired power boiler. Right: characteristic deposits along the leading edges of superheaters in a power boiler fueled with agricultural residues (wood, shells, and pits)

Individual Rankine cycle power plants principally using biomass fuels typically range up to about 50 MWe electrical generating capacity, which is reasonably small in comparison with coal-fired power plants more typically in the 500 MWe