thermochemical processing of biomass

Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, First Edition... Deswarte Biofuels Wim Soetaert & Erick Vandamme Handbook of Natural Colorants Thomas Be

Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, First Edition Edited by Robert C Brown

© 2011 John Wiley & Sons, Ltd Published 2011 by John Wiley & Sons, Ltd ISBN: 978-0-470-72111-7

in Renewable Resources Series Editor

Christian 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 & Ingeg€ard Johansson

Industrial Applications of Natural Fibres - Structure, Properties

and Technical Applications

J€org M€ussig

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

Processing of Biomass Conversion into Fuels, Chemicals and Power

Edited by ROBERT C BROWN Department of Mechanical Engineering,

Iowa State University, USA

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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.

thermochemical processing programs of the Center for Sustainable EnvironmentalTechnologies and the Bioeconomy Institute at Iowa State University.

1 Introduction to Thermochemical Processing of Biomass into Fuels,

3 Gasification 47Richard L Bain and Karl Broer

3.5 Classifying Gasifiers According to Transport Processes 58

David C Dayton, Brian Turk and Raghubir Gupta

5 Fast Pyrolysis 124Robbie H Venderbosch and Wolter Prins

6.6.3 Other Methods for Chemical Upgrading of Bio-oil 180

7.2.2 History of Hydrothermal Liquefaction Process Development 2027.2.3 History of Hydrothermal Gasification Process Development 203

7.4.7 Conclusions Relative to Hydrothermal Liquefaction 216

7.5.2 Process Description 217

7.5.4 Hydrothermal Gasification in Supercritical Water 2217.5.5 Conclusions Relative to Hydrothermal Gasification 2237.6 Pumping Biomass into Hydrothermal Processing Systems 223

Geoffrey A Tompsett, Ning Li and George W Huber

8.3.4 Supercritical Reactions – Reforming of Sugars 246

8.5.2 Biphasic Dehydration Reactions (HMF and Furfural Production) 255

8.7.3 Aromatics from Sugar Fragments in the Aqueous Phase 271

9.1.1 Biorefineries 2809.1.2 Hybrid Thermochemical/Biochemical Processing 281

9.2.7 Current Industrial Progress of Syngas Fermentation 291

10 Costs of Thermochemical Conversion of Biomass to Power

Mark M Wright and Robert C Brown

10.3.5 Gasification and Syngas Fermentation to PHA and

10.4.2 Bio-oil Upgrading to Gasoline and Diesel 316

Series Preface

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

The area interconnects several scientific disciplines (agriculture, biochemistry, try, technology, environmental sciences, forestry, etc.), which makes it very difficult to have

chemis-an expert view on the complicated interaction Therefore, the idea to create a series ofscientific books, focusing on specific topics concerning renewable resources, has been veryopportune 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 politicalstandpoints Even science is not free from hypes and buzzwords The use of renewableresources is again more important nowadays; however, it is not part of a hype or a fashion Asthe 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 newmaterial sources

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

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

Although our society was much more (almost exclusively) based on renewable resourcescenturies ago, this disappeared in the Western world in the 19th century Now it is time tofocus again on this field of research However, it should not mean a retoura la nature, but itshould be a multidisciplinary effort on a highly technological level to perform researchtowards 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 moresustainable ways to create prosperity and to fight poverty and hunger in the world A globalapproach is certainly favoured

This challenge can only be dealt with if scientists are attracted to this area and arerecognized for their efforts in this interdisciplinary field It is therefore also essential thatconsumers 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, theseries 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 ondifferent aspects of renewable resources I hope that readers become aware of thecomplexity, the interaction and interconnections, and the challenges of this field and thatthey will help to communicate the importance of renewable resources

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

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

Christian V StevensFaculty of Bioscience EngineeringGhent University, BelgiumSeries Editor Renewable Resources

June 2005

The genesis of this book was an invitation by Christian Stevens to describe the

‘thermochemical option’ for biofuels production at the Third International Conference onRenewable Resources and Biorefineries at Ghent University in 2007 At that time, manypeople working in the biofuels community viewed thermochemical processing as little morethan an anachronism in the age of biotechnology I was very appreciative of Chris’ interest inexploring alternative pathways After the conference, he followed up with an invitation tosubmit a book proposal on thermochemical processing to the Wiley Series in RenewableResources, for which he serves as Series Editor At the time I was busy with otherresponsibilities and declined his invitation A year later Chris repeated his offer and Iagreed to edit a volume on thermochemical production of biofuels, biobased chemicals, andbiopower I am very grateful that several prominent colleagues in the field agreed tocontribute chapters: Bryan Jenkins, Richard Bain, David Dayton, Wolter Prins,Tony Bridgwater, Douglas Elliott, George Huber, Mark Wright, and DongWon Choi Theproject editors at Wiley were extremely helpful and patient during the 2 years that mycolleagues and I struggled to find time to write on a subject that was rapidly moving fromobscurity to prominence and was presenting us with a variety of distractions These steadfastproject editors include Richard Davies, Jon Peacock, and Sarah Hall I am also indebted toseveral people who helped me with administrative and management responsibilities at theBioeconomy Institute (BEI) and the Center for Sustainable Environmental Technologies(CSET) at Iowa State University while this book was being prepared: Jill Euken, deputydirector of the BEI; Ryan Smith, deputy director of CSET; Becky Staedtler, businessmanager 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 allduring 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, USARobert C Brown, Department of Mechanical Engineering, Iowa State University, USADavid 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&MUniversity – Commerce, Commerce, TX 75429, USA

David C Dayton, Center for Energy Technology, RTI International, North Carolina, USAAlan A DiSpirito, Department of Biochemistry, Biophysics and Molecular Biology, IowaState University, USA

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

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

1 Introduction to Thermochemical

Processing of Biomass into Fuels,

Chemicals, and Power

“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 thefirst thermal transformations of biomass controlled by humans, while fermentation of fruits,honey, grains, and vegetables was practiced before recorded time Despite their longrecords of development, neither is mature, as the application of biotechnology to improvingbiochemical processes for industrial purposes has revealed [1] The petroleum andpetrochemical industries have accomplished similar wonders in thermochemical processing

of hydrocarbon feedstocks, although the more complicated chemistries of plant moleculeshave not been fully explored

Ironically, the domination of thermochemical processing in commercial production offuels, 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

Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, First Edition Edited by Robert C Brown

© 2011 John Wiley & Sons, Ltd Published 2011 by John Wiley & Sons, Ltd ISBN: 978-0-470-72111-7

pollutants from thermochemical processing of fossil fuels is an indelible icon from thetwentieth 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 sciencesalso offer opportunities for dramatic advances in biomass processing Thermochemicalprocessing has several advantages relative to biochemical processing As detailed inTable 1.1, these include the ability to produce a diversity of oxygenated and hydrocarbonfuels, 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 notrequire the sterilization procedures demanded for biological processing The data in Table 1.1also suggest that thermochemical processing can be done with much smaller plants than ispossible for biological processing of cellulosic biomass Although this may be true for somethermochemical options (such as fast pyrolysis), other thermochemical options (such asgasification-to-fuels) are likely to be built at larger scales than biologically based cellulosicethanol plants when the plants are optimized for minimum fuel production cost [3].The first-generation biofuels industry, launched in the late 1970s, was based onbiochemically processing sugar or starch crops (mostly sugar cane and maize respectively)into ethanol fuel and oil seed crops into biodiesel These industries grew tremendously inthe first decade of the twenty-first century, with worldwide annual production reachingalmost 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, anddebate over the environmental impact of biofuels agriculture, including uncertainties aboutthe 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 thatcan be grown on marginal or waste lands These alternative crops are of two types: lipidsfrom alternative crops and cellulosic biomass

Lipids are a large group of hydrophobic, fat-soluble compounds produced by plants andanimals 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 theproduction of biodiesel As illustrated in Figure 1.1, triglycerides consist of three long-chainfatty 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

Table 1.1 Comparison of biochemical and thermochemical processing Adapted from NSF,

2008, Breaking the Chemical and Engineering Barriers to Lignocellulosic Biofuels: Next GenerationHydrocarbon Biorefineries, Ed George W Huber, University of Massachusetts Amherst Courtesy:National Science Foundation

hydrotreaters at petroleum refineries [6], although the high price of traditional vegetable oilshas discouraged companies from producing transportation fuels in this manner Commercialdeployment 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 promisingalternative is microalgae, which can be highly productive in natural ecosystems with oilyields 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 largequantities of inexpensive lipids rather than upgrading them

Cellulose, on the other hand, is the most abundant form of biomass on the planet In theform of lignocellulose, a composite of cellulose, hemicellulose, and lignin [12], it dominatesmost natural ecosystems and is widely managed as sources of timber and animal forage Asillustrated in Figure 1.2, cellulose is a structural polysaccharide consisting of a long chain ofglucose molecules linked by glycosidic bonds Breaking these bonds releases the glucose

H R2

H R3

OH

O

O

CH2OH OH

OH

O

CH2OH

O OH

OH

O

O

CH2OH OH OH

Glycosidic Bonds Glucose Unit

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

and makes it available for either food or fuel production A variety of microorganismssecrete enzymes that hydrolyze the glycosidic bonds of cellulose (and hemicellulose) Manyanimals, like cattle and other ruminants, have developed symbiotic relationships with thesemicroorganisms to allow them to digest cellulose.

However, cellulose is usually found in nature as lignocellulose, a composite of cellulosefibers in a matrix of hemicellulose and lignin The lignin, which few microorganisms areable to digest, protects the carbohydrate against biological attack Thus, even ruminantanimals 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 thepolysaccharides and all of the lignin pass through the gut undigested Biochemicalprocessing has many similarities to the digestive system of ruminant animals Physicaland chemical pretreatments release cellulose fibers from the composite matrix, makingthem more susceptible to enzymatic hydrolysis, which releases simple sugars that can befermented 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 oreven days to complete even in the presence of biocatalysts

Thermochemical processing occurs at temperatures that are at least several hundreddegrees Celsius and sometimes over 1000C above ambient conditions At these tem-peratures, thermochemical processes occur very rapidly whether catalysts are present ornot 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 indis-criminate as is sometimes attributed to it Thermal depolymerization of cellulose inthe absence of alkali or alkaline earth metals produces predominately levoglucosan, ananhydrosugar of the monosaccharide glucose [14] Under certain conditions, it appearsthat lignin depolymerizes to monomeric phenolic compounds [15] Under conditions ofhigh-temperature combustion and gasification, chemical equilibrium among products isattained Thus, thermochemical processing offers opportunities for rapid processing ofdiverse 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(800–1600C) suitable for electric power generation Gasification generates both moderate-temperature thermal energy (700–1000C) and a flammable gas mixture known commonly

as producer gas or syngas, which can be used to generate either electric power or tosynthesize fuels or other chemicals using catalysts or even microorganisms (syngasfermentation) [16] Fast pyrolysis occurs at moderate temperatures (450–550C) in theabsence 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 steamreforming or into liquid hydrocarbons via hydroprocessing Whereas fast pyrolysis requiresrelatively dry feedstocks (around 10 wt% moisture), hydrothermal processing is ideal forwet 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 500C, dependingupon whether the desired products are fractionated plant polymers [18], a partiallydeoxygenated liquid product known as biocrude [19], or syngas [20] Finally, hydrolysis

of plant polysaccharides yields simple sugars that can be catalytically or biocatalyticallyconverted into fuels Concentrated acid or the combined action of dilute acid and heat arewell known to hydrolyze polysaccharides to monosaccharides The biotechnology revolu-tion 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 cellulosicbiofuels by this biochemical route [21] Although acid hydrolysis qualifies as thermochem-ical 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 ofthe cellulose can produce glucose solutions Fast pyrolysis also yields significant quantities

of sugars and anhydrosugars under suitable processing conditions [22] These “thermolyticsugars” can either be fermented or catalytically upgraded to fuel molecules

1.2 Direct Combustion

Much of the focus on bioenergy in the USA has been production of liquid transportationfuels in an effort to displace imported petroleum Recently, it has been argued that a betteruse of biomass would be to burn it for the generation of electricity to power battery electricvehicles (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 andmoisture content of the fuel, the amount of air used to burn the fuel, and the construction ofthe furnace, flame temperatures can exceed 1650C Direct combustion has the advantagethat it employs commercially well-developed technology It is the foundation of much of the

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

electric power generation around the world In principle, existing power plants could bequickly and inexpensively retrofitted to burn biomass, compared with greenfield construc-tion 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 thelong term, high efficiency combined-cycle power plants based on gasified or pyrolyzedbiomass will provide power for long-range electric vehicles based on advanced batterytechnology [25].

However, combustion is burdened by three prominent disadvantages These includepenalties associated with burning high-moisture fuels, agglomeration and ash fouling due

to alkali compounds in biomass, and difficulty of providing and safeguarding sufficientsupplies 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 andunder 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 thegasification of coal to produce so-called “manufactured gas” or “town gas” for heatingand lighting Coal gasification has also been used for large-scale production of liquidtransportation fuels, first in Germany during World War II and then later in South Africaduring 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 molecularweight gas mixtures In fact, the high volatile matter content of biomass allows it to begasified more readily than coal Biomass gasification has found commercial applicationwhere waste wood was plentiful or fossil resources were scarce An example of the formerwas Henry Ford’s gasification of wood waste derived from shipping crates at his earlyautomotive plants [27] An example of the latter was the employment of portable woodgasifiers 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 theemergence 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 thetwenty-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 itsflexibility of application, including thermal power generation, hydrogen production, andsynthesis of fuels and chemicals This offers the prospect of gasification-based energyrefineries, producing a mix of energy and chemical products or allowing the stagedintroduction of technologies as they reach commercial viability

The simplest application of gasification is production of heat for kilns or boilers Often thesyngas can be used with minimal clean-up because tars or other undesirable compoundsare consumed when the gas is burned and process heaters are relatively robust to dirty gasstreams The syngas can be used in internal combustion engines if tar loadings are not toohigh 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 bined-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 moresensitive to contaminants than are gas turbines.

gasification–com-Chapter 3 describes gasification technologies, gasification–com-Chapter 4 covers gas stream clean-up andcatalytic upgrading to fuels and chemicals, and Chapter 9, which covers hybrid thermo-chemical–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 ofoxygen to produce liquids, gases, and char [17] The distribution of products depends on thebiomass composition and rate and duration of heating Liquid yields as high as 72% arepossible for relatively short residence times (0.5–2 s), moderate temperatures (400–600C),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 bothboilers and gas turbine engines, although its cost, corrosiveness, and instability duringstorage have impeded its commercial deployment

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

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

Hydroprocessing bio-oil into hydrocarbons suitable as transportation fuel is similar tothe process for refining petroleum Hydroprocessing was originally developed to convertpetroleum into motor fuels by reacting it with hydrogen at high pressures in the presence ofcatalysts Hydroprocessing includes two distinct processes Hydrotreating is designed toremove sulfur, nitrogen, oxygen, and other contaminants from petroleum When adapted tobio-oil, the main contaminant to be removed is oxygen Thus, hydrotreating bio-oil isprimarily 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-chainmolecules into lower molecular weight compounds Although fast pyrolysis attempts todepolymerize plant molecules, a number of carbohydrate and lignin oligomers are found

in bio-oil, which hydrocracking can convert into more desirable paraffin or naphthenemolecules Some researchers are attempting to add catalysis to the pyrolysis reactor to yieldhydrocarbons directly Similar to the process of fluidized catalytic cracking used in thepetroleum industry, the process occurs at atmospheric pressure over acidic zeolites A yield

of 17% of C5–C10hydrocarbons has been reported in a study of upgrading of pyrolyticliquids from poplar wood [33] Although superior to conventional bio-oil, this product stillneeds refining to gasoline or diesel fuel Fast pyrolysis of biomass to bio-oil is described inChapter 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 elevatedpressures to produce carbohydrate, liquid hydrocarbons, or gaseous products dependingupon the reaction conditions As illustrated in Figure 1.5, processing temperature must be

Figure 1.5 Temperature/pressure regimes of hydrothermal processing

increased as reaction temperature increases to prevent boiling of water in the wet biomass.

At temperatures around 100C, extraction of high-value plant chemicals such as resins, fats,phenolics, and phytosterols is possible At 200C and 20 atm (
2 MPa), fibrous biomassundergoes a fractionation process to yield cellulose, lignin, and hemicelluloses degradationproducts such as furfural Further hydrothermal processing can hydrolyze the cellulose toglucose At 300–350C and 120–180 atm (
12.2–18.2 MPa), biomass undergoes moreextensive 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 600–650C and 300 atm (30.4 MPa)the primary reaction product is gas, including a significant fraction of methane

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

1.6 Hydrolysis to Sugars

Although biochemical processing is sometimes referred to as the “sugar platform,” it ispossible to thermally depolymerize biomass into monosaccharides and catalyticallysynthesize fuel molecules from these carbohydrate building blocks Thus, the so-calledsugar platform can be a pure play in biochemical processing (enzymatic hydrolysis of plantcarbohydrates to sugar followed by fermentation), a hybrid thermochemical–biochemicalprocess (thermally or chemically induced hydrolysis followed by fermentation of thereleased sugar), a hybrid biochemical–thermochemical process (enzymatic hydrolysisfollowed by catalytic synthesis of the sugar to hydrocarbons), or a pure play in thermo-chemical processing (thermal depolymerization followed by catalytic upgrading of thesugar to fuel molecules)

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

ferment-These sugars can also be catalytically converted to fuels Sugars that exist as five-memberrings, like the five-carbon sugar xylose or the six-carbon sugar fructose, are readilydehydrated to the five-member rings of furan compounds [35], some examples of whichare illustrated in Figure 1.6 Furans are colorless, water-insoluble flammable liquids withvolatility comparable to hydrocarbons of similar molecular weight Some kinds of furanshave heating values and octane numbers comparable to gasoline, making them potentialtransportation fuel [36] Catalysts can improve yields by making furan-producing pathwaysmore selective among the large number of competing reactions that can occur duringpyrolysis of biomass 2,5-Dimethyl furan in particular has received recent interest becausenew 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

A more promising approach known as aqueous-phase processing reacts monomericsugar or sugar-derived compounds in the presence of heterogeneous catalysts at200–260C 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 overfermentation, including higher throughputs, ready conversion of a wide range of sugars,and the immiscible hydrocarbon products could be recovered without the expensivedistillations required in ethanol plants Chapter 8 explores the possibilities of catalyticallyconverting 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 biomassfeedstocks, 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 commer-cial-scale facilities, despite the well-known limitations of such analysis Although by nomeans comprehensive, Chapter 10 provides cost estimates for a wide range of thermochemi-cal processes, ranging from electric power generation to the production of biopolymers andhydrogen via syngas fermentation Although differences in basis years, feedstock costs,financing options, and granularity of the analyses make it difficult to make comparisonsamong the various technology options, these analyses provide a useful starting point forexploring the feasibility of different approaches to thermochemical processing

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

[3] Wright, M and Brown, R.C (2007) Establishing the optimal sizes of different kinds ofbiorefineries Biofuels, Bioprocessing, and Biorefineries,1, 191–200.

[4] US Energy Information Agency (2008) International Energy Statistics, Renewables, http://tonto.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid¼79&pid¼79&aid¼1 (accessed 28 November2010)

[5] Farrell, A.E., Plevin, R.J., Turner, B.T et al (2006) Ethanol can contribute to energy andenvironmental goals Science,311, 506–508

[6] Kram, J.W (2009) Aviation alternatives Biodiesel Magazine (January), magazine.com/article.jsp?article_id¼3071 (accessed 25 May 2009)

http://www.biodiesel-[7] Klass, D.L (1998) Biomass for Renewable Energy, Fuels, and Chemicals, Academic Press,San Diego, CA, p 340

[8] Trabucco, A., Achten, W.M.J., Bowe, C et al (2010) Global mapping of Jatropha curcas yieldbased on response of fitness to present and future climate GCB Bioenergy,2, 139–151.[9] Basiron, Y (2007) Palm oil production through sustainable plantations European Journal ofLipid Science and Technology,109, 289–295

[10] Sheehan, J., Dunahay, T., Benemann, J., and Roessler, P (1998) A look back at the U.S.Department of Energy’s Aquatic Species program, US DOE National Renewable EnergyLaboratory Report, NREL/TP-580-24190, July

[11] Lundquist, T.J., Woertz, I.C., Quinn, N.W.T., and Benemann, J.R (2008) A realistic technologyand engineering assessment of algal biofuel production, Technical Report, Energy BiosciencesInstitute, University of California, Berkeley, CA, October

[12] Sjostrom, E (1993) Wood Chemistry: Fundamentals and Applications, second edition,Academic Press, San Diego, CA

[13] Brown, R.C (2003) Biorenewable Resources: Engineering New Products from Agriculture,Iowa State Press, Ames, IA pp 169–179

[14] Patwardhan, P.R., Satrio, J.A., Brown, R.C., and Shanks, B.H (2009) Product distribution fromfast pyrolysis of glucose-based carbohydrates Journal of Analytical and Applied Pyrolysis,86,323–330

[15] Patwardhan, P.R., Johnston, P.A., Brown, R.C., and Shanks, B.H (2010) Understanding fastpyrolysis of lignin Preprint Papers – American Chemical Society, Division of Fuel Chemistry,

55 (2), 104

[16] Brown, R.C (2005) Biomass Refineries based on hybrid thermochemical/biological processing –

an overview, in Biorefineries, Biobased Industrial Processes and Products (eds B Kamm,P.R Gruber, and M Kamm), Wiley-VCH Verlag GmbH, Weinheim

[17] Bridgwater, A.V and Peacocke, G.V.C (2000) Fast pyrolysis processes for biomass Renewableand Sustainable Energy Reviews,4, 1–73

[18] Allen, S.G., Kam, L.C., Zemann, A.J., and Antal, M.J., Jr., (1996) Fractionation of sugar canewith hot, compressed, liquid water Industrial & Engineering Chemistry Research, 35,2709–2715

[19] Elliott, D.C., Beckman, D., Bridgwater, A.V et al (1991) Developments in direct cal liquefaction of biomass: 1983–1990 Energy and Fuels,5 (3), 399–410

thermochemi-[20] Elliott, D.C., Neuenschwander, G.G., Hart, T.R et al (2004) Chemical processing in pressure aqueous environments 7 Process development for catalytic gasification of wet biomassfeedstocks Industrial & Engineering Chemistry Research,43, 1999–2004

high-[21] Service, R.F (2010) Is there a road ahead for cellulosic ethanol? Science,329, 784–785.[22] Brown, R.C., Radlein, D., and Piskorz, J (2001) Pretreatment processes to increase pyrolyticyield of levoglucosan from herbaceous feedstocks, in Chemicals and Materials fromRenewable Resources (ed J.J Bozell), ACS Symposium Series No 784, American ChemicalSociety, Washington, DC, pp 123–132

[23] Campbell, J.E., Lobell, D.B., and Field, C.B (2009) Greater transportation energy and GHGoffsets from bioelectricity than ethanol Science,324, 1055–1057

[24] Gifford, J and Brown, R.C., personal communication, December 2, 2010

[25] Brown, R.C and Wright, M (2009) Biomass conversion to fuels and electric power, in Biofuels:Environmental Consequences and Interactions with Changing Land Use, Proceedings of the

Scientific Committee on Problems of the Environment (SCOPE) International Biofuels ProjectRapid Assessment, 22–25 September 2008 Gummersbach, Germany (eds R.W Howarth and

S Bringezu), Cornell University, Ithaca, NY (http://cip.cornell.edu/biofuels/)

[26] Rezaiyan, J and Cheremisinoff, N.P (2005) Gasification Technologies: A Primer for Engineersand Scientists, Taylor & Francis, Boca Raton, FL

[27] Reigel, E.R (1933) Industrial Chemistry, 2nd edn, The Chemical Catalog Company, Inc.,New York, p 253

[28] Cummer, K and Brown, R.C (2002) Ancillary equipment for biomass gasification Biomassand Bioenergy,23, 113–128

[29] Czernik, S., French, R., Feik, C., and Chornet, E (2002) Hydrogen by catalytic steam reforming

of liquid byproducts from biomass thermoconversion processes Industrial & EngineeringChemistry Research,41, 4209–4215

[30] Elliott, D.C (2007) Historical development in hydroprocessing bio-oils Energy and Fuels,

21, 1792–1815

[31] Marker, T.L., Petri, J., Kalnes, T et al (2005) Opportunities for Biorenewables in Oil Refineries,Final Technical Report, US Department of Energy, Prepared by UOP, Inc., 12 December, http://www.osti.gov/bridge/purl.cover.jsp;jsessionid¼CE524ACAABE8C174BD29C25416E6C780?purl¼/861458-Wv5uum/ (accessed 28 November 2010)

[32] Wright M.M., Daugaard D.E., Satrio J.A., and Brown R C (2010) Techno-economic analysis ofbiomass fast pyrolysis to transportation fuels Fuel,89 (Supplement 1), S2-S10 DOI: 10.1016/j.fuel.2010.07.029

[33] Carlson, T., Vispute, T., and Huber, G (2008) Green gasoline by catalytic fast pyrolysis of solidbiomass derived compounds ChemSusChem,1, 397–400

[34] So, K.S and Brown, R.C (1999) Economic analysis of selected lignocellulose-to-ethanolconversion technologies Applied Biochemistry and Biotechnology,77, 633–640

[35] Lewkowski, J (2001) Synthesis, chemistry and applications of 5-hydroxymethylfurfural and itsderivatives ARKIVOC,1, 17–54

[36] Bayan, S and Beati, E (1941) Furfural and its derivatives as motor fuels Chimica e Industria,

[40] Kunkes, E.L., Simonetti, D.A., West, R.M et al (2008) Catalytic conversion of biomass tomonofunctional hydrocarbons and targeted liquid-fuel classes Science,322, 417–421

2 Biomass Combustion

Bryan M Jenkins,1Larry L Baxter2and Jaap Koppejan3

1University of California, Davis, CA, USA2

Brigham Young University, UT, USA3

Procede Biomass BV, Enschede, Netherlands

Nomenclature

Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, First Edition Edited by Robert C Brown

© 2011 John Wiley & Sons, Ltd Published 2011 by John Wiley & Sons, Ltd ISBN: 978-0-470-72111-7

2.1 Introduction

Since humans first learned to manage fire a quarter of a million years ago or more [1], theburning 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 increaseand concentration of populations, the waging of world wars, and the globalization of tradeand culture As the world population continues to expand, the environmental impacts ofcurrent 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 emission alternatives, and the future design of combustion systems will be heavilychallenged to adapt to more stringent regulations affecting environmental performancewhile maintaining economic competitiveness

zero-Biomass resources of wood and straw supported early industrialization efforts untillargely supplanted by fossil energy resources – coal, petroleum, and natural gas – andhydroelectric 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, anduncontrolled emissions are responsible for high levels of respiratory and other diseasesmostly among women and children [2] Firewood use in fireplaces and woodstoves forheating purposes is a major demand sector for biomass These uses of biomass are typicallyassociated with low conversion efficiency and high pollutant emissions Although thesustainability of biomass production and conversion to fuels and power has recently seenincreasing scrutiny due to indirect land use change and other effects associated with globalfood and energy markets [3], as a well-managed renewable resource biomass has thepotential to contribute more substantially to the development of a sustainable economy Thecombined processes of plant photosynthesis and respiration produce in biomass a chemi-cally complex resource supporting a wide range of uses Emulating these processes inmanufacturing fuels and chemicals from sunlight but without the need of life processes isnow viewed as one of the scientific grand challenges [4] The energy storage in biomassalso enables its use as a renewable resource for baseload power generation, an integralcomponent in managing electricity distribution systems as generating capacity increasesamong 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 heatwhich is useful in and of itself, and which may be transformed by heat engines of varioustypes into mechanical and, hence, electrical energy Direct conversion of biomass toelectricity by magnetohydrodynamic energy conversion has been investigated, but thetechnology is still speculative at this time Burning of wood and agricultural materials inopen fires and simple stoves for cooking and space heating is common around the worldand a vital source of heat, although less desirable than advanced conversion techniquesfrom the perspective of atmospheric pollution and undue health impacts from incompletecombustion

Electricity generation using biomass fuels expanded rapidly in the USA followinglegislation changing utility regulatory policy in 1978, but stalled for economicand environmental reasons after the mid 1990s US generating capacity at present is

approximately 10 GWeof electricity, with global capacity about five times that amount [5].Combustion plays a major role in waste disposal, complementing other waste managementpractices 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 and33% by 2020 [6] Integration of power and heat generation in biorefinery operations willalso lead to capacity expansions for biomass combustion and related systems Meetingenvironmental and economic performance requirements into the future will prove chal-lenging, however, and there continues to be the need for targeted research in advancedsystem design.

This chapter outlines technologies and performance issues in biomass combustion,summarizing system designs, feedstock properties, and environmental impacts Combus-tion fundamentals are also briefly reviewed, including combustion stoichiometry, equilib-rium, and kinetics As highlighted by the simple burning of logwood, combustion is

a complex process involving multiple simultaneous phenomena More detailed predictivecapability facilitating analysis, design, operation, control, and regulation remains a goal forfurther research and development

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 ofselected fuel types are listed in Table 2.1

2.2.1.1 Solids

Solids constitute the primary class of biomass fuels, including woody and herbaceousmaterials 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 ofunseparated wastes or combustion of separated wastes in which recyclables and otherconstituents have first been sorted from the waste stream Properties of biomass feedstocksare 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 lowermoisture and volatile content than the original feedstock Charcoal production is an ancienttechnology in which a large fraction of the volatile matter in biomass is first driven off byheating and pyrolysis Charcoal yields from traditional processes are often below 10% of thebiomass dry matter, with industrial charcoal making in the range up to about 30%, althoughmore modern techniques can increase this substantially [7] Charcoal is widely usedthroughout the world as a “smokeless” cooking and heating fuel, although pollutant

emissions are still high in most applications using open fires and simple stoves Traditionalcharcoal making as practiced in many countries is a heavily polluting process due touncontrolled venting of volatiles to the atmosphere In some applications, charcoal hasadvantages 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 Thebiological conversion of biomass through anaerobic digestion generates a biogas consistingprimarily of methane (CH4) and carbon dioxide (CO2) with much smaller amounts ofhydrogen sulfide (H2S), ammonia, and other products The CH4concentration typicallyranges 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 digestersfor grain, energy crop, and other agricultural biomass in addition to wastes, especially inEurope The anaerobic conditions in landfills also result in the production of a similarbiogas 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 stringentnitrogen oxides (NOx) emission limits must be met and post-combustion catalysts em-ployed, a common problem for reciprocating engines used for power generation Scrubbing

of the biogas to remove CO2and contaminants generates biomethane (or renewable naturalgas), which in some cases is suitable for injection into utility natural gas pipelines.Pyrolysis and gasification produce fuel gases, although pyrolysis is more generallyoptimized for solids or liquids production Gasifiers generate fuel gases of variablecomposition 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, H2Oalong with hydrocarbons (HCs) and a large fraction of N2 Oxygen-blown units incur thecost of oxygen separation but eliminate nitrogen dilution in the gas to produce a synthesisquality gas, or syngas, useful for burning as well as chemical synthesis or electrochemicalconversion via fuel cells after reforming to hydrogen (some fuel cells are internallyreforming) Steam gasifiers also produce low nitrogen syngas, and several dual reactordesigns have been developed to provide heat demand and energy for steam raising throughresidual char combustion Syngas can be used to make substitute natural gas (SNG), anothertype of biomethane, and reformed to produce hydrogen Details on gasification processesare 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 alcoholsproduced 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 biodieselsthrough transesterification or enzyme-mediated reactions, or through hydrotreating to makeHCs similar to petroleum-based fuels with higher heating value than the oxygenatedbiodiesels Black liquor from chemical pulping is commonly burned in recovery boilers forchemical 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 andtraditional cooking stoves to highly controlled furnaces used for power generation andcombined heat and power (CHP) applications These span a wide range of scales, fromkilowatt-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 EJand 16.6 EJ respectively [8] The largest use of biomass by combustion is still in traditionalcooking, heating, and lighting applications, mostly in developing nations Pollutantemissions from these systems are a major health concern [2, 9] and contribute to greenhousegas emissions More modern uses for power generation and CHP are roughly equallydeployed around the world among developed and developing nations Cofiring of biomasswith 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 andother biomass combustion appliances for heating and cooking, both to reduce fuel demandand emissions Developments in stove design for these types of application are the subject ofactive discussion and debate around the world [10] More sophisticated stoves have beendeveloped for residential and small commercial and industrial heating applications Theseoften involve automatic control and the use of preprocessed fuels, such as pellets, tomaintain good control over the combustion and reduce emissions Despite many improve-ments in combustor design, biomass remains one of the most difficult heating fuels to burncleanly [11] Small biomass systems typically emit considerable amounts of CO, particulatematter (PM), polycyclic aromatic hydrocarbons (PAHs), and other products of incompletecombustion 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 substantiallygreater control over heat output rates while maintaining adequate air supply with reducedemissions compared with stick- or log-wood-fueled furnaces The inclusion of a catalyticcombustor in some designs improves emissions performance by continuing to reactcombustion products to lower temperatures (around 260C) than would occur otherwiseoutside the primary firebox Reductions in emissions have accompanied improvements instove design, test standards, flue gas cleaning systems, system installation, and bettereducation of users on stove operations [9] Average emissions of CO, for example, have beenreduced by half over the last decade PM emissions from advanced pellet stoves now rangefrom 15 to 25 mg MJ
1compared with log-wood boilers and stoves that commonly exceed

300 mg MJ
1 Electrostatic precipitators and cloth baghouses are now being deployed foremission control on small systems in addition to their more conventional use on large-scalebiomass combustors The International Energy Agency (IEA) coordinates research and

outreach on these and other combustion technologies through its Task 32, Biomasscombustion 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 MWeincluding large-scale solid fuel combustion as well as smaller scale digesterand 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 toincrease uses for fuels, heat, and power [13]

The most common type of biomass-fueled power plant today utilizes the conventionalRankine 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 efficiency units designed for biomass fuels utilize steam temperatures and pressures of up to

medium-540C and 6–10 MPa, although installed systems include pressures up to 17 MPa [14] Thesteam is expanded through one or more turbines (or multistage turbines) that drive anelectrical generator In smaller systems, reciprocating and screw-type steam engines aresometimes used in place of the steam turbine The steam from the turbine exhaust iscondensed and the water recirculated to the boiler through feedwater pumps Combustionproducts exit the combustor, are cleaned, and vented to the atmosphere Typical cleaningdevices 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

Emission Control

Fly ash Stack Exhaust

Electricity

Steam Turbine Superheated Steam

Water

Boiler

Fuel

Bottom Ash Air

Cooling Medium

Condensor Boiler

Feedwater Pump

Generator

Figure 2.1 Schematic Rankine cycle

removal Selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) of

NOxmay also be included Low CO and HC emissions are generally maintained by propercontrol of air/fuel ratio in the furnace and boiler Organic fluids can also be used with theRankine cycle instead of water, in which case the system is referred to as an organic Rankinecycle (ORC) These are typically applied to lower temperature operations such as waste heatrecovery or solar thermal systems

Fireside fouling of steam superheaters and other heat exchange equipment in boilers byash is a particular concern with biomass fuels (Figure 2.2), and larger boiler designsfrequently incorporate soot-blowing capacity for intermittent cleaning Severe fouling mayrequire an outage (shutdown) of the plant to remove deposits more aggressively Suchoutages reduce operating time and thus increase the cost of delivered energy Corrosion isalso an issue with many biomass types, especially those containing higher concentrations ofchlorine Feedstock pretreatment to remove chlorine prior to firing has distinct advantages

in reducing corrosion and fouling, but also increases cost

Individual Rankine cycle power plants principally using biomass fuels typically range up

to about 50 MWeelectrical generating capacity, which is reasonably small in comparisonwith coal-fired power plants more typically in the 500 MWe region Larger sizes arepossible, and size selection is accomplished through an analysis of fuel resource availability,plant design and economy, electricity and heat markets, and local regulations Thedistributed nature of biomass fuels and the limited economy of scale associated withplants of this type have kept the size of individual facilities relatively small in comparisonwith coal or nuclear generating stations A 350 MWebiomass power plant has been proposedfor Wales burning wood from North America [15], and optimization studies have suggestedlarger sizes are feasible than most currently built [16, 17] The efficiencies of biomass powerplants are generally lower than comparable fossil-fueled units because of higher fuelmoisture content, lower steam temperatures and pressures to control fouling at highercombustion gas temperatures, and to some extent the smaller sizes, with a proportionately

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 andcoal Center: ash deposits on a superheater in a wood-fired power boiler Right: characteristicdeposits along the leading edges of superheaters in a power boiler fueled with agriculturalresidues (wood, shells, and pits)

higher parasitic power demand needed to run the pumps, fans, and other components of thepower station Biomass integrated gasification combined cycles (BIGCCs) are projected toexceed 35% electrical efficiency Cofiring biomass in higher capacity, higher efficiencyfossil (e.g coal) stations can also lead to higher efficiency in biomass-fueled powergeneration [12] CHP applications realize much higher efficiencies (80% or better) due

to the use of much of the heat that is otherwise rejected in power-only applications.Power boilers utilize three principal types of combustor: grate burners, suspensionburners, and fluidized beds [18] The differences in these units relate primarily to the relativevelocities of fuel particles and gas and the presence of an intervening heat transfer medium

as in the fluidized bed They also differ in their abilities to handle fouling-type fuels, theirlevels of emissions, and a number of other operating considerations

2.2.2.3 Cofiring

Cofiring is the simultaneous burning of two or more fuels in the same combustor [5].Cofiring is an attractive option for reducing greenhouse gas emissions associated with thecombustion of coal and for utilizing biomass at higher efficiencies than in most biomass-dedicated power plants Cofiring has advanced rapidly in a number of countries, with morethan 230 operations around the world at present [9] Roughly half of these are in pulverizedcoal facilities, with the rest principally in bubbling and circulating fluidized beds along with

a few grate-fired units Typical biomass cofiring rates without derating the plant are 5–10%

of total fuel input energy Even at these fractions, cofiring in a large coal plant requires

a substantial biomass supply, similar to a 25–50 MWebiomass plant or larger Moderateinvestments are needed for storage and handling equipment until the fraction of biomassbegins to exceed about 10% Beyond this, or if the biomass is fired separately into thefurnace, changes are needed to mills, burners, dryers, and other equipment which increasecost New pulverized coal units now cofire up to about 40% biomass [9]

Biomass can be added in a cofiring application by pre-mixing with the coal prior toinjection into the furnace, by direct injection with the coal, or by burning separately in thesame furnace [19] A special case is that of firing biomass in the upper levels of the furnace as

a reburning fuel to help control NOxemissions Reburning is a multistage (normally two)fuel-injection technique which uses fuel as a reducing agent to react with and remove

NOx[20] Metal oxide promoters (Na-, K-, Ca-containing additives) can be injected with ordownstream of the reburning fuel to enhance the NOxreduction, although modeling studieshave shown that neither of these locations is as effective as co-injection with the mainfuel [21] Gasification of biomass and cofiring of producer gas have also been tested forreburning purposes [22]

Various technical concerns associated with biomass cofiring in coal facilities include fuelpreparation, storage, handling, and supply, ash deposition, fuel conversion, pollutantformation, corrosion, ash utilization, impacts on SCR systems for emission control, andformation of striated flows in the boiler, and research is directed toward understanding andmitigating adverse impacts [23]

Addition of biomass ash may also influence the value of coal fly ash used for constructionand other materials, such as concrete additives [12], although there appear to have been fewquality concerns with cofired wood fuels [11] Higher concentrations of alkali metals andchlorine in straw and other herbaceous fuels may be of more concern, but most impacts

appear to be manageable [23] Restrictions on comingled ash in various standards forconcrete admixtures pose significant penalties on cofiring operations, and research iscontinuing on this issue [24] New standards have been developed that include testingprocedures to ensure quality of comingled ash is not reduced compared with coal ash forthese applications.

2.2.2.4 Alternative Combustion and Power Generation Concepts

Steam Rankine cycles are the dominant power generation concept employed at present forsolid biomass fuels Alternatives to the conventional steam cycle include various enhance-ments, including supercritical Rankine cycles which operate at higher temperatures andpressures (above the critical point of water at 22.1 MPa, 647 K), and are more commonlyused with coal and other fossil fuels and for biomass cofiring Ash fouling and superheatercorrosion are a primary concern with higher temperature systems, so most biomass-dedicated plants remain subcritical

ORCs operate on the same cycle as steam power stations, but they replace water withanother working fluid such as ammonia or an HC like propane or butane Operatingtemperatures are generally lower for ORC units that are now being deployed with biomass

at scales of around 400 to 1500 kWewith efficiencies of up to 20 % [25] ORC units canalso be deployed to take advantage of waste heat in cogeneration applications to improveoverall efficiency

Conversion of biomass by gasification to make producer gas is another alternative forpower generation using conventional spark-ignited or dual-fuel compression-ignitedreciprocating engines The technology has a long history of use and development, andtypically suffers from inadequate gas purification for small distributed and transportation-related applications New designs are emerging, however, that offer improved performanceand longer service

Other cycles used with solid biomass fuels include Brayton (gas turbine) and Stirlingengines Attempts have also been made to direct-fire powdered biomass into Dieselengines [26], but success is limited due to scoring of the cylinder walls and other problems

in handling solids and ash Of these, the most advanced for use with solid fuels is the Stirlingengine, although this class of engines remains mostly developmental in this application.Direct-fired gas-turbine engines have received considerable attention, but cleaning thecombustion products sufficiently to run through the turbine blading has proved difficult [27].Indirect-fired (hot air) turbines have also been investigated, but in these engines high-temperature heat exchange becomes a limiting issue Compression-ignited (Diesel), spark-ignited (Otto), and Brayton engines, especially microturbines, are currently used withbiogas and landfill gas, biomethane, biodiesel, and alcohols, and should also be compatiblewith HCs, mixed alcohols, SNG, and other clean fuels made from syngas by Fischer–Tropsch synthesis and other techniques Pyrolysis oils (bio-oils) and vegetable oils can also

be used after hydrotreating or other refining to improve viscosity and stability, removeoxygen, and reduce corrosivity BIGCCs, in which biomass is gasified to generate a fuel gas(producer gas or syngas) that can be used in a combined gas-turbine–steam cycle (combinedcycle), similar to the use of coal in an integrated gasification combined cycle, have beenunder active investigation for improving power plant efficiency and potentially repoweringexisting steam plants [28, 29] The V€arnamo BIGCC plant demonstrated in Sweden wasdesigned for a net electrical efficiency of 32% (lower heating value) while simultaneously

generating 6 MWeof electricity and 9 MWtof heat for district heating Overall efficiencywas rated at 83% in cogeneration mode [28].

Other advanced power generation options include fuel cells, in which the oxidation iscarried out electrochemically rather than thermally or thermocatalytically Five major types

of fuel cell have been developed, with all practical fuel cells at present using hydrogen as theenergy carrier Alkaline, acid, and the solid polymer (or polymer electrolyte membrane)-typefuel cells require any HC, syngas, or biomass feed to first be reformed to hydrogen Moltencarbonate and solid oxide fuel cells are higher temperature types and internally reforming, sothat a reforming stage upstream of the fuel cell may not be needed when using biomethane,biogas, or syngas from biomass as long as purity is high The solid oxide fuel cell operates attemperatures in the range of 600 to 1000C and could be used to replace the gas turbine in acombined cycle operation In such cases, the peak net electrical efficiency might be improvedfrom about 50% to close to 70% at low loads, including parasitic demands of the celloperation, and from 30% to about 55% at high loads [30] Significant research remains forbiomass integrated fuel cell applications and many other advanced options

Combustion is a complex phenomenon involving simultaneous coupled heat and masstransfer with chemical reaction and fluid flow For the purposes of design and control,thorough knowledge is required of fuel properties and the manner in which these propertiesinfluence the outcome of the combustion process Combustion conditions must also bespecified, including type of oxidant (air, oxygen, oxygen-enrichment), oxidant-to-fuel ratio(stoichiometry), type of combustor (e.g., pile, grate, suspension, fluidized bed), emissionlimits, and many other factors Fully detailed models of the combustion process includepyrolysis and gasification of solid feedstock along with homogeneous and heterogeneousoxidation involving a substantial number of reactions and reaction intermediates Compre-hensive models have been developed for combustion of fuels such as hydrogen and CH4, buthave not so far been completed for more complex fuels such as biomass Fortunately,simpler approaches involving more global reaction processes can be used to account forspecific feedstock properties and combustion conditions

2.3.1 Combustion Properties of Biomass

Combustion of biomass is heavily influenced by the properties of the feedstock and thereaction conditions (e.g., air/fuel ratio) The amount of heat released during combustiondepends on the energy content of the fuel along with the conversion efficiency of thereaction The organic matter assembled by photosynthesis and plant respiration containsthe majority of the energy in biomass, but the inorganic fraction also has importance for thedesign and operation of the combustion system, particularly in regards to ash fouling,slagging, and in the case of fluidized bed combustors, agglomeration of the bed medium

2.3.1.1 Composition of Biomass

Photosynthesis and plant respiration result in the production of a diverse and chemicallycomplex array of structural and nonstructural carbohydrates and other compounds,