handbook of combustion (online) || gasification of biomass and waste

10Gasification of Biomass and WasteAlberto Gómez-Barea and Bo Leckner

10.1Introduction

Gasification of biomass offers a combination of flexibility, efficiency, and environ-mental acceptability that is essential for meeting future energy requirements.Gasification has benefits over direct combustion in certain applications, becausethe fuel is converted into a gaseous fuel, which increases the opportunities for usingbiomass as an energy source. This process leads to a fuel gas suitable for co-firing inexisting boilers and also, when sufficiently cleaned, for feeding gas engines andturbines to generate electricity, and in some applications it may serve as a raw gas forthe synthesis of fuels or chemicals.

There are factors, however, that can limit the use of gasification compared tocombustion: firstly, the carbon conversion efficiency is lower if part of the fuel energyremains as unconverted char, especially in fluidized-beds; secondly, in cold gasapplications, such as theuse of the gas in engines, the sensible heat of the gas could belost unless a heat recovery system is included. Finally, if the temperature in thegasifier is not high enough, the tar content in the gas produced canmake the processunfavorable from a technical or/and economical point of view.

There are various gasificationmethods and technologies. The selection of the bestgasification system depends on the nature and availability of biomass, as well as onthe product desired. To successfully convert biomass into a product, a chain ofprocesses must be designed; the most important ones are indicated in Figure 10.1.The product, for instance, process heat, electricity, or gas for the synthesis of otherproducts, requires a specific gas quality. To achieve such a quality, a gas cleaningdevicemust be designed. This gas-cleaning equipment depends on the gas generatedin the gasifier, which in turn depends on the type of gasifier and on the biomass used.The quality of the biomass, defined by its physical and chemical properties, can beimproved before feeding it into a gasifier by several pretreatment methods. Notably,not only technical aspects decide the type of gasifier, but also economic ones areconcerned, such as the availability of biomass. For instance, the viability of atreatment intended for a particular biomass can be reduced if sufficient amounts

Handbook of Combustion Vol. 4: Solid FuelsEdited by Maximilian Lackner, Franz Winter, and Avinash K. AgarwalCopyright � 2010 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-32449-1

j365

of biomass cannot be guaranteed. Similarly, the type of gasifier to be selected dependson the scale of the process, because the investment in some systems is only justifiedfor large plant. In summary, for a given biomass and a desired product, thegasification system should be optimized by a successful choice of the subsystemspresented in Figure 10.1.

Biomass gasification has been the subject of a considerable effort during the lasttwo decades, but various difficulties have prevented it from reaching full commercialstatus. Still, technical efforts must be made to better understand the technical andeconomical aspects of the gasification process to optimize the process successfully.An introduction to this matter is the main aim of the following survey.

This chapter is organized as follows: firstly, the properties of fuels for gasificationare presented, emphasizing the characteristics of biomass and wastes compared tocoal, aswell as the technical reliability of gasification of themost important biomassesand wastes. The fundamentals of thermochemical aspects of gasification are thenreviewed as well as themain gasification reactors andmethods. The requirements ofthe gas for the various uses and the cleaning technologies employed to achieve thesespecifications are discussed. Tar removal and conversion are dealt with due to theirimportance for applications that require cooling of the gas. Finally, the mainapplications are presented with examples of representative installations. A map ofbiomass and waste gasification is drawn in the last section, where the state of the artand the degree of commercialization of biomass gasification technology are outlined.

10.2Biomass as a Fuel for Gasification

10.2.1Impact of Biomass Characteristics on Gasifier Performance

Here, the impact of the properties of biomass and waste on gasifier design andoperation is outlined and advantages and difficulties associated with biomassgasification compared to coal are explained.

Biomass conversion is similar to that of coal in the sense that biomass can beregarded as a young coal. However, there are also significant differences betweenbiomass and coal as well as between one biomass and another. Each type of biomasshas its own specific properties, which determine its performance as a fuel ingasification plants. Biomass properties have been determined and reported for a

GASIFIERGAS

UTILISATION

GAS CLEANING

AND PREPARATION

BIOMASS PRODUCTBIOMASSPREPARATION

Figure 10.1 Main processes in the biomass gasification system, including the main stages frombiomass reception to product utilization.

366j 10 Gasification of Biomass and Waste

wide range of fuel types [1, 2]. Compared to coal, biomass fuels have a higher contentof oxygen and volatiles, and the nature of the ash differs substantially from that ofcoal ash.

The high volatile content in biomass [about 80% on a dry and ash-free (daf) fuel]leads to rapid conversion into a gaseous product. This can be compared with coalwhose volatile content ranges from 5% for anthracite to about 40% for low-rank sub-bituminous coal and lignite. The amount of volatiles has a major impact on the levelof tar production in gasifiers. The remainder after devolatilization is char. Biomassfuels produce small amounts of char (20% of the mass of the original daf fuel), andthis char is more porous and reactive and easier to gasify that that of coal.

The moisture content of biomass can be much higher than that of coal, and therelease of moisture leads to an even higher fraction of volatile matter with respect tothe solid fraction remaining. High moisture content of the fuel lowers the temper-ature of a gasifier, setting an upper limit of moisture content for satisfactoryoperation. Moisture in typical fixed-bed gasifiers is limited to 35%, while fluidizedand entrained-bed gasifiers are less tolerant to moisture, requiring ideally moisturecontents of the feedstock of less than 5–10%. The moisture contained in biomasstogether with the lower heating value [20 compared with 33MJ kg�1 (daf) for coal],which is related to the higher oxygen content of biomass fuels, affects the gasificationprocess in lowering the heating value of the product gas. Therefore, the throughput ofa biomass gasifier has to be increased to produce a gas with a power output that isequivalent to that produced by coal.

The chemical composition of the ash is also important, because it affects thegasification rate and the melting behavior of the ash. Biomass ash may have acomparatively low melting point compared to that of coal. Ash melting can causeslagging and channel formation in the reactor that can eventually block the entirereactor. The inorganic constituents are critical for the tendency of fouling andslagging. Alkali and alkaline earth metals, in combination with other fuel elements,such as sulfur, and facilitated by the presence of chlorine, are responsible for manyundesirable reactions in gasification and other conversion devices processingbiomass [1].

In fluidized-bed gasifiers, in contrast to entrained-flow gasifiers, a large portionof the bed material is ash, which is normally removed as a solid. For this reason,fluid-bed-gasifiers operate at temperatures below the softening point of the ash,which is typically in the range 950–1100 �C for coal and 800–950 �C for biomass.This limits the operation of systems with biomass of high-alkali content, i.e., straw,olive stone, etc. An important aspect of biomass gasification is that operationat lower temperature enhances the tar yield. Moreover, the tar is more difficultto convert. These characteristics have a notable effect on gas cleaning andapplications.

The amount of ash differs widely in different types of biomass (0.1% inwood, up to15% in some agricultural products, and 40% on a dry basis in residues, such assewage sludge and fractions of municipal solid waste). This influences the design ofthe reactor, particularly the ash removal system, which becomes a critical designelement. In fluidized beds of fuels with high ash content it is necessary to employ a

10.2 Biomass as a Fuel for Gasification j367

relatively high removal rate of bedmaterial to refresh the bed to avoid agglomeration.This is also true for other contaminated biomasses or for high-alkali biomasses likesome agricultural wastes.

Physical properties, such as density, particle size distribution, and the fibrouscharacteristics, may also differ substantially between biomass fuels. They are keyproperties for reactor design, including the feed system and other ancillary devices.Thefibrous character, particularly in vegetable biomass, is an important aspect to takeinto account as it may have a strong impact on handling. Bulk density differs widelybetween biomass and coal and between different types of biomass. Together with theheating value, it determines the energy density of the gasifier feedstock. Biomass oflow bulk density is expensive to handle, transport, and store. Apart from the handlingand storage behavior, the bulk density is important for the performance of thebiomass as a fuel inside the reactor: high voidage tends to channeling, bridging,incomplete conversion, and reduced capacity of the gasifier. The bulk density varieswidely (100–1000 kgm�3) between biomass feedstocks, also as a result of the wayused to prepare the biomass (chips, loose, baled, pellets, etc.).

There are other factors that make gasification of coal different from biomass, suchas the amount of sulfur, nitrogen, chlorine, and tracemetals. This can varywidely but,in general, biomass has amuch lower sulfur content and higher content of free alkalimetals (freemeans not bound in themineral substance of the ash, such as in coal). Incontaminatedwastes the amount of trace elements and heavymetals can be of specialconcern. This leads to pollutant formation and the need for removal systems.

10.2.2Biomass Classification and Standardization

Gasification technologies require well-defined feedstock in terms of moisture, size,and ash. Standards facilitate the production of dedicated fuels for gasifiers. Stan-dardized engineering practices setting out protocols of analysis and interpretationmay prove useful in reducing unfavorable impacts and costs.

According to CEN (Center Europ�een de Normalisation)/TS 1496, biomass fuelscan be grouped into four primary classes: (i) woody materials, including wood,fibrous waste from the pulp and paper industry, forest and plantation wood, shortrotation forest (SRF), and so on; (ii) herbaceous biomasses, including crops of cerealand oils seeds, annual growthmaterials such as straws, grasses, leaves, and so on; (iii)fruit biomass, including agricultural by-products and residues, including shells,hulls, pits, and also residue from animal farms, that is, manures; and (iv) blends andmixtures.

Refuse-derived fuels (RDF) and waste or non-recyclable paper, often mixed withplastics, are excluded from the category of biomass. An important source of wastefuels are those included in the so-called solid recovered fuels, having their ownEuropean technical specifications (CEN/TS 343). These fuels have been widely usedover the years in cement industry, limekilns, and,more recently, directly co-firedwithcoal in pulverized coal (PC) boilers and, indirectly, by generating a fuel gas in acoupled gasifier, which is fed into the PC boilers (indirect co-firing).


10.2.3Biomass Reliability

Besides the technological reliability of different biomasses andwastes, one importantaspect for adequate penetration of all biomass conversion technologies is that of anadequate resource supply. Figure 10.2, adapted from Reference [3], depicts themarket potential versus reliability of the technology, involving the most importantbiomass and waste fuels in gasification applications.

Woody biomass has the highest reliability, since most problems related to bedsintering in fluidized-bed gasifiers or slag formation on heat-exchanger surfaces arerelatively well understood for such fuels and the industry has sufficient confidence inmost types of woody biomass. However, clean biomass fuels for new conversionplants are scarce because they compete with other markets, so there is hardly anyreliable supply. Therefore, other fuels, such as wastes and energy crops, includingshort rotation forestry and grass, are becoming popular for thermochemical con-version plants.

Waste recovered fuels (WRFs) present the advantage that they often have a negativecost associated with their disposal, which can significantly decrease the operatingcosts of a plant. The principal difficulties of solid waste gasification, especiallymunicipal solid waste (MSW), are related to the heterogeneity of wastes. A possiblesolution is the production of a refuse-derived fuel (RDF) from thewastes, resulting inhomogeneous and controlled characteristics. In any case, gasification is particularlysuitable formany homogeneous agricultural and industrial wastes (waste tires, paperand cardboard wastes, wood wastes, food wastes, etc.). Refuse-derived fuel hassignificant potential for gasification applications, since gasification does not havesuch a negative public image as incineration and has been successfully applied [3],as will be illustrated in Section 10.7.

Low

High

LowHigh

OVERALL TECHNOLOGY RELIABILITY

MARKETPOTENTIAL

RDF WRF and

Grasses

biomassWoody

Straw

cropsEnergy

Sludge

Figure 10.2 Assessment ofmarket potential and technological availability of various biomasses forgasification systems. (Adapted from Reference [3].)

10.2 Biomass as a Fuel for Gasification j369

Sludge is receiving increasing interest as a fuel for gasification. There is littleexperience, though, and the technical reliability has not yet been demonstrated, but itis expected that the application of sludge will increase in the future.

Energy crops, especially short rotation forestry (SRF), are becoming popularbecause they are seen a mean to increase the production of biomass fuels whilesimultaneously creating new jobs for the farming community. Despite the currentefforts in some countries to develop SRF schemes (Brazil, USA), there is still littleknowledge on the technical reliability of SRF gasification. A sensitive area is that ofheavy metals, some of which are easily taken up by the plants. Similarly, grassespresent technical problems, such as need for size reduction, storage, and drying –

even their relatively fast biodegradability may limit their suitability for storage. Theirlow bulk density results in solids-flow problems and can create local hot spots in thegasifier.

10.3Thermochemistry of Biomass Gasification

The thermochemistry of gasification is similar to that of other thermochemicalconversion routes. Some differences, however, must be recognized in order to selectthe type of equipment and to understand the properties of the gas generated and theenvironmental issues in practice. Figure 10.3 illustrates the connection betweengasification and combustion of a fuel. The figure is a hypothetical representationbased on equilibrium.Moreover, it is supported by the physics of the processes andbyexperience. Oxygen (air) is added to the hypothetical reactor to burn sub-stoichio-metrically part of the fuel. Then the temperature (not shown) increases and volatiles

Stoichiometric ratioStoichiometric ratio

MJ/

kgM

J/kg

ChemicalChemical

SensibleSensible

TotalTotal

2020

1010

00 0.50.5 1100

Figure 10.3 Energy conversion from solid fuel into gas, illustrating the distribution of the totalenergy in the gas between sensitive heat and chemical energy (gas heating value).


are released in the form of a chemical product, whose heating value is shown. Thechemical production increases with the oxygen supply (temperature) up to astoichiometric ratio of about 0.3 because as the temperature of the gasifier increasesand so do the gasification reactions with char and the gases (CO2 and H2O)converting the char residue into CO and H2. However, for higher oxygen supply,more fuel is burned to CO2 andH2O, and the heat production increases at the cost ofuseful gas production.

Processes with stoichiometric ratio (SR) higher than unity are usually known ascombustion,whilst thosewithSRrangingfrom0.15 to0.40areoftencalledgasification;infact,apartialcombustionofthefuel takesplaceinthelattercases.Sincetheoxidationisonly a small part of the overall process, and reforming reactions are rate-controlling,gasification of biomass is also named reforming of biomass. Finally, processes withSR¼ 0, that is, when oxygen is not added to the system, are called pyrolysis.

Processes occurring during biomass gasification are illustrated in Figure 10.4,while the chemical reactions involved are presented in Table 10.1. In Figure 10.4 themain processes are distinguished: drying and devolatilization, volatile and charcombustion, and gasification and tar reforming with steam and carbon dioxide.These processes can be identified in certain spatial regions in fixed bed gasifiers. Influidized beds, however, they may proceed all over the bed [4].

During the devolatilization, the fuel is thermally decomposed into a carbonaceoussolid named char, while it releases volatiles, expressed in a simplified way by (10.1) inTable 10.1. The volatiles include non-condensable gases, such as CO2, H2, CO, CH4,H2, and so on, condensable gases (tar) and water vapor (both from chemically boundand from the free water in the fuel). The devolatilization step, chemical decompo-sition by heating in the absence of oxygen, is also termed pyrolysis. During pyrolysisvolatile material is released from the fuel. This is a thermal process driven by theheating of the fuel and oxygen is supposed not to penetrate into the fuel particle, sothe process proceeds in an atmosphere free of oxygen. This is due to the high release

Figure 10.4 Main processes during biomass conversion in a gasifier: drying and devolatilization,volatile and char combustion, and gasification and tar reforming with steam and carbon dioxide.

10.3 Thermochemistry of Biomass Gasification j371

Table 10.1 Main reactions in biomass gasification.

Stoichiometry Heat ofreaction(kJ mol�1)

Name

Biomass ! charþ light gasðCOþCO2 þH2 þ

CH4 þC2þ þN2 þ � � � ÞþH2Oþ tar ð10:1Þ

Endothermic Biomassdevolatilization

Cþ 12O2 !CO ð10:2Þ �111 Complete

combustion

CþO2 !CO2 ð10:3Þ �394 Partialcombustion

CþCO2 ! 2CO ð10:4Þ þ 173 Boudouardreaction

CþH2O!COþH2 ð10:5Þ þ 131 Steamgasification

Cþ 2H2 !CH4 ð10:6Þ �75 Hydrogengasification

COþ 12O2 !CO2 ð10:7Þ �283 Carbon

monoxideoxidation

H2 þ 12O2 !H2O ð10:8Þ �242 Hydrogen

oxidation

CH4 þO2 !CO2 þ 2H2O ð10:9Þ �283 Methaneoxidation

COþH2O $ CO2 þH2 ð10:10Þ �41 Watergas-shiftreaction

CnHm þðn=2ÞO2 ! nCOþðm=2ÞH2 ð10:11Þ Highlyendothermic(200–300)

Partialoxidation

CnHm þ nCO2 !ðm=2ÞH2 þð2nÞCO ð10:12Þ Dryreforming

CnHm þ nH2O!ðm=2þ nÞH2 þ nCO ð10:13Þ Steamreforming

CnHm þð2n�m=2ÞH2 ! nCH4 ð10:14Þ Hydrogenreforming

CnHm !ðm=4ÞCH4 þðn�m=4ÞC ð10:15Þ Thermalcracking


of volatiles frombiomass fuels, which obstructs the transport of oxygen from the bulkfluidization agent to the interior of the fuel particle. However, oxygenmay be presentin the gas around the particle and can react with the volatiles, or it can reactwith a charparticle after devolatilization.

After the primary decomposition, various gas–gas and gas–solid reactions takeplace: secondary pyrolysis, during which the tar may reform [(10.12) and (10.13)],oxidize (10.11), and crack (10.15). The light hydrocarbons (CH4, C2þ ) and othercombustible gases (CO, H2) may react with O2 by reactions (10.7)–(10.9). The charcan be burned [(10.2) and (10.3)] or gasified [reforming with CO2, H2O, and less withH2, by reactions (10.4)–(10.6)], depending on the oxygen concentration in the specificlocation of the gasifier where the char is present.

The rates of char gasification with H2O, CO2 are orders of magnitude lower thanthose of devolatilization or char oxidation. Typically, it takes a few seconds todevolatilize most of the volatiles in a biomass fuel particle, as well as to burn thevolatiles and char with oxygen. In contrast, it takes a fewminutes to gasify more thanhalf of the char at temperatures below 900 �C. When the pyrolysis takes place in anatmosphere containing steam and oxygen, the latter is consumed preferentially bycombustion of the volatiles, leaving steam and char. This is the main cause ofunconverted solid fuel in biomass gasification, especially in fluidized beds.

In fluidized beds, the biomass particles are also affected by shrinkage and primaryfragmentation, occurring immediately after injection of the fuel particles into the hotbed. These phenomena are provoked by thermal stresses and internal pressurescaused by the release of volatiles. Secondary and percolative fragmentation andattrition of char particles take place together with char conversion. These diminutionprocesses are important, since small pieces of fuel and char aremore easily entrainedfrom the bed, thus removing the fuel from the gasifier partially unconverted. The gasvelocity is an important parameter; it dictates the fluid-dynamics of the bed, andthereby the distribution of char anddevolatilizing particles throughout the bed aswellas the fraction of entrained particles [4].

10.4Gasification Technologies

Gasification methods can be classified in various ways depending on type of gasifier,heat source for reaction, gasification agent, pressure of operation, and others. Thecombination of these aspects yields the various types of gasification technologies onthe market [5]. They are discussed in the following.

10.4.1Types of Gasifiers

There are three main types of gasifier: fixed or moving beds, fluidized beds, andentrained-flow gasifiers. Among these designs there are variations, such as spoutedbed, draught tube, internally circulating fluidized-bed gasifier, and so on.

10.4 Gasification Technologies j373

10.4.1.1 Fixed-Bed GasifiersIn fixed-bed gasifiers, the fuel is gasified in a layer of the bed. The fuel goes throughdifferent zones where the gasification reactions take place (drying, pyrolysis, oxida-tion, and reduction). A distinction regarding the flow is made between counter-current and co-current gasifiers (Figure 10.5).

In updraft gasifiers, the fuel bedmoves downward and the gasification agent flowsfrom the bottom upward (updraft). This arrangement is a counter-current one. Thegas leaves the gasifier with a relatively low temperature, leading to high gasificationefficiency. The solid carbon in the fuel is completely converted into gas and tar.Besides the high conversion and efficiency, updraft gasifiers have the advantage thatthey do not require any special fuel preparation and allow gasification of a wide rangeof biomass types with different particle sizes andmoisture contents. However, as thegas leaves the reactor near the pyrolysis zone, the gas generated in updraft gasifiersexhibits a high content of organic components (tar).

In downdraft gasifiers, the fuel and the gasification agent flow in the samedirection. They are then co-current gasifiers. The biomass is first dried and pyrolyzednearly in the absence of oxygen. Further down in the bed, there is a hot oxidationzone. In this zone the biomass is converted into char that falls into the reductionzone, where is gasified by CO2 and H2O. The gases that are mainly produced in thepyrolysis zone are heated tomore than 1000 �C in the oxidation zone. In this process,gaseous compounds with high content of tar are to a great extent converted into low-tar components, which then react with the char in the subsequent reduction zone,producing additional gas. In contrast to counter-current gasification, the raw gasleaving the gasifier has a relatively high temperature, and this reduces the processefficiency. The throat situated in the oxidation zone leads to a uniform distribution ofthe gas over the cross section.With increased dimension of the gasifier, however, coldzones appear, increasing the tar concentration in the producer gas. Several designshave been developed to avoid this problem, but the maximum size is limited to a fewMWof fuel power. In downdraft gasifiers, the requirement of fuel quality is crucial fora proper temperature distribution and gas–solid contact. Therefore, co-current

Ash

Gas

Biomass

Air

Drying

Pyrolysis

Reduction

Oxidation

70011001300 300500900

Temperature T [ K } Temperature T [ K }

70011001300 300500900Ash

Biomass

Air

Drying

Pyrolysis

Reduction

OxidationAir

Gas

Figure 10.5 Two main types of fixed-bed gasifiers: (a) counter-current and (b) co-current, with anindication of the main zones and temperature profiles.


gasifiers demand careful fuel preparation with regard to particle size and moistureand ash contents.

10.4.1.2 Fluidized-Bed GasifiersGasification of biomass and waste in fluidized-bed offers advantages over fixed bed,since fluidized beds can be built in medium- and large-scale that may overcomeproblems found at smaller scale, typically related to fixed-bed designs [6]. However,the gasification efficiency of a FB (fluidized bed) may be limited if part of the fuelenergy remains unconverted in the char, and in cold gas applications if the sensibleheat of the gas cannot be recovered. Finally, if the temperature is not high enough inthe gasifier, the tar in the product gas can make the process unfeasible from atechnical and economical point of view.

The two types of FB, bubbling (BFBG) and circulating (CFBG), differ in the sensethat the latter type is always built with recirculation of particles. Recycling of finesleads to a greater efficiency of carbon conversion by increasing the residence time ofparticles. Recycling is a generic solution that can be applied even if the bed is called abubbling bed gasifier, as demonstrated, for example, by theHTW (HighTemperatureWinkler) process for lignite gasification. Figure 10.6 shows the main characteristicsof the two types of FBG analyzed in this chapter. CFBG (circulating fluidized-bedgasifier) is taller and provided with a continuous solid recycling system for re-injection of particles into the bed (particle separator, return leg and particle seal).

CFBG operates with higher superficial velocities, typically in the range 2–5ms�1,by maintaining the ratio of fuel-to-fluidization gas, which makes the throughput

Figure 10.6 Two types of FBG: bubbling and circulating.


higher than in BFBG (bubbling fluidized-bed gasifier). Therefore, for the same crosssection, the gasifier is fed with higher fuel flow-rate in CFBG than in BFBG. Theentrainment of material from the bottom bed then increases significantly comparedto a BFBG, and recycling is necessary. This increases the solids flow, as well as thecontact time in the freeboard. Further details can be found in detailed reviews [4, 7].

10.4.1.3 Entrained-Flow GasifiersEntrained-flow gasifiers (EFGs) operate with fine fuel and no inert material ispresent. The operating temperature ismuch higher than in other classes of gasifiers,usually in the range 1200–1500 �C, depending onwhether air or oxygen is employed.As a result, the product gas has low concentrations of tars and condensable gases andconversion approaches 100%. However, this high-temperature operation createsproblems of materials selection and ash melting. EFGs are well known for coal, butthe process has not been developed for biomass yet.

In principle, an entrained-flow gasifier seems to be an attractive way to processbiomass, since high temperature allows the production of a tar-free gas, and the lowmelting point of the biomass ash would keep the oxidant demand low. However,various aspects speak against this design for biomass. Firstly, the short residencetimes of entrained-flow reactors require small particle size to ensure full gasificationof the char. Methods for size reduction that perform satisfactorily on fibrous biomasshave not yet been found. Secondly, the aggressive nature of themolten slag generatedby biomass ashmakes such a solution difficult. Furthermore, one has to consider theinherent limitations in size of biomass equipment caused by restrictions in thequantities of biomass that can be delivered to a plant. Hence, because of economicscale effects the introduction of systems based on EFG has been prevented to date. Itseems that in the short term most processes for biomass and waste gasification atmedium- and large-scale will use FB designs [8].

Development of entrained bed gasifier for biomass is a matter of great interest forlarge-scale systems (>200 MWth). This technology is especially attractive for thedevelopment of second-generation biofuels. An EFGwith liquid ash removal wouldbe an attractive alternative to bubbling or circulating FBG, because it allows removalof the two main drawbacks of FBG systems (low carbon conversion and high tarconcentration in the gas). Various solutions are being investigated to provide biomasswith the necessary requirements: torrefaction, decentralized pyrolysis units, and soon [5].

10.4.2Direct and Indirect Gasification

The gasification concept can be grouped into two approaches, depending on the waythe heat for gasification is provided to the gasifier: autothermal and allothermalgasification. In autothermal or direct gasification the heat is released by partialoxidation of the fuel in the gasifier itself. The partial oxidation can be carried out usingair or oxygen. To some extent, steam can also be added to these oxidants. Airgasification produces a low heating-value (LHV) gas (4–7MJNm�3) suitable for


nearby boiler, engine, or turbine operation. In oxygen gasification there is nodilution of the gas by nitrogen from air and a medium heating-value (MHV) gas(10–18MJNm�3) is produced that is suitable for pipeline distribution and as a basisfor synthesis of liquid biofuels.

Allothermal or indirect gasification, on the other hand, uses steam as gasificationagent, obtaining the heat necessary for gasification without burning the fuel inthe gasifier itself. This concept allows generating gas of medium heating value(14–18MJNm�3), rich in hydrogen without the need for oxygen. This is a greatadvantage, since direct gasification with oxygen is generally unfavorable due to thepenalty caused by oxygen production.

In principle, there are two concepts of indirect gasification, depending on whetherheat is supplied from internal or external sources, yielding external or internalindirect gasification. Figure 10.7a shows a general scheme that illustrates the threeways of providing the heat to the gasifier in indirect gasification: by external heat or byinternal recirculation of either gas or char.

In external indirect gasification the heat is delivered from an external source suchas in plasma or solar gasification. In contrast, in indirect internal gasifiers the energycomes from the process itself. In fact, if the process is maintained without anyexternal heat support, the overall gasification process is autothermal. Indirectinternal gasifiers are grouped as char-indirect gasifiers and gas-indirect gasifiersdepending on the type of internal energy source: char or gas, as indicated by therecirculation streams in Figure 10.7a. A char-indirect gasifier (Figure 10.7b) consistsof two separate reactors: an FB steam devolatilizer that produces the product gas andan FB combustor that burns the residual char to provide the necessary heat for thedrying and devolatilization. Bed material is circulated between the two reactors to

(b)(a)

Externalheat

Product gas

SteamBiomass

HEAT

Gas recirculation

Char recirculation

GASIFIER

COMBUSTIONCHAMBER

Air

COMBUSTIONCHAMBER

AIR

COMBUSTION

CHAMBERGASIFIER

Product gasFlue gas

SteamBiomassAir

Bed material + char

Bed material + heat

Figure 10.7 (a) Principle of various modes ofindirect gasification (IG): IG by external heataddition, IG by combustion of part of the gas(gas recirculation) and IG by combustion of part

of the char produced (char recirculation); (b)operational principle of indirect gasification bychar recirculation (twin-bed gasification).


transfer heat. In a gas-indirect gasifier the heat is provided by recirculation of afraction of the combustible gas. The hot combustion products are led through heattransfer tubes in the bed.

External indirect gasification has not been the subject of sufficient commercialinterest, probably due to the economic limitations and technical problems related toerosion of the heat transfer tubes. In contrast, indirect internal char gasifiers arecurrently offered commercially [9, 10]. They have proven good performance andscalability, producing a gas with a heating value up to 18MJNm�3 with a highproportion of hydrogen. Internal recirculation of gas is sometimes made in gasifiers(whether direct or indirect, although it is rarely reported) to maintain enoughtemperature when the fuel moisture is high. However, it seems not to be feasibleto recirculate gas to maintain the overall process as autothermal.

10.4.3Pressured Gasification

Pressurized gasification is of interest if the gas producedhas to be compressed beforethe utilization, as for use in gas turbines or for the synthesis of synthetic natural gas,methanol, or other chemicals. In gas turbine applications, the gas is supplied to theturbine at the required pressure, removing the need for gas compression and alsopermitting relatively high tar content in the gas. Hot gas cleaning is usually applied,which reduces thermal and pressure energy losses. A few demonstration units havebeen built and operated, but the technical experience available is limited. Someexperience with pressurized gasification is available from coal gasification wheresimilar problems arise. The alkali content of the gas, though, requires specialattention in connection with many biomass fuels.

A few difficulties arise when pressurizing gasifiers. The gasification reactor isexpensive and needs appropriate design. Special concern is the introduction of thefeed material into the gasifier. Several solutions have been suggested and some havebeen successfully tested in demonstration units but are rather expensive. Capitalcosts of pressurized gasifiers aremuchhigher than atmospheric gasifiers so only verylarge availability of biomass can be make pressurized gasification feasible for large-scale systems. So far only the fluidized bed has been used in pressurized gasificationdemonstration units. Despite the drawbacks in the development of pressurizedgasification, this is a potential option for the future, if entrained-flow gasification isfinally developed for biomass.

10.5Gas Requirements for Utilization

The gas from the gasifier contains various contaminants, such as tar, alkalinemetals,and dust, which have to be removed by cleaning to some extent, depending on thetype of gas utilization. Alkaline metals, dust, and tars cause corrosion and erosion ofcylinder walls and pistons in engines.


The intended end use of the gasifier gas is a key issue for the selection of anoptimal, integrated clean-up strategy. The most important end uses so far, practicedcommercially or under research study, are: (i) close-coupled combustion (kilns,ovens, furnaces, dryers, boiler firing, and �town gas� for local distribution); (ii)internal combustion like diesel and Otto engines; (iii) gas turbine internal combus-tion; (iv) external combustion for power: steam engines, externally fired turbines,Stirling engines, and thermo-electric systems; (v) fuel cells: molten carbonate, solidoxide, and others; and (vi) chemical synthesis: methanol, ammonia, methane, andFischer–Tropsch (FT) liquids [11].

Inconsistencies have been found, however, when comparing between manufac-turers and even for the samemanufacturer, where specifications vary widely betweenprior commercial offers and actual guarantees once the end-use device (turbine ormotor) has been delivered. In addition, information about the performance ofindustrial installations is scarce, and sometimes contradictory. Therefore, despitethe existence of publications of lists of contaminant levels that can be tolerated inend-use applications [11, 12], there are no definitive tolerances that can be used withsufficient confidence for a given application. Nonetheless, there is agreement on thequalitative tolerance that each end-use requires for proper operation. This is dis-cussed in the following for the main applications.

The requirements on gas quality for gas used in heat applications are not extremelystrict, especially when the gas remains at high temperature during transportation tothe burner, which prevents tars and alkalinemetals condensing. Standard technologyseems to be available to ensure that a biomass gasifier coupled to a modern kiln orboiler (with burners designed for its purpose) will meet stringent environmentalemissions guidelines and regulations. Figure 10.12 below presents an arrangementof this concept, specifically an updraft gasifier coupled with a boiler, which isintegrated in a Rankine steam-cycle to produce electricity. This case demonstratesthat there is no urgent need to further address gas cleanup in close-coupledcombustion.

The above is true for clean biomass but is questionable for contaminatedbiomass or waste. For waste gasification, especially for high alkali fuels such asagro-residues, energy crops, or waste-derived fuels, gas cleaning is recommendedto avoid severe fouling and corrosion in downstream equipment, as well as highemission of pollutants in the flue gas generated after combustion of the producergas. Notably, after burning the producer gas, the flow rate of the flue gas increases,so, if the producer gas is heavily contaminated, it is, in principle, a good option toclean the gas before burning. This aspect is discussed below in relation to variousindustrial installations.

Engine applications require that particles and tars be reduced before the producergas can be effectively utilized. Limits have been expressed over the years in terms ofspecies concentration, in the range of about 30mgNm�3 for particulates and100mgNm�3 for �tar.� However, for tar it has been demonstrated that the dewpoint ismore appropriate as ameasure: the recommendation being a dewpoint in therange of 30–50 �C. Amixture of a gas with 100mgNm�3 of tar that has a dew point ofover 70 �Chas been found to cause problems in the engine. Conversely, gas with a tar

10.5 Gas Requirements for Utilization j379

concentration above 5000mgNm�3 has been used in engines without technicalproblems. This shows that the nature of the tar, and not the tar concentration, is thekey parameter for proper assessment of the suitability of a gas for engine applica-tions. The range of the limiting particle concentration in gas turbines is 0.1–120mgNm�3, depending on the design and the operating conditions. Alkalis are also criticalcontaminants, and the reduction of these to acceptable levels (usually below 0.1mgNm�3) remains one of the greatest challenges for successful commercialization. Verylittle has been published with respect to tolerable �tar� concentrations in gas turbineapplications. As discussed, gas turbine application requires that the hot gas be fullycleaned and remain hot (and under pressure) before use.

Synthesis gas applications have high gas cleaning requirements. Before the gasenters the final synthesis loop, particulates should be less than 0.02mgNm�3, andthe �tar� concentration less than 0.1mgNm�3. Hydrocarbons also pose potentialproblems in processes for methanol synthesis. If the methane concentration isgreater than 10%, the entire syngas stream must be reformed to CO and H2. If it isless than 3%, no reforming is necessary. In the intermediate range, reforming of arecycle stream is required. To preclude catalyst poisoning (particularly copper/zinc-based catalysts), the total olefin content should be less than 6mgNm�3 and theethylene concentration should be below 4mgNm�3. Synthesis catalysts are also veryintolerant to the presence of sulfur and chlorine, normally present in MSW-derivedgas and in gas produced from some herbaceous species, with a concentration limit ofabout 0.1mgNm�3 reported for both species [11].

10.6Gas Cleaning

Gas cleaning in biomass gasification is focused on eliminating tars, dust, and gaspollutants, such as alkali, chlorine, nitrogen, and sulfur compounds. A detailedreview on gas cleaning for biomass gasification systems has been published [12].Elimination of dust and gas pollutants can be effectively carried out by existingmethods like cyclones, filters, and wet scrubbers. Removal of tar in biomassgasification is a greater challenge, since tar introduces additional inconveniencesdepending on the end-use application and the method used for removal. Therefore,the main focus of the present chapter is on tar removal.

10.6.1Dust Removal

Equipment used for dust removal includes cyclones, bag filters, electrostatic pre-cipitators, and other devices commonly used for combustion applications. Depend-ing on the upstream processing of the producer gas, the fabric of bag filters maybecome clogged by tar. Packed beds filled with fine granular particles, like sand,sawdust, rice husk, and so on, are also proposed for dust removal. Suchmaterialsmayultimately be used as a fuel when the filter bed is renewed. This option is not as


efficient as fabricfilters, but it is relatively cheap and insensitive for tar deposits. As analternative, ceramic filters may be considered. Such filters could withstand a hightemperature and they have high chemical resistance. A drawback of ceramics is theirsensitivity to temperature differences between different parts of the filter, whichcould, for example, occur due to fast changes in heat load.

10.6.2Removal of Contaminants

The removal ofN, S, andCl components and other trace elements volatilized from thebiomass during gasification is usually required in most end-uses of the gas [12].Depending on the contamination of the fuel, the gas cleaning effort varies and sodoesits relative cost. Therefore, for waste-derived material and high-alkali fuels, such asagro-residues and energy crops, the gas cleaningmay be allowed to be costly, and thesize of the installation for economical feasibility of a plant is greater.

In the following the main techniques for removal of contaminants in the gas aresummarized, presenting methods without paying attention of the effect on theeconomy of the process. Integration of gas cleaning for difficult (contaminated)biofuels is dealt with in the section on applications.

10.6.2.1 NitrogenNitrogen compounds are present mainly as ammonia, but some hydrogen cyanidemay also be present. These compounds cannot be removed by filtration, but requirewet scrubbingwithwater or aqueous solutions, which cool the gas to about 50 �C. Thealternative to wet scrubbing is to leave the N compounds and to use low-NOx

techniques during combustion or selective catalytic reduction of the nitrogen oxidesin the flue gas after oxidation.

10.6.2.2 ChlorineChlorine arises from sea salt, pesticides, and herbicides, as well as waste materials.Chlorine contained in the biomass is converted mostly into HCl in the gas from thegasifier, the concentration depending on feedstock and gasification conditions.Chlorine and its compounds can be removed by absorption in active material, eitherin the gasifier or in a secondary reactor, or by dissolution in a wet scrubbingsystem. Dolomite and related materials are less effective in removing chlorine thansulfur.

10.6.2.3 AlkalisThe alkali components in the biomass, particularly Na and K compounds, are volatileat high temperatures, but it is uncertain which compounds are actually present in thegas. Alkali metals cause high-temperature corrosion of turbine blades, stripping offtheir protective oxide layer, and for this reason it is widely believed that the alkaliconcentration must not exceed 0.1 ppm at the entry to the turbine.

At high temperatures alkalimetal compounds are in the vapor phase and thereforepass through particulate removal devices, unless the gas is cooled. The maximum

10.6 Gas Cleaning j381

temperature that is considered to be effective for condensing metal species is in therange of 500–600 �C. However, alkali compounds also damage ceramic filters atthese temperatures, and a cleanup system will, thus, need a cooler upstream of thehot-gas filter. Hence, it is possible that gas cooling to this level will cause alkali metalsto condense on to entrained solids and be removed in the particulate-removal stage.However, they also nucleate homogeneously, forming dust in the formof sub-micronparticles that may pass a filter depending on the fractional efficiency of the filter, orclog the filter if the temperature is unsuitable.

For high-alkali fuels, dry-gas cleaning is necessary to yield a gas with sufficientquality even for the less exigent requirements, such as direct firing in stand-aloneboilers or co-firing in large PC boilers.

10.6.2.4 SulfurSulfur is not considered to be a problem inbiomass fuels because they have low sulfurcontent. However, some waste-derived materials, such as sludge contain largeamounts of sulfur that must be taken into account. Sulfur can often be removedwith a conventional sulfur guard in gas turbine applications. If a dolomite (CaO.MgO) tar cracker is included in the process it will absorb significant proportions ofsulfur and reduce the levels considerably, but then the sulfur acts as a poison thatreduces the catalytic performance of the material. A sulfur guard, consisting of a hotfixed bed of zinc oxide, is likely to be adequate for the concentrations expected. Thiswould be relatively inexpensive to install but would create a waste disposal problemwith the zinc sulfide produced.

10.6.3Tar Removal and Conversion

Considerable efforts have been directed to tar removal from fuel gas. A recent reviewon tar removal and conversion has been published [13]. Broadly speaking, tar removaltechnologies can be divided into two approaches [11]: gas cleaning after the gasifier(secondary methods) and treatment inside the gasifier (primary methods).Figure 10.8 illustrates the difference between primary and secondary methods.Primary methods include measures taken during the gasification to prevent orconvert tar formed in the gasifier. Secondary methods are measures to improve thehot product gas issuing from the gasifier. An ideal primary method concepteliminates the need for secondary treatment. The secondary methods are beingstudied widely and are well understood [11]. In contrast, primarymethods are not yetfully understood and are currently investigated a great deal [14].

10.6.3.1 Secondary MethodsSecondarymethods are conventionally used to treat the hot product gas of the gasifier.The methods can be chemical or physical. Secondary chemical methods include tarcracking downstream of the gasifier, either thermally or catalytically. Secondaryphysical methods include the use of cyclone, filters of various types (baffle, ceramic,and fabric), rotating particle separator, electrostatic filter, and scrubber.


Removal of tars by condensation is, in principle, the least complicated way toremove tars. Various cooling methods have been applied in biomass gasificationsystems: scrubbers, venturies, humidified packed beds, and so on [12]. Table 10.2summarizes the approximate tar and particle reduction efficiency of variousmethods [15].

In the direct water-cooled tar-removal systems also some of the dust, HCl, sulfuroxides, and alkaline metals are removed by the water. A major drawback of directwater-cooled systems is the stream of waste water contaminated by tar, which needs

Gasifier+

TarConversion

Application

Air/Steam/O2

GasCleanup

Tarfree gas

Biomass

Dust N, S, Halogen Compounds

Gasifier

Air/steam/O2

Syn. Gas+ Tar

Biomass

Downstream Cleaning(Tar, Dust, N, S, Halogen Compounds)

TarRemoval/

Conversion

GasCleanup

Tarfree gas

Application

SECONDARY MEASURES FOR TAR ELIMINATION

PRIMARY MEASURES FOR TAR ELIMINATION

Figure 10.8 Comparison of primary and secondary measures for gas cleaning.

Table 10.2 Reduction efficiency of particle and tar in various gas cleaning systems.

Particle reduction (%) Tar reduction (%)

Sand bed filter 70–99 50–97Wash tower 60–98 10–25Venturi scrubber 50–90Wet electrostatic precipitator >99 0–60Fabric filter 70–95 0–50Rotational particle separator 85–90 30–70Fixed-bed tar adsorber 50


treatment before disposal. Various designs have been developed over the years tosolve this problem.

A novel solution is the OLGA Process [16] developed by the Energy ResearchCenter of TheNetherlands. This process removes the tar prior to water condensation,which makes it possible to recirculate the tar to the gasifier, improving thermalefficiency and, more importantly, reducing the cost for treating the waste-waterstream. The tar is removed by contacting the tar-loaded product gas with a speciallydeveloped scrubbing liquid (�oil�) in an absorption column. Tar aerosols and heavyand light tars are removed from the product gas in the absorption column. Thecurrent design of the OLGA technology requires a dust-free gas so a hot gas filter anda gas cooler are used to ensure this. At present there is not enough information on thedemonstration projects currently conducted using this technology.

In methods based on scrubbing with water, an important factor to consider is theamount of soluble tars in the water stream, mainly phenols. Phenols are relativelyeasily destroyed at high temperature in the gasifier. Properly linked primarymeasures with this secondary wet cleaning concept could be a sound technicalsolution for small- to medium-scale plants for power generation.

Conversion of tar in the hot gas by thermal cracking is generally preferred, sincethe energetic value of tar remains in the gas phase, while it is decomposed intolight fuel gases. Thermal cracking is only effective at high temperatures, >1200C, whilst a catalytic technique is more effective at the thermal level of the hot gas.However, the latter is more expensive and still has technical shortcomings, such asinactivation caused by deposited carbon and H2S. Novel catalysts can overcomethese disadvantages, but they need demonstration prior to industrial implemen-tation. Therefore, tar cracking systems are not yet commercially available, but areunder development.

In summary, although some secondary gas cleaning methods are reported to beeffective, they are often not economically viable.

10.6.3.2 Primary MethodsPrimary methods include: (i) selection of operating conditions, (ii) bed additives orcatalysts in the gasifier, and (iii) gasifier design.

The operating conditions play a crucial role during biomass gasification in manyways: carbon conversion, product gas composition, tar formation, and tar reduction.Themost important influencing parameters are temperature, gas concentration, andresidence time. For a given gasifier design and biomass type, these variables are theresult of setting the following variables: air ratio (oxygen feed in relation to thestoichiometric), composition of the gasifyingmedium (flow rates of oxygen/steam inrelation to the flow rate of biomass), and the addition of catalysts and additives.

The key variable in gasification is the temperature. Figure 10.9 summarizes theeffect of temperature in the gasifier on key variables (heating value of the gas and tarand char conversion) and processes, such as sintering, when gasifying differentbiomasses and wastes. The temperature range 800–900 �C is identified as the mostcommon one in biomass gasification, considering the balance of the benefits and thedrawbacks associated with the thermal level.


Addition of steam enhances the reforming reaction of tar and char gasification,improving the gas quality and reducing the tar content. However, these reactionsconsume heat, which must be added to the reactor, for example, by addition ofoxygen. If the oxygen is added by air, the gas is diluted with nitrogen and the heatingvalue of the gas produced is reduced. If pure oxygen is used a better gas quality isachieved, but oxygen is expensive and affects the economical feasibility of thegasification process. Therefore, the composition of the gasification agent must beassessed by appreciating the technical benefits and the economical drawbacks.Use ofoxygen produced at low cost, such as bymembranes with a purity limited to 40–50%,has not beenmuch investigated but could show that operation with enriched-air andsteam is feasible [17].

In-bed additives in fluidized-bed gasifiers have received much attention. Additionof catalysts, such as carbonate rocks (dolomite and limestone), olivine, and metal-based catalysts improve significantly the gas quality by reducing the tar content.However, loss of catalyst by entrainment, inhibition caused by carbon and sulfur, andcontamination of the ashes greatly limit these options. Again, an economic evalu-ation of the process decides which technique is feasible in practice. Themeasure thatyields the maximum reduction of tar in the bed is not necessarily the best: primarymeasures should be understood as a primary action in conjunction with secondarymeasures. The overall process then should be optimized in terms of technicalreliability and economical feasibility.

Gasifier design measures to minimize the tar content in the gas have been triedduring years of development. The idea is to stratify the process, both in terms ofgasification agent and temperature, to optimize the overall performance. Stratifica-tion of the gasification agent can be achieved, for example, by secondary air injection(Figure 10.10a), resulting in a high-temperature zone where the tars are thermally

Figure 10.9 Effect of temperature adjustment on various parameters and processes duringgasification of various fuels.


cracked. However, at a given air ratio and composition of the gasification agent theresults of this action are limited [17]. If the air ratio increases, the tar is significantlyreduced, but the heating value of the gas is also affected. Stratification by steaminjection at high temperature in specific zones of the gasifier has been found to givesignificant tar reduction [18], but the economic feasibility of this measure isquestionable.

Stratification has also been tried by two-stage gasification [19]. Figure 10.10bpresents this approach, which separates the pyrolysis zone from the char gasificationzone (where the gas is reduced, i.e., reduction zone). Tars formedduring the pyrolysis(first stage) are decomposed in the reduction zone (second stage). The hightemperature achieved in the second zone due to the addition of secondary air helpsin reducing the tar.

10.6.4State of the Art of Gas Cleaning Technology

Available technologies offer solutions to clean polluted producer gas in compliancewith the requirements for gas utilization. Dust and contaminants can be removedfrom the gas in a reliable way, but reduction of tar in the gas still needs improvement.Interesting new methods for tar removal, such as catalytic tar cracking and tarremoval by organic solvents are available, although some of the technical solutionsare unfeasible on economic grounds.Moreover, no satisfactory technical informationhas been published to give confidence in commercial large-scale application. Inaddition, the residual materials accumulating from gas cleaning have to be pro-cessed. This limits some processes like those based on water scrubbing, which have

Figure 10.10 Stratification of the gasification process: (a) secondary air injection; (b) two-stagegasifier.


been implemented in developing countries. In general, gas cleaning in biomassgasification systems needs further development.

10.7Applications

The gasification of biomass has to be seen in connectionwith the utilization of the gasproduced. Economic competitiveness depends to a great extent on using the specificadvantages in theutilization of the gas. So, the proper combination of gasification andutilization is an important technical task. The various gasification applications areshown in Figure 10.11 in terms of their market potential and overall technologicalreliability. It is an updated versionmade by the authors from the originally publishedfigure of Maniatis [3]. Each of the applications involved in the figure is discussedbelow as examples of end-uses. Detailed information on existing demonstrationprojects can be found in the literature [3, 5].

The main applications have been discussed in Section 10.5, in terms of the gasrequirements needed in each type of application.

10.7.1Direct Firing

10.7.1.1 Direct Firing for Thermal ApplicationsFiring the raw gas in thermal applications like kilns after removal of dust andparticulates is the simplest application: the gas is kept hot and the tar problem isavoided. The first commercial Ahlstr€omPyroflowCFB gasifier was commissioned in1983 at the present Wisa Forest Pulp and Paper Mill in Pietarsaari, Finland. The fuelfor the 35MWth (about 150 tonnes per day of biomass) gasifier is primarily bark andsawdust. Between 1985 and 1986, three more gasifiers, two in Sweden (25 MWth at

Low

High

LowHigh

OVERALL TECHNOLOGY RELIABILITY

MARKET

POTENTIAL

Firing

BIGCC

FiringCo-

biofuelsLiquid

& Chemicals

Hydrogen

Engines

Figure 10.11 Evaluation of the main gasification applications for biomass and waste.

10.7 Applications j387

Norrsundet Bruk, and 27MWth at ASSI, Karlsborg) and one in Portugal (15 MWth atPortucel, Rodao Mill), were built and commissioned to replace combustion of oil inlime kilns [3, 20]. The CFB gasifier at V€ar€o Bruk, manufactured by G€otaverken in thebeginning of the 1980s, is still in operation.

10.7.1.2 Direct Firing in Stand-Alone Gas Boiler for Electricity ProductionFigure 10.12 presents the direct combustion of the gas produced in an updraft fixed-bed gasifier generating steam, which drives a steam turbine to produce electricity.The heat of the condenser is used for district heating,making the overall efficiency ofthe process high. Several such updraft gasifiers are in operation in Finland incombination with small district heating plants. The gas is immediately burnedwithout cooling. It is claimed that this combination method is competitive withcombustion of biomass using grate furnaces, because the gasifier is more flexible asto feedstock quality: it can gasify wood chips, strawwaste, and even peat. In Finland ahigh number of annual operating hours can be expected, so the economic compet-itiveness of this process in other locations with less favorable conditions of operationand biomass supply remains to be demonstrated.

Gasification with gas cleaning is an option to be compared with direct combustionof the solid biofuel in stand-alone boilers, where the corrosive nature of theflue gas indirect combustion limits steampressure and superheating temperature. The cleanedproduct gas can be burned in stand-alone gas boilers with high steam values. It hasbeen claimed that this type of new waste-to-energy process may have an electricefficiency on the order of 35–42% at the same (or lower) investment costs asincineration, which hasmuch lower electric efficiency (only 17–27%) [6]. This option

Figure 10.12 Direct firing of the gas produced in an updraft fixed-bed gasifier, in a steam boiler,which drives a steam turbine to produce electricity.


is especially favorable for waste-derived fuels, energy crops, straw, and other fuelscontaining alkali and chlorine. It is not clear if this process is economically viable,though, because purification of the gas is technically complicated and expensive. InFinnish conditions, a CFB gasifier with gas cooling and filtration has been claimed tobe economically applicable in the capacity range of 40–150 MW fuel [6]. For smallersizes, high-efficiency waste utilization concepts have been applied with the Novelfixed-bed gasifier in Finland, where demolition wood and RDF have been success-fully tested [6].

A combination of the two direct firing concepts presented above has been testedon a commercial scale in Chianty, where a CFB gasification process for pellets ofRDF began operation in 1993 [3]. The process consists of two 15 MWth CFBgasifiers, generating a fuel gas with a heating value of 8MJNm�3. The raw gasfrom one of the gasifiers passes through two stages of solids separation beforebeing fed to a boiler to generate steam for producing 2.3 MWe in a condensingsteam turbine. The overall power generation efficiency was about 19 to 20%. Thegas produced in the second gasifier is supplied to the neighboring cement factoryfor direct combustion in the cement kiln. The gas leaves the cyclone at atemperature of about 850 �C and is sent to an oil-filled heat exchanger to becooled to about 450 �C before it is sent to the cement factory. The plant has beenoperated intermittently due to difficulty in obtaining continuous supply of RDFpellets.

10.7.2Co-Firing

In co-firing, the gas is burned in an existing boiler to partially replace the primary fuelof the boiler. Two versions can be distinguished: co-firing with coal in PC boilers andco-firing with natural gas.

Co-firing in an existing PC boiler is illustrated in Figure 10.13, where the case ofintermediate hot gas cleaning is shown for a process with contaminated biomass orwaste [20]. With clean biomass the gas cooler and the gas filtration unit could beremoved.

Co-firing in PC boilers is interesting as the overall costs are relatively low due to theavailable power cycle. The concept consists of a gasifier connected to a largeconventional boiler with a high-efficiency steam cycle (typically 40%). The greatadvantage is the utilization of biofuels with the high efficiency mentioned. Fig-ure 10.13 shows the essential idea of this application. Co-firing by gasification (alsotermed indirect co-firing) has additional advantages. The ash of the biomass is notmixed with the coal ash, in contrast to direct co-combustion, where the biomass fuelsare mixed with coal before or during the combustion process. This allows using theexistingmarket for ash as a constructionmaterial togetherwith the ashes from thePCboiler. In addition, the problem of milling of biomass is avoided. In reburningapplications, when the fuel gas is introduced almost at the top of the coal boiler, it hasbeen shown that the environmental performance of the power station is significantlyimproved in addition to the replacement of fossil fuels by renewable biomass fuels.


Also the technical risks are low, as the gas is utilized hot and therefore there is no tarproblem.

In the simplified concept, clean biomass or �well-defined waste� fuel is gasified,and the gas is directly fed to a PCboiler and co-firedwith coal. Installations of this typehave been successfully operated in Lahti [20] and Zeitweg [3]. In Lahti, for instance,the gasifier uses industrial waste wood, chips, peat, and recycled materials as fuel.The gas produced by a gasifier is burned in a PC boiler with high flame temperature,guaranteeing the purity of the flue gases. The gasifier replaces 50MW of the powerstation�s 350MWfuel power by biofuels. One-third of the fuel supplied to the gasifieris recycled fuel, which is classified refuse fromhouseholds and industry and the othertwo-thirds are composed of different biomasses. By using biofuels the powerstation�s emissions are reduced and the environmental hazards are diminished.The decrease of the carbon dioxide emission is calculated to be 60 000–80 000 t a�1.

Contaminated biomass or waste fuels can also replace coal in large-scale PCboilers. In fact, the Lahti plant has been successfully run with waste gasification.However, the feed has been carefully pretreated (homogenization, removal ofmetals,mixing with wood and peat to control the chlorine in the final feed, etc.). This is whathas been called above �well-defined waste.� Contaminated wastes without such asevere pretreatment have to be cleaned of most of the harmful contaminants prior toco-combustion of the gas. This option is illustrated in Figure 10.13, where thesimplified gas-cleaning section consists of a gas-cooler and a hot filter, the two keycomponents of this installation. Cleaning of product gas allows utilization of fuelswith even higher chlorine and metal contents. The dry gas cleaning technology hasbeen developed in Finland at pilot scale, while testing the potential technical risksrelated to the gas cooler design and filter operation at 300–500 �C. Demonstrationexperiences are still unknown.

Figure 10.13 Co-firing of the gas in a coal pulverized boiler with intermediate hot gas cleaning.


A similar scheme (but based on cold-gas cleaning) was applied in the Amerplant [3], where a gas cleaning section was designed to clean the gas produced by aCFB gasifier fed by demolitionwood. Severe condensation of tar in the gas-cooler wasdetected and the plant was modified to more conventional direct firing, keepingthe gas at 450 �C. No data about the experience of this plant in its new form areavailable.

Aswith coal, fuel gas produced by biomass gasification can be co-firedwith naturalgas, either directly in turbines, boilers, or duct burners or as a reburning fuel.Biomass/waste gasification coupled to gas-fired combined-cycle power plants hasprobably an economically attractive future market, since inmany countries there is amajor medium-term trend to replace coal-fired power plants by natural-gas com-bined-cycle plants. Demonstration plants will require extensive public support, andthe projects should be organized so that required technical and scientific expertise isavailable, for example, through R&D programs designed around the demonstrationprojects. Very little work has been published on this issue.

10.7.3Power Production in Engines

Internal combustion engines of the otto or diesel type can be operated with gasproduced by gasification of biomass. Standard natural gas engines can be used withgas from biomass, but some changes are necessary to adapt the engines to theconsiderably lower heating value of the gas, according to some operating experiencewith landfill gas and biogas.

Themain problem is the efficient removal of tar because the enginemanufacturershave not been able to design more robust engines, which can tolerate tar in the gas.The operational diagram of a gasification process for power generation by engines isillustrated in Figure 10.14, showing a fluidized-bed gasifier with hot gas cleaning.The gas cleaning system consists of tar cracker, gas cooler, and afilter unit atmediumtemperature. The gas is further cooled before introduction in a gas deposit, fromwhich the gas is fed to the engines. Cold gas cleaning based on water or solventabsorption has also been implemented [15, 16]. In this case the gas is first cooled,filtered, and finally scrubbed. Gas cooling and contaminated effluent streams are themain operational problems when water is used as a scrubbing liquid [21]. Whenorganic solvents are used, the cost is high, and installation requires a certain size to befeasible. Owing to the lack of open information on industrial experiences, a clearjudgment is not possible for the hot gas cleaning or for the cold option based onscrubbing with solvents.

In developing economies like China or India there are examples where engines arecarefully and continuously maintained and operated with a relatively dirty gas. Thesedesigns result, however, in significant quantities of condensate that accumulate andcause an environmental hazard. This is actually not acceptable from an environ-mental point of view.

The energy efficiency of the conversion of biomass to power using internallyignited 4-stroke gas motors running on gas from biomass gasification is 35–40%,


which is higher than the efficiency of the conventional steam cycle but less thanachieved in conventional power plants using fossil fuels.Motorswith capacities of upto 30 MW gas input per engine are available. To improve the energy efficiency, low-temperature heat can be recovered from the cooling system of the motor in the formof hot water and used for district heating or for industrial applications. In this way theenergy efficiency can be raised to 75–85%. Modules for the catalytic treatment of theexhaust gases are available. Gas motors can be used for power production in lowcapacity plants as they are standard equipment.

Gasification of biomass combined with gas motors has been operated in pilot anddemonstration plants, but still some operational problems seem to exist and fullcommercialization of the process has not been achieved. Information on operationfor longer periods from recent demonstration projects, such as that in Skive inDenmark [5], is confusing.

10.7.4Biomass Gasification Integrated in Combined Cycles

The main market for IGCC (integrated gasification combined cycle) plants based onbiomass is in combined heat and electricity production in a medium-size range(30–100 MWe). In this range the IGCC based on oxygen-blown gasification andmultistage wet gas cleaning has been shown not to be economically attractive and,consequently, simpler process configurations are needed to keep the specificinvestment costs at a reasonable level. The most promising process alternative isthe so-called simplified IGCC based on air gasification and subsequent hot gascleaning [21]. In the simplest case of the IGCC concept (Figure 10.15), biomass isgasified in a bubbling or circulating fluidized-bed at a temperature of 800–1000 �C

Figure 10.14 Electricity production by burning the gas produced inmotor engines.Heat recovery isundertaken in a combined heat and power plant.


and at a pressure of 1.8–2.5MPa, the produced fuel gas is first cooled to 350–550 �Cand then cleaned from particulates and condensed alkali metals by ceramic filtersbefore leading into the combustion chamber of the gas turbine.

The first-biomass IGCC demonstration plant in the world located in V€arnamo,Sweden [20], has already demonstrated the technical feasibility of pressurizedbiomass gasification based IGCC technology. However, the low electricity pricetogether with deregulation of electricity markets made this process insufficientlyinteresting economically. The commercial breakthrough of this technology wouldrequire expensive large-scale demonstration, which is difficult to finance in thepresent market situation. By applying the new findings developed in low-pressurewaste gasification R&D to pressurized gasification, further improvement and costreduction of the biomass IGCC process are likely to be achieved [6].

10.7.5Production of Liquids by Chemical Synthesis

Synthesis gas with H2 and CO as the main components can be achieved from allbiomass fuels, and it can beused to produce different types offinal products: diesel oilbased on FT liquids, methanol, mixed alcohols, H2, and synthetic natural gas. Theadvantage of these chemicals is that they can be used either in fuel cells for electricityor transport applications, or alternatively, they can be processed to liquid transportfuel additives such as dimethyl ether (DME) and dimethoxymethane (DMM).

Figure 10.15 BIGCC: biomass gasificationintegrated into a combined cycle. The gasproduced in a pressurized gasifier is burned inthe combustion chamber of a gas turbine,

producing electricity (Brayton cycle). Thesensitive heat from the turbine gas is recoveredin a steam recovery boiler, fromwhich the steamis fed to a steam turbine (Rankine cycle).


The Biofuel Directive from the EC [22] promotes the use of biofuels or otherrenewable fuels for transportation, and this sets national indicative targets forutilization of bio-components in gasoline and diesel fuels. The targets set by theEC Directive (5.75%, calculated on the basis of energy content of all gasoline anddiesel for transportation purposes, by 2010) are only achievable by large-scaleimplantation of energy crops, and then biomass gasification seems to be anappropriate technology. Despite other routes for the production of these chemicalsfrombiomass, thermal biomass gasification is the only one that can be fuel-flexible toprocess any potential biomass feedstock as well as biodegradable waste fractions,yielding a product with high quality standard. Biomass gasification could be suitablefor large-scale production to achieve significant CO2 reduction effects and to benefitfrom the economy of scale.

Awide variety of chemical synthesis processes are already available for using cleansyngas and gas conditioning methods developed for coal or oil gasification andnatural gas conversion plants can also be utilized for the final gas treatment in thebiofuel plants. Therefore, efforts can be concentrated on improving the biomassgasification and gas-cleaning processes and on optimizing the integration of thebiomass conversion processes with the liquid fuel and by-product energy productionsystems.

To attain the above targets, various routes of technical development are currentlyunder consideration: (i) gigantic-scale (1–5GW) production of FT-diesel by pressur-ized entrained-flow gasification (for instance, by decentralized production of inter-mediate flash-pyrolysis oil in 50–60MW units with pyrolysis units coupled to localcombined heat and power production), (ii) large-to-medium-scale (100–1000MW)production of liquids (FT-diesel, DME, or methanol) based on pressurized oxygen-blown fluidized-bed gasifier, and (iii) medium-scale (100–300MW) co-production ofliquid biofuels, heat, and electricity in a large pulpmill, where the biomass feedstocklogistics already exist.

Various countries are developing processes of liquid biofuels for transportation,such in VTT (Finland), V€arnamo (Sweden), and Germany. A large demonstrationproject is currently in preparation in Sweden to gasify 100MWth of forest residues tosynthetic natural gas (SNG). Considerable effort is also under way in The Nether-lands, focused on SNG development.

10.7.6Fuel Cell Applications

Fuel cell applications are also under development for biomass gasification. The bestsuited fuel-cells for biomass gasification gas are themolten carbonate fuel cell and thesolid oxide fuel cell, which have been developed mainly for natural gas. The gascleaning requirements are order of magnitude harder than in other power produc-tion applications, but similar to those set to by chemical synthesis. Optimizedproduction of ultra-clean gas for fuel cells probably requires completely newtypes of gasification and gas cleaning processes. The real commercialization ofhigh-temperature fuel cells technologies even with natural gas will probably still take


some years. Consequently, the gasification developments to bemade for other powerapplications as well as for the liquid biofuels concepts will be useful for the biomass-gasification-fuel cell applications.

10.8Summary and Outlook

Gasification technologies offer the advantage of producing energy and chemicalvectors from various lignocellulosic materials. These vectors can be used in numer-ous applications directly or after further processing and upgrading and can be eitherin the gaseous or liquid state, depending on the processes and applications.

Fixed-bed gasification has been proven to be a viable technology in small-scaleapplications for some biomasses. However, this technology has a limited potential ofscaling-up; furthermore, fixed bed gasification imposes severe restrictions on thequality of the biofuels that can be employed. These aspects limit the development ofreliable gasification applications for biomass and residues, since these materials arehighly diversified and heterogeneous.

Fluidized-bed biomass gasification presents advantages compared to gasificationin fixed beds, especially regarding possibilities for scaling-up, automation, andadaptability to different biomasses and residues. It is, thus, especially useful forindustrial processes. This technology has been technically demonstrated in recentyears in several projects for heat and electricity production.

Gas cleaning and tar cracking for removal of tar with economically viable methodsremain the largest problem in biomass gasification for applications that differ fromdirect firing for heat applications. The development of the technology has movedbeyond the element of the �gasifier� to the critical area of the supply of a �clean gas,�free from particulates and tar. Primary measures to reduce the content of tar andcontaminants in the gas require special attention to avoid expensive and complexsecondary measures.

A limiting factor for these biomass gasification projects is the high costs ofbiomass with sufficient quality for reliable operation of plants with a certain size incomparison with other fuels. Lack of standardization, penalties imposed on thetransport, seasonality and other logistic limitations explain the obstacles to con-structing large-size plants designed for a specific biomass. An interesting option forovercoming these limitations is the co-gasification of several biomasses andbiomass with wastes. This option can reduce the effects of temporary shortagesof certain biomasses.

At present, the most reliable applications for gasification of clean biomass are co-firing and direct firing of the fuel gas in a boiler for heat or steam cycle. Theseapplications present the least technical risks, as the problem of tar is avoided.

Power production in small- and medium-scale by engines has high marketpotential but technological feasibility seems to be still limited. A large number ofdemonstration projects have been undertaken, but there is not yet a reliablecommercial technology.

10.8 Summary and Outlook j395

The biomass IGCC process almost reached commercial breakthrough in 1990s,but the very low electricity price together with deregulation of electricity marketsmade this process less interesting economically.

Current efforts and demonstration projects in Europe are focused on develop-ments of systems for producing fuels for the transport sector and synthetic naturalgas. In the longer term, it seems that the high-temperature fuel cell technologies areof interest. In these applications the gas cleaning requirements are an order ofmagnitude higher than in other power production applications. The production ofultra-clean gas probably requires completely new types of gasification and gascleaning processes.

A question remains to be answered: Despite the potential interest of biomassgasification, after two decades of extensive research and development and muchfunding, why are there still only a few commercial operating plants?

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