Energy 32 (2007) 124

sim

ta

Tec

ec

f g

briu

g c

hat

residues and municipal solid waste may be considered.During the oil crisis in the late 1970s and through to the

interest in gasication. Focus is less on coal, but more onbiomass gasication as a form of renewable energy.

a decision was made to co-gasify up to 50% of biomass inorder to generate a high proportion of green energy [1].

energy saving and environmental point of view to ndout whether biomass can be gasied with the sameefciency as coal.

ARTICLE IN PRESSAn interesting difference between coal and biomass liesin the composition of their organic matter: woody biomasscontains typically around 50wt% carbon and 45wt%

0360-5442/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.energy.2006.07.017

Corresponding author. Tel.: +3140 2473734; fax: +31 40 2446653.E-mail address: M.J.Prins@tue.nl (M.J. Prins).{Deceased.middle of the 1980s, coal was regarded as a very importantsubstitute for oil. Reserves of coal are abundant and moregeographically spread over the world than crude oil. In thisperiod, several coal gasiers were developed and commer-cialized. However, due to a large drop in the oil price, coalgasication did not gain a much larger share of the energymarket, although heavy oil gasication is commerciallypracticed at several reneries.During the last decennium, there has been renewed

The Kyoto protocol emphasizing the need to combatcarbon dioxide emission has also been an impetus for theinterest in biomass gasication. Carbon dioxide emissionsfrom using biomass as a fuel are perceived as neutralbecause this carbon dioxide is xed by photosynthesis in arelatively short period. Nevertheless, it has been arguedthat society should try to use biomass with the samethermodynamic efciency as fossil fuels, regardless of itsperceived CO2-neutrality [2]. It is important from anwhich corresponds to a lower heating value above 23MJ/kg. For gasication at 1227 1C, a fuel with O/C ratio below 0.3 and lowerheating value above 26MJ/kg is preferred. It could thus be attractive to modify the properties of highly oxygenated biofuels prior to

gasication, e.g. by separation of wood into its components and gasication of the lignin component, thermal pre-treatment, and/or

mixing with coal in order to enhance the heating value of the gasier fuel.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Energy; Entropy; Biomass; Coal; Gasication; Exergy; Thermodynamics process

1. Introduction

Gasiers can be designed to gasify almost any kind oforganic feed. A wide variety of fuels, such as many types ofwood, agricultural residues, peat, coal, anthracite, oil

Introduction of biomass as a renewable energy source isvital for the transition to a more sustainable society. Anexample for the shift from coal to biomass is the powerstation in Buggenum, the Netherlands, which integratescoal gasication and combined cycle technology. Recently,than those for coal (O/C ratio around 0.2). At a gasication temperature of 927 1C, a fuel with O/C ratio below 0.4 is recommended,From coal to biomass gathermodyna

Mark J. Prins, Krzysztof J. P

Department of Chemical Engineering, Eindhoven University of

Received 1 D

Abstract

The effect of fuel composition on the thermodynamic efciency o

model is used to describe the gasier. It is shown that the equili

possibly attained for a given fuel. Gasication of fuels with varyin

illustrated in a Van Krevelen diagram, is compared. It was found t81259

cation: Comparison ofic efciency

sinski, Frans J.J.G. Janssen{

hnology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

ember 2004

asiers and gasication systems is studied. A chemical equilibrium

m model presents the highest gasication efciency that can be

omposition of organic matter, in terms of O/C and H/C ratio as

exergy losses in gasifying wood (O/C ratio around 0.6) are larger

www.elsevier.com/locate/energy

ARTICLE IN PRESSrgyoxygen, whereas coal contains (depending on coal rank)6085wt% carbon and 520wt% oxygen. One might arguethat the high oxygen content of biomass is benecialbecause less oxygen needs to be added for gasication; onthe other hand, biomass has a relatively low caloric value,which would not be benecial for gasication. Thefundamental question which fuel composition is moreadvantageous for gasiers is addressed in this study.The evaluation of gasication efciency is based on

exergy analysis (formerly known in the USA as availabilityanalysis). The exergetic efciency is based on the rst aswell as the second law of thermodynamics, which is usefulbecause it considers not only the decrease in energy ofcombustion of the product gas compared to the solid fuel(due to partial oxidation) but also increases in entropy (as a

Nomenclature

C mole fraction of carbonH mole fraction of hydrogenI irreversibility (MJ/s); subscript indicates the

type of (sub-) processHHVfuelhigher heating value of fuel (MJ/kg)LHVfuel lower heating value of fuel (MJ/kg)O mole fraction of oxygenzA weight fraction of ashes (wt% dry)zC weight fraction of carbon (wt% dry)zH weight fraction of hydrogen (wt% dry)zN weight fraction of nitrogen (wt% dry)zO weight fraction of oxygen (wt% dry)zS weight fraction of sulfur (wt% dry)b ratio of chemical exergy and lower heating value

of fuel [-]

M.J. Prins et al. / Enesolid fuel is decomposed into many smaller molecules). Inthe 1980s, exergy analysis was applied to evaluate coalgasication processes [35]. Later, biomass gasiers wereanalyzed [68]. These analyses show that second-lawefciencies are considerably lower than rst-law efcien-cies, e.g. for a coal gasier, the rst-law efciency is nearly100% for a well-insulated gasier with negligible heatlosses, whereas the second-law efciency equals 79% [3].However, the results of these analyses are difcult tocompare because the gasication processes are not alwaysanalyzed on the same basis, e.g. both autothermal andallothermal gasiers have been considered, complete andincomplete carbon conversions, etc. Therefore, this papercomprises a comprehensive analysis of the effect of the fueltype and related fuel composition on the efciency that canbe attained in a gasier.The paper starts with a description of the applied

methodology, notably: which fuels are considered, whattheir thermodynamic properties are, and how the gasica-tion efciency is dened. As a chemical equilibrium modelis applied throughout this study to describe the perfor-mance of gasiers, its main assumptions and their validityfor practical gasiers is considered in a separate section. Itis demonstrated that the equilibrium model indicates themaximum efciency that can possibly be attained whengasifying a fuel. Finally, results are given that compare theattainable gasication efciency for the various fuelsconsidered.

2. Methodology

2.1. Gasifier fuel properties

As a gasier fuel, biomass and coal vary in manyproperties, such as their heating values, proximate analyses(xed carbon, volatile material, ash content and moisturecontent), ultimate analyses (amounts of carbon, hydrogen,

ech,fuel chemical exergy of fuel (MJ/kg)ech,fuel_unconv chemical exergy of unconverted fuel

(MJ/kg)ech,oxygen chemical exergy of oxygen (MJ/kg)ech,steam chemical exergy of steam (MJ/kg)ech,gas chemical exergy of gas (MJ/kg)eph,fuel_unconv physical exergy of unconverted fuel

(MJ/kg)eph,steam physical exergy of steam (MJ/kg)eph,gas physical exergy of gas (MJ/kg)Fm,fuel mass ow of fuel (kg/s)Fm,fuel_unconv mass ow of unconverted fuel (kg/s)Fm,oxygen mass ow of oxygen (kg/s)Fm,steam mass ow of steam (kg/s)Fm,gas mass ow of gas (kg/s)C exergetic efciency

32 (2007) 12481259 1249oxygen, sulfur, nitrogen, chloride and other impurities) andsulfur analyses (type of sulfur present). For example, theorganic matter in biomass contains a large fraction ofvolatile material, namely 7080wt%. Coals range fromlignite with approximate volatile matter of 27% toanthracite with an average of 5%, with sub-bituminousand bituminous coals intermediate between these values.The change in composition from biomass to coal is

illustrated using a diagram developed by Van Krevelen [9].Fig. 1 shows the change in atomic ratios H/C and O/Cfrom biomass to peat, lignite, coal and anthracite. Thegure specically shows the composition of lignocellulose,i.e. wood, since this is the most abundant biofuel. Wood isrelated to coal because it is the precursor for its formationin nature by various coalication processes. Vitrinite, themost important constituent of coal, descends from woodytissue [9]. This paper aims to compare gasicationefciencies that can be attained for all fuels mentioned inthe Van Krevelen diagram. Focus is on the composition ofthe organic material in the fuel, i.e. on dry and ash-freefuels, taking into account only the most importantconstituents: carbon, hydrogen and oxygen.

ARTICLE IN PRESS

0.4ic

gram

rgyIn order to perform thermodynamic calculations forreacting fuels with oxygen, their thermodynamic propertiesmust be available, such as the heating values and chemicalexergies (or alternatively, enthalpy and entropy of forma-tion). Thermodynamic properties of most fuels are notexactly known, except those for cellulose, because thestructure of these fuels is not well dened. Statisticalcorrelations are used to overcome this problem. The higherheating value can be accurately predicted by the correlationdeveloped by Channiwala and Parikh (in MJ/kg) [10]:

HHVfuel 0:3491zC 1:1783 zH 0:1034 zO 0:0151zN 0:1005zS 0:0211zA. 1

This equation was developed for a wide spectrum of fuels,including the whole range from coal to biomass. It has a

0

0.2

0.2

0.4

1.0

1.2

1.4

1.8

0.8

0.6

1.6

Atom

Ato

mic

H/C

ratio

Peat

Lignite

Coal

Anthracite

Fig. 1. Van Krevelen dia

M.J. Prins et al. / Ene1250standard deviation from experimentally determined valuesof only 1.45% and the bias error is negligible. Since thegasier is regarded as a CHO system, only the rst threeterms apply. The higher heating value is converted to lowerheating value using the enthalpy of evaporation for waterformed during combustion. Finally, the statistical correla-tion of Szargut and Styrylska [11] was used to calculate thechemical exergy of the fuel:

ch; fuel bLHVfuel, (2)with

b 1:0438 0:0158HC 0:0813 O

Cfor O

Cp0:5 , (3)

b 1:04140:0177H=C0:3328O=C 10:0537H=C 10:4021O=C for 0:5o OCp2 .

(4)

Fig. 2 displays the lower heating values and the ratios ofchemical exergies to lower heating values as a function ofthe fuel composition shown in the Van Krevelen diagram.The effect of H/C ratio on these parameters is muchsmaller than that of the O/C ratio. Therefore, they areplotted only as a function of O/C ratio with the arearepresenting realistic H/C ratios, as these occur in the VanKrevelen diagram. This approach is also followed in therest of this paper. Obviously, fuels with high O/C ratiohave a smaller heating value than those with low O/Cratio. However, the factor b increases with increasing O/Cratio, which indicates that by decomposing a fuel with highO/C ratio, relatively more work may be delivered. Thiseffect is known from structurally well-dened compounds,e.g. when comparing the ratio between chemical exergy andlower heating value for methane (O/C of 0) and methanol(O/C of 1). For methane, this ratio is 1.037 (831.65 kJ/mol/802.33 kJ/mol), whereas for methanol the ratio is 1.125(718/638.4).

0.80.6O/C ratio

Biomass

WoodLigninCellulose

Increased Heating Value

for various solid fuels.

32 (2007) 124812592.2. Gasification efficiency

The gasication efciencies are determined for a gasieras well as a gasication system. For the gasier, shown inFig. 3a, the thermodynamic efciency is dened as theexergy increase of the gas divided by the exergy decrease ofthe solid fuel. The efciency of gasifying a fuel with pureoxygen and optionally steam is therefore given by

Cgasifier

Fm;gas ch;gas ph;gas

Fm;oxygench;oxygenFm;steam ch;steam ph;steam

Fm;fuelch;fuel

Fm;fuel_unconv ch;fuel_unconv ph;fuel_unconv

. (5)

The chemical exergy of all gaseous components isobtained from Szargut et al. [12]. For the calculation ofphysical exergy, it is essential to use accurate data for theenthalpy and entropy in order to obtain accurate results.Thermodynamic data depend on the heat capacity as a

ARTICLE IN PRESS

ra

ratio

rat

rati

rat

rgy0.40.20.015

20

25

30

35

40

45

50

Low

er

heat

ing

valu

e (M

J/kg)

Atomic O/C

low H/C

high H/C

low H/C

high H/C

M.J. Prins et al. / Enefunction of temperature. Commonly used third-orderpolynomial equations give large deviations at temperaturesabove 1000K. To overcome this problem, a more advancedequation developed by Barin [13] is used for all compo-nents. The physical exergy of the product gas consists onlyof a temperature-dependent term, i.e. the work content ofthe sensible heat of the product gas. A pressure-dependentterm is not included because gasication at atmosphericpressure is considered.The gasier is part of a gasication system, shown in

Fig. 3b, which includes steam, oxygen and electricityproduction units. Since these units consume exergy,irreversibilities also take place outside of the gasier. Toenable a fair comparison between all fuels, the above-mentioned efciencies have to be corrected with theirreversibilities occurring in these production units. Thiscorrection is done on the basis that electricity requiredfor oxygen production and process steam are generatedfrom gasication product gas. This leads to the following

Fig. 2. Lower heating value and ratio of chemical exergy to lowe

(a)

(b)

Fig. 3. (a) Block diagram of a gasier. (b) Block diagram of a gasica0.80.6tio of fuel

0.92

0.96

1.00

1.04

1.08

1.12

1.16

1.20

io

o

io

(-)

32 (2007) 12481259 1251denition:

Csystem

Fm;gas ch;gas ph;gas

Ioxygen_production I electricity_productionI steam_production

Fm;fuelch;fuel Fm;fuel_unconv

m;fuel_unconv m;fuel_unconv

. (6)

For large-scale oxygen production, cryogenic separationof air is the most widely applied process. The electricityconsumption of large-scale cryogenic oxygen plants isapproximately 380 kWh/t oxygen [14]. This is equivalent toan exergetic efciency of 9.1%. Electricity required foroxygen production can be generated from gasicationproduct gas in gas turbines or fuel cells; an exergeticefciency of 50% is assumed for this process. Steam can beproduced by heat exchange with the product gas. Thisstudy considers steam at 500K and atmospheric pressure.

r heating value of fuels shown in the Van Krevelen diagram.

tion system, including production of oxygen, steam and electricity.

ARTICLE IN PRESSrgyThis relatively low steam quality is preferably produced byheat exchange with product gas, rather than in a boiler.For a well-designed heat exchanger, a thermodynamicefciency of 50% can be achieved. Finally, the exergy of airand water, required for oxygen and steam production, isnegligible.This paper also uses the so-called chemical efciency,

which is calculated by neglecting the physical exergy ofproduct gas in Eq. (5) and/or Eq. (6). The chemicalefciency indicates how well the chemical exergy of fuel ispreserved in the chemical exergy of product gas.

2.3. Process losses

To gain more understanding of the observed thermo-dynamic efciency, it is possible to analyze the various sub-processes occurring in a gasier. This methodology isdescribed in more detail in [15]. Gasication is conceptuallyassumed to be a sequence of the following steps: heating ofsolid fuel and reactant gases to the gasication tempera-ture, mixing of reactant gases, instantaneous chemicalreaction, heat transfer to the reactant molecules, andproduct mixing. The rate of heat transfer determines atwhich temperature the chemical reaction takes place. Ifheat transfer is very fast, the gasier is isothermal so thatthe product molecules have the same temperature as thereactant molecules. However, if heat transfer is slow, thetemperature of product molecules may be much higherthan the reactant molecules. For practical purposes, onlythe isothermal case is considered here.For each sub-process, the irreversibilities can be calcu-

lated. The overall irreversibilities equal the sum of theirreversibilities of the subprocesses:

Ioverall Iheating_fuel Iheating_O2 Iheating_steam I reactant_mixing I chemical_reaction Iproduct_mixing: 7

The irreversibilities for heating of the solid fuels cannotbe calculated separately because the heating capacities ofcomplex fuels are not known as a function of temperature,and also pyrolysis takes place when these fuels are heated.Therefore, the irreversibilities for heating of fuel andchemical reaction are combined. The irreversibilities can bedivided by the expended exergy, so that they are expressedas relative exergy losses. The sum of these relative exergylosses is the fraction of the expenditures lost throughirreversibility. Irreversibilities occurring in oxygen, electri-city and steam production unit gasication systems canalso be taken into account as relative exergy losses.

3. Gasier models

3.1. Equilibrium model

M.J. Prins et al. / Ene1252A chemical equilibrium model is applied to predict theproduct gas composition, gas amount and carbon conver-tar, which is not considered in equilibrium models, andmuch more hydrocarbons (especially methane) thanpredicted. For entrained ow gasiers, which operate attemperatures in the range 10501400 1C, the approach toequilibrium is much better. E.g., for a Shell coal gasier,all predicted gas species are within 0.7% absolute of themeasured values, and equilibrium temperatures are veryclose to the gasier exit temperatures [24].stuchemical reactions may occur [19]. Although it is difcultto determine the intrinsic kinetics of these reactions, manyresearchers agree that steam and carbon dioxide reform-ing reactions of char are kinetically limited at gasicationtemperatures lower than 1000 1C [23]. Furthermore, theproduct gas of uidized bed gasiers generally containstimes differ. For example, if wood is gasied in a counter-current moving bed gasier, devolatilization takes placebefore the particles reach the hot zone in the bottom ofthe gasier, volatiles are contained in the product gasstream, and the composition of this gas stream will bevery different from the equilibrium composition. In acocurrent moving bed gasier, these volatiles passthrough a narrow cross-section, the so-called throat, inwhich the temperature is high. The equilibrium model hasmuch better predictive potential for this gasier [22]. Theuid dynamics of large-scale gasiers, such as uidizedbed and entrained ow gasiers, are more favorable thanfor moving bed gasiers, but the residence time of thesolids is much shorter.The model assumes that gasication reaction rates arefast enough and residence time is sufciently long to reachthe equilibrium state. The kinetics of gasication reac-tions is complicated: in the gasifying process, thermalpyrolysis, homogeneous gas phase and heterogeneousgassolid reactions take place, so that thousands ofsion in gasiers. Equilibrium models are valuable becausethey predict the thermodynamic limits of the gasicationreaction system. The equilibrium model has been exten-sively documented in literature [1619] and applied forperformance evaluations [20,21]. However, it remainsimportant to realize that the main assumptions behind thismodel may not always be valid for practical gasiers. Theseassumptions are discussed below:

The gasier is often regarded as a perfectly insulatedapparatus, i.e. heat losses are neglected. In practice,gasiers have heat losses to the environment, but thisterm can be incorporated in the enthalpy balance of theequilibrium model.

Perfect mixing and uniform temperature are assumed forthe gasier. Different hydrodynamics are observed inpractice, depending on the design of the gasier.Gasication technologies utilize xed bed, moving bed,uidized bed or entrained bed reactors, in which thecontacting pattern of gas and solid, and their residence

32 (2007) 12481259Table 1 summarizes all the assumptions on which thisdy was based.

ARTICLE IN PRESS

th v

sulfu

and

om

, i.e

ers

ch i

eali

rgy3.2. Quasi-equilibrium models

In order to describe the behavior of uidized bedgasiers more accurately, modications have been madeto the equilibrium model. Empirical parameters have beenadded, such as the amount of methane in the product gas[25] and/or the carbon conversion [26]. Inclusion ofempirical parameters leads to a better agreement withexperimental data, but the model looses much of itspredictive capabilities.Another approach is the use of quasi-equilibrium

temperatures, whereby the equilibria of the reactionsdened in the model (see Eqs. 810) are evaluated at atemperature, which is lower than the actual processtemperature.

C 2H2 CH4, (8)

Table 1

Simplifying assumptions in this study

Fuel CHO considered wi Presence of nitrogen, Higher heating value

Gasifying agent Pure oxygen (O2), in sOperating conditions Atmospheric pressure

Negligible heat lossesGasier Perfect mixing

No kinetic limitations Complete carbon conv

Thermodynamic efciency Gasier efciency System efciency, whi

electricity (for which r

M.J. Prins et al. / EneCH2O COH2, (9)

C CO2 2CO. (10)This approach was introduced by Gumz [16]. For

uidized bed gasiers, the average bed temperature canbe used as the process temperature, whereas for downdraftgasiers, the outlet temperature at the throat exit should beused. Li et al. [26] found that the kinetic carbon conversionfor pressurized gasication of subbituminous coal in thetemperature range 747877 1C is seen to be comparable toequilibrium predictions for a temperature about 250 1Clower. Bacon [27] dened quasi-equilibrium temperaturesfor each independent chemical reaction. Based on 75operational data points measured in circulating uidisedbed (CFB) gasiers operated on biomass, Kersten [23] hasshown that for operating temperatures in the range740910 1C, the reaction equilibria of Eqs. (8)(10) shouldbe evaluated at much lower temperatures (respectively,457729 1C, 531725 1C, and 583725 1C). These quasi-equilibrium temperatures appear to be independent ofprocess temperature in this range.An important subject, which has not yet been studied

thoroughly in the gasication literature, is whether kineticlimitations increase or decrease the efciency of gasiers.To this end, Appendix A compares gasication atequilibrium conditions with quasi equilibrium conditions.The conclusion is reached that the gasication efciency isseverely affected when the gasication reactions of Eqs. (9)and (10) are kinetically limited and do not contributesufciently to the carbon conversion. The equilibriummodel therefore indicates the maximum efciency that canpossibly be attained when gasifying a fuel. This model isapplied in the next section to study the effect of changes infuel composition on the maximum gasication efciency.As a word of caution, it must be realized that it is difcultto reach this maximum at gasication temperatures below

arying composition as in Van Krevelen diagram

r, minerals and ashes is ignored

chemical exergy of fuel estimated by empirical correlations

e cases together with steam (H2O)

. chemical equilibrium is reached

ion

ncorporates irreversibilities incurred in production of oxygen, steam and

stic values are assumed)

32 (2007) 12481259 12531000 1C, even for reactive fuels such as biomass. This mayrequire operation at prolonged residence times (e.g., in abubbling uidized bed, the carbon conversion appears tobe higher than in a CFB [23]), higher pressures to increasethe char reforming rates (although less favorable for theequilibrium of these reactions), which is demonstrated bythe relatively high carbon conversion of the Varnamogasier [28], and/or catalytically active bed materials suchas dolomite or olivine [29].

4. Results

4.1. Gasification temperatures and equivalence ratios

The gasication efciency that can be achieved as afunction of fuel composition is evaluated at three differenttemperatures: the carbon boundary temperature (thetemperature obtained when exactly enough oxygen isadded to achieve complete gasication), and referencetemperatures of 927 and 1227 1C. The reference tempera-ture of 927 1C is benecial to preserve the chemical exergy

of the fuel in the product gas [21]. However, it is evidentfrom the previous section that higher temperatures mayvery well be required in practice, in order to reduce kineticlimitations. Therefore, a temperature of 1227 1C is alsoincluded as a reference temperature. Fig. 4 shows thecarbon boundary temperatures for gasication of fuelswith different O/C and H/C ratios, as well as the referencetemperatures. It can be observed that lower oxygen contentin the fuel corresponds to a higher carbon boundarytemperature. Optimum gasication temperatures werefound to increase from 782 1C for biomass (averagecomposition CH1.4O0.6) to 1668 1C for gasication of coal(average composition CH0.95O0.2) gasication.Fig. 5 shows the equivalence ratio required, depending

on the desired gasication temperature, for different O/C

and H/C ratios of the fuel. The equivalence ratioexpresses the amount of oxygen required for gasicationrelative to the amount required for combustion. Equiva-lence ratios may range from 0.244 for cellulose to 0.500 forpure carbon (graphite), as exemplied in the equationsbelow:Cellulose gasication:

C6H2O5 1:461O2 4:567CO 1:295CO2 0:138CH4 3:958H2 0:766H2O:

11

Cellulose combustion:

C6H2O5 6O2 6CO2 5H2O: (12)

ARTICLE IN PRESS

0.80.40.30.20.0500

1000

1500

2000

2500

Gas

ificat

ion

tem

pera

ture

(C)

Atomic O/C ratio of fuel

low H/C ratio

high H/C ratio

927C

1227C

carbon boundarytemperature

Fig. 4. Gasication temperatures for fuels of varying O/C and H/C ratios.

c

0.4

0.44

om

M.J. Prins et al. / Energy 32 (2007) 124812591254arbon boundary temperature

0.20.0

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

0.40

0.42

Equi

vale

nce

ratio

(-)

AtFig. 5. Minimum equivalence ratios for gasication of fuels ohigh H/C ratio

high H/C ratio

high H/C ratio

low H/C ratio

low H/C ratio927C

1227C

0.80.6

low H/C ratio

ic O/C ratiof varying O/C and H/C ratios at different temperatures.

Carbon gasication:

C 1=2O2 CO: (13)Carbon combustion:

CO2 CO2. (14)As expected, it becomes clear from Fig. 5 that coal

gasiers operated at the carbon boundary temperaturerequire much higher equivalence ratios than biomassgasiers. For fuels with O/C ratios above 0.4 (correspond-ing to carbon boundary temperatures below 927 1C), therequired equivalence ratio increases only moderately withdecreasing O/C ratios. This happens because the increasedextent of oxidation is accompanied by an increased extentof reforming, as the reaction equilibria of Eqs. (9) and (10)become more favorable at higher temperature. If gasica-tion is carried out at a xed reference temperature, the

difference in equivalence ratios levels out. The equivalenceratios become fairly constant for all fuels, around 0.29 for agasication temperature of 927 1C and 0.320.33 for agasication temperature of 1227 1C. At low O/C ratios,steam partly replaces oxygen, so that lower equivalenceratios are sufcient, whereas at high O/C ratios, moreoxygen is required to reach the desired temperatures. Inother words, biomass is gasied above its carbon boundarytemperature by over-oxidizing the fuel, where coal may begasied below its carbon boundary temperature bymoderation with steam.

4.2. Gasification efficiencies

Fig. 6a shows the thermodynamic efciencies correspond-ing to oxygen-blown gasication of fuels with different O/Cand H/C ratios. For gasication at the carbon boundary

ARTICLE IN PRESS

Ther

mo

dyna

mic

effic

ienc

y (%

)

Atomic O/C ratio of fuel0.80.60.40.20.0

60626466687072747678808284868890

/C

(a)

M.J. Prins et al. / Energy 32 (2007) 12481259 12550.40.20.060626466687072747678808284868890

Ther

mo

dyna

mic

effic

ienc

y (%

)

Atomic O(b)Fig. 6. (a) Thermodynamic efciency of gasication of fuels of varying O/C a

gasication, including steam and oxygen production, of fuels of varying O/C0.80.6ratio of fuelnd H/C ratios at different temperatures. (b) Thermodynamic efciency of

and H/C ratios at different temperatures.

temperature, the overall efciency as well as chemicalefciency rises when decreasing the O/C ratio from 0.8 to0.4. Beyond this point, the overall efciency increases onlymarginally, whereas the chemical efciency decreases. This iscaused by the sharp temperature increase at O/C ratiosbelow 0.4 (see Fig. 4), which results in a high physical exergyof the product gas, but low chemical exergy. For gasicationat 927 1C, respectively, 1227 1C, the chemical efciency in theO/C region below 0.4 is improved substantially due tomoderation of the temperature with steam, with the chemicalefciency at 927 1C approximately 2% points higher than at1227 1C. Moderation of temperature hardly changes theoverall efciency. For fuels with high O/C ratios, such asbiomass, the efciencies at a gasication temperature of 927or 1227 1C are considerably lower than at the carbonboundary temperature. These fuels need to be over-oxidizedin order to reach the desired gasication temperature.Therefore, more oxygen is used than necessary for completegasication, which causes thermodynamic losses.

occurring in sub-processes in the gasier, also exergy lossesfor production of oxygen, electricity and steam are shown.The exergy losses due to heating of fuel and chemicalreaction are by far the largest contribution to the overallexergy losses. These losses correlate very well with b, theratio of chemical exergy to lower heating value. Therefore,it can be concluded that fuels with a low b are preferred inorder to achieve high exergetic efciency.Exergy losses due to oxygen and electricity production

consume 56% of the fuels exergy. These losses increaseonly slightly with decreasing O/C ratio because therequired equivalence ratio is nearly constant. Fig. 7explains the slight attening of efciency curves below anO/C ratio of 0.4, observed in Fig. 6b. Below this ratio,steam is introduced to moderate the gasication tempera-ture. Although the exergetic losses due to heating of fueland chemical reaction continue to decrease below 0.4, theseare partially compensated by the losses incurred for steamproduction, and to a smaller extent for heating of steam

ARTICLE IN PRESS

C r0.4

M.J. Prins et al. / Energy 32 (2007) 124812591256Fig. 6b shows thermodynamic efciencies for gasifyingfuels of varying composition, when exergetic losses forproduction of oxygen, electricity and steam are taken intoaccount. The effect of these additional exergetic losses issubstantial. The losses incurred for producing oxygen aremost obvious for gasication of fuels with low O/C ratiosat the carbon boundary temperature, which need highequivalence ratios to be gasied. The curves for overall andchemical efciency atten somewhat; this happens belowan O/C ratio of 0.4 for gasication at 927 1C and an O/Cratio of 0.3 for gasication at 1227 1C.

4.3. Process losses

Fig. 7 shows, as an illustration, the relative exergy lossesfor gasication at 927 1C. Apart from exergy losses

heating steam

mixing

Rel

ative

ther

mody

nam

ic lo

sses

(%)

O/

0.00.0 0.2

0.4

0.8

4

8

12

16

20

24Fig. 7. Relative exergy losses for sub-processes in gasication of fuels (includinand mixing of reactant gases.

5. Conclusion and discussion

It was shown that the presence of kinetically limited chargasication reactions in gasiers negatively inuences theefciency of a gasier. Therefore, a relatively simpleequilibrium model predicts the maximum efciency thatcould be attained.In order to gasify fuels with high thermodynamic

efciency at atmospheric pressure, it is recommended touse a gasication temperature around 927 1C, and fuelswith O/C ratio smaller than 0.4 (corresponding to apreferred lower heating value above 23MJ/kg). To mini-mize kinetic restrictions, higher temperatures may bepreferred. At a gasication temperature of 1227 1C, the

atio of fuel0.6 0.8g steam and oxygen production) of varying O/C and H/C ratios at 927 1C.

recommended O/C ratio of the fuel is 0.3 or less(corresponding to a preferred lower heating value above26MJ/kg). Fuels with higher O/C ratios, such as wood,have larger exergy losses because of their high ratio ofchemical exergy to lower heating value. Furthermore, suchfuels are over-oxidized in the gasier in order to reach therequired gasication temperature. In practice, due to heatlosses, the presence of ash in the fuel, and of nitrogen in thegasifying agent if air or enriched air is used, the extent ofover-oxidation will be more severe. However, for gasica-tion at elevated pressures, the minimum temperaturerequired for gasication increases, and the gap withkinetically preferred temperatures (i.e. the extent of over-

temperatures around 250 1C (a process known as biomasstorrefaction [31]), which lowers the O/C ratio prior togasication. Additional irreversibilities in the extra processstep must be carefully taken into account [32]. Lignin ortorreed wood could also be mixed with coal to enhancethe gasication fuel properties further.

Acknowledgments

This research project was conducted within the scope ofthe Incentives Program for Energy Research, founded byNWO (Netherlands Organisation for Scientic Research)and Novem (Netherlands Agency for Energy and Environ-

thermodynamic efciency of air or oxygen-blown gasica-

ARTICLE IN PRESSM.J. Prins et al. / Energy 32 (2007) 12481259 1257oxidation) can be reduced.Based on the above, it becomes clear that highly

oxygenated biofuels are not ideal fuels for gasiers froman exergetic point of view. However, this is a purelytheoretical conclusion, based on thermodynamic equili-brium and 100% carbon conversion, which does not implythat woody biomass is less attractive than coal as agasication fuel. Apart from the composition of theorganic matter in a fuel, there are many other factors thatneed to be taken into consideration, such as the lower ash,sulfur and nitrogen content of biomass and the highreactivity of biomass char compared to coal char. Inpractice, at the same temperature, biomass gasication mayhave a higher carbon conversion than coal gasication,which means that kinetic restrictions may outweigh thepotential thermodynamic advantage of coal over biomass.What this study does teach us is that methods to modify

the properties of solid biofuels prior to gasication couldbe considered. An effective way to do so, is mixing ofbiomass with coal (co-gasication, see e.g. [30]), a processwith negligible additional irreversibilities. Alternatively,wood could be separated into its components, e.g. byextraction of lignin with phenol, and only the lignincomponent could be gasied. However, this greatly reducesthe amount of fuel available for gasication, and would beattractive only if the C5 and C6 polysugars present in thecellulose and hemi-cellulose fractions are desired products.Another approach uses thermal pretreatment of wood atFig. 8. Biomass (CH1.4O 0.6, point A), wet biomass (containing 10wt% moistu

(to point C) and (b) at nonequilibrium conditions (to point D).tion, based on the rst and second law of thermodynamics,reaches a maximum at the carbon boundary temperaturement). NWO and Novem do not guarantee the correctnessand/or the comprehensiveness of the research data.

Appendix A. Gasication efciency at equilibrium andnonequilibrium conditions

Fig. 8a illustrates gasication of biomass at equilibriumconditions at typical gasication temperature of 877 1C.Preheating of fuel or gasifying medium is not considered,which means that the fuel and oxygen enter the gasier at atemperature T0 25 1C. The biomass fuel is represented bya general formula of CH1.4O0.6 (with net heat of combus-tion of 19.6MJ/kg) and indicated by point A in thetriangular diagram. Assuming that 10% moisture is presentin the fuel, the composition of the wet biomass is given bypoint B. When oxygen is added, the composition movesinto the direction of point C; at this point, all carbon ispresent in the gaseous phase as carbon monoxide, carbondioxide or methane. The required equivalence ratio,dened as the amount of oxygen added for gasicationrelative to the amount of oxygen required for completecombustion, is 0.295. Notice that point C does not lie onthe carbon boundary line (line I), because this line has to becrossed over in order to reach the desired temperature. There, point B) and gasication of wet biomass, (a) at equilibrium conditions

numbers are higher for the total gasication systems, butthe conclusion that the equilibrium case is more efcientstill holds. Analysis of the process losses, also shown inTable 2, indicates that the losses for heating the fuel andchemical reaction have the largest contribution. Theselosses are higher for the quasi-equilibrium case. The reasonthat gasication at equilibrium conditions is more efcientis that the exothermic oxidation reactions are effectivelycoupled with endothermic reforming reactions, so that thedriving force for the overall chemical reaction (difference inchemical potential) is lower.In the denitions of Eqs. (5) and (6), unconverted carbon

is not regarded as a loss because, in principle, it can berecycled into the gasier. However, this is problematic inpractice as unconverted carbon is contained in the ashes. Ifunconverted carbon is regarded as a loss, the thermo-dynamic efciency for gasication at quasi-equilibriumconditions drops below 50%. This number may be some-what improved by processing the unconverted carbonrather than to dispose of it, e.g. it can be burned out in aseparate reactor.

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ARTICLE IN PRESS

conditions conditions

rgy[8]. Theoretically, the biomass could be gasied moreefciently at a slightly lower equivalence ratio.The triangular diagram in Fig. 8b shows the carbon

boundary lines that we have determined, based on theresearch of Li et al. (line II; this is the carbon boundary lineat 627 1C) and the research of Kersten (line III; includinglines to indicate uncertainty). These lines have shifteddownwards compared to line I in Fig. 8a, which indicatesthat as a result of kinetic limitations, more oxygen isrequired for complete gasication. This is the combined netresult of two effects. If the exothermic reaction of Eq. (8) iskinetically limited and more methane (and higher hydro-carbons) is formed than predicted, it is favorable becausehydrocarbon formation contributes to the carbon conver-sion. However, if the endothermic reactions of Eqs. (9) and(10) are also kinetically limited, it is not favorable becausethis means that these reactions are effectively frozen in,and more oxygen must be added to obtain complete carbonconversion.Fig. 8b illustrates gasication of biomass at a gasication

temperature of 877 1C at quasi-equilibrium conditions, withthe reaction equilibria evaluated at 627 1C. Again, thebiomass fuel considered in this work is represented by ageneral formula of CH1.4O0.6, indicated by point A in thetriangular diagram, and the composition of the wet biomassincluding 10% moisture is given by point B. When oxygenis added, the composition moves into the direction of pointD. At this point, 60% of the carbon is present in thegaseous phase and 40% remains unconverted. Due to thelow carbon conversion, the required equivalence ratio toreach the desired process temperature is only 0.151, which islower than for the unmodied equilibrium model. This isbecause the endothermic char reforming reactions, whichnormally act as a temperature ceiling in a gasier, hardlyoccur. In practice, higher equivalence ratios need to be usedto compensate for heat losses from the gasier and warm upinert components (such as ash in fuel, and nitrogen if air isused as gasifying medium). More carbon is thus converted,but the total carbon conversion in uidized bed gasiersoperating at atmospheric pressure and bed temperaturesbelow 910 1C remains rather low, in the range of 7085% atequivalence ratios of 0.20.3 and complete carbon conver-sion only above 0.4 [23].The process parameters for atmospheric gasication at a

temperature of 877 1C at equilibrium and quasi-equilibriumconditions are compared in Table 2. The gas composition isvery different for these scenarios. At quasi-equilibriumconditions, much less carbon monoxide is present in thegas, and more methane, steam and carbon dioxide.Methane formed in gasiers, probably by thermal crackingof tar, reforms too slowly and hence its concentration ishigher than at equilibrium conditions. The concentrationsof steam and carbon dioxide are higher due to kineticlimitations of the char reforming reactions.Table 2 compares the thermodynamic efciency of the

M.J. Prins et al. / Ene1258gasication processes. It shows that the thermodynamicefciency is highest at equilibrium conditions. For thegasier, the process losses amount to 19.3% for theequilibrium case and 23.0% for the quasi-equilibrium case.Due to losses incurred for oxygen production, these

Equivalence ratio 0.295 0.151

Carbon conversion (%) 100 60

Gas composition (mol%)

H2O 9.6 17.9

H2 36.0 37.0

CO 44.3 19.5

CO2 10.0 21.3

CH4 o 0.1 4.3Gasier efciency (%)

Chemical 75.2 70.3

Physical 5.5 6.7

Total 80.7 77.0

Process losses (%)

Heating oxygen 0.4 0.3

Heating fuel and chemical

reaction

17.9 21.4

Product mixing 1.0 1.3

Gasication system efciency

(%)

Total 75.6 73.0

Total if unconverted carbon

is lost

48.7Table 2

Gasication at 877 1C, equilibrium and quasi-equilibrium conditions

Equilibrium Quasi-equilibrium

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From coal to biomass gasification: Comparison of thermodynamic efficiencyIntroductionMethodologyGasifier fuel propertiesGasification efficiencyProcess losses

Gasifier modelsEquilibrium modelQuasi-equilibrium models

ResultsGasification temperatures and equivalence ratiosGasification efficienciesProcess losses

Conclusion and discussionAcknowledgmentsGasification efficiency at equilibrium and nonequilibrium conditionsReferences