oehra

a mcatitypturegib) ancysin

2013 Elsevier Ltd. All rights reserved.

y avairimaryrlds arts bioromismbusticess th

through experiments [46]. Many models, also are developed forbiomass gasication in uidized bed gasiers which Gomez-Bareaand Leckner presented a comprehensive review of them [7].Models can be categorized into three types; equilibrium, nite rate,and neural network models [8]. Equilibrium models due to theirsimplicity and few input requirements are suitable for preliminarystudies and are widely used by researchers [914]. Due to

ture content andal. studiedans of an eGibbs free

minimization, was used to study effect of air preheating onmance of a circulating uidized bed biomass gasier [20]. Scet al. perform an extensive parametric study and surveyedences of parameters such as temperature, fuel composition andamount of uidizing agent. [21].

In this study a modied equilibrium model based on Gibbs freeenergy minimization approach is presented. Modications forcarbon conversion and tar formation are applied. Effects of gasi-cation parameters such as equivalence ratio, process temperature,fuel type and its moisture content, and gasifying agent are studied.

Corresponding author. Tel.: +98 913 2324628.

Energy Conversion and Management 79 (2014) 1824

Contents lists availab

n

seE-mail address: rshahsavan@mecheng.iust.ac.ir (R. Shahsavan-Markadeh).suitable technologies for biomass conversion. Many parameterscan affect biomass gasication performance, such as reactor pres-sure and temperature, fuel quality, type of gasifying agent, particlesize. Some researchers studied the effects of various parameters

equilibrium model and studied effects of moistemperature on syngas quality [18]. Altani etof various parameters on wood gasication by meriummodel [19]. Aspen Plus simulation based on0196-8904/$ - see front matter 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.enconman.2013.12.007effectsquilib-energyperfor-husterinu-liquid fuels into gaseous fuel using gasifying agents such as air andoxygen. Based on how and where gasifying agent and fuels meeteach other, there are three major types of gasiers; xed bed,uidized bed, and entrained ow. Fluidized bed gasiers, that havegood heat and mass transfer between gas and solid phase and tol-erate variations in fuel quality and particle size [3], are one of the

sion of 60% [16]. Barman et al. in their equilibrium model assumed4.5% of product to be tar and consider its formula as CH1.003O0.33[17]. These kinds of modication lead to better agreement betweenmodels and experiments.

By means of equilibrium models various parameters effects areinvestigated. Antonopoulos et al. developed a non-stoichiometric1. Introduction

Biomass as a renewable, widelenergy resource, is the third largest pcoal and oil. It provides 14% of the wotion [1]. Gasication which convevarious applications, is one of the phas many advantages over direct co

The gasication is a chemical prolable and CO2-neutralenergy resources after

nnual energy consump-mass to syngas withing technologies whichon [2].at converts the solid or

existence of non-equilibrium phenomena in the gasier, equilib-rium models results differ from experimental data. Hence, someof modelers applied modications on their models to obtain betteragreement between predicted and actual results. Li et al. assumedcomplete conversion for all elements other than carbon and hydro-gen and imposed an availability function on these two elements tomodify element abundance vector [15]. Jarungthammachote andDutta considered heat loss to be 1% of the higher heating valueof biomass and modied the model by considering carbon conver-Fuel typeAir O2 enriching

O/C ratio shows adverse effect. Moisture content of biomass has negative effects on cold gas efciencyand HHV. For higher equivalence ratio this negative effect decreases.Effects of various operational parametersprocess; a modied equilibrium model

Hojat Ghassemi, Rasoul Shahsavan-Markadeh School of Mechanical Engineering, Iran University of Science and Technology, Narmak, T

a r t i c l e i n f o

Article history:Received 3 May 2013Accepted 5 December 2013

Keywords:BiomassGasicationEquilibrium modeling

a b s t r a c t

In this paper by means offormance of biomass gasiication temperature, fuelresults show that temperatemperature and has neglihigher heating value (HHVincreases. Cold gas efcienequivalence ratio. By increa

Energy Conversio

journal homepage: www.eln biomass gasication

n 16846-13114, Iran

odied equilibrium model based on Gibbs free energy minimization, per-on process is studied. Effects of parameters such as equivalence ratio, gas-e and its moisture content, and gasifying agent, are investigated. Modelhas signicant inuence on cold gas efciency more remarkably at lower

le effect on syngas heating value. Enriching the air with O2 increases gasd for larger amount of oxygen it shows a maximum as equivalence ratiofor higher oxygen amount is larger and its maximum obtained at lowerg the H/C molar ratio of biomass, cold gas efciency increases but biomass

le at ScienceDirect

and Management

vier .com/locate /enconman

y CoPerformance of gasication process is determined by syngas higherheating value and gasication cold gas efciency. By studying theeffects of gasication parameters on optimum condition this ideathat if gasifying agent consumption varies for different operatingcondition, is investigated.

2. Model description

Thermodynamic equilibrium calculations based on Gibbs freeenergy minimization is chosen to model biomass gasication pro-cess. In thermochemical equilibrium state total Gibbs free energyof system is minimum [22]. The total Gibbs free energy of systemis dened as follows

G XNi1

nili 1

where li and ni are chemical potential and number of moles of ithspecies, respectively. Chemical potential is dened as [23]:

li @G@ni

T;P;nji

2

For gas phase (i = 1, 2, . . ., NG) and condensed phase (i = NG + 1,. . ., N) chemical potential can be calculated as follows [24]:

li li RTln nintot RTln PP i 1;2; . . . ;NGli i NG 1; . . . ;N

(3

where P, T, R, and ntot are pressure, temperature, gas constant, andtotal number of moles, respectively and superscript means stan-dard conditions. li is the chemical potential at standard conditionsand equals to Gibbs energy of formation at given temperature

Nomenclature

CP specic heat at constant pressureG total Gibbs free energyh0f enthalpy of formationH total enthalpyHHV higher heating valueLHV lower heating value_m mass ow raten number of molesN number of speciesNG number of gaseous species

H. Ghassemi, R. Shahsavan-Markadeh / Energwhich can be calculated as follows:

DG DH TDS 4where DH and DS are enthalpy and entropy of formation at the gi-ven temperature. For minimizing Gibbs free energy under followingconstraint,

XNSi1

ajini b0j 0 5

Lagrange multiplier method is used and nal equations ob-tained as follows:

li XNEj1

kjaji

! 0 6

XNSi1

ajini b0j 0 7XNSi1

ni ntot 8

where aji and b0j are number of atoms of element j in species i and

number of moles of element j in reaction, respectively. NE, NS, andkj are respectively, number of elements, number of species, and La-grange multiplier.

According to the rst law of thermodynamics, enthalpy equa-tion is obtained as:

HP HR Qin Qout 9Qin and Qout are exchanges of heat between reactor and surroundingwhich is neglected in this study. HP and HR are enthalpy of productsand reactants, respectively. Assuming the ideal gas behavior,enthalpies are temperature dependent and can be calculated bymeans of each species enthalpy at given temperature.

HR;P XR;P

nih0f ;298 XR;P

ni

Z Ti298

CpidT 10

where Cpi and h0f ;298 are specic heat and enthalpy of formation ofith species. In order to calculate the thermodynamic properties ofspecies, such as Gibbs energy and enthalpy, at a specic tempera-ture following equations are used [25].

H0TRT

a1T2 a2 ln TT a3 a4T2 a5 T

2

3 a6 T

3

4 a7 T

4

5

b1T

11

S0TR a1 T

2

2 a2

T a3 ln T a4T a5 T

2

2 a6 T

3

3 a7 T

4

4 b2 12

P pressureQ heat exchangeR gas constantT temperature

Greek lettersdc carbon conversion efciencyu equivalence ratiogcg cold gas efciencyl chemical potential

nversion and Management 79 (2014) 1824 19where coefcients of this equation are available for several species[25].

Enthalpy of formation of biomass is calculated by means of itsheating value [26]:

h0f ;biomass LHVbiomass Xk

nkh0f ;k 13

where h0f ;k is the enthalpy of formation of product k under completecombustion of biomass. Heating value of biomass in terms of MJ/kgcan be determined by knowing its elemental analysis [27].

HHV 0:3491C 1:1783H 0:1005S 0:1034O 0:0151N 0:0211A 14

where C, H, S, O, N, and A are mass percentage of carbon, hydrogen,sulfur, oxygen, nitrogen, and ash in dry biomass, respectively. Forcalculation of LHV of biomass following correlation is used [16]:

LHV HHV 9mHhfg 15

Coz NMCCMN

19

where in these equations H, C, O and N are mass fraction of hydro-gen, carbon, oxygen and nitrogen in ultimate analysis of biomass,respectively and M is their molecular weight.

Performance of gasication usually is determined by means ofsyngas heating value and cold gas efciency. The dry syngas higherheating value at the standard state is determined from syngascomposition as follows [15]:

HHV 12:75H2 12:63CO 39:82CH4 63:43C2H4 . . . 20

where species contents are in mole fraction and heating value is inMJ/N m3.

Cold gas efciency is dened as the ratio between energy con-tent of produced gas in standard state and energy content ofbiomass:

gcg LHVg _mgLHVf _mf

21

2.1. Model modication

Thermodynamics models results deviate from experimentaldata due to presence of nonequilibrium phenomenon in reactor.Therefore, some modications for improving the results are neces-sary. Some of basic modications which used by other researchersare considering the carbon conversion and tar formation.

Under actual conditions usually some of carbon remains uncon-verted. Lv et al. reported carbon conversion between 0.6 and 0.95in airsteam gasication in a uidized-bed gasier [29]. Lapuertaet al. for different type of biomass observed carbon conversion be-tween 0.5 and 0.98 [30]. Kaewluan et al. reported carbon conver-sion varying between 0.79 and 0.87 for wood by differentmoisture content [31]. Since equilibrium models overestimatethe carbon conversion efciency Azzone et al. calculated the molarquantity of carbon that convert to gas by means of following equa-tion [10]:

dc 1 nC 0:32 0:82 1 eu0:229

22where mH is the hydrogen mass fraction in the biomass and hfg isthe enthalpy of vaporization of water.

Under gasication conditions only CO, CO2, H2, H2O, CH4 and so-lid carbon exist in remarkable amount [28]. Thus, overall reactionof biomass gasication with air can be written as:

CHxOyNz wH2OmO2 3:76mN2 !nCC nH2H2 nCOCO nCO2CO2 nH2OH2Og nCH4CH4nN2N2 nO2O2

16

where m is the number of air moles per mole of biomass which canbe calculated by stoichiometric air multiplied by equivalence ratio.w is the water introduced to reactor in the form of moisture of bio-mass and/or steam injected as gasifying agent. x, y and z are deter-mined by ultimate analysis of biomass as follows [17]:

x HMCCMH

17

y OMCCMO

18

20 H. Ghassemi, R. Shahsavan-Markadeh / Energywhere u is the equivalence ratio and the coefcients were derivedbased on comparison between equilibrium models prediction andexperimental data of biomass gasication. Hence, this model isvalid for gasication conditions and if operating conditions, suchas temperature, are not in suitable range for gasication it can beinaccurate. In this study, we used Eq. (22) to take into accountthe carbon conversion efciency. For obtaining results withacceptable accuracy, temperature varies in appropriate range forgasication.

The product gas from equilibrium model is tar free, but underactual conditions in gasication process always tars are produced.Gil et al. reported amount of tars in a uidized bed biomass gasierup to 9.5% of fuel mass [4]. Herguido et al. observed tar yield is28% of fuel mass in temperature range of 650800 C [32]. Chem-ical composition of tar produced in biomass gasication, varieswith different fuels and operating conditions. Liu et al. accordingto GCMS analysis results considered phenol as main compoundof tar [33]. Haseli et al. in modeling of wood combustion particlesconsider tar as C3.878H6.426O3.561 [34]. Because toluene and naph-thalene are more stable than other compound of tar, Fourcaultet al. considered these as representative compound of tar [35]. Bar-man et al. in their equilibrium model assumed 4.5% of product tobe tar and consider its formula as CH1.003O0.33 [17]. All the abovemodels are considered and as it can be observed in Table 1, Barmanet al. model give results with better agreement with experiment.Thus, in this study this model is applied for modeling the tarformation.

For calculating the equilibrium temperature and composition ofsyngas, rst a temperature is guessed. Then by calculating proper-ties in this temperature, gas composition by use of Eqs. (6)(8)to-gether with Eq. (22) and Barman et al. model for tar is determined.This procedure is repeated until temperature difference betweentwo iterations becomes negligible. This procedure is developed ina FORTRAN code.

3. Results

3.1. Model validation

For validating the model, results of original and modied modelis compared to two experimental datas reported by Narvaez et al.[5] and Subramanian et al. [36]. In Ref. [5] gasication of pine saw-dust with air at 800 C and equivalence ratio of 0.32 and in Ref. [36]gasication of rice husk with air at 887 C and equivalence ratio of0.4 are selected for comparison. Pine sawdust and rice husk analy-sis is given in Table 2. Comparison of experimental data with re-sults of model is given in Table 3. The RMS error is calculated asfollows,

RMS PN

i1ni;exp ni;modelN

s

As it can be observed modifying the model leads to betteragreement between models predictions and experiment. Never-theless, model predicts no methane content which differs fromexperiment.

3.2. Parametric study

For studying effects of various parameters on gasicationperformance, Pine sawdust with analysis shown in Table 2 is se-lected [5]. Although mineral matters in the ash can affect theneeded energy for heating up, in this study this effect is neglected.For all the simulations, input temperature is assumed 25 C (ambi-ent temperature) and for constant temperature runs. Gasicationtemperature is considered 800 C, biomass HHV is calculated by

nversion and Management 79 (2014) 1824means of Eq. (14). For determining gas composition at constanttemperature, enthalpy equation (Eq. (9)) is not solved. Variousparameters effects on cold gas efciency and gas HHV are studied.

heating value of syngas, due to decrement of H2 as a valuablegas, decreases, but cold gas efciency has a maximum atER = 0.29. This is because of increasing of the gas yield.

3.2.2. Effects of O2-enriching the airUsing of O2-enriched air as a gasifying agent, received some

attention lately [37,38]. For study effects of enriching the air onsyngas HHV, temperature keep xed at 800 C, and mass fractionof O2 in the gasifying agent (O/A) varied from 0.23, as in air, to 1for pure oxygen. In Fig. 3 syngas heating value is plotted versusequivalence ratio for different values of O/A ratio. It can be ob-

Table 1Comparison of error of model by using different tar compositions.

Tar composition C6H6 C6H6O C7H8 C10H8 CH1.003O0.33 C3.878H6.426O3.561RMS error 3.47 3.60 3.45 3.50 2.19 3.97

Table 2Ultimate analysis of biomass wt% (daf basis).

C H O N S

Pine sawdust 50.0 5.7 44.1 0.17 0.03Rice husk 38.92 5.10 53.69 2.17 0.12

Table 3Comparison between experimental data and results of model.

Experimental study Original model Modied model

Case a [5] Case b [36] Case a Case b Case a Case b

H. Ghassemi, R. Shahsavan-Markadeh / Energy Conversion and Management 79 (2014) 1824 213.2.1. Effects of equivalence ratioEquivalence ratio (ER) is dened as ratio of actual oxygen

amount in gasication process to stoichiometric amount. Effectsof equivalence ratio on syngas composition and temperature areshown in Fig. 1. It can be seen that increasing of equivalence ratioleads to decreasing of H2 content. It is probably because of furtheroccurring of hydrogen oxidation, due to presence of more oxygenin system. Other modelers reported same results too [13,17]. COcontent of syngas increases by increasing of equivalence ratioand then decreases and CO2 content of syngas shows inverse trend.N content of syngas increases due to increasing of air introduced

CO 14.0 16.13 14.4 19.48 12.7 15.35H2 7.0 7.77 14.9 11.46 10.2 8.30CO2 13.5 10.68 14.6 11.54 14.8 13.85CH4 3.0 2.78 0.0 0.0 0.0 0.0N2 61.3 62.58 56.1 57.52 62.3 60.06O2 0.0 0.06 0.0 0.0 0.0 0.0RMS error 4.47 3.43 2.19 1.952

to reactor. As it can be observed, with increasing of equivalence ra-tio temperature increases. It is probably due to occurring exother-mic reaction because of existence of more oxygen in the system. Italso shows that for higher equivalence ratio temperature is highand for keeping it in the feasible range for uidized bed gasiers,it is necessary to remove heat from the reactor. Effects of equiva-lence ratio on syngas heating value and cold gas efciency areshown in Fig. 2. Although by increasing of the equivalence ratio

Fig. 1. Syngas composition variation with equivalence ratio.3.2.3. Effects of fuel typeTo classication of fuels due to their chemical constituents

there are three methods: atomic ratios, the ratio of ligno-celluloseconstituents, and the ternary diagram [39]. The atomic ratios showthe ratio of carbon, hydrogen and oxygen contents of fuels. VanKrevelen diagram plots atomic ratios (H/C) against (O/C) for allfuels and shows that for biomasses these ratios are in the followingrange [39]:

0:4 6 O=C 6 0:9; 1 6 H=C 6 1:8

For investigate the inuence of fuel atomic ratio on gasicationperformance temperature assumed 800 C. In Fig. 5 cold gas ef-ciency of gasication process is plotted for different values of H/Cserved that increasing of the O/A ratio increases the HHV of syngas.Also for O/A ratio larger than 0.4 a maximum point is observed inHHV and larger the O/A ratio is, ER corresponding to maximumHHV is larger. Due to the signicant effects of oxygen on gasica-tion temperature and consequent effects of temperature on gasi-cation performance, determination of cold gas efciency in xedtemperature gives no reasonable results. Therefore, cold gas ef-ciency is determined by considering non-constant temperature.As it can be observed in Fig. 4 cold gas efciency increases as O/Aincreases. Maximum value of cold gas efciency for higher O/A ra-tios occurs at lower equivalence ratio; so it can be deduced thatoptimum performance of gasication process for higher O/A ratio,obtained with less oxygen consumption.Fig. 2. Effects of equivalence ratio on syngas heating value and cold gas efciency ofgasier.

Co22 H. Ghassemi, R. Shahsavan-Markadeh / Energymolar ratio of biomass. In this plot O/C ratio of biomass is assumedto be 0.65. It can be observed that for larger H/C ratio, cold gas ef-ciency is larger and the maximum of efciency occurs at lowerequivalence ratios. This means for biomasses with larger H/C ratio,the maximum efciency obtains with lesser oxygen consumption.But for ER approximately higher than 0.37, efciency of gasicationprocess for lower H/C ratio biomasses is a little larger. In Fig. 6 coldgas efciency is plotted for various O/C molar ratio of biomass withH/C = 1.4. Increasing the O/C ratio leads to decreasing of the coldgas efciency. The equivalence ratio corresponding to maximumefciency is almost the same for different O/C ratio.

3.2.4. Effect of moistureMoisture content of various biomasses is remarkable and it can

affect the performance of gasication process. For study the effectof moisture content of biomass on gas heating value, temperaturekeep xed at 800 C. In Fig. 7 effects of moisture on HHV of syngasis shown. It can be seen that lower moisture content leads to high-er HHV. But the difference between HHV for various moisture con-tent decreases as equivalence ratio increases. Since moisture

Fig. 3. HHV of syngas for different O/A ratio.

Fig. 4. Cold gas efciency for different O/A ratio.nversion and Management 79 (2014) 1824content of biomass affects signicantly on gasication temperaturedue to heat required for evaporation of moisture, studying the ef-fects of moisture content on cold gas efciency at xed tempera-ture does not give reasonable results. Thus for investigating theeffects of moisture on cold gas efciency enthalpy equation alsohave been solved. As it can be seen in Fig. 8, for lower equivalenceratio as moisture content increases cold gas efciency decreases. Inhigher equivalence ratio cold gas efciency is the same for differ-ent moisture content. This is because of presence of further oxygenand consequently further occurring of exothermic reactions whichleads to complete evaporation of moisture and higher gasicationtemperature.

3.2.5. Effect of gasication process temperatureGasication, as a thermochemical process, is highly affected by

temperature. Temperature inuences various aspects of processand consequently syngas quality. In Fig. 9 can be observed thatincreasing of the temperature rst increases the cold gas efciency.

Fig. 5. Variations of cold gas efciency for different H/C molar ratio of fuel byequivalence ratio.

Fig. 6. Cold gas efciency for different O/C molar ratio of fuel.

y CoH. Ghassemi, R. Shahsavan-Markadeh / EnergFor temperatures higher than 700 C variations of cold gas ef-ciency by temperature became so small which can be neglected.This behavior reported in other model [10] and experimental study[40] too. Also maximum point of efciency for higher temperatureoccurs at lower equivalence ratio which means decrease in oxygenconsumption. For describing the cold gas efciency behavior ver-sus temperature we need to know the variations of gas heating va-lue by temperature. In Fig. 10 HHV of syngas is plotted for differenttemperature and observed it remains almost constant. For lowerequivalence ratio HHV for T = 600 C is a little higher but its valuefor higher equivalence ratio is lesser than other temperatures. Itcan be deduced that cold gas efciency increment by increasingof temperature is not because of heating value enhancement butit is due to increasing the amount of gas produced. Gas yieldincreasing is lesser in high temperature, as it reported in experi-mental studies [29], so cold gas efciency shows slight increase.For higher temperatures gas yield reach its maximum and coldgas efciency decreases due to decreasing the HHV.

Fig. 7. HHV of syngas for different moisture content.

Fig. 8. Cold gas efciency for different moisture content.nversion and Management 79 (2014) 1824 234. Conclusion

In this study we investigated effects of various parameters ongasication process via a modied equilibrium model based onGibbs free energy minimization. Results show that enriching theair with O2 can improve both cold gas efciency and syngasHHV. Enriching the air also decreases the oxygen consumptionfor obtaining the maximum cold gas efciency. Gasication of bio-masses with higher H/C molar ratio, in equivalence ratio lowerthan ER 0.37, have higher cold gas efciency but for higherequivalence ratio this trend diverses. Higher the H/C molar ratiois lower the optimum equivalence ratio become. Increasing theO/C molar ratio of biomass decreases the cold gas efciency. Effectof biomass moisture content also studied and observed thatincreasing the moisture content decreases cold gas efciency andsyngas HHV. But in higher equivalence ratio due to enhancementof exothermic reactions, this decrease became lesser. Temperatureof gasication process has no remarkable inuences on syngasHHV. But increasing the temperature rst increases the cold gasefciency and then cold gas efciency remains almost constant.

Fig. 9. Cold gas efciency for different gasication temperature.

Fig. 10. Effects of gasication temperature on HHV of syngas.

It is also observed that in higher temperature maximum cold gasefciency can be obtained in lower equivalence ratio.

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Effects of various operational parameters on biomass gasification process; a modified equilibrium model1 Introduction2 Model description2.1 Model modification

3 Results3.1 Model validation3.2 Parametric study3.2.1 Effects of equivalence ratio3.2.2 Effects of O2-enriching the air3.2.3 Effects of fuel type3.2.4 Effect of moisture3.2.5 Effect of gasification process temperature

4 ConclusionReferences