Two phase biomass air-steam gasification model for fluidized bed reactors: Part II—model sensitivity

Biomass and Bioenergy 22 (2002) 463–477

Two phase biomass air-steam gasi�cation model for !uidizedbed reactors: Part II—model sensitivity

Samy S. Sadakaa ; ∗, A.E. Ghalyb, M.A. Sabbahc

aAgricultural and Biosystems Engineering Department, Iowa State University, NSRIC, Ames, IA 50011, USAbBiological Engineering Department, Dalhousie University, Halifax, Nova Scotia Canada B3J 2X4

cDesert Development Center, American University, Cairo, Egypt

Received 29 April 1999; received in revised form 26 November 2001; accepted 16 January 2002

Abstract

A sensitivity analysis was performed on the two phase biomass gasi�cation model developed by Sadaka et al. (BiomassBioenergy) to test its response to variations in three operating parameters (!uidization velocity, steam !ow rate and biomassto steam ratio). The model performance criteria included bed temperature, gas compositions, higher heating value and gasproduction rate. The results showed that the model was sensitive to changes in all operating parameters. The temperatures ofthe reactor were more in!uenced by changes in the steam !ow rate than those of !uidization velocity and biomass to steamratio. The steam !ow rate has the most e7ect on the mole fractions of CH4, and CO2 followed by the biomass to steam ratioand then the !uidization velocity. In the case of H2, and CO the biomass to steam ratio has the most e7ect on their molefractions followed by the steam !ow rate and then !uidization velocity. The !uidization velocity has the most e7ect on themole fraction of N2. The biomass to steam ratio has the most e7ect on the gas higher heating value and the gas productionrate. ? 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Fluidization velocity; Air-steam; Gasi�cation; Higher heating value; Model; Straw

1. Introduction

In the previous study [1], a two phase biomassair-steam gasi�cation model for !uidized bed reactorswas developed. The model is capable of predictingthe bed temperature, gas compositions, higher heat-ing value in both the bubble and emulsion phasesas well as gas production rate under a wide range

∗ Corresponding author. Tel.: +1-515-294-4330; fax: +1-515-294-4250.

E-mail address: [email protected] (S.S. Sadaka).

of operating conditions. It is, therefore, essential totest the sensitivity of the model to variations in theparameters which most likely in!uence the gasi�erperformance.

2. Objectives

The goal of this study was to test the sensitivity ofthe model predictions of bed temperature, gas compo-sitions, higher heating value of the gas and gas pro-duction rate to changes in !uidization velocity, steam!ow rate and biomass to steam ratio.

0961-9534/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S 0961 -9534(02)00024 -7

464 S.S. Sadaka et al. / Biomass and Bioenergy 22 (2002) 463–477

Table 1The base values and ranges of variables used in the model sensi-tivity analysis as well as the air and biomass !ow rates

Fluidization Steam Biomass to Air Biomassvelocity !ow rate steam ratio !ow rate !ow rate(m=s) (kg=min) (kg=kg) (m3=min) (kg=min)

0.30 0.25 4 0.40 1.000.35 0.25 4 0.56 1.000.40 0.25 4 0.72 1.000.45 0.25 4 0.87 1.000.50 0.25 4 1.02 1.00

0.40 0.15 4 0.92 1.000.40 0.20 4 0.82 1.000.40 0.25 4 0.72 1.000.40 0.30 4 0.61 1.000.40 0.35 4 0.51 1.00

0.40 0.25 2 0.72 0.500.40 0.25 3 0.72 0.750.40 0.25 4 0.72 1.000.40 0.25 5 0.72 1.250.40 0.25 6 0.72 1.50

3. Range of variables

The developed model was used to predict the bedtemperature, mole fractions of the species, higherheating value and gas production rate under variousoperating conditions. The variables considered in thisstudy are: !uidization velocity, steam !ow rate andbiomass to steam ratio (with 5 levels each). A basecase was used for each variable and this variablewas independently changed around its base valuein the range shown in Table 1. It also shows theair !ow rate as well as the biomass !ow rate. In-creasing the !uidization velocity at �xed steam !owrate was applied by changing the inlet air !ow rate.Increasing the steam !ow rate at �xed !uidizationvelocity was applied by decreasing the inlet air !owrate.

4. Results and discussion

4.1. Bed temperature

The temperature pro�les in the bubble, emulsionand solid phases were sensitive to changes in !uidiza-tion velocity, steam !ow rate and biomass to steam

ratio. Fig. 1 shows the e7ects of !uidization velocityon the temperature pro�le in the bubble, emulsion andsolid phases. Table 2 shows the temperature responseto changes in !uidization velocity, steam !ow rate andbiomass to steam ratio. The model results showed thatthe higher the !uidization velocity and=or the lowerthe steam !ow rate and=or the lower the biomass tosteam ratio, the higher the temperature of the threephases.

Increasing the !uidization velocity from 0:3 to0:5 m=s (by 67%), increased the bubble temperatureby 47 K (4.59%), the emulsion temperature by 48 K(4.66%) and the solid temperature by 36 K (3.67%).The increase in temperatures with increases in the !u-idization velocity resulted from the increased amountof air introduced into the reactor which in turn in-creased the rate of the exothermic reactions (oxidationreactions) and release of energy. Similar results werereported by Ergudenler and Ghaly [2]. The emulsiontemperature is lower than the solid temperature andhigher than the bubble temperature due to the factthat heat is transferred from the solids (sand) to theemulsion and then to the bubbles.

Increasing the steam !ow rate from 0.15 to0:35 kg=min (by 135.35%) decreased the bubble tem-perature by 131 K (−12:14%), the emulsion temper-ature by 134 K (−12:29) and the solid temperatureby 130 K (−11:88%). These signi�cant reductions inthe temperatures, caused by increases in the steam!ow rate, were due to the e7ect of the endothermicreactions (methanation, water gas shift and water gasreactions). Similar results were reported by Singh etal. [3], Walawender et al. [4] and Corella et al. [5] forthe steam gasi�cation of cotton wood branchs, strawand di7erent crop residues, respectively.

When the biomass to steam ratio increased from2.0 to 6:0 kg=kg (by 200%), the bubble temperaturedecreased by 93 K (−8:53%), the emulsion tempera-ture decreased by 95 K (−8:6%) and the solid tem-perature decreased by 98 K (−8:82%). The decreasein the solid temperature is due to the increases inthe endothermic reactions which capture the energyfrom the bed and to the high energy required to dryand pyrolyze the biomass. The oxygen fed is rapidlyconsumed by the carbon present in the feed mate-rial and the heat released from the exothermic oxi-dation process is utilized to pyrolyze the remainingsolid material which contributed to the reduction in

S.S. Sadaka et al. / Biomass and Bioenergy 22 (2002) 463–477 465

Fig. 1. The e7ects of !uidization velocity on the temperature in the bubble, emulsion and solid phases.


Table 2Temperature response to changes in operating parameters

Parameter Value Temperature response

Bubble zone Emulsion zone Solid zone

(K) (%) (K) (%) (K) (%)

Fluidization velocity 0.30 1023 0.00 1029 0.00 1036 0.00(m=s) 0.35 1035 1.17 1041 1.66 1048 1.16

0.40 1045 2.15 1059 2.92 1064 2.700.45 1053 2.93 1060 3.01 1068 3.090.50 1070 4.59 1077 4.66 1074 3.67

Steam !ow rate 0.15 1079 0.00 1090 0.00 1094 0.00(kg=min) 0.20 1057 −2:04 1065 −2:29 1076 −1:65

0.25 1045 −3:15 1059 −2:84 1064 −2:740.30 961 −10:94 966 −11:38 975 −10:880.35 948 −12:14 956 −12:29 964 −11:88

Biomass to steam ratio 2.00 1090 0.00 1105 0.00 1111 0.00(kg=kg) 3.00 1082 −0:73 1097 −0:72 1094 −1:53

4.00 1045 −4:13 1059 −4:16 1064 −4:235.00 1014 −6:97 1027 −7:06 1032 −7:116.00 997 −8:53 1010 −8:60 1013 −8:82

the bed temperature. Ergudenler [6] reported similarresults.

From the above results it appears that, on a per-centage increase basis (1% increase) of each ofthe operating variables, the steam !ow rate has themost e7ect on the reactor temperature followed bythe biomass to steam ratio and then the !uidizationvelocity.

4.2. Gas compositions

4.2.1. Combustible gasesFigs. 2–4 show the e7ects of the steam !ow rate

on the mole fractions of CH4, H2 and CO in the bub-ble and emulsion phases (as illustration of the modelresults). Table 3 shows the response of the com-bustible gases to changes in !uidization velocity,steam !ow rate and biomass to steam ratio. Themodel results showed that the lower the !uidizationvelocity and=or the higher biomass to steam ratio,the higher the mole fractions of the combustiblegases.

Increasing the !uidization velocity from 0.3 to0:5 m=s (by 67%) decreased the mole fractions ofCH4, H2 and CO in the bubble phase by 0.090

(−60:81%), 0.020 (−15:50%) and 0.061 (−26:34%)and in the emulsion phase by 0.043 (−29:45%),0.021 (−16:94%) and 0.060 (−29:29%), respec-tively. This was due to several reasons: (a) the highamount of oxygen being introduced to the systemwhich resulted in burning some of the combustiblegases, (b) the shorter residence time for the char inthe gasi�er which resulted in the loss of some of itsenergy content as reported by Beaumont and Schwob[7] and Ghaly et al. [8] and (c) the increases of themole fractions of the noncombustible componentsof the producer gas (O2; N2 and CO2) due to thehigh amount of air being introduced to the system[2,9,10].

Increasing the steam !ow rate from 0.15 to0:35 kg=min (by 133%) increased the mole frac-tions of CH4 and H2 in the bubble phase by 0.086(179.17%) and 0.065 (65.0%) and in the emulsionphase by 0.087 (193.33%) and 0.058 (58.0%), re-spectively, and decreased the mole fractions of COby 0.112 (−58:03%) and 0.119 (−62:63%) in thebubble and emulsion phases, respectively. Theincreases in the mole fractions of H2 and decreases inthe mole fraction of CO with increases in the steam!ow rate indicated that the gas shift reaction had


Fig. 2. The e7ects of steam !ow rate on the mole fraction of CH4 in the bubble and emulsion phases.

a substantial e7ect in steam gasi�cation. Similar re-sults were reported by Richard et al. [11] and Hos andGroeneveld [12].

Increasing the biomass to steam ratio from 2.0to 6:0 kg=kg (by 200%) increased the mole frac-tions of CH4, H2 and CO by 0.063 (80.77%),0.053 (69.74%) and 0.076 (66.09%) in the bubblephase, respectively and by 0.053 (70.67%), 0.045(63.38%) and 0.072 (66.06%) in the emulsion phase,

respectively. At the highest biomass to steam ratio(6:0 kg=kg), the mole fractions of all species otherthan N2 were at their maximum level because ofthe high amount of biomass being introduced to thesystem.

From the above results it appears that, on a percent-age increase basis of the operating variables, the steam!ow rate has the most e7ect on the mole fractions ofCH4 and CO followed by the biomass to steam ratio


Fig. 3. The e7ects of steam !ow rate on the mole fraction of H2 in the bubble and emulsion phases.

and then the !uidization velocity. In the case of H2,the biomass to steam ratio has the most e7ect on itsmole fraction followed by steam !ow rate and then!uidization velocity.

4.2.2. Noncombustible gasesFigs. 5 and 6 show the e7ects of the biomass

to steam ratio on the mole fractions of CO2 andN2 in the bubble and emulsion phases. Table 4shows the response of the noncombustible gasesto changes in !uidization velocity, steam !ow rate

and biomass to steam ratio. The results showedthat the lower the !uidization velocity and=orthe higher the steam !ow rate and=or the higherthe biomass to steam ratio, the higher the molefraction of CO2 and the lower the mole fractionof N2.

Increasing the !uidization velocity from 0.30 to0:50 m=s (by 67%) decreased the mole fraction of CO2

by 0.027 (−15:17%) and 0.048 (−26:82%) and in-creased the mole fraction of N2 by 0.196 (60.87%) and0.147 (42.00%) in the bubble and emulsion phases,


Fig. 4. The e7ects of steam !ow rate on the mole fraction of CO in the bubble and emulsion phases.

respectively. The decrease in the mole fraction of CO2

and the increase of the mole fraction of N2 are dueto the increase in the amount of air introduced to thereactor and the reaction of O2 with the C and CO toform CO2.

Increasing the steam !ow rates from 0.15 to0:35 kg=min (by 133%) increased the mole fractionof CO2 by 0.124 (228.30%) and 0.120 (244.90%)and decreased the mole fraction of N2 by 0.160(−26:40%) and 0.179 (−23:21%) in the bubble

and emulsion phases, respectively. The increase inthe mole fraction of CO2 was due to the conver-sion of CO to CO2 owing to the gas shift reactionwhereas the decrease in the mole fraction of N2

was due to the relative increase in the combustiblegases.

Increasing the biomass to steam ratio from 2.0 to6:0 kg=kg (by 200%) increased the mole fraction ofCO2 by 0.089 (85.58%) and 0.086 (85.15%) and de-creased the mole fraction of N2 by 0.281 (−44:82%)

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Table 3Combustible gases response to changes in operating parameters

Parameter Value Mole fractions

CH4 H2 CO

Bubble phase Emulsion phase Bubble phase Emulsion phase Bubble phase Emulsion phase

(dimensionless) (%) (dimensionless) (%) (dimensionless) (%) (dimensionless) (%) (dimensionless) (%) (dimensionless) (%)

Fluidization 0.30 0.148 0.00 0.146 0.00 0.129 0.00 0.124 0.00 0.224 0.00 0.205 0.00velocity 0.35 0.139 −6:08 0.137 −6:16 0.125 −3:10 0.122 −1:61 0.220 −1:79 0.198 −3:41(m=s) 0.40 0.129 −12:84 0.129 −11:64 0.123 −4:65 0.120 −3:23 0.176 −21:43 0.169 −17:56

0.45 0.065 −56:08 0.114 −21:92 0.114 −11:63 0.108 −12:90 0.173 −22:77 0.175 −14:630.50 0.058 −60:81 0.103 −29:45 0.109 −15:50 0.103 −16:94 0.163 −26:34 0.145 −29:29

Steam !ow 0.15 0.048 0.00 0.045 0.00 0.100 0.00 0.100 0.00 0.193 0.00 0:190 0.00rate 0.20 0.114 137.5 0.112 148.89 0.105 5.00 0.102 2.00 0.189 −2:07 0.192 1.05(kg=min) 0.25 0.129 168.75 0.129 186.67 0.123 23.00 0.120 20.00 0.176 −8:81 0.169 −11:01

0.30 0.132 175.00 0.130 188.89 0.145 45.00 0.136 36.00 0.110 −43:01 0.107 −43:680.35 0.134 179.17 0.132 193.33 0.165 65.00 0.158 58.00 0.081 −58:03 0.071 −62:63

Biomass to 2.00 0.078 0.00 0.075 0.00 0.076 0.00 0.071 0.00 0.115 0.00 0.109 0.00steam ratio 3.00 0.116 48.72 0.106 41.33 0.099 30.26 0.092 29.58 0.158 37.39 0.150 37.61(kg=kg) 4.00 0.129 65.38 0.129 72.00 0.123 61.84 0.120 69.01 0.176 53.04 0.169 55.05

5.00 0.131 67.95 0.119 58.67 0.125 64.47 0.120 69.01 0.185 60.87 0.176 61.476.00 0.141 80.77 0.128 70.67 0.129 69.74 0.116 63.38 0.191 66.09 0.181 66.06


Fig. 5. The e7ects of biomass to steam ratio on the mole fraction of CO2 in the bubble and emulsion phases.

and 0.261 (−40:53%) in the bubble and emulsionphases, respectively. The increase in the mole fractionof CO2 and the reduction of the N2 mole fraction wasdue to the high amount of biomass (carbon) being in-troduced to the system which resulted in the produc-tion of more mole fractions of the combustible gasesas well as CO2.

From the above results it appears that, on a per-centage increase basis of the operating variables, thesteam !ow rate has the most e7ect on the mole frac-tion of CO2 followed by biomass to steam ratio and

then !uidization velocity where the !uidization ve-locity has the most e7ect on mole fraction of N2

followed by biomass to steam ratio and then steam!ow rate.

4.3. Gas higher heating value

The e7ects of the steam !ow rate on the higherheating value of the producer gas in the bubble andemulsion phases are presented graphically in Fig. 7.The gas higher heating value response to changes in


Fig. 6. The e7ects of biomass to steam ratio on the mole fraction of N2 in the bubble and emulsion phases.

!uidization velocity, steam !ow rate and biomass tosteam ratio is presented in Table 5.

The results showed that the lower the !uidizationvelocity and=or the higher the steam !ow rate and=orthe higher the biomass to steam ratio, the higher thegas heating value.

Increasing the !uidization velocity from 0.30 to0:50 m=s (by 67%) decreased the higher heating valueby 4.57 (−44:12%) and 2:424 MJ=m3 (−24:38%) in

the bubble and emulsion phases, respectively. Thesewere due mainly to the decrease in the combustiblegases and the increase in the mole fraction of N2 in-troduced to the reactor in the air which further dilutedthe combustible gases.

Increasing the steam !ow rate from 0.15 to0:35 kg=min (by 133%) increased the higher heat-ing value of the gas in the bubble phase by2:841 MJ=m3 (50.46%) and in the emulsion phase by


Table 4Noncombustible gases response to changes in operating parameters

Parameter Value Mole fractions

CO2 N2

Bubble phase Emulsion phase Bubble phase Emulsion phase

(dimensionless) (%) (dimensionless) (%) (dimensionless) (%) (dimensionless) (%)

Fluidization 0.30 0.178 0.00 0.179 0.00 0.322 0.00 0.350 0.00velocity 0.35 0.177 −0:56 0.174 −2:79 0.340 5.59 0.374 6.86(m=s) 0.40 0.165 −7:30 0.160 −10:61 0.407 26.40 0.421 20.29

0.45 0.153 −14:04 0.136 −24:02 0.494 53.42 0.467 33.430.50 0.151 −15:17 0.131 −26:82 0.518 60.87 0.497 42.00

Steam !ow 0.15 0.053 0.00 0.049 0.00 0.606 0.00 0.616 0.00rate 0.20 0.069 30.19 0.055 12.24 0.523 −13:70 0.540 −12:34(kg=min) 0.25 0.165 211.32 0.160 226.53 0.407 −32:84 0.421 −31:66

0.30 0.172 224.53 0.164 234.69 0.441 −27:23 0.436 −29:220.35 0.174 228.30 0.169 244.90 0.446 −26:40 0.473 −23:21

Biomass to 2.00 0.104 0.00 0.101 0.00 0.627 0.00 0.644 0.00steam ratio 3.00 0.106 1.92 0.103 1.98 0.521 −16:91 0.550 −14:60(kg=kg) 4.00 0.165 58.65 0.160 58.42 0.421 −32:85 0.421 −34:63

5.00 0.188 80.77 0.182 80.20 0.371 −40:83 0.407 −38:806.00 0.193 85.58 0.187 85.15 0.346 −44:82 0.383 −40:53

3:150 MJ=m3 (57.58%). These increases were causedby the gas shift reaction as well as the low amountof N2 being introduced to the system. Similarresults were reported by Maniatis et al. [13] andSadaka [14].

Increasing the biomass to steam ratio from 2.0to 6:0 kg=kg (by 200%) increased the higher heat-ing value of the gas by 4:148 MJ=m3 (74.97%) inthe bubble phase and by 3:666 MJ=m3 (69.54%)in the emulsion phase. These increases were dueto the increases in the mole fractions of the com-bustible gases (CH4, H2 and CO) caused by thepyrolysis process and cracking of the hydrocarbons.Font et al. [15] and Mudge et al. [16] reportedincreases in the higher heating value with increasesin the biomass to steam ratio as a result of in-creases in the mole fractions of hydrocarbons(CH4, C2H2, C2H4 and C2H6) which were releasedfrom the tar.

From the above results it appears that, on a per-centage increase basis of the operating variables, thebiomass to steam ratio has the most e7ect on the gashigher heating value followed by steam !ow rate andthen !uidization velocity.

4.4. Gas production rate

Figs. 8–10 show the e7ects of the !uidization ve-locity, steam !ow rate and biomass to steam ratio onthe normalized gas production rate. Table 5 presentedthe response of the gas production rate to changes in!uidization velocity, steam !ow rate and biomass tosteam ratio.

Increasing the !uidization velocity from 0.30 to0:50 m=s (67%) and the biomass to steam ratio from2.0 to 6:0 kg=kg (200%) increased the normalizedgas production rate by 0:833 Nm3=min (51.45%)and 1:272 Nm3=min (97.85%), respectively. The pro-ducer gas production rate increased with increasesin the !uidization velocity and biomass to steamratio due to the large amount of volatiles resultingfrom biomass devolatilization. Higher temperaturesincreased the eKciency of tar and char conversion togases.

On the other hand, increasing the steam !ow ratefrom 0.15 to 0:25 kg=min (by 67%) increased thegas production rate from 1.753 to 2:079 Nm3=min(18.60%). Further increases in the steam !ow ratefrom 0.25 to 0:35 kg=min (by 40%) decreased the


Fig. 7. The e7ects of steam !ow rate on the higher heating value in the bubble and emulsion phases.

normalized gas production rate to 1:451 Nm3=min(by 30.20%). The increase in the steam !ow ratedecreased the gasi�er temperature sharply whichin turn decreased the conversion of char and tarinto combustible gases. Corella et al. [5] reportedincreased gas yield with increases in the bedtemperature.

From the above results it appears that, on a per-centage increase basis of the operating variables, thebiomass to steam ratio has the most e7ect on the nor-

malized gas production rate followed by !uidizationvelocity and then steam !ow rate.

5. Conclusions

The bed temperature, gas composition, higher heat-ing value and gas production rate were all sensitiveto changes in all the operating parameters tested inthis study. The predicted temperatures in the bubble,


Table 5Gas higher heating value and gas production rate response to changes in operating parameters

Parameter Value Higher heating value (MJ=Nm3) Gas production rate(Nm3=min)

Bubble phase Emulsion phase

(dimensionless) (%) (dimensionless) (%) (dimensionless) (%)

Fluidization 0.30 10.357 0.00 9.942 0.00 1.169 0.00velocity 0.35 9.905 −4:36 9.457 −4:88 1.857 14.70(m=s) 0.40 8.932 −13:76 8.817 −11:32 2.079 28.41

0.45 6.235 −39:80 8.135 −18:18 2.281 40.890.50 5.787 −44:12 7.518 −24:38 2.452 51.45

Steam !ow 0.15 5.630 0.00 5.471 0.00 1.753 0.00rate 0.20 8.273 46.94 8.173 49.39 1.826 4.16(kg=min) 0.25 8.932 58.65 8.817 61.16 2.079 18.60

0.30 8.503 51.03 8.535 56.00 1.617 −7:760.35 8.471 50.46 8.621 57.58 1.451 −17:23

Biomass to 2.00 5.533 0.00 5.272 0.00 1.300 0.00steam ratio 3.00 7.885 42.51 7.279 38.07 1.659 27.62(kg=kg) 4.00 8.932 61.43 8.817 67.24 2.079 59.92

5.00 9.152 65.41 8.855 67.96 2.212 7.156.00 9.681 74.97 8.938 69.54 2.572 97.85

Fig. 8. The e7ects of the !uidization velocity on the normalized producer gas.

emulsion and solid phases were more sensitive tochanges in the steam !ow rate then the biomass tosteam ratio and !uidization velocity. The steam !owrate has the most e7ect on the mole fractions of CH4,and CO2 followed by the biomass to steam ratio andthen !uidization velocity. In the case of H2, and CO

the biomass to steam ratio has the most e7ect on itsmole fraction followed by steam !ow rate and then!uidization velocity. The !uidization velocity has themost e7ect on the mole fraction of N2. The biomassto steam ratio has the most e7ect on the gas higherheating value and the gas production rate.


Fig. 9. The e7ects of steam !ow rate on the normalized producer gas rate.

Fig. 10. The e7ects of biomass to steam ratio on the normalized producer gas.

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Documents

Two phase biomass air-steam gasification model for fluidized bed reactors: Part II—model sensitivity