304
Ind. Eng. Chem. Res. 1988,27, 304-310
Process Analysis of a Dual Fluidized Bed Biomass Gasification System Burugupalli V. R. K. Prasad and James L. Kuester* Department of Chemical and Bio Engineering, Arizona State University, Tempe, Arizona 85287
Process data from a biomass dual fluidized bed gasification system were utilized for process modeling and analysis. The objective of the study was to elucidate the effect of the process operating conditions, i.e., the temperature, the steam, the residence time, and biomass feedstock composition, on the performance of the gasifier. The results indicate that there were two distinct regions, the residence time and the equilibrium-dominated regimes. The breakpoints of the two regions were not contiguous. The temperature-residence time interactions determined the temperatures a t which these occurred. For optimum performance, the gasification system temperature was chosen to be at an intermediate point where both of the phenomena prevail. Product gas compositions were found to be dependent, based on experimental and equilibrium studies with a number of feedstocks, on the composition of the feedstock. T h e hydrogen-to-oxygen ratio in the biomass was determined to be the most important parameter that affected the product distribution. Biomass, being renewable, has generated a considerable amount of interest in a number of energy utilization processes. A number of thermochemical conversion processes that utilize biomass feedstocks to produce synthesis or fuel gases are reported in the literature (Walawender et al., 1985a). These synthesis gases can be used in the manufacture of methanol, ammonia, or Fischer-Tropsch liquid hydrocarbons. Fuel gases, with high methane content, are of pipeline quality and can substitute for natural gas. Product gas compositions, heating values, yields, etc., can be tailored to suit the requirements by manipulating the process operating conditions, addition of oxygen and f or steam, heating rate, recycle, etc. A thermochemical conversion process to convert various biomass materials to diesel-type fuels has been under development in the Biomass Conversion Laboratory, at Arizona State University, since 1975. An indirect liquefaction approach as shown in Figure 1is used, where gasification of biomass to synthesis gas is followed by liquefaction of the synthesis gas. The primary virtue of this indirect liquefaction approach for cellulosic-type feedstocks is that oxygen contained in the material is easily separated. Thus, the liquid hydrocarbon product is free of oxygenated compounds and can therefore be tailored to match transportation-grade fuel products derived from petroleum. Different feedstocks, approximately 100 different biomass materials, that include industrial wastes, agricultural and forest residues, and energy crop materials were investigated. The primary product of this process, the liquid hydrocarbon product, is a transportation-grade fuel similar to diesel fuel. This can be upgraded to high-grade gasoline-type fuel by catalytic reforming. The major virtue is the utilization of renewable, often considered waste, biomass feedstocks to produce a quality product. Current efforts are involved in maximizing the product yields, alternate feedstock assessment, alternate product development, process system simplification, gasification and liquefaction catalyst development, environmental assessment, and scale-up studies. Details on some of these studies were discussed by Kuester (1982, 1984) in the interim reports submitted to the Department of Energy. A schematic of the ASU indirect liquefaction process is shown in Figure 2. The existing system is laboratory scale with a capacity of approximately 10 kg f h of biomass feedstock. The gasification system is comprised of a dual fluidized bed reactor system with connecting circulating solid transfer loops. One fluidized bed is used as a feedstock pyrolyzer, while the second bed operates in a combustion mode to heat the circulating solid media. The
fluidized bed approach allows for efficient heat transfer, continuous solid recirculation, and elimination of a combustion zone in the pyrolyzer. Cellulosic materials are continuously fed to the pyrolyzer and flashed to a synthesis gas consisting of paraffins, olefins, carbon monoxide, hydrogen, and carbon dioxide. The gas passes through a cyclone-scrubbersystem to a compressor. From the compressor, the gas can be distributed to the pyrolyzer and f or the liquefaction reactor. Additional gas candidates for fluidizing the pyrolyzer are steam and off-gas from the liquefaction reactors. Currently a mixture of steam and recycle pyrolysis product gas is used as the fluidizing medium. Recycle of the product pyrolysis gases increases the residence time and moves the system closer toward equilibrium. Steam as a fluidizing gas decreases the residence time and moves the system further from equilibrium. The liquefaction system consists of a catalytic reactor to produce paraffinic liquid fuel. The reactive components in the pyrolysis product gas, namely olefins, carbon monoxide, and hydrogen, are converted to a primary paraffinic hydrocarbon phase and a secondary alcohol-water phase. The off-gas from this reactor accumulates an appreciable amount of n-paraffins and thus exhibits an enhanced heating value. Process analysis studies of the FischerTropsch system have indicated that for optimum liquid hydrocarbon product yields, high ethylene composition and a high H2/C0 mole ratio (>1)in the feed gas are required. Optimization of the pyrolysis reactor operating conditions, to produce desired synthesis gas, can lead to the maximization of the liquid hydrocarbon yields in the FischerTropsch reactor. Some of the factors that can affect the performance of the pyrolysis reactor system are the reactor bed temperature, the fluidizing gas (steam vs recycle pyrolysis gas), the feedstock properties and characteristics, the biomass feed rate, the residence time, and the hot solids circulation rate (heating rate of biomass). Feedstocks vary by the heating value, the carbon-to-hydrogen ratio, the carbonto-oxygen ratio, and polyphenol, oil, protein, ash content, etc., present in the biomass. A number of studies have been carried out at ASU to evaluate the effect of some of the above factors on pyrolysis product gas yields. Sabin (1979), through a two-factor factorial study, found that temperature has the greatest effect on the pyrolysis gas yields, followed by the temperature and biomass feed rate interactions. Scott (1982) observed that feedstock composition had a pronounced effect on the pyrolysis gas yields. Ethylene yields were
0888-5885/88/ 2627-0304$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 305 Biomass Feedstock
---I-Fluidized Bed Pyrolysis Process
I
Fluidized Bed
Sluny Phase Liquefaction Process
j
Water Propanol
Liquid Hydrocarbon
Figure 1. Indirect liquefaction process scheme a t Arizona State University.
Figure 2. Biomass conversion system schematic.
reported to vary from 2.6% for pine sawdust to 19.3% for polyethylene feedstock. Hunter (1980) observed that the addition of steam and catalysts was found to increase the gas yields and also alter the gas compositions. He postulated that steam addition could possibly affect the gasphase reactions in two ways: steam dilution could decrease the gas-phase partial pressures of the pyrolysis products, and it would reduce the actual retention time of the products. Kuester (1982) reported that higher gas yields were obtained in the presence of water gas shift catalysts, e.g., dolomite. Ash from the biomass pyrolysis, with traces of metal oxides, was also suspected to catalyze the water gas shift reaction. Shariat (1984) reported that different mixed metal catalysts increased the product gas yields. Biomass gasification is said to occur by three process steps: (1) pyrolysis producing volatile matter, (2) secondary reactions of the volatile products, and (3) char gasification (Antal et al., 1979; Rensfelt et al., 1978). These studies found that pyrolysis of the biomass takes place at moderate temperatures (573-773 K) and is extremely rapid (0.5 s). Char gasification reactions occur at high temperatures (993 K). High heating rate, high temperature, and low pressures are said to favor low char productions (Rensfelt et al., 1978). Secondary gas-phase reactions were investigated by a number of researchers (Walawender, 1985b). Antal et al. (1979) modeled the steam gasification of biomass by simple first-order rate equations (Arrhenius plots). The results for cellulose feedstock indicate that the rate of conversion of the product gases exhibits a breakpoint at about 950 K. Walawender et al. (1985b) observed the conversion rates to change at about 940 K for cellulose
gasified in a steam gasification system. Walawender et al. (1985a) found through process modeling and statistical studies that steam-to-biomass ratio is an important factor influencing the product gas distribution, heating values, and yields. Walawender et al. (1980) and Antal et al. (1979) gasified several biomass feedstocks and found that the product gas yields are correlated to the cellulose content. These studies, in the past, have been of a limited range and have not fully elucidated the effect of residence time, temperature, steam, and biomass feedstock on biomass gasification. The objective of the current analysis study is to find the effect of these factors for a broad range of process conditions, to optimize the performance of the dual fluidized bed gasification system, and to subsequently maximize liquid hydrocarbon product yields. Empirical process model development, analysis of the model, and a comparison of the model predictions with the theoretical equilibrium computed compositions are the three phases of this study.
Process Model Development Process data acquired over the years are masked by the variations in the process operating conditions. Screened process data for pine sawdust with aluminum oxide as the hot circulating solids medium were found to be the best available data set. These process data were collected at steady-state conditions, with constant feed conditions maintained for at least 1 h. The recycle pyrolysis gas (sparge gas) flow was nearly constant in all these runs. Under these conditions, the two main process factors that could affect the product gas yields are the reactor bed temperature and the steam-to-biomass ratio. Steam-tobiomass ratio (dependent on the steam flow) is an indirect function of the residence time. Steam-to-biomassratio was chosen as the independent variable, in this study, for modeling convenience. Pressure was held constant in all the above runs at about 2 psig, as low pressures are favorable from a thermodynamic point of view, to reduce the formation of high molecular weight substances like tars and saturated hydrocarbons like methane (Antal et al., 1979). A linear regression program, LINREG (Kuester and Mize, 1973),was used to fit a linear polynomial expression relating the dependent variables Y (e.g., CO, C02, etc.) with the two independent variables, i.e., the temperature and the steamlbiomass weight ratio. Models of varying order, with up to 10 parameters, were investigated. All the models with less than six parameters resulted in unsatisfactory fits. This suggests that there is a strong interaction between the two factors of interest, namely the temperature and the steam/biomass ratio. The 10-parameter cubic model was chosen to represent the pyrolysis system. The model coefficients for the various dependent variables, e.g., H2, CO, etc., along with the coefficient of regression terms are listed in Table I. The calculated values from this empirical model were compared with the theoretical equilibrium calculated compositions, as discussed in the next section. Evaluation of the pyrolysis model with the theoretical equilibrium limits, to elucidate a better understanding of the process, are the principle goals of the study. Equilibrium Studies Equilibrium studies are used to predict the maximum possible conversions in any chosen chemical reacting system. These calculations also serve to assess how these compositions will change with the manipulation of the process operating conditions such as the temperature, the
306 Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 Table I. Linear Regression Models for the Pyrolysis Reactor System" Moles of Hydrogen Produced per Mole of Biomass (R2 = 0.9627) Yr = 0.94424 X 10' - 0.92124 X 10-2X1- 0.48633 X 10-'Xz - 0.13860 X 10-3X,Xz- 0.30640 X 104X12- 0.63433 X 10-2X,2+ 0.15245 X 10-8X,3 + 0.13512 X 10-3X,3 + 0.12921 X 10-BXzX12 + 0.22974 X 10-5X1X2 Moles of Carbon Monoxide per Mole of Biomass ( R 2 = 0.9636) Y, = -0.47427 X 10' + 0.66132 X 10-2Xl + 0.11490 Xz + 0.46680 X 10-4X1Xz- 0.24228 X 10-5X12- 0.22282 X lO-'X? + 0.13224 X 10-9X13+ 0.54617 X lO-'X2' - 0.86936 X 10-7XzX12+ 0.25670 X 104X1X22 Moles of Carbon Dioxide per Mole of Biomass (R2 = 0.9987) Y3 = +0.38460 X 10' - 0.60414 X 10-2X, 0.38557 X 10-'Xz - 0.10081 X 10-3X,X2+ 0.30670 X 10-5X,2- 0.76852 X 10-2Xz2- 0.48885 X 10-9X,3+ 0.12558 X 10-3X23 0.62340 X 10-7XzX12+ 0.32208 X 10-5X1X2 Moles of Methane per Mole of Biomass (R2= 0.9589) Y4 = -0.91024 + 0.21109 X lO-'X, + 0.87064 X 10-2Xz 0.43265 X 10-4X1Xz- 0.16335 X 10dX,2 + 0.17320 X 10-2X,2+ 0.43849 X 10-4X,3- 0.58211 X 10-6X23- 0.35407 X 10-7XzX12- 0.11843 X 10-5X1Xz2 Moles of Ethylene per Mole of Biomass (Rz = 0.9882) Y5= +0.14166 - 0.22933 X 10-3X1+ 0.64835 X 10-2Xz 0.28890 X 10-4X,X2+ 0.11908 X 104X,2 + 0.14542 X 10-*Xz2- 0.13550 X lO-'"XI3 - 0.20782 X 10-4X2 - 0.25087 X lO-'XZXl2 - 0.69057 X 104X1X,2 Moles of Ethane per Mole of Biomass (R2= 0.9943) Ye = +0.78710 X lo-' - 0.88841 X 10-4X1+ 0.24247 X 10-2Xz+ 0.61072 X 10-5X1Xz+ 0.17929 X 10-7X,2+ 0.17346 X 10-3X2 + 0.52925 x 10-11X13+ 0.30461 X 104Xz3- 0.55684 X 10-8X2X12- 0.13611 X 104X1Xz2
+
+
+ +
'XI = temperature, O F ; X, = steam-to-biomass (wet basis) weight ratio.
pressure, and the feed compositions. Equilibrium studies are also useful to provide a goal by which to measure improvements, to choose the operating conditions, and to know which of the two mechanisms (equilibrium vs reaction rate) are prevalent in the present gasification system. Equilibrium calculation methods are grouped as either stoichiometric or nonstoichiometric methods. Both of these methods are based on the fact that the Gibbs free energy of the system must be a t the minimum for equilibrium to occur a t a given temperature and pressure, and these have been shown to be the same by Smith and Missen (1982). Since pyrolysis reactions are complex and difficult to understand, a very large set of all possible reactions have been proposed in the literature (Kuester, 1983). Since in the existing pyrolysis reactor oxygen is not present in the elemental form, a large number of these reactions can be eliminated. Further, previous studies on the pyrolysis of wood or cellulosetype chemical compounds by Antal et al. (1979) and Desrosiers (1981) have indicated that only C, CO, COz, HP,HzO, and CHI exist at equilibrium conditions at the temperature range of interest, 600-2000 K. Cellulose or any other feedstock was considered to contribute only the chemical elements to the system. Carbon was assumed to be present in the product only as elemental graphite. The number of independent reactions are reduced to three, namely, C(S) + HZO = CO + Hz (1) CO
+ H20 = COZ + H2
(2)
For the above three reactions, the equilibrium constants for an ideal chemical reacting system (Le., fugacity coefficients, equal unity) a t a given temperature, T, and pressure, P , are (4) (5)
where [CO], etc., are the mole fractions of the gaseous species and P is the system pressure. These equilibrium constants are related to the Gibbs free energy of formation.
Equilibrium reaction constants K,-K3 were calculated a t different temperatures in the range, and the data were fitted by a polynomial function in temperature to facilitate interpolation or extrapolation of these constants to any intermediate temperature. The sum of the mole fractions of a gas is 5
C Y , = 1.0 i=l
(7)
where YL'sare the mole fractions of the gas species. The elemental constraint equations are given by N
Calknl = Ak
i=l
(8)
where k = 1-3 represents the three equations for carbon, hydrogen, and oxygen. n,'s are the number of mqles of the species produced. These seven equations (eq 4-8) were solved by using SNGINT, a nonlinear equation solver from the AMDLIB library (Hillstrom, 1976) to get the equilibrium gas compositions, the molar rates of carbon, and the total gas produced. Pressure, in this study, was 1 atm, as higher pressures that favor formation of higher saturated hydrocarbons (Antal et al., 1979),e.g., methane, tars, etc., are undesirable in the indirect liquefaction process. Antal et al. (1979) carried out equilibrium studies on the pyrolysis of cellulose (CH1.6600.833) by using the nonstoichiometric method. They used a computer package from NASA that utilizes the JANAF tables and can test for any number of specified species. Their studies are however limited to a single feedstock (cellulose) and a fixed amount of steam. They studied the effect of various temperatures, (573-1073 K) and pressures (0.1-100 atm). Desrosiers (1981), on the other hand, studied the effect of varying amounts of steam, oxidant/fuel ratio, temperatures, and pressures on the equilibrium compositions using wood as a feedstock. His studies indicate that steam addition affects the quantity of product gas produced but the compositions are fixed by the water gas shift and methanation reactions. However, his studies are of a limited range (temperatures, steam, etc.) and do not draw out any conclusive results on the effect of the amount of steam used and the system temperatures on biomass gasification. Composition analysis of the pine sawdust biomass feedstock was determined by an independent laboratory (Guelph Chemical Laboratories, Guelph, Ontario) and is presented in Table 11. Heating values of the feedstock were calculated based on the chemical composition (Gra-
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 307 Table 11. Pine Sawdust Characterization Data elemental anal., wt % basis C 47.36 H 5.84 N 0.0 S 0.04 0 46.76 ash 0.5 formula (dry, S, ash-free basis) MW = 25.3368 CH1.469200.7411 MW = 7.899 C0.3115H0.457600.2308 high heating value -19.9548 MJ/kg heat of formation -2.3609 MJ/kg
boski and Bain, 1981). To facilitate a comparison between the equilibrium results for different feedstocks, equilibrium compositions were obtained with the pine sawdust, cellulose, and wood. The molecular formulas were determined based on the chemical analysis of the feedstocks and were on a dry, ash-free basis. For this comparison study, stoichiometric amounts of steam and 1mol of biomass were used. The steam required for cellulose, wood, and pine sawdust feedstocks was determined from CH1.66700.833+ 1.167H20 COP + 2H2 (9)
-
CH1.469200.7411+ 1.2589H2O
-P
c02
1.9935H2
(11)
The equilibrium compositions obtained using the above three feedstocks are presented as a function of temperature (573-1473 K) in Table 111. These results show the gas composition, the total gas flow, and the amount of carbon produced to be nearly the same for the cellulose and the pine sawdust feedstock (at each of the temperatures). This can be explained by the fact that the H/O ratios of the two are very close (2.0, 1.9825) in the feedstock and (2.0,
1.9935) in the total feed mixture. However, all the gas compositions obtained with Desrosiers' feedstock (wood) are different from the other two. The mole fractions of CHI and H2are higher, while all the other compositions are lower. The total gas flow is also higher, while moles of elemental carbon present are lower than those obtained with the other feedstocks. The H/O ratio in the wood feedstock is 2.37, while in the feed mixture it is 2.11. These ratios for wood are relatively higher than those of the other two feedstocks used. Actual experimental biomass gasification studies (Kuester, 1984) with a number of feedstocks indicated that the H/O ratio has a strong effect on the amount of ethylene produced (regression correlation coefficient = 0.9841) and on the H2/C0 ratio in the product gas (regression correlation coefficient = 0.9141). Some of these experimental process results are tabulated in Table IV for different feedstocks. The equilibrium compositions and H2/C0 ratios calculated for the same feedstocks are listed in Table V. The experimental data were obtained for near identical process conditions, i.e., temperatures (- 1400 O F ) and steam-to-biomass ratios (2-3). A comparison of these two results further supports the conclusions from the initial equilibrium studies on the effect of the feedstock composition on the product gas yields. These studies confirm the experimental evidence of the dependence of the pyrolysis gas compositions on the type of feedstock used and especially the H/O ratio in the biomass. Higher ethylene and H,/CO gas yields could be obtained with feedstocks having a high H/O ratio. Thus, feedstocks could be screened to select the ones with high H/O compositions, to provide optimum pyrolysis/liquefaction reactor yields. Designed experimental studies, with a number of feedstocks, would be needed in the future to develop empirical models that are feedstock independent. These models
Table 111. Equilibrium Compositions for Three Feedstocks"
Hz
2
co
3
eoz
4
CH4
5
HzO
6
carbon, mol/h
7
a
variable
no. 1
total gas, mol/h
biomass sawdust wood cellulose sawdust wood cellulose sawdust wood cellulose sawdust wood cellulose sawdust wood cellulose sawdust wood cellulose sawdust wood ce11u 1o8e
300 0.0295 0.0305 0.0296 0.0002 0.0002 0.0002 0.2078 0.1988 0.2070 0.1916 0.2050 0.1924 0.5707 0.5702 0.5707 0.1897 0.1667 0.1885 2.0269 2.0871 2.0303
400 0.1054 0.1086 0.1056 0.0042 0.0041 0.0042 0.2309 0.2170 0.2301 0.1788 0.1900 0.1794 0.4805 0.4801 0.4805 0.1253 0.1045 0.1241 2.1125 2.1778 2.1162
500 0.2419 0.2487 0.2423 0.0315 0.0306 0.0315 0.2361 0.2230 0.2354 0.1295 0.1370 0.1300 0.3607 0.3605 0.3607 0.0809 0.0666 0.0801 2.3130 2.3887 2.3173
600 0.3887 0.3998 0.3894 0.1210 0.1169 0.1207 0.1936 0.1827 0.1929 0.0585 0.0612 0.0586 0.2381 0.2392 0.2381 0.0000 0.0000 0.0000 2.6798 2.7709 2.6849
temperature, "C 700 800 0.4541 0.4454 0.4690 0.4620 0.4550 0.4464 0.1929 0.2190 0.1888 0.2148 0.1926 0.2188 0.1388 0.1148 0.1298 0.1065 0.1383 0.1143 0.0070 0.0005 0.0077 0.0006 0.0071 0.0005 0.2071 0.2201 0.2043 0.2160 0.2069 0.2199 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 2.9516 2.9903 3.0625 3.1063 2.9578 2.9968
900 0.4306 0.4480 0.4316 0.2350 0.2302 0.2348 0.0989 0.0912 0.0984 0.0001 0.0001 0.0001 0.2352 0.2304 0.2349 0.0000 0.0000 0.0000 2.9931 3.1096 2.9996
1000 0.4186 0.4365 0.4196 0.2472 0.2418 0.2469 0.0867 0.7965 0.0863 0.0000 0.0000 0.0000 0.2473 0.2419 0.2469 0.0000 0.0000 0.0000 2.9934 3.1099 2.9999
1100 0.4040 0.4275 0.4102 0.2567 0.2509 0.2564 0.0772 0.0706 0.0768 0.0000 0.0000 0.0000 0.2568 0.2509 0.2565 0.0000 0.0000 0.0000 2.9935 3.1100 3.0000
Basis: 1 mol/h of biomass and stoichiometric amounts of steam.
Table IV. Experimental Results from Different Feedstocks chemical formula feedstock guayule cork Portugese cork shrub live oak euphorbia lathyris sawdust Russian thistle
C 1 1 1 1 1 1
H 1.726 1.7256 1.8445 1.9833 1.4692 1.9798
0 0.3565 0.397 0.7649 0.9015 0.7411 1.081
H/O 4.8415 4.345 2.4112 2.2 1.9835 1.8314
10.72 10.43 3.62 4.63
W C O 3.63 2.41 2.23 3.63
1.86
6.02
C2H4
1200 0.4016 0.4040 0.4026 0.2643 0.2564 0.2639 0.6973 0.0635 0.0693 0.0000 0.0000 0.0000 0.2643 0.2580 0.2639 0.0000 0.0000 0.0000
2.9935 3.1100 3.0000
308 Ind. Eng. Chem. Res., Vol. 27, No. 2,1988 Table V. Equilibrium Compositions for Different Feedstocks and at a Temperature of 1040 KO feedstock co n, CH, 64.2978 0.2254 guayule cork 24.2288 63.8248 0.2198 Portugese cork 24.4966 shrub live oak 26.139 59.8235 0.1774 euphorbia lathyis 26.510 58.838 0.1679 sawdust 27.047 51.352 0.1545 0.1427 Russian thistle 27.5200 55.965
Temperature 1040 K ~ ~ d r o g e( n~ 2 ) .cb. Y Carbon Dioxide (C02)
I r-
, * **
n,p 2.647 2.605 2.288 2.219 2.084 2.033
-c + :
'Basis: stoichiometric amounts of steam. Steam t o Biomass Ratio = 4.0 Hydrogen (H2). CO. 8. Carbon Dmoide (GO21
~- . ~
~
9 n. .
,
f~~~ ~~~~
~
~~
~
0
:,;
L
.2 5 a
E
v
1
,000
1~
1020
,013
,060
,:~>
81..
Temperalure (K 1100 K, only the water gas shift reaction determines the gas compositions. By use of the empirical model, developed in the preceding section, the gas compositions (on dry basis) were calculated for different temperatures (1000-1100 K) and for various amounts of steam-to-biomass mole ratios (1-10). To elucidate a better understanding of the operating conditions under which equilibrium or residence time prevail, the experimental model predictions are compared with calculated equilibrium compositions. Figure 3 depicts the equilibrium compositions as a function of temperature, with steam-to-biomassratio of 4.0. These compositions are shown on a dry basis to facilitate a better comparison with the experimental data. These curves indicate very little change in the gas compositions with temperature. The equilibrium gas compo-
,
3
2
1
5
6
I
8
'i
10
Steom to Bsomass Ratso
Figure 5. Effect of steam on hydrogen at different temperatures.
- 35
1
2
I
B Steam to B8amars Mote Ratio I
i
6
I
9
I
10
Figure 6. Effect of steam on carbon monoxide at different temperatures.
sitions, on a dry basis, as a function of the steam-tobiomass ratio (and at a fixed temperature of 1040 K) are depicted in Figure 4. This indicates that because of the effect of dilution, i.e., with the increase in steam usage, on the water gas shift reaction, the concentrations of Hz and COz increase while that of CO decreases. Negligible amounts of carbon, methane, and other higher order hydrocarbons were found to exist at equilibrium. Figures 5-10 depict the effect of temperature and steam-to-biomassmole ratio on hydrogen, carbon monoxide, carbon dioxide, methane, ethylene, and ethane. The approach to equilibrium (C - Cev) is plotted vs steamto-biomass mole ratio for 3fferent temperatures. The compositions are on a dry mole fraction basis. At lo00 K, the compositions for the higher hydrocarbons CH,, C2H,, and C2H6tend to increase with an increase in steam flow
Ind. Eng. Chem. Res., Vol. 27, No. 2, 1988 309 14
+ io00 K
C a r b o n Dioxide
1
I
+ 1020 + 1040
h
1063
