Biomass Flow Rate Ratio on Gas

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Ind. Eng. Chem. Res. 1998, 37, 3812-3819

Influence of Catalyst Weight/Biomass Flow Rate Ratio on Gas Production in the Catalytic Pyrolysis of Pine Sawdust at Low Temperatures Lucia Garcia, Marı´a L. Salvador, Jesu ´ s Arauzo, and Rafael Bilbao* Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain

Pine sawdust catalytic pyrolysis has been studied in a fluidized bed at temperatures of 650 and 700 °C. The experimental work was carried out in a bench-scale plant based on Waterloo Fast Pyrolysis Process (WFPP) technology. The Ni-Al catalyst used was prepared by coprecipitation with a molar ratio 1:2 (Ni-Al) and calcined at 750 °C for 3 h. The catalyst was not reduced prior to the biomass reaction. The influence of the catalyst weight/biomass flow rate ratio (W/ mb) on the product distribution and on the quality of the gas product obtained was analyzed. An increase of the W/mb ratio increases the total gas yield and diminishes the liquid yield. When the W/mb ratio increases, H2 and CO yields increase while CO2, CH4, and C2 yields decrease. For W/mb ratios g 0.4 h, no significant modifications are observed on the initial yields of different gases, and it is confirmed that under these conditions the initial gas composition is similar to that for thermodynamic equilibrium. For W/mb ratios < 0.4 h, a simple first-order kinetic equation has been suggested for H2 and CO formation. Introduction Nowadays there is an increasing interest in gas production from the thermochemical processing of biomass. Hydrogen and carbon monoxide are some of the valuable gases that can be produced from biomass. Hydrogen constitutes a potential option for obtaining energy by its use in fuel cells and as a transport fuel in internal combustion engines. Hydrogen, like CO, is useful as a raw material in the chemical industry (for ammonia, methanol, and hydrocarbon synthesis, hydrogenation reactions, and other reactions where it is involved because of its reduction capability). The present demand for these valuable gases has resulted in the study of several processes for their production from the thermochemical processing of biomass, such as pyrolysis and steam gasification. The reaction temperature is one of the most influential variables in pyrolysis and steam gasification processes. Both processes are endothermic, and an input energy is required. Therefore, a decrease in the operating temperature allows a significant savings in energy, which also decreases costs. However, this temperature decrease causes lower gas yields and more tar production. In the case of pyrolysis, the tar production is significant at temperatures lower than 700-750 °C. One possibility for eliminating tars proceeding from the pyrolysis process is the use of a catalyst in a secondary reactor down stream.1-8 In these works, steam, high temperatures, and catalysts (mainly dolomites and nickel catalysts) are used in a secondary reactor with the objective of decreasing the tar content in the product gas. Catalytic steam reforming of liquid generated from pyrolysis has been also studied, for example, by Wang et al.9 and Czernik et al.10 using different nickel catalysts. * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (+34) 976 762142.

Instead of using secondary reactors, the possibility has been proposed of placing the catalyst in the same reactor where the pyrolysis of biomass is performed. This option has also been used in the process of gasification of biomass.11-15 Of the various catalysts, the use of nickel catalysts is a promising option that the literature refers to as one of the most encouraging prospects in the processing of tars.16,17 These catalysts have been widely used for the upgrading of gases obtained in the thermochemical processing of biomass, from the work of Blackader and Rensfelt18 to the more recent studies of Simell et al.19 and Caballero et al.20 Previous works21,22 showed that Ni-Al catalysts prepared by coprecipitation and placed in the pyrolysis reactor allow an increase in the gas yield at low pyrolysis temperatures (e700 °C). In this context, it has been considered of interest to continue studying the catalytic pyrolysis of biomass at low temperatures using Ni-Al coprecipitated catalysts in the pyrolysis reactor. These studies are of great interest not only for the pyrolysis process itself but also because pyrolysis is a prior step in other important processes such as steam gasification. The present paper focuses on the analysis of the influence of the catalyst weight/biomass flow rate ratio and of the reaction time on the product distribution and on the yields of different gases. It has also been interesting to quantify the experimental results obtained at the initial time of the experiment. For this purpose, the results have been compared with those corresponding to the thermodynamic equilibrium, and a simple kinetics has been used to fit the results obtained when the thermodynamic equilibrium was not achieved. Experimental Procedure Experimental System. The experimental system is a bench-scale installation using Waterloo Fast Pyrolysis Process (WFPP) technology.23,24 The principal

S0888-5885(98)00196-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/10/1998

Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3813

Figure 1. Scheme of the experimental system.

components of this technology are a biomass feeder, allowing continuous feeding, and a fluidized-bed reactor. A schematic of the installation is shown in Figure 1. The biomass feeder supplies biomass flow rates between 5 and 100 g/h. The inner section of the fluidized-bed reactor is 4.35 cm2. In the experiments two nitrogen streams are introduced into the reactor. One transports the biomass into the reaction bed, while the other enters at the bottom and reaches the bed through the distributor. The total nitrogen flow rate into the reactor is 1715 cm3(STP)/ min, of which 1215 cm3(STP)/min transports the biomass to the reaction bed while the rest enters through the distributor. The product gas is cleaned of char particles using a cyclone. A system of two condensers and a cotton filter allows us to retain liquid products. The gas flow rate is then measured using a dry testmeter, and CO and CO2 concentrations are continuously determined by an infrared analyzer. In addition, gas samples are taken at regular time intervals and analyzed by chromatography, showing the content of H2, CO, CO2, CH4, and C2 (C2H2, C2H4, and C2H6) in the product gas. Reaction temperature, total gas flow, and CO and CO2 concentrations are registered by a data acquisition system.

Pyrolysis experiments were carried out at 650 and 700 °C and atmospheric pressure. The reaction bed was composed of mixtures of sand and catalyst. The catalyst weight, W, ranged from 0.25 to 20 g. All the experiments were performed with the same bed volume, 34.39 cm3. Biomass flow rates, mb, ranged from 8.5 to 28 g/h. The catalyst weight/biomass flow rate, W/mb, has been studied from 1.19 to 0.016 h. The particle sizes of catalyst used were between 150 and 350 µm. The biomass processed was pine sawdust with a moisture content of about 10% and a particle size of -350 + 150 µm. The results of the elemental analysis (in percent mass) of the pine sawdust are as follows: carbon, 48.27%; hydrogen, 6.45%; nitrogen, 0.09%; oxygen (by difference), 45.19%. Catalyst. The catalyst employed was prepared in our laboratory by coprecipitation using a method similar to that described by Al-Ubaid and Wolf,25 with a molar ratio Ni-Al of 1:2. All of the experiments were carried out with the catalyst calcined in an air atmosphere, with a low heating rate, at a final temperature of 750 °C for 3 h. The precursor and the calcined catalyst were characterized by various techniques such as mercury porosimetry, nitrogen adsorption, X-ray diffraction (XRD), atomic emission spectrometry by inductively coupled plasma (ICP), thermogravimetric analysis, and temperature-programmed reduction (TPR). One of the most significant results obtained in this characterization study is the presence of NiO and a spinel phase (NiAl2O4), shown by XRD. Analysis by TPR shows a significant reduction at 550-600 °C and the total reduction of the catalyst at 900 °C. More details about characterization results and catalyst preparation can be found in a previous work.22 The catalyst employed in this work was not reduced prior to the biomass reaction. This is due to the results obtained in the aforementioned paper,22 in which it is concluded that the gases generated in the thermal decomposition of biomass at 650 and 700 °C, H2 and CO, have the ability to reduce the Ni-Al catalyst prepared by coprecipitation and calcined at 750 °C. The results have also been obtained by other authors.15,26,27 The use of catalyst without reduction simplifies the catalyst preparation and can have an economic advantage due to the absence of hydrogen costs.

Table 1. Results of Pyrolysis at 650 °C run catalyst wt, W (g) sawdust feeding rate, mb (g/h) W/mb (g of catalyst h/g of biomass) reaction time (min) yields (mass fraction of original biomass) gas liquid solids recovery gas yields (mass fraction of original biomass) H2 CO CO2 CH4 C2 gas composition (% mol, N2 free) H2 CO CO2 CH4 C2

1

2

3

4

5

6

7

8

9

20 20.16 0.992 96.9

20 21.9 0.913 88.9

20 24.98 0.801 49

20 27.98 0.715 49.1

5 12.78 0.391 120

2.5 13.14 0.190 61

1 13.08 0.076 120

0.5 15.19 0.033 52.3

0 14.42 0 60

0.670 0.166 0.091 0.927

0.748 0.071 0.100 0.919

0.909 0.091 0.058 1.058

0.698 0.224 0.062 0.984

0.590 0.302 0.053 0.945

0.514 0.376 0.049 0.939

0.372 0.486 0.049 0.907

0.336 0.497 0.050 0.883

0.338 0.506 0.038 0.882

0.043 0.459 0.155 0.012 0

0.053 0.522 0.159 0.014 0

0.063 0.689 0.142 0.021 0

0.042 0.518 0.116 0.022 0

0.027 0.429 0.101 0.019 0.014

0.023 0.375 0.090 0.017 0.009

0.009 0.244 0.083 0.022 0.014

0.006 0.220 0.075 0.021 0.014

0.005 0.216 0.077 0.024 0.016

50.78 38.92 8.38 1.92 0

53.58 37.43 7.25 1.74 0

52.12 40.45 5.30 2.13 0

48.00 42.69 6.09 3.22 0

41.29 46.58 7.00 3.52 1.61

40.62 47.31 7.19 3.66 1.22

25.38 51.83 11.19 8.02 3.58

21.63 53.95 11.62 8.81 3.99

16.91 55.43 12.58 10.63 4.45

18.83 55.56 9.65 10.59 5.37 20.50 54.72 9.70 10.00 5.08 21.78 53.55 9.84 9.73 5.10 27.65 51.68 8.07 8.15 4.45 30.84 50.99 7.15 7.11 3.91 32.56 50.80 6.71 6.34 3.59 34.83 50.52 5.80 5.71 3.14 36.66 49.97 5.68 5.14 2.55 38.71 48.59 5.25 4.85 2.60 37.05 50.11 5.14 4.70 3.00 47.50 45.00 4.10 2.60 0.80 37.70 50.31 5.04 4.34 2.61 47.41 46.12 3.00 2.38 1.09

0.008 0.330 0.090 0.036 0.032 0.008 0.318 0.089 0.033 0.029 0.009 0.325 0.094 0.034 0.030 0.014 0.365 0.090 0.033 0.029 0.017 0.397 0.087 0.032 0.029 0.019 0.416 0.086 0.030 0.028 0.021 0.426 0.077 0.028 0.025 0.025 0.477 0.085 0.028 0.022 0.028 0.499 0.085 0.028 0.026 0.027 0.501 0.081 0.027 0.029 0.026 0.486 0.076 0.024 0.025 0.055 0.747 0.076 0.022 0.016

0.050 0.669 0.095 0.022 0.013

0.496 0.388 0.036 0.920 0.477 0.395 0.021 0.893 0.492 0.376 0.037 0.905 0.531 0.325 0.052 0.908 0.562 0.299 0.039 0.900 0.579 0.287 0.049 0.915 0.577 0.279 0.036 0.892 0.637 0.237 0.042 0.916 0.666 0.222 0.051 0.939 0.665 0.210 0.037 0.912 0.849 0.073 0.072 0.994 0.637 0.211 0.045 0.893 0.916 0.031 0.069 1.016

0 23.49 0 120 0.25 15.29 0.016 61 0.5 15.08 0.033 120 1 14.04 0.07 120 1.5 16.03 0.09 120 2 15.07 0.13 120 2.5 13.96 0.18 120 5 15.93 0.31 180 10 19.20 0.52 275.2 5 8.45 0.59 345 20 19.27 1.04 259.4 10 9.42 1.06 513

16

run

15 14 13 12 11 10 Table 2. Results of Pyrolysis at 700 °C

Global Results. Table 1 presents global results obtained in experiments carried out at 650 °C. In this table are shown the values of some experimental variables such as catalyst weight, sawdust feeding rate, and reaction time. Yields of gas, liquids, and char, expressed as mass fractions of the original biomass, are indicated and so are the recovery and yields of different gases, H2, CO, CO2, CH4, and C2. The gas composition, expressed as molar percent (nitrogen free), is also shown. Experimental results corresponding to 700 °C are presented in Table 2. The analysis of the pyrolysis results shown in Tables 1 and 2 must be done with care, because not all the experiments were carried out with the same sawdust flow rate and experimental time. However, some general trends can be observed. For both pyrolysis temperatures, the increase of the W/mb ratio produces an increase in gas yields and a decrease in liquid yield. CO, expressed as the mass fraction of the original biomass, is the preponderant gas obtained in all of the experiments. It is also observed that H2 and CO yields increase when the W/mb ratio increases. Effect of Reaction Time. The results can be analyzed in more detail by studying the evolution of gas yields with reaction time. As an example, Figure 2 shows gas yield evolution with time for an experiment carried out at 700 °C (run 15) with a W/mb ratio of 0.31 h. In accordance with this figure, total gas, H2, and CO yields diminish with experimental time, while CO2, CH4, and C2 yields increase. This evolution of gases with time is a consequence of the loss of catalyst activity. The deactivation of the catalyst is caused mainly by the formation of carbon deposits on the catalyst surface.16,28,29 For a reaction temperature of 650 °C, the trends of gas yields variation with experimental time are similar to those obtained at 700 °C. Effect of the W/mb Ratio. The influence of the W/mb ratio on different gas yields at 650 and 700 °C has been studied. Figures 3-7 show the results corresponding to 700 °C, with the trends obtained for 650 °C being similar. These figures also include the results of the noncatalytic pyrolysis process for the purposes of comparison. It can be observed that the presence of the catalyst exerts a significant influence on the gas yields. H2 and CO yields increase significantly with the use of the catalyst (Figures 3 and 4). The differences between catalytic and noncatalytic processes are more pro-

17

Results and Discussion

20 16.78 1.19 181

18

Figure 2. Gas yield versus time for W/mb ) 0.31 h.

catalyst wt, W (g) sawdust feeding rate, mb (g/h) W/mb (g of catalyst h/g of biomass) reaction time (min) yields (mass fraction of original biomass) gas liquid solids recovery gas yields (mass fraction of original biomass) H2 CO CO2 CH4 C2 gas composition (% mol, N2 free) H2 CO CO2 CH4 C2

19

20

21

22

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Figure 3. Influence of the W/mb ratio on the H2 yield.

Figure 6. Influence of the W/mb ratio on the CH4 yield.

Figure 4. Influence of the W/mb ratio on the CO yield.

Figure 7. Influence of the W/mb ratio on the C2 yield.

Figure 5. Influence of the W/mb ratio on the CO2 yield.

Figure 8. Influence of the W/mb ratio on the total gas.

nounced at the initial time of the experiment and increase with the W/mb ratio. H2 and CO yields decrease with time, but higher yields are achieved than without a catalyst. Yields of CO2 (Figure 5), CH4 (Figure 6), and C2 (Figure 7) decrease when the W/mb ratio increases. For a given W/mb ratio, these yields increase with the reaction time. The initial CO2 yield has not been considered due to its high value. These high initial yields can be caused by CO2 formation during the reduction of the catalyst by the reaction atmosphere.22 Total gas yield evolution versus time is presented in Figure 8 for the experiments at 700 °C. It is observed that the total gas yield increases with the W/mb ratio

but decreases during the experiment. The knowledge of the influence of the catalyst on the total gas yield obtained is insufficient due to the different influence of the catalyst on the various gases. Similar total gas yields can correspond to different gas compositions. Initial Gas Yields. The influence of the use of catalysts on the yield of several gases at the initial time of the experiment has also been studied. For this study, the values of the yield evolution of the different gases versus time have been extrapolated to zero time. In Figures 9 and 10, H2 and CO initial yields obtained at 650 and 700 °C are represented versus the W/mb ratio. Gas yields corresponding to noncatalytic pyrolysis experiments are also included. It is observed that H2 and

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Figure 9. Initial H2 yield for different W/mb ratios.

Figure 10. Initial CO yield for different W/mb ratios.

Figure 11. Initial CO2, CH4, and C2 yields for different W/mb ratios.

CO initial yields increase significantly with the presence of the catalyst. For a W/mb ratio > 0.4 h, the values of these yields can be considered as constant. CO2, CH4, and C2 initial yields versus different values of the W/mb ratio are shown in Figure 11. It is observed that these yields decrease when the W/mb ratio increases. The decrease of CO2 yields is asymptotic, and for a W/mb ratio > 0.3 h, the changes are not noticeable. CH4 and C2 yields tend to zero for high values of W/mb. The value of the C2 yield can be considered zero for a W/mb ratio > 0.8 h. In addition, the colorless gas obtained at the beginning of the experiments carried out for a W/mb ratio >

Figure 12. Comparison between initial experimental results and thermodynamic equilibrium predictions for W/mb > 0.4 h.

0.4 h could indicate that tars are not produced and the catalyst is capable of transforming all of the liquid products generated. For a W/mb ratio < 0.4 h the yellow-brownish color of the gas at the initial time could show the presence of tars in the product gases. Taking into account that for a W/mb ratio > 0.4 h the presence of tar is negligible at the beginning of the experiment and that the H2, CO, and CO2 initial yields are not modified significantly with the W/mb ratio, it was considered useful to compare the experimental results with those corresponding to thermodynamic equilibrium. The values of H2, CO, CO2, and CH4 contents in the product gas (N2 and H2O free) are shown in Figure 12. A good agreement can be observed between the experimentally obtained initial gas composition and that corresponding to thermodynamic equilibrium. It can consequently be deduced that, at the beginning of the experiment, the catalyst not only achieves an increase in the gas yields but also allows us to reach thermodynamic equilibrium. Simple Kinetic Study for Initial Yields of H2 and CO. For those results of initial yields that do not correspond to thermodynamic equilibrium (experiments with a W/mb ratio < 0.4 h), it was considered of interest to carry out a simple kinetic study that allows us to fit the initial yields of several gases. For this study, it must be taken into account that thermal decomposition of biomass is a complex process. Gases, liquids, and char are obtained, and furthermore the liquid products generated are a mixture of a high number of compounds. The inclusion of the catalyst increases the complexity of this process. Considering this complexity, in this work a simple way of analyzing the influence of the catalyst has been attempted. As can be observed in Figure 13, the process has been divided into two stages, the first being pyrolysis and the second considering the influence of the catalyst. In the pyrolysis step the biomass, with an inlet flow rate of mb (g/h), is transformed into gases, with a flow rate of mio (g/h), tars, mao (g/h), and char, mco (g/ h). These products of the pyrolysis step are modified in the catalytic step, and flow rates of gases, mif (g/h), tars, maf (g/h), and char, mc (g/h), are obtained. In the pyrolysis stage, gases (H2, CO, CO2, CH4, and C2), liquids, and char are obtained. The quantities and yields of these products correspond to the results obtained in noncatalytic pyrolysis experiments. The presence of the catalyst, in accordance with the experimental results, decreases the liquid yields and

Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998 3817

Bilbao et al.30 made use of a gas (H2, CO, CO2, and CH4) formation first-order kinetics for a flash pyrolysis process in a fluidized bed with discontinuous solid feeding. The materials used were wheat straw and pine sawdust. Other similar kinetics have been employed for gas (H2 and CO2) formation in the thermal decomposition of cellulose and pine sawdust in a fixed-bed reactor discontinuous for the solid (Bilbao et al.31). Equations 3 and 4 are grouped as

d(mi/mb) Figure 13. Schematic representation of the two stages of the biomass catalytic pyrolysis process.

increases the global gas yields. Although the catalyst interacts with gases, tars, and char, it has been assumed that the presence of the catalyst does not modify the char yield obtained in the pyrolysis stage, so mc ) mco is assumed. Considering that the catalyst used mainly operates on gas and tar reactions, this assumption seems to be appropriate. With regard to gases, the CH4, CO2, and C2 yields decrease with the presence of the catalyst, while H2 and CO are produced also in the catalytic stage, since their yields are higher than those in noncatalytic pyrolysis. For this reason and also because of the interest in these gases, the study focuses on the H2 and CO formation during the catalytic stage at the initial time of the experiment, when deactivation of the catalyst has not occurred. Applying a mass balance for gas product formation, i, in a differential element of catalyst weight and assuming a plug flow of gases, the following equation is obtained:

dmi ) ri dW

(1)

where ri is the formation rate of H2 and CO (g of gas/g of catalyst h) and mi is the flow rate of each gas in the catalytic step (g/h). From eq 1 it is obtained that

dmi dW

ri )

(2)

Equation 2 is transformed by dividing both terms by the flow rate of the inlet biomass:

ri )

d(mi/mb) d(W/mb)

(3)

A first-order kinetic equation of gas formation is proposed, where the gas formation rate is determined by multiplying the kinetic constant by a driving force. This driving force is the difference between the maximum yield of a specific gas at a reaction temperature and the experimental yield obtained. In accordance with the results presented in Figure 12, the maximum yield would correspond to the values of thermodynamic equilibrium. The equation is

ri ) ki

[( ) mi mb

( )]

mi max mb

(4)

where ki is the kinetic coefficient for the formation of gas i (g of sawdust/g of catalyst h) and (mi/mb)max is the maximum yield of a specific gas. A first-order reaction is frequently found in thermal decomposition processes of biomass and its constituents.

d(W/mb)

[( )

) ki

mi mb

max

-

( )] mi mb

(5)

Applying eq 3 to the specific case of initial time, the gas formation rates at initial time of H2, rH2, and CO, rCO, can be obtained from the experimental data of (mi/ mb) versus W/mb (Figures 9 and 10) by deriving the data. For temperatures of 650 and 700 °C, the gas formation at initial rate of H2 and CO has been represented versus the difference between the maximum yield of each gas, corresponding to thermodynamic equilibrium data, and the experimental yields obtained (eq 5). From these representations the kinetic constants of H2, kH2, and CO, kCO, obtained and the regression coefficient, r, are

kH2(650 °C) ) 11.2 g of sawdust/g of catalyst h r ) 0.997 (6) kH2(700 °C) ) 11.6 g of sawdust/g of catalyst h r ) 0.965 (7) kCO(650 °C) ) 10.0 g of sawdust/g of catalyst h r ) 0.994 (8) kCO(700 °C) ) 9.4 g of sawdust/g of catalyst h r ) 0.899 (9) The kinetic constants of each gas formation at initial time have similar values at both temperatures. A priori, an increase of the reaction temperature should involve an increase in the kinetic constant value. However, it can be observed that for H2 the values of kH2 are very similar, and in the case of CO, kCO even decreases when the temperature increases. This fact can be explained by considering that the values of ki depend on the formation rate of gases and on the thermodynamic equilibrium data. When the temperature increases, the gas formation rate also increases (Figures 9 and 10). An increase in temperature also produces an increase in (mi/mb)max values, and as a consequence this factor can balance the increase of gas formation or even exceed it. Conclusions The main conclusions obtained from the experimental results of biomass pyrolysis at 650 and 700 °C carried out with a Ni-Al coprecipitated catalyst and calcined at 750 °C without previous reduction are as follows: (1) The W/mb ratio has a very significant influence on the yields of different gases in the pyrolysis process. When the W/mb ratio increases, the total gas, H2, and CO yields increase and CO2, CH4, and C2 yields decrease.

3818 Ind. Eng. Chem. Res., Vol. 37, No. 10, 1998

(2) For a given W/mb ratio, H2 and CO yields decrease with the reaction time, while the yields of CO2, CH4, and C2 increase. (3) It is proposed that the complex process of catalytic pyrolysis can be simplified into two sequence stages (pyrolysis and catalytic steps). In the second stage a significant increase in H2 and CO yields is obtained. (4) For a W/mb ratio higher than 0.4 h, the initial yields of several gases do not vary significantly. For these values of W/mb, the experimental gas composition of pyrolysis at 650 and 700 °C is very close to thermodynamic equilibrium. (5) For W/mb ratios smaller than 0.4 h, the initial yields of several gases are strongly influenced by the W/mb ratio. A simple first-order kinetic equation for H2 and CO formation is proposed. Acknowledgment The authors express their gratitude to DGICYT for providing financial support for the study (Project PB930593) and also to the Ministerio de Educacio´n y Ciencia (Spain) for the research grant awarded to L.G. The authors thank D. S. Scott of University of Waterloo and D. Radlein, J. Piskorz, and P. Majerski of RTI, Ltd. (Waterloo, Ontario, Canada), for their assistance and interest in the research program in which this study has been developed. Nomenclature i ) individual gas (H2, CO, CO2, CH4, and C2) ki ) kinetic coefficient for the formation of gas i (g of sawdust/g of catalyst h) kH2 ) kinetic coefficient for the formation of hydrogen (g of sawdust/g of catalyst h) kCO ) kinetic coefficient for the formation of CO (g of sawdust/g of catalyst h) maf ) flow rate of tars after the catalytic step (g/h) mao ) flow rate of tars after the noncatalytic step (g/h) mb ) inlet flow rate of biomass (g/h) mc ) flow rate of char after the catalytic step (g/h) mco ) flow rate of char after the noncatalytic step (g/h) mf ) flow rate of each gas after the catalytic step (g/h) mi ) flow rate of each gas in the catalytic step (g/h) mio ) flow rate of each gas after the noncatalytic step (g/ h) ri ) gas formation rate (g of gas/g of catalyst h) rH2 ) hydrogen formation initial rate (g of H2/g of catalyst h) rCO ) carbon monoxide formation initial rate (g of CO/g of catalyst h) t ) time (h) W ) catalyst weight (g) W/mb ) catalyst weight/biomass flow rate ratio (h)

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Received for review March 31, 1998 Revised manuscript received June 25, 1998 Accepted July 23, 1998 IE9801960