Catalytic Steam Gasification of Pine Sawdust. Effect of Catalyst Weight

Energy & Fuels 1999, 13, 851-859

851

Catalytic Steam Gasification of Pine Sawdust. Effect of Catalyst Weight/Biomass Flow Rate and Steam/Biomass Ratios on Gas Production and Composition L. Garcıá, M. L. Salvador, J. Arauzo, and R. Bilbao* Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain Received November 12, 1998

Pine sawdust catalytic steam gasification has been studied in a fluidized bed at a relatively low temperature, 700 °C. The Ni-Al catalyst used was prepared by coprecipitation and calcined at 750 °C for 3 h. The influence of the catalyst weight/biomass flow rate (W/mb) and steam/ biomass (S/B) ratios 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, H2, CO, and CO2 yields, while CH4 and C2 yields decrease. An increase of the S/B ratio increases H2 and CO2 yields while CO and CH4 yields decrease. This fact can be explained by steam reforming and water-gas shift reactions. The increase of the S/B ratio also has a positive effect on the life of the catalyst. The gas composition and gas yields at initial time have also been studied. For W/mb ratios > 0.5 h, the gas composition at initial time is similar to that for thermodynamic equilibrium for different S/B ratios. The influence of the S/B ratio on gas yield at initial time is more marked, up to a ratio of 1.5.

Introduction

CO + 3H2 T CH4 + H2O

(1)

The current significant demand for hydrogen may well increase due to the amount of hydrogen needed in refineries and its possible utilization as a fuel. The most common process for hydrogen and synthesis gas production is the catalytic reforming of natural gas, liquified petrol gas, or naphtha with steam. Nowadays, there is a great interest in the use of renewable energy sources to replace fossil fuels which harm the environment. Hydrogen can be obtained from renewable energy sources through processes such as water electrolysis and thermal conversion of biomass. The latter include the catalytic reforming of liquids from pyrolysis of lignocellulosic wastes (Wang et al.1,2) and direct biomass gasification which can be used to obtain CO, hydrogen, or synthesis gas. Steam is one of the most commonly used gasification agents due to the good quality of the gas obtained, with a high percentage of hydrogen. In biomass gasification, the gas can contain significant amounts of tars which can be eliminated by thermal cracking (high-temperature gasification) or by the use of a catalyst (catalytic gasification). The catalyst can increase the reaction rate of the steam with char and/or can participate in the secondary reactions. The main secondary reactions involved are

CO + H2O T H2 + CO2

(2)

tar + H2O f H2 + CO

(3)

tar + H2 f light hydrocarbons + gases

(4)

tar f coke + gases

(5)

* Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Wang, D.; Czernik, S.; Montane´, D.; Mann, M.; Chornet, E. Ind. Eng. Chem. Res. 1997, 36, 1507-1518. (2) Wang, D.; Czernik, S.; Chornet, E. Energy Fuels 1998, 12, 1924.

Therefore, the catalyst improves the quality of the gas obtained and allows us to diminish the tar content in the gas. High- and low-temperature gasification processes can be distinguished, the demarcation being around 700750 °C. An increase in the gasification temperature causes an increase in the gaseous product yield and a decrease in the tar yield. Despite this lower tar content, the remaining tars must be eliminated in order to be able to use the gas obtained. These tars are mainly polyaromatic hydrocarbons which are difficult to convert, and the relative proportion of PAH increases with the gasification temperature (Chornet et al. 3). In hightemperature gasification processes, the catalysts are placed in a second reactor downstream. Among the studies performed using catalysts in a second bed can be cited those of Simell et al.,4 Aznar et al.,5,6 Corella (3) Chornet, E.; Wang, D.; Montane´, D.; Czernik, S. Hydrogen Production by Fast Pyrolysis of Biomass and Catalytic Steam Reforming of Pyrolysis Oil. In Bio-oil Production & Utilisation; Bridgwater, A. V., Hogan, E. N., Eds.; CPL Press: Berkshire, 1996; pp 246-262. (4) Simell, P.; Kurkela, E.; Stahlberg, P. Formation and Catalytic Decomposition of Tars from Fluidized-Bed Gasification. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Ed.; Blackie Academic & Professional: Londres, 1994; Vol. 1, pp 265-279.

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852 Energy & Fuels, Vol. 13, No. 4, 1999

et al.,7,8 Narvae´z et al.,9,10 Orio et al.,11 Kinoshita et al.,12 Paisley and Gebhard,13 and Paisley.14 Low-temperature gasification is an interesting alternative from the energy point of view. This process causes a higher tar yield than the high-temperature process but this tar can be eliminated by placing a catalyst in the same reactor where the biomass gasification is performed (Baker et al.,15 Rei et al.,16-18 Tanaka et al.,19 and Arauzo et al.20). The most frequently used catalysts are metallic, basically based on Ni. Some are commercially available, but others are prepared in the laboratory specifically for the process. In this context, it has been considered of interest to study low-temperature gasification using a Ni-Al coprecipitated catalyst prepared in the laboratory and placed in the gasification reactor itself. Of the different techniques used to prepare gasification catalysts, impregnation (Tanaka et al.19 and Garg et al.21) and coprecipitation (Arauzo et al.20,22), coprecipitation has been chosen because it allows us to obtain a catalyst with a big metallic area and high thermal stability in the metallic phase (high resistance to sinterization). The behavior of this catalyst has also been studied in other processes: methanation at high temperature (Alzamora et al.23 and Kruissink et al.24), hydrogenation reactions (5) Aznar, P.; Delgado, J.; Corella, J.; Lahoz, J.; Araguë´s, J. L. Fuel and Useful Gas by Steam Gasification of Biomass in Fluidized Bed with Downstream Methane and Tar Steam Reforming: New Results. In Biomass for Energy, Industry and Environment; Grassi, G., Collina, A., Zibetta, H., Eds.; Elsevier Applied Science: London, 1992; pp 707713. (6) Aznar, P.; Corella, J.; Delgado, J.; Lahoz, J. Ind. Eng. Chem. Res. 1993, 32, 1-10. (7) Corella, J.; Aznar, P.; Delgado, J.; Aldea, E. Ind. Eng. Chem. Res. 1991, 30, 2252-2262. (8) Corella, J.; Aznar, P.; Delgado, J.; Martıńez, M. P.; Araguë´s, J. L. The Deactivation of Tar Cracking Stones (Dolomites, Calcites, Magnesites) and of Commercial Methane Reforming Catalysts in the Upgrading of the Exit Gas from Steam Fluidized Bed Gasifiers of Biomass and Organic Wastes. In Catalyst Deactivation 1991; Bartholomew, C. H., Butt, J. B., Eds.; Elsevier Science Publishers: Amsterdam, 1991; pp 249-252. (9) Narvaéz, I.; Orıó, A.; Aznar, P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110-2120. (10) Narvaéz, I.; Corella, J.; Orıó, A. Ind. Eng. Chem. Res. 1997, 36, 317-327. (11) Orıó, A.; Narvaéz, I.; Corella, J. Characterization and Activity of Different Dolomites for Hot Gas Cleaning in Biomass Gasification. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; Vol. 2, pp 1144-1157. (12) Kinoshita, C. M.; Wang, Y.; Zhou, J. Ind. Eng. Chem. Res. 1995, 34, 2949-2954. (13) Paisley, M. A.; Gebhard, S. C. Gas Cleanup for Combined Cycle Power Generation Using a Hot Gas Conditioning Catalyst. Proceedings of the Second Biomass Conference of the Americas, Portland, Oregon, August 21-24, 1995; pp 617-629. (14) Paisley, M. A. Catalytic Hot Gas Conditioning of Biomass Derived Product Gas. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; Vol. 2, pp 1209-1223. (15) Baker, E. G.; Mitchell, D. H.; Mudge, L. K.; Brown, M. D. Energy Prog. 1983, 3, 226-228. (16) Rei, M.; Lin, F.; Su, T. Appl. Catal. 1986, 26, 27-37. (17) Rei, M.; Yang, S. J.; Hong, C. H. Agric. Wastes 1986, 18, 269281. (18) Rei, M.; Su, T.; Lin, F. Ind. Eng. Chem. Res. 1987, 26, 383386. (19) Tanaka, Y.; Yamaguchi, T.; Yamasaki, K.; Ueno, A.; Kotera, Y. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 225-229. (20) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Energy Fuels 1994, 8, 1192-1196. (21) Garg, M.; Piskorz, J.; Scott, D. S.; Radlein, D. Ind. Eng. Chem. Res. 1988, 27, 256-264. (22) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Ind. Eng. Chem. Res. 1997, 36, 67-75. (23) Alzamora, L. E.; Ross, J. R. H.; Kruissink, E. C.; van Reijen, L. L. J. Chem. Soc., Faraday Trans. 1 1981, 77, 665-681.

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(Penã et al.25), and methane reforming (Ross and Steel26 and Ross et al.27). When Ni-Al catalysts are used in the catalytic steam gasification of biomass, usually commercial steam reforming catalysts are tested and placed in a fixed bed or a slightly fluidized bed downstream of the gasifier in a second or third reactor. In these cases, the atmosphere around the catalyst (tar and gas composition) is completely different from the case in which the catalyst is placed in the same fluidized bed reactor where the gasification is performed. Also, demands on the catalyst for its use in fluidized or fixed bed are different. As a consequence, the behavior of the catalyst (decrease of liquid yield, increase of gas yield, and upgrading of the gas quality) cannot be predicted from the results obtained in previous works with other Ni catalysts and other experimental systems, because the circumstances are different. In addition, the amount and quality of the gas obtained in the steam biomass gasification process depends on different factors such as the steam/biomass ratio. For example, the work of Herguido et al.28 analyzes the influence of this ratio in the non catalytic gasification of several types of biomass. In this context, it has been considered of interest to analyze the influence of the steam/biomass (S/B) and catalyst weight/ biomass flow rate (W/mb) ratios on the product distribution and on the yields of different gases obtained during the experimental time. Experimental Procedure Experimental System. The experimental system is a bench-scale installation using Waterloo fast pyrolysis process (WFPP) technology (Scott and Piskorz29 and Scott et al.30). The principal components of this technology are a biomass feeder, allowing continuous feeding, and a fluidized bed reactor of 4.35 cm2 inner section. A schematic of the installation is shown in Figure 1. The biomass feeder supplies biomass flow rates between 5 and 100 g/h. Two streams are introduced into the reactor. One is of nitrogen and transports the biomass into the fluidized bed, while the other is of steam or mixtures of nitrogen and steam entering at the bottom and reaching the bed through the distributor. The total flow (nitrogen and steam) were the same in all the experiments in order to work with a constant residence time of the gases in the reaction bed. The water is supplied in liquid state by a syringe pump that allows a constant and accurate flow rate between 0.1 and 99.9 mL/h. The steam is generated while the water flows toward the reaction bed crossing the electric furnace. The product gas is cleaned of char particles using a cyclone. The liquid products (24) Kruissink, E. C.; van Reijen, L. L.; Ross, J. R. J. Chem. Soc., Faraday Trans. 1 1981, 77, 649-663. (25) Penã, J. A.; Rodrı´guez, J. C.; Herguido, J.; Santamarıá, J.; Monzoń, A. Influence of the Catalyst Pretreatment on the Relative Rates of the Main and Coking Reactions during Acetylene Hydrogenation on a NiO/NiAl2O4 Catalyst. In Catalyst Deactivation 1994; Delmon, B., Froment, G. F., Eds.; Elsevier Science Publishers B. V.: Amsterdam, 1994; pp 555-560. (26) Ross, J. R. H.; Steel, M. C. F. J. Chem. Soc., Faraday Trans. 1 1973, 69, 10-21. (27) Ross, J. R. H.; Steel, M. C. F.; Zeini-Isfahani, A. J. Catal. 1978, 52, 280-290. (28) Herguido, J.; Corella, J.; Gonza´lez-Saiz, J. Ind. Eng. Chem. Res. 1992, 31, 1274-1282. (29) Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1982, 60, 666674. (30) Scott, D. S.; Piskorz, J.; Radlein, D. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581-588.

Catalytic Steam Gasification of Pine Sawdust

Energy & Fuels, Vol. 13, No. 4, 1999 853

Figure 1. Scheme of the experimental system. and the water that has not reacted are retained in a system of two condensers and a cotton filter. The gas flow rate is then measured using a dry testmeter, and the 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 to determine the percentages 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. Gasification experiments were performed at 700 °C and atmospheric pressure. The reaction bed was composed of mixtures of sand and catalyst. The particle sizes of catalyst and sand used were between 150 and 350 µm. In all the experiments the total bed volume was the same, 34.39 cm3. The catalyst weight, W, ranged from 0, in the experiment of non catalytic gasification, to 20 g. Inlet biomass flow rates, mb, ranged from 8.4 to 24.5 g/h. The catalyst weight/biomass flow rate ratio, W/mb, was varied from 0 to 0.87 h. The different steam flows used allowed us to carry out experiments with a steam/biomass ratio, S/B (g/g), ranging from 0.49 to 2.74. The total gas flow rate into the reactor is 1715 (STP) cm3/ min. The maximum steam flow rate used is 24 g/h, and in this case the stream through the distributor is steam only. Therefore, the minimum nitrogen flow rate is 1215 (STP) cm3/min. The biomass processed was pine sawdust with a moisture content of about 10% and a particle size of -350 + 150µm. The results of the elemental analysis (in % mass) of the pine sawdust are carbon 48.27%, hydrogen 6.45%, nitrogen 0.09%, and oxygen (by difference) 45.19%. Catalyst. The catalyst with a molar ratio 1:2 (Ni:Al) was prepared in the laboratory by coprecipitation following the method described by Al-Ubaid and Wolf.31 Previously to the experiments and in the same reactor the catalyst was calcined

in air atmosphere at a low heating rate until a final calcination temperature of 750 °C was achieved and maintained at this temperature for 3 h. The precursor and the calcined catalysts were characterized by various techniques such as nitrogen adsorption, mercury porosimetry, X-ray diffraction (XRD), atomic emission spectrometry by inductively coupled plasma (ICP), thermogravimetric analysis, and temperature-programmed reduction (TPR). The presence of NiO and a spinel phase (NiAl2O4) is one of the most significant results obtained in the characterization study. Analysis by TPR shows a significant reduction at 550600 °C and the total reduction of the catalyst at 900 °C. In a previous work (Garcia et al.32), more details about characterization results and catalyst preparation can be found. The catalyst employed was not reduced before the biomass reaction due to the results obtained in previous works (Garcia et al.32,33), in which it was concluded that the gases generated in the thermal decomposition of biomass at 700 °C have the ability to reduce the Ni/Al catalyst prepared by coprecipitation and calcined at 750 °C. Other authors (Tanaka et al.19) have also found the same behavior in steam biomass gasification. In industrial processes the activation of the steam reforming catalyst is carried out using a mixture of steam and hydrocarbons with a steam/carbon ratio of 5/10 (Rostrup-Nielsen34). The use of a catalyst without previous reduction makes its preparation simpler and cheaper. (31) Al-Ubaid, A.; Wolf, E. E. Appl. Catal. 1988, 40, 73-85. (32) Garcia, L.; Salvador, M. L.; Bilbao, R.; Arauzo, J. Energy Fuels 1998, 12, 139-143. (33) Garcia, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Ind. Eng. Chem. Res. 1998, 37, 3812-3819. (34) Rostrup-Nielsen, J. R. Catalytic Steam Reforming. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; SpringVerlang: Berlin, 1984; Vol. 5; pp 3-110.


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Table 1. Results of Steam Gasification at 700 °C run catalyst wt, W (g) sawdust feeding rate, mb (g/h) W/mb (g of catalyst h/g of biomass) S/B (g/g) reaction time (min)

1 2 3 4 5 6 7 8 9 10 20 20 10 10 10 10 10 10 7 5 23.04 24.50 15.25 16.53 14.92 17.34 15.48 20.38 8.40 11.64 0.868 0.816 0.656 0.605 0.670 0.577 0.646 0.491 0.833 0.430 1.27 0.94 1.51 1.39 1.37 1.33 0.51 0.49 2.74 1.98 181 181 61 240 222 260 61 266 49.15 180

yields char/biomass char/(biomass + steam) tar + H2O/biomass tar + H2O/(biomass + steam) gas/biomass gas/(biomass + steam) recovery

0.021 0.009 0.682 0.301 1.623 0.716 1.026

0.018 0.009 0.382 0.197 1.600 0.827 1.033

0.020 0.008 0.999 0.383 1.575 0.604 0.995

0.016 0.007 1.022 0.427 1.400 0.586 1.020

0.017 0.007 0.831 0.351 1.560 0.659 1.017

0.022 0.009 1.052 0.452 1.290 0.557 1.018

0.030 0.019 0.410 0.091 1.343 0.866 0.976

0.028 0.019 0.342 0.230 1.110 0.746 0.995

0.014 0.004 2.216 0.567 1.640 0.420 0.991

0.022 0.007 1.569 0.528 1.350 0.453 0.988

gas yields (mass fraction of original biomass) H2 CO CO2 CH4 C2

0.142 0.540 0.935 0.006 0

0.137 0.676 0.778 0.007 0.002

0.139 0.466 0.964 0.005 0.001

0.111 0.443 0.826 0.012 0.008

0.132 0.571 0.843 0.009 0.005

0.101 0.488 0.672 0.018 0.011

0.107 0.752 0.477 0.006 0.001

0.082 0.590 0.409 0.016 0.013

0.136 0.361 1.136 0.007 0

0.113 0.371 0.847 0.010 0.009

gas composition (% mol, N2- and H2O-free) H2 CO CO2 CH4 C2

63.63 17.28 19.05 0.04 0

61.80 21.78 15.95 0.40 0.07

64.11 15.35 20.21 0.29 0.04

60.90 17.36 20.60 0.82 0.32

62.09 19.19 18.02 0.53 0.17

59.60 20.57 18.02 1.33 0.48

58.40 29.32 11.83 0.41 0.04

56.28 28.93 12.76 1.37 0.66

63.46 12.03 24.10 0.41 0

62.81 14.73 21.40 0.69 0.37

Table 2. Results of Steam Gasification at 700 °C run 11 12 13 14 15 16 17 18 19 20 21 catalyst wt, W (g) 5 5 5 5 5 5 3 2 1 0.5 0 sawdust feeding rate, mb (g/h) 11.61 14.89 16.49 11.83 16.47 18.80 17.26 16.58 16.48 17.37 23.21 W/mb (g of catalyst h/g of biomass) 0.431 0.336 0.303 0.423 0.306 0.266 0.174 0.121 0.061 0.029 0 S/B (g/g) 1.55 1.54 1.40 1.27 0.91 0.53 0.93 0.84 0.93 0.92 0.99 reaction time (min) 181 170 180 240 180 181 60 65 81 56 121 yields char/biomass char/(biomass + steam) tar + H2O/biomass tar + H2O/(biomass + steam) gas/biomass gas/(biomass + steam) recovery

0.015 0.006 1.231 0.483 1.250 0.492 0.981

0.021 0.008 1.343 0.528 1.240 0.488 1.024

0.017 0.007 1.177 0.492 1.150 0.480 0.979

0.028 0.012 1.092 0.481 1.060 0.469 0.962

0.006 0.003 0.826 0.433 1.010 0.527 0.963

0.022 0.014 0.550 0.359 0.890 0.581 0.954

0.016 0.008 0.746 0.387 1.130 0.574 0.969

0.019 0.010 1.072 0.583 0.659 0.347 0.940

0.021 0.011 1.100 0.570 0.679 0.344 0.925

0.022 0.011 1.238 0.645 0.545 0.273 0.929

0.025 0.013 1.308 0.657 0.616 0.309 0.979

gas yields (mass fraction of original biomass) H2 CO CO2 CH4 C2

0.101 0.377 0.755 0.012 0.005

0.084 0.346 0.781 0.016 0.013

0.090 0.394 0.634 0.018 0.014

0.075 0.363 0.593 0.016 0.013

0.070 0.372 0.536 0.016 0.016

0.054 0.447 0.346 0.023 0.020

0.087 0.437 0.585 0.013 0.008

0.032 0.293 0.285 0.025 0.024

0.034 0.292 0.303 0.023 0.027

0.019 0.275 0.195 0.028 0.028

0.016 0.342 0.181 0.041 0.036

gas composition (% mol, N2- and H2O-free) H2 CO CO2 CH4 C2

61.54 16.41 20.91 0.91 0.23

57.07 16.79 24.12 1.36 0.66

59.90 18.73 19.18 1.50 0.69

Results and Discussion Tables 1 and 2 present global results obtained in experiments performed with different W/mb and S/B ratios. In these tables are shown the values of some experimental variables such as catalyst weight (W), biomass flow rate (mb), S/B ratio, and reaction time. Also indicated are yields of total gas, char, and (tar + H2O), expressed as mass fractions of the original biomass and as mass fractions of the sum of biomass and steam, the yields of different gases as mass fraction of the original biomass, and the gas composition expressed as molar percentages (N2 and H2O free), together with the recovery. The recovery is the ratio between the sum

57.32 19.82 20.60 1.53 0.73

56.40 21.41 19.63 1.61 0.95

50.94 30.12 14.83 2.71 1.40

59.17 21.23 18.09 1.11 0.40

45.21 29.57 18.30 4.41 2.51

46.25 28.38 18.74 3.91 2.72

35.79 37.01 16.70 6.59 3.91

28.34 43.28 14.58 9.08 4.72

of char, tar + H2O and gases obtained, and the sum of biomass and steam introduced. Before analyzing the influence of the S/B and W/mb ratios on the catalytic steam gasification, it is interesting to know what modification in the yields of each gas in the gasification process is caused by the presence of the catalyst. Furthermore, a comparative study of the pyrolysis and gasification results shows the influence of the steam in the process. In Figure 2 the total gas, H2, CO, CO2, and CH4 yields are represented for the catalytic and non catalytic pyrolysis and gasification. H2O is not included in the total gas yield because it is condensed together with the liquid products. All the



Figure 3. Gas yield versus time for W/mb ) 0.816 h.

Figure 4. Gas yield versus time for W/mb ) 0.306 h. Figure 2. Total gas, H2, CO, CO2, and CH4 yields in steam gasification and pyrolysis for non catalytic and catalytic processes.

experiments were carried out at 700 °C, and in the catalytic processes the catalyst weight was 20 g. In the steam gasification experiments the S/B ratio was close to 1. A comparison of non catalytic processes of pyrolysis and steam gasification shows that the presence of steam increases the total gas, H2, CO, CO2, and CH4 yields. This rise is more significant for H2 and CO2 than for the other gases. This is caused by the participation of the steam in gas-phase reactions and in the char reactions. If the results of catalytic steam gasification are compared with those of catalytic pyrolysis, an important increase in the gas total, H2, and CO2 is observed, while CO and CH4 yields diminish. In the catalytic process the effects originated by the steam are greater than those of the non catalytic process due to the presence of catalyst, and the decrease of CO and CH4 yields can be explained by water-gas shift and steam reforming of methane reactions. In the steam gasification process the presence of the catalyst causes a significant increase of the gas yield from 0.62, in the non catalytic process, up to 1.60. There is also an increase in the H2 yield (from 0.016 to 0.137), CO yield (from 0.342 to 0.676), and CO2 yield (from 0.181 up to 0.778), but the CH4 yield decreases (from 0.041 down to 0.007) with the use of the catalyst.

Effect of the W/mb Ratio. The analysis of the global 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 an S/B ratio close to 1 (runs 2, 15, 17, 19, 20, and 21), a decrease of the W/mb ratio produces a decrease of total gas, H2, CO, and CO2 yields and an increase of CH4 and C2 yields. This trend is also observed in experiments with an S/B ratio close to 0.5 (runs 7, 8, and 16). The results can be analyzed in more detail by studying the evolution of total gas and individual gas (H2, CO, CO2, CH4, and C2) yields with reaction time. As examples, Figures 3 and 4 show the gas yield evolution with time for two experiments (runs 2 and 15) carried out with different W/mb ratios. In Figure 3 (run 2), with a W/mb ratio of 0.816 h, it is observed that total gas, H2, CO, and CO2 yields are maintained constant throughout the experiment and CH4 and C2 yields show very low values, close to zero. These results indicate that the catalyst does not deactivate under these operating conditions. A different performance is observed in the gas yields in Figure 4 (run 15), with a W/mb ratio of 0.306 h. Total gas, H2, CO, and CO2 yields decrease while CH4 and C2 yields increase over the course of reaction time. The performance of the CH4 yield with reaction time and the W/mb ratio can be explained taking into account the influence of the catalyst on the steam-methane reforming reaction. The increase in


Figure 5. Influence of the W/mb ratio on the H2 yield for an S/B ratio close to 1.

Figure 6. Influence of the W/mb ratio on the CH4 yield for an S/B ratio close to 1.

reaction time and the decrease of the W/mb ratio diminish CH4 reforming. In relation to the C2 performance, a similar explanation is proposed. The influence of the W/mb ratio over time on total gas, H2, CO, CO2, CH4, and C2 yields has been studied for the experiments with an S/B ratio close to 1 (runs 2, 6, 15, 17, 19, 20, and 21). The evolution of total gas, H2, CO, and CO2 yields shows a similar tendency over reaction time for the different W/mb ratios. As an example, the H2 yield evolution with time is represented in Figure 5. An initial H2 yield of about 0.12 for W/mb ratios higher than 0.174 h is observed. For smaller ratios, the initial H2 yield decreases when the ratio diminishes. The H2 yield evolution with reaction time is caused by catalyst deactivation. No significant variations in H2 yield with reaction time are observed for the highest W/mb ratio (0.816 h). For W/mb ratios smaller than or equal to 0.061 h, the decrease of the H2 yield is very sharp and tends toward the values of H2 yields for the noncatalytic process (0.016 h). For intermediate values of the W/mb ratio, the initial hydrogen yield is maintained during a period of time and subsequently diminishes. The time period at which the decrease of H2 yield occurs is longer when the W/mb ratio increases. The CH4 yield shows a different tendency, as can be observed in Figure 6. The CH4 yield increases when the W/mb ratio diminishes and also increases with the

Garcıá et al.

Figure 7. Influence of the W/mb ratio on the H2/CO ratio of product gas for an S/B ratio close to 1.

reaction time. In Figure 7 the H2/CO (vol/vol) ratio evolution of product gas with time is represented. It is observed that for an S/B ratio close to 1, the initial H2/ CO ratio is about 3. The experiment with a W/mb ) 0.577 h (run 6) shows a higher initial value of the H2/ CO ratio, which can be due to the fact that the S/B ratio in this experiment is 1.33, slightly higher than in the others. The experiments with W/mb ratios smaller than or equal to 0.061 h show an initial H2/CO ratio less than 3. The decrease of the H2/CO ratio with reaction time is observed in most of the experiments, except in the experiment where W/mb is 0.816 h, and this can be caused by the catalyst deactivation. It is also observed that the H2/CO ratio tends toward the values of the non catalytic gasification process (H2/CO ) 0.64) when the catalyst deactivation is significant. The influence of the W/mb ratio has also been studied for the experiments with an S/B ratio close to 0.5 (runs 7, 8, and 16). The results obtained of the evolution of total gas, H2, CO, CO2, and CH4 yields with time show trends similar to that observed with an S/B ratio close to 1. The H2/CO ratio evolution with an S/B ratio close to 0.5 presents a similar tendency to that with an S/B ratio close to 1. But the initial H2/CO ratio is higher when the S/B ratio increases: for an S/B ratio close to 0.5, the initial H2/CO ratio is about 2. The analysis of gas evolution with time for all the experiments allows us to conclude that the experiments with W/mb ratios higher than 0.65 h do not show deactivation of the catalyst during the reaction time. For smaller W/mb ratios, the deactivation of the catalyst produces a decrease in total gas, H2, CO, and CO2 yields and an increase in CH4 and C2 yields with reaction time. The experiments with W/mb ratios smaller than 0.12 h show a very fast deactivation of the catalyst from the beginning of the experiment. The results also reveal an important effect of the W/mb ratio on initial yields to different gases. This influence is now analyzed in more detail for an S/B ratio close to 1. In this study, the values of the yield evolution of total gas, H2, CO, CO2, CH4, and C2 versus time have been extrapolated to zero time. The values obtained at zero time correspond to initial yields. These initial yields have been represented versus the W/mb ratio. In Figure 8, H2, CO, and CO2 initial yields are shown for the gasification experiments with an S/B



Figure 8. Initial H2, CO, and CO2 yields for different W/mb ratios and an S/B ratio close to 1.

Figure 10. Influence of the S/B ratio on the H2 yield for 5 and 10 g of catalyst.

Figure 9. Initial CH4 and C2 yields for different W/mb ratios and an S/B ratio close to 1.

ratio of about 1. Gas yields corresponding to the non catalytic gasification experiment (with S/B ) 1) are also included. In Figure 9, CH4 and C2 initial yields are shown for the same S/B ratio. Figure 8 shows an asymptotic increase of H2, CO, and CO2 yields with the increase of the W/mb ratio. For a W/mb ratio > 0.4 h, the values of these yields can be considered constant. An important increase in these yields compared with non catalytic yields is also observed. The initial yields of CH4 and C2 (Figure 9) show an asymptotic decrease with the increase of the W/mb ratio. The initial yield of C2 presents a value close to zero for W/mb ratios g 0.3 h, while the initial yield of CH4 decreases asymptotically to a value of 0.004 CH4/ sawdust (g/min)/(g/min). Effect of the S/B Ratio. From the global experimental results presented in Tables 1 and 2 it is possible to analyze the effect of the S/B ratio for two different catalyst weights, 5 and 10 g. The general trend shows an increase of total gas, H2, and CO2 when the S/B ratio increases, while CO and CH4 yields diminish. The influence of the S/B ratio on gas yields can be analyzed in more detail comparing the evolution of total gas, H2, CO, CO2, CH4, and C2 with reaction time for a catalyst weight in the reaction bed of 5 or 10 g. In Figures 10 and 11, H2 and CO yields are represented versus the sawdust/catalyst (g/g) ratio for different S/B

ratios. The sawdust/catalyst ratio is useful to compare experiments with different sawdust feeding rates. In Figure 10, for 5 and 10 g, it is observed that the increase of the S/B ratio produces an increase in the H2 yield. The decrease of the H2 yield with reaction time, caused by catalyst deactivation, is more pronounced when the S/B ratio diminishes. This fact indicates the positive effect of steam to achieve a longer lifetime of the catalyst. The trends found in H2 yields are also observed in total gas and CO2 yields. A different tendency is found when the CO yield is studied (Figure 11). For 5 and 10 g, the CO yield at the beginning of the experiment decreases when the S/B ratio increases; this could be because of the effect of the water-gas shift reaction (eq 2). The minor increases in H2 and CO yields at short times, as observed in Figures 10 and 11, can be due to the catalyst reduction by the reaction atmosphere (Garcia et al.32). In Figure 12, the CH4 yield evolution is represented versus the sawdust/catalyst ratio for the different S/B ratios using 5 g of catalyst. An increase of the sawdust/ catalyst ratio causes an increase in the CH4 yield, consistent with the results obtained when the influence of the W/mb ratio was analyzed. The increase of the S/B ratio diminishes the CH4 yield, being more marked for higher sawdust/catalyst ratios. With respect to the H2/CO (vol/vol) ratio, an increase is observed when the S/B ratio increases. This result was to be expected because the H2 yield increases and the CO yield diminishes when the S/B ratio increases. The results obtained in the study of the influence of the S/B ratio can be explained by considering the steam reforming reaction of tars (eq 3), water-gas shift (eq 2),


Garcıá et al.

Figure 13. Influence of the S/B ratio on the initial H2, CO, and CO2 yields, with W/mb ratios > 0.5 h.

Figure 11. Influence of the S/B ratio on the CO yield for 5 and 10 g of catalyst.

Figure 12. Influence of the S/B ratio on the CH4 yield using 5 g of catalyst.

steam reforming of methane (eq 1), as well as the following reactions:

C + H2O T CO + H2

(6)

C + CO2 T 2CO

(7)

The increase of H2 and CO2 yields and the decrease of the CO yield when the S/B ratio increases can be explained taking into account principally the water-gas shift reaction (eq 2). The increase of the S/B ratio also increases the methane reforming reaction and as a consequence the methane yield diminishes. Equations 6 and 7 represent the gasification reactions of carbon deposits with H2O and CO2. These reactions become significant at 700 °C, and the increase of the S/B ratio is positive because it favors these reactions.

Figure 14. Comparison between initial experimental results and thermodynamic equilibrium predictions for different S/B ratios, using W/mb ratios > 0.5 h.

This fact can explain the diminishing of catalyst deactivation with the increase of the S/B ratio.


The initial yields of different gases have been obtained for several S/B ratios. In Figure 13, initial yields of H2, CO, and CO2 are represented versus S/B, corresponding to experiments with W/mb ratios > 0.5 h because for these values it can be assumed that at the beginning of the experiment the catalyst is capable of transforming all the liquid products generated in the biomass thermochemical decomposition. Figure 13 also includes gas yields corresponding to catalytic pyrolysis (S/B ) 0). An increase in H2 and CO2 initial yields can be observed while CO diminishes when the S/B ratio increases. H2, CO, and CO2 initial yields remain almost unchanged for S/B ratios > 1.5. In non catalytic steam gasification experiments of lignocellulosic wastes in a fluidized bed, Herguido et al.28 found that the most significant change in total gas yield occurs at an S/B ratio of 1.5. This fact is in accordance with the results of this work. Thermodynamic Equilibrium. Taking into account that for experiments with an S/B ratio close to 1 and W/mb ratio > 0.4 h (Figures 8 and 9) initial yields of different gases are not significantly modified with the W/mb ratio, it was considered useful to compare the experimental gas composition with that corresponding to thermodynamic equilibrium. In Figure 14 bar graphs are used to show this comparison for an S/B ratio of 0.51, 1, 1.51, and 2.74 with W/mb ratios > 0.5. 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 the catalyst modifies the initial product gas composition so as to reach thermodynamic equilibrium. Conclusions This paper studies catalytic steam gasification of pine sawdust in fluidized bed at 700 °C. The catalyst used was prepared by coprecipitation, and was calcined at


750 °C for 3 h. No reduction prior to the biomass reaction was carried out. From the experimental results obtained, the following conclusions can be presented: ‚The presence of the catalyst in the steam gasification process increases significantly total gas, H2, CO, and CO2 yields, while CH4 and C2 yields decrease. ‚The increase of the W/mb ratio in catalytic steam gasification influences catalyst deactivation. Working at W/mb ratios g 0.65 h, catalyst deactivation is not observed in experiments of 3-4 h length. For smaller W/mb ratios, catalyst deactivation decreases total gas, H2, CO, and CO2 yields, while CH4 and C2 increase. ‚The increase of the S/B ratio increases H2 and CO2 yields, while CO and CH4 yields diminish. These results can be explained as a consequence of the reactions involved in this process, mainly water-gas shift and steam reforming of tars and methane. ‚Initial yields of H2, CO, and CO2 increase asymptotically with the increase of the W/mb ratio, while initial yields of CH4 and C2 decrease. H2 and CO2 initial yields increase while the CO initial yield diminishes when the S/B ratio increases. ‚The initial product gas composition for W/mb ratios > 0.5 h is similar to that corresponding to thermodynamic equilibrium for the different S/B ratios. Acknowledgment. The authors express their gratitude to D.G.I.C.Y.T. for providing financial support for the study (Project PB93-0593) and also to the Ministerio de Educacioń y Ciencia (Spain) for the research grant awarded to L. Garcıá. The authors thank D. S. Scott of the 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. EF980250P

Catalytic Steam Gasification of Pine Sawdust. Effect of Catalyst Weight

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