Hydrogen Production by Steam Gasification of Biomass Using NiâAl

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Energy & Fuels 2002, 16, 1222-1230

Hydrogen Production by Steam Gasification of Biomass Using Ni-Al Coprecipitated Catalysts Promoted with Magnesium L. Garcia, A. Benedicto, E. Romeo, M. L. Salvador, J. Arauzo, and R. Bilbao* Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain Received February 15, 2002

Ni-Al coprecipitated catalysts promoted with magnesium have been prepared using the rising and the constant pH techniques, two precipitant agents [(1) KOH and K2CO3, and (2) NH4OH)] and different metal contents. Catalyst characterization by temperature-programmed reduction and CO2 reforming of methane as a test reaction served to select the appropriate catalysts for use in the steam gasification of biomass. The catalysts selected were NiMgAl2O5, prepared at constant pH and precipitated with KOH and K2CO3; NiMgAl4O8 and NiMgAl1.24O3.86, both prepared at increasing pH with NH4OH. Biomass steam gasification experiments were carried out at 700 °C and at atmospheric pressure using different steam/biomass (S/B) and catalyst weight/ biomass flow rate (W/B) ratios. From an analysis of the results obtained, the initial activity and stability of the catalysts have been studied. The NiMgAl2O5 catalyst presents the best performance showing the highest initial activity and stability. This work evidences an improvement of the NiMgAl2O5 catalyst with respect to the previously studied NiAl2O4 catalyst.

Introduction Hydrogen is an important raw material in chemical synthesis and plays a significant role in refinery processes among other uses. Nowadays, there is an increasing interest in hydrogen as a fuel because it creates almost no pollution. The use of hydrogen in fuel cells has another advantage: high energy efficiency. Such reasons contribute to research into hydrogen production particularly from renewable sources, of which biomass could be the most promising. In the transport sector, some estimations indicate that the costs of running a conventional gasoline car and a hydrogen fuel cell car, using biomass as a raw material, are similar.1 Two hydrogen production processes currently being explored are catalytic steam reforming of biomass fastpyrolysis oil (bio-oil)2-4 and, most recently, steam reforming of vegetable oils.5,6 Hydrogen can also be generated by steam gasification of biomass. Extensive research has being carried out in * Author to whom correspondence should be addressed. Fax number: 34-976 762142. E-mail: [email protected]. (1) Ogden, J. Hydrogen. In Solar Energy Today’s Technologies for a Sustainable Future; McIntyre, M., Ed.; American Solar Energy Society: Boulder, 1997; pp 52-54. (2) Wang, D.; Czernik, S.; Montane´, D.; Mann, M.; Chornet, E. Ind. Eng. Chem. Res. 1997, 36, 1507-1518. (3) Wang, D.; Czernik, S.; Chornet, E. Energy Fuels 1998, 12, 1924. (4) Czernik, S.; French, R.; Feik, C.; Chornet, E. Fluidized Bed Catalytic Steam Reforming of Pyrolysis Oil for Production of Hydrogen. In A Growth Opportunity in Green Energy and Value-Added Products; Overend, R. P., Chornet, E., Eds.; Elsevier: Amsterdam, 1999; pp 827832. (5) Marquevich, M.; Coll, R.; Montane´, D. Ind. Eng. Chem. Res. 2000, 39, 2140-2147. (6) Marquevich, M.; Medina, F.; Montane´, D. Catal. Commun. 2001, 2, 119-124.

biomass gasification using different reactor configurations, catalytic materials, temperatures, gasifying agents, and steam/biomass ratios among other operating variables.7-23 Because of the endothermic nature of steam gasification of biomass, working at low temperatures (650-700 °C) requires less energy input for the process, which is of economic interest. However, when temperatures (7) Tanaka, Y.; Yamaguchi, T.; Yamasaki, K.; Ueno, A.; Kotera, K. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 225-229. (8) Baker, E. G.; Mudge, L. K. J. Anal. Appl. Pyrol. 1984, 6, 285297. (9) Baker, E. G.; Mudge, L. K.; Brown, M. D. Ind. Eng. Chem. Res. 1987, 26, 1335-1339. (10) Rei, M. H.; Su, T. S.; Lin, F. S. Ind. Eng. Chem. Res. 1987, 26, 383-386. (11) Corella, J.; Aznar, M. P.; Delgado, J.; Martıńez, M. P.; Araguë´s, J. L. The Deactivation of Tar Cracking Stones (Dolomites, Calcites, Magnesites) and of Commercial Methane Steam 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. (12) Corella, J.; Orıó, A.; Toledo, J. M. Energy Fuels 1999, 13, 702709. (13) Herguido, J.; Corella, J.; Gonza´lez-Saiz, J. Ind. Eng. Chem. Res. 1992, 31, 1274-1282. (14) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Energy Fuels 1994, 8, 1192-1196. (15) Kinoshita, C: M:; Wang, Y.; Zhou, J. Ind. Eng. Chem. Res. 1995, 34, 2949-2954. (16) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 3637-3643. (17) Paisley, M. A. Catalytic Hot Gas Conditioning of Biomass Derived Product Gas. In Developments in Thermochemical Biomass Conversion, Vol. 2; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie A&P: London; U.K., 1997; pp 1209-1223. (18) Caballero, M. A.; Aznar, M. P.; Gil. J.; Martıń, J. A.; France´s, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5227-5239. (19) Olivares, A.; Aznar, M. P.; Caballero, M. A.; Gil, J.; France´s, E.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 5220-5226.

10.1021/ef020035f CCC: $22.00 © 2002 American Chemical Society Published on Web 07/25/2002

Hydrogen Production by Steam Gasification of Biomass

decrease, the tar yield increases and less gas is produced. This inconvenience is dealt with by using catalysts to convert tars into gases and improve the quality of product gas. Our biomass gasification research has been carried out at low temperatures (650-700 °C). A fluidized bed reactor was selected due to the high heat transfer and constant temperature. The catalyst is introduced into the same reaction bed where the biomass is fed, and no postreactor equipment is used. In a previous work,24 Ni-Al coprecipitated catalyst with a molar ratio 1:2 (Ni/ Al) was shown to display an appropriate performance and initial activity. An H2 content of 60% in the product gas (N2 and steam-free), was achieved, and the catalyst activity was maintained for 3-4 h using catalyst weight/ biomass flow rate ratios g0.65 h. For smaller catalyst weight/biomass flow rate ratios, the catalyst deactivation led to a decrease in the total gas, H2, CO, and CO2 yields, while the CH4 and C2 yields increased. The principal reason for catalyst deactivation is the formation of carbon (coke) on the catalyst surface. To develop more stable catalysts, magnesium was selected as a promoter. This metal has some advantages because it enhances steam adsorption, while solid solutions of NiO/MgO stabilize nickel and prevent catalyst sintering.25,26 In addition, the formation of magnesium aluminate spinel considerably increases the mechanical strengh of the catalysts.27,28 The performance of magnesium catalysts has been studied in the CO2 reforming of methane. This reaction is currently being investigated and the study of an appropriate catalyst sufficiently stable and active for this process is being carried out. A review by Wang et al.29 analyzes the thermodynamics, catalyst selection and activity, reaction mechanism, and kinetics of this reaction. However, the main problem of catalyst deactivation is again the formation of carbon deposits. Magnesium appears as a support for nickel catalysts showing high stability.30 Other studies indicate that the basicity of the support and the particle size of nickel can explain the stability of a nickel-magnesia solid solution.31 The addition of magnesium to nickel catalysts prepared by impregnation and used in the steam reforming of an aqueous fraction of bio-oil showed a higher hydrogen yield and slower deactivation.32 Other results (20) Simell, P. A.; Hepola, J. O.; Krause, A. O. I. Fuel 1997, 76, 1117-1127. (21) Rapagna`, S.; Jand, N.; Foscolo, P. U. Int. J. Hydrogen Energy 1998, 23, 551-557. (22) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. Int. J. Hydrogen Energy 1998, 23, 641-648. (23) Garcıá, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Fuel Process. Technol. 2001, 69, 157-174 (24) Garcıá, L.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy Fuels 1999, 13, 851-859. (25) Tottrup, P. B.; Nielsen, B. Hydrocarb. Process. 1982, March, 89-91. (26) Ross, J. R. H. Metal Catalysed Methanation and Steam Reforming. In Catalysis, Vol. 7, The Royal Society of Chemistry: London, 1985; pp 1-45. (27) Bangala, D. N.; Abatzoglou, N.; Chornet, E. AICHE J. 1998, 44, 927-936. (28) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Ind. Eng. Chem. Res. 1997, 36, 67-75. (29) Wang, S.; Lu, G. Q.; Millar, G. J. Energy Fuels 1996, 10, 869904. (30) Wang, S.; Lu, G. Q. Energy Fuels 1998, 12, 248-256. (31) Tomishige, K.; Yamazaki, O.; Chen, Y.; Yokoyama, K.; Li, X.; Fujimoto, K. Catal. Today 1998, 45, 35-39.

Energy & Fuels, Vol. 16, No. 5, 2002 1223

have indicated a slight decrease in catalytic activity using coprecipitated Ni-Al catalysts promoted with magnesium in the pyrolysis of biomass.28 However, restoring the initial activity of the catalysts after regeneration was easier. Magnesium is also present in natural olivine ((Mg,Fe)2SiO4) which has been used as a catalyst support for biomass gasification.33 To study the effect of magnesium on Ni-Al coprecipitated catalysts, two coprecipitation techniques have been employed: rising pH and the constant pH. Different precipitant agents have also been used and catalysts with various nickel contents have been prepared. For all the catalysts a molar ratio of Ni/Mg 1:1 was selected. Catalyst characterization by temperature-programmed reduction and X-ray diffraction, among others techniques, has been used to provide some information about the preparation and to explain the performance of these catalysts in reaction. The selected catalysts have been tested for the steam gasification of biomass with different S/B ratios. The initial activity and the stability of these catalysts are compared with those corresponding to the Ni/Al coprecipitated catalyst previously studied. Experimental Section Experimental System. The experiments have been carried out in a bench scale installation based on Waterloo Fast Pyrolysis Process (WFPP) technology.34,35 The most relevant items of equipment are the biomass feeder and the fluidized bed reactor of 4.35 cm2 inner section. The biomass feeder supplies biomass flow rates of up to 100 g/h. Two streams are introduced into the reactor. One is of nitrogen and transports the biomass from the feeder into the fluidized bed, while the other is of a mixture of nitrogen and steam entering at the bottom and reaching the bed through the distributor. The water is supplied in liquid state by a syringe pump that allows a constant and accurate flow rate. The steam is generated while the water flows toward the reaction bed passing the electrical furnace. The product gas is cleaned of char particles using a cyclone. The liquid products, tar and water, 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 (C2H4, C2H6, and C2H2). A scheme of this experimental system with minor modifications can be found elsewhere.24 The experiments were carried out at 700 °C and at atmospheric pressure. The reaction bed was composed of catalyst diluted with sand. These solids had the same particle size of between 150 and 350 µm. All the experiments were performed with 2 g of catalyst and 50.47 g of sand, and the total bed volume was 34.39 cm3. The total nitrogen flow rate into the reactor was 1465 (STP)cm3/min and the water flow rate was 12 g/h. The biomass flow rate varied from 1 to 42 g/h. As a consequence, the steam/biomass (S/B) ratio and catalyst weight/biomass flow rate (W/B) ratio changed from 11.28 to 0.28 and from 1.88 to 0.05 h, respectively. The catalyst weight/ biomass flow rate (W/B) ratio employed in this work can be (32) Garcıá, L.; French, R.; Czernik; S.; Chornet, E. Appl. Catal. A 2000, 201, 225-239. (33) Courson, C.; Makaga, E.; Petit, C.; Kiennemann, A. Catal. Today 2000, 63, 427-437. (34) Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1982, 60, 666674. (35) Scott, D. S.; Piskorz,; Radlein, D. J. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581-588.

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Table 1. Composition and Preparation of Studied Catalysts nominal composition

Ni (wt %)

technique

precipitant agent

NiMgAl2O5 NiMgO2 NiMgAl4O8 NiMgAl1.24O3.86 NiAl2O4 NiMgAl2O5 NiMgO2

27 51 18 33 33 27 51

constant pH constant pH rising pH rising pH rising pH rising pH rising pH

K2CO3/KOH K2CO3/KOH NH4OH NH4OH NH4OH K2CO3/KOH K2CO3/KOH

easily converted to WHSV value, because the W/B ratio is the inverse of WHSV. 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 % mass) of the pine sawdust were 48.27% carbon, 6.45% hydrogen, 0.09% nitrogen, and (by difference) 45.19% oxygen. Catalysts. Catalyst Preparation. The catalysts were prepared in the laboratory by coprecipitation. Table 1 shows the nominal composition, nickel content, preparation technique, and precipitant agent used. The nickel content is expressed as a weight percentage in the calcined catalyst. Nickel nitrate hexahydrate, magnesium nitrate hexahydrate, and aluminum nitrate nonahydrate were employed in all these preparations. High quality deionized water was also used. The catalyst samples prepared at constant pH follow the method described by Rodrı´guez et al.36 Two aqueous solutions, one containing the metallic nitrates (1 M) and the other the precipitant, K2CO3 and KOH (2.4 M), were simultaneously poured into a glass reaction vessel that initially contained 650 mL of water. The addition was performed dropwise with moderate stirring at 60 °C and a constant pH ) 10.2 ( 0.2. After precipitation, the final mixture was slowly stirred for 2 h at constant pH while more acid solution was added. The precipitates obtained were filtered, washed at 60 °C, and dried at 70 °C for 12 h. A different procedure was followed in the preparation using the rising pH technique. The precipitant, ammonium hydroxide or KOH and K2CO3, was added to the aqueous solution (600 mL) containing the metallic nitrates (1.4 M). The precipitation was carried out at 40 °C with moderate stirring until the final pH was reached. A final pH of 10 was fixed for the preparations with KOH and K2CO3. For the samples precipitated with NH4OH, the final pH was 8.3, except for those without magnesium for which the final pH was 7.9. The precipitates were filtered, washed with water at 40 °C, and dried at 105 °C for 15 h. Further details about the preparation of the NiAl2O4 catalyst using the increasing pH technique with NH4OH can be found in a previous work.37 The precursors, obtained after the drying step, were calcined in air atmosphere at a low heating rate until a final calcination temperature of 750 °C was achieved and maintained for 3 h. Catalyst Characterization. The calcined catalysts were characterized by various techniques such as temperatureprogrammed reduction (TPR), nitrogen adsorption, X-ray diffraction (XRD), and atomic emission spectrometry by inductively coupled plasma (ICP). Also, CO2 reforming of methane was used as a test reaction. The results of the NiAl2O4 catalyst previously studied are included for comparison.37 Figure 1 presents the TPR analysis of all the catalysts studied. The catalysts prepared using the constant pH technique show maximum reduction peaks at 750 °C for NiMgAl2O5 and temperatures higher than 1000 °C for NiMgO2. In addition, small reduction signals are observed at around 550 °C. A different reduction performance is observed for the catalysts (36) Rodrı´guez, J. C.; Romeo, E.; Fierro, J. L. G.; Santamarıá, S.; Monzoń, A. Catal. Today 1997, 37, 255-265. (37) Garcia, L.; Salvador, M. L.; Bilbao, R.; Arauzo, J. Energy Fuels 1998, 12, 139-143.

Figure 1. TPR plots for different catalysts: (a) prepared by the constant pH technique, (b) prepared by the rising pH technique precipitated with NH4OH, and (c) prepared by the rising pH technique precipitated with KOH and K2CO3. prepared with increasing pH depending on the precipitant. For the KOH and K2CO3 precipitant, the maximum reduction peaks are about 410 and 550 °C for the NiMgO2 and NiMgAl2O5 catalysts, respectively. The catalysts precipitated with NH4OH present maximum reduction peaks at temperatures higher than 700 °C. The results obtained indicate that the catalysts prepared at rising pH precipitated with KOH and K2CO3 show very low metal-support interaction. Nickel crystallites after reduction are very unstable for working at the reaction temperature of 700 °C. The NiMgO2 catalyst prepared at constant pH must have formed a solid solution that could explain its low reducibility. The formation of a NiO/MgO solid-solution phase has been reported in the literature.38,39 The analysis of the results for the catalysts prepared at rising pH precipitated with NH4OH shows that the presence of magnesium diminishes the reducibility of the samples. For NiMgAl1.24O3.86 and NiMgAl4O8 catalysts, the increase in nickel content decreases the temperature of the maximum reduction peak. Clause et al.40 observed that the presence of magnesium in coprecipitated Ni-Al catalysts causes a shift of the reduction peak toward higher temperatures. Matsuo et al.41 explain that the reducibility of nickel over NiO-MgO solid solution catalysts is related to nickel content on the surface and close to the surface. CO2 reforming of methane at 700 °C was used as a test reaction. The experiments carried out in a thermogravimetric installation (C. E. I. Electronics) indicate very low activities (38) Hu, Y. H.; Ruckenstein, E. Catal. Lett. 1996, 36, 145-149. (39) Ruckenstein, E.; Hu, Y. H. Chem. Innovation 2000, March, 3943. (40) Clause, O.; Goncalves Coelho, M.; Gazzano, M.; Matteuzzi, D.; Trifiro`, F.; Vaccari, A. Appl. Clay Sci. 1993, 8, 169-186. (41) Matsuo, Y.; Yoshinaga, Y.; Sekine, Y.; Tomishige, K.; Fujimoto, K. Catal. Today 2000, 63, 439-445.


Energy & Fuels, Vol. 16, No. 5, 2002 1225 Table 3. Results of Steam Gasification Using NiMgAl2O5 run

1

2

3

4

5

6

sawdust feeding rate (g/h) W/B (g catalyst h/g biomass) S/B (g steam/g biomass) reaction time (min) yields (mass fraction) gas/biomass gas/(biomass + steam) recovery gas yields (mass fraction of original biomass) H2 CO CO2 CH4 C2 gas composition (% mol, N2- and H2O-free) H2 CO CO2 CH4 C2

25.68 0.078 0.47 30

20.63 0.097 0.58 68

10.76 0.186 1.12 72

9.93 0.201 1.21 86

8.75 0.228 1.37 61

2.12 0.94 5.66 90

0.815 1.011 1.108 1.315 1.178 2.122 0.621 0.639 0.524 0.595 0.497 0.319 0.926 0.976 0.936 0.906 0.954 0.949 0.054 0.462 0.328 0.032 0.034

0.061 0.474 0.422 0.029 0.025

0.083 0.345 0.644 0.017 0.020

0.106 0.472 0.695 0.022 0.020

0.096 0.525 0.518 0.026 0.014

0.187 0.277 1.630 0.00 0.00

49.79 30.45 13.76 3.74 2.25

51.16 28.33 16.03 2.99 1.50

58.97 17.57 20.88 1.54 1.04

60.34 19.24 18.03 1.58 0.81

59.56 23.24 14.60 1.98 0.62

66.51 7.06 26.43 0.00 0.00

Table 4. Results of Steam Gasification Using NiMgAl4O8 run

Figure 2. XRD spectra of selected catalysts: (a) NiMgAl2O5, (b) NiMgAl4O8, (c) NiMgAl1.24O3.86, and (d) NiAl2O4. Table 2. Surface Area of Catalysts catalyst

surface area (m2/g)

NiMgAl2O5 NiMgAl4O8 NiMgAl1.24O3.86 NiAl2O4

172 108 77 150

of the catalysts prepared at rising pH precipitated with KOH and K2CO3. The reduction temperatures (Figure 1c) indicate a low support-metal interaction, therefore it is possible that the sintering of nickel crystallites takes place. The NiMgO2catalyst prepared at constant pH and reduced at 600 and 700 °C also showed low activity, which could be due to the low catalyst reducibility. These catalysts were considered inadequate for use in gasification experiments and no further characterization study was carried out. For the remaining catalysts, the ICP analysis showed a good correspondence of metal content with the nominal composition. The surface area of selected catalysts calculated using the BET equation is presented in Table 2. Results Obtained Using XRD Analysis. Figure 2, show important differences in both the crystallinity and the presence of spinel phases (NiAl2O4 and MgAl2O4) in the catalysts studied. The symbols 0, O, and 2 correspond to XRD patterns of NiO, NiAl2O4, and MgAl2O4, respectively, and the lines are the XRD results obtained with the catalysts. A higher degree of spinel phases and crystallinity are observed in the NiMgAl4O8 catalyst (b) (at rising pH) than in the NiMgAl2O5 catalyst (a) (at constant pH). High crystallinity and a high proportion of NiO phase are detected in the NiMgAl1.24O3.86 catalyst (c) (at rising pH) while very small signals of spinel phases are observed. The higher Ni/Al ratio of this catalyst could explain the high proportion of NiO phase. From the XRD analysis it is not possible to know the proportion between both spinel phases (NiAl2O4 and MgAl2O4) presented in the catalysts promoted with magnesium, because both of them

7


8

9

10

11

12

13

32.64 17.1 9.48 4.92 2.6 1.48 1.06 0.061 0.117 0.211 0.406 0.769 1.351 1.881 0.37 0.70 1.27 2.43 4.62 8.11 11.28 30 40 60 60 60 60 60 0.621 0.891 1.08 1.148 1.521 1.582 1.623 0.454 0.524 0.476 0.334 0.271 0.174 0.132 0.922 0.947 0.976 1.019 0.965 0.987 0.959 0.019 0.363 0.161 0.042 0.038

0.048 0.419 0.339 0.042 0.043

0.065 0.431 0.505 0.035 0.042

0.065 0.528 0.489 0.037 0.031

0.133 0.342 1.010 0.021 0.015

0.163 0.153 1.260 0.008 0.00

0.134 0.00 1.489 0.00 0.00

31.41 43.24 12.18 8.71 4.46

47.29 29.44 15.16 5.14 2.98

51.36 24.42 18.20 3.51 2.51

49.20 28.71 16.94 3.47 1.67

64.31 11.78 22.14 1.24 0.53

70.17 4.70 24.71 0.42 0.00

66.52 0.00 33.48 0.00 0.00

have very similar XRD pattern (angles and intensities). Clause et al.40 indicate the preferential formation of MgAl2O4 over NiAl2O4.

Results and Discussion Tables 3, 4, and 5 show the global results of steam gasification of biomass using NiMgAl2O5, NiMgAl4O8, and NiMgAl1.24O3.86 catalysts, respectively. All the experiments were performed with 2 g of catalyst in the reaction bed. The biomass flow rate varied from 1.06 to 42 g/h, resulting in different catalyst weight/biomass flow rate (W/B) and steam/biomass (S/B) ratios. The values of some experimental variables such as biomass flow rate, W/B and S/B ratios, and reaction time are shown in the tables. Also indicated are yields of total gas, expressed as mass fraction of original biomass and as mass fraction 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 of gases, liquids (tar + H2O), and char obtained, and the sum of biomass and steam introduced.

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Table 5. Results of Steam Gasification Using NiMgAl1.24O3.86 run

14

15

16

17


42.12 0.048 0.28 10

9.38 0.213 1.41 35

7.75 0.258 1.55 50

1.29 1.55 9.30 25

0.848 0.659 0.922

1.362 0.597 0.965

1.2 0.471 1.050

1.148 0.334 0.944

0.058 0.536 0.223 0.019 0.012

0.092 0.516 0.704 0.028 0.023

0.080 0.509 0.550 0.034 0.028

0.124 0.494 0.990 0.000 0.000

52.75 35.04 9.27 2.15 0.80

55.32 22.24 19.32 2.14 0.97

54.11 24.74 16.99 2.92 1.24

60.76 17.25 21.99 0.00 0.00

Although these tables present experiments with different reaction times and biomass flow rates, some main tendencies can be observed. For each catalyst the simultaneous increase of W/B and S/B ratios causes the increase of the total gas yield as a mass fraction of original biomass. This yield is higher than 1 in most of the experiments, due to the fact that the steam converts in the product gases. The main tendency also indicates an increase in H2 and CO2 yields while CH4 and C2 yields decrease with the increase of the W/B and S/B ratios. The increase in the total gas, H2, and CO2 yields when these ratios increase can be a consequence of steam participation in the reforming reactions of tars and light hydrocarbons (CH4 and C2) together with the water gas shift reaction. In addition, the increase in the W/B ratio diminishes catalyst deactivation. A more detailed analysis has been carried out studying the evolution of the total gas yield and the yields of different gases versus the reaction time. Most of the experiments with biomass flow rates higher than 10 g/h show the loss of catalyst activity. For each catalyst, the total gas, H2, and CO2 yields decrease while the CH4 and C2 yields increase when the W/B ratio diminishes. The same evolution in gas yields is observed with the progression of reaction time. These results are in accordance with those obtained in a previous work.24 The evolution of gas yields with reaction time is a consequence of catalyst deactivation that produces the decrease in total gas, H2, CO, and CO2 yields while the CH4 and C2 yields increase. Moreover, a comparison of CO yields obtained for various S/B and W/B ratios shows a peak in the CO yield for S/B ratios of around 0.580.7 (Figure 3). At higher S/B ratios (high W/B), the water gas shift reaction results in a low CO yield while at lower W/B ratios (low S/B) no significant steam reforming reactions occur. In this figure, the sawdust/catalyst ratio is employed to compare experiments with different biomass flow rates. This ratio allows us to analyze the catalyst deactivation, and then, provides some information related to the lifetime of the catalyst. Initial Gas Yields. To analyze the behavior of the catalysts for steam gasification of biomass the initial

Figure 3. CO yield evolution versus sawdust/catalyst ratio, using NiMgAl2O5 and NiMgAl4O8 catalysts and various S/B ratios.

yield to different gases has been determined. For the experiments that showed catalyst deactivation, the values of the yield evolution of total gas, H2, CO, CO2, CH4, and C2 versus time have been extrapolated to zero time to determine the initial gas yields. For the experiments without loss of catalyst activity, the yield of different gases did not change with time and corresponded to the initial value. Some experiments with very low biomass flow rates did not present a constant flow rate. From the elemental analysis of the biomass and the exit gas composition, the specific inlet biomass flow rate could be determined, and subsequently the S/B and W/B ratios. All these data were included in this study. Figures 4 to 9 show H2, CO, CO2, CH4, C2, and total gas initial yields. In each figure, the initial gas yields for the three catalysts studied, NiMgAl2O5, NiMgAl4O8, and NiMgAl1.24O3.86, are represented versus the S/B ratio. For S/B ratios higher than 5, initial gas yields do not significantly change when the S/B ratio increases. In this range, the NiMgAl2O5 and NiMgAl4O8 catalysts show similar initial yields for different gases. H2, CO2, and total gas initial yields are almost constant with values of around 0.16, 1.5, and 1.9 g gas/g biomass. The NiMgAl1.24O3.86 catalyst presents smaller initial yields to these gases, of around 0.12, 0.9, and 1.6 g gas/g biomass for H2, CO2 and total gas, respectively. CH4 and C2 initial yields tend to be zero as the S/B ratio increases for all the catalysts. CO initial yields obtained with NiMgAl2O5 and NiMgAl4O8 catalysts tend to zero as the



Figure 4. H2 initial yield versus S/B ratio using the selected catalysts.

Figure 7. CH4 initial yield versus S/B ratio using the selected catalysts.

Figure 5. CO initial yield versus S/B ratio using the selected catalysts.

Figure 8. C2 initial yield versus S/B ratio using the selected catalysts.

Figure 6. CO2 initial yield versus S/B ratio using the selected catalysts.

Figure 9. Total gas initial yield versus S/B ratio using the selected catalysts.

S/B ratio increases while the value for the NiMgAl1.24O3.86 catalyst is around 0.5 g gas/g biomass. For S/B ratios lower than 5, all the catalysts show an increase in H2, CO2, and total gas initial yields while CO, CH4, and C2 initial yields decrease when the S/B ratio increases. In this range a slightly better performance is observed for the NiMgAl2O5 catalyst, which generates the highest initial yields to H2, CO2, and total gas and produces the lowest initial yields to CO, CH4, and C2. These results indicate that NiMgAl2O5 is a very

active catalyst for steam reforming reactions of tars and light hydrocarbons (CH4 and C2) and for the water gas shift reaction. The NiMgAl4O8 catalyst presents initial yields to H2, CO, and CO2 comparable to the NiMgAl2O5 catalyst except for very slow S/B ratios (0.3 h. For W/B g 0.3 h the initial gas yields can be considered constant when the W/B ratio increases. The NiMgAl2O5 catalyst presents higher initial yields to H2, CO2, and total gas compared with those obtained with the NiAl2O4 catalyst for the three W/B ratios studied. The initial activity of the NiAl2O4 catalyst tends to be similar to that of the NiMgAl2O5 catalyst when the W/B ratio increases, and for W/B > 0.3 h are very similar. The CO initial yield is also higher for the NiMgAl2O5 catalyst than for the NiAl2O4 catalyst. H2, CO, CO2, and total gas initial yields obtained with NiMgAl4O8 does not follow a clear tendency when compared with those corresponding to the NiAl2O4 catalyst. CH4 and C2 initial yields produced with the NiMgAl4O8 catalyst in most cases are higher than those obtained with the NiAl2O4 catalyst. The NiMgAl1.24O3.86 catalyst presents the highest CH4 and C2 initial yields, especially for W/B > 0.3 h. H2, CO2, and total gas initial yields are lower than those obtained with the other catalysts, particularly at W/B > 0.3 h, while the CO initial yield is the highest. The experimental results of initial gas yields obtained for W/B ratios > 0.4 h have been compared with those corresponding to the thermodynamic equilibrium. A good agreement was observed for the NiMgAl2O5 and NiMgAl4O8 catalysts. However, the NiMgAl1.24O3.86 catalyst showed poor agreement for all the S/B ratios checked. Evolution of Gas Yields with Time. To compare the performance of the different catalysts in the steam gasification of biomass, the initial activity study has to

Figure 10. H2 yield evolution versus sawdust/catalyst ratio using a S/B ratio close to 1.

Figure 11. CO yield evolution versus sawdust/catalyst ratio using a S/B ratio close to 1.

Figure 12. CO2 yield evolution versus sawdust/catalyst ratio using a S/B ratio close to 1.

be completed with a study of the stability of the initially more active catalysts (NiAl2O4, NiMgAl2O5, and NiMgAl4O8). The evolution of different gases with time is therefore compared for the catalysts studied. As an example, Figures 10 to 15 present the evolution of H2, CO, CO2, CH4, C2, and total gas yields versus the sawdust/catalyst ratio using similar W/B ratios, and a S/B ratio close to 1. In these figures it can be observed that the H2, CO, CO2, and total gas yields decrease while


Figure 13. CH4 yield evolution versus sawdust/catalyst ratio using a S/B ratio close to 1.


the NiMgAl2O5 and NiAl2O4 catalysts while a more rapid increase is observed for the NiMgAl4O8 catalyst when the sawdust/catalyst ratio increases. The catalysts are placed in the reaction bed without reduction, and during the steam gasification of biomass, the catalysts are reduced by the reaction atmosphere generating an active surface that is used in the steam reforming of tars, CH4, C2H4, and other light hydrocarbons. Simultaneously with these reactions, carbonaceous intermediates can be formed. The use of catalysts without previous reduction allows us an easier and safer operation because no hydrogen is employed. Moreover, the results can be compared to those obtained in a previous work24 using the NiAl2O4 catalyst without reduction. NiMgAl2O5 appears to be the most stable catalyst. The properties of this catalyst (high surface area, low crystallinity, relatively low proportion of spinel phase, reducibility) could influence its initial activity and stability. The NiMgAl4O8 catalyst, with a higher proportion of spinel phase, high crystallinity, and less reducibility, presents lower initial activity especially for the CH4 and C2 yields and lower catalyst stability.

Conclusions

Figure 14. C2 yield evolution versus sawdust/catalyst ratio using a S/B ratio close to 1.

Figure 15. Total gas yield evolution versus sawdust/catalyst ratio using a S/B ratio close to 1.

the CH4 and C2 yields increase. This is a consequence of catalyst deactivation. Although all the catalysts lose activity as the reaction progresses, the NiMgAl2O5 catalyst shows the best performance with the highest yields to H2, CO2, and total gas. This catalyst also presents the lowest CO yield for sawdust/catalyst ratios < 5. The NiAl2O4 catalyst shows slightly lower yields to H2 than those obtained with NiMgAl2O5. CH4 and C2 yields increase slowly for

Biomass steam gasification experiments have been carried out at 700 °C and at atmospheric pressure in a bench scale installation using coprecipitated Ni-Al catalysts promoted with magnesium. Different precipitation techniques, precipitant agents and metal contents have been used with the purpose of developing a more active and stable catalyst than the coprecipitated NiAl catalyst previously studied. The main conclusions obtained are the following: 1. The precipitation technique and precipitant agent have a very significant influence on catalyst properties. The catalysts prepared at constant pH (precipitated with KOH and K2CO3) and at rising pH (precipitated with NH4OH) can be suitable for use in the steam gasification of biomass, except for the NiMgO2 catalyst prepared at constant pH, which displays a low reducibility in the TPR characterization study. 2. The NiMgAl2O5 catalyst presents the highest initial activity of all the catalysts tested. The high values of initial yields to H2, CO2, and total gas and the low values of initial yields to CH4, C2, and CO indicate a high activity in steam reforming reactions of tars and light hydrocarbons (CH4 and C2) and the water gas shift reaction. The NiMgAl4O8 catalyst shows slightly lower initial yields to H2, CO, and total gas than the NiMgAl2O5 catalyst. NiMgAl1.24O3.86 has a considerably lower initial activity than the other catalysts, which is corroborated by the fact that it is not close to the thermodynamic equilibrium. 3. The NiMgAl2O5 catalyst shows a slightly more stable performance than the reference catalyst, NiAl2O4. The H2 and CO2 yields reach and maintain the highest values during the experiment compared with the other

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catalysts, while the CH4 and C2 yields increase more slowly. 4. The properties of the NiMgAl2O5 catalyst (a surface area of 172 m2/g, low crystallinity, relatively low proportion of spinel phase, and a maximum peak reduction temperature of 750 °C) can influence its performance. 5. From the conclusions obtained, it can be affirmed that the NiMgAl2O5 catalyst presents a better perfor-

Garcia et al.

mance, initial activity, and stability than the catalyst previously studied. Acknowledgment. The authors express their gratitude to the “Comisioń Interministerial de Ciencia y Tecnologıá (C.I.C.Y.T.)” and to the European Commission for providing financial support for the study (Project 2FD97-0890). EF020035F

Hydrogen Production by Steam Gasification of Biomass Using NiâAl

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