Energy & Fuels 1998, 12, 139-143
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Influence of Calcination and Reduction Conditions on the Catalyst Performance in the Pyrolysis Process of Biomass L. Garcia, M. L. Salvador, R. Bilbao,* and J. Arauzo Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain Received June 23, 1997X
The influence of several preparation parameters on the performance of a coprecipitated nickel alumina catalyst for use in the pyrolysis of lignocellulosic residues has been studied. The variables considered were calcination temperature (750 and 850 °C), reduction time (1, 2, and 3 h), and hydrogen flow in the reduction step (1740 and 3080 cm3 (STP)/min). The catalyst performance was evaluated on a bench scale plant equipped with a continuous fluidized bed reactor using the Waterloo fast pyrolysis process (WFPP) technology. The biomass used was pine sawdust and the reaction temperature was 650 °C. The results show that when the higher calcination temperature is applied, more severe operating conditions on the reduction process must also be applied, but catalyst sintering can appear when very severe reduction conditions are used.
Introduction Processes such as pyrolysis or gasification of biomass are being proposed as possible alternatives to the conventional processes used to produce synthesis gas or gas with a high hydrogen content. Nowadays fast pyrolysis provides a real possibility of transforming biomass into valuable products (Chornet et al.).1 This is confirmed by the development of different technologies of fast pyrolysis: Waterloo fast pyrolysis process, WFPP, (Scott et al.2), rapid thermal processing (Graham et al.3), entrained-flow pyrolysis (Kovac et al.4), vortex ablative pyrolysis (Diebold and Scahill5), rotating cone reactor (Wagenaar et al.6), ablative pyrolysis reactor (Peacocke and Bridgwater7). * To whom correspondence should be addressed. E-mail: qtarauzo@ posta.unizar.es. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) 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 & Utilization; Bridgwater, A. V., Hogan, E., Eds.; CPL Press: Berkshire, UK, 1996; pp 246-262. (2) Scott, D. S.; Piskorz, J.; Radlein, D. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 581-588. (3) Graham, R. G.; Freel, B. A.; Bergougnou, M. A. The production of pyrolytic liquids, gas and char from wood and cellulose by fast pyrolysis. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 629-641. (4) Kovac, R. J.; Gordon, C. W.; O’Neil, D. J. Entrained flow pyrolysis of biomass. Thermochemical Conversion Program Annual Meeting; Solar Energy Research Institute: Golden, CO, 1988; pp 5-20. (5) Diebold, J. P.; Scahill, J. Production of primary pyrolysis oils in a vortex reactor. In Pyrolysis Oils from Biomass; Soltes, E. J., Milne T. A., Eds.; American Chemical Society: Washington, DC, 1988; pp 31-40. (6) Wagenaar, B. M.; Kuipers, J. A. M.; van Swaaij, W. P. M. Chem. Eng. Sci. 1994, 49, 927-936. (7) Peacocke, G. V. C.; Bridgwater, A. V. Design of a novel ablative pyrolysis reactor. In Advances in Thermochemical Biomass Conversion; Bridgwater, A. V., Eds.; Blackie A&P: Glasglow, UK, 1994; pp 11341150.
Depending on the operating conditions, in particular the pyrolysis temperature, these technologies allow us to produce different products. The use of relatively low temperatures (600-700 °C) is of commercial interest because of the decrease in the input energy necessary for the process. At these temperatures, significant amounts of liquids are produced, the gas yield obtained being relatively low. One recent possibility of transforming these pyrolytic oils into gases is through their subsequent catalytic steam reforming (Wang et al.).8 The use of catalysts in the pyrolysis process itself could be an interesting approach for increasing the gas yield by decreasing the amount of liquid. Catalysts have been extensively studied as a means to eliminate tar generated in the thermal decomposition of biomass (Mudge et al.)9 These works focus on the cracking of tars and on the steam reforming of the gas generated in the processing of lignocellulosic residues (Aznar et al.10 and Kinoshita et al.11). The use of Ni-Al oxide catalysts is a promising option that the literature quotes as one of the most encouraging prospects in the processing of tars (Baker et al.,12 Arauzo et al.13,14). There are other metals (e.g. Pt, Ru, (8) Wang, D.; Czernik, S.; Montane´, D.; Mann, M.; Chornet, E. Ind. Eng. Chem. Res. 1997, 36, 1507-1518. (9) Mudge, L. K.; Baker, E. G.; Brown, M. D.; Wilcox, W. A. “Bench scale studies on gasification of biomass in the presence of catalysts”. Contract DE-AC06-76RLO-1830, Final Report, Pacific Northwest Laboratory, Battelle Memorial Institute, Richland, WA, November 1987; PNL-5699. (10) Aznar, M. P.; Corella, J.; Delgado J.; Lahoz, J. Ind. Eng. Chem. Res. 1993, 32, 1-10. (11) Kinoshita, C. M.; Wang, Y.; Zhou, J. Ind. Eng. Chem. Res. 1995, 34, 2949-2954. (12) Baker, E. G.; Mudge, L. K.; Brown, M. D. Ind. Eng. Chem. Res. 1987, 26, 1335-1339. (13) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Energy Fuels 1994, 8, 1192-1196. (14) Arauzo, J.; Radlein, D.; Piskorz, J.; Scott, D. S. Ind. Eng. Chem. Res. 1997, 36, 67-75.
S0887-0624(97)00097-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998
140 Energy & Fuels, Vol. 12, No. 1, 1998
Figure 1. Schematic of the experimental system.
Rh, ...) as active as Ni, but Ni is cheaper and sufficiently active. The Al2O3 presence causes high metal dispersion and acts as an acid catalyst to crack tars. The use of several Ni-Al2O3 catalysts gives good initial results but they experience a deactivation problem caused mainly by carbon deposition. For given pyrolysis conditions, the performance of the catalyst will be influenced by different aspects, such as its preparation and pretreatment. In the literature there are several studies about the preparation and characterization of these catalysts (Kruissink et al.,15 Clause et al.,16,17 Alzamora et al.18). However, there are other less studied features of these catalysts, such as the calcination and reduction conditions, a knowledge of the influence of which can contribute to the development of an appropriate catalyst. In this context it has been considered useful to analyze the influence of the calcination and reduction conditions of the catalyst on its performance. A coprecipitated nickel-alumina catalyst was prepared and the pyrolysis was performed in a fluidized bed using the WFPP technology. Experimental Procedure Experimental System. A schematic of the installation is shown in Figure 1. Biomass is continuously fed from a feeder at flow rates between 5 and 100 g/h. The reactor used is a fluidized bed of 4.35 cm2 inner section. The bed is composed of 20 g of catalyst and 20 g of sand with a bed volume of 34 cm3. The particle sizes of the catalyst and sand used are between 150 and 350 µm. The pyrolysis experiments are carried out at 650 °C and at atmospheric pressure. In these experiments two nitrogen streams are introduced into the reactor. One transports the biomass into the fluidized 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, corresponding to a superficial velocity of 12.5 cm/s. The theoretical minimum fluidization velocity value of the mixture (15) Kruissink, E. C.; van Reijen, L. L.; Ross, J. R. H. J. Chem. Soc., Faraday Trans. 1 1981, 77, 649-663. (16) Clause, O.; Gazzano, M.; Trifiro´, F.; Vaccari, A.; Zatorski, L. Appl. Catal. 1991, 73, 217-236. (17) Clause, O.; Rebours, B.; Merlen, E.; Trifiro´, F.; Vaccari, A. J. Catal. 1992, 133, 231-246. (18) Alzamora, L. E.; Ross, J. R. H.; Kruissink, E. C.; van Reijen, L. L. J. Chem. Soc., Faraday Trans. 1 1981, 77, 665-681.
Garcia et al. of solids at 650 °C is 2.0 cm/s. Elutriation of catalyst and sand is not observed. 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. Afterwards, the gas flowrate is measured using a dry testmeter, and CO and CO2 concentrations are continuously determined. In addition, gas samples are taken at regular time intervals and analyzed by chromatography. Reaction temperature, total gas flow, and CO and CO2 concentrations are registered by a data acquisition system. Pine sawdust with a moisture content of about 10% and a particle size of -350 + 150 µm was processed. 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 Ni-Al catalyst employed was prepared in the laboratory by coprecipitation by a method similar to that described by Al-Ubaid and Wolf.19 Ammonium hydroxide is added to a solution of Ni(NO3)2‚6H2O and Al(NO3)3‚9H2O in water until the pH reaches 7.9. The precipitation medium is maintained at 40 °C and moderately stirred. The salts are mixed in appropriate proportions to obtain a molar ratio 1:2 (Ni:Al). The precipitate obtained is filtered and washed at 40 °C and dried for about 15 h at 105 °C. Following these steps, the precursor is obtained. This precursor is calcined in air atmosphere at a low heating rate, as suggested by Alzamora et al.,18 until a final calcination temperature is achieved, and maintained for 3 h. In most cases, just before the pyrolysis is performed, the calcined catalyst is reduced in the reactor at a temperature of 650 °C using hydrogen. By carrying out pine sawdust pyrolysis experiments at 650 °C, the influence has been studied of the calcination temperature (750 and 850 °C), reduction time (1, 2, and 3 h), and hydrogen flow rate in the reduction step (1740 and 3080 cm3 (STP)/min) on the pyrolysis performance. Each experiment is characterized by a set of conditions, e.g. calcination temperature 750 °C, reduction time 1 h, and hydrogen flow rate 1740 cm3 (STP)/min. Experiments without previous reduction of the catalyst were also performed. The calcined catalyst is characterized by different techniques: atomic emission spectrometry, mercury porosimetry, nitrogen adsorption, thermogravimetric analysis, X-ray diffraction (XRD), and temperature-programmed reduction (TPR). Thermogravimetric analysis and atomic emission spectrometry have been used to analyze the precursor. The elemental analysis of metals (Ni and Al) in the catalyst, carried out using atomic emission spectrometry by inductively coupled plasma (ICP), has shown good agreement with the theoretical formulation, being Ni/Al 0.49 for the precursor and 0.48 for the calcined catalyst. The analysis of the calcined catalyst by mercury porosimetry has determined a total intrusion volume of 0.7222 and 0.5864 mL/g and a median pore diameter of 44 and 48 Å for the calcination temperatures of 750 and 850 °C, respectively. Surface areas of adsorption of 150 and 131 m2/g, respectively, for the catalyst calcined at 750 and 850 °C have been obtained by adjusting the nitrogen adsorption data to the BET equation. The results obtained from the thermogravimetric analysis indicate that the weight loss observed during the calcination occurs in two steps. In the first step, at about 114 °C, molecular water is lost from the interlayer of the layer structure. In the second step, at about 298 °C, the layer structure decomposes with the evolution of nitrogen oxides. It is in this latter step that the maximum decomposition takes place. These results are similar to those obtained by Alzamora et al.18 Results obtained using XRD analysis, Figures 2 and 3, show differences in the catalyst structure depending on whether the precursor is calcined at 750 or 850 °C. An increase in the calcination temperature causes an increase in the crystallinity (19) Al-Ubaid, A.; Wolf, E. E. Appl. Catal. 1988, 40, 73-85.
Catalyst Performance in the Pyrolysis Process of Biomass
Energy & Fuels, Vol. 12, No. 1, 1998 141
Results and Discussion
Figure 2. XRD spectrum of a catalyst calcined at 750 °C.
Figure 3. XRD spectrum of a catalyst calcined at 850 °C.
Figure 4. TPR plot for catalyst calcined at different temperatures. of the catalyst and in the proportion of spinel phase in it. Similar trends were observed by Alzamora et al.18 and Clause et al.17 Temperature-programmed reduction (TPR) analysis of the catalyst calcined at 750 and 850 °C were also carried out. In these experiments the hydrogen consumption was determined using a thermal conductivity detector (TCD) and expressed in a normalized way. The results, Figure 4, show that reduction is more difficult as the calcination temperature is increased, which could be due to the higher proportion of spinel phase existing in the catalyst calcined at 850 °C. Alzamora et al.18 found similar results for this kind of catalyst subjected to calcination temperatures between 750 and 900 °C.
Before the influence of the use of catalysts was analyzed, pine sawdust pyrolysis experiments at 650 °C without catalyst were carried out. In these experiments significant amounts of liquid were obtained. Some representative results of the yields of the products, expressed as mass fractions of the original biomass, obtained in these experiments are 0.32 gas, 0.20 char, and 0.48 liquid. Representative values of exit gas composition (vol %, nitrogen free) were 15.3% H2, 55.0% CO, 11.1% CO2, 11.0% CH4, and 7.6% C2. The yields of the different gases, also expressed as mass fractions of the original biomass, are 0.004 H2, 0.201 CO, 0.064 CO2, 0.023 CH4, and 0.028 other gases. In addition, in these experiments without catalyst a significant agglomeration was observed in the bed and operational problems appeared at short reaction times. The use of catalysts significantly changes the results obtained. Table 1 presents the product distributions corresponding to pyrolysis experiments performed at 650 °C with catalyst. The calcination temperature and the reduction conditions (hydrogen flow rate and reduction time) have been varied in these experiments. In general, a decrease in the liquid yield and an increase in the gas yield is observed when these results are compared with those corresponding to experiments without catalyst. With respect to the yield of the different individual gases, the use of catalysts produces a significant increase in the yields of H2 and CO. The CO2 yield also increases but more slightly, while the CH4 yield decreases. Experiments with the catalyst calcined at 850 °C and without previous reduction were also performed. The results are not shown because operational problems occurred and agglomeration of the bed was observed. The results shown in Table 1 correspond to the overall products obtained for each experiment during the reaction time indicated in the table. However, for a given experiment the yields of the different products vary with the reaction time. The overall results are also affected by variables such as the flow rate of the biomass fed and the duration of the experiment. Therefore, and in order to analyze the influence of the calcination and reduction conditions of the catalyst on the product yields, it was considered useful to look at the variation of the gas yields as the sawdust/catalyst mass ratio changed. The influence of the catalyst reduction conditions on the gas yields at the same calcination temperature has been analyzed. Figure 5 shows the results obtained when the catalyst is calcined at 750 °C and different hydrogen flow rates are used in the reduction. These results are also compared with those corresponding to the experiments performed with a catalyst not previously reduced. The experimental results obtained for a hydrogen flow of 1740 cm3 (STP)/min and reduction times of 2 and 3 h are not shown because they are very similar to those observed for a reduction time of 1 h and it is therefore considered that under the above conditions the product distributions are not affected by changes in reduction time. This is consistent with the work of Chen and Shiue,20 who, by chemisorption measures, found that there is a certain reduction time beyond which no significant changes occur to the
142 Energy & Fuels, Vol. 12, No. 1, 1998
Garcia et al.
Table 1. Results Obtained with Catalyst run calcination temp (°C) H2 flow rate (cm3 (STP)/min) reduction time (h) sawdust feeding rate (g/h) reaction time (min) yields (mass fraction of original biomass) gas liquid solids recovery gas yields (mass fraction of original biomass) H2 CO CO2 CH4
1
2
3
4
5
6
7
8
9
21.9 88.9
750 1740 1 15.3 120.0
750 1740 2 20.2 97.1
750 1740 3 15.2 124.0
750 3080 1 19.8 109.4
850 1740 1 16.3 96.7
850 1740 2 15.8 124.2
850 3080 1 15.9 122.5
850 3080 2 21.9 71.6
0.748 0.169 0.062 0.979
0.755 0.083 0.114 0.952
0.717 0.155 0.098 0.970
0.759 0.060 0.132 0.951
0.694 0.180 0.084 0.958
0.652 0.234 0.092 0.978
0.678 0.142 0.122 0.942
0.799 0.110 0.089 0.998
0.620 0.218 0.115 0.953
0.053 0.522 0.159 0.014
0.054 0.596 0.096 0.009
0.040 0.576 0.074 0.027
0.046 0.613 0.070 0.018
0.046 0.538 0.093 0.017
0.030 0.498 0.104 0.020
0.041 0.498 0.116 0.023
0.045 0.635 0.097 0.022
0.033 0.448 0.123 0.016
750
Figure 5. Total gas yield obtained for different hydrogen flow rates at a calcination temperature of 750 °C.
catalyst structure. It can be observed in Figure 5 that, compared with the nonreduced catalyst, no significant differences are obtained when the catalyst is reduced with a hydrogen flow rate of 1740 cm3 (STP)/min, but when this flow rate is higher, 3080 cm3 (STP)/min, lower total gas yields are obtained. Besides the total gas formed, the yields of the main gas products, H2, CO and CO2, have also been compared for the experiments with the catalyst reduced at a hydrogen flowrate of 1740 cm3 (STP)/min, and with no reduction, Figure 6. It can be observed that the reduced catalyst allows us to achieve higher yields of H2 and CO, but lower CO2 yields, than those obtained without reduction of the catalyst. These results, together with the fact that operational problems do not appear when using catalysts calcined at 750 °C without previous reduction, allow us to assume that this catalyst can be reduced during the reaction. The reduction of the catalyst by the reaction products during the first moments of the experiments was also observed by other authors, Ross and Steel,21 and Ross et al.22 This reduction can be produced both by the H2 and the CO formed in the pyrolysis. The catalyst reduction (20) Chen, I.; Shiue, D.-W. Ind. Eng. Chem. Res. 1988, 27, 429434. (21) Ross, J. R. H.; Steel, M. C. F. J. Chem. Soc., Faraday Trans. 1 1973, 69, 10-21. (22) Ross, J. R. H.; Steel, M. C. F.; Zeini-Isfahani, A. J. Catal. 1978, 52, 280-290.
Figure 6. H2, CO2 and CO yields obtained without and with previous reduction of the catalyst at a hydrogen flow rate of 1740 cm3 (STP)/min during 1 h.
with CO was mentioned by Ross and Steel21 when they suggested that the CO evolved from a reaction is available to produce a complete reduction of the external shells of the catalysts when they were not previously reduced by H2. This occurs through the reaction
NiO + CO f Ni + CO2 The significant amount of CO2 observed at the beginning of the experiments carried out in this work with the catalyst without reduction could justify the presence of this reaction. The total gas yields obtained when the catalyst is calcined at 850 °C are shown in Figure 7. The results corresponding to the nonreduced catalyst are not shown due to the above-mentioned operational problems and because it is assumed that the catalyst calcined at 850 °C is not reduced during the reaction, because its reduction is more difficult. It can be observed in Figure 7 that when the catalyst is reduced with a hydrogen flow rate of 3080 cm3 (STP)/min for a reduction time of 1 h, higher gas yields are obtained than those corresponding to a flow rate of 1740 cm3 (STP)/min, which is contrary to the results observed for a calcination temperature of 750 °C. Similar trends are obtained for the yields of H2 and CO, Figures 8 and 9. It can be also observed in Figures 7-9 that when the reduction time with a hydrogen flow rate of 3080 cm3 (STP)/min is 2 h, the yields of the different gases are lower than those observed with a reduction time of 1 h.
Catalyst Performance in the Pyrolysis Process of Biomass
Figure 7. Total gas yields obtained for different hydrogen flow rates and reduction times at a calcination temperature of 850 °C.
Energy & Fuels, Vol. 12, No. 1, 1998 143
deduced that very severe reduction conditions can produce a decrease in the gas yields. This fact can be observed by comparing the results obtained for a calcination temperature of 750 °C and hydrogen flow rates of 1740 and 3080 cm3 (STP)/min, and also by comparing the results corresponding to a calcination temperature of 850 °C, a hydrogen flow rate of 3080 cm3 (STP)/min and reduction times of 1 and 2 h. Sintering of the catalyst due to the drastic operating conditions would be the probable reason for these gas yield decreases. Figures 8 and 9 show the H2 and CO yields obtained for different conditions of calcination and reduction of the catalysts. It can be observed that at the beginning of the reaction the yields obtained with Tcal ) 750 °C and QH2 ) 1740 cm3 (STP)/min are the highest. For higher sawdust/catalyst ratios the highest yields are obtained with Tcal ) 850 °C and QH2 ) 3080 cm3 (STP)/ min and the decrease of these yields with increases in the reaction time is significantly lower. The highest initial yields obtained with the catalyst calcined at 750 °C and QH2 ) 1740 cm3 (STP)/min could be due to the high amount of superficial active Ni originating from a more reducible structure. However, a fast deactivation occurs, probably caused by the coke deposition. The good performance of the catalyst calcined at 850 °C and QH2 ) 3080 cm3 (STP)/min during the reaction could be explained by its spinel structure which, although less reducible, is capable of maintaining a sufficient amount of active superficial Ni, causing a stabilizing effect. Conclusions
Figure 8. H2 yield for different calcination and reduction conditions of the catalyst.
Figure 9. CO yield for different calcination and reduction conditions of the catalyst.
From these results it can be deduced that the optimum values of time reduction and hydrogen flow rate for obtaining the maximum catalyst activity depend on the calcination temperature used. When the higher calcination temperature is used, more severe reduction conditions can be applied, probably due to the presence of the spinel phase in the catalyst. It can also be
The operating conditions applied during the calcination and reduction steps when producing nickel aluminate catalysts significantly affects the catalytic pyrolysis process of biomass at low temperatures. This influence is observed in changes in the exit product distribution. A catalyst calcined at 750 °C can be reduced due to the presence of CO and H2, which are being generated, in situ, during the pyrolysis reaction. Higher calcination temperatures make this reduction more difficult. Severe reduction conditions may produce a decrease in the gas yields due to sintering of the catalyst. From the results obtained with catalysts calcined at 750 and 850 °C, it can be observed that the performance of the former is better during an initial short processing time. However, when the entire reaction period is considered, better performances are obtained with the catalyst calcined at 850 °C. 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 Educacio´n y Ciencia (Spain) for the research grant awarded to L.G. 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. EF970097J
