Energy &Fuels 1994,8, 1192-1196
1192
A New Catalyst for the Catalytic Gasification of Biomass J. Arauzo,? D. Radlein, J. Piskorz, and D. S. Scott* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and Department of Chemical Engineering, Universidad de Zaragoza, Zaragoza, Spain Received July 31, 1992. Revised Manuscript Received July 20, 1994@
Various kinds of catalysts are being developed and tested for the pyrolytic gasification of biomass using the Waterloo Fast Pyrolysis Process (WFPP) technology. The present report describes gasification tests with wood in a continuous bench scale fluidized bed reactor but with no added air or oxygen in the region of 500-700 "C and at short gas contact times, using a crystalline nickel aluminate catalyst. The objective was to determine the most appropriate operating conditions for this catalyst and its performance for the production of a synthesis gas in high yields. Experiments were carried out with this catalyst in both inert and reactive gasification media, but without any oxygen or air addition. The results show the influence of the catalyst on the nature of the gasification products and the effect of operating variables. Gas compositions are given for typical WFPP operating conditions. Evidence is presented which indicates that the gasification mechanism is a fast thermal pyrolysis followed by catalytic reforming of the vapors with a high yield of synthesis gas.
Introduction Gasification of biomass has been extensively studied and a voluminous journal and patent literature exists. Among the many excellent reviews available, two recent publications may be mentioned as useful references: Bridgwater and Double1 and Beenackers and Bridgwater.2 In the great majority of autothermal gasification studies air, oxygen, steam, or mixtures of these three have been used as gasifying media. In most developments, no catalysts have been employed. Only in more recent years have reports appeared concerning the effects of catalysts in various pyrolytic biomass gasification schemes. A distinction must be made between conventional gasification in which a reactive gas, usually oxygen or steam, is a major factor, and pyrolytic gasification, which does not require any additional reactants, although limited amounts may be used, particularly of steam or carbon dioxide. Pyrolytic gasification is endothermic, and heat must be added from some external source. Extensive studies by workers at Battelle of pyrolytic gasification using catalysts have been described by Mudge et al.3 and the catalytic methanation of wood with carbon conversions to methane of over 80% has been described by Garg et to give only two recent examples. * Author to whom correspondence should be addressed.
Universidad de Zaragoza. Abstract published in Advance ACS Abstracts, September 1,1994. (1)Bridgwater, A. V.; Double J. M. A strategic assessment of liquid fuels from biomass. In Research in Thermo-chemical Biomass Conuersion; Bridgwater, A. V., Kuester, J. L. Eds.; Elsevier Applied Science Publishers: London, 1988; pp 98-110. (2)Beenackers, A. A. C. M.; Bridgwater, A. V. Gasification and pyrolysis of biomass in Europe. In Pyrolysis and Gasification; Ferrero, G. L., et al., Eds.; Elsevier Applied Science Publishers: London, 1989; pp 129-157. (3)Mudge, L. K.; Baker, E. G.; Brown, M. D.; Wilcox, W. A. Bench scale studies on gasification of biomass in the presence of catalysts. Final report, Pacific Northwest Laboratory, Battelle Memorial Institute, Contract DE-AC06-76RLO-1830, Richland, WA, November 1987, PNL-5699. @
0887-0624/94/2508-1192$04.50/0
There are two advantages apparent in the use of catalysts for the pyrolytic gasification of biomass. First, the gasification reactions can be made to occur at much lower temperatures and with only small amounts of steam or carbon dioxide, if any, resulting in energy economies as well as the possibility of altering product compositions because of equilibrium relationships. Second, the catalyst, because of the greater reaction rates possible, may give a gas product more nearly at equilibrium yields and thus allow more accurate predictions of product composition to be made. In some early tests with nickel on alumina-supported catalysts4, we had observed that even in an inert gas atmosphere (nitrogen), high gas yields could be obtained in the surprisingly low temperature range of 500-650 " C . It is also an interesting fact that the hydrogen which could be produced from a pyrolytic biomass gasification plant of feasible size, say 1000 tonnedday, would be adequate for the hydrocracking or hydrotreating of about 15 000-30 000 bbVday of heavy petroleum refinery residue, tar sand stripped bitumen, or other heavy crudes or residues. This would appear to be one of the few instances in the energy field in which the feasible scale of operation of a biomass conversion plant can be matched to an economical operating level for a petroleum process. For these reasons, it was decided t o undertake more extensive investigations of catalytic gasification of biomass with the objective of producing maximum yields of synthesis gas components, that is, carbon monoxide and hydrogen, while at the same time not employing any auxiliary oxygen or steam production facilities and not diluting the gas product with inerts. The present report describes a part of this study in which a new (4) Garg, M.; Piskorz, J.; Scott, D. S.; Radlein, D. The hydrogasification of wood. Ind. Eng. Chem. Res. 1988,27,256-264. (5) Scott, D. S. Production of hydrocarbons from biomass using the WFPP. Final Report, Contract 06152-23283-8-6067, Renewable Energy Div., Energy, Mines & Resources Canada, Ottawa, Canada, 1990. (6) Scott, D. S.;Piskorz, J. The flash pyrolysis of aspen-poplar wood. Can. J . C h e n . Eng. 1982,60,666-674.
0 1994 American Chemical Society
Energy & Fuels, Vol. 8, No. 6,1994 1193
Catalytic Gasification of Biomass Chart I extractives, wt % lignin, wt % hemicellulose, wt % cellulose, wt % ash, wt %
3.0 23.5 20.0 49.1 0.46 96.1wt %, dry other substances, losses, analytical errors, wt % 3.9 100 wt % elemental analysis, wt % maf carbon 50.5 6.2 hydrogen oxygen 43.2 nitrogen 0.06 0.46 ash 4661 higher heating value, maf, caVg
catalyst was evaluated in a fluid bed reactor. The fluid bed reactor was chosen for this initial work because it allows continuous removal of the primary char formed from the pyrolysis of biomass, and possibly secondary carbon also.7
Experimental Section Materials Used. Biomass. All the gasification tests reported were done with a standard hybrid poplar wood obtained from the International Energy Agency. This wood was ground and dried to give a feed material with a -0.5 mm particle size and a moisture content between 4.8 and 5.8%. The analysis of this material as given by ScottS is shown in Chart 1. Catalyst. The catalyst used in this work was a stoichiometric NiAl204, prepared in our laboratory, with a nickel content of 33%. It was a green crystalline compound with a spinel structure and a bulk density of 1100 kg/m3. The structure of the fresh catalyst was determined by mercury intrusion porosimetry, which gave the pore size distribution for pore sizes above 8 pm. The bulk of the macropores were in the diameter range of 30-80 pm, with an average value of 45 pm. Apparatus. A process flow diagram of the experimental system is shown in Figure 1. For convenience, nitrogen or COZ was used to transport the wood particles continuously into the bench scale fluidized bed reactor at feed rates of 10-100 g h . Nitrogen entered the base of the reactor to fluidize the bed of catalyst particles (-250 75 pm). The process operated at atmospheric pressure. All solid, liquid, and gaseous products were collected, and a material balance was attempted. The concentrations of carbon monoxide and carbon dioxide were continuously monitored as the experiment progressed by an on-line infrared gas analyzer t o give an indication of any changes in catalyst activity. The product recovery system was designed t o permit a complete material balance to be obtained since each unit, as well as the connecting lines, could be disassembled and weighed before and after the experiment. The condensers and all the lines were washed with methanol at the conclusion of a run t o obtain the “methanol solubles” composed of tar and water. Analysis of gases was carried out by gas chromatography and water was determined by Karl Fischer titration. No tar analyses were normally required, because of the negligible amounts produced. A detailed description of the methods and procedures used in this “bench scale” pyrolysis unit as well as of analytical methods has been given by Scott et aL6s7 Product yields are reported as percent by weight of feed on a moisture-free basis. Gas compositions are reported as percent by weight of feed on a moisture-free basis.
+
(7) Scott, D. S.; Piskorz, J.; Radlein, D. Liquid products from the continuous flash pyrolysis of biomass. Ind. Eng. Chem. Process Des.
Dew. 1986,24, 581-588.
Results Effect of Activation Time. Initial tests in which the catalyst was reduced for 30 min in hydrogen at the operating temperature suggested on the basis of an oxygen balance that incomplete reduction was occurring. Therefore, a series of experiments were done in which the catalyst used had been given different activation times from 20 to 180 min at 650 “C. In these gasification tests the fluidizing gas was nitrogen at 650 “C, the initial catalyst charge was 50 g of nickel aluminate, and the runs were all 60 min in length. Results are shown in Table 1. The designation in Table 1of “char, coke, soot” includes all solid residues, where (‘char’’refers t o primary pyrolysis char, “coke”to apparent secondary carbon deposition on the catalyst, and ‘(soot” to secondary carbon in the gas product collection system or in the lines. Runs 148, 149, and 150 were done using catalyst from the same batch, whereas runs 122 and 123 were from an earlier batch. The negative values given for water yield mean that there was less water in the products than was fed as moisture in the wood. The apparent lower char yields a t low activation times may be misleading, inasmuch as the catalyst would continue to lose weight in these runs during the gasification test. Other differences are relatively small, but an activation time of 60 min appeared to give near optimal yields of CO Hg,and so this activation time was adopted as a standard for succeeding runs. It is noteworthy also that no measurable amounts of tar were produced in any of the runs, and the methane yield was small, about 2.1 wt % of feed (about 2.4 vol %, nitrogen-free basis) at 60 min activation time. Effect of Temperature. Gasification tests were carried out at comparable conditions a t 500, 600, 650, and 700 “C using the nickel aluminate catalyst. Results are given in Table 2. Run 59 was carried out with a catalyst activated for 60 min and with gasification run time of 45 min. Run 115 had a catalyst activation time of only 30 min and a run time of 150 min. Both runs 148 and 155 had activation times of 60 min and run times of 60 min. Initial weight of catalyst was 50 g in all runs. An additional test, G-42, with the same feed material but carried out in a noncatalytic fluidized sand bed, is also shown in Table 2 for comparison. The very high yield of tar water of 73.3% compared to run 59 shows the great effect of the catalyst on the nature of the thermal decomposition products obtained. At temperatures above 600 “C, little or no tar is produced from this catalyst. The higher temperature also gave a significant reduction in the yield of carbon and soot and an equally significant reduction in COz production with a corresponding increase in CO yield. Methane production is also reduced at higher temperatures to low values. An operating temperature of 650 “C may be near the optimal for this catalyst. Effect of Gasification Media. The influence of the fluidizing gas composition was studied in an attempt to improve the char gasification mechanism and so reduce the secondary carbon production, and also assist in determining the best operating conditions for an integrated process in which the recycling of fluidizing gas might be considered.
+
+
1194 Energy & Fuels, Vol. 8,No.6, 1994
Arauzo et al.
FEEDER
PROCESS FLOW DIAGRAM
rh t:
ANALYSER ICEMIATER CONDENSERS
REACTOR AND CYCLONE ASSCMOLY
-*
t
FURNACE
0
NITROGEN
%'$$$qqv co,lco
I/
'-
t
f
LIQUID PRODUCTS
I
TOVENT
@ '
Figure 1. Schematic drawing of bench scale gasification unit. Table 1. Experiments at Different Activation Times (IEA Poplar, -0.5 mm,N2 Atmosphere, 650 "C, NiAl204 Catalyst) run no. 150
moisture, % feed rate, g/min time of reduction with H2, min weight of catalyst in run, g FK, h-l= residence time, s yields, wt % mf feed gas water tar
solids (char,coke, soot) recovery gas yields, wt % feed H2
co co2 CHq
C to gas, wt % HdCO vol ratio vol % (H2 + CO), N2 free a
122
148
123
149
5.8 0.31 20 45.8 0.38 0.85
4.9 0.37 30 44.4 0.47 0.81
5.8 0.45 60 44.6 0.58 0.81
4.9 0.48 100 42.0 0.65 0.83
5.8 0.52 180 44.0 0.67 0.79
90.24 -3.47 0.00 10.67 97.44
89.57 0.00 0.00 7.84 97.41
87.23 -2.97 0.00 15.67 99.93
83.74 -3.36 0.00 16.98 97.36
84.24 -3.55 0.00 17.39 98.08
4.68 63.53 14.87 1.88 69.7 1.02 91.1
4.90 64.86 13.65 1.78 74.8 1.05 91.9
5.28 62.52 12.13 2.08 67.4 1.15 92.4
4.36 58.46 14.46 2.37 65.1 1.03 90.5
4.59 56.63 15.35 2.78 70.0 1.12 89.3
Feed rate to catalyst weight in bed.
Table 3 shows the results obtained using the nickel aluminate catalyst, with nitrogen, nitrogen-steam, carbon dioxide, and carbon dioxide-steam as the gasification media. Operating conditions established were as follows: temperature 650 "C; initial catalyst weight 50 g; catalyst activation time 100 min; run time 60 min. While results for gas yields from runs 123 and 127 are presented on the basis of wood as the feed, in run 129 and run 147 when C02 was also fed the feed was taken as equal to wood COS, and gaseous product yields were based on this quantity. Char yields are reported as a percent of the moisture free wood fed for both these runs, however. In Table 3, values are not given for product COSbecause the high concentrations of this gas did not allow accurate small incremental changes to be measured. According to the gas analysis for runs 112 and 127 a t 650 "C,steam in the inert fluidizing gas also gasifies char to some degree producing a lower char yield and some increase in CO content. Simultaneously, the water-gas shift reaction is taking place generating
+
Table 2. Experiments at Different Temperatures (IEA Poplar Wood,4.92%Moisture, -0.5 nun, N2 Atmosphere) run no. G42
feed rate, g/mi.n temperature, "C weight of catalyst in run,g F/C, h-l residence time, s yields, wt % &feed gas water tar solids (char, coke, soot) recovery gas yields, wt % feed H2
co co2
CH4 C to gas, wt % HdCO vol ratio vol% (Ha CO), Nzfree
+
59
-
0.46
497
500
115
148
155
0.63
0.80
0.49 600 43.4 0.65 0.78
14.0 12.2 61.1 14.7 102.0
72.93 -2.95 5.39 21.40 96.07
84.78 -1.62 0.37 14.94 98.44
87.23 -2.97 0.00 15.67 99.93
90.12 -3.68 0.00 14.11 100.55
-
3.56 24.47 37.77 6.46 50.4 2.00 68.5
4.20 53.73 19.70 2.98 64.9 1.08 86.2
5.28 63.52 12.13 2.08 67.4 1.15 92.4
4.27 68.25 10.20 2.16 71.9 0.86 92.6
(sand) 48 0.53 0.58
5.03 6.96 0.34
-
0.45 650 44.6 0.58 0.81
0.46 700 43.1 0.61 0.80
additional COS and H2. The small increase in CHq content also suggests that steam reforming of methane is not taking place at this temperature. This point is in agreement with preliminary thermodynamic calculations. The use of carbon dioxide or carbon dioxide-steam as a gasifying medium (runs 129 and 147)suggests that it is possible to accomplish a nearly complete gasification of secondary coke or soot which might deposit on the catalyst surface. For noncatalyzed pyrolysis of poplar wood, the normal primary char yield a t 650 "C is about 6% of the moisture-free wood feed7. Assuming that catalytic gasification occurs by a process of thermal fast pyrolysis of the wood followed by catalytic cracking and reforming of the pyrolysis vapors on the catalyst surface in the fluidized bed, the primary char yield would be largely unaffected by the presence of the catalyst. At the relatively low temperatures used (650 "C) and short reaction times, little gasification of these discrete primary char particles could be expected. However, secondary carbon deposits on the catalyst surface might be readily gasified by a reactive fluidizing gas. The results in Table 3 suggest that carbon dioxide or carbon
Energy & Fuels, Vol. 8, No. 6,1994 1196
Catalytic Gasification of Biomass
Table 3. Experiments with Different Gasification Media (EAPoplar, 4.9%moisture, -O.Smm, Nickel Aluminate Catalyst) run no. 123 117 127 129 147 0.48 0.51 0.58 0.57 0.45 feed rate, glmin 650 650 650 650 650 temperature, "C 42.4 43.8 44.8 weight of catalyst in run, g 42.0 43.4 0.65 0.68 0.78 0.75 0.74 FIC. h-l atmosphere ratio s t e a d w o o d , g residence time, s yields, wt % mf feed gas water
N2 NA
0.83
tar solids (char, coke, soot) recovery gas composition, % of feed a s is
Hz
co
coz CHI %C to gas
Nz-steam
N2 -steam
1:6.7 0.77
1:4.4 0.70
96.92 -14.18
Tr. 11.79 94.5
109.97 -22.18 0.00 12.20 99.99
4.36 58.46 14.46 2.37 65.1
3.58 56.87 27.20 4.66 73.9
6.14 68.53 27.17 2.70 82.2
k7.2 0.74
a
a
U
U
0.00 6.08
0.00 8.18
0.246 16.07 84.39 0.00 88.7
0.196 12.24 92.61 0.00 83.5
-
+ COz) feed.
-
+ C + (11/2)H2
(1)
+ 2H,O - 6CO + (13/2)H2(theoretical)
(2)
C&gO4 (wood) C,H904
0.74
-
Accurate yields not available because of high C02 content. Based on weight of (wood
+
COz-steam
NA
83.74 -3.36 0.00 16.98 97.36
dioxide-steam could effectively reduce the secondary carbon deposition to low values. The proportion of steam to wood used in runs 117, 127, and 147 was purposely kept low in order to simulate water addition only as moisture in the wood feed. Because of the possibility of recycling carbon dioxide from the gasification products in order to improve carbon utilization, the use of a fluidizing medium rich in carbon dioxide would appear to be a promising possibility.
coz
H,O (in wood)
5CO
-
C+H,O*H,+CO
Discussion
c + CO,
2co
(4)
(5)
Results from biomass obtained with the nickel aluminate catalyst suggest strongly that the overall gasification proceeds at lower temperatures (
