Energy & Fuels 1994,8, 637-642
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Steady-State Gasification of an Alberta Subbituminous Coal in a Microfluidized Bed Z.-G. Zhang,' D. S. Scott, and P. L. Silveston' Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Received August 3,1993.Revised Manuscript Received March 8, 1994"
Use of a fluidized bed is an important design option if advantage is to be taken of the lower operating temperatures that catalytic coal gasification offers. Catalytic gasification, however, has been conducted with fixed bed systems whose performance is often quite different from that expected from fluidized beds. Experiments were performed in a microfluidized bed with continuous overflow to compare rates of coal gasification or pyrolysis, and rates of formation of the product gas components with and without an impregnated Fe catalyst. A Forestburg (Alberta) subbituminous coal was used. Variables were bed temperature, coal feed rate, and catalyst loading. The Fe catalyst becomes active above 750 "C in the overflow fluidized bed regardless of catalyst loading, but the catalytic activity of the ash in the coal obscures the Fe contribution below 800 "C. Coal pyrolysis occurs as the coal enters the bed so it is a devolatilized char that is gasified. Methane found in the product gas arises only from coal pyrolysis. Under the conditions used, methanation does not take place over Fe. For this high-ash coal, an optimum catalyst loading was observed a t 3-5 w t % Fe based on the carbon in the coal.
Introduction One advantage offered by the use of a catalyst in coal gasification is that a lower temperature can be employed. Lowering temperature means, however, that ash must be removed as a solid implying that gasification on a commercialscale would have to be carried out in a fluidized bed or a circulating fluidized bed. Gasification in a fluidized bed provides better heat transfer than other gasifier designs, which is important if gasification is performed under endothermic conditions. A further potential advantage is that coal can be fed to the gasifier well below softening temperature because of the rapid mixing and heating that fluidized beds offer so that feed line blockagescan be avoided. A large literature is available on fluidized beds and there have been many applications to coal and coke gasification.l4 However, only a few studies of catalyzed coal gasification in fluidized beds exist.7~8This paper describes experiments on a fluidized bed operating with significant carryover, but well below gas velocities typical of fast fluidization. The work was undertaken to examine continuous catalytic gasification. Much of the catalytic gasification research reported in the literature has employed microbalances or fixed beds. Fixed bed experiments are transient because the char content of the bed changes with time and thus the product gas also varies. A second objective was to create a data base in + Current address: Research Institute of Chemical Reaction Science, Tohoku University, Sendai, Japan. * Author to whom correspondence should be addressed. Abstract published in Adoance ACS Abstracts, April 1, 1994. (1)Morris, J. P.; Keairns, D. L. Fuel, 1979,58, 465. (2)Patel, J. G. Energy Res., 1980,4,149. (3)Shires, M.J. Fuel, 1981,60,809. (4)Furimsky, E. Fuel Process. Technol. 1985,11, 167. (5)Goyal, A.;Rehmat, A.; Knowlton, T. M.; Leppin, D.; Waibel, R. T.; Patel, J. G . Fuel Rocess. Technol., 1987,17, 169. (6)Rubel, A. M.; Robi, T. L.; Carter, S. D. Fuel 1990,69,992. (7)Gallagher, J. E., Jr.; Euker, C. A., Jr. Energy Res. 1980,4, 137. (8)Tomita, A.; Watanabe, Y.; Takarada, T.; Ohtauka, Y.; Tamai, Y. Fuel 1985,64,795.
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order to evaluate the application of composition modulation to gasifi~ation.~ Mechanism and assessment of the catalytic contribution to gasification were not the main purposes of our investigation. Candidate coals for gasification are the low-rank, surface-mined coals, many of which have high ash levels. Specific objectives of the study were to compare the gasification and pyrolysis of catalyst impregnated coals with raw coals, determine the temperature at which catalytic gasificationbecomes important, and search for an optimal catalyst loading. Thus, the coals were not deashed prior to use. Iron was the gasification catalyst chosen for our modulation studygJObased on the experiments of Suzuki et al." Thus, it was the catalytic system used in this investigation. Iron is of interest because it is potentially a low cost catalytic material. The catalyst has been discussed by several researchers, often together with an alkali promoter.l"l5 Herman and HuettingerlBdeal with the mechanism and suggest a redox pathway in which iron is reduced by the char and reoxidized by the gas phase. A later paper identified the iron species participating in the redox ~yc1e.l~Spreading of the metal to maintain contact with the shrinking carbon mass is dealt with by Baker.'8 Utilization of the redox mechanism was explored (9)Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Proc. Znt. Conf. Coal Sci. 1993a. (10)Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Roc. Znt. Conf. Coal Sci. 1993b. (11)Suzuki, T.; Chouchi, H.; Naito, K.; Watanabe, Y.Energy Fuels 1989, 3,535. (12)Suzuki, T.; Inoue, K.; Watanabe, Y. Energy Fuels 1988,2,673. (13)Suzuki, T.; Inoue, K.; Watanabe, Y. Fuel 1989,68,626. (14)Haga, T.; Nishiyama, Y. Znd. Eng. Chem. R o d . Res. Deu. 1987, 26,1202. Jpn 1987,66, (15)Ohtauka, Y.;Hosoda, K.; Nishiyama, Y. J.Fuel SOC. 1031. (16)Herman, G.; Huettinger, K. J. Carbon 1986,24,429. (17)Furimsky, E.; Sears, P.; Suzuki, T. Energy Fuels 1988,2,634. (18)Baker, R. T. K. In Carbon and Coal Gasification;Figueoredo, J. L.; Moulijn, J. A., Eds.; NATO AS1 Series E Applied Sciences No. 105; Kluwer, Dordrecht, The Netherlands, 1986.
0 1994 American Chemical Society
Zhang et a1.
638 Energy & Fuels, Vol. 8, No. 3, 1994 Feeder L
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Figure 1. Flow sheet and schematic diagram of the feed, gasifier, sample collection, and analytical system.
by Suzuki et al.11 as mentioned above. All of the experimental work in the papers cited used either a microbalance or a fixed bed.
Experimental Section All experiments employed a Forestburg subbituminous coal [proximate analysis (by weight): moisture 18.9%,VM 31.9%, FC 33.3%, and ash 15.9%. Ultimate analysis (dry): carbon 59.8%,hydrogen 3.6%,nitrogen 1.2%,and oxygen 15.8% by difference]. Forestburg is a surface-mined,commercialcoal. The coal was received from the mine in a drum and stored under N2 until needed. Prior to use, the raw coal was ground in a ball mill and sieved to obtain a -35 by +SO US.mesh sample (size range 147-425 pm). This sample was dried under vacuum at 110 "C. Samples were also stored under N2. To prepare the Fe-loaded material, the sieved and dried sample was impregnated with an amount of Fe(NO& solution sufficient to form a dense, mudlike slurry. This mixture was thoroughly kneaded, then water was removed by evaporation at ambient temperature and drying at 110 OC under vacuum. The Fe loading was varied by the choice of the concentration of the Fe salt and samples containing from 0.1 to 5 wt % Fe based on the carbon content of the coal were prepared. The microfluidized bed built for this study was developed fiom a design for biomass pyrolysis.*g As shown in Figure 1, the system consists of a continuous, micro coal feeder which uses C& flow through a tube fitted with an inlet port at the vena contracta of a nozzle which sucks in coal particles from the fluidized feed container, the microfluidized bed, a collector to catch particle carryover, and a tar knockout train. Details of the micro coal feeder are given by Piskorz and Scott.lg Details of the microfluidized bed, furnace, and tar knockout train are contained in a paper by Smuk et al.20 An online HP 5890-11 GC equipped with a carbosieveS-I1column was used for product analysis; flow through the gasifier was measured by a volumetric, rotating vane, gas flow meter. Gasification and pyrolysis experiments were performed using a common procedure. The empty gasifier was brought to temperature with either C02 or N2 flowing at 1-1.3 X the minimum fluidizing velocity. This required about 1.5 h. N2 was used for the pyrolysis experiments, while C02 was the oxidizing gas for gasification. Leak checks were performed during warm up to ensure oxygen was not entering the flow system. Once the designated gasifier temperature was reached, the coal feed was started at a rate between 22 and 40 g/h. As indicated in Figure 1, the 1.5 mm (i.d.) annular feed tube extended to just above the distributor plate of the fluidized bed to ensure rapid mixing. Flow of COz through the annulus provided cooling to prevent softening and decomposition of the powdered coal feed. The char product accumulated until the bed height reached the (19) Piskorz, J.;Scott, D. S. Ind. Eng. Chem. Fundam. 1982,21,319. (20) Smuk, D.;Zhang, 2.-G.; Furimsky, E.; Suzuki, T.; Scott, D. S.; Silveston, P. L. In Proceedings of the C-JJARP Research Conference (Vancouver, 1990); Chambers, A. J., Watkinson, A. P., Silveston, P. L., Eds.; Alberta Research Council: Edmonton, Canada, 1992, 118.
overflow port. At this point, char overflow into the solids collector began. Steady state was achieved quickly thereafter and data collection was initiated. Accumulation required about 1.5-2 h, about half of a typical run duration. There was a short transient period after overflow into the collector started, but this had only a small influence on the carbon balances obtained. These were prepared from the gas flow rates leaving the bed, the GC analysis, the weight of char and tar collected, and the char analyses. Carbon conversion was based on the difference between the weight of coal fed and the weight of char collected, while gas production rates used the GC and flow meter data. The measured conversion and production rates characterized the continuous, steady-state performance of the gasifier. The column and gas chromatograph were capable of measuring ethane, ethylene, and propane to a detection limit of 0.05 % (TCD detector), but none of these possible products were observed. Helium was used as the carrier gas for the analysis of all components except Hz. To measure this important product, the carrier was switched to N2. Changing carrier gases allowed a single chromatograph to be used for the experiments. The char collection vessel was equipped with a thimble type sample collectorthat could be activated after steady state was established. In thia way, a char representative of continuous operation was obtained for ultimate analysis. The achievement of steady state was monitored by observing the char overflow rate, the gas flow through the volumetric flow meter, and the composition of the off gas. Steady state was achieved minutes after overflow began and was maintained with just minor fluctuations through each 1-2 h experimental run. Key considerations in maintaining steady state were the gasifier temperature and the coal feed rate. Feed rate was measured before each run by discharging the coal samples into a weighing vessel instead of the bed and checked after the run by the weight loss of the fluidized coal container. Good agreement was obtained and feed rates were found to be constant once the over pressure in the coal container was adjusted. Bed temperature was controlled through the furnace which exhibited a variability of about f 2 "C. Reproducibility was checked by replicates run throughout the experimental program. Measurement reproducibilitywas about f10 % ,based on replicate experiments. Variables explored for raw and Fe impregnated coal were feed rate and gasification temperature. Fe loading was varied from 0.1 to 5 wt % based on carbon in the coal. A parallel sequence of pyrolysis experiments were also performed.
Results and Discussion Pyrolysis is a rapid process in the temperature range 700-900 "C used in our investigation. Consequently, it is the coal char that is gasifying in the fluidized bed. Nevertheless, pyrolysis contributes to the product gas and allowance for its contribution must be made. Figure 2a shows the product gas yields for raw coal vs fluidized bed temperature with Nz as the fluidizing gas while Figure 2b gives the yields for coal impregnated with 3 w t % Fe. Yields correlate with the coal feed rate and not the char accumulated in the fluidized bed so yields are given in the figures as moles per gram of carbon fed. The correlation confirms that pyrolysis occurs rapidly as coal enters the hot fluidized bed. As expected, Hz and CO yields per gram of carbon fed increase with temperature, while the yields of COz drops and that of CHd seems unaffected. In the 700-900 "C range studied, these yields appear to vary linearly with temperature. Comparison of the figures shows an effect of the catalyst on all the products. The reduction in methane production in the presence of the added Fe is significant at about 35% and does not depend on temperature. A decrease in the tar produced was also observed. Changes in the formation rates of the other
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Bed Temperature ("C) Bed Temperature ("C) Figure 2. Productgas yields for steady-statepyrolysis in a micro fluidized bed as a function of temperature: (a) raw coal, (b) coal impregnated with 3 wt % Fe.
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Bed Temperature ("C) Figure 3. Influence of bed temperature on conversion of a coal loaded with 3 wt % Fe for steady-state pyrolysis in a microfluidized bed.
products are about 10% and are within the reproducibility of the experimental data. Nevertheless, the changes are consistent in direction and in the effect of temperature. Clearly, the increase in H2 results from the decrease in CHI and tar. Increased cracking should raise the Hz and carbon formed. The increase in CO and decrease in COz are consistent with the reverse water gas shift reaction as the coals were fed bone dry. This change can also be explained by COz gasification of the char. We believe, however,that gasification makes just a small contribution because the COz content in the off gas is quite low. Much larger catalytic effects were observed by Tomita et al.8 for the fluidized bed pyrolysis of a Yallourn coal (47.3 wt 5% VM) loaded with a nickel oxide. Hydrogen production depended strongly on temperature, just as in
Bed Temperature ("C)
Figure 2, and in the presence of 12-14 wt % Ni was better than 2 X production when just raw coal was used. Tomita et al. observed much lower tar formation and attributed the extra Hz to Ni-catalyzed cracking of tar, light hydrocarbons, and some gasificationby HzO formed during pyrolysis. Figure 3 shows the variation in carbon conversion (as wt % ) with bed temperature for the Fe-impregnated coal. A linear conversion-temperature relation occurs just as in Figure 2. Results for raw coal fall on almost the same curve within the precision of the measurements. The figure does not show a catalyst effect, but then the plot is based upon weight data and the analysis of a sample of the carryover from the fluidized bed. These data are less accurate than the flow rate and gas composition measurements used to prepare Figure 2. Conversion at 900 "C exceeds slightly the volatile matter content of the coal, but this is not surprising in view of coal rank and the conditions under which volatile matter is determined. Gasification is a much slower process than pyrolysis in the temperature range studied so its rate is expected to depend on the carbon inventory in the fluidized bed. Figure 4a-c give the CO, Hz and CH4 production rates for raw coal and coal impregnated with 3 wt % Fe, respectively. Corrections have not been made for products formed by pyrolysis. Some variations of feed rate occurred for the data shown, but this was small. The mean feed rate for all gasification experiments was 33 g/h and the standard deviation was just 129%. COz gasifies the Forestburg char even at 700 "C. The CO production in Figure 4a at 700 "C is about 3 times the contribution from pyrolysis; by 800 "C CO from char gasification is 6 times the pyrolysis contribution. If the replicated measurements at 800 OC in Figure 4a are averaged, the CO production rates for coal impregnated with 3 wt % Fe fall above those for raw coal at all temperatures, even though the differences below 800 "C are within the reproducibility of the experiments. At 850 "C, the CO production rate difference exceeds 35%. Further results bearing on the question of the temperature at which the catalytic contribution of iron becomes evident are shown in Figures 6, 7, and 9, and will be discussed later. Allowing for pyrolysis accounts for the Hz production at 700 "C in Figure 4b, most of the production at 800 "C, but only 50-7096 at 850 "C. The difference between impregnated and raw coal at the latter temperature must result from Fe-catalyzed gasification. Methane from pyrolysis accounts for the production shown in Figure 4c at 750 and 800 "C and exceeds
Bed Temperature ("C)
Bed Temperature ("C)
Figure 4. Product gas yields for steady state gasification in a microfluidized bed as a function of temperature for raw coal and for Fe-impregnated coal: (a) CO, (b) hydrogen, (c) methane.
Zhang et al.
640 Energy & Fuels, Vol. 8, No. 3, 1994
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production at 700 "C. The figure shows about a 30% increase in CH4 formation with the 3 wt 5% Fe coal at 850 "C. This is at variance with Figure 2 if CH4 is produced only through pyrolysis. Because the question of CH4 formation in Fe-catalyzed gasification was of interest, the 850 OC observation was investigated further. All the methane production data collected are plotted vs coal feed rate in Figure 5. The pyrolysis and gasification data overlap and seem to confirm that methane is formed primarily in fast pyrolysis when coal is introduced into the bed of hot char. We attribute the difference shown in Figure 4c to error magnified by plotting CH4 production per gram of carbon in the fluidized bed when production per gram of coal fed appears to be the better normalization variable for methane. Nevertheless, a small amount of Fe-catalyzed CH4 formation during gasification can be ruled out at 850 "C because of reproducibility considerations. If the pyrolysis and gasification data shown in Figure 5 are averaged without regard for the coal feed rate, methane formation during pyrolysis is somewhat greater than it is during COn gasification suggesting the COZmay suppress volatile5 cracking. This conclusion would be supported by differences in the C/H ratio in the char carried over from the fluidized bed. Unfortunately, the C/H analysis of the chars showed too much variability due to the low H content in the char to identify what should be just a small difference. Figure 5 showsdecreasing CH4 production per gram of coal fed with increasing flow rate. A possible explanation for this observation is that the CHd formation during pyrolysis is not instantaneous. If this is the case, then partially devolatilized coal will be carried out of the bed and into the unheated overflow collector to a greater extent at the higher feed rates. Proof for this would be the C/H ratio in the overflow. As just mentioned, variability in the measured ratios prevented the observation of any trend. Problems with interpreting the methane results are due in some part to the low concentration of CH4 in the off gas. Figure 6 summarizes the temperature effect on gasification for runs at various Fe loadings. The data in this figure are taken from the flow rate and product gas composition measurements. Replicates are shown. Gasification rate rises rapidly with temperature for both raw coal and the impregnated samples. The emergence of a strong catalytic effect on rate at 850 "C is clearly shown. Addition of just 0.1 wt % Fe does not affect the gasification rate so the
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Bed Temperature ("C) Figure 7. Conversion ofa subbituminous coal in amicrofluidized bed as a function temperature and iron loading. point should be averaged with the two raw coal points. When this is done, there is a 35% increase between the rates for raw coal and a coal impregnated with 3 w t % Fe. When the 0.1 wt % Fe and raw coal results are averaged, the gasification data at 800 "C show rates that are higher with 3 and 1wt 5% Fe just as do the data at 850 "C where Fe clearly catalyzes gasification. This is no longer true at 750 OC forcing us to conclude that the addition of iron does not affect the gasification rate at this temperature. The data spread at 750 "C, thus, is an estimate of the experimental reproducibility. Based on the four data points, the average gasification rate is 0.36 g/(h g carbon) in bed and the sample standard deviation is 315% . The gasification rate is based on about 10 independent measurements. Even if error in individual measurements is small, the operation of error propagation places a high uncertainty on the gasification rate and occasional large variations in this rate must be expected. The sample standard deviation compares with &lo% estimated from replicate measurements. (21)Kunii, D.;Levenepiel, 0.Fluidization Eng. 1990.
Gasification of an Alberta Subbituminous Coal
Energy & Fuels, Vol. 8,No. 3, 1994 641
periodically switching the gas environment from C02 to 3 0 1 N2 and back with char samples of this coal show catalytic
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activity at 750 "C, and some activity even a t 700 "C.10 In these fixed bed experiments, however,the iron contribution is evident even in the presence of ash. In the only other fluidized bed study of catalytic gasification,'l albeit with steam, a higher rank coal and a Ni catalyst, activity was evident a t 500 "C. Effect of temperature on the rate of production appeared to be similar to Figure 4. The investigators comment that the gas-phase composition is controlled by the water gas shift equilibrium. The apparent insensitivity of the fluidized bed measurements as suggested by our difficulty in establishing the threshold for iron catalytic activity was investigated. I t was not possible to control precisely the coal feed rate of an experiment due to pressure variations and so feed rates varied within a f 6 range. This variation may also have occurred between experiments. Feed rate was also varied intentionally (but this data was not included in Figures 4, 6, and 7) to see if there was an effect on the solids holdup in the fluidized bed. Figure 8 shows only a small effect on the carbon inventory in the bed: the inventory appears to climb at 800 and 850 "C with rate above a feed rate of 35 glh. A distinct difference between the bed inventory under pyrolysis and gasification conditions is evident. This is likely due to particle size differences. Particle size in the bed is stable in pyrolysis but will decrease with the char residence time during gasification. Particle size effects on the solids fraction in fluidized beds are well established.21 Temperature differences are clearly evident in Figure 8. Carbon holdup is greater at the lower gasification temperatures and this is probably the source of the scatter in Figure 8 as indicated by a sample standard deviation of over 20% around a mean carbon amount. A particle size effect arising from lower gasification rates at 700 and 750 "C probably explains the temperature segregation of the data. Nevertheless, an examination of the gasification data in Figure 8 even after allowance for temperature show 1 6 1 5 % scatter so in our opinion measurement imprecision with respect to bed inventory appears to be the probable source of our difficulty infixing the threshold for catalytic activity rather than variation in the feed rate. Figures 4 and 6 plot rates per gram of carbon in the bed. Product gas measurements at different iron loadings indicate an optimal loading at 850 "C of between 1 and 3 wt % Fe based on the carbon content of the coal. This can be seen in Figure 9a. The CO results show a small maximum in the 800 "C data a t 3 wt 7% which suggests that the optimal loading may be temperature dependent.
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Open Symbols: Gasification 1 2 00 700%, 0 3o 750% V , 800°C 4 o i 850°C
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Conversion data, based on weight measurements and shown in Figure 7, basically agree with the gasification data given in Figure 6. Temperature has a strong effect on conversion for raw coal as well as Fe-loaded coal. The conversion improvement at 800 "C between raw coal and the coal loaded with 3 wt % Fe catalyst amounts to about 10% compared to about 30% at 850 "C. There is no increase in conversion at or below 750 "C. There is an effect of iron loading on gasification: coal with 3 w t % Fe gasifies more rapidly at 850 "C than coal with 1w t % and the latter coal more rapidly than coal with either 5 wt % or 0.1 wt %. The difference between 3 and 1 w t % Fe disappears at 800 "C. Suzuki et al." and Furimsky et al.,17 who used coal impregnated with Fe in the same manner as in this work, observed that a redox cycle involving Fe proceeds at 750 "C so that iron must be active at this temperature. Suzuki et al. used a Yallourn coal with just 1.1 w t % ash (db), while Furimsky et al. employed a Balmer, low-volatile bituminous coal with 9.6wt % ash (db). Both investigators used fixed beds in their experiments. It seems likely, therefore, that Fe is active catalytically at 750 "C, but that its contribution is overshadowed by the activity of the ash in the Forestburg coal. The Forestburg samples used for these experiments contained 15.9 wt %I ash on a wet basis which works out to an ash level of 33 wt % based on the carbon content of the coal. The high value is the ash level that would be encountered in the char contained in the bed. Following the response of CO production to , ; : ,
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Figure 9. Product gas yields for steady-state gasification in a microfluidized bed as a function of Fe loading and bed temperature for a subbituminous coal: (a) CO, (b) hydrogen, (c) methane.
642 Energy & Fuels, Vol. 8, No. 3, 1994
The Ha measurements indicate an optimal loading at 800 and 850 OC as well, but none a t lower temperatures. Lack of an optimal loading was anticipated because virtually all of the H2 measured arises from pyrolysis. We surmise that the apparent optimum comes from pyrolysis;possibly thereverse water gas shift reaction contributesto the effect. Methane data in Figure 9 are given as mols of CHI per gram of carbon fed rather than per gram of carbon in the bed. In agreement with our discussion of the CHI data in Figure 2, a and b, Figure 9c suggests increasing the Fe loading suppresses CH4 formation up to a level of 3 wt ?6 . This is consistent with the Hz increase in Figure 9b. However, the data point at 850 OC and 5 wt ?6 Fe suggests further suppression of CH4, but the corresponding point in Figure 9b shows the Hz production decreases. Existence of optimal metals loading in catalysis is well established and the effect is usually attributed to pore blockage at high loadings or to agglomeration of the metal
Zhang et al.
or ita oxide at lower loadings. The optimal iron level observed in our experiments is surprising in view of the high ash content of the Forestburg coal. Pore blockage seems unlikely because devolatilization occurs when coal is fed to the fluidized bed and this is a pore opening process. We speculate that fusion with the ash may be happening so that at the highest Fe level the activity of the ash is reduced. Pore blockage, however, could be important for the apparent decrease in CH4 production with increased Fe loading. Any retardation of the escape of volatile5 from the fusing and decomposingcoal particles would trap more methane precursors inside the particle and increase cracking of these species. Acknowledgment. The work reported was funded by a Strategic Grant from the Canadian Natural Sciences and Engineering Research Council.
