Energy & Fuels 1995,9, 479-483
479
Iron-Catalyzed Gasification of Coal Char under Composition Modulation Z.-G. Zhang,? D. S. Scott, and P. L. Silveston" Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1 Received September 6,1994@
Experiments are described which attempt to exploit the redox or oxygen transfer mechanism of iron-catalyzed coal gasification by periodically alternating between an oxidizing and a reducing gas environment. A low-rank, Canadian coal was used and carbon dioxide was the oxidizing gas. A 15%increase in the yield of CO based on C 0 2 supplied compared t o the steady-state operation of the gasifier was observed a t 800 "C for a coal loaded with 3 w t % Fe using a cycle period of 60 s. The advantage for the modulation operation decreased with increasing temperature and became negligible at 900 "C.
Introduction Iron-catalyzed gasification of carbonaceous matter has been under study for at least four decades.1,2 Exploration of the iron mechanism began early, but progress became rapid after McKee3observed that it is a reduced or partially reduced form of the oxide that is active. Over the last decade, evidence has accumulated supporting iron oxide catalysis by a redox mechanism in which the oxide is reduced by the carbonaceous matter it is in contact with and then reoxidized by a gas-phase ~ x i d a n t . ~ -Several l~ contributions just cited show that the predominant state of iron on the carbon surface changes with temperature and composition of the gas p h a ~ e . ~ - l l JIndeed, ~ J ~ Ohtsuka et al.1° observe that the different iron states have markedly different catalytic activity. Consequently, process-dictated choices of temperature and gasifier feed composition may result in the presence of iron in a low activity state. Instead of gasifying under steady state, it might be advantageous to alternate between reducing and oxidizing environment so as t o maintain iron in its catalytically most active state. It may be possible as well t o exploit the apparent cyclic operation of the catalyst.
' Now with the Institute of Chemical Reaction Science, Tohoku University, Sendai, Japan. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, March 15, 1995. (1)Tudenham, W. H.; Hill, G. R. Ind. Eng. Chem. 1955,47,2129. (2)Prakash, S.;Shanker Ray, H. Fuel 1991,70, 17-23. (3)McKee, D.W. Carbon 1974,12,453-464. (4)Hippo, E. J.; Jenkins, R. G.; Walker, P. L. Jr. Fuel 1979,58, 341-344. (5) Tomita, A.; Takarada, T.; Tamai, Y. Fuel 1983,62,62-68. (6) Huettinger, K. J. Fuel 1983,62,166-169. (7)Adler, J.; Huettinger, K. J.; Minges, R. Fuel 1986,1215-1219. (8)Moreno-Castilla, C.; Rivera-Utrilla, J.;Lopez-Peinado,A,; Fernandez-Morales, I.; Lopez-Garzon, F. J. Fuel 1985,1220-1223. (9)Hermann, G.; Huettinger, K. J. Fuel 1986,65, 1410-1418. (10)Ohtsuka, Y.; Kurodea, Y.; Tamai, Y.; Tomita, A. Fuel 1986,65, 1476-1477. (11)Ohtsuka, Y.; Tamai, Y.; Tomita, A. Energy Fuels 1987,1,3236. (12)Suzuki, T.; Inoue, K.; Watanabe, Y. Energy Fuels lS88,2,673679. (13)Furimsky, E.;Sears, P.; Suzuki, T. Energy Fuels 1988,2,634639. (14)Yamashita, H.;Ohtsuka, Y.; Yoshida, S.; Tomita, A. Energy Fuels 1989,3,686-692. (15)Suzuki, T.;Inoue, K.; Watanabe, Y. Fuel 1989,68, 626-630. @
Several composition modulation experiments, that is, alternating between oxidizing and reducing conditions, were carried out by Suzuki et a1.16 who found that gasification rates could be elevated in this way. This paper extends the work of Suzuki and co-workers. The role of cycle period (z) and temperature on CO formation are examined and the magnitude of the rate improvement under composition modulation is assessed. Only symmetrical cycles are considered. In these, the duration of exposure to an oxidizing atmosphere is the same as the exposure duration to a reducing one. The application of composition modulation to heterogeneous catalytic reactions to increase catalyst activity or selectivity is w e l l - k n o ~ n . ~Besides ~ J ~ the Suzuki et al. paper just mentioned, this forcing technique has not been heretofore applied to gasification. Successive pulse methods, which are much like composition modulation, have been used on gasification by several research teams12J5J9-21to obtain mechanistic information.
Experimental Section The simplest means of exploring composition modulation is t o expose a char sample t o periodically changing gas compositions. Different cycling and operating conditions can be examined with fresh samples to make results comparable at the same carbon conversion. Suzuki et a1.I6 used this procedure in their earlier study of composition forcing. To carry out our modulation experiments, a gasifier was constructed from a 3 mm (id.) quartz tube, which was mounted horizontally in a split casing tube furnace. A weighed char sample was tightly packed into the center of the gasifier tube between quartz wool wadding so that the sample was situated at the middle of the furnace. Contents of the tube were weighed on completion of an experiment. An empty vessel, serving as an integrator, was placed between the gasifier and an IR Industries dual wave length infrared spectrophotometer (16)Suzuki, T.; Chouchi, H.; Naito, K.; Watanabe, Y. Energy Fuels 198Si3;535-536. (17)Silveston, P. L. Sadhana (India) 1987,10,217-246. (18)Silveston, P. L. Can. J. Chem. Eng. 1991,69,1106-1120. (19)DeGroot, W. F.; Shafizadeh, F. Fuel 1984,210-216. (20)Freund, H. Fuel 1986,65, 63-66. (21)Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Energy Fuels 1994, 8,767-771.
0887-0624/95/2509-0479$09.00/00 1995 American Chemical Society
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480 Energy & Fuels, Vol. 9, No. 3, 1995
Key:
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Figure 1. Schematic diagram of the cyclically operated tube gasifier. Table 1. Coal Analysisa ultimate (dry basis) (wt %) proximate (wt %) ~~
volatile matter fixed carbon ash moisture
31.9 33.3 1E1.9~ 18.9
carbon hydrogen nitrogen oxygen (by dim
59.8 3.6 1.2 16.1
As reported by Forestburg Open Pit Mine. Ash measurements showed 5.7 wt % ash in some of the samples used.
char sample and these ranged from 650 to 900 "C. An experiment consisted of loading a weighed sample into the tube, fxing it securely between quartz fiber wadding, bringing the sample to operating temperature in Nz, and then initiating periodic switching of the gas flow through the sample between COz and Nz. Measurement of the CO and the COz concentrations and the gas flow rate leaving the tube gave the rate of CO production and the yield per gram of sample. Yield is here defined as millimoles of CO produced per gram of char placed in the gasifier. Weighing of the sample after the run was used t o calculate carbon conversion. Steady-state measurements were made for comparison purposes using a 50-50 mixture of COz and Nz. Carbon balances were used to check the measurements. These balances closed reasonably well, although carbon recovered in the gas consistently exceeded carbon loss from the sample. Differences were as much as 1520% at 800 "C, but fell to 10-18% at 850 "C, and to 7-14% at 900 "C. The same differences were found in steady-state operation so comparisons of the two operation modes can be made. The comparison, not the absolute rates or yields, was the object of our experiments. Furthermore, the CO production and carbon conversion data were analyzed separately and, as will be seen below, showed the same advantages of the periodic over the continuous mode of operation.
a
(IR) used to measured CO and COz in the gasifier off gas. This vessel back mixed gas emerging from the quartz tube thus providing a constant time average composition of the off gas which otherwise changed constantly because of modulation of the COz and Nz stream fed to the gasifier. Alternatively, the integrator could be bypassed in order t o monitor the time variation of the CO and COZconcentrations leaving the gasifier tube. The latter arrangement was used for steady-state measurements. A bubble meter, after the IR, gave the volumetric flow rate. The experimental system is shown in Figure 1. In the experiments the flow rate was maintained at 85 mL/ min by mass flow controllers in the COz and the Nz feed lines. Composition change was affected by three-way solenoid valves activated by a timer. Symmetrical cycling was used; that is, the sample was exposed to COz and t o Nz for equal durations. All observations were made using a Forestburg subbituminous coal, an abundant, surface mined coal that would be a candidate for gasification. Analysis of this coal is to be found in Table 1. Mine run coal was received in drums and stored under Nz. Just prior to use, samples were ground and then sieved to the 147 x 425 U.S. mesh size desired. All sieved samples were kept under Nz and refrigerated. Iron-loaded material was prepared by impregnation with a solution prepared from Fe(N0&9HZO. To make a sample containing a set weight percent iron, an appropriate amount of the nitrate solution was mixed with a weighed quantity of sieved coal to form a wet mass. The mass was thoroughly kneaded then water was removed by evaporation at ambient temperature and drying at 110 "C under vacuum. Because of the high oxygen content of the Forestburg coal, CO evolved during devolatilization. Thus, to avoid the devolatilization contribution during gasification, chars were used in place of coal. Both Fe-loaded and raw coal char were prepared by carbonizing the dry coal in a microfluidized bed at 900 "C under NZ pressure. This preparation procedure gave reproducible results. Measurements are reported in this contribution for a 3 wt 7c Fe loaded char based on the carbon content of the coal. Sample size placed in the quartz gasification tube ranged from 250 to 540 mg and depended on the furnace temperature setting. Larger samples increased the CO concentration and thus measurement accuracy, but size was limited by the range of the IR which could read only up t o 5 vol % CO. Temperatures were measured by a thermocouple inserted into the the
Observations The two parts of Figure 2 compare the time average CO yield and carbon conversion under composition modulation at t = 60 s with yield and conversion in steady-state operation at the same temperature and time average feed composition for gasification runs of different durations. Space velocity was the same in both experiments. Periodic operation of the gasifier always increases the CO yield and char conversion over what can be attained by operating at steady state. Results in the two parts of the figure appear to be identical but they are drawn from separate and independent measurements in the same experiments. Yields are based on IR and flow rate measurements, while conversion uses weight loss ones. The results confirm the pioneering observations of Suzuki et a1.16 Working only at 800 "C, Suzuki and co-workers found up to &fold increase in total CO yield, whereas in Figure 2a, the yield difference is just about 15%. The char conversion difference, at 20% in Figure 2b, is somewhat larger. Cycle periods (z) were the same, but Suzuki et al. used char obtained from a Yallourn (Australian)brown coal with an iron loading of about 2.5 wt % based on carbon in the coal. Figure 2 extends the results of Suzuki et a1.I6 by showing that the advantage for modulation with a cycle period (t)of 60 s depends on temperature. In Figure 2 , the CO yield increase drops from about 15% at 800 "C, to about 5% at 850 "C, and to 3% a t 900 "C. With respect to char conversion, the 20% difference at 800 "C falls to about 9% at 850 "C and to ca. 6% at 900 "C. One reason for the temperature effect discussed above is that the 60 min cycle period used in Figure 2 is not the optimum period at 850 and 900 "C. This can be seen from rate of CO production data shown in Figure 3. Data for all modulation experiments were collected with the integrator in place and are thus time average results. The figure shows the IR and flow rate data acquired in the experiments interpreted as the CO production rate. The effect of the integrator can be seen in the 60 min needed t o flush out the integrator after cycling has terminated and just N2 flows through the reactor. It is
Energy & Fuels, Vol. 9, No. 3, 1995 481
Iron-Catalyzed Gasification of Coal Char
____CYCLING MODE -STEADY MOO€ TIME ( m i n )
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CO yield increase through composition modulation is about 20% as mentioned above, whereas a t 850, the increase in CO yield is 14% at t = 30 s and may be higher if the optimal cycle period is less than 30 s. At 900 "C, the increase is 17% at z = 30 s, but the optimal cycle period is certainly less than 30 s as Silvestonls explains, so the improvement because of composition modulation is probably more than 17%. Char conversion results are shown in the bottom part of Figure 4. With the exception of the 800 "C data, the increases in carbon conversion through feed composition modulation exceed the increases in total CO yield. These independent measurements confirm conclusion drawn from the CO yield data. Figure 4 demonstrates that cycle period has a large effect on CO yield and char conversion. Increasing the
Zhang et al.
482 Energy & Fuels, Vol. 9, No. 3, 1995 170 (0)
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Figure 4. Influence of cycle period on performance in gasification runs of fixed duration at BOO, 850, and 900 "C: (a) total CO yield and (b) carbon conversion in the char.
period forces the cycling system toward quasi-steady state in which the gasifier switches between steadystate operation corresponding to a C02 feed and one corresponding to a N2 feed where gasification ceases. If the rate of gasification is proportional to the COz partial pressure, the time average quasi-steady-state total CO yield and the carbon conversion will be the same as the steady-state yield and conversion at the time average feed composition. These are given by the broken horizontal lines in the figure. Thus, quasi-steady state is approached at cycle periods greater than 180 s at 800 "C, while at 850 and 900 "C the limit is approached at periods greater than 120 s. Modulation, thus, at cycle periods greater than 120 or 180 s at 800 "C will not increase CO yield and char conversion above steadystate values. At the other extreme of cycling period, as t approaches zero, mixing in the gasifier smooths the partial pressure variation and the system behaves as though steady state exists. Mixing observations, however, suggest this high-frequency limit will be reached with cycle periods of about 1 s. Thus, the maximum in the 800 "C curve in Figure 4 demonstrates that there is an optimal period due to physical or chemical effects. Figures 3 and 4 show that the optimal period is about 60 s at 800 "C and evidently less than 30 s at the higher temperature. It is conceivable from the data in the
figure that improvements in total CO yield and char conversion of the order of 20% are possible at temperatures above 800 "C if short cycle periods are used. Figure 4 does not indicate that gasification proceeds faster at 800 than at 850 "C. Yields and conversion depend on the duration of sample exposure t o COz as well as on rate. Exposure was varied so that the effect of cycle periods at different temperatures could be compared. Thus, the exposure duration at 800 "C was greater than 3 x the duration at 850 "C. Experiments discussed above where carried out with symmetrical cycling. Removing the integrator between the gasifier and the IR makes it possible to follow CO evolution during a cycle. Results for these experimentsZ1show that higher CO yields and char conversions can be obtained using an unsymmetrical cycle in which the duration of the reducing environment exceeds the duration of the oxidizing one with COn flowing through the sample. We speculate that the improvement over steady state using short cycle periods and unsymmetrical cycles could reach 30% for the Forestburg coal chars. Results of Suzuki et al. for Yallourn coal char suggest improvements may be much higher for other coals.
Discussion An earlier paper22discusses the catalytic mechanism under feed composition modulation. This contribution is restricted to the engineering aspects. Nevertheless, mechanism must be introduced to discuss the optimal cycle period that has been found and the effect of temperature on this optimum. By following CO evolution from the char, Zhang et a1.22were able to observe at least part of the redox cycle through which it is believed iron acts as a catalyst in carbon gasification. They found that reduction of the iron oxide at 800 "C by the carbon of the char proceeds more slowly than its reoxidation by COz. Reduction occurs when only NZflow through the gasifier. Between 30 and 90 s exposure to Nz seems to be required to reduce iron oxide in the reduction part of the redox cycle. Thus, at t = 30 s, the catalyst is not fully reduced; whereas at t = 120 s, the reduction time is exceeded and gasification ceases. Both conditions reduce CO yield and carbon conversion. On the other hand, at 850 and 900 "C, the reduction step is much faster so that reduction is complete in an exposure of less than 30 s which forces the optimal cycle period below 60 s and probably below 30 s. There appear to be two reasons for the improvement under composition modulation. The first of these is that iron oxide in the reduced state reacts with COz, while in the oxidized state it reacts with carbon to produce CO during the alternating exposures to Nz and C02. Probably, this cycle is not catalytic but what is significant is that the cycle cannot take place at steady state. The iron oxide states involved may be similar to the ones proposed by Suzuki et a1.16 for iron-catalyzed gasification. Rapid evolution of CO when switching from CO2 to N2 and back againz2is evidence for the existence of this cycle. The second reason is that the catalyst is rendered more active for gasification with COz when periodically exposed t o a reduction environment. This (22) Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Energy Fuels 1994, 8, 943-946.
Energy & Fuels, Vol. 9, No. 3, 1995 483
Iron-Catalyzed Gasification of Coal Char
C har/As h ‘Discharge Figure 5. Schematic for a circulating solids fluidized bed system for coal gasification.
activity declines only slowly to the steady-state level in the presence of CO2. Evidence for this activation is given by Zhang et a1.22 Periodic operation of coal char gasification has been shown to increase CO yield and char conversion by at least 15%for the Forestburg subbituminous coal, and improvements up to 30% may be possible for this coal. How could these process advantages be realized on a commercial scale? The gasifying feed could be steam or C02 or a steam, oxygen, and C02 mixture depending on the gas composition desired and whether or not a C02 stream is available. We visualize a catalytic gasification process utilizing a two fluidized beds or a circulating fluidized bed system in which the riser reactor of the circulating system is fed the oxidizing mixture producing a H2-CO syn gas heavily contaminated with steam and unconverted CO2 if CO2 or 0 2 are included in the feed. These contaminants would be removed by cooling and scrubbing with water and CO2
recycled. The resulting syn gas serves as the reducing gas for the second fluidized bed. Figure 5 shows a schematic of.the two bed gasifier. Coal char circulates between the riser and the fluidized bed. Coal is fed to the latter. A carbon-rich ash stream is withdrawn from the fluidized bed and could be used as a fuel. We intend to build and test such a two-bed design.
Acknowledgment. The work reported was funded from a Strategic Grant to the senior authors from the Canadian Natural Sciences and Engineering Research Council. We are pleased to acknowledge the advice and guidance provided by Professor Suzuki of Kansai University. This was a project of the Canada-Japan Joint Academic Research Program on Finding Improved Methods for Coal Utilization. EF940166J
