Energy & Fuels 1994,8, 943-946
943
Successive Pulsing of an Iron-Loaded Canadian Subbituminous Coal Char Z.-G. Zhang,? D.S. Scott, and P. L. Silveston* Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, Canada, N2L 3Gl Received December 15, 1993. Revised Manuscript Received April 18, 1994'
Successive, alternating pulses of COPand N2 fed to beds of iron-impregnated coal char confirm the existence of a redox cycle with iron catalysts. Additional experiments with chars from raw and demineralized coals establish that the catalytic gasification dominates and that the coal ash also functions through a redox cycle. Extending the duration of the COZpulse leads to a plateau in the rate of CO formation that has not been previously observed. A redox cycle involving easily oxidized and reduced iron oxide species is proposed as the mechanism for gasification in this plateau region of the pulse.
Introduction During the 1980'5, the redox mechanism for oxidecatalyzed gasification of carbon gathered much support. The mechanism aasumes that gasification proceeds through a cycle in which the oxide is reduced by the carbon in the char forming CO and then reoxidized either by H2O to yield hydrogen or by COZforming further CO. Various investigators14 suggested this mechanism for alkalicatalyzed gasification, while Herman and Huettinger6 proposed it for iron-catalyzed gasification. Using a pulse technique and labeled COZ,Suzuki et al.6 provided evidence for the mechanism with iron catalysts. Later, Mirssbauer observations7 added further evidence. Indeed, Furimsky et al.7 contend that the redox cycle is the dominant mechanism in steam and C02 gasification. Our topic in this contribution is the redox mechanism. Use was made of equipment built to explore the application of composition modulation to the catalytic gasification of coal chars.* Suzuki in an earlier paperg proposed and tested this periodic switching of the gas composition for gasification of a coal char. The novel feature in this contribution is the use of an in-line IR to followthe evolution of CO during a cycle of gas composition variation.
Experimental Section All observations were made on a Forestburg subbituminous coal (proximateanalysis (byweight): moisture 18.9%,VM 31.9%, FC33.9%,andash15.3%;ultimateanalysis(dry): carbon59.8%, hydrogen 3.6%, nitrogen 1.2%, and oxygen 16.1 %). Coal in the t Presentlywith the Institute for Chemical Reaction Science,Tohoku University,Sendai, Japan. * To whom correspondence should be addressed. *Abstract published in Aduance ACS Abstracts, May 15, 1994. (1) Yokoyama, S.; Miyahara, K.; Tanaka, K.; Tashiro, J.; Takakuwa, I. J. Chem. SOC. Jpn. 1980,6, 974. (2) Wood, B. J.; Sancier, K. M. Catal. Rev.-Sci. Eng. 1984, 26, 233. (3) Suzuki, T.; Miehima, M.; Watanabe, Y. Fuel 1985, 64, 661. (4) Saber, J. M.; Falconer, J. L.; Brown, L. F. Fuel 1986,65,1356; J . Chem. SOC. Commun. 1987,445. (5) Herman, G.; Huettinger, K. J. Carbon 1986, 24, 42-35, (6) Suzuki, T.;Inoue, K.; Watanabe,Y. Energy Fuels 1988,2,673-679. (7) Furimsky, E.;Sears, P.; Suzuki,T. Energy Fuels 1989,2,634-639. (8)Zhang, 2.-G.;Scott,D. S.;Silveaton,P. L. Submittedfor publication in Energy Fuels. (9) Suzuki, T.; Chouchi, H.; Naito, K.; Watanabe, Y. Energy Fuels
1989,2,536-536.
size range 147-425 gm was impregnated with an Fe(NO& solution. This was done by thoroughly kneading a paste of solution and coal powder, removing water by evaporation at 25 "C and f i i d y drying at 110 OC under vacuum. The Fe-loaded char was prepared from the dry coal by devolatilization in a microfluidized bed under Nz pressure. Measurements are reported for a 3 w t % Fe-loaded char based on the carbon content of the coal, name1yFe:C = 3100. Asmaller number of additional measurements were made with chars prepared in the same way from raw coal and a coal demineralized by washing with an HC1 solution. Sample size placed in the quartz gasification tube ranged from 250 to 550 mg depending on the furnace temperature setting. A sample was held in place in the 3 mm (i.d.) tube by quartz wool. The tube was mounted in a split casing radiant furnace that could be controlled to about f2 OC at the temperatures used. Sample temperatures were measured by a TC mounted adjacent to the tube and these ranged from 650to 900 "C. Timer-operated solenoid valves periodically switched flow through the tube from N2 to COZand back. A dual-wavelength IR was attached to the end of the tube and the signal from the IR was continuously recorded. Further details of the equipment are given in a companion paper.8 An experiment consisted of bringing the sample to operating temperature in N2 and then initiating periodic switching of the gas flow through the sample between COz and N2. IR absorptions at standard wavenumbers for CO and C02 were recorded continuously beginning with the first cycle. Variables in the experiments were the char sample (chars from raw, demineralized and Fe-loaded coal were used), run length, temperature, and exposure durations within a period. Space velocity was held constant. By measuring the gas flow rate and converting the IR readings to volume fractions, carbon conversionswere calculated and used to correct the absorption signals to a constant carbon sample size.
Observations Figure 1 shows CO evolution with time from an experiment in which the duration of the NZexposure was held constant at 120s and the exposure to C02 was changed successively on each cycle from 10 to 120 s. The temperature was 800 OC, and the pattern is for a char containing 3 w t % Fe. Salient features are a virtually instantaneous (ca. 5-10 s) rise in CO formation when the flow switches to COZfollowed by a sharp decline (ca. 1015 s) to a plateau provided the COZexposure duration is
0887-062419412508-0943$04.50/0 0 1994 American Chemical Society
944 Energy & Fuels, Vol. 8, No. 4, 1994
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Figure 1. CO signal versus time for periodic switching between COz and N2. Duration of the Nz exposure is constant, but the duration of the C02 exposure varies. Experimental conditions given on the figure. Peak heights are corrected to the initial weight of the Fe-loaded char sample. sufficiently long. Once the flow switches back to N2, there is a second sharp decline (ca. 20-25 s), if the plateau appeared. A smaller CO evolution peak follows during the N2 exposure. The slopes of the rise and decline in the first peak are similar and are attributable to mixing in the gasification tube and the IR cell. The slope of the decline following the plateau is about half that of the first decline and indicates continued CO evolution after C02 has been taken out of the feed. The mechanism is probably desorption. The area under the second peak is greater than the area under the first peak for the 10-s C02 exposures, but for the 60-sexposurethe area under the second peak becomes about 40% less than the first peak area allowing for the contribution of the initial rise from zero to the maximum and then the decay to the plateau for the first peak. With the 90-sexposure, the area under the second peak is about 75 76 smaller. The two peaks correspondingto durations of C02 exposure of 20 s or less are similar to the pulse and identified by responses observed by Suzuki et them as the reactions
-
+ CO, Fe,O,+l + C
Fe,O,
Fe,O,+l Fe,O,
+ CO
+ CO
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respectively. Shape of the second peak suggests that reduction, eq 2, is the slow step. On the other hand, the rapid rise in the CO signal after the introduction of C02 indicates reoxidation is rapid. The area under the second peak, occurring during NPexposure, increases by only 5 9% for each increase in the COz duration in Figure 1. Thus, since the CO, exposure is just 10 s in the experiment, reoxidation must be complete in just over 10 s at 800 "C. Suzuki et al. did not observe the plateau because of the small, 41 pmol C02 sample used in their experiments. A t the flow rates used in our experiments, the peaks in Figure 1 correspond to millimoles passing through the bed. Figure 2 furnishes further evidence for a relationship between the first and second peaks in CO evolution. This figure plots the IR signal as CO volume percent in the off gas as a function of time for the first peak as the COZ portion of the cycle begins, for the plateau region, and for the second peak after the switch to N2 has been made. The
I
l
Y
l
I
60 80 100 120 140 160 TIME (min)
Figure 2. CO concentration in the gasifier off gas as a function of time on stream for symmetricalcompositionswitchingbetween COz and Nz at 800 "C and 1bar with a cycle period of 120 s using a 3 w t % Fe char curve A, f i t peak in the CO evolution pattern; curve B, plateau; curve C, second peak.
CO concentration drops with time as gasification progresses and either the carbon surface area and/or the interfacial surface with the catalyst decreases. From the figure, it can be seen that the two peaks disappear at the same time. The first peak merges into the plateau. That these peaks disappear before gasification is complete suggests that it is the loss of interfacial surface that is responsible. Temperature variation of the response while varying the duration of the C02 exposure was not investigated. The temperature effect was explored, however, with variation of the length of the N2 exposure. Responses for a constant duration of C02 exposure are shown in Figure 3 . For the evolution pattern at 800 it is evident that the magnitude of and area under the initial peak is independent of the N2 exposure duration after a 60-s exposure. Thus, reduction of the active Fe requires less than 60 s. The pattern in the upper portion of the figure suggests less than 30 s is needed at 850 "C and less than 15 s at 900 "C. The second peak during the N2 portion of the cycle cannot be distinguished and the reduction reaction now appears as tailing on the evolution pattern during the C02 exposure. CO evolution in the plateau region increases with successive pulses. We believe this is caused by a slow redistribution of the catalyst on the carbon surface. The increase is not observed below 850 "C. At 750 "C, the effect of N Pexposure is dramatic. About 240 s is needed to reduce the active oxide at this temperature. The second burst of CO evolution has disappeared by 700 "C. At 750 "C,a lag of about 120 s can be seen before the second CO formation peak appears. CO evolves over 120 s. The temperature dependence and the times suggest that reduction may be controlled by a surface diffusion step, possibly the spreading of the oxide on the carbon surface, or, assuming reduction to iron is possible, the rate-controlling step may be diffusion of metal atoms from the oxide cluster exposing oxide which can then react with the carbon of the char. The presence of a-Fe (martenistic) and y-Fe (austenitic) during char gasificationwith H2O and C02 above 700 "Cand in graphite gasification has been claimed1&12using XRD and Miissbauer techniques. Furimsky et a1.I observed these iron species as well using Mhsbauer spectroscopy.
"e,
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(IO) Ohtauka, Y.;Tamai, Y.;Tomita, A. Energy Fuels 1987,1,32-35. (11)Ohtauka, Y.;Kuroda, Y.;Tamai, Y.;Tomita, A. Fuel 1986, 65, 1476-1478. (12)Baker, R.T. K.;Chludzineki, J. J.; Lund,C. R.Carbon 1987,25, 296-303.
Successive Pulsing of an Iron-Loaded Coal Char 601
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Energy & Fuels, Vol. 8, No. 4, 1994 945 9°C
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Figure 4. CO signal at various temperatures versus time for periodic switching between COZ and Nz.Duration of the COZ exposure is constant at 60 a, but the duration of the Nz exposure varies. Experimental conditions are given in the figure. Peak heights are corrected to the initial weight of the demineralized char sample.
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z
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RAW COAL CHAR
20
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0 0
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Figure 3. CO signal at various temperatures versus time for periodic switching between COz and Nz. Duration of the COz exposure is constant at 60 a, but the duration of the Nz exposure varies. Experimental conditions are given in the figure. Peak heights are corrected to the initial weight of the Fe-loaded char sample. Although dropping the temperature to 700 "C causes the reduction peak to disappear, the magnitude of the initial peak on introducing COZ still depends on the duration of the NZexposure. Evidently, some catalyst reduction must proceed, but the CO evolution was below the detection limit of our IR unit. On further decreasing the gasification temperature to 650 OC, this peak becomes independent of the Nz exposure. The behavior for temperatures below 750 "C suggest that more than one chemical entity participates in the redox cycles. One possibility is that the oxide may be reduced to iron stepwise. This has been suggested by Ohtsuka et a1.l0J1 and Mossbauer spectra7 has established the presence of FeaOr and FeO (wustite) during COZexposure, and FeO and Fe during NZexposure. It is possible that ash, not iron, is responsible for or at least contributes to the CO evolution pattern. Our measurement gave an ash content of 5.7 w t % ,much lower than the 15.3% value provided with the samples. Even the lower measurement means that the char contains about 11 wt ?6 ash. The contribution of ash was tested by repeating the experiments with chars prepared from demineralized and raw coal. Results for the demineralized char are shown in Figure 4. Raw char results in Figure 5 as well as the patterns in Figure 4 confirm that the peaks are associated with the Fe addition. The magnitude of the CO formation peaks on introducing COZat 700, 750, and 800 "C in Figure 3 for Nz exposure durations where peak height is constant are 7X, 8X, and 1 l X the peak heights for raw coal at the same temperature. Conse-
TIME ( m i n )
Figure 5. CO signal at various temperatures versus time for periodic switching between COz and Nz.Duration of the COz exposure is constant at 60 a, but the duration of the Nz exposure varies. Experimental conditions are given in the figure. Peak heights are corrected to the initial weight of the raw char sample. quently, the large peaks seen in Figure 3 come primarily from iron impregnation. The CO evolution patterns for raw coal without iron addition in Figure 5 at 700-800 "C show the initial peak on introducing COZ and, at 800 "C, the second CO formation peak after Nz has begun to flow. This seems to indicate that ash catalyzes gasification via a similar redox mechanism. Observations with chars from acid washed coals (Figure 4) show that at 800 "C uncatalyzed gasification can occur. CO evolution plateaus can be seen which are independent of the Nz exposure duration. The plateau height was also independent of the COZexposure duration. Comparison of Figures 1,2, and 3 show that 3 w t 7% Fe increases the CO rate in the plateau region by 40X at 800 "C, 45X at 850 "C, and 50X at 900 "C. Figure 4 for char prepared from raw coal shows CO formation in the plateau region is greater for Fe-loaded char, but nevertheless the increase over char without Fe impregnation is not an order of magnitude. The increase in the CO concentration, that
946 Energy & Fuels, Vol. 8, No.4,1994
is the CO formation rate, is about 19% at 900 "C, rises to 29% at 850 "C, and then climbs slowly with decreasing temperature to 41 % at 700"C. The decreasing importance of iron addition with increasing gasification temperature reflects, we believe, (1) the growing contribution of noncatalytic gasification and (2) the relative contribution of CO formed in oxidizing the iron catalyst to its working state in the presence of C02 and in reduction by the char when C02 is not in the gas feed. In terms of the nominal CO concentration unit per 100 mg of carbon, the height of the CO formation peak after introducing C02 in Figure 3 does not change significantly between 750 and 900 "C. The ratio of peak height (and thus area) to plateau height (area) decreases from 2.6 at 750 OC to 0.83 at 800 OC, and to 0.16 at 900 "C. For char prepared from raw coal (Figure 5), the height ratios are 0.27 at 700 "C, 0.16 at 800 "C, and zero for 850 "C and above. It is tempting to associate the plateau region with steadystate gasification, but measurementsgiven in a companion papers indicate that the plateau with periodic feed gas switching is some 20-30% higher than CO formation at steady state. A feature of the IR signal patterns in Figures 3 and 5 for impregnated and raw coal chars is the tailing at 700 and 750 O C that begins some 15-20 s after C02 has been replaced by N2. It is our view that this is CO evolution from reduction. It suggests that there may be two or more redox cycles involving iron oxides during gasification; one of these cycles involves oxides which are rapidly reduced and reoxidized. CO evolution during reduction of this easily reduced oxide may be the tailing that can be seen in the patterns in Figure 3 at 650, 700,and 750 O C just after the composition change.
Discussion The CO formation peaks observed after composition switching have been described and interpreted previ0usly.~J3 This contribution adds further data on the 113) Suzuki, T.;Inoue, K.;Watanabe,
Y.Fuel
1989,626630,
Zhang et al.
influence of exposure duration, Fe addition, and temperature which reinforce the earlier interpretations of pulsing experiments, namely that the peaks indicate a redox cycle in which reoxidation of iron is much more rapid than reduction below 8W850 "C. At 900 "C, however, iron reduction by carbon in the char also proceeds rapidly. Rate limitation by a diffusion process probably accounts for the lower rate of the reduction step. The new observation in this contribution is the appearance of a rate of CO formation plateau for an extended C02 pulse which is not a steady-state condition, although it is not greatly different from steady state. It is our conjecture that gasification in this rate plateau proceeds through a redox cycle involving easily oxidized and reduced species, possibly through wustite and magnetite. This cycle is probably also responsible for steady-state gasification. We believe the dispersion of the iron phase on the char accounts for the difference between the plateau and steady-state rates. We would anticipate a slow decay of the plateau rate toward the steady-state rate as the C02 exposure duration lengthens. Unfortunately, the techniques available to us in this study did not permit the marshalling of support for our views. The importance of the catalyst reoxidation contribution to CO formation at gasification temperatures below 800 OC may explain the increase in the gasification rate observed in feed composition modulation experiments.9 A companion paper in thisjournalsextends the exploration of the application of feed composition modulation to catalytic gasification undertaken by Suzuki and coworker~.~
Acknowledgment. The work reported was funded from a Strategic Grant from the CanadianNatural Sciences and Engineering Research Council to P.L.S.and D.S.S.It was carried out as part of the Canada-Japan Joint Academic Research Program on Developing Advanced Processes for the Efficient Use of Coal. It is a pleasure to acknowledge consultation with Professor T. Suzuki, Kansai University, Osaka, Japan.
