Energy & Fuels 1995,9, 45-52
45
C02 Gasification of Iron-Loaded Carbons: Activation of the Iron Catalyst with CO Shinji Tanaka, Takashi U-emura, Ken-ichi Ishizaki, Kouichi Nagayoshi, Na-oki Ikenaga, Hiroyuki Ohme,? and Toshimitsu Suzuki" Department of Chemical Engineering. Faculty of Engineering, Kansai University, Suita, 564, Japan
Hiromi Yamashita and Masakazu Amp0 Department of Applied Chemistry, Faculty of Engineering, University of Osaka Prefecture, Sakai, 593, Japan Received May 19, 1994@
Carbon dioxide gasification of various iron-loaded carbon species such as Yallourn coal char, carbon black, and activated carbon were investigated. Catalytic effect of iron was pronouncedly observed when less reactive carbons such as carbon black or activated carbon was gasified at 800 "C. Cyclic gasification in which a gasification agent and a product CO were fed alternately was carried out in order to enhance activities of iron catalyst. When appropriate intervals in the feeding of C02 and CO was selected ((202 75 s and CO 120 s), gasification rate increased considerably, even with the shorter effective gasification time. Moreover, instantaneous gasification of iron-loaded carbons occurred, if the iron-loaded carbons were treated with CO at 780 t o 800 "C for 120 s before introducing C02. Such phenomenon is called rapid gasification and is only observed when highly dispersed a-iron (elucidated by EXAFS)was produced on the carbon surface. The most significant rapid gasification was observed and about 70% of original carbon was gasified within 4 min at 800 "C, when 1.8 mmol of iron was loaded on 1g of carbon black (surface area 101 m2/g).
Introduction Development of coal gasification processes for clean fuels and chemical feed stocks is an important subject for efficient utilization of fossil energy resource. Most coal gasification processes are operated at a high temperature. However, for economic reasons, lowtemperature gasification has been paid much attention. To carry out coal gasification at a low temperature, use of catalyst was proposed. Alkali metal salts have been known to be highly active catalysts for coal gasification.1-6 It is well-known that mineral matter in coal act as catalyst during gasification r e a ~ t i o n . ~Iron - ~ which is one of the mineral matter is an active and abundant catalyst for coal gasification.lOJ1 Iron catalyst was reported t o show high activity in the + Present address: Toray Industries, Plastic Research Laboratories, Nagoya, Japan. e Abstract published in Advance ACS Abstracts, November 15,1994. (1)McKee, D.W. Chem. Phys. Carbon 1981,16,1-118. (2)Haynes, W. P.; Gasior, S. J.;Forney, A. T. Adu. Chem. Ser. 1974,
131,179-202. (3)Jhonson, J. L.Catal. Rev.-Sci. Eng. 1976,14,131-152. (4)Kayembe, N.; Pulsifer, A. H. F w l 1976,55,211-216. ( 5 ) Veraa, M. J.; Bell, A. T. Fuel 1978,57,194-200. (6)Wood, B. J.; Sancier, K M. Catal. Rev.-Sci. Eng. 1984,26,233. (7)Hippo, E. J.; Jenkins, R. G.; Walker, P. L. Jr. Fuel 1979,58, 338-344. (8)Otto, K.;Shelef, M. Fuel 1979,58,85-91. (9)Morales, I. F.;Garzon, F. J. L.; Pernado, A. L.; Castilla, C. M.; Utrilla, J. R. Fuel 1985,64, 666-673. (lO)Tuddenham, W. H.; Hill, G, R. Ind. Eng. Chem.. 1955,47, 2129-2131. (11)Ohtsuka, Y.; Tamai, A. Energy Fuels 1987,1, 32-36.
initial stage of gasification, but its activity decreased immediatelyunder the high partial pressure of steam.7 The reason for this is considered t o be that the catalytic cycle did not complete, due to the slower reaction of the reduction of higher-valent iron oxide to metallic iron or lower-valent iron oxides. Recently, use of iron catalyst coloaded with different metal species12-15such as sodium tetracarbonylferrate was proposed for the activation of iron catalyst.16 Dispersion and nature of iron species of iron-loaded brown coal char and its heat treatment conditions were elucidated by Tomita et aZ.17 One of the present authors illustrated the following redox cycle of iron catalyst loaded on Yallourn coal char, with pulse reaction technique by using isotope labeled W02.1*
+ CO, - FemOnfl+ CO Fe,O,+l + C - Fe,O, + CO
Fe,O,
(1) (2)
(12)Baker, R.T. K.; Chludzinski, J . J. Jr.; Sherwood, R. D. Carbon 1985,23,245-254. (13)Haga, T.; Nishiyama, Y. 2nd. Eng. Chem..Res. 1987,26,12021206. (14)Adler, J.; Huttinger, K. J. Fuel 1984,63,1393-1396. (15)Inui, T.;Otowa, T.; Okazumi, F. Carbon 1985,23,195-208. (16)Suzuki, T.; Mishima, M.; Kitaguchi, J.; Watanabe, Y. Chem. Lett. 1982,985-986. (17)Yamashita, H.; Ohtsuka, Y.;Yoshida, S.; Tomita, A. Energy Fuels 1989,3,686-692. (18)Suzuki, T.; Inoue, K.; Watanabe, Y. Energy Fuels 1988,2,673679.
Q887-Q624l95I25Q9-QQ45$09.QQlQ0 1995 American Chemical Society
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46 Energy &Fuels, Vol. 9, No. 1, 1995
Such catalytic cycle of iron prompted us to develop cyclic gasification process in which a gasification agent and an inert gas were alternately fed to the iron-loaded char bed. In this process, during an inert gas atmosphere, eq 2 was forced to occur in order to regenerate catalytic activity. Thus the enhancement of prolonged catalytic activity was obtained.lg In this work, we carried out the cyclic gasification by supplying CO instead of an inert gas. Carbon monoxide is a product of gasification of carbon. From practical viewpoints, it is highly desirable to use product gas for activating catalyst. For comparison, the gasification behavior of iron-loaded various carbon species under the steady flow of CO2 was studied. The effect of CO pretreatment before C02 gasification on the ironcatalyzed gasification was investigated. Characterization of iron catalyst was carried out with X-ray difiaction (XRD)and extended X-ray absorption fine structure (EXAFS).
Experimental Section Materials. Yallourn coal, YL (C 67.2%, ash l.l%, 100200 mesh), activated carbon, AC (ash 0.1%, Wako,Darco G-60, surface area 938 m2/g),and carbon black, CB (ash 4.1%, surface area 101 mz/g, Mitsubishi Kasei, No 30 B) were used. Gasification samples were prepared by following procedure: (1)iron nitrate (Fe(N0&.9H~0) was loaded onto coal and carbon materials by the usual impregnation method, and typical loading levels were 0.15-1.2 mmol of Fe/g of dry coal and 0.3-2.4 mmoVg of carbon, respectively (hereafter designated as -Fe). (2) Demineralization of Yallourn coal was effected by stirring the coal with hydrochloric acid (6 M) at 60 "C for 24 h. The coal was repeatedly washed with water until no chloride ion was detected in the filtrate and it was dried in vacuo. Demineralized coal was also impregnated with iron (0.6 mmoVg of coal) (-HCl-Fe). (3)Acetic acid (3 M) was also used for demineralization in order to examine the effect of the acid (-AcOH-Fe). (4) To examine whether a small amount of foreign additive affects the iron-catalyzed gasification, Fe(N03)~and mo3,Fe( N o & and NaN03, or Fe(NO& and Ca(N03)~were simultaneously impregnated on the HC1-demineralized coal. The amounts of loading of iron and additives were 0.6 and 0.12 mmol of metal per g of dry coal, respectively (-HC1-Fe-M; M: K, Na, Ca). Coal char was prepared by heat treatment at 800 "C at a heating rate of 25 "C/min from room temperature under an argon atmosphere for 30 min of holding time. The yield of the char is approximately 50% in all case. (5) Yallourn coal char was impregnated with iron (0.3-2.4 mmoVg of carbon) as above after COz pretreatment a t 800 "C for 15 min (-Char-Fe). (6) Active carbon was also impregnated with iron (0.3-2.4 mmoVg of carbon) as above after air oxidation at 350 "C for 1 h (-fir-Fe). The nomenclature in the abbreviations of samples used in this paper is as follows: carbon material followed by symbol which expresses method of treatment and followed by catalyst metal species and the amount of catalyst loading. For example, YL-HC1-Fel.2 denotes the hydrochloric acid demineralized Yallourn coal, impregnated with 1.2 mmol of irodg of carbon. Apparatus and Procedure. The schematic diagram of the experimental apparatus is similar to the previously reported one.lg Weight loss of the sample during gasification was (19)Suzuki, T.; Chouichi, H.; Naito, K.; Watanabe, Y.Energy Fuels 1989,3, 535-536.
measured by using a thermogravimetric balance (Shimadzu, TGA-50 ; further as TG). COz, CO, and Ar were used as a gasification agent, a reductant and a n inert gas, respectively, in a flow rate of 40, 20, and 30 ml/min. The sample (5 mg) was set into a pan of TG and heated in an Ar stream at a heating rate of 50 "C/min. All runs were performed at a constant temperature (800 "C) under a n atmospheric pressure. 1. In a steady flow gasification, reaction started with introducing COz at 800 "C. 2. Before COz gasification in certain runs, CO was supplied from 780 to 800 "C for 120 s and then the gasification reaction was carried out at 800 "C by supplying COZ. 3. Cyclic gasification was done by feeding COz and CO alternately through three-way solenoid valves which were operated by a timer after CO supplied from 780 to 800 "C for 120 s. The solenoid valve was periodically switched to introduce CO into the reactor. During this period gasification with COz was halted. The reaction was carried out with two different COz feeding periods with varying CO feeding time as follows: (i) COz feeding period 75 s, CO feeding period 20, 30,40, and 70 s; (ii) COZfeeding period 120 s and CO feeding period 300 s. Analysis of Catalyst. Iron species before and after gasification reaction were analyzed by powder X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS). The samples for XRD and EXAFS measurements were prepared by using a quartz reactor (7 mm i.d.) placed in a furnace in the same manner as for the gasification runs. Surface areas of catalyst loaded carbons were measured with BET single point method using nitrogen a s a n adsorbate at -196 "C.
Results and Discussion Effects of Iron Catalyst on the COZSteady Flow Gasification of Various Carbon Sources. As standard experiments, gasification of various carbons without or with iron catalyst was carried out. Nearly 100% carbon conversion was obtained with Yallourn coal char. Carbon conversion of AC and CB without catalyst after reaction of 150 min was 6.9 and 0.7 wt %, respectively, indicating very low reactivity of CB. The reactivities of all iron-loaded samples were much enhanced as compared to those without catalyst. AC and CB which showed very low reactivity in the gasification without added catalyst were rapidly gasified in the iron-loaded samples. Carbon conversion reached 90 and 83 wt % after 150 min for AC and CB, respectively. The average gasification rates of ironloaded samples for the initial 60 min in YL, YL-HC1, AC, and CB were 96, 45, 56, and 51%/h, respectively. The gasification rate of iron-loaded Yallourn coal (a)was the fastest among all the samples. As compared to the average gasification rates without iron, those of ironloaded ones were enhanced by factors of 1.4,10,28, and 222 for YL, YL-HC1, AC, and CB samples, respectively. These results clearly indicate that in the catalyzed gasification, catalysts provided the active center by transferring oxygen from metal oxide to carbon as shown in eq 2. If carbon species have sufficient amounts of active centers as in the case of Yallourn coal char, the effect of loaded-iron was not observed or only slightly observed. Similar results have been reported by Takarada et al., in the potassium-catalyzed gasification of 34 different rank coals.20 Figure 1illustrates gasification rates of various ironloaded Yallourn coal char with different loading levels (20) Takarada, T.; Tamai, Y . ;Tomita, A. Proc. Int. Conf: Coal Sci.,
Sydney 1985,273-276.
COz Gasification of Iron-Loaded Carbons
Energy &Fuels, Vol. 9,No. 1, 1995 47
125 I
7
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200
'
0.0
0.5
1.0
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Figure 1. 1. Effect of iron-loading level on the gasification rate of Yallourn coal char at various conversion. 800 "C. Fe 1.2 mmol/g of carbon. Arc02 = 30:40 mumin. (0) 20 w t %; (A)
50 wt %; ( 0 )70 wt %. 50
I
0.0
ri 0.5
1.0
1.5
2.0
I
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Iron loadlng (mmoVg ofcarbon)
Figure 2. Effect of iron-loadinglevel on the gasification rate of activated carbon at various conversion. 800 "C. Fe 1.2 ~30:40 mumin. (0)20 wt %; (A) mmol/g of carbon. h : c o= 50 wt %, (0)70 wt %. 45
1
..
0.0 0 . 5
1.0
1.5
2.0
2.5
Iron loading (mmoUg of carbon)
Figure 3. Effect of iron-loadinglevel on the gasificationrate of carbonblack at various conversion. 800 "C. Fe 1.2 mmoVg of carbon. k C O 2 = 30:40 mumin. (0) 20 w t %; (A) 50 wt %; (0)70 wt %.
at conversion of 20, 50, and 70 wt %. At all the conversion level, the maximum gasification rate was observed with a loading level of 1.2 mmoVg of carbon. On the contrary, with an increase in the amount of iron loading to 2.4 mmoVg of carbon, gasification rate decreased. The gasification rates of different amounts of ironloaded AC and CB a t carbon conversion of 20, 50, and 70 wt % are shown in Figures 2 and 3 (note that gasification rates of uncatalyzed samples were negligibly small as compared to those of catalyzed samples). As shown in these figures, the gasification rates increased more pronouncedly with increasing amount of iron than that observed for Yallourn coal. However, the absolute gasification rates of these samples were lower than the gasification rate of Yallourn coal in all cases.
0' 0
1
2
1
3
Iron loadlng (mmoUg of carbon)
Figure 4. Effects of iron-loadinglevel on the surface area of various carbons. BET surface area Nz at 77 K. (0) YL; (A) AC; (0)CB.
In the cases of AC and CB with low iron loading, conversion did not reach 70 wt % within the reaction time of 150 min. Tomita et al. discussed structures of iron species dispersed on brown coal at different loading levels and heat treatment conditions. With an increase in the loading level, decreases in the amount of finely dispersed iron species were reported.17 These results seem to imply that the dispersion of catalyst decreases with increasing loading level. Consequently, contact between the iron active center and carbon decreased, resulting in the decrease of the gasification rate at a higher loading level or at a higher conversion. Figure 4 shows the BET surface area of various ironloaded samples. The specific surface areas vary significantly with carbon materials. In the case of YL coal char, the specific surface area increased with the amount of iron loading up to loading level of 1.8 mmoVg of carbon and decreased with a further increase in the amount of iron loading. The reason for the increases in the surface area is considered to be the reaction of the carbon and iron oxide to give open pore structures during char preparation stage. On the contrary, in the case of AC sample, the specific surface area decreased with increasing the amount of iron loading to 1.2 mmoVg of carbon, and further increase in the amount of iron did not change the surface area of the sample. Decreases in the surface area of AC or Yallourn coal char at 2.4 mmol of Fe/g of carbon seem to indicate filling of pore structure with loaded iron. The specific surface areas of CB showed a constant value in all cases. Judging from the smaller surface area of CB, the pore diameter of CB would be larger than other carbon sources used here. Thus even a t a higher loading level the pore was not filled with the loaded iron species. In general, the absolute gasification rate of carbon species decreased with increasing burn-off, and in many cases the absolute gasification rate was corrected for the residual carbon to be gasified as shown in eq 3. Figure 5 shows the COa gasification rate of iron-loaded samples (Fe 1.2 mmoVg of carbon) a t 800 "C as a function of carbon conversion of the samples. In this figure, the gasification rate ( r )was calculated as follows:
r = (dX/dt)(l/(l - X))
(3)
where X is the fractional carbon conversion a t time t . The gasification rates of iron-loaded Yallourn coals (YL and YL-HC1) calculated from eq 3 exhibited the steady value up to carbon conversion of 50 wt % and that of
Tanaka et al.
48 Energy &Fuels, Vol. 9, No. 1, 1995
4.0
1
0.0 0
40 60 80 Carbon conversion (%)
20
100
Figure 6. Changes in the gasification rate of iron-loaded various carbons against carbon conversion. Fe 1.2 mmoVg of carbon. Ar:CO2 = 30:40 mumin. ( 0 )n,(0)YL-HCl; (A) AC; (0) CB.
iron-loaded AC or CB also showed constant values during gasification reaction. The constant values of the gasification rates indicate that the amount of effective active sites for gasification decreased in proportion to the carbon conversion. Thus, the apparent gasification rates of all the samples appear to decrease with increasing the carbon conversion. This is different behavior from the case of alkali metal catalyzed gasification.21p22 Cyclic Feed Gasification. We have previously reported that in the COz gasification of iron-loaded brown coal char, alternate feed of COZand an inert gas (Ar) provided great enhancement in the gasification rate. Gasification rate was measured by analyzing the produced gas at certain intervals, using a microflow reactor.lg In this work, we have examined activation of an iron catalyst using CO, and we have measured a weight loss of iron-loaded carbon species with TG. Thus we could observe more detailed information about the gasification behavior. On the contrary, due to the larger volume of the reactor, internal diffusion of gases prohibited prompt exchange of alternately fed gases. Figure 6 illustrates typical results of gasification profiles of iron-loaded Yallourn char by feeding CO and COZ alternately (c, d, e). As comparison, results of steady flow (a) and CO pretreated (b) gasifications are shown. CO pretreatment was carried out by introducing CO at temperature range of 780-800 "C for 2 min before COZ gasification. In this case sudden weight loss amounting to about 20% of carbon was observed immediately after switching over to COZfeed (b). Here after we call this phenomenon as rapid gasification. Such rapid gasification was never observed in the pretreatment with an inert gas (a). During the course of alternate feeding of CO and COZ, weight loss patterns are not smooth as with steady gasification. This is enlarged in the upper section of Figure 6. After sudden weight loss, weight increase and weight loss are periodically observed in accordance with CO and COZfeeding. In the course of COZ feeding period, weight increase was first observed due to the oxidation of lower valent iron oxide or metallic iron on the char (eq 1). Gasification of carbon with COZor oxygen transfer from iron oxides to carbon occurred simultaneously. During the (21) Wigmam, T.; Haringa, H.; Moulijn, J. A. Fuel 1983,62,185189. (22) Schumacher, W.; Muhlen, H. J.; Van Heek, K. H.; Juntgen, H. ~~~
Fuel 1986,65, 1360-1363.
0
20
40
60
Reactlon time (min)
Figure 6. COz gasification profile of iron-loaded Yallourn coal char under different gasification conditions. 800 "C. Fe 1.2 mmoVg of carbon. Ar:COz:CO = 30:40:20 mumin. (a) COZ steady flow gasification;(b) COz steady flow gasification after CO pretreatment. Cyclic gasification CO cycle time at (c) 20 8 , (d) 40 s, (e) 120 s.
CO feeding period, reduction of iron oxide of higher valency state seemed to occur, together with oxygen transfer from iron oxide to carbon. The net carbon weight loss is expressed in the difference of the weight between a bottom and the next bottom of the waves of the TG weight loss curve. The effects of CO feeding period on the gasification profile are shown in Figure 6(c-e). The behavior of gasification profile was affected by CO feeding period (further designated as CO cycle time). The apparent gasification rate a t CO cycle time of 20 s appeared to be lower than the steady flow gasification. Two reasons for the lower gasification rate are considered: (1)Due to the short feeding period of CO, higher-valent iron oxide could not be reduced to active form, and consequently this period was not effectively used for gasification. (2) The concentration of COZ was lowered by introducing CO gas to the reactor. If CO cycle time became sufficiently long, successive rapid gasification was observed (e) with highly activated iron catalyst. The overall gasification rate became higher than that of steady flow gasification. Carbon conversion against reaction time in the cyclic gasification is shown in Figure 7. In cyclic gasification, conversion increased in the initial stage of gasification. However, above conversion of 90%, carbon conversion of steady flow gasification was larger than that of cyclic gasification. Due to the rapid gasification a t the initial stage, contact between activated catalyst and carbon would decrease and consequently metallic iron tended to sinter t o a larger sized one. The effects of the amount of iron loading and CO cycle time on the gasification time to achieve 50 and 70% carbon conversions are summarized in Table 1. The reaction time required t o reach conversion of 50% became short with increase in the amount of iron loading. When iron loading is 1.2 mmoVg of carbon, the
COz Gasification of Iron-loaded Carbons
Energy &Fuels, Vol. 9, No. 1, 1995 49
a
n "
0
20
40
60
Reaction time (min)
Figure 7. Conversion vs reaction time in the cyclic gasification of Yallourn coal char. 800 "C. Fe 1.2 mmol/g of carbon. = 30:40:20 m u m i n . (a) COz steady flow gasificaA~:COZ:CO tion; (b) COz steady flow gasification after CO pretreatment. Cyclic gasification CO cycle time at (c) 20 s, (d) 70 s, (e) 120 s. reaction time was the shortest in all the cases. When the amount of iron loading increased to 2.4 mmol/g of carbon, gasification rate decreased as compared to 1.2 mmoVg of carbon loaded Yallourn char. This behavior is the same as that observed in the steady gasification. The effect of CO cycle time on the gasification rate was small when a small amount of iron was loaded but was significant a t a higher catalyst loading. Considerable decreases in the time required t o reach burn-off of 50 or 70%were observed when 1.2 mmol Fe was loaded to YL char with cycle times of 40-120 s. In the case of the 1.2 mmol Fe-loaded Yallourn char, the required time of 20 min for 50 w t % carbon conversion in the steady flow gasification was shortened to 9 min in the cyclic gasification with CO cycle time of 120 s. Thus, the cyclic gasification process with CO and COZ is the effective and promising method for activation of iron catalyst. In a cyclic gasification, it must be noted that the reaction time was expressed with the sum of CO and COZfeeding time (nominal reaction time); therefore real gasification time, that means COn feeding period, should be shorter than the nominal reaction time. The real gasification time was 78.9, 71.4, 65.2, and 51.7% of
nominal reaction time for CO cycle times of 20, 30, 40, and 70 respectively, when COZ feeding period is set at a constant time of 75 s. Similarly, the real gasification time was estimated to 71.4%of nominal reaction time, when CO cycle time was set at 120 s with C02 feeding period of 300 s. The effect of carbon materials on the cyclic gasification is shown in Table 2. Good to fair shortening in the reaction time was observed by applying the cyclic gasification at a proper CO cycle time. Increases in the gasification rate of iron-loaded carbon black are remarkable as compared to the steady flow gasification. This is mainly due to the rapid gasification discussed below. In the cyclic gasification of feeding C02 and Ar and feeding C02 and CO, the gasification rate was much enhanced in the initial stage of the gasification in both cases. Enhancement of the gasification rate with the cyclic gasification using CO was superior to that using Ar, since the rapid gasification was observed. Rapid Gasification. Sudden weight loss (here we call it rapid gasification) after introducing CO2 followed by CO treatment will be discussed in this section. Typical results are shown in Figure 8. Tomita et aLZ3 first observed a low-temperature fast gasification of nickel-catalyzed brown coal under steam. In their case the gasification occurred rapidly above 500 "C up to conversion of 80%followed by the devolitilization of coal. In the rapid gasification observed by us (Figure 8a), about 20 w t % of the sample was gasified within several tens of seconds. Thus the gasification rate was much larger than that found by Tomita, although the gasification occurred at higher temperature, 800 "C. This phenomenon was not observed when the gasification was carried out at 800 "C followed by pretreatment under an inert gas t o 800 "C (Figure 6 a). The catalytic activity of Co and Ni for the rapid gasification was examined. However, as shown in Figure 8, d and e, no such phenomena were observed. The reason why such rapid gasification was only observed for the iron-loaded case is not clear at present. reported the catalytic activity of Fe, Ni, Walker et
Table 1. Time (in min) Required To Reach 50 and 70 wt % Carbon Conversions in the COz Steady Gasification and Cyclic Gasification of Iron-Loaded Yallourn Chap conversion to 50% conversion to 70% amount of steady gasificn cyclic gasificn CO steady gasificn cyclic time (s) treatment with CO cyclic gasificn CO cyclic time (s) iron loading treatment with CO no yes (mmoVg of carbon) 20 30 40 70 120 no yes 20 30 40 70 120 0.3 30 31 25 27 29 43 42 46 47 53 55 55 69 81 0.6 24 25 23 23 22 29 28 37 36 44 43 40 57 48 1.2 20 16 34 14 13 13 9 31 31 32 19 22 67 19 2.4 21 58 27 25 29 56 22 36 38 114 52 50 58 128 a COz gasification at 800 "C, 5 mg; k C 0 ~ : C = 0 30:40:20 mumin. Table 2. Time Required To Reach 60 and 70 wt % Carbon Conversions in the COz Steady Gasification and Cyclic Gasification at Difference Cycle Time" conversion to 50% conversion to 70% steady gasificn cyclic gasificn steady gasificn treatment with CO CO cycle time (s) treatment with CO cyclic gasificn CO cyclic time (s) no yes 20 30 40 70 120 no Yes 20 30 40 70 120 Yallourncoal 20 16 34 14 13 13 9 31 31 67 32 29 29 22 active carbon 51 59 106 42 41 31 50 88 110 >150(69) 125 108 68 >150(55) carbon black 59 10 7 4 7 4 Co =- Ni was given. However, the differences are not so remarkable as to account for the occurrence of the rapid gasification only with Fe catalyst. Detailed studies on the pulsed gasification of Fe-, Ni-, and Co-loaded carbons are now in progress.26 This will shed light on the mechanism of group Vlll metal catalyzed gasification. For Yallourn coal char, only the sample containing 1.2 mmol Fe/g of carbon exhibited rapid gasification behavior. Rapid gasification was not observed for ironloaded Yallourn coal demineralized with hydrochloric acid. When iron and alkali metals or alkaline-earth metal were coloaded, these metals acted as promoters and these metals enhanced gasification rate, but they did not affect the rapid gasification phenomenon. When Yallourn coal was charred at 800 "C and then iron was impregnated, not significant but considerable rapid gasification was observed in the loading level of 0.32.4 mmol Fe/g of carbon. However, this phenomenon was not observed when acid-treated Yallourn coal char was impregnated with iron. The rapid gasification of iron-loaded AC was observed at higher iron loading level of 1.2-2.4 mmol/g of carbon. The rapid gasification of iron-loaded CB was most significant and even a t lower loading level of 0.3 mmol Fe/g of carbon is sufficient to see this phenomenon. These results suggest that existence of other mineral matter is not an important factor for the rapid gasification. (23)Tomita, A.;Ohtsuka, Y.; Tamai, Y. Fuel 1981 60,992-993; 1983,62,150-154. (24)Walker, P.L.;Shelef, M.; Anderson, R. A. Chen. Phys. Carbon 1968,4, 287-383. (25)Ohme, H.; Suzuki, T., to be published.
The carbon conversion during rapid gasification of various carbons was plotted against iron loading level in Figure 9. In the case of CB, with an increase in the iron loading level, conversion increased and the maximum carbon conversion was obtained a t 1.8 mmol Fe/g of carbon loaded sample. Approximately 70 wt % of carbon was gasified within 150 s. The carbon conversion of Yallourn coal char was 20% in the loading level of 1.2 mmol Fe. The carbon conversions of CO2 pretreated YL char followed by iron loading were lower in a wide range of loading level. The reason why only ironloaded CB was so efficiently gasified under rapid gasification conditions cannot be accounted at present stage. One possibility is ascribed to smaller surface area of CB as shown in Figure 4. Consequently, better dispersion of iron on the carbon surface could be achieved without plugging pores of the carbon. The phenomenon of rapid gasification which was found in this study is the new phenomenon which had never observed in the iron catalyzed gasification. It was possible to enhance the gasification rate repeatedly if iron-loaded sample was alternately treated with CO2 and CO (see Figure 6). Further improvement in enhancing gasification rate a t a higher conversion level is required. This idea can be transferred to a practical process by using a circulating fluidized bed reactor, where in the downcomer region activation of catalysts could be achieved. X-ray Diffraction. Figure 10 illustrates X-ray diffraction patterns of iron-loaded carbon black treated under different conditions. The iron-loaded sample before gasification reaction showed no sharp XRD peaks (a, b). f i r heat treatment under an argon atmosphere t o 800 "C, the iron species decomposed to exhibit peaks ascribed to FeO or Fel-,O and P-Fe203 (probably oxidized in air during measurement of XRD) (c). After CO pretreatment before C02 gasification, the iron species showed Fe& peaks and weak a-iron peak (d) due to the reduction of iron oxide with CO. It was expected that the a-iron would be produced by the reduction of iron oxide with carbon monoxide, but a-iron species was slightly detected in the CO treated sample by XRD. Probably this is ascribed to the good dispersion of a-iron species on the carbon black surface. In iron species examined after the end of rapid gasification, the sharp peak a t 28 = 57" would be
CO, Gasification of Iron-Loaded Carbons
Energy & Fuels, Vol. 9, No. 1, 1995 51
nFe-. I
a-Fe p-Fe203 (A'
0
2
4
Distance (A)
0
2
4
6
Distance ( A )
Figure 11. FTs of EXAFS of iron-loaded carbons. Sample: (A-C) CB-Fe 2.4 mmoVg of carbon; (D)YL-Fe 1.2 mmoVg of char; (E) YL-HC1-Fe 1.2 mmoVg of char; (A) heat treatment under Ar at 800 "C; (B-E) CO pretreatment at 780-800 "C; (C) COz gasification aRer CO pretreatment. a) 40
50
60
70
80
90
28
Figure 10. X-ray diffraction patterns of iron-loaded carbon black. Sample: (a) CB-Fe 1.2 mmoVg of carbon, (b-e) CB-Fe 2.4 mmoVg of carbon. Conditions: (a, b) Fe(NO)a-loadedcarbon without heat treatment; (c) heat treatment under Ar at 800 "C; (d) CO pretreatment at 780-800 "C; (e) COz gasification after CO pretreatment.
assigned to a-iron in the sample after the reaction with CO2 for 5 min (e). Possibly weak peaks assignable to P-Fe2O3 were observed as in the case of Figure 1Oc. Such assignments are partly consistent with the results reported on the in situ XRD analyses of iron loaded carbon in CO/CO2 mixed Since most of the diffraction peaks observed here were very broad and certain peaks appeared as overlapping peaks, assignments are tentative and not exclusive. EXAFS Spectra. Figure 11 illustrates the Fourier transforms of EXAFS spectra of iron-loaded samples. Parts A, B, and C in this figure correspond to the samples c, d, and e shown in Figure 10. EXAFS spectrum (Figure 11A) of the heat-treated sample which showed diffraction peaks assignable to FeO or Fel-,O and ,&Fez03 (Figure 1Oc) exhibited only absorption assignable to wustite (Fe1,O). This indicates that most of the iron species decomposed from Fe(N03)~were partially reduced to Fel-,O with carbon and exist in a finely dispersed phase, which was weakly detected with XRD analysis. After treatment of this sample with carbon monoxide for 5 min at 780-800 "C, wustite was further reduced to a-iron. Again a-iron was finely dispersed on the carbon surface well below the size of detection limit (2 nm) of XRD analysis. This stage is considered to be the same stage that rapid gasification takes place. Again, the a-iron peak was observed in the EXAFS spectrum (26) Ohtsuka, Y.;Kuroda, Y.; Tamai, Y.; Tomita, A. Fuel 1986, 65, 1476-1478.
of after C02 gasification for 5 min.27 This sample exhibited sharp diffraction peaks assignable to a-iron in the XRD (Figure 10e). Therefore, during rapid gasification, a-iron agglomerate to a large crystallite size due to the loss of carbon support by gasification. Fourier-transformed EXAFS spectra of iron-loaded Yallourn coal char (Fe 1.2 mmoVg of carbon) and acidtreated Yallourn coal char (HC1 treated Fe 1.2 mmoVg of char) are shown in Figure 11,D and E. Both of the samples were heat-treated under argon and CO at 800 "C for 5 min. Both spectra are quite similar but are slightly different in the appearance of very weak Fe-0 peak at 1.5 8, (D). Only Yallourn coal char loaded Fe onto original coal afforded rapid gasification. From these spectra it is difficult to clarify the reason why HC1treated iron-loaded Yallourn char did not undergo rapid gasification. We can safely say that well-dispersed a-iron species was a key species in the rapid gasification. However, the dispersion state of iron and contact between a-iron and carbon varies according to the carbon species and heat-treatment stage. Conclusion In the gasification of various iron-loaded carbons, alternate feed of CO and C02 (cyclic gasification) significantly enhanced gasification rate, as compared to gasification with steady feed of C02. The so-called rapid gasification was observed where up to 70% of carbon was gasified within a few minutes at 800 "C when iron-loaded carbon black was treated with CO at 780-800 "C for 2 min. XRD and EXAFS analyses suggest that the active form of iron species would be atomically dispersed a-iron species which are produced during CO pretreatment of (27) A reviewer suggested the possibility of existence of Fe3C in the
FT spectra of EXAFS shown in Figure 11 (B and C). This might be possible from the results of XRD (Figure 10d). However, the principal species observed in Figure 11 (Band C) would be a-Fe.
Tanaka et al.
52 Energy & Fuels, Vol. 9,No. 1, 1995
Fe(NO&loaded carbon black. The properties of the carbon surface affect dispersion state of loaded iron species on the carbon.
Acknowledgment. This work was carried out as a research project of the Japan Petroleum Institute com-
missioned by the Petroleum Energy Center with the subsidy of the Ministry of International Trade and Industry. Partial support of cooperative research fund from Kansai University to T. Suzuki and N. Ikenaga is acknowledged. EF940094W
