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Energy & Fuels 1996, 10, 980-987
Mechanisms of CO2 Gasification of Carbon Catalyzed with Group VIII Metals. 1. Iron-Catalyzed CO2 Gasification Hiroyuki Ohme† and Toshimitsu Suzuki* Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, 564, Japan Received December 13, 1995X
Mechanisms of iron-catalyzed CO2 gasification of various carbon materials were investigated. The investigation was conducted by using steady-state gasification with thermogravimetric analysis, 13CO2 pulsed reactions, temperature-programmed desorption (TPD), and powder X-ray diffraction (XRD) technique. Useful information obtained from the above techniques allowed us to elucidate reaction mechanisms. The activities and mechanisms of the iron catalyst were affected by carbon materials. Iron loaded on activated carbon or carbon black had high activity in the CO2 gasification showing “rapid gasification”. It was elucidated from the responses to the pulsed reaction that both steps in the oxidation and reduction of iron species proceeded very fast and the key step for carbon gasification was the oxidation step of iron metal in the redox cycle. It was also clarified by the TPD spectra and XRD that the active species is highly dispersed iron metal and the deactivated species are sintered iron and highly oxidized iron. For coal chars, in addition to the iron active site showing fast reaction, another reaction site showing slow reaction giving “satellite peaks” exists and a larger amount of this site was contained on coal char. In the steady-state gasification, only the fast reaction on highly dispersed iron metal proceeds in all carbons. The amount of active site for the fast reaction determines the gasification rate.
FemOn + CO2 f FemOn+1 + CO
(1)
also proved by Mo¨ssbauer observation.8 Zhang et al. suggested the contribution of redox cycle in the steadystate gasification.9 In a previous paper, one of the present authors reported the very high activity of Fe catalyst.10 About 70% of original carbon was gasified within 3 min at 800 °C. The appearance of high reactivity was affected by pretreatment condition and carbon species. These phenomena cannot be explained by the simple redox cycle described above. We have investigated the mechanisms of CO2 gasification of carbon with alkali or alkaline-earth catalyst by the combination of pulsed reaction and TPD technique and proposed the new mechanism of gasification involving rates of oxidation of a catalyst metal and reduction of metal oxide with carbon species.11,12 In this work, we applied these techniques to the investigation of Fe-catalyzed CO2 gasification.
FemOn+1 + C f FemOn + CO
(2)
Experimental Section
Introduction Coal gasification is an important process in producing clean fuels and a chemical feed stocks. In the present process, coal gasification is operated at a high temperature. From economical viewpoints, low-temperature gasification is desirable. The use of catalyst was proposed for low-temperature gasification to overcome slow reaction of carbon with H2O or CO2. Group VIII metals as well as alkali and alkaline-earth catalysts are effective catalysts. Among them, iron is the most practical catalyst for its activity and abundance. For iron catalyst, the following oxygen transfer mechanism was proposed.1-5 Hermann and Hu¨ttinger explained that the rate-determining step was the catalyst reduction step (eq 2).5
We also elucidated directly this redox cycle by using pulsed reaction technique,6,7 and the redox cycle was † Present address: Toray Industries, Plastic Research Laboratories, Nagoya, Japan. X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Walker, P. L.; Shelef, M.; Anderson, R. A. Chem. Phys. Carbon 1968, 4, 287. (2) McKee, D. W. Carbon 1974, 12, 453. (3) McKee, D. W. In Chemistry and Physics of Carbon; Marcel Dekker: New York, 1981, Vol. 16, p 1. (4) Kasaoka, S.; Sakata, Y.; Yamashita, H.; Nishino, T. J. Fuel. Soc. Jpn. 1979, 58, 373. (5) Hermann, G.; Hu¨ttinger, K. J. Carbon 1986, 24, 429. (6) Suzuki, T.; Inoue, K.; Watanabe, Y. Energy Fuels 1988, 2, 673. (7) Suzuki, T.; Inoue, K.; Watanabe, Y. Fuel 1989, 68, 626.
S0887-0624(95)00255-6 CCC: $12.00
Materials. Five carbon materials, activated carbon (AC) (Norit A), carbon black (CB) (Mitsubishi-Kasei #30B), Yallourn coal (YL), Wandoan coal (WD), and Hongay coal (HG) wee used with loading with Fe. Three coals (100-200 mesh) and AC were demineralized using 6 M hydrochloric acid and 48% hydrofluoric acid. The catalyst was impregnated onto AC, CB, and coals from aqueous solution of Fe(NO3)3 and dried at 80 (8) Furimsky, E.; Sears, P.; Suzuki, T. Energy Fuels 1988, 2, 634. (9) Zhang, Z.-G.; Scott, D. S.; Silveston, P. L. Energy Fuels 1994, 8, 943. (10) Tanaka, S.; Uemura, T.; Ishizaki, K.; Nagoyoshi, K.; Ikenaga, N.; Ohme, H.; Suzuki, T.; Yamashita, H.; Ampo, M. Energy Fuels 1995, 9, 45. (11) Suzuki, T.; Ohme, H.; Watanabe, Y. Energy Fuels 1992, 6, 343. (12) Suzuki, T.; Ohme, H.; Watanabe, Y. Energy Fuels 1994, 8, 649.
© 1996 American Chemical Society
CO2 Gasification of Carbon
Energy & Fuels, Vol. 10, No. 4, 1996 981 Table 1. Rates of CO2 Gasification of Activated Carbona run
Figure 1. Gasification profiles of Fe(NO3)3-loaded activated carbon at 800 °C; solid line: In CO2 flow from room temperature (type A, run 1), dashed line: in argon flow to 800 °C and CO2 flow from 800 °C (type B, run 5). °C under vacuum. Dried coals were heat-treated at 800 °C under argon (Ar) for 30 min. CO2 Gasification. CO2 gasification experiments were carried out with a thermobalance (Shimazu TG-31). The sample (ca. 15 mg) was heated in a furnace in Ar or CO2 flow (20 mL/min). The temperature profiles are shown in Figure 1. Two types of experiments, type A and B, were carried out. In type A experiment, the sample was heated in CO2 flow from room temperature. In type B experiment, the sample was heated in Ar flow from room temperature to 800 °C and maintained for a while at 800 °C. Then, Ar flow was changed to CO2 flow. The rate of gasification was defined as r ) (dX/ dt)/(1 - X), where X is carbon burnoff. Pulsed Reaction, TPD, and XRD Analysis. Details of the apparatus for pulsed reaction and TPD were published elsewhere.11,12 The apparatus was assembled by modifying a commercial gas chromatograph equipped with a quartz reactor between a gas injection port and analyzers. Prior to the reaction, sample was heat-treated to 1000 °C at a heating rate of 50 °C/min under helium. At a certain temperature, 13CO2 (1.0 mL ) 41 µmol) was injected and the effluent gas was analyzed by mass spectrometer and gas chromatograph. Successive pulsed reaction was done by injecting ten times of 0.25 mL (10.2 µmol) of 13CO2 every 5 s. TPD was carried out at a heating rate of 50 °C/min from room temperature to 1000 °C after 13CO2 pulsing and followed by rapid quench. The cooled sample after heat treatment of gasification was subjected to powder X-ray diffraction (XRD) analyses by using Cu KR radiation.
Results and Discussion CO2 Gasification Profiles. Figure 1 shows the CO2 gasification profiles of Fe(NO3)3-loaded activated carbon at 800 °C. In this figure, results of two types of reaction were shown. Solid line shows the reaction in CO2 flow from room temperature (type A). Dashed line shows the reaction after heat treatment in argon from room temperature to 800 °C, at which CO2 was introduced after heat treatment for 6 min at 800 °C (type B). In both cases, weight loss was observed below 200 °C and constant weight was observed at 200 °C. On increasing the temperature above 200 °C, gradual weight loss was observed in both cases. In type A reaction, gasification proceeded at an almost constant rate at 800 °C. In type B reaction, a large weight loss was observed at about 780 °C and gradual weight loss continued at 800 °C even under Ar flow. When CO2 was introduced, a very large weight loss was observed. However, weight loss was terminated shortly and a slight weight increase was observed. After this phenomenon, the reaction pro-
1 2 3 4 5 6 7 8 9 10 11 12 b
catalyst Fe
Na K Ca Ba none
temp (K)
type of reaction
600 700 750 800 800 850 850 800 800 800 800 800
Bc Bc Bc B A B A A A A A A
0%b 0.004 0.13 0.35 0.36 0.050 0.39 0.075 0.082 0.11 0.079 0.079 0.002
rates (min-1) 10%b 30%b
50%b
0.004 0.080 0.31 0.29 0.030 0.37 0.064 0.083 0.12 0.065 0.065
0.004 0.020 0.026 0.044 0.071 0.098 0.19 0.025 0.047
0.003 0.18 0.020 0.022 0.30 0.065 0.082 0.15 0.042 0.060
a CO 20 mL/min; sample 15 mg; catalyst 1.2 mmol/(g carbon). 2 Conversion. c Pre-heat treatment at 800 °C before reaction.
Figure 2. Responses to 13CO2 pulsed reaction of Fe-loaded AC: (a) reaction at 800 °C after heat treatment to 1000 °C, (b) reaction at 700 °C after heat treatment to 1000 °C, (c) reaction at 800 °C after CO2 gasification at 800 °C for 3 min. Sample, 50 mg; catalyst, 1.2 mmol/(g carbon); 13CO2, 41 µmol.
ceeded similar to that in type A reaction. This type of large weight loss was observed as described in the previous paper and called as “rapid gasification”.10 Total weight loss in type A reaction was smaller than that in type B reaction. The rates of CO2 gasification of AC loaded with Fe are shown in Table 1. Iron catalyst with pre-heat treatment showed much higher gasification rate in rapid gasification (run 4) than that of alkali or alkaline earth metal catalyst (runs 8-11). Without pre-heat treatment or after the rapid gasification stage, the rates with Fe were smaller than that with alkali or alkaline-earth metal catalyst. Rapid gasification was observed above 700 °C and the rates in rapid gasification were constant above 750 °C. High reactivity, effect of pretreatment, and deactivation of Fe catalyst are considered to be related to the reaction mechanisms. To investigate the reaction mechanisms, pulsed reaction and TPD were performed and the reaction mechanisms were proposed. Pulsed Reaction of Fe-Loaded Activated Carbon. Figure 2a shows responses to the 13CO2 pulsed reaction of the Fe-loaded activated carbon at 800 °C after heat treatment in He. Such conditions correspond to that of rapid gasification in type B reaction. In the
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pulsed reaction with 13CO2, the oxidation and reduction steps of the catalyst can be observed separately. The symmetrical shapes of unreacted 13CO2 response and no tailing pattern were observed. The responses of 13CO appeared as symmetrical one without tailing. The symmetrical responses and passing time of 13CO2 and 13CO were identical to those of pulsing 13CO without 2 sample. This shows that the oxidation of iron proceeded very fast. The responses of 12CO and 12CO2 profiles were also identical to those of 13CO and 13CO2 without tailing. The oxygen recovery described below was almost 100%.
oxygen recovery )
[12CO] + [12CO2] × 2 [13CO]
× 100
These results show that the catalyst reduction step is also very fast. Figure 2b shows the responses of pulsed reaction at reaction temperature at 700 °C for Fe-loaded AC after heat treatment in He. Symmetrical responses at 700 °C were similar to those at 800 °C. The amount of 13CO formed at 700 °C were smaller than that at 800 °C buty oxygen recovery was again nearly 100%. This shows that the oxygen transferred to Fe (eq 1) is the key step to determine reaction rate. In pulsed reaction at 600 °C, only a small amount of 13CO formation was observed. This is clearly related to the lower rate of gasification at 600 °C (Table 1). Figure 2c shows the responses of pulsed reaction at 800 °C of Fe-loaded AC after CO2 gasification at 800 °C for 3 min. Such conditions corresponded to that was observed after rapid gasification in type B reaction. The amount of 13CO was very small but symmetrical responses of all four carbon oxides were observed similar to those in Figure 2a. Responses of 12CO and 12CO2 did not show any tailing after 13CO production and oxygen recovery was nearly 100%. Both catalyst oxidation and catalyst reduction steps proceeded while 13CO2 passed through carbon bed. The responses of pulsed reaction without pre-heat treatment (with similar conditions in type A reaction) were identical to those of Figure 2c. The differences between Figure 2a and Figure 2c reveal the differences between rapid gasification and slow gasification. The activity for catalyst oxidation is the most distinct point. In the oxygen transfer mechanism, it was considered that catalyst oxidation step (eq 1) was faster than catalyst reduction step (eq 2) and catalyst reduction step was the rate-determining step.5 However, results of pulsed reaction described above show different aspect. The symmetrical responses and complete oxygen recovery regardless of reaction temperature show that the key step is catalyst oxidation step of iron on carbon. The most important factor in the rate-determining step would be the activity for the oxidation of iron. Temperature-Programmed Desorption and XRD of Fe-Loaded Activated Carbon. To investigate the reaction mechanisms in detail, TPD were carried out. Figure 3 shows the TPD spectra of Fe-loaded AC after 13CO treatment at 500 °C. No 13C containing gas was 2 observed. This denies the existence of a stable intermediate like MxOy-13CO which was observed for alkali and alkaline-earth metal catalysts.11,12 Large broad 12CO peak was observed at around 650 °C. This peak
Figure 3. TPD spectra of Fe-loaded activated carbon after pulsing 13CO2 at 500 °C under He flow. Sample, 50 mg; catalyst, 1.2 mmol/(g carbon); 13CO2; 41 µmol; heating rate, 50 °C/min.
is considered to reflect the catalyst reduction step in rapid gasification. The catalyst reduction step is considered to proceed above 600 °C. In addition to the peak at 650 °C, two sharp 12CO peaks were observed above 850 °C. These peaks were ascribed to the highly oxidized Fe species as described later. XRD spectra of iron-loaded AC prepared under various conditions were measured. The reaction conditions and identified species were summarized in Table 2. The high dispersion state of iron was maintained in the impregnated sample and in the sample after heat treatment below 500 °C. Figure 4a shows the XRD pattern after heat treatment to 1000 °C, and strong peaks of Fe3C were observed. Figure 4b shows the XRD pattern after heat treatment to 1000 °C followed by O2 treatment at 200 °C. Oxidized Fe species were not observed. By contrast, large peak of CO were observed at around 700 °C in the TPD of the same sample as shown in Figure 5. These results show that the observed species by XRD did not reflect the active species for gasification and seemed to be a deactivated species for gasification. Highly dispersed species which cannot be detected by XRD might be considered to be active species. To investigate the state of catalyst before reaction the TPD of Fe(NO3)3 loaded AC was carried out without preheat treatment (Figure 6). CO and N2 correspond to m/z ) 28, NO to m/z ) 30, N2O and CO2 to m/z ) 44, and NO2 to m/z ) 46. The desorption of NO (m/z ) 30) occurred below 300 °C and desorptions of m/z ) 28 and m/z ) 44 showed peaks at the same temperatures as NO. Production of NO2 (m/z ) 46) was not observed. The desorptions of nitrogen containing gases seemed to cease below 300 °C and the desorptions of m/z ) 28 and m/z ) 44 above 300 °C would be ascribed to CO and CO2. CO2 desorbed from 300 to 700 °C and showed a small peak at 750 °C. CO desorbed from 600 °C and showed large sharp peak at 760 °C. The large amounts of desorbed CO and CO2 show that Fe contains a large amount of oxygen atom after the decomposition of nitrate. Large CO peak at 760 °C reveals that the highly oxidized Fe species described above is stable to this temperature. Yamashita et al. investigated the structure of Fe species loaded on Australian brown coal by Mo¨ssbauer spectroscopy and extended X-ray absorption fine structure and detected finely dispersed FeOOH like species which was related to the rate of gasification.13,14 Our results also show the existence of highly dispersed higher valent Fe species containing large
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Energy & Fuels, Vol. 10, No. 4, 1996 983
Table 2. Species Identified by XRD at Various Reaction Stages for Fe-Loaded ACa
a
condition
chemical formb
1. iron nitrate loaded fresh sample 2. heat treatment to 500 °C 3. heat treatment to 1000 °C 4. heat treatment to 1000 °C followed by O2 treatment at 200 °C 5. heat treatment to 1000 °C after O2 treatment at 200 °C 6. CO2 gasification for 1 min at 800 °C in CO2 flow from room temperature
not detected not detected Fe3C(s) Fe3C(s) Fe3C(s) Fe2O3(w)
Catalyst 1.2 mmol/(g carbon). b s, strong; w, weak. Table 3. Rates of CO2 Gasification at 800 °Ca rates at 10% conversion (min-1) carbon
Ab
AC CB CBd YL YLe WD HG
0.045 0.016 0.025 0.013 0.015 0.003
Fe Bb 0.32 0.015 0.23 0.021 0.015 0.014 0.003
Bc 0.030
K Ab
Ca Ab
0.082 0.041
0.065 0.016
0.11
0.097
0.011 0.12 0.058
a CO 20 mL/min; sample 15 mg; catalyst 1.2 mmol/(g carbon). 2 Type of reaction. c After rapid gasification. d Pre-heat treatment at 900 °C. e Fe impregnated onto demineralized YL char.
b
Figure 4. XRD pattern of Fe-loaded activated carbon: (a) after heat treatment at 1000 °C, (b) after heat treatment at 100 °C followed by pulsing O2 at 200 °C. Sample, 50 mg; catalyst, 1.2 mmol/(g carbon); O2, 41 µmol.
Figure 5. TPD spectra of Fe-loaded activated carbon after heat treatment at 1000 °C followed by pulsing O2 at 200 °C under He flow. Sample, 50 mg; catalyst, 1.2 mmol/(g carbon); O2; 41 µmol; heating rate, 50 °C/min.
Figure 6. TPD spectra of Fe(NO3)3-loaded activated carbon without heat treatment before TPD. Conditions are the same as shown in the caption to Figure 3.
amount of oxygen. However, this Fe species is not the active species for gasification. In the pre-heat treatment in Ar, decomposition of the highly oxidized Fe complex (13) Yamashita, H.; Ohtsuka, Y.; Yoshida, S.; Tomita, A. Energy Fuels 1989, 3, 686. (14) Yamashita, H.; Tomita, A. Ind. Eng. Chem. Res. 1993, 32, 409.
proceeded and highly dispersed metallic Fe species were yielded. This highly dispersed metal species seems to be the active species in the rapid gasification. By XRD, weak Fe2O3 peaks were observed in type A reaction (Table 2). It is considered that in CO2 flow from room temperature (without pre-heat treatment in Ar) highly oxidized state was maintained at 800 °C so that the reaction with CO2 was slow. Pulsed Reaction and TPD of Fe-Loaded Other Carbons. Table 3 shows the rate of CO2 gasification at 10% conversion of Fe-loaded various carbon materials measured with TG. In type A reaction, the gasification rate was low for all carbons. In type B reaction, the rate of carbon black was high but those of coal chars were low. The rates of gasification were much affected by carbon species. Figure 7a shows pulsed reaction with 13CO2 and Figure 8a shows TPD spectra of Fe-loaded CB. In the pulsed reaction, remarkable responses of 13CO and 12CO were observed and such behavior was quite similar to those of AC. In the TPD, 12CO peak was observed at about 650 °C and sharp 12CO peaks were observed above 850 °C similar to the case of AC. The similarity of pulsed reaction and TPD of CB to AC shows that the same reaction mechanisms could be considered in both AC and CB. In the TPD of Fe(NO3)3-loaded CB without pre-heat treatment a sharp large CO peak was observed at 910 °C, which was 150 °C higher than that observed in the case of AC. This difference would be caused by the weak interaction between iron oxide and carbon, because of smaller surface area of CB.15 In the CO2 gasification, the rate of Fe-loaded CB was small after pre-heat treatment at 800 °C but large after pre-heat treatment at 900 °C (Table 3). This shows that the active species for gasification was produced after the decomposition or the reduction with carbon of highly oxidized Fe complex. Figure 7b-d shows the responses of 13CO2 pulsed reactions of Fe-loaded various coal chars. For YL char (Figure 7b) the responses of 13CO2 and 13CO were (15) Surface area of AC was 920 m2/g and that of CB was 90 m2/g.
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Figure 8. TPD spectra of Fe-loaded various carbons after pulsing. 13CO2 at 500 °C: (a) CB, (b) YL, (c) WD, (d) HG. Sample, 50 mg; catalyst, 1.2 mmol/(g carbon), 13CO2, 41 µmol; heating rate, 50 °C/min. 12CO
Figure 7. Responses to 13CO2 pulsed reaction of Fe-loaded various carbon materials at 800 °C: (a) CB, (b) YL, (c) WD, (d) HG. Reaction was conducted after heat treatment to 1000 °C. Sample, 50 mg; catalyst, 1.2 mmol/(g carbon); 13CO2, 41 µmol.
similar to the case of AC and CB. By contrast, the responses of 12CO and 12CO2 were much different. Small 12CO and 12CO2 peaks appeared in accordance to the responses of 13CO and 13CO2. Following these responses, second broad 12CO and 12CO2 peaks appeared as reported in previous papers, in which these second peaks were called satellite peak.6,7 The peak top of the 12CO satellite peak appeared at the same position as the 12CO2 satellite peak. In WD and HG chars, similar patterns to that of YL char were observed, although a large amount of unreacted 13CO2 was observed in these chars. In Fe-loaded coal chars, the positions of the first peaks were not varied when the reaction temperature was varied. However, the position of satellite peaks of 12CO and 12CO2 appeared shortly after the first peaks with increasing reaction temperature. Two different responses of 12CO and 12CO2 show existence of different reaction paths. Figure 8b-d shows TPD spectra of Fe-loaded various coal chars. In YL char, small 12CO peak at 740 °C, large
peaks at 810 and 850 °C and 12CO2 peak at 800 °C were observed. The large peak below 700 °C and sharp peak at around 900 °C of 12CO were not observed different from the cases of AC and CB. In WD and HG chars, 12CO peaks were observed at about 730 and 820 °C and the 12CO2 peak was observed at 820 °C. When the TPD was conducted after 13CO2 pulsing at 750 °C followed by rapid quench, only the peaks above 800 °C were observed, and in this condition satellite peaks were not observed in the pulsed reaction. The reaction site showing peaks above 800 °C in the TPD were considered to be identical to the site showing satellite peaks in the pulsed reaction. The similar patterns of first peaks in the pulsed reaction show similar active species on coal chars to those on AC and CB. However, the amount of active species seems to be small, judging from the small first peaks of 12CO and 12CO2 in the pulsed reaction and the small desorption peak below 800 °C in TPD on coal chars. For the appearances of satellite peaks, different reaction paths must be considered. The multistep reaction described below seems to be suitable for the formation of satellite peaks. slow
FeOx+1 + Cf 98 FeOx+1-Cf slow
FeOx+1-Cf 98 FeOx + CO
(3) (4)
where Cf indicates free carbon active sites. This multistep reaction was observed in alkali and alkaline-earth metal loaded cases.11,12 In these cases, the interaction between metal oxide and carbon surface
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Energy & Fuels, Vol. 10, No. 4, 1996 985
Table 4. Comparison of Gasification Rate with the Amount of Gases Produced in the Pulsed Reactiona carbon
gasificn rate at 10% conversn (min-1)
AC ACb CB CBb YL WD HG
0.29 0.020 0.23 0.011 0.020 0.012 0.002
a
amount of 12C containing gases in pulsed reaction 12CO (µmol) (µmol) total 12C (µmol) 2
amount of 13CO in pulsed reaction (µmol)
12CO
35.5 2.1 29.5 1.5 9.4 3.0 1.2
1.8 0.7 2.6 0.5 2.3 0.6 0.3
37.3 2.8 32.1 2.0 11.7 3.6 1.5
39.2 3.6 35.0 2.5 14.5 4.9 2.8
Catalyst 1.2 mmol/(g carbon). b After CO2 gasification for 3 min (after rapid gasification). Table 5. Comparison of Gasification Rate with the Amount of Gases Produced in the Pulsed Reaction of Coal Chars
carbon
gasificn rate at 10% conversna (min-1)
YLd YLe YLf WDe HGe
0.01 0.02 0.004 0.012 0.002
total amount of in the pulsed reactionb (µmol)
amount of 12C in the first peakc (µmol)
7.7 11.7 4.9 3.6 1.5
2.5 3.8 1.4 2.5 1.0
12C
a At 800 °C. b 12CO plus 12CO , 50 mg of char, at 800 °C with 2 41 µmol 13CO2 as pulse. c Except satellite peaks and tailing production. d Catalyst 0.6 mmol/(g carbon). e Catalyst 1.2 mmol/ (g carbon). f Catalyst 2.4 mmol/(g carbon).
Figure 9. Responses to successive pulsed reaction of Feloaded carbons: (a) YL, (b) AC. 13CO2, 0.25 mL × 10. Other conditions are the same as in Figure 2.
is weak and shift of oxygen from metal oxide to carbon surface seems to proceed slowly. When Fe was impregnated onto YL char, the satellite peak was not observed. Heat treatment to prepare char is considered to relate the formation of satellite peak. For example, agglomeration of catalyst or combination with heteroatoms contained in coal during heat treatment is considered to weaken the interaction between catalyst and carbon. Estimation of Reaction Mechanisms in the Steady-State Gasification. The reaction mechanisms of CO2 gasification for Fe-loaded carbons were estimated by using pulsed reaction and TPD. However, pulsed reaction is a non-steady state reaction and it does not reflect the steady state reaction perfectly. In Table 4, the rates of CO2 gasification of various carbons are compared with the material balances in the 13CO2 pulsed reaction. For AC and CB, the gasification rate was correlated to the amount of 13CO with oxygen recovery of approximate 100%. However, for coal chars, the total amount of 12CO and 12CO2 in the produced gases in the pulsed reaction does not correlate to the rate of steady-state gasification. A single reaction path was considered for AC and CB but two reaction paths would be considered for coal chars. In the pulsed reaction of coal chars, the amount of 12C-containing gases produced in the satellite peak was significant. To estimate the reaction mechanisms in steady-state gasification, successive pulsed reaction was conducted. Figure 9 shows the responses to successive pulsed reaction of Fe-loaded YL char and AC. In YL, the socalled 13CO overshoot was observed and the intensities
of 13CO peak gradually decreased. Correspondingly, the formation of 12CO decreased and 12CO2 increased. The total amount of oxygen in 12C-containing gas ([12CO] + [12CO2] × 2) was almost constant in every peak in spite of 13CO overshoot. After the last pulse of 13CO2, satellite peaks of 12CO and 12CO2 appeared. These results show that the second path giving satellite peak does not proceed in the presence of 13CO2. In AC, most of the injected 13CO2 reacted to give corresponding 13CO peaks. The corresponding peaks of 12CO to 13CO and no tailing production were observed. During successive pulsed reactions, peak sizes of 13CO and 12CO were in the same order within experimental error. The oxygen recovery was approximate 100% in every peak. These findings suggest that reaction in steady state seems to proceed as single pulsed reaction. In Table 5, the amounts of 12C-containing gas produced when 13CO2 passed through the carbon bed (produced in first peak) are shown for coal chars. This reveals the activity only for fast path because second path does not proceed in the presence of 13CO2 as observed in successive pulsed reaction. The amount of 12C through fast path exhibits good correlation to the rate of steady-state gasification as compared with the total amount of 12C in the pulsed reaction. A good correlation was observed not only among carbon species but among different catalyst contents. These results show that fast path on highly dispersed metallic Fe species correlates the gasification rate in steady state. For coal chars, it is considered that the amount of the reaction site showing fast path is small and rapid gasification is not observed. In our previous work, YL char showed rapid gasification without demineralization.10 YL coal contains small amounts of inherent minerals like Na, Ca, and Fe. It is well-known that coloading of alkali and alkaline-earth metals with Fe
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Figure 10. Responses to 13CO2 pulsed reaction of Fe-loaded AC at 800 °C: (a) after CO2 gasification for 3 min, (b) after CO2 gasification and followed by TPD to 1000 °C. Conditions are the same as in Figure 2.
Figure 11. TPD spectra of Fe-loaded AC after CO2 gasification at 800 °C for 3 min. Conditions are the same as in Figure 3.
enhances the reactivity of char.16-21 Inherent minreals in coal seem to increase the active site for fast path. Reactivation of Catalyst. Metallic Fe species shows high activity for CO2 gasification of carbon causing rapid gasification but the activity of iron gradually decreased during reaction. Many researches intensively studied the development of deactivation-free catalyst. Mixing of reducing gas like CO or H2 is attempted to maintain the metallic state of the catalyst.2,5,22 However, it is undesirable to use a large amount of reducing gas to maintain metallic state from the economical point of view. Now, we propose two methods of reactivation without using a large amount of reducing gas. The first method of reactivation is the heat-treatment method. Figure 10a shows the responses of pulsed reaction at 800 °C of Fe-loaded AC after CO2 gasification at 800 °C for 3 min. Judging from the large unreacted 13CO peak, the catalytic activity of iron species was very 2 low. Figure 11 shows the TPD of the above sample after CO2 gasification. A large sharp 12CO peak was observed at 880 °C. This pattern is similar to that of TPD of Fe(NO3)3 loaded AC (Figure 4). Figure 10b shows the responses of pulsed reaction after TPD. The amounts of 13CO and 12CO increased and catalytic activity was recovered. These results show that the catalytic activity was recovered after a large amount of 12CO production in TPD and the deactivation of iron catalyst occurred by its transformation to a highly oxidized state. The (16) Ohtuka, Y.; Tamai, Y.; Tomita, A. Energy Fuels 1987, 1, 32. (17) Haga, T.; Nishiyama, Y. J. Catal. 1983, 81, 239. (18) Baker, R. T. K.; Chludzinski, J. J. Jr.; Sherwood, R. D. Carbon 1985, 23, 245. (19) Alder, J.; Hu¨ttinger, K. J. Fuel 1984, 63, 1393. (20) Inui, T.; Otowa, T.; Okazumi, F. Carbon 1985, 23, 195. (21) Suzuki, T.; Mishima, M.; Kitaguchi, J.; Watanabe, Y. Chem. Lett. 1982, 985. (22) Ohtsuka, Y.; Kuroda, Y.; Tamai, Y.; Tomita, A. Fuel 1986, 65, 1476.
Figure 12. Responses to pulsed reaction of Fe-loaded AC at 800 °C: (a) 13CO2 pulsed reaction after CO2 gasification for 5 min, (b) 12CO pulsed reaction after the operation of (a), (c) 13CO pulsed reaction after the operation of (b). 13CO , 41 2 2 µmol; 12CO, 20 µmol.
peak top temperature of 880 °C observed in Figure 11 is 120 °C higher than that observed in Figure 4. Sintering due to the decrease in the surface area of carbon affected the reaction activity of highly oxidized iron species with carbon. The second method of reactivation of iron catalyst is the CO pulsing method. Figure 12a shows the responses of 13CO2 pulsed reaction of Fe-loaded AC at 800 °C after CO2 gasification at 800 °C for 5 min. A large amount of unreacted 13CO2 was observed and the catalytic activity of iron species seemed to be very low. Figure 12b shows the responses to the pulsing of 20 µmol of 12CO. A large amount of 12CO and a small amount of 12CO2 were observed. The amount of produced 12CO was 38 µmol and that of 12CO2 was 3.8 µmol, even though 20 µmol of 12CO was pulsed. This result shows that reduction of highly oxidized iron to the metallic state was achieved not with the pulsing CO but with the carbon on AC initiated by the introduced 12CO. Figure 12c show the responses of 13CO2 pulsed reaction after CO pulsing. The amounts of 13CO and 12CO increased and catalyst activity was recovered. In our previous work, the cyclic gasification of Fe-loaded carbon between CO2 and CO increased reaction rate and similar reactivation of iron catalyst was proposed.10 The mechanism of reactivation both by heat treatment and by CO pulsing seems to be essentially the same. In case of CO pulsing, the decomposition of highly oxidized Fe species is initiated by the reduction of a part of iron oxide with CO and it triggers decomposition of highly oxidized iron. In the case of heat treatment, small amount of CO production through oxygen transfer from oxide to carbon seems to induce
CO2 Gasification of Carbon
the decomposition of whole complex. In the earlier method to maintain catalyst activity, reduction of highly oxidized iron species to metallic iron species was conducted in CO or H2 flow or by mixing CO or H2 with reactant gas and required a large amount of these reducing gases. By contrast, carbon may reduce oxidized iron species by introducing a small amount of CO gas in our method. In addition, activation is recovered during a short treatment time. For example, fluidized bed reactor which is designed for carbon particle to pass through the reducing atmosphere or through the hightemperature region seems to be utilizable to keep high reactivity of Fe catalyst as mentioned in previous paper.10 Conclusion Iron-catalyzed CO2 gasification was investigated on various carbon materials and the reaction mechanism was elucidated in connection with catalyst activity. Iron species on activated carbon or carbon black exhibited high activity for CO2 gasification showing rapid gasification with pre-heat treatment. On these carbons, the oxidation of Fe with CO2 and reduction of iron oxide with carbon are very fast and the rate determining step seems to be the oxidation step of Fe.
Energy & Fuels, Vol. 10, No. 4, 1996 987
It is elucidated that the activated state of iron catalyst is highly dispersed metallic iron which cannot be detected by the XRD method and the deactivated state of iron catalyst seems to be sintered metallic iron and highly oxidized iron species. Highly oxidized iron would be decomposed to metallic iron with the treatment of small amount of CO at an elevated temperature and the catalytic activity could be recovered. For coal chars loaded with Fe, two different reaction paths were detected. One is the fast path that was observed on an activated carbon or a carbon black and the other process is the slow path providing satellite peaks. In steady-state gasification, only contribution of the fast path is prevailing in the iron-catalyzed gasification. The amount of active site for the fast path is larger on AC and CB than those on coal chars. Gasification rate of iron-loaded carbon is determined by the number of active sites in highly dispersed metallic iron. Acknowledgment. A part of this work was carried out as a research project of The Japan Petroleum Institute commissioned by the Petroleum Energy Center with the subsidy of the Ministry of Trade and Industry. EF950255B
