Energy & Fuels 1988,2, 679-684 amounts of CO and C02 were desorbed in the TPD runs, by liberating CO as a satellite peak during pulsed gasification. The logarithms of CO produced (including the satellite peak above 750 OC) in the first pulsed gasification were plotted against the inverse of temperature to give a straight line.29 The apparent activation energy was estimated at 77 kJ/mol, indicating difficulty in reaction 3 as compared to reaction 8. Similar to the Na2C03-loadedcase above 700 "C (as shown by the dotted line in Figure 5 ) , a straight line having a different slope may also be drawn. The apparent activation energy was estimated as 1.3 X lo2 kJ/mol. This indicates that activation energy for the reduction of iron oxide by carbon is the same order of magnitude as that for the reduction of Na20 by carbon. At 700 "C, the amount of COP reacted exceeded the amount of CO produced, and repeated experiments gave similar results. The reason for this cannot be clarified yet, but the formation of cementite (Fe3C) is one plausible reason:30 5Fe + COz 2Fe0 + Fe3C (13)
-
(29)Since a freah surface of iron-loaded char was prepared by the heat treatment up to lo00 "C, the results obtained from the first C02pulse offered significant physical meaning.
679
Conclusion By the use of pulse gasification and the TPD technique, the oxidation-reduction cycle between metal and metal oxide species on the char surface is elucidated. Oxidation of iron or reduced-state iron oxide after heat treatment of char under an He flow can be observed by pulsed C02 gasification as the formation of CO (CO main peak in the on-line gas chromatograph). Reduction of iron oxide of higher valency by carbon can be observed by the CO satellite peak followed by the CO main peak in the COz gasification of Fe(N03)3-loadedcoal char. The active form of Na2C03-catalyzedchar gasification seems to involve the following redox cycle: Na + COz Na20 + CO Na20 C Na + CO
+
-
-
Acknowledgment. This work was financially supported by a grant for "Special project for energy" from the Ministry of Education, Science and Culture of Japan. Critical comments from reviewers of this manuscript are also appreciated. Registry No. Na2C03,497-19-8;Fe(CO&, 10421-48-4; Na20, 1313-59-3;COz, 124-38-9;Fenom,1332-37-2;CO, 630-08-0. (30)Ohtsuka, Y.;Kuroda, Y.; Tamai, Y.; Tomita, A. Fuel 1986,65, 1476.
TPD Study on Coal Chars Chemisorbed with Oxygen-Containing Gases Zhan-Guo Zhang, Takashi Kyotani," and Akira Tomita Chemical Research Institute of Non-Aqueous Solutions, Tohoku University, Sendai 980, Japan Received March 1, 1988. Revised Manuscript Received April 14, 1988
The surface complexes formed by the chemisorption of oxygen-containing gases (02,COz,and H20) on Morwell coal char were investigated by using a temperature-programmed desorption (TPD) technique. TPD patterns of the chemisorbed chars indicated that there are many types of oxygen complexes both on carbon and mineral matter (Ca and Fe), and the amount and feature of these complexes strongly depends on the adsorption conditions. The study on Ca-derived surface complexes revealed that oxygen easily moved around over carbon and Ca. The Ca-catalyzed gasification mechanism is discussed in relation to the present TPD results. The solid-state reaction between CaC03 and C does not occur during the gasification, but Ca catalyst functions as the medium for oxygen transfer.
Introduction The mechanism of gasification of carbonaceous materials with oxygen-containing gases (02,C02,and H20) has been widely studied. It is generally accepted that the first step in the reaction is the chemisorption of the gases on carbon to form surface oxygen complexes and that such complexes act as reaction intermediates. The oxygen-chemisorption study was carried out to estimate the amount of surface active sites. Walker and his co-workers' first determined this amount and termed it active surface area (ASA). They correlated it with the reactivity of carbon black in 0 2 and (1) Laine, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67,2030-2034.
0887-0624/88/2502-0679$01.50/0
emphasized the importance of this amount in the reaction kinetics. For other carbonaceous materials, the correlation between the rate and the ASA has also been demonstrated by many worker^.^-^ The temperature-programmed desorption (TPD) technique gives useful information about the surface complexes on carbon.&1° Surprisingly, however, most of these studies (2)Chen, C.-J.; Back, M. 'H. Carbon 1979,17, 495-503. (3)Radovic, L R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983,62, 849-856. (4)Ahmed, S.; Back, M. H. Carbon 1985,23,513-524. (5)Su,J.-L.; Perlmutter, D. D. AlChE J. 1985,31, 1725-1727. (6)Wigmans, T.;van Doorn, J.; Moulijn, J. A. Fuel 1983,62,190-195. (7)Causton, P.; McEnaney, B. Fuel 1985,64, 1447-1452.
0 1988 American Chemical Society
Zhang et al.
680 Energy & Fuels, Vol. 2, No. 5, 1988
have been conducted by using pure carbons, and essentially no work has been done on coal char. In the case of coal char, the amount and the nature of surface complexes would be greatly influenced by the presence of the inherent mineral matter. Our previous paper described a preliminary study on this aspect:" we examined TPD patterns of the surface oxygen complexes for partially gasified coal char and 02-chemisorbedchar and attempted to clarify the correlation between the amount of these complexes and gasification reactivity. The results are as follows: (1)TPD patterns of gasified char and O2-chemisorbedchar from various types of coals showed some sharp desorption peaks and/or broad desorptions; (2) the appearance of a sharp peak depends on whether the sample contains catalytically active mineral matter; (3) on the other hand, the broad desorptions were thought to originate from carbon substrate itself; (4) the larger the amounts of (C02 + CO) desorbed from char in the TPD experiment, the higher the reactivity of char in the steam gasification. Among the five coal chars used in that study, Morwell char showed the highest reactivity because of the presence of highly dispersed Ca. This paper reports in details the effects of the type of adsorption gas and adsorption conditions on the TPD pattern of Morwell coal char in order to elucidate the formation and decomposition of the surface complexes. Special attention is paid to the interaction between adsorption gas and inorganic species, particularly Ca, and the TPD data obtained are discussed in relation to the function of inorganic species as gasification catalysts.
Experimental Section Materials. The coal used in this study was Morwell brown coal. The proximate analysis (wt %) was as follows: moisture, 25.9; volatile matter, 38.2; fiied carbon, 34.1; ash, 1.8. The ultimate analysis (wt %, daf) was as follows: C, 67.9; H, 5.0; N, 0.5; S, 0.3; 0,26.3. The ash analysis (wt %) was as follows: MgO, 33; CaO, 28; FezO3,lO; SO3, 8; S O 2 , 6; Na20, 5; K20, 2; A1203,1; Ti02, 0.2; MnO, 0.1; P20s,0.1. The raw coal (32 X 60 mesh) was devolatilized in N2 at 1100 K for 30 min in a small fluidized-bed reactor as described in the previous paper." The ash content in the resulting char was 3.6 wt %. In order t o check the effect of Ca, Ca-loaded coal was prepared with demineralized Morwell coal by the ionexchange method.12 Apparatus. The apparatus used in this study basically consists of two parts: a reactor unit and a gas analysis unit. Steam gasification, chemisorption with 02,C02, or H20, and T P D were sequentially carried out in the reactor. This made it possible to carry out desorption experiments without taking out a sample to an ambient atmosphere. In the case of C 0 2 chemisorption, a trace amount of O2in C 0 2was removed by a deoxygenator. H20 chemisorption was performed under a saturated vapor pressure of water at 300 K. The gas analysis was made with a quadrupole mass spectrometer. A large portion of the gas evolved during TPD was evacuated with a rotary pump and a slight portion was introduced to the vacuum chamber through a leak valve. Although the results are presented in arbitrary units, they are corrected by considering the pattern coefficient and the ionization efficiency of each gas. Procedure. About 50 mg of devolatilized char was placed on a quartz basket, which was hung by a quartz spring. The sample was gasified with H 2 0 / N 2 (1/1) a t a total flow rate of 100 mL(STP)/min. After the char conversion reached 50 wt %, the sample was cooled down t o 570 K with flowing H20(N2 gas, followed by cooling to 420 K in N2. This partially gasified char (8) Ersolmaz, C.;Falconer, J. L. Fuel 1986, 65, 400-406. (9) Hermann, G.; Huttinger, K. J. Fuel 1986,65, 1410-1416. (10) Kapteijn, F.; Porre, H.; Moulijn,J. A. AIChE J. 1986,32,691-695. (11) Kyotani, T.; Zhang, Z.-G.; Hayashi, S.; Tomita, A. Energy Fuels
-1988.2. ___
136-141. --(12) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983, 1 - 1
82,382-394.
Temperature ( K )
Figure 1. T P D patterns of 02-chars chemisorbed at various temperatures: (a) 420 K; (b) 470 K; (c) 520 K; (d) 570 K; (e) 620 K. Key: (-) C 0 2 X 5; CO. (-e-)
was subjected to TPD up t o 1100 K. The results have already been reported." This TPD operation removed most of the surface oxygen complexes formed during the gasification and cooling stage. This char (referred to as "clean char") was used as a starting material throughout this study and was repeatedly subjected to a series of chemisorptions and desorptions. Three series of TPD experiments were performed. Procedure 1. After being cooled to a desired temperature in vacuo, the clean char was exposed to either 02,COz, or H20. The chemisorption of O2or C02was performed at 0.1 MPa for 60 min. On the other hand, H 2 0 was chemisorbed at 4 kPa for 30 min. These 02-, C 0 2 - and H20-chemisorbed chars, referred to as 02-, C02-, and H20-char respectively, were then outgassed t o 0.1 P a at the same temperature as in the adsorption run. They were subjected to TPD in vacuo at a linear rate of 10 K/min up to 1100 K. Procedure 2. The clean char was exposed to H 2 0 vapor of 4 kPa at 1100 K for 30 min. Some gasification reaction took place under these conditions. The sample was then cooled to 420 K in the presence of the H20 vapor and the product gas. This char was then outgassed a t 420 K and subjected t o TPD. Procedure 3. The clean char was cooled to 420 K in vacuo, and H 2 0 vapor a t 4 kPa was allowed to contact it for 30 min, followed by outgassing. Then O2 chemisorption was further performed on this H20-char for 60 min a t the same temperature. After outgassing to 0.1 Pa, this doubly chemisorbed char (abbreviated as H20/02-char) was subjected to TPD. A similar sample was prepared by reversing the order of chemisorption (02/ H20-char).
Results 02-Chars. The TPD patterns of 02-chars prepared at different temperatures are shown in Figure 1. The COz desorption pattern can be divided into two groups depending on the adsorption temperature. When O2 was adsorbed at 420,470, and 520 K, the desorption patterns were similar to each other. A broad desorption was observed in a wide temperature range, which started from a slightly higher temperature than the adsorption temperature and reached about 1040 K. In addition, a small peak was observed at around 720 K. On the other hand, the chars chemisorbed at 570 and 620 K showed a different TPD pattern. The desorption in the low-temperature region disappeared and a large and sharp peak appeared at around 840 K with a shoulder to higher temperature. In these cases, a small weight decrease was observed during adsorption, indicating that the combustion of carbon took place to some extent. A broad CO desorption in a wide temperature range up to 1100 K was also observed, and the amount of desorbed CO gas was fairly large. It would be noted that the intensity for COz desorption was multiplied by 5 in Figure 1.
TPD Study on Coal Chars
Energy & Fuels, Vol. 2, No. 5, 1988 681
.->I ' . 400
600
800
1000
400
Temperature (K)
Figure 2. TPD patterns of C02-chars chemisorbed at various temperatures: (a) 420 K; (b) 620 K; (c) 770 K. Key: (-) C 0 2 x 5; (-. -1 co.
600
800
Temperature
1000 (K)
Figure 4. TPD patterns of H20-chars chemisorbed at various temperatures: (a) 420 K; (b) 620 K; (c) 820 K; (d) 970 K. Key: (--) HzO X 5; (-*-) CO.
>I L
m
Temperature (K)
T e m p e r a t u r e (K)
Figure 3. TPD patterns of Ca-loaded coal char after COz chemisorptionat 620 K. Adsorption time: (a) 5 min; (b) 60 min. Key: (-) C 0 2 X 1/3; (---) CO x 1/3.
COz-Chars. Figure 2 shows the T P D patterns of COz-chars prepared a t different temperatures. The char chemisorbed a t 420 K gave almost the same COPdesorption pattern as that of OZ-char (Figure la). The char chemisorbed at 620 K gave two COz peaks at 770 and 830 K, whereas the char chemisorbed at 770 K exhibited only one COz peak at 850 K. In every case, the amount of CO desorption was much smaller than that of Oz-char. A small sharp CO peak at around 1000 K was observed. In order to understand the origin of the sharp C02peak, the TPD experiment was carried out with the C02-chemisorbed char derived from Ca-loaded Morwell coal. The amount of Ca was 0.6 mmol/g (dry), which is 8 times more than that in the Morwell raw coal. As shown in Figure 3, the char chemisorbed at 620 K, with an adsorption time of 5 min, exhibited two COz peaks at 720 and 770 K. With an adsorption time of 60 min, the intensity of the latter peak significantly increased and the peak temperatures shifted to 750 and 830 K, respectively. The pattern was similar to those of Figure 2b. H20-Chars. TPD patterns of H20-chemisorbed chars are shown in Figure 4. In contrast with the Oz- and C02-chars, H20-chars exhibited neither broad COz desorption nor a sharp COz peak. A sharp H 2 0 peak was observed for the char chemisorbed at 420 K. When the H 2 0 adsorption was carried out at 620, 820, and 970 K, no HzO peak appeared. Only CO desorption, similar to that for COz-char, was observed a t above 900 K. Figure 5 shows the T P D pattern of the char prepared by procedure 2. This char was cooled down in an atmosphere of product gas after H 2 0 gasification. Besides the
Figure 5. TPD pattern of char prepared by procedure 2: (-) coz x 5; (- -) co.
.
.-
1
~~
a\,
I
I
-I
Temperature (K)
Figure 6. TPD patterns of H20/02-and 02/HzO-charschemisorbed at 420 K (a) HzO/Oz-char;(b) 02/H20-char.Key: (-) COP X 5; (--) H2O X 5 ; CO. (-e-)
CO desorption that appeared for the normal H,O-chars, a sharp C02peak was observed at 840 K, but an HzO peak as is seen in Figure 4a was not detected. HzO/Oz- and Oz/HzO-Chars. Figure 6 shows the TPD patterns of HzO/Oz-char and Oz/H20-char chemisorbed at 420 K by procedure 3. Essentially similar TPD patterns were obtained despite the reverse order of chemisorption for the two chars. An HzO peak was observed a t almost the same temperature as for the H20-char (Figure 4a). The C02 desorption may be attributable to OZ-adsorption. However, its pattern was completely different from that observed for OZ-char prepared at 420 K (Figure la), as both sharp and broad desorptions were
Zhang et al.
682 Energy & Fuels, Vol. 2, No. 5, 1988
observed. The peak temperature for the sharp one was at about 770 K. It is noteworthy that the broad desorption curve exhibited a temporary downturn at the temperature corresponding to the evolution of H20 gas. With respect to the CO evolution, a similar pattern was observed as that for 02-char (Figure la).
Discussion CO Evolution. A sharp CO peak was observed at a high temperature in Figures 2,4, and 5 . In 02-chars, the CO peak is not clear, because there was broad and very large CO desorption as background around the peak. It was concluded that the presence of Fe compounds in the char played the most important role on the appearance of a rather sharp CO peak." The mechanism may be as follows. When the clean char was exposed to oxidizing gas at a relatively high temperature, the Fe compounds in the char, Fe3C or Fe, were partly oxidized to Fe304. When such a char was heated again, this small amount of Fe304was reduced back to FeO, Fe, or Fe3C by carbon. CO gas was released as a result of this reduction.13J4 On the other hand, the broad CO desorptions were from the decomposition of oxygen complexes on carbon. They were formed by the chemisorption of oxidizing gas at free active sites on carbon. In the case of 02-chars (Figures 1 and 6), this desorption started at relatively low temperatures. On the contrary, C02- and H20-chars produced CO only at relatively high temperatures (Figures 2 and 4). This fact is in agreement with the results reported by Kelemen et al., who examined the interaction of H20 vapor as well as COz with carbon.16J6 They found that the exposure of carbon to H 2 0 vapor at 300 K or to C02 at 570 K resulted in the formation of stable oxygen species that were desorbed as CO at high temperature. H 2 0 Evolution. The H 2 0 desorption observed at around 600 K in Figures 4 and 6 can be ascribed to the reaction Ca(OH)2= CaO + H20. This is supported by the TPD results for pure Ca(OH)2in vacuo. The formation process of Ca(OH)2in the char may be described as follows. If the H20-gasified char is heated up to 1100 K in vacuo, the Ca compound in the clean char would become CaO. When the char was cooled to 420 K, the CaO in the char remained as it was. When such chars were allowed to contact HzO vapor of 4 kPa at 420 K, which is much lower than the decompositiontemperature of Ca(OH)2,the CaO was hydrated to Ca(OH)2. It evolved H 2 0 during the subsequent TPD run. In the case of the char whose TPD is given in Figure 5, Ca(OH)2was not formed even though the atmosphere contained a sufficient amount of HzO. All Ca compounds were converted to CaC03 during cooling, and CaO was not available in the lower temperature range where the formation of Ca(OH)2was possible. COz Evolution. (a) Decomposition of Pure CaC03. A TPD of pure CaC03 was carried out to check whether the C02 desorption observed in our TPD experiments can be ascribed to the decomposition of CaC03. A sharp C02 peak was found at 930 K, which is higher by about 100 K than observed in the above figures. McKee reported that the decomposition of bulk CaC03 took place at around 1120 K in He flow and at around 1220 K in COz flow.17J8 (13)Cypres, R.;Soudan-Moinet, C. Fuel 1981,60,33-39. (14)Suzuki, T.;Inoue. K.; Watanabe, Y. Chem. Express 1987,2, 365-368. (15)Kelemen, S.R.;Freund, H.;Mims, C. A. J. Vac. Sci. Technol. A 1984,2,987-990. (16) Kelemen, S. R.; Freund, H.Carbon 1985,23, 723-729. (17)McKee, D.W.Fuel 1980,59,308-314.
These discrepancies can be explained by the difference in C02 partial pressure in the gas phase. Naturally the decomposition temperature becomes low at a low C02 pressure, especially when the gas phase is evacuated as in the present study. (b) Sharp C 0 2 Evolution at around 800 K. The sharp COz peak at around 800 K in Figures 1-3,5, and 6 was regarded as the result of the decomposition of CaC03. First, such sharp peaks were not observed in the demineralized char but in the raw coal char and the Ca-loaded char (Figure 3). Second, the amount of Ca estimated from the C02 peak area in Figure Id or Figure le, by assuming that the C02 comes from CaC03, corresponds to the Ca content in the raw coal. However, the peak temperature was lower by about 100 K than that of pure CaC03, as mentioned above. Possible explanations for this difference are as follows: (1)the size of CaC03 in char is so small that it decomposes more easily than a lump of CaC03, and/or (2) CaC03 has some chemical interaction with carbon, which may lower the decomposition temperature. This type of interaction has also been suggested in the literature.8J7 The CaC03 in char can be formed through the reaction between CaO and COP gas. This reaction occurs at a temperature range high enough to promote the reaction and low enough to prevent the decomposition of CaC03. In the case of 02-chars, the sharp COPpeak was observed only from chars treated at high temperatures (Figure ld,e). This is because C02 is unavailable in low-temperature adsorptions, whereas at higher temperatures the combustion of carbon with O2 occurred and the C02 gas that was produced was caught by CaO in the char. For COZ-chars, the char chemisorbed at 420 K did not exhibit such a C02peak (Figure 2a). The reaction between CaO and COz is too slow at 420 K, and temperatures of 620 and 770 K are sufficient for the CaC03 formation reaction to take place.Ig Sharp C02 peak was also observed in Figure 5. When the clean char was exposed to H 2 0 at 1100 K, the H20gasification reaction occurred and CO, COP,and H2 gases were produced. When this char was cooled in this gas atmosphere, CaC03 was formed from CaO and C02 in the temperature range where CaC03 is stable. In the H20-chars (Figure 4) no C02peak was observed. A t 420 or 620 K, the temperature was so low that H 2 0 gasification did not occur during adsorption, and therefore no CaC03 was formed. In the case of 820 and 970 K (Figure 4c,d), the H 2 0 gasification reaction probably occurred and CaC03 was formed to some extent. But the system was outgassed at the same temperature prior to TPD, and therefore the CaC03, if any, would decompose back to CaO. Thus the Ca compound in char changes its chemical form among Ca(OH)2,CaC03, or CaO depending on the conditions, including temperature, pressure, and atmosphere. CaO was transformed to Ca(OH)2in Figure 4, while the same CaO was changed to CaC03 in Figure 5. The results shown in Figure 6 show another example of the delicate balance among these Ca compounds. In the case of H20/02-char, the clean char was first exposed to HzO, and the CaO in the char became Ca(OH),. This Ca(OH), remained as it was during outgassing at 420 K, because it is stable at this temperature. The char was then exposed to 02,the formation of surface oxygen complexes on carbon substrate being expected, as discussed below. Such surface complexes and Ca(OH)2are also formed in (18)McKee, D.W. Carbon 1979,17,419-425. (19)Nitsch, W. 2.Elektrochen. 1962,66,703-708.
TPD S t u d y on Coal Chars 02/H20-char as well as in H20/Oz-char. When doubly chemisorbed chars were heated in vacuo, C02was evolved from a low temperature due to the decomposition of oxygen complexes on the carbon surface, simultaneously Ca(OH)2decomposed to CaO, releasing H 2 0 gas. Since the COPevolution and the CaO formation occurred in the same temperature range, the evolved C02gas was captured by CaO and produced CaC03. This was supported by the fact that the amount of COz evolution temporarily decreased when Ca(OH)2decomposed. Upon further heating, this CaC03decomposed with the formation of a C02peak (Figure 6). In 02-and COz-chars prepared at low temperature, there is no COz peak at around 800 K in spite of the presence of CaO and the broad COP evolution. Therefore, the nascent CaO is important for the formation of CaC03. (c) Small COz Peak at around 700 K. The small peaks observed in Figures 1 and 2 are also regarded as being associated with the Ca species. The Ca-loaded char exhibited two similar COPpeaks after C02 chemisorption at 620 K (Figure 3), and the demineralized char showed no such peak. This small peak should be distinguished from the sharp COPpeak discussed above, since the desorption temperature was considerably lower. However, a detail discussion is not possible at the present time. (d) Broad COP Evolution. In addition to the COP peaks due to Ca compounds, broad desorption was observed in a wide temperature range from an adsorption temperature to about 1000 K (Figures 1,2, and 6). This C02 evolution, together with the broad CO desorption, is attributable to the decomposition of surface oxygen complexes on carbon. The desorption over a wide temperature range implies that the origin consists of a number of oxygen-containingspecies with different desorption energies. Similar desorption was observed in an 02-chemisorbedchar of a demineralized Morwell coal" and pure carbons.8~~ The surface complexes may be formed on 02-chars in the following process. When a clean char was exposed to O2 at 420 K, various kinds of surface complexes with different thermal stabilities may be formed at free active sites. The type and amount of surface oxygen complexes were influenced by the adsorption temperature. As shown in Figure 1,the adsorption at higher temperature produced a larger amount of complexes. When the temperature was further raised where the combustion reaction occurred, the C02 evolution from complexes with relatively low thermal stabilities were no longer observed. This is because the surface temperature of char became higher than the given temperature due to the local overheating, and only thermally stable complexes were retained. The origin of the broad desorption from a COz-char (Figure 2a) is not clear, since the temperature, 420 K, is too low for the reaction of carbon with COPto take place. The presence of active CaO probably assists the chemisorption of C02 on carbon. A further experiment is required to clarify this point. A temporary decrease in C02evolution in Figure 6 is due to the capture of C02 by CaO as discussed above. This is interesting because this indicates the exchange of surface oxygen between carbon and a metallic impurity. Such an exchange may be related to the nature of the catalysis by metal in the gasification reaction. Relation to the Gasification Mechanism. There are two approaches so far to estimate ASA on carbon. In addition to a conventional TPD approach, Freund has proposed a transient kinetic method.20 He attempted to (20) Freund, H.Fuel 1986, 65, 63-66.
Energy & Fuels, Vol. 2, No. 5, 1988 683
determine the amount of surface oxygen under gasification conditions to obtain more direct information about the active site. However, this technique only gives information on the total amount of oxygen complexes, and the detailed characterization of complexes is impossible. The TPD method, on the other hand, presents information on the binding energy of each oxygen complex. Furthermore, it is generally approved as a reasonable method to evaluate ASA. The amount of gas adsorbed at low temperature (ASA) has, at least semiquantitatively, a correlation with the amount of actual active sites during gasification. In other words, when gasified sample was cooled in vacuo and exposed to oxygen at 420 K, the amount of resulted surface complexes may be indirectly correlated to the amount of actual active sites during gasification. Of course, some of the more tightly bound oxygen, which is only desorbed at higher temperatures, does not desorb under gasification conditions, and some of the weakly bound oxygen, which is desorbed at low temperatures, may not even be formed during gasification. The broad desorptions of C02 and CO in the present study have a similar meaning and limitation in relation to the actual active site on carbon. With respect to the gas evolution from mineral matter, there is almost no accumulation of data so far. When one determines the amount of oxygen complexes on the catalyst-bearing sample by heating it quickly from ambient to, e.g., 1273 K, the amount of gas desorbed contains oxygen both from carbon itself and a metal-carbon complex.21 The slow heating method can reveal more detailed features as are shown in this study and several studies with metal-loaded pure carbon.6@10 The mechanism of Ca-catalyzed gasification is still a matter of argument in spite of many studies. McKee investigated the thermogravimetric behavior of a CaC03-graphite mixture in He under a linear heating rate.17J8 He found that the presence of graphite caused a small lowering in the CaC0, decomposition temperature, and the weight loss was slightly more than expected for the complete conversion of CaC0, to CaO. Therefore, he proposed the following solid-state reaction as the initial step of carbon gasification: CaC03 + C = CaO + 2CO (1) Suppossing that reaction (eq 1)initially occurs, CO would be the main reaction product in our TPD experiment. However, in fact, only C02was evolved as a result of the decomposition of CaCO3 This COPmight escape from the sample very quickly before reacting with C, since the present experiment was carried out under very unique conditions, that is, under high vacuum. The present results thus suggest that the above solid-state reaction (eq 1)does not occur. The decrease in the decomposition temperature observed by McKee does not necessarily support the occurrence of the reaction (eq 1). It can also be explained by the decrease in the local C02 pressure due to the consumption via the Boudouard reaction. A similar effect of C02pressure on the decomposition temperature of CaCO, was discussed above. The peak temperature for bulk CaC03 decomposition was 930 K, which is lower by about 300 K than that in an atmospheric pressure e~periment.'~ Our TPD results clearly indicate that many types of surface oxygen compounds are formed on carbon and Ca, and oxygen easily moves around among them. Considering the large mobility of oxygen in the presence of Ca, we would like to support the following oxygen-transfer mechanism for the Ca-catalyzed gasification. (21) Hashimoto, K.; Miura, K.; Xu, J.-J.;Watanabe, A.; Masukarni, H. Fuel 1986,65,489-494.
Energy & Fuels 1988,2, 684-692
684
CaO
+ 0 (from 02,COz, or HzO) = CaO(0) CaO(0) + Cf = CaO + C(0) C(0) = CO (and/or COO)+ Cf
(2) (3) (4)
Oxygen-containinggas, whatever it is, first adsorbs on fine CaO particles, forming surface oxygen complexes, CaO(0) (reaction 2). This surface oxygen spills over to a free site on the carbon surface, Cf, to form an oxygen complex on carbon, C(0) (reaction 3). Excellent dispersion of the Ca species and good contact with carbon is essential for these steps. In reaction 4,C02and CO would be produced from C(0) through many reaction paths as described in the literat~re.~J~
Conclusions The type and amount of surface comlexes on coal char, after being exposed to oxygen-containing gases at various conditions, were evaluated by TPD measurement using a
slow heating rate. Many types of oxygen complexes, both on mineral matter and carbon, were identified. The features of these complexes are strongly affected by the adsorption conditions and also by the treatment conditions after adsorption. Oxygen can be transferred from carbon to metal and vice versa. The gasification mechanism was discussed in relation to the present TPD results. It is proposed that the solid-state reaction between CaC03and C does not occur during the Ca-catalyzed gasification, but the oxygen transfer from CaO to carbon is the principal function of a Ca catalyst.
Acknowledgment. The partial financial support of a Grant-in-Aidfor Scientific Research on Priority Areas from the Ministry of Education, Science acd Culture, Japan (62603014), is acknowledged. Registry No. 02,7782-44-7; C02, 124-38-9;HzO, 7732-18-5; Ca, 7440-70-2.
Characterization of Titanium in United States Coals Gary L. Steinmetz, Mysore S. Mohan,* and Ralph A. Zingaro Department of Chemistry and t h e Coal and Lignite Research Laboratory, Texas A&M University, College Station, Texas 77843 Received July 6, 1987. Revised Manuscript Received April 11, 1988
Detailed knowledge of the modes of occurrence of titanium in coals is necessary to understand the mechanism(s) by which the element is deposited on the catalysts in the hydroliquefaction and hydrodesulfurization of coal. The modes of occurrence of titanium in six US.coals (0.28-0.67% Ti) have been investigated. The samples (-100 mesh) were extracted with a variety of solvents (benzene/MeOH, methyl isobutyl ketone, pyridine, and dimethyl sulfoxide). DMSO was found to be the most efficient solvent, extracting up to 6.3% of the titanium in one case. In 70% of the cases, the solvents extracted less than 1% of the total titanium. Extractions with NH,Ac indicate that less than 1% of the titanium was ion-exchangeable. Examination of the coal samples by scanning electron microscopy-energy-dispersive X-ray spectrometry showed that titanium in the coals occurred principally in the following modes: (1)as discrete, elongated grains of Ti02(
