Energy & Fuels 1992,6, 287-293
287
A Temperature-Programmed Reaction Study of Calcium-Catalyzed Carbon Gasification D. Cazorla-Amor6s, A. Linares-Solano,* and C. Salinas-Martinez de Lecea Departamento de Quimica Inorgcinica e Ingenieria Qulmica, Universidad de Alicante, Alicante, Spain
J. P. Joly Laboratoire de Catalyse Organique, ESCIL, Universitd Claude Bernard de Lyon, Villeurbanne, France Received November 1 1 , 1991. Revised Manuscript Received February 24, 1992 The present work extends previous investigations, related to the calcium-catalyzed carbon gasification, by using a technique called temperatureprogrammed reaction (TPR) under COP The purpose is to confirm and to complete the carbon gasification mechanism catalyzed by calcium deduced from earlier TPD studies. The TPR technique has the advantage of studying the catalyst behavior under conditions closer to the gasification reaction than in the case of TPD experiments. TPR results, obtained from calcium-containingcarbon samples with different calcium contents and with increasing calcium sintering degree, show (i) CaC0, is the active species during C02-carbon reaction and (ii) with increasing calcium sintering, the beginning of the gasification reaction is determined by the possibility for C02 to reach the calcium-carbon interface. The results allow us to rule out the Ca02-CaO cycle mechanism and support the CaC0,-CaO cycle mechanism.
Introduction The knowledge of the role of an heterogeneous catalysts implies first the identification of the catalytically active solid phase and second the description of the elementary steps of the reaction mechanism. In the field of calciumcatalyzed carbon gasification, some important differences between various authors have been observed about these two points. Essentially, two mechanisms have been considered: the CaO-CaC03 cycle1p2and the CaO-Ca02 cycle,H consequently, more research is needed to clarify this discrepancy. In earlier studies, the calcium-catalyzed carbon gasification has been investigated with special attention being devoted to the nature of the active phase and to the related mechanisms of the reaction.+l4 A relevant subject such as the extent of carbon-calcium contact (scarcely studied up to date) was analyzed in detail since it may be the key for a better understanding of the mechanisms of carbon gasification catalysis. It is worth noting that TPD patterns from C a h b o n samples previously contacted with C02 appear to be very useful and lead us to propose a threezone model for the particles of the catalyst.12 It appears to be important to differentiate between the external surface of catalyst particles, which seems to be inactive, and the contact zone (Ca-carbon interface) that has been shown to be responsible for catalytic activity.12J4 These experiments, carried out with 12C02or 13C02,12J4 provided results that support the presence of a catalyst-carbon interface where a CaC0,-CaO cycle occurs in the course of gasification catalysis. The conclusion was that particles of the catalyst act as a medium for the transport of “active C02”to the catalyst-carbon interface where it dissociates when temperature is adequate. Experiments with 13C02 made it possible to distinguish two steps of the h i a t i o n of this “active C02” at the catalyst-carbon interface:14 CaC03-C + CaO-C(0) + CO (1) CaO-C(0)
-
CO
+ CaO-C
* Author to whom correspondence should be addressed.
(2)
These experiments were carried out in a helium flow, that is, under conditions far from those of the gasification catalytic tests performed in a C02 atmosphere. The present work extends previous investigations by using a technique called temperature-programmedreaction (TPR) in COP. The purpose of this study is to confirm and to complete the mechanism deduced from earlier TPD studies. The TPR techniques, in which the sample is heated at a nearly constant rate under a stream of reactants, have been essentially limited to the determination of kinetic parameter^.'^ However, the use of this technique to catalyzed carbon gasification has been proven to be an interesting tool.298J6J7 Furthermore, the literature shows that TPR may provide information about the reaction mechanism and the active phase of the catalyst.8J7 In that sense, the interesting works of Falconer et al.?17 in which (1) McKee, D. W.; Chatterji, D. Carbon 1976,13,381-390. (2) Ohtauka, Y.; Tomita, A. Fuel 1986,65, 1653-1657. (3) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1983,82, 382-394. (4) Sears,J. T.; Muralidhara, H. S.; Wen, C. Y. Znd.Eng. Chem. Des. Rev. 1980,19, 358-364. (5) Baker, R. T. K.; Chludzinski, J. J., Jr. Carbon 1985,23,635-644. (6) Kapteijn, F.; Porre, H.; Moulijn, J. A. AZChE J. 1986,32,691-695. (7) Zhang, 2.;Kyotani, T.;Tomita, A. Energy Fuels 1988,2,679-684. (8) Chang, J.; Adcock. J. P.: Lauderback. L. L.: Falconer. J. L. Carbon 1989.27. 593-602.
(9) Lkares-Solano, A.; hela-Alarcbn, M.; Salinas-Madnez de Lecea, C. J. Catal. 1990,125,401-410. (10)Linares-Solano, A.; Almela-Alarcbn,M.; Salinas-Martinez de Lecea, C.; Cazorla-Amorbs, D. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989, 34, 136-143. (11) Linares-Solano, A.; Salinas-Martinez de Lecea, C.; CazorlaAmorh, D.; Joly, J. P.; Charcosset, H. Energy Fuels 1990, 4, 467-474. (12) Cazorla-Amorbs, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Joly, J. P. Carbon 1991,29, 361-369. (13) Cazor!a-Amor6a, D.; Linares-Solano, A.; Marcilla-Gomis, F. A.; Salinas-Martinez de Lecea, C. Energy Fuels 1991,5, 796-802. (14) Cazorla-Amorbs, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Meijer, R.; Kapteijn, F. Prepr. Pap-Am. Chem. Soc., Diu. Fuel Chem. 1991,36,975-981. (15) Miura, K.; Silveston, P. L. Energy Fuels 1989, 3, 243-249. (16) Joly, J. P.; Cazorla-Amorbs,D.; Charcosset, H.i Linares-Solano, A.; Marcilio, N. R.; Martinez-Alonso, A.; Salinas-Martinez de Lecea, C. Fuel 1990,69, 878-884. (17) Ersolmaz, C.; Falconer, J. L. Fuel 1986, 65, 400-406.
0887-0624/92/2506-0287$03.00/00 1992 American Chemical Society
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288 Energy & Fuels, Vol. 6,No. 3, 1992 T(K)
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Figure 1. Experimentalprocedure type 1: TPD (He, 20 K/min) C02chemisorption at 573 K, 5 min TPD (He, 100 K/min) TPR (5 or 10% C02,50 or 100 K/min).
+ +
+
the calcium-carbon and the barium-carbon systems were studied using TPR, have to be mentioned because of their relation with the present study. The authors draw quite different conclusionsabout the n.ature of the active phase depending on the alkaline earth element concerned, in spite of their chemical similarity. A carbonate-oxide cycle is proposed for barium because the decomposition temperature of BaC03 allows this compound to be stable and therefore to be present under carbon gasification condit i o n ~ . ~ 'On the contrary, on the basis of the fact that CaC03, in the presence of carbon, has already decomposed at the reaction temperatures used during the gasification reaction, they justified the use of the CaO-Ca02 cycle mechanism.8 In the present work, TPR results with C02are presented and interpreted on the basis of earlier TPD result^.'^-'^ The aim is to confirm the chemical nature of the catalyst as CaC03 in the calcium-carbon system. Besides, an explanation of the apparent differences found by Falconer et al.SJ7between the chemical natures and behaviors of the active phases of the Ca-carbon and the Ba-carbon systems is proposed.
Experimental Section Sample Origins. Samples used in TPR experimentswere CaO
produced from calcium acetate or calcium carbonate, thermally decomposed in situ, and calcium-bon sampleslargely described in previous works.loJ1The latter consist of a phenolformaldehyde char oxidized by nitric acid and then loaded with calcium by ion exchange (for example,A2-11-2.9) or impregnation (for example, A2-1-9.4).The momenclature includes the calcium content (wt % 1.
60
40
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TPR Experiments. TPR experiments consist of heating samples from 298 K to a maximum temperature of 1223 K at a rate of 50 or 100 K/min under a continuous flow (60 mL/min) of the gas mixture: C02(5 or lo%), argon (2%), and helium (93 or 88%). The analysis of the gas at the outlet of the reactor was made with a quadrupole mass spectrometer. In situ decomposition of calcium acetate or CaC03was performed under a stream of He, the temperature being raised from 298 to 1223 K at a rate of 20 K/min. Prior to TPR, Ca-carbon samples undergo one of the treatmenta schematidy represented in Figures 1 and 2 and described as follows: type 1,a TPD at 20 K/mh in He up to 1223 K followed by C02chemisorption at 573 K for 5 min and then by another TPD, in order to transform it into CaO, to eliminate the surface oxygenated groups, and to assess the dispersion of the catalyst (Figure 1);type 2,similar sequence of treatments as in type 1 followed by one or various treatments to sinter the catalyst. For instance, a sample gasification by C02is carried out at constant temperature for a few minutes and a TPD under He is immediately performed (Figure 2).
80
120
100
(mid
Figure 2. Experimentalprocedure type 2 TPD-(He,20 K/mF) + C02chemisorption at 573 K, 5 min + TPD (He, 100 K/min) + C02reaction + TPR (5 or 10% C02,50 or 100 K/min). (!bVol/g Ca) x0.06
. T(K)
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Figure 3. TPR pattern (5% C02)of CaO from CaC03. (%Vol/gCa) x 0 0 5 80 _--_
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Figure 4. TPR pattern (10% C02)of CaO from calcium acetate.
TPR patterns of pure CaO consist of curves showing the variation of C02level at the outlet of the reactor and the reaction temperature as a function of time. TPR patterns obtained with Ca-carbon samples give the levels of C02, CO, and also the quantity C02+ C0/2 that represents the C02balance which is used to derive the chemicalstate of the catalyst. The application of the TPR technique to the Ca-carbon system has been described and discussed in detail in a previous publication.16 The results are expressed per gram of carbon for Ca-carbon samples and per gram of CaO for pure CaO samples.
Results and Discussion TPR of CaO. Figures3 and 4 present the TPR patterns obtained with CaO proceeding from CaC03 (CaO excarbonate) and from calcium acetate (CaO ex-acetate), respectively. These patterns exhibit two depressions in the COP level: the first one located at about 573 K is J ~ Jsecond ~ one at attributed to C 0 2c h e m i s o r p t i ~ n , ~the 900 K corresponds to the bulk carbonation of the CaO (18)Cazorla-Amor6s, D.;Joly, J. P.; Linares-Solano, A.; SalinasMartinez de Lecea, C.; Marcilla-Gomis, A. J. Phys. Chem. 1991, 95, 66114617.
Calcium-Catalyzed Carbon Gasification ~~~
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Energy & Fuels, Vol. 6, No. 3, 1992 289
~ -T(K) - 1400
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970 K the decomposition of CaC03starts from the external surface of such particles where, according to
Cazorla-AmorBs et al.
290 Energy & Fuels, Vol. 6,No. 3, 1992 2
onvovg 0 7
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.
.
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-
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Figure 6. TPR pattern (10% COz) of sample A2-1-9.4(-B,COz + '/&O; -, CO; bold line, C02). previous TPD results,12 it is less stable than the bulk CaC03 and that at the interface with carbon. This process yields C02 in the gas phase which is responsible for the lower temperature peak in the balance curve. However, gasification occurs simultaneously as a consequence of the CO, in the Ca-carbon interface and the subsequent COz diffusion from the gaseous phase, as it is observed from the decrease in the C02 curve in Figure 5. The result, at this point of the TPR, is a partial decarbonation of the CaC03 particles, probably, with the formation of CaO layers in the regions which are not in contact with carbon. The overall process can be written as CaC03(sup)== CaO(,",) + COz(g)
(8)
2. When the temperature is increased further, the gasification rate increases as well as the extent of particle decarbonation (see the higher temperature peak in the balance curve in Figure 5). It is worth noting that, despite the COz production due to carbonate decomposition (as indicated by the maximum in the balance curve), a significant amount of C02is also consumed by the gasification reaction (as shown by the drop in the COP level). The variations of both balance and COz curves stress the fact that decomposition is more favored through the Ca-carbon interface than toward the gas phase and that the continuous transport of CO, toward the Ca-carbon interface is not fully compensated for by the carbonation of CaO particles by COz from the gas phase (in current experimental conditions (10% CO,)). As a consequence, the C02 held in the catalyst particles in form of carbonate is being consumed during this process. The net effect is the slow transformation of CaC03 into CaO and the predominant consumption of CO, through the Ca-carbon interface. 3. At temperatures higher than 1170 K (not frequently used for the carbon gasification catalyzed by calcium), decarbonation of CaC03 is completed and the catalyst is transformed into CaO. Thermodynamic conditions prevent the formation of bulk CaC03. Nevertheless, this does not mean than C032-at the Ca-carbon interface has to be ruled out as a possible labile intermediate in the gasification mechanism. This point will be further discussed at the end of this paper. Our interpretation, according to which CaC03is present during the carbon gasification and it decomposes preferentially through the Ca-carbon interface, is also in agreement with previous results of TPD performed after isothermal interactions of COPwith CaO at various temperatures.lZ In the following, examples including the effect of calcium loading and catalyst sintering are presented. The interpretation presented above permits us to explain the results obtained in this study. TPR of A2-1-9.4. Figure 6 presents the TPR pattern of sample A2-1-9.4 which contains a large amount of cal-
cium. A previous TPD study made with this sample showed that about 6 wt % of its calcium content is initially as calcium acetate, whereas the remaining 3.4 wt % is ion-exchanged with carboxylic groups of the carbon surface." The calcium acetate yields, after pyrolysis, inactive calcium oxide particles without any significant contact with carbon.11J2 Thus, the reactivity of this sample, due to its ion-exchangedcalcium, is nearly the same as that of samples containing 3.3 w t % of ion-exchanged calcium.14 The TPR pattern (Figure 6) shows important differences with respect to that of A2-11-2.9. The main features that are observed are as follows: (i) the COz + C0/2 balance presents two depressions at about 650 K and 860 K and two peaks at 1080 and 1115 K (ii) the C02 level exhibits a maximum at 1080 K; and (iii) the CO level presents a large peak at 1115 K. The two depressions in the balance, which are superimposed on the COz curve in the lower temperature part of the pattern, are comparable to the COz evolution found with CaO ex-carbonate(Figure 4). This indicates that CaO particles are large enough to allow the superficial and bulk carbonation to be easily distinguished. The peaks found in the balance curve result from the decomposition of CaC03just formed and a good agreement is found between the carbonation and the decarbonation process (102 and 98%, respectively). It is worth noting that the first peak is much higher than the second one, contrarily to the case of A2-11-2.9. This behavior presents a great similarity with that observed in TPD after an isothermal chemisorption or carbonation at various temperatures.12 The interpretation was that the first peak results from the decarbonation of the inactive calcium, that is, the external surface of CaC03, while the second composite (CO, l/&O) peak was due to the active calcium, that is, the Ca-carbon interface, the diffusion of C02 to this interface, and its reaction with carbon during heating.12 These observations allow us to interpret the TPR pattern of A2-1-9.4 as follows. When the sample is heated in the reactant flow, the chemisorption of C02and a partial carbonation occur first up to about 750 K; at this temperature, in agreement with a previous interpretation,12 COz may redistribute and reach the Ca-carbon interface. These processes originate the first depression of the balance curve (steps 5-7). A new increase in temperature (5" > 750 K) results in the evolution of CO, which means that gasification starts at about 800 K. This temperature is similar to that observed in the case of A2-11-2.9, as expected since these samples present nearly the same isothermal steady-state reactivity (3.2 and 3.7 h-', respectively, measured at 1073 K and 0.1 MPa of COP). Simultaneously, the second depression in the balance curve is observed with a minimum a t about 860 K, in such a way that the reaction is not important until most of the calcium oxide is carbonated (2' = 970 K). It is important to stress here the experimental fact that gasification occurs to a significative extent when the whole CaO is carbonated because, as we shall discuss later on, this is an observation that favors the CaO-CaCOs cycle in respect to the CaOCa02one. Carbon gasification (at this temperature range where carbonation is favored from the thermodynamic and kinetic points of view) would occur by steps 5-7 and 1-2, in which transport of C02 to the Ca-carbon interface plays an essential role. At temperatures higher than 970 K, the decomposition of the carbonate formed in the earlier stage of the TPR occurs, as shown by the balance curve which evidences an exof producta with regard to reactant. Again C02may leave the carbonate either as C02, through the surface in
+
Calcium-Catalyzed Carbon Gasification contact with the gas phase, or as CO through the Cacarbon interface. The presence of a maximum in the COz level at 1080 K (not observed for A2-11-2.9), which coincides with a maximum in the balance curve, indicates that one part of CaC03 (probably involving the zones near the external surface of the particles) decomposes directly toward the gas phase, while the other part decomposes through the Ca-carbon interface giving the CO corresponding to the peak at 1115 K. The decarbonation toward the gas phase through the particle free surface implies a size reduction of the CaC03nucleus formed in the lower temperature part of the TPR (T< 970 K). At around 1080 K, it is observed that the evolution of CO increases and that, simultaneously, the COz and the balance peaks are similar in shape and magnitude; this means that carbonate decomposes directly toward the gas phase giving the major contribution to the balance peak and hence the consumption of C02 by carbon (originating CO) does not involve an additional decarbonation of catalyst particles. When carbonate decomposition toward the gas phase is completed, Le., at the temperature at which the COz peak and the first balance peak are just over, the CO production is still important (see CO peak in Figure 6); this indicates that the rest of the decarbonation evidenced by the second balance peak is achieved through the Ca-carbon interface. These results suggest that the decarbonation toward the gas phase takes place before the nucleus of CaC03 has reduced sufficiently to favor the decomposition through the Ca-carbon interface. Finally, at the end of the TPR (T> 1200 K in Figure 6) CaC03is completely decomposed into CaO. From the preceding discussions, the following points may be emphasized in relation to the reaction mechanism that is going to be commented later on: (i) The gasification reaction takes place when the CaO particles are completely carbonated, suggesting that calcium carbonate could be the active phase. (ii) This carbonation implies that, under the experimental conditions used, the diffusion of C02 is fast and hence is not the limiting step. (iii) At high temperatures (T> 1200 K), calcium carbonate is completely decomposed and the CO level could indicate that the activity of the catalyst as CaO is lower than that of CaC03. Further results are needed at this high-temperature reaction zone to confirm this hypothesis because diffusional problems of the reactant gas and the fact that bulk carbonate formation is not thermodynamically possible in these conditions also have to be considered. The role of sintering will be examined in the following, in order to study the effect of the particle size and that of the extent of the interface Ca-carbon on the catalytic behavior of calcium. The expected results would be intermediate between those of A2-11-2.9 and those of A2-1-9.4 samples presented above, since the experiments begin with a sample whose properties are near to those of A2-11-2.9 and sintering makes the charaderistia of the sample closer to those of A2-1-9.4. TPR of Samples with Increasing Sintering Degree. Figure 7 shows the TPR patterns for sample A2-1-3.7. Figure 7a corresponds to the unsintered sample and Figure 7b to the sample that underwent a partial gasification in COz (about 30% burn-off). This sample contains the maximum amount of calcium which can be exchanged by the protons of the superficial carboxylic groups." In Figure 7a, in contrast to the other TPR, the sample only has suffered a treatment with COz at 573 K for 5 min without a subsequent TPD in He before the TPR run. Hence, the lower part of the TPR pattern in Figure 7a (T< 800 K) cannot be compared with other TPR patterns.
Energy & Fuels, Vol. 6, No. 3, 1992 291
Kvollg
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Figure 7. TPR patterns (10% COP)of samples: (a, top) A2-1-3.7 and (b, bottom) A2-1-3.7 after a 30% B.O. in COz (-E, COz '/zCO; -, CO; bold line, COP).
+
Figure 7 evidences the following main features: (i) the sintered sample shows a depression at 900 K in the balance curve which corresponds to the bulk carbonation of larger CaO particles; this depression is not observed in the unsintered sample (Figure 7a). (ii) The appearance of CO is shifted from 800 to 970 K for the sintered catalyst, which indicates the decrease in reactivity with increasing calcium sintering degree. (iii) Two peaks are clearly shown in the balance curve of the sintered sample (at about 1080 and 1140 K), while they can be hardly distinguished in the balance curve of the unsintered catalyst; the maximum at 1080 K in the COz level, which characterizes the decomposition of CaC03 toward the gas phase, only appears for the sintered sample. These results, together with previous ones12in which a progressive catalyst-carbon contact loss was observed with sintering, indicate the following transformations: (i) The growth of the catalyst particles occurs and makes it possible to distinguish surface and bulk carbonation as in sample A2-1-9.3. (ii) Another consequence of particle growth is that a higher temperature is required to achieve a complete carbonation and consequently for COz to reach the CaOcarbon interface. This explains the temperature shift of the appearance of CO to higher temperatures. This reactivity loss with increasing sintering degree has been previously observed in isothermal reactivity measurements.l0 Nevertheless, it has been shown previ~usly'~ that activity per calcium site is constant and does not depend on the particle size; thus, if COPwas equally present on the reaction sites, the temperature at which CO appears should be the same in all the cases (a decrease in the number of sites would lower the CO evolution curve without shifting it). The important shift observed in the CO appearance temperature confirms that a slow diffusion of COz toward the Ca-carbon interface delays the gasification reaction in the TPR conditions (100 K/min, 10% COz). An extreme example of what could occur during TPR would be a catalyst presenting big particles with very small contact with carbon: the decarbonation (yielding
292 Energy & Fuels, Vol. 6, No. 3, 1992
COPdirectly into the gas phase) of partially carbonated particles would occur before carbonate could reach the Ca-carbon surface to produce CO. (iii) During sintering, the surface of the Ca-carbon interface decreases in relation to the particle surface in contact with the gas phase.12 Thus, for sintered particles, CaC03 decomposition above 970 K is expected to occur preferentially towards the gas phase with a release of COz. This explains the clear presence of the lower temperature peak in the balance curve for the sintered sample. These observations will be used later on, to explain the differences with the results of Falconer et a1.8 At this point it is worth noting that, if one consider the CaO-CaOz cycle mechanism, in which COPis dissociated by CaO to yield 0 that goes to the carbon surface by a spillover mechanism:* the dissociation temperature would be independent of the sintering degree of CaO, since there will be necessarily some CaO exposed to gaseous CO,; as a consequence, on TPR the evolution of CO should start at a temperature independent of the sintering degree of CaO particles and only the magnitude of this evolution would be different. Our results, showing an increase in the temperature of CO evolution with sintering, seem to be against this mechanism. Discussion of the Gasification Mechanisms. To further discuss the consequencesof our TPR experiments in the mechanisms of carbon gasification, it is useful to recall the two main mechanisms found in the literature. In one of these mechanisms, CaO, is considered to be the active species and oxygen is transferred to carbon by a spillover p r o c e s ~ ~ - ~ C-CaO + COz + C-Ca02 + CO (9) C-Ca02 + C C-CaO + CO (10) while in the other mechanism a CaC03-Ca0 cycle is proposed to explain the catalytic CaC03-C + CaO + 2CO (11) Falconer et al. support the assumption of Ca02 as the active species on the basis of TPR resultss while, in the case of barium, they consider a BaC03-BaO cycle." Samples were prepared, in both cases, by a physical mixture of carbonate and carbon, in which either the catalyst or the carbon used was isotopically labeled. The difference in the gasification mechanism is based on the gap between decomposition temperatures of carbonates: It was observed by TPR that CaC03 decomposes before the gasification starts (at about T = 1100 K) and hence they ruled out the CaC03-Ca0 cycle. On the contrary, BaC03 would be present during usual gasification temperature since BaC03 decomposes at higher temperatures (T > 1300 K). However, the present TPR study, in agreement with earlier TPD results with 12C02and 13C02,12J4 shows that CaC03 is one of the species involved in calcium-catalyzed carbon gasification because, when the catalyst particle size is very small, the whole calcium is readily carbonate at temperatures lower than 900 K and there is a temperature range (in our TPR conditions) in which both carbonate stability and carbon gasification are observed. The apparent discrepancy between our results and those found by Falconer et al. in the case of calciums can be related with the fact that they used mechanical mixtures of catalyst and carbon with a catalyst particle size larger than ours and a very reduced Ca-carbon interface (their TPR experiments are a much more extreme case of sintering commented above (Figure 7b)). In this sense, the use of a small Ca-carbon interface will introduce difficulties to determine and to elucidate the active phase. In fact, the presence of a small amount of CaC03 at the interface
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Cazorla-Amorbs et al.
cannot be ruled out as it is observed in all our TPR in which the beginning of the CO evolution appears before the carbonate decomposition starts. From our point of view, the calcium-catalyzed carbon gasification may be explained by the decarbonation-carbonation mechanism orginally proposed by McKee et al.' (step 11)if some improvements and steps are added owing to its extreme simplicity. Thus, catalysis by calcium is due to the formation of CaC03 in presence of COz which, by diffusion through the catalyst particles, reaches the Cacarbon interface to yield CaC03 in contact with carbon. The formation of this CaC03 involves the destabilization of C-0 bonds in CO2- anion, with regard to the same bonds in COz molecule (the order of C-0 bond in carbonate anion is lower than that in CO,). This facilitates its subsequent dissociation through the contact with carbon (steps 1 and 2).'* This mechanism involves the regeneration of CaO at the interface which is again transformed to CaC03 by the continuously transferred COz from the external surface of the catalyst particles. The mechanism of transport supposes the breaking and formation of C-0 bonds in carbonate anions.ls Therefore, the external layer of CaC03 is decarbonated as a consequence of the COz consumption at the Ca-bon interface and readily carbonated by the COz in the gas phase. The conclusion is that CaC03 appears to be the active phase in calcium-catalyzed carbon gasification, owing to the reversibility of the CaC03-Ca0 transformation which facilitates the reaction between CO, and carbon. The role of CaO particles is to transport CO, to the Ca-carbon interface; the larger the catalyst particles, the slower the diffusion of COzto the interface. The consequence in TPR is the shift of the gasification to a higher temperature. In accordance with this mechanism, the calcium-bon contact is the responsible of the catalytic activity13and not the external surface of the CaO particles as should be inferred from the CaOz-CaO mechanism (steps 9 and 10). On the other hand, from a chemical point of view, the CaC03-Ca0 mechanism is much more reliable than the CaOz-CaO one. In fact, CaO, in presence of COz and at the reaction temperatures frequently used in the COPcarbon gasification, is not stable and it will be transformed into ita stable form of CaC03 (i.e., AH = -169.5 kJ/mol at 973 K19). Contrarily, the CaO-Ca02mechanism (steps 9 and 10) implies the formation of Ca02which is not stable. In fact, if Ca02decomposes in 1atm of O2at T = 548 K,% its presence at the COz-carbongasification temperatures, seems not to be justified. The experimental results presented here complete those of Falconer et al.sJ7and allow interpretation of the apparent discrepancies found by them between the mechanisms of the gasification catalyzed by Ca and by Ba. Conclusions The results of temperature-programmed reaction show the usefulness of this technique for studying the mechanism of the calcium catalyzed carbon gasification. It has been observed that (i) during gasification, the catalyst particles exist as calcium carbonate if the dispersion of calcium is large enough to allow a fast transport of C02 by diffusion through the particles and (ii) when the catalyst sintering degree is very high, the temperature a t which gasification starts is determined by the possibility for C02 (19) Barin, I.; Knacke, 0. Thermochemical Properties of Inorganic Substances; Springer-Verlag: Berlin, 1973. (20) Handbood of Chemistry and Physics, 59th 4.;CRC Presa: Boca Raton, FL, 1978 p B106.
Energy & Fuels 1992,6, 293-300
to reach the calcium-carbon interface. The consequence of sintering in TPR is a shift of the temperature at which the gasification starts to higher values. The following points seem to be against the mechanism involving a CaOz-CaO cycle: (a) CaC03appears to be the active phase in C02-carbon gasification; (b) from a chemical point of view, CaC03 is stable in the presence of C02 and at the gasification temperatures, while Ca02 is not; (c) the calcium-carbon contact, and not the external surface, is the responsible of the catalytic activity; and (d) when calcium sintering degree increases, the temperature at which gasification starts is shifted to higher values. Finally, our resulta support that a CaC03-Ca0 cycle takes place during catalysis, as originally proposed by McKee. The mechanism involves the following steps:
293
CaO-C + C02 + CaC03-C (7) CaC03-C + CaO-*COZ-C ==CaO-C(0) CO (1) CaO-C(0) CaO-C + CO (2) where (7) is the carbonation of the Ca-carbon interface, (1)is the decomposition of the carbonate at the interface through a CaO-*C024 intermediate which yields CO, and (2) is the decomposition of an oxidized site of carbon that constitutes the determining step of the process. Previous results,14in which two kinds of CO were observed, support steps 1 and 2 of the mechanism.
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+
Acknowledgment. We thank the DIGCYT (Project PB-880295) and the MEC for Diego Cazorla's thesis grant. Registry No. C, 1440-44-0; COP,124-38-9;Ca, 1440-10-2.
In Situ X-ray Absorption Fine Structure Spectroscopy Investigation of Sulfur Functional Groups in Coal during Pyrolysis and Oxidation M. Mehdi Taghiei, Frank E. Huggins, Naresh Shah, and Gerald P. Huffman* 233 Mining and Mineral Resources Building, University of Kentucky, Lexington, Kentucky 40506 Received January 13, 1992. Revised Manuscript Received March 2, 1992
Sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy has been utilized to conduct the first direct characterization and quantification of sulfur functional groups in coal during in situ high-temperature oxidation and pyrolysis. The behavior of all major sulfur forms during such treatments was derived for two U.S. bituminous coals and a low-rank Australian brown coal from least-squares analysis of sulfur K-edge XANES spectra taken at constant temperatures up to 600 "C. During pyrolysis, pyrite began to convert to pyrrhotite at temperatures above 400 OC. The organic sulfides decreased significantly above 300 "C, while thiophenic sulfur remained nearly constant throughout the measurements. The formation of sulfate during oxidation experiments was observed above 300 "C.
Introduction It is well recognized that a full understanding of the characterization and behavior of all major forms of sulfur, both inorganic and organic, is essential for the solution of many of the significant research problems involving sulfur in coal. ASTM' methods D2492 and D3177 can be used to determine the total, pyritic, sulfatic, and, by difference, organic sulfur concentrations in coal. Scanning and transmission electron microscopies and electron microprobe2+ provide information on the local concentration and physical distribution of both organic and inorganic sulfur. Mossbauer spectro~cop@~ provides direct quantitative analyses of all iron-sulfur compounds (pyrite, iron sulfates, (1) Gaseous Fuels, Coal and Coke Annual Book of ASTM Standards; ASTM: Philadelphia, PA, 1986; Vol 05.05. (2) Wert, C. A.; Hsieh, K. C.; Tseng, B. H.; Ge, Y. P. Fuel 1987, 66, 915. (3) Straszheim, W. E.;Greer, R. T.; Markuszewski, R. Fuel 1983,62, 1070. (4) Karner, F. R.; Hoff, J. L.; Huber, T. P.; Schobert, H. H. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1986, 31, 29. (5)Raymond, R.; Gooley, R. Scanning Electron Microsc. 1978, I, 93.
(6) Huffman, G. P.; Lin, M. C.; Huggins, F. E.; Dunmyre, G. R.; Pignocco, A. G. Fuel 1985, 64,849. (7) Huffman, G. P.; Huggins, F. E. Fuel 1978, 57, 592.
0887-0624/92/2506-0293$03.00/0
pyrrhotite, etc.) in coal and coal-derived materials. X-ray photoelectron spectroscopy (XPS)"13 can distinguish between unoxidized and oxidized (divalent, tetravalent, and hexavalent) sulfur concentration on the surface of the coal. Until recently, information about the different organic forms of sulfur in coal could not be obtained directly but was based on indirect methods involving pyrolysis of the coal. Analytical pyrolysis and oxidative techniques emerged as a new discipline from the converging applications of two techniques: pyrolysis-gas chromatography (GC) and pyrolysis-mass spectrometry (MS), to analyze different forms of sulfur present in coal. Pyrolysis-GC/MS techniques eventually led to specific techniques for analysis of sulfur forms in coal such as flash pyrolysis ('pyroprobe") (8) Frost, D. C.; Leeder, W. R.; Tapping, R. L. Fuel 1974, 53, 206. (9) Frost, D.C.;Leeder, W. R.; Tapping, R. L.; Walbank, B. Fuel 1977, 56, 277. (10) Perry, D. L.; Grint, A. Fuel 1983, 62, 1024. (11) Pillai, K. C.: Young, V. Y.; Bockris, J. M. Colloid Interface Sci. 1985, 103, 145.
(12) Kelemen, S.R.;Gorbaty, M. L.; George, G. N.; Kwiatek, P. J. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, 36, 1213. (13) Kelemen, S.R.;Gorbaty, M. L.; Kwiatek, P. J.; George, G. N. Fuel 1991, 70, 396.
0 1992 American Chemical Society
