Znd. Eng. Chem. Res. 1991,30, 1735-1744
1735
Transient Kinetics Study of Catalytic Char Gasification in Carbon Dioxide Anthony A. Lizzio and Ljubisa R. Radovic* Department of Materials Science and Engineering, Fuel Science Program, The Pennsylvania State University, University Park, Pennsylvania 16802
The deactivation behavior of K, Ca, and Ni catalysts during carbon (char) gasification in C 0 2 was investigated. Correlations were sought between gasification rates and reactive surface areas (RSA) of the chars. In addition, the results allowed some speculation on recently proposed mechanisms of catalysis. An excellent correlation was found in the case of K catalysis, suggesting the rate-determining step in the o v e r d mechanism to be the same as in the uncatalyzed reaction, i.e., desorption of the reactive C(0) intermediate. For the Ca-catalyzed reaction, the quality of the correlation depended on catalyst dispersion, suggesting that an additional process, besides the direct decomposition of the reactive C(0) intermediate, contributed to the transient evolution of CO (e.g., oxygen spillover). No correlation was found for Ni-catalyzed gasification; an oxygen-transfer mechanism is proposed to explain these findings. Mixed catalyst systems (Ca/K, K/Ni, Ca/Ni) were also studied. An excellent correlation between reactivity and RSA was observed in cases where the K-catalyzed reaction was dominant.
Introduction The lack of a quantitative understanding of the rates and mechanisms of catalyst deactivation is a major impediment to the commercialization of catalytic coal gasification processes. Despite the importance of the subject, no comprehensive review is available on this issue; a recent general review of catalytic coal gasifcation (Pullen, 1984) discusses it only briefly. The most commonly reported (or proposed) mechanisms of deactivation are loss of contact between catalyst and ’support” (solid reactant) (Wigmans et al., 1983a), particle sintering (Radovic et al., 1983a) unfavorable interaction. with mineral matter (Formella et al., 1986), loss of catalyst by volatilization (McKee and Chatterji, 19751, change in oxidation state of the catalyst (Walker et al., 1968), and catalyst ‘encapsulation” by carbon deposition (Lund, 1985). A decrease in the catalyzed rate with the extent of reaction has also been attributed to pore blockage by the catalyst, i.e., a reduction in the accessible surface area of the porous carbon due to the accumulation of catalyst in the micropores (Abel et al., 1985). The degree of Occurrence of one or more of these deactivation processes probably depends on the gasification conditions and the nature of both the catalyst and the char (carbon). No single experimental technique has been available to quantify all of them. Recently we proposed a new fundamental expression for the rate (R)of uncatalyzed char gasification (Lizzio et al., 1990; Radovic et al., 1991a):
RSA - k(RSA) (1) TSA ASA where C, is the concentration of carbon (re)activesites and TSA, ASA, and RSA are the total, active, and reactive surface area of the char, respectively. Only a portion of the surface of the char is active (Le., chemisorbs oxygen); furthermore, only a portion of the active surface-or, conceivably, a different surface altogether-is reactive in uncatalyzed carbon gasification (Radovic et al., 1991a). In catalyzed gasification, we propose that the same expression should apply; using the terminology of Carberry (1987), ASA and RSA correspond to the catalyst and catalytic surface area, respectively. The former is typically referred to as the dispersion (fraction exposed) of the catalyst and is determined by low-temperature chemisorption of a suitable gas (e.g., 02, CO,); the latter represents the concentration of carbon atoms that form the reactive interR = kC, = k(TSA)-
mediate under reaction conditions. The commonly observed deviations from unity of the ratio ASA/TSA make carbon gasification, even when uncatalyzed, more similar to a heterogeneous catalytic reaction than to a conventional noncatalytic gas/solid reaction (Radovic et al., 1983b). Deviations from unity of the RSA/ASA ratio lead to the concept of structure sensitivity of carbon gasification (Lizzio et al., 1990; Radovic et al., 1991b). In many studies of catalyzed carbon gasification, the performance of the catalyst is evaluated, and the Catalysts are compared, on the basis of char reactivities expressed per unit mass of char (or catalyst) or per unit total surface area of the char. This information is important for practical purposes but reveals hardly anything fundamental about the catalyst or its interaction with the char (support). Other studies correlate a decrease in the gasification rate with a decrease in the catalyst surface area (dispersion) and assume that the catalytic surface area is equal, or at least directly proportional, to the catalyst surface area. This, of course, is not necessarily a good assumption; the dissociated reactive gas needs to gasify the reactive carbon atoms (located at the catalyst/char interface or at the edges of the carbon crystallites) and the concentration of these may or may not be related to the catalyst surface area. If a method for measuring RSA were available, a true turnover frequency (k),i.e., rate per catalytic site, could be obtained. From a practical standpoint, by now probably most of the elements in the periodic table have been tested as potential gasification catalysts. It seems unlikely, therefore, that a commercially viable catalyst will emerge from studies on entirely new catalysts; rather, it will probably emerge as a result of optimization studies on some of the already identified leading candidates, if these are more thoroughly characterized and understood. The establishment of a definitive correlation between the catalyzed gasification rate and the catalytic (or reactive) surface area of a coal char would be an important step toward understanding the mechanisms of catalyst deactivation. It is well-known that potassium may deactivate because of partial volatilization (Sams et al., 1985) and undesirable interaction with the mineral matter in coal (Bruno et al,, 1988). Calcium may sinter and lose contact with the char support (Radovic et al., 1983a; Kapteijn et al., 1986). Nickel may deactivate in an oxidizing atmosphere if it cannot maintain its metallic form (Walker et al., 1968; 0 1991 American Chemical Society
1736 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
Yamada et al., 1983; McKee, 1974). At the present time, a quantitative description of these processes does not exist. The present study offers a new approach to achieving this goal. It seeks to obtain correlations between gasification rates in COOand catalytic surface areas for a number of catalyst systems prepared from alkali, alkaline-earth-, and transition-metal precursors, all of which have considerable commercial potential. The transient kinetics technique, often used in heterogeneous catalysis studies (Biloen, 1983; Peil et al., 1990) and shown recently to be a powerful characterization tool in uncatalyzed char gasification (Adschiri et al., 1987; Zhu et al., 1989; Lizzio et al., 1990, Radovic et al., 1991b), is used in pursuit of this goal. Experimental Section Sample Preparation. Two coals were used in this study: a high volatile A Illinois No. 6 bituminous coal (PSOC-1098) and a North Dakota lignite (PSOC-1406P). Their thorough characterization is on file at the Penn State/DOE Sample and Data Bank. Two high-purity microporous carbons, Saran char and Carbosieve,were also used. Saran, supplied by the Dow Chemical Company, was used as the precursor for the Saran char. This copolymer of vinylidene chloride and vinyl chloride (mole ratio of 9:l) was heated to 1173 K and held at this temperature for 4 h in a flowing stream of nitrogen. The char produced is microporous, has a high total surface area (-1200 m2 g-9, and is similar in pore structure to some bituminous coal chars. Carbosieve (S-11, supplied by Supelco, Inc.) is a carbon molecular sieve having a surface area of lo00 m2 g-' and a relatively narrow pore size distribution (1.0-1.2 nm). All carbon (char) samples were ground with agate mortar and pestle and a -270-mesh ( 0.90, in contrast to the behavior observed for the Ca-loaded chars. Figure 6 also shows the reactivity profile for the Saran-K-IE char at the same temperature. It is very similar, but the rate is -50% lower than that of the former, as expected (see Table I). These findings are consistent with the suggestion that potassium redisperses itself quite readily during gasification and that the K-catalyzed gasification behavior is independent of the chosen catalyst preparation method (Radovic et al., 1984; Spiro et al., 1984). It is interesting to note that, if one considers only the percent K increase with increasing conversion and assumes that the rate is directly proportional to the potassium concentration over the entire conversion range, a reactivity profile can be generated that is remarkably similar to those of Figure 6. The result of this simple but revealing calculation, shown in Figure 7 and being in agreement with the findings of Wigmans et al. (19841, suggests that the (often reported) catalyst loss due to volatilization (Samset al., 1985; Wigmans et al., 1983b) is insignificant under our reaction conditions. The effect of catalyst loss on the observed gasification behavior was minimal probably because most of the volatilization (if any) occurred prior to gasification, Le., during heat treatment of the chars (Shadman et al., 1987). Figure 8 shows the reactivity profiles for Carbosieve and the demineralized bituminous coal char, both loaded with 3% K by IW impregnation. The behavior of the former is seen to be nearly identical with that of Saran-K-IW char
0.6
Figure 6. Reactivity profiles for Saran-K-IW (a) and Saran-K-IE (b) chars gasified in 1 atm of COz at 1023 K. 4.0
40 h
0
.
0
'
0 0.0
0.2
0.4
0.6
0.8
1.0
Conversion Figure 7. Correlation between potassium content and reactivity for Saran-K-IW char gasified in 1 atm of COPat 1023 K.
over the entire conversion range. The latter is significantly less reactive and shows a maximum at an earlier conversion range. This may be attributed to unfavorable interaction of potassium with the remaining mineral matter in the char (Formella et al., 1986; Sams et al., 1985). Hamilton et al. (1984) have observed similar rate variations with conversion for a bituminous coal char, loaded with 1.5 and 3.0 w t % K by IW impregnation and gasified in 0.15 atm of COz at 1073 K. They could not explain the reactivity profiles (which showed a minimum at X = 0.20 and a maximum at X = 0.70) using some of the recently proposed random pore models of char gasification. They concluded that K-catalyzed char gasification is complex and depends mainly on three factors: the change in total surface area of the char, the change in K/C ratio due to gasification, and the loss of catalyst. Figure 9 presents the RSA variations with conversion for the Saran-K-IW and -1E chars. They are seen to be remarkably similar to their respective reactivity profiles. Figure 10 presents a plot of turnover frequency (or RIRSA) vs conversion for these two chars. Considering the rather
1740 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991 140 120
3.0 -
-.
100
c
v
2.0-
a
20
0.04 0.0
"
"
"
"
0.2
0.4
0.6
0.8
'
I 1.0
0 0.0
0.2
0.4
0.6
0.8
1.0
Conversion
Figure 10. Turnover frequency vs conversion for gasification of K-catalyzed Saran chars in 1 atm of C02 at 1023 K: w, Saran-K-IW, O , Saran-K-IE (40% error bare shown).
0.0 0.0
0.2
0.4
0.6
0.8
1.0
Conversion
Figure 8. Reactivity profiles for Carbosieve-K-IW (a) and PSOC1098-Dem-K-IWchars gasified in 1 atm of COPat 1023 K. I
60 1
G . CD
V
P
v
d
v1
a
0.0
0.2
0.4
0.6
0.8
1.0
I
120 I '
(b)
100 -
G . V
80 -
M
-
