Energy & Fuels 1996, 10, 431-435
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Ion-Exchanged Calcium from Calcium Carbonate and Low-Rank Coals: High Catalytic Activity in Steam Gasification Yasuo Ohtsuka* and Kenji Asami Research Center for Carbonaceous Resources, Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan Received August 25, 1995X
Interactions between CaCO3 and low-rank coals were examined, and the steam gasification of the resulting Ca-loaded coals was carried out at 973 K with a thermobalance. Chemical analysis and FT-IR spectra show that CaCO3 can react readily with COOH groups to form ion-exchanged Ca and CO2 when mixed with brown coal in water at room temperature. The extent of the exchange is dependent on the crystalline form of CaCO3, and higher for aragonite naturally present in seashells and coral reef than for calcite from limestone. The FT-IR spectra reveal that ionexchange reactions also proceed during kneading CaCO3 with low-rank coals. The exchanged Ca promotes gasification and achieves 40-60-fold rate enhancement for brown coal with a lower content of inherent minerals. Catalyst effectiveness of kneaded CaCO3 depends on the coal type, in other words, the extent of ion exchange.
Introduction publications1-4
In earlier we have shown that inexpensive Ca(OH)2 is suitable as a catalyst for the steam gasification of low-rank coals, since the Ca2+ ions in saturated Ca(OH)2 solution can readily be ion-exchanged with COOH groups when mixed with coal at room temperature, and the exchanged Ca is very active for the gasification. As is well-known, Ca(OH)2 is manufactured by calcination of limestone (CaCO3) at high temperatures and subsequent hydration of CaO formed. Utilization of CaCO3 in place of Ca(OH)2 as a catalyst raw material can therefore lead to a simplified process. We have previously reported that CaCO3 shows almost the same rate enhancement in steam gasification as Ca(CH3COO)2 when kneaded with a brown coal in water.2 However, it has been remained mysterious why CaCO3 which is practically insoluble in water can work as effectively as water-soluble Ca(CH3COO)2. Therefore the present work aims first at making clear interactions between CaCO3 and low-rank coals in water, and second at examining in detail the catalyst effectiveness of CaCO3 for the steam gasification at low temperatures. Experimental Section Coal Sample. Low-rank coals with almost the same carbon contents were used in the present study. All the samples were air-dried, ground, and sieved to coal particles with size fraction 75-150 µm. The analyses are shown in Table 1, where the content of the calcium inherently present in coal is also given as inherent Ca and ranges 0.03-1.3 wt %. Loy Yang brown coal, denoted as LY, was used mainly. X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Nabatame, T.; Ohtsuka, Y.; Takarada, T.; Tomita, A. J. Fuel Soc. Jpn. 1986, 65, 53-58. (2) Ohtsuka, Y.; Tomita, A. Fuel 1986, 65, 1653-1657. (3) Takarada, T.; Ohtsuka, Y.; Tomita, A. J. Fuel Soc. Jpn. 1988, 67, 683-692. (4) Ohtsuka, Y.; Asami, K. Energy Fuels 1995, 9, 1038-1042.
0887-0624/96/2510-0431$12.00/0
Catalyst Material and Addition. Two crystalline forms of CaCO3, hexagonal-rhombohedral calcite and orthohombic aragonite, were used. Calcite was of research grade from Kanto Chemical Co., the assay after dryness at 383 K being >99%. Aragonite of >99% pure was supplied by Yoshizawa Lime Industry Co. Each CaCO3 was received in a powder form and sieved to 97% purity was also kneaded with LY coal.4 The ion-exchange method using CaCO3 was carried out under a flow of high-purity N2. First, 10 g of LY coal and 150 cm3 of deionized water were mixed in a flat-bottomed flask; a large amount of pure N2 was bubbled into deionized water before use in order to remove CO2 naturally dissolved in it. Then, 1.3 g of calcite or aragonite was added into the flask and the resulting aqueous mixture was stirred magnetically at room temperature. The change in the pH during stirring was monitored, and all the effluent from the flask was collected into a plastic bag for the analysis of CO2 by gas chromatography. After stirring for 60 min and subsequent standing, the Ca-exchanged coal was carefully separated from the remaining CaCO3 slurry containing a small amount of coal by repeated decantations. Finally, the exchanged coal was dried in the same manner as in the kneading method. Characterization. Actual loadings in all the Ca-loaded samples were determined by atomic absorption spectroscopy (AAS) after the acid leaching at 330 K. The content of inherent Ca in the original coal was also analyzed by the leaching method in the same manner as above. Fourier transform infrared (FT-IR) spectra of some Ca-loaded coals were measured by the diffuse reflectance method to examine interactions between coal and CaCO3 in the catalyst addition step. The X-ray diffraction (XRD) measurements of Ca-loaded coals
© 1996 American Chemical Society
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Table 1. Analyses of Coals and Actual Calcium Loadings coal (country)
code
Loy Yang (Australia) Wyoming (U.S.A.) Zalainuoer (China) Bienfait (Canada)
LY WM ZN BF
ultimate analysis (wt %, daf) C H N S O 68.1 68.1 69.3 69.9
4.7 4.9 4.9 4.7
0.6 0.9 1.7 1.1
0.3 0.7 0.3 0.6
26.3 25.4 23.8 23.7
ash (wt %, dry)
inherent Caa (wt %, dry)
0.7 5.4 6.3 13.5
0.03 0.79 1.0 1.3
Ca loadinga,b (wt %, dry) phys knead ion 3.9 4.3 4.0
3.9 4.2 4.0 3.9
2.2
a
Determined by atomic absorption spectroscopy. b Corrected by the amount of inherent Ca. Phys, physical mixing; knead, kneading; ion, ion exchange. Table 2. Amounts of Ca Loaded Actually on Loy Yang Coal and CO2 Evolved during Ion Exchange with CaCO3 type of CaCO3
Ca loaded (mmol/g of coal)
CO2 evolved (mmol/g of coal)
calcite aragonite
0.56 0.84
0.41 0.63
and the devolatilized chars were carried out with Ni-filtered Cu KR radiation (45 kV, 30 mA) to clarify the crystalline form and dispersion state of calcium catalyst. The char samples for XRD were prepared under a stream of high-purity N2 in the same manner as the gasification experiments described below. The Ca-loaded coals were devolatilized at 973 K for 10 min, and the resulting chars were subjected to XRD measurements immediately after they were quenched to room temperature. Steam Gasification. Isothermal gasification experiments were performed with a thermobalance attached with infrared lamps. This made it possible to heat 20 mg of the sample at 300 K/min up to 973-1023 K in a flowing stream of steam (80 kPa) diluted with high-purity N2. The details of the procedure have been described elsewhere.5 The thermogravimetric curves revealed that the coal devolatilization completed within a few minutes, and the char gasification proceeded subsequently. The effectiveness of Ca catalyst at the latter stage will be discussed throughout the present paper. Char conversion and specific rate are used as the indexes to describe the reactivity of char. Char conversion is expressed as wt % on a dry ash-free, catalyst-free basis. Specific rate is defined as the gasification rate per unit weight of remaining char, and given as the average rate in the conversion range of 10-30 wt %.
Figure 1. Change in pH of the aqueous mixture of Loy Yang coal and CaCO3 in the ion-exchange process.
Results Calcium Loading. Actual Ca loadings determined by AAS are summarized in Table 1, where calcite is used and the loading is corrected by the amount of inherent Ca present in the original coal. As expected, most of the calcium nominally added into coal could be actually loaded by the physically mixing and kneading methods irrespective of the coal type. When CaCO3 was ionexchanged with LY coal, however, half of the calcium was loaded, the remainder being recovered as CaCO3 by decantation after the ion exchange process. Ion-Exchange Process. Figure 1 illustrates the change in pH during stirring the aqueous mixture of LY coal and CaCO3. The pH of the mixture of coal and water before CaCO3 was added into it was approximately 4. When calcite was added, the pH increased gradually with increasing time and became 6 after 60 min. On the other hand, on addition of aragonite, the pH increased steeply from the initial 4 to 6.5 in a few minutes and then at a slow rate, and finally reached about 7 after 60 min. No significant changes in pH with time were observed without addition of CaCO3 or with (5) Asami, K.; Ohtsuka, Y. Ind. Eng. Chem. Res. 1993, 32, 16311636.
Figure 2. FT-IR spectra of Loy Yang coal with and without CaCO3.
an aqueous slurry of CaCO3 alone, the pH of the latter being 9.8-9.9. Therefore, these observations suggest some reaction between coal and CaCO3 at different rates. Table 2 shows the amounts of both Ca loaded and CO2 evolved. The actual Ca loading was higher with aragonite than calcite; 70% of the calcium added was actually loaded on LY coal with aragonite, whereas only 50% with calcite. Interestingly, a large amount of CO2 was evolved in both cases. The amount was also larger with aragonite, but the ratio of CO2 evolved to Ca loaded was almost the same between the two, that is, 0.75 and 0.73 with aragonite and calcite, respectively. It is evident that CO2 comes from the reaction of coal and CaCO3 in water. FT-IR Spectra. Figure 2 shows the FT-IR spectra of LY coal with and without CaCO3. The raw coal without CaCO3 had a strong, carboxylic CdO stretching
Interactions between CaCO3 and Low-Rank Coals
Figure 3. X-ray diffraction profiles for CaCO3 loaded on Loy Yang coal by different methods.
band at 1700 cm-1. The same spectrum was observed in the coal physically mixed with CaCO3, though not given in Figure 2. When CaCO3 was added by the ionexchange method, the intensity of the absorption band at 1700 cm-1 decreased drastically, and instead the intensity of the peak near 1600 cm-1 increased, the latter band including the carboxylate anions as well as the carbonyl bonds naturally present in coal. The spectrum was independent of the crystalline form of CaCO3. As is seen in Figure 2, the CaCO3-kneaded coal showed almost the same spectrum as in the exchanged sample. These observations confirm that ion-exchange reactions between CaCO3 and COOH groups take place in both the ion exchange and kneading process. X-ray Diffraction Profiles. Figure 3 illustrates the XRD profiles for Ca-loaded coals prepared by three different methods. No diffraction lines due to Ca species were present when CaCO3 was incorporated into LY coal by the ion-exchange method. Contrarily, the peaks of CaCO3 appeared with the kneaded carbonate, and the intensities increased considerably with the physicallymixed carbonate. Figure 4 shows the XRD results for Ca-bearing chars after devolatilization at 973 K. When Ca-exchanged LY coal was devolatilized, no significant XRD lines from Ca species existed on the char, which means that the calcium is too highly dispersed to be detected by XRD. On the other hand, very small peaks of CaO and Ca(OH)2 were observed on the char derived from CaCO3kneaded LY coal. It is reasonable to understand that Ca(OH)2 is formed by hydration of CaO with moisture upon exposure of the char to laboratory air for recovery, since high-temperature XRD measurements have revealed complete transformation of Ca(OH)2 into CaO and CaCO3 in an inert gas at e800 K.2 Easy hydration of CaO at room temperature suggests that some of Ca catalyst loaded on coal is present at the outer surface. As is seen in Figure 4, the XRD intensities of Ca(OH)2 increased on the char from CaCO3-kneaded WM coal. Almost the same profiles were observed on Ca-bearing chars from BF and ZN coal. Reactivity in Steam. Figure 5 shows the gasification profiles for LY coal with and without CaCO3 at 973 K. The reactivity of the original coal was low, and char conversion was 20% after 90 min. The catalytic effect
Energy & Fuels, Vol. 10, No. 2, 1996 433
Figure 4. X-ray diffraction profiles for Ca-bearing chars prepared at 973 K from different CaCO3-loaded coals: A and B, from Loy Yang coal ion-exchanged and kneaded with CaCO3, respectively; C, from Wyoming coal kneaded with CaCO3.
Figure 5. Steam gasification profiles at 973 K for Loy Yang coal with and without CaCO3: A, original coal; B, physicallymixed with CaCO3; C and D, ion-exchanged with CaCO3 of calcite and aragonite, respectively; E, kneaded with CaCO3.
of the physically mixed calcite was low, and only a small degree of the rate enhancement was observed. On the other hand, the presence of the exchanged catalyst increased the gasification rate dramatically, and char conversion reached 90% at 20 min. The use of aragonite in place of calcite further enhanced the rate, which leads to higher conversion of 95% at 20 min. The catalytic activity of the kneaded calcite was almost the same as that of the exchanged Ca from aragonite. Figure 6 illustrates the gasification profiles for WM, ZN, and BF coal kneaded with CaCO3. Compared with the original LY coal, the reactivities of these coals without CaCO3 were much higher. The carbonate promoted the gasification of all the coals, but the degree of rate enhancement was lower than that with LY coal. Gasification Rate. Figure 7 shows the specific rate at 973 K of the Ca-catalyzed gasification of LY coal as a function of char conversion. The rate with the physically mixed carbonate was small and decreased with increasing char conversion. In contrast, the larger rates with the exchanged carbonates were almost constant in the conversion range of e50% regardless of the crystalline form and tended to rather increase at the latter stage of the gasification. The change in rate with conversion for the kneaded carbonate had the same tendency as for the exchanged catalysts. When WM,
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gave higher RCa, which was nearly equal to that (7.6 1/h) for the kneaded calcite. Discussion Reaction of CaCO3 with COOH Groups. A significant amount of CO2 evolved in the ion-exchange process (Table 2), and the intensity of the IR band due to COOH groups decreased dramatically in the Caexchanged coal (Figure 2). These observations point out that ion-exchange reactions between CaCO3 and COOH groups proceed according to the following equation. Figure 6. Steam gasification of the original and CaCO3kneaded coals at 973 K. Key as in Table 1.
Figure 7. Specific rate of the Ca-catalyzed gasification of Loy Yang coal against char conversion at 973 K. Key as in Figure 5. Table 3. Gasification Rates for Different Samples RCaa (1/h) temp Rnone coal (K) (1/h) phys knead ion LY 973 LY 1023 WM 973 ZN 973 BF 973
0.13 0.38 1.2 2.9 2.6
0.30
7.6b
1.7
3.8 6.6 4.5
2.6
5.4 (8.0)c 18.1
(RCa - Rnone)/Rnone knead ion 58
40 (61)c 47
2.2 1.3 0.7
a
Calcite was used unless otherwise described. Phys, physical mixing; knead, kneading; ion, ion exchange. b 8.5 for kneaded Ca(OH)2. c Use of aragonite in place of calcite.
ZN, and BF coals kneaded with CaCO3 were gasified, there was a similar relationship between specific rate and char conversion as with LY coal. Table 3 summarizes the specific rates with and without CaCO3, denoted as RCa and Rnone, for all the samples examined in the present study. A rate enhancement index, defined as (RCa - Rnone)/Rnone, is also given in Table 3 to compare the rate enhancement by catalyst addition quantitatively among the samples. The rates were slightly increased or unchanged in the presence of the physically mixed carbonate. The kneaded carbonate enhanced the gasification rates of all the coals. The degree of the rate enhancement depended strongly on the coal type, and the carbonate on LY coal, which had the lowest rate without catalyst, gave the largest rate enhancement index of 58. It is noteworthy that the rate for kneaded CaCO3, 7.6 1/h, was comparable to that (8.5 1/h) for kneaded Ca(OH)2. When CaCO3 was added to LY coal by the ionexchange method, as is seen in Table 3, the exchanged catalyst worked as effectively as the kneaded carbonate, the enhancement index being 40 and 47 at 973 and 1023 K, respectively. The use of aragonite instead of calcite
CaCO3 + 2(-COOH) ) -(COO)2Ca + CO2 + H2O (1) The formation of Ca carboxylates is also supported by the increased intensity of the absorption band at ≈1600 cm-1, though the contribution of carboxylate anions to the intensity could not be determined because of overlapping CdO bonds. Equation 1 shows that the amount of CO2 evolved should be equal to that of exchanged Ca. However, the amount of CO2 was equivalent to about 75% of the calcium, irrespective of the crystalline form of CaCO3. Since it is difficult to separate the Caexchanged coal completely from unreacted CaCO3 slurry by decantation, some of unreacted CaCO3 would be physically adsorbed in the coal matrix, which results in higher amount of the calcium observed than calculated. It is probable from these considerations that eq 1 proceeds almost stoichiometrically. The calcium incorporated in the exchange process would have the same coordination sphere as the calcium exchanged by using water-soluble Ca(CH3COO)2, since ion-exchange reactions between Ca2+ ions in the acetate solution and COOH groups occur with the same stoichiometry of 1:2 as in eq 1.6 When CaCO3 was kneaded with LY coal, the drastic decrease in the IR intensity of COOH groups and the corresponding increase in the carboxylate band were observed as in the Ca-exchanged coal (Figure 2). It is evident that ion-exchange reactions also take place during kneading. Since the extent of the exchange is controlled by the content of COOH groups which are not associated with inherent metal cations, LY coal with an extremely low ash content (0.7 wt %) can hold a larger amount of exchanged Ca than other low-rank coals with 8-20-fold ash contents. Scheme for Ion-Exchange Reactions. Ion-exchange reactions may proceed according to the following scheme.
CaCO3 ) Ca2+ + CO32Ca2+ + 2(-COOH) ) (-COO)2Ca + 2H+ 2H+ + CO32- ) H2O + CO2 CaCO3 would initially dissolve in deionized water, because the direct reaction between CaCO3 and COOH groups is unlikely. The solubility of CaCO3 in water is quite low (14 mg/L of water), but it is increased in the (6) Linares-Solano, A.; Salinas-Martı´nez de Lecea, C.; CazorlaAmoro´s, D.; Joly, J. P.; Charcosset, H. Energy Fuels 1990, 4, 467474.
Interactions between CaCO3 and Low-Rank Coals
acidic mixture of LY coal and water. Then, Ca2+ ions undergo exchange reactions with COOH groups. Since the H2CO3 formed is unstable in the present pH region of 6-7 (Figure 1), CO2 can be released and further expelled completely from the aqueous mixture by bubbling N2. The removal of CO2, that is, CO32- ions, from the aqueous system can promote the dissolution of CaCO3 in water. Nevertheless, the dissolution process would be a rate-determining step for the present exchange reaction. The pH of the aqueous mixture after completion of eq 1 should be equal probably to 4.5 for an saturated aqueous solution of CO2 unless CO2 evolved is retained in the mixture. Since CO2 was actually purged by N2, however, the pH increased with increasing time and approached 7 (Figure 1). The steep increase in the pH observed with aragonite suggests that the rate of ion exchange is higher with aragonite than calcite. The reason is not clear at present, but it may be related to a slightly larger solubility of aragonite in water. As shown in Figure 1, the pH at completion of the ion exchange step was also higher with aragonite. This lead to higher Ca loading (Table 2). Since LY coal contains 2.4 mequiv/g of COOH groups,7 the amount of the calcium exchanged by eq 1 was equivalent to 70 and 45% of the COOH content for aragonite and calcite, respectively. In the ion-exchange method using a saturated solution of Ca(OH)2, on the other hand, the extent of the exchange is nearly equal to the COOH content.1 The larger extent observed with Ca(OH)2 may be caused by the higher solubility in water. Catalyst State and Activity. It has been shown by X-ray absorption fine structure spectroscopy that the calcium ion exchanged with COOH groups in low-rank coals is essentially atomically dispersed.8,9 On devolatilization at low temperatures of around 1000 K, the calcium is transformed into the well-dispersed species with crystallites of
