Energy & Fuels 1995,9, 1043-1050
1043
Effect of Catalyst Impregnation Conditions and Coal Cleaning on Caking and Gasification of Illinois No. 6 Coal Robert L. McCormick” and Mahesh C. &at Amax Research and Development Center, Golden, Colorado 80403 Received June 7, 1995@
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The effect of catalyst impregnation conditions on activity of Ca, Na, and Na Ca composite catalysts for swelling reduction and gasification of Illinois No. 6 coal was investigated. The asreceived coal and samples cleaned by panning with water and heavy liquid separation were studied. The coals were impregnated with catalyst solutions at various levels of pH, catalyst loading, and NdCa molar ratio. Catalyst loadings of a few tenths to about 4 wt % metals were examined. Impregnated coals were characterized by free swelling index, diffise reflectance infrared spectroscopy, and X-ray powder diffraction. Catalyst initial dispersion was measured by C02 chemisorption a t 573 K after pyrolysis in argon at 1073 K. Carbon dioxide gasification rates were measured by TGA at 1073 K. Impregnation of Illinois No. 6 coal with catalysts consisting of Ca or Na Ca using a pH 3 solution leads to better wetting of the coal and enhanced interaction of catalyst with coal surface functional groups relative to impregnation at pH 9. Higher activity for decaking and gasification results. Solution pH had no effect on activity of Na. Synergistic effects were observed for Na Ca composites prepared a t low pH and NdCa molar ratio greater than one. Calcium and Na Ca catalysts deactivated by reaction with sulfur and mineral matter. Sodium-containing catalysts retained their activity to high conversion levels but were less active than Ca or Na Ca. Deactivation was reduced significantly by heavy liquid cleaning of the coal to remove most of the inorganic sulfur and more than 80 w t % of the ash. Panning (water washing) t o remove roughly 50%of the ash and 30%of the sulfur was not effective.
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Introduction Among the most pressing technical problems in gasification of Illinois coals are coal swelling and low carbon conversion and consequent high temperature and large char recycle required in most systems.l One approach to overcoming these obstacles is t o add a gasification catalyst t o the coal. Gasification catalysts can substantially lower the temperature required for a given reaction rate,2s3enhance ultimate carbon conversion, and reduce or eliminate coal swelling and consequent caking and aggl~meration.~,~ Swelling can be eliminated through the formation of cross-links at temperatures below that required for pyrolysis. It has been proposed that alkali and alkaline earth metals affect swelling through reaction with coal phenolate and carboxylate gr0ups.~9~ Swelling elimination may occur because these metals catalyze formation of ether cross-links at temperatures near 573 K. Another possibility is that the decaking activity of alkali
* Corresponding author. Department of Chemical Engineering and Petroleum Refining, Colorado School of Mines, Golden, CO 80401-1887. Phone: (303) 273-3967. FAX: (303) 273-3730. Internet:
[email protected]. + Present address: Entech Global, Inc., Golden, CO 80403. Abstract published in Advance ACS Abstracts, October 15, 1995. (1) Penner, S. S., Ed. “Coal Gasification: Direct Applications and Synthesis of Chemicals and Fuels”; DOEER-0326, June 1987. (2) Johnson, J. L. Cutal. Rev.-Sci. Eng. 1976,14(1), 131-152. (3) Nishiyama, Y. Fuel Process. Technol. 1991,29,31-42. (4) Khan, M. R.; Jenkins, R. G. Fuel 1986,65,1203-1208. (5) Khan, M. R.; Jenkins, R. G. Fuel 1989,68,1336-1339 . (6)Bexley, K.; Green, P. D.; Thomas, K. M. Fuel 1986,65,47-53. (7) Tromp, P. J. J.; Karsten, P. J. A.; Jenkins, R. G.; Moulin, J. A. Fuel 1986,65,1450-1456. @
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and alkaline earth additives is a physical effect and results from separation of plastic coal particles from one another by inert additive particles. Khan and Jenkins4s5 investigated this hypothesis by comparing physically mixed SiO2-, Ca-, and K-containing additives at loadings of 5-50 wt % t o distinguish between diluent and catalytidchemical effects. They observed a small reduction in swelling for Si02. The effects of Ca and K compounds were much more significant. Considering these and published results, the chemical nature of the decaking phenomena for alkali and alkaline earth additives would seem to be well established. To mitigate the problem of low carbon conversion, high gasification temperatures ( > 1250 K) are usually employed. High temperatures can lead t o increased oxygen requirement (lower heating value product gas), may require expensive materials of construction, can increase agglomeration problems, and can increase levels of alkali vaporization. Therefore, it is desirable to develop improved methods for coal gasification that allow high carbon conversion to be obtained a t lower temperature. This is particularly desirable for Illinois coals which exhibit a high caking tendency. It is well-known that alkali, alkaline earth, and transition metals found naturally in coal can catalyze gasification.1° By increasing the gasification rate, a catalyst can reduce required operating temperatures (8) Gouker, T. R.; Liotta, R. Fuel 1986,64,200-208. (9) Crewe, G. F.; Gat, U.; Dhir, V. K. Fuel 1976, 54, 20-23. (10)Walker, P. L.; Matsumoto, S.; Hanzawa, T.; Muira, T.; Ismail, I. M. K. Fuel 1983,62 140-142.
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1044 Energy & Fuels, Vol. 9, No. 6, 1995
and pressures, reduce residence times, reduce char recycle rates, or lead to a reduction in gasifier size. In conventional catalytic gasification, an alkali metal carbonate is physically mixed with the coal at loadings of 10-20 wt This high loading necessitates recovery and recycle of the catalyst. Gasification catalysts have also been added by ion exchange12J3and by i m p r e g n a t i ~ n . ~These > ~ J ~approaches may provide high gasification activities a t lower catalyst loadings. If catalyst loading is low enough the catalyst could be used on a throwaway basis. Both Na and K carbonates have shown high activity when physically mixed with coal or char.15J6 The catalysts eventually deactivate by reaction with mineral Meijer and matter to form inactive aluminosili~ates.~~J~ co-workerslg have reported that up t o 40 wt % of the potassium or sodium can be lost by evaporation during gasification at temperatures as low as 1000 K. Calcium has long been known to be an active gasification catalyst. Unlike the alkali metals, calcium must be highly dispersed to be active.20 If high dispersion is obtained by ion exchange to form surface carboxylate or phenolate salts, calcium has a catalytic activity equal to that of potassium.21 However, calcium rapidly deactivates by sintering or agglomeration.21,22Under slow heating conditions as might be encountered in a moving bed gasifier large CaO particles can form long before gasification temperatures are reached.23 Furthermore, reaction of calcium with sulfur to form Cas deactivates the catalysts.24 Advanced catalysts consisting of alkali (Na or K) and Ca have recently been reported by two research group^.^^,^^ Synergistic effects are observed and may be caused by prevention of Ca agglomeration by Na or K.25 A n initial atomic dispersion of Ca is not thought to be required for high activity. These catalysts are also reported to be resistant to poisoning by reaction with ash or sulfur up to at least 900 K.27 Resistance to (11)Nahas, N. C. Fuel 1983, 62, 239-241. (12) Abotsi, G. M. K.; Bota, K. B.; Saha, G. Fuel Sci. Technol. Int. 1993, 11(2), 327-348. (13) Matsumoto, S.; Walker, P. L., Jr. Carbon 1986, 24(3), 277285. (14) Haga, T.; Sato, M.; Nishiyama, Y.; Agarwal, P. K.; Agnew, J. B. Energy Fuels 1991,5, 317-322. (15) Probstein, R. F.; Hicks. R. E. Synthetic Fuels, 2nd ed.; McGrawHill: New York, 1990. (16) Sears, R. E.; Timpe, R. C.; Galegher, S. J.; Willson, W. G. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986,31, 166-175. (17)Lang, R. J.; Neavel, R. C. Fuel 1982, 61, 620-626. (18) Bruno, G.; Buroni, M.; Carvani, L.; Del Piero, G.; Passoni, G. Fuel 1988, 67, 67-72. (19) Meijer, R.; Weeda, M.; Kapteijn, K.; Moulijn, J . A. Carbon 1991, 29,929-941. (20) Carzorla-Amoros, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Nomura, M.; Yamashita, H.; Tomita, A. Energy Fuels 1993, 7, 625-631. (21) Kapteijn, F.; Porre, H.; Moulijn, J. A. M C h E J. 1986,32,691695. (22) Linares-Solano, A.; Almela-Alarcon, M.; Salinas-Martinez de Diu.Fuel Lecea, C.; Cazorla-Amoros,D. Prepr. Pap.-Am. Chem. SOC., Chem. 1989, 34, 136-143. (23) Shah, N.; Huggins, F. E.; Shah, A,; Huffman, G. P.; Jenkins, R. G.; Piotrowski, A. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1989,34, 30-35. (24) Yamashita, H.; Nomura, M.; Tomita, A. Energy Fuels 1992, 6, 656-661. (25) Pereira, P.; Somorjai, G. A.; Heinemann, H. Energy Fuels 1992, 6, 407-410. (26) Haga, T.; Nogi, K.; Amaya, M.; Nishiyama, Y. Appl. Catal. 1991, 67, 189-202. (27) Heinemann, H.; Somorjai, G. “Fundamental Studies ofCatalytic Gasification”;Report LBL-30015, Lawrence Berkeley Laboratory, June 1991.
McCormick and Jha deactivation at higher temperatures and alkali vaporization rates have not been determined. Sodium can be substituted for potassium in these catalysts with no effect on reactivity or deactivation. Limited research has been reported on the effects of the conditions of impregnation on catalyst dispersion or activity for decaking and gasification. Nishiyama and co-workers3J4suggest that oxygen-covered hydrophilic surfaces promote crystallization of large catalyst particles during impregnation of Ca or K. Hydrophobic surfaces that are poorly wetted were thought to lead to highly dispersed catalysts via the formation of very small droplets of solution during drying. Ginter and coworkers,28however, suggest that good wetting of the coal or char by the impregnating solution is required for highly dispersed K Ca composites. None of these groups examined the effect of impregnating solution pH, although the important effect of carbon or coal surface properties and solution pH on catalyst loading has been demonstrated by Solar and co-workers for equilibrium adsorptionz9 and should also have important consequences in impregnation. Solar and co-workers found that the charge of the catalyst precursor species and the charge of the carbon surface in the catalyst addition solution determined equilibrium catalyst loading. For example, at pH values where the surface is negatively charged, positive ions are attracted to the surface leading to a higher level of catalyst loading. Impregnation is a somewhat different situation where the coal pore structure is filled with catalyst solution and the moisture is then removed. Catalyst loading is determined by solution volume and concentration. The concentration of catalyst and the solution pH may change as drying proceeds, and the catalyst may be deposited on the surface by precipitation rather than adsorption or ion exchange. Abotsi and c o - w ~ r k e r shave ~ ~ shown that for an Illinois No. 6 coal the surface is negatively charged over a wide range of pH. They also demonstrated that calcium could not be ion exchanged onto this coal in the pH range of 1-11. It was possible to exchange potassium at pH greater than about 7. Because ion-exchange procedures appear t o limit the amount of catalyst that can be added t o Illinois No. 6 coal,30we have investigated catalyst addition by impregnation. The catalysts investigated are calcium, sodium, and calcium mixed with sodium. Note that this coal has a significant oxygen content (6.5 wt %) of which roughly 50% is present as hydroxyl with the balance as ether, carbonyl, and c a r b o ~ y l . ~ l - ~ ~
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Experimental Section The coal used in this study was a Herrin seam coal (Illinois No. 6) obtained from the Illinois Coal Sample Program (designated IBC-105). The coal was screened to 0.42 by 0.074 mm when received and stored in sealed plastic bags. The coal (28) Ginter, D. M.; Somorjai, G. A.; Heinemann, H. Energy Fuels 1993, 7,93-398. (29) Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. Carbon 1990, 28 (2/3),369-375. (30) Abotsi, G. M. K.; Bota, K. B.; Saha, G. Energy Fuels 1992, 6 , 779-782. (31)Abdel-Baset, Z.; Given, P. H.; Yarzab, R. F. Fuel 1978,57,9599. (32) Dela Rosa, L.; Pruski, M.; Lang, D.; Gerstein, B.; Solomon, P. Energy Fuels 1992, 6 , 460-468. (33) Gethner, 3. S. Fuel 1987.66, 1091-1096.
Energy & Fuels, Vol. 9, No. 6, 1995 1046
Caking and Gasification of Illinois No. 6 Coal was cleaned by water washing and heavy liquid gravity separation. Water washing was performed by panning several hundred grams to remove the heavier mineral constituents. Approximately 80 wt % of the starting coal was recovered after drying in air at 323 K. Heavy liquid cleaning was performed by floating the coal off the mineral matter in a heavy liquid of specific gravity 1.4. This liquid was prepared from a petroleum fraction of gravity 0.75 and perchloroethylene of gravity 1.6. Approximately 40 wt % of the starting coal was recovered as the float fraction. The heavy liquid was removed from the float fraction by drying in air at 323 K followed by vacuum drying at 323 K. Catalyst addition was performed by impregnation from aqueous solutions. Sodium hydroxide and calcium acetate were used as the catalyst precursors. Composite catalysts were produced by impregnating Ca and Na from a solution containing both components. The procedure followed was to prepare 50 mL of a solution of the desired concentration based on a target catalyst loading. The pH of this solution was then adjusted to the desired level using nitric acid (12 M) or ammonium hydroxide (28% as NH3). This pH adjusted solution was mixed with 100 g of coal. Calcium and/or sodium concentration varied with the target catalyst loading and was as high as 0.9 M for the most highly loaded catalysts. The impregnated coal was dried in air at 323 K overnight (16 h). These drying conditions were chosen because they are similar to conditions that might be used in a coal preparation plant. Control samples were prepared by drying the raw coal under identical conditions and by impregnating the raw coal with pH adjusted water containing no catalyst followed by drying under identical conditions. Free swelling index (FSI) was selected for measurement of caking tendency. Comparison of FSI with pyridine swelling and qualitative measures of bench scale gasifier agglomerate hardness indicated that this is a reasonably accurate and sensitive measure of coal swelling under gasification conditions, although it is certainly not quantitative. Infrared spectra were obtained using a Nicolet 510P FTIR equipped with a Spectra Tech diffise reflectance cell. Spectra were obtained on neat coal samples (not diluted with KBr) that had been ground for 1min in a Wig-L-Bug mill. Typically 400 scans were acquired with a resolution of 4 cm-l to produce the reported spectra. X-ray diffraction patterns were obtained ,on a Philips XPert System using Cu K a radiation. Na and Ca catalyst content was determined by flame atomic absorption. The sample was first dried at 383 K for 1h, ashed in air at 898 K, and fused with LiB02. The melt was dissolved in 5% nitric acid. The atomic absorption analysis yields weight percent of the catalytic metals. The weight percent catalyst loading reported has been adjusted by subtracting the Na and Ca present in the coal before catalyst addition. Measurements of catalyst initial dispersion and gasification rate were accomplished in a TGA experiment. Roughly 100 mg of coal was loaded into the apparatus and air was purged by evacuation (rough vacuum)/Ar flushing cycles. The sample was then heated t o 1073 K at 30 Wmin and held for 10 min. After the 10 min soak the sample was cooled to 573 K. Argon containing 10% carbon dioxide was introduced and the weight gain was monitored. After 30 min the gas was switched back to argon. The weight gain was taken as the amount of C02 chemisorbed and was used to calculate catalyst dispersion. This technique was first employed by Ratcliffe and Vaughn34 and has been investigated extensively by L i n a r e s - S ~ l a n o . ~ ~ Dispersion was calculated as the mole fraction of Ca, Na, or Na Ca added to the coal that chemisorbs COZ:
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dispersion = mmol CO, adsorbed(mmo1 Na
+ Ca)
(34) Ratcliffe, C. T.; Vaughn, S. N. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1986,30,304-310. (35)Linares-Solano, A,; Almela-Alarcon, M.; Salinas-Martinez de Lecea, C. J. Catal. 1990,125, 401-410.
Table 1. Analytical Data for Raw and Cleaned Coals air water heavy liquid dried washed cleaned proximate analysis, wt % moisture 9.1 4.8 2.3 volatile (mf) 36.1 39.0 43.2 ash (mf) 3.2 18.4 8.9 45.6 52.1 fxed carbon (mf) 53.7 ultimate analysis, (mf)wt % carbon 65.4 69.9 74.7 4.3 5.2 hydrogen 5.3 1.3 1.4 nitrogen 1.5 8.0 6.4 7.7 oxygen 4.68 3.08 sulfur 2.88 sulfur forms, (mf)wt % 2.29 ND" pyritic 0.20 sulfate 0.15 0.20 ND 2.52 2.19 ND organic (bd) selected elements, (mf)wt ?k sodium 0.115 0.058 0.072 0.278 ND potassium 0.060 0.803 0.083 calcium 0.071 iron 1.96 ND 0.406 free swelling index 3.0 2.5 3.0 a ND = not determined. The results of replication of this measurement on one sample indicate that the dispersion results may be considered accurate to f0.03 mole fraction. Note that heating rate during pyrolysis can have a large effect on the catalyst dispersion obtained.36 However, our results should provide a relative indication of catalyst dispersion even a t the low heating rate employed (30 Wmin).35 After dispersion measurement the sample was heated at 30 Wmin t o 1073 K under argon. Upon attaining 1073 K the argon flow was stopped and pure carbon dioxide was introduced to gasify the sample. Most gasification runs were continued until complete carbon conversion was achieved. Gasification results are reported as normalized gasification rate (mg/mgi,iti.l - min) vs conversion.
Results Table 1lists relevant analytical data for the dried and cleaned coal samples. Water washing and heavy liquid cleaning were successful a t removing large fractions of the mineral matter and inorganic sulfur. Analysis of pyritic sulfur in the heavy liquid cleaned sample indicated that virtually all had been removed. The starting coal contains significant amounts of naturally occurring catalytic species and their concentration was also reduced by washing or heavy liquid cleaning. Note that cleaning of the coal had little effect on FSI relative to the air dried coal. FSI for the as-received coal was 4.5 and this was reduced to about 3.0 by air drying. Impregnation with water adjusted to pH of 3 or 9 and air drying had the same effect. This reduction in FSI is most likely a result of low-temperature oxidation during the drying process. Oxidation will inevitably occur in any practical process for adding catalyst to coal and the results reported here are intended to model such a process. Infrared spectra of the air dried and cleaned samples are shown in Figure 1. Spectra of control samples prepared at pH 3,6,and 9 were also obtained and were identical to that reported for the dried coal. Differences in the spectra of untreated and cleaned samples are ~~
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(36)Cazarla-Amoros, D.; Linares-Solano, A,; Salinas-Martinez de Lecea, C.; Yamashita, H.; Kyotani, T.; Tomita, A.; Nomura, M. Energy Fuels 1993,7 , 139-145.
McCormick and Jha
1046 Energy & Fuels, Vol. 9, No. 6, 1995
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Figure 1. DRIFTS spectra of coal samples: (a) dried coal, (b) water-washed coal, (c) heavy liquid cleaned coal.
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