Energy & Fuels 1994,8, 294-300
294
Carbon Monoxide Pretreatment of Subbituminous Coals S. C. Lim, R. F. Rathbone, A. M. Rubel, E. N. Givens,* and F. J. Derbyshire University of Kentucky Center for Applied Energy Research, 3572 Iron Works Pike, Lexington, Kentucky 40511-8433 Received August 24, 1993. Revised Manuscript Received December 1, 1 9 9 P
Thermal CO pretreatment of Wyodaksubbituminous coalsfrom the Clovis Point and Black Thunder mines in aqueous media at 300 OC produces a water-insoluble product having a higher hydrogen content, lower oxygen content, higher volatility, and increased solubility in pyridine. Oxygen-rich, water-soluble organic acids are formed in the absence of CO or at low CO pressures. However, at 800 psig of CO, the water-soluble organic product is absent. Both coal hydrogenation and CO conversion were examined in the presence of added OH-, C032-and HC03-. CO conversion is related to formation of formate in the aqueous phase and the generation of COz and Hz. Chemical, thermogravimetric, and optical microscopic data of the pretreated material indicate major modification of the coal structure during pretreatment. Liquefaction of the pretreated material at 400 "C in hydrogen/tetralin with or without catalysts (Fe and Ni-Mo) indicates that pretreatment gives higher conversion and, in most cases, higher oil yields and lower hydrogen consumption than with the untreated coal.
Introduction The process of coal liquefaction involves the cleavage of bonds within the coal structure and the stabilization of the resulting fragments with hydrogen to progressively reduce molecular weight and ultimately to produce distillable liquid products. The attainment of this goal requires minimizing the cross-linking and condensation reactions that are important to coal carbonization and lead to the formation of high molecular weight refractory species. Solomon et al.l identified at least two distinct cross-linking events when coals are heated. One applies primarily to low-rank coals and occurs a t lower temperatures. There is simultaneous COz and HzO evolution2 that correlates with the loss of carboxyl groups and occurs prior to bridge-breaking or depolymerization reactions. A second event that is exhibited by higher rank coals occurs at moderate temperatures simultaneously with methane formation. It follows the initial bridge-breaking reactions and correlates best with methane formation. For lignites, these cross-linking reactions begin at about 200 "C while the corresponding reactions for higher rank coals begin at temperatures above 400 OC.l Pretreating coal prior to direct liquefaction has been shown to result in higher yields of product through a net reduction of retrograde reactions. The methodology can vary from treatment by demineralization,3t4solvent swelling,6.6 alkylation,'-Q dissolution with strong acids and bases,lOJl or catalytic reactions,12 or by the presence of a Abstract published
in Advance ACS Abstracts, January 15, 1994. (1) Solomon, P. R.; Serio, M. A,; Deshpande, G. V.; Kroo, E. Energy Fuels 1990,4, 42. (2) Suuberg, E. M.; Lee, D.; Larson, J. W. Fuel 1985,64, 1669. (3) Serio,M. A.;Solomon,P. R.; Kroo,E.;Bassilakia,R.;Malhotra, R.; McMillen, D.Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1990,35(1), el. (4) Deshpande, G. V.; Solomon, P. R.; Serio, M. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1988, 33(2), 310. (5) Matturo, M. G.; Liotta, R.; Reynolds, R. P. Energy Fuels 1990,4, 346. (6) Warzinski, R. P.; Holder, G. D. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1991, % ( I ) , 44. (7) Schlosberg, R. H.; Neavel, R. C.; Maa, P. S.; Gorbaty, M. L. Fuel 1980, 59, 45.
0887-0624194125O8-0294$04.50/ 0
hydrogen.13 Aqueous treatment at around the critical temperature of water, i.e., -374 OC, is also known to cause significant changes to the coal structure which can enhance coal conversion under a variety of liquefaction process conditions.14J6Illinois No. 6 coal, pretreated with steam a t 5.0 MPa between 320 and 360 O C , showed a 2-fold increase in liquid yield and a 20% increase in total volatile yield when pyrolyzed at 740 OC.16 The pretreated coal had a reduced oxygen content," exhibited reduced hydrogen bonding and an increased pore volume,ls and had The twice as many hydroxyl groups as the starting presence of steam inhibited the retrograde reactions of the phenolic groups that were formed.18120 Generally, coal conversion in aqueous systems to which CO and base are added is higher than in pure aqueous especially for systems or in hydrogen donor low-rank coal^.^^^^ In the CO/HZO/NaOH system, CO (8) Baldwin, R. M.; Nguanpraegert, 0.; Kennar, D. R.; Miller, R. L. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1990,36(1), 70. (9) Solomon, P. R.; Squire, K. R. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1985, 30(4), 346 (10) van Bodegom, B.; van Veen, J. A. Rob; van Kesael, G. M. M.; Sinnige-Nijssen, M. W. A.; Stuiver, H. C. M. Fuel 1984, 63, 346. (11) Kasehegen, L. Znd. Eng. Chem. 1937,29,600.
(12) Solomon,P.R.;Serio,M.A.;Deshpande,G.V.;Kroo,E.;Schobert,
H.; Burgess, C. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1989, 34(3), 803. (13) Bockrath, B. C.; Illig, E. G.;Wassell-Bridger,W. D. Energy Fuels 1987, 1 , 227. (14) Tome, S . E.;Shah, Y.; Holder, G.D.;Deshpande, G. V.; Cronauer, D. C. Fuel 1985,64,883. (15) Bienkowski, P. R.; Naragan, R.; Greenkorn, R. A.; Chao, K. C. Ind. Eng. Chem. 1987,26,202. (16) Graff, R. A.; Brandes, S.D. Energy Fuels 1987,1,84. (17) Khan, M. R.; Chen, W. Y.; Suuberg,E. Energy Fuels 1989,3,223. (18) Tse, D. S.; Hirschon, A.; Malhotra, R.; McMillan, D. F.; Roes,D. S . Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1991,36(1), 23. (19) Brandes, S. D.; Graff, R. A.; Gorbaty, M. L.; Siskin, M. Energy Fuels 1991,3,494. (20) Trewhella, M. J.; Grint, A. Fuel 1988,67, 1135. (21) Lim, S. C. Ph.D. Theais, Monash University, 1990. (22) Appell, H. R.; Wender, I. Prepr. Pap.-Am. Chem. SOC., Diu. Fuel Chem. 1968, 12(3), 220. (23) Appell, H. R. Energy 1976,1(4), 24. (24) Jackson. W. R.; Larkina, F. P.; Lim, S.C.; Redlich, P.; Stray, G.;
Chaffee, A. In Coal Science and Technology; Moulijn, J. A., Na&, K. A., Chermin, H. A. G. Eds.; Elsevier: New York, 1987; Vol. XI, p 46ff.
0 1994 American Chemical Society
Pretreatment of Subbituminous Coals was found to quantitatively react with the base at approximately 100 "C to produce formate.27 Subsequently, -4% of the starting CO was converted to H2 and CO2 at 180 "C over a 2.5 h period, whereas in the absence of base the water-gas-shift reaction was not observed.28 The data of Elliott et aL29 appears to conflict with these reports in that they found that up to 90 ?6 of the CO was consumed at 300 "C to give hydrogen and C02 in the presence of various alkali metal salts. Even in the absence of any alkali metal salt, a sizable quantity of the CO has been observed to disappear over a 1h period. In this case, the extent of the water-gas-shift reaction was not reported. Farnum et reported a -6% water-gas-shift conversion at 300 "C in the presence of a Wyodak coal, and Safianos et a1.31observed an approximate 15% CO conversion after 30 min at 350 "C in the presence of a Waterberg coal and a pyrite-H2S catalyst. Although the noncatalyzed watergas-shift reaction has been regularly observed at temperatures of 400 "C or higher,32*33 there is confusion in the literature regarding the extent of the reaction at temperatures between 200 and 400 "C. In the study reported here the effect of aqueous CO pretreatment on subbituminous coals at temperatures around 300 "C has been investigated and the findings are consistent with recently reported results.34 Substantial changes to the coal structure have been identified and the liquefaction of the pretreated coal in a hydrogen donor solvent has been evaluated. Experimental Section Materials. Reagents were purchased as follows: 99% purity UVgrade tetralin, high-purity tetrahydrofuran (THF),and highpurity pentane were Burdick & Jackson Brand from Baxter S/P; dimethyl disulfide was from Fluka A G sodium hydroxide, bicarbonate, and carbonate were from Aldrich Chemical. UHP 6000# hydrogen was supplied by Air Products and Chemicals, Inc. Samples of two Wyodak subbituminous coals were used in this study. One was from the Clovis Point mine near Gillette, Wyoming, and was supplied by the Wilsonville Advanced Coal Liquefaction Research and DevelopmentFacility (WACLF).The other was from the Thunder Basin Coal Company in Wright, Wyoming, and was supplied by Consol, Inc. Each had been ground to -200 mesh, riffled, and stored in tightly sealed containers. Analyses of the coals are presented in Table 1. Shell 324M (2.8 w t % Ni, 12.4 wt % Mo on alumina) catalyst was provided by WACLF and ground to -100 mesh. Nanometer size iron oxide (63 wt % Fe) was provided by MACH I, Inc., King of Prussia, PA. Procedures. Pretreatment experiments were conducted by adding coal, distilled water, and a sodium salt, as specified, to a 25-mL microautoclave which was sealed and leak tested. In all of these experiments, sufficient water is present to maintain a liquid phase a t reaction temperature. The reador was pressurized and submerged in a fluidized sandbath at 300 "C and shaken a t (25) Oelert, H. H.; Siekmann, R. Fuel 1976, 55, 39. (26) Takemura, Y.; Ouchi, K. Fuel 1983,62, 1133. (27) King, A. D.; King, R. B.; Yang, D. B. J.Am Chem. SOC.1980,102, 1028. (28) Ng, F. T. T.; Tsakiri, S.K. Fuel 1993, 72, 211. (29) Elliott, D. C.; Sealock, Jr., L. J. 2nd. En#. Chem. Prod. Res. Deu. 1983,22,426. (30) Farnum,S. A,; Wolfson,A. C.; Miller, D. J.; Gaides,G. E.; Messick, D. D. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1986,30(2), 354. (31) Sofianos, A. C.; Butler, A. C.; Louwrens, H. B. Fuel Process. Technol. 1989,22, 175. (32) Appell, H. R.; Miller, R. D.; Illig, E. G.; Moroni, E. C.; Steffgen,
F. W. "Coal Liquefaction with Synthesis Gas", PETC/TR-79/1, Sept. 1979. (33) Amestica, L. A.; Wolf, E. E. Fuel 1986, 65, 1226. (34) Vaughn, S. N.; Siskin,M.; Katritzky,A.; Brons, G.; Reynolds, N.; Culross, C. C.; Neskora, D. R. United States Patent No. 5,151,173; Sept. 29,1992.
Energy & Fuels, Vol. 8, No. 2, 1994 295 Table 1. Ultimate Analysis of Wyodak Coals Clovis Point Black Thunder composition,w t % maf carbon 71.0 73.9 hydrogen 4.9 5.2 nitrogen 1.3 1.3 sulfur 1.1 0.6 oxygen, by difference 21.7 19.0 ash w t % dry coal 6.94 6.12 H/Cratio 0.83 0.84
a rate of 400 cycledmin. The pressure of saturated steam at this condition is 8.6 MPa (-1230 psig). At the end of the reaction period, the reactor was rapidly quenched to room temperature. Gaseous products were vented into a collection vessel and analyzed by gas chromatography. The solid and liquid products were scraped and washed from the reactor using distilled water and separated by filtration. A material referred to as the "water-insolubleproduct" was prepared by drying in air for 18 h at -4 'C. The water-soluble product was separated by extracting with tetrahydrofuran (THF) in a Soxhlet thimble for 18 h and drying overnight a t 80 "C/l6 Wa. The soluble fraction was concentrated by removing excess THF in a rotary evaporator. A 5 0 1 excess volume of pentane was added and the mixture placed in an ultrasonic bath for 3 min. The pentane insoluble material was filtered and dried. After the pentane was evaporated, only trace quantities of material were obtained, indicating the absence of any measurable yield of pentane-soluble product. The pH of the aqueous phases before reaction was dependent upon the sodium salt that was added but was typically 6 after completion of the reaction. The water-soluble fraction was removed from the reactor, acidified with HC1 and centrifuged. The precipitated solid phase, which was regenerated humic acids, was collected and dried and the acidifiedaqueouslayer was further extracted with ether. In every case a t least two runs were made at the same conditions to prepare sufficient product for characterization and evaluation. The precision of the experimentally measured yields was *3 wt % maf coal for the water-insoluble product and f 2 wt % mafcoal for the THF-soluble and -insoluble products. Product yields are reported on a moisture- and ash-free (maf) basis. Elemental recoveries of carbon, hydrogen, and oxygen, found in the recovered water-insoluble product and humic acids, when they were formed, are reported relative to the starting coal. Ashing of the water-insoluble product indicated that >95% of the sodium added as NaOH reported to the aqueous layer. Larger scalecoalpretreatments were performed in a 1-Lstirred autoclave by adding 80 mL of water to the reactor which was sealed and pressurized with CO to 800 psig. The reactor was heated to 310 5 "C and a slurry containing 150 g of dry coal and the required amount of NaOH or NazCOs in an additional 120 mL of water was injected into the hot reactor. After approximately 7-8 min, the temperature of the reactor had recovered to the reaction temperature. After 1h the reactor was cooled and depressurized and the solid product was removed from the reactor and handled in the manner described above. Liquefaction experiments were performed in duplicate by placing 2 g of raw or pretreated coal and 2 g of tetralin into the microautoclave. Iron oxide or Shell 324 catalyst was added a t a level of 1 w t % iron or 0.1 wt % molybdenum on dry coal, respectively, along with dimethykdisulfide to provide a sulfur to added metal@) atomic ratio of 2. Hydrogen (5.62 MPa) was added, and the reactor sealed and then heated to 400 "C in the manner described above. Solid and liquid products were removed from the reactor and extracted with tetrahydrofuran (THF) in a Soxhlet thimble for 18 h and dried overnight at 80 "C/16 kPa. The THF insolubles contained insoluble organic matter (IOM) plus ash. The soluble fraction was concentrated by removing excess THF in a rotary evaporator. A 5 0 1 excess volume of pentane was added and the mixture placed in an ultrasonic bath for 3 min. The pentane-insolublepreasphaltenes and asphaltenes (PA+A) were filtered and dried. The soluble fraction was
*
Lim et al.
296 Energy & Fuels, Vol. 8,No. 2, 1994
Table 2. Product Distribution from Pretreatment of Coal. I I1 I11 IV V gas MPa at 23 "C added water, mL products, wt. % maf coal THF solubles THF insolubles water soluble humic acids ether extract mass recoveries elemental recovery carbon
hydrogen oxygen'
VI
VI1
VI11
2.86 4
co
5.62 2
co
co
5.62b 6
Hz 5.62 6
Nz 5.62 6
5.5 69.2
17.2 72.8
17.6 74.1
18.0 71.7
16.0 70.0
3.8 83.0
-
5.5 0.5 80.7
-
-
-
-
-
4.0
-
91.7
89.7
86.0
-
92.7
-
90.8
87.0
92 83 63
101 99 66
101 103 60
99 108 60
96 92 52
101 95 57
97 81 56
co
co
co
2.2
1.48 2 78.6 5.1 1.1
87.0 93 84
64
1.48 6
2.7
5.62 6
80.0 7.0
2 g of dry Clovis Point Wyodak coal, 4.4 wt % NaOH on moisture- and ash-free ( m a coal. No NaOH. Oxygen by difference.
recovered after the pentanewas evaporated. The product is then separated into pentane solubles, PA+A and IOM plus ash. The molydenum, nickel, and iron in the catalysts are presumed to convert to MoSz, NiS, and pyrrhotite ( F Q . ~ )respectively, , and report to the ash fraction. The product distribution was calculated assuming complete recovery of the ash plus catalyst. Oils are calculated by difference, and as a result, water produced during liquefaction,as well as any experimentalerror, is included in these yield data. Coal conversion equals 100 minus the yield of IOM. Analyses. FTIR absorption spectra were obtained on the water-insoluble products using a Nicolet Model 2OSXC spectrometer. Samples were demineralized with dilute 0.02 M HCl and dried overnight in avacuumoven at 60 O C and then pelletized with KBr at a concentration of 5 w t % . Spectra were obtained with a resolution of 4 cm-l over the interval 4000-400 cm-l by madding 128 interferograms. Carbon, hydrogen, and nitrogen were determined on aLecoModel 600combustion analyzer.Total sulfurwas determinedaccordingto ASTM D4239-84with a Leco Model SC32 combustion-IR analyzer. Optical Microscopy. Samples of water-insoluble product were prepared for microscopical examination by mixing the residues with epoxy resin, placing the mixture into a small cylindrical mold, and allowing the epoxy to harden. The surface was then prepared by grindingand polishing on a polishing wheel, using a series of Sic grits and alumina polishing compounds. Microscopical observations and 50 huminite reflectance measurementa were acquired on each sample using a Leitz MPV Compact microscope photometer. A petrographic method for detecting weathering in bituminous coal^^^ was used as a qualitative measure of acidic oxygen functional groups on the coals and pretreated coals. Freshly polished surfaces of the epoxy pellets were immersed in aqueous KOH (pH=13),rinsed and then placed into a solution of Safranin-0 cationic dye. The intensity of the staining this produced is related to the concentration of acidic oxygen functional groups.
Results and Discussion Pretreatment Conditions. Visual inspection of the products after coal pretreatment with CO and water indicated that a significant transformation had occurred. The material acquired a brownish appearance, similar to that of brown coals, sprinkled with tiny bright orange specks. The speckled appearance was particularly evident for the Clovis Point coal. Depending on the pretreatment reaction conditions, the physical characteristics of the solids range from being either soft and pliable to being highly friable. On heating to 60 "C,the solids turn black. In contrast, materials produced in the absence of CO, but in the presence of steam, were charred, granular, and very (36)Gray, R. J.; Rhoads, A. H.; King, D. T. Tram. SOC.Mining Eng.
1976, 260, 934.
(36) Marchioni, D. L. Int. J. Coal Geol. 1983, 2, 231.
Table 3. Hydrogen/Carbon Ratio of Products products pressure, gas MF'a humic acids water-insoluble I I1 I11 IV V
VI VI1 VI11
co co co co co co Hz Nz
1.5 1.5 2.9 5.6 5.6 5.6 5.6 5.6
N.D.a 0.71 b C C C
0.82 0.52
0.75 0.75 0.83 0.85d 0.92 0.80 0.78 0.72
*
N.D. = not determined. Insufficient sample ( -4% ) for analysis. None formed. d Calculated from THF-solubleand THF-insoluble
fractions.
hard. None of the pretreated products consisted of the finely divided particles characteristic of the original coal. The appearance of the corresponding aqueous phases ranged from being lightly tinted to being quite dark, the latter indicating the presence of significant amounts of dissolved organic matter. The reaction products were recovered using water to remove the solid/liquid material from the reactor. The respective yields of materials soluble and insoluble in THF are shown in Table 2. The quantity of THF-soluble product that was soluble in pentane was so small in the high CO pressure runs that it could not be determined. At low CO pressures (runs I, 11, and 111),as well as in the absence of CO (runs VI1 and VIII), regenerated humic acids and ether-extracted fulvic acids were recovered from the aqueous phase. The remaining unaccounted material in the mass balance, in addition to the pentane solubles, includes CO+CO2 and water. Only trace quantities (
