Energy & Fuels 1998, 12, 589-597
589
A Kinetic Scheme for Catalytic Coliquefaction of Coal and Waste Tire Ramesh K. Sharma, John W. Zondlo, and Dady B. Dadyburjor* Department of Chemical Engineering, P.O. Box 6102, West Virginia University, Morgantown, West Virginia 26506-6102 Received September 26, 1997
Catalytic liquefaction of two bituminous coals and one subbituminous coal was studied separately and with waste tire as solvent at different temperatures and tire/coal ratios. The tire sample represented a mixture of waste, recyclable tires. A ferric-sulfide-based catalyst impregnated in situ in the coal was used as catalyst. Runs were made in a batch tubing-bomb reactor at 350-450 °C. The results indicate that the primary products of coal liquefaction are the asphaltenes, but a significant fraction of the subbituminous coal is also converted directly to oil and gas. The primary products of tire liquefaction are in the oil range. For all the coals, the addition of tire shows a synergistic effect on coal conversion, the synergism being mainly manifested in an increase in the yield of asphaltenes. The extent of synergism depends on process conditions and the coal type. The conversions of all coal types increase dramatically using the catalyst, the catalytic effect being mainly on the coal-to-asphaltenes reaction. The importance of retrogressive reactions is evident for all processes involving subbituminous coal. In the case of bituminous coals, the retrogressive reactions are important mainly at high temperatures. The data analysis suggests that the coliquefaction of different coals with waste tire may be described by essentially similar reaction schemes, although the relative significance of the various steps is different for different coals.
Introduction Disposal of waste tires is a major environmental problem. The liquefaction of such tires in conjunction with coal was suggested as an alternative for the disposal of the tires in landfills, etc.1 An improvement in conversion has been noted1-3 during coliquefaction, often greater than the algebraic sum of the separate conversions of coal alone and tire alone, i.e., a synergism. The liquefaction of coal is known to produce free radicals: recombination reactions among these radicals are considered to be the major retrogressive reactions,4 forming char which limits the conversion of the coal.5 The rubber portion of the tire comprises about 43 wt % of the tire (mainly styrene-butadiene polymer, but with some polybutadiene1) and this fraction probably depolymerizes during liquefaction to produce additional free radicals.6-8 Early work from our laboratory2 suggested that the conversion of coal increases synergisti* To whom correspondence should be addressed. DD voice: (304) 293-2111 ext 411; Fax: (304) 293-4139. E-mail: dadyburjor@ cemr.wvu.edu. (1) Farcasiu, M.; Smith, C. M. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37, 472. (2) Liu, Z.; Zondlo, J. W.; Dadyburjor, D. B. Energy Fuels 1995, 9, 673. (3) Tang, Y.; Curtis, C. W. Fuel Process. Technol. 1996, 46, 195. (4) Malhotra, R.; McMillen, F. Energy Fuels 1993, 7, 227. (5) Whitehurst, D. D.; Mitchell, D. O.; Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (6) Ibrahim, M. M.; Seehra, M. S. Prepr. Pap.sAm. Chem. Soc., Div. Fuel. Chem. 1993, 38, 841. (7) Chambers, C.; Larsen, J. W.; Li, W.; Wiesen, B. Ind. Eng. Chem. Proc. Des. Dev. 1984, 23, 648. (8) Williams, P. T.; Besler, S.; Taylor, D. T. Fuel 1990, 69, 1474.
cally during coliquefaction with tire due to the stabilization of coal radicals by the tire radicals and the consequent decrease of the retrogressive reactions and char formation. We have recently9 studied the effect of process conditions on the coliquefaction kinetics of Blind Canyon coal and waste tire. In these experiments, a separate solvent was not used, in order to avoid swamping the effects of the tire with the effects of the solvent. We observed that the extent of synergism is dependent on the temperature and the tire-to-coal ratio and that the effects of hydrogen pressure and reaction time are small. The results were analyzed using a simple consecutive irreversible reaction scheme for the liquefaction of coal to asphaltenes and oil + gas, the conversion of tire to oil + gas, and a synergism reaction between coal and tire when both coal and tire are present. The synergism reaction was found to be significant, especially in the absence of a catalyst. Since a solvent was not used, the tire may take on the additional roles of a solvent, including4,5 hydrogen addition, hydrogen abstraction, or bond scission, as well as radical addition. The relative magnitude of these roles may be different for different types of coals. Further, the chemistry of coal liquefaction is complex and not well understood, and liquefaction for different coals varies, both in nature and in magnitude:4,10 the relative significance of the synergism reaction may be dependent on the nature of the coal. Hence, it is of (9) Sharma, R. K.; Yang, J.; Zondlo, J. W.; Dadyburjor, D. B. Catal. Today, in press. (10) Keogh, R. A.; Davis, B. H. Energy Fuels 1994, 8, 289.
S0887-0624(97)00187-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/18/1998
590 Energy & Fuels, Vol. 12, No. 3, 1998
Sharma et al.
Table 1. Analysis of Coal and Tire Samplesa proximate analysis
sample DECS-6 DECS-24 DECS-26 tire a
ash volatile fixed water (% matter carbon (%) dry) (% daf) (% daf) 1.8 0.8 0.4
6.3 13.4 6.6 8.1
49.0 47.0 48.5 71.0
51.0 53.0 51.5 29.0
ultimate analysis (% daf) C 81.9 76.3 75.5 79.7
H
N
6.3 1.5 5.3 1.3 6.1 1.0 7.5
