Ramesh K. Sharma, Jianli Yang, John W. Zondlo and Dady B. Dadyburjor
Department of Chemical Engineering, West Virginia University, PO Box 6102 Morgantown, WV
June 1998.
Abstract
Thermal and catalytic liquefactions of waste (recycled) tire and coal were studied both separately and using mixtures with different tire/coal ratios. Runs were made in a batch tubing bomb reactor at 350–425C. The effect of hydrogen pressure on the product slate was also studied. Mixtures of tire components and coal were used in order to understand the role of the tire as a solvent in co-liquefaction. In the catalytic runs, a ferric-sulfide-based catalyst impregnated in situ in the coal was used. Both the tire components and the entire tire exhibit a synergistic effect on coal conversion. The extent of synergism depends on temperature, H2 pressure and the tire/coal ratio. Experiments with coal and tire components show that the synergistic effect of tire is due to the rubber portion of the tire and not the carbon black. The synergism mainly leads to an increase in the yields of asphaltenes, which are nearly double those in the coal-only runs at 400°C. The conversion of coal increases dramatically using the catalyst, but the catalytic effect is attenuated somewhat in the presence of tire, especially at high tire/coal ratios. The data were analyzed using a consecutive reaction scheme for the liquefaction of coal to asphaltenes and thence to oil+gas, both reactions being of second order; a second-order conversion of tire to oil+gas; and an additional synergism reaction when both coal and tire are present, first-order in both coal and tire. Parallel schemes were assumed for thermal (uncatalyzed) and catalyzed reactions. The uncatalyzed liquefaction of coal has a low apparent activation energy, 36 kJ/mol, compared to those for the synergism reaction (84 kJ/mol) and the catalytic coal liquefaction (158 kJ/mol). The conversion of asphaltenes to oil+gas is relatively independent of temperature and of the presence of the catalyst. The catalyst appears to play a significant role in the conversion of coal to asphaltenes, but a negligible role in the synergism reaction."
Again, as with the other research we've cited, adding scrap auto tires to coal in a liquefaction process improves the yield of liquid fuel raw material; the practice is "synergistic". And: "the synergistic effect of tire is due to the rubber portion of the tire and not the carbon black"; a conclusion drawn by other researchers, wherein the "rubber" contributes hydrogen to the hydrogenation and liquefaction reaction, and can reduce the amount, and associated costs, of synthetic hydrogen donor solvent, such as tetralin, or others, which might otherwise be needed to hydrogenate the coal.
So effective is the addition of "rubber" to coal, that it can "increase in the yields of asphaltenes, which are nearly double those in the coal-only runs". In other words, by adding "waste" rubber to coal in a liquefaction process, we can "nearly double" the production of liquid fuel raw materials from coal.
We were careful above to use the word "rubber" parenthetically, since modern tires can be made of either natural or synthetic polymers. But, we must note that, where natural rubber tire waste would be used in a coal co-liquefaction process, such use would represent yet another route of atmospheric carbon dioxide recycling, via the initial photosynthetic production of botanical latex.