Synergistic effects between light and heavy solvent components during coal liquefaction

As part of research to examine coal conversion in solvents containing high-boiling-point components, experimental studies were carried out with model compound solvents. The dissolution of bituminous and subbituminous coals was investigated in pyrene-tetralin and 2-methylnaphthalene-tetralin mixtures. The effects of donor level, gas atmosphere, hydrogen pressure and conversion temperature were determined. At 400 °C, in the presence of hydrogen gas, pyrene-tetralin solvent mixtures show synergism in coal conversion. At donor concentrations as low at 15 wt%, the degree of conversion was almost as high as in pure tetralin. This phenomenon was not apparent in 2-methylnaphthalene-tetralin mixtures. The relative ease of reduction of pyrene and its ability to shuttle hydrogen is considered to be a principal reason for this difference in behaviour. Conversion in pure pyrene and in pyrene-tetralin mixtures at low donor concentrations increased with increasing hydrogen pressure. At 427 °C, bituminous coal conversion was higher in a 30 wt% tetralin-70 wt% pyrene mixture than in either pure compound. It was found that in the absence of coal pyrene can be hydrogenated by H-transfer from tetralin as well as by reaction with hydrogen gas. This can provide a means to increase the rate of transfer of hydrogen to the dissolving coal through the formation of a very active donor (dihydropyrene). During coal liquefaction, several pathways appear to be available for hydrogen transfer for a given coal, the optimal route being dependent upon the solvent composition and the conditions of reaction.