The apparent viscosity of coal slurry prepared with hydrogenated anthracene oil was measured at temperatures between 323 and 473K, and the mechanism of viscosity change with heating was investigated. Depending on the coal rank, the slurry viscosity increased with increasing temperature, time and coal concentration at temperatures above 423K. The slurry viscosity of Miike coal, a bituminous coal, increased much more than that of Wandoan coal, a sub-bituminous coal, with heating time at 453K, although Taiheiyo coal, a lower rank coal, and Beatrice coal, a semi-anthracite, showed no remarkable changes in slurry viscosity. The effect of coal dissolution on the increase in slurry viscosity was relatively small, and the viscosity increase of coal slurry was found to be caused mainly by solvent absorption into the coal particles.
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