Abstract Using the self-developed viscosity measuring device, the viscosity variations of coal–oil slurries with temperature increasing during coal–oil co-processing were studied. The results show that the viscosity of coal–oil slurries prepared by different kinds of oil varies differently during heating. The viscosity of the coal–oil slurry prepared by the catalytic cracking slurry (FCC) generally decreases during heating. However, the viscosity of the coal–oil slurry prepared by the high-temperature coal tar (CT) will peak at 338?°C during heating. The differences in viscosity variations of coal–oil slurries are analyzed. In addition to the temperature, the properties of the solvents and coal are the main influencing factors. Because the used coal contains a large number of polar functional groups, the swelling behavior of the coal in polar solvent (CT) is stronger than that in non-polar solvent (FCC). The swelling effect of the coal can result in the appearance of the viscosity peak. Therefore, before 100?°C, the solvent molecules entering into the coal pores is the main influencing factor of coal–oil slurries viscosity variations. After 100?°C, the increasing of particle size of coal particles is the main influencing factor of coal–oil slurries viscosity variations.
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