Liquid fuels derived from direct coal liquefaction are a promising alternative to petroleum-derived fuels due to the increased demand on petroleum, reduced worldwide oil reserves, and abundant availability of coal. For the successful implementation of this technology, information on various transport properties, such as rheological characteristics, of coal-derived liquids (CDLs) is important for the design of coal liquefaction process equipment. To the best of our knowledge, correlations of CDL viscosity have been studied to a limited extent and the parameters that influence viscosity and the mechanisms of viscosity change have not been explored in depth. Consequently, the effect of various variables on rheological properties of CDLs was studied in this work. The direct coal liquefaction experiments were carried out in microreactors (tubing bombs). The results showed that preasphaltenes and asphaltenes in CDLs have a significant affect on the viscosity of liquid products. The viscosity dramatically decreased with removal of preasphaltenes and asphaltenes from the liquid products. In addition, the effect of solvent type on coal conversion and CDL viscosity was investigated using three solvents -tetralin, a medium oil, and a heavy oil. Tetralin was found to be a good hydrogen-donor solvent. In contrast, the medium oil and the heavy oil are not able to effectively enhance the conversion of coal. Comparisons of the coal conversions and the viscosities of CDLs that were generated using two coals illustrated that processing a bituminous coal (Illinois No.6) resulted in higher conversion, higher preasphaltene and asphaltene yields, and therefore higher viscosity of the products than when processing a subbituminous coal (Dietz seam from Montana). Moreover, the viscosities of CDLs decreased with an increase in both reaction temperature (370-430°C) and initial hydrogen pressure (800-1200 psi). The experiments performed with three different coal particle sizes (60, 100, and 200 mesh) verified that the conversion is independent of coal particle size, while coal with smaller particle size is beneficial in generating lower viscosity liquid products. In this study, it is concluded that a change in CDL viscosity depends on various parameters, such as concentrations of asphaltene and preasphaltene components, reaction temperature, hydrogen pressure, solvent type, coal rank, and coal particle size.
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