Effect of Direct Coal Liquefaction Conditions on Coal Liquid Quality

Solvent extraction of coal was investigated with a focus on the quality of the coal liquids rather than coal conversion. The aim was to determine how the hydrogen/carbon ratio and other quality measures were influenced by liquefaction conditions. Liquefaction was performed using Canadian Bienfait lignite in the temperature range of 350–450 °C, 4 MPa H₂, solvent/coal ratio of 2:1, and residence times up to 30 min at liquefaction temperature. An industrial hydrotreated coal liquid was used as the solvent. The hydrogen/carbon ratio of the coal liquids decreased with an increase in coal conversion, so that coal liquid quality decreased with an increase in the maximum liquefaction temperature. Selective extraction of hydrogen-rich material during the initial stages of liquefaction could be explained in terms of the low solubility parameter of the solvent, the weaker association of less polar molecules, and the limited extent of hydrogen transfer between phases. At longer residence times, especially at higher temperature, the coal liquids became heavier (>550 °C boiling material) and more aromatic and had a higher density and refractive index. These changes were partly due to increased coal conversion and partly due to increased time for hydrogen transfer, cracking, and recombination reactions to take place. It was further found that the nitrogen content of the coal liquids increased with increasing temperature and residence time. Some industrial implications of the changes in coal liquid quality on process development for coal liquefaction were discussed.