Solvent–Coal–Mineral Interaction during Solvent Extraction of Coal

Abstract

Solvent extraction of coal is a coal-to-liquids conversion process whereby the organic matter in coal is dissolved in a solvent to produce a substantially mineral matter free product. The solvent extraction of Poplar lignite coal was studied with three model solvents (tetralin, quinoline, and 1-naphtol) and one industrial coal liquid derived solvent. Of interest was the fate of hydrogen, nitrogen, sulfur, and mineral constituents during physical dissolution at 200 °C and during reactive dissolution at 400 °C. The hydrogen redistribution in all solvents at 400 °C was within a narrow range, even though hydrogen-donor solvents resulted in higher extraction yield and in a higher H/C ratio of the residue. Quinoline strongly interacted with the coal, which masked its high physical extraction yield at 200 °C and enabled more mineral matter to be stabilized in solution. This behavior is typical of nitrogen bases in the coal, but was amplified by quinoline. Nitrogen redistribution took place with all solvents to nitrogen enrich the residue. Little or no sulfur redistribution took place during solvent extraction at 200 °C with any of the solvents, but at 400 °C, sulfur was redistributed. Evidence that sulfur is removed from the mineral matter during solvent extraction was provided by an observed decrease in S/Fe ratio, decrease in sulfur content of the ash, and increase in S/C ratio of extraction residues.