Room-temperature extraction of direct coal liquefaction residue by liquefied dimethyl ether

Direct coal liquefaction residue (DCLR) is the main byproduct during the direct coal liquefaction process. The efficient recovery of organic components from DCLR at low temperatures is beneficial for improving the economy and reducing energy consumption and environmental pollution. Here, DCLR was extracted using liquefied dimethyl ether (DME), acetone, and hexane as the solvents. Compared with the other two solvent Soxhlet extraction, the DME extraction process was performed at room temperature with the shortest extraction time, the lowest energy consumption, and the highest extraction yield (16.2%). Owing to the high carbon contents, low sulfur and oxygen contents, and low ash contents (<0.1%), the extracts obtained using liquefied DME and acetone naturally became the feedstock of carbon materials. Based on the results of the gas chromatography-mass spectrometry analysis, the extracts obtained using the three different solvents had similar compositions in light compounds and were abundant in polycyclic aromatic hydrocarbons with two-, three-, four-, five-, and six-membered benzene rings, indicating that all three DCLR extracts are potential raw materials for preparing high value-added carbon materials. Furthermore, the molecular composition analysis revealed that the room-temperature extraction using liquefied DME was as good as high-temperature Soxhlet extraction using acetone, considering the similarity of their compositions in high molecular weight species and the considerably higher efficiency than that of high-temperature Soxhlet extraction using hexane. Due to the low energy consumption, short extraction time, high extraction yield, and high performance of the extract, liquefied DME is an efficient and economic solvent for extracting DCLR.