Evaluation of Coal Extraction with Supercritical Carbon Dioxide/1Methyl-2-pyrrolidone Mixed Solvent

A novel coal extraction method using supercritical carbon dioxide (scCO(2))/1-methyl-2-pyrrolidone (NMP) mixed solvent, which has the advantages of high efficiency, diffusibility, separability, and environmental friendliness, is proposed in this paper. The physical and chemical properties of the extracts and residues in scCO(2)/NMP mixed solvent extraction of coal were studied. The results showed that 8 bituminous coals had relatively higher extraction yields. The bituminous coal with 85.12% carbon content had the maximum observed extraction yield. The mixed solvent of scCO(2) and NMP had a good extractability for hydroxyl-rich substances, oxygen, sulfur, and other heteroatom-containing compounds. The extracts of Bei Su (BS) and Qu Jin (QJ) coals with higher extraction yields contained more aromatic ring structures. Studies of coal structure revealed that BET surface area of residues were generally lower than that of raw coals, especially for low-rank coals. After extraction, BET and pore volume declined whereas the pore size increased with increasing pressure. It could also be concluded that stacking heights of four coals (QJ, BS, Nan Lu Tian, and Yang Chang Wan) decreased after the extraction procedure. The degree of decline of NLT lignite was larger than that of other coals according to the carbon minicrystal research. A microcrystalline structure similar to raw coal was formed in the process of the solvent evaporation, with a lower stacking height and a higher interplanar spacing. The gasification reactivities of the four coals have been improved by the extraction with scCO(2)/NMP mixed solvent, especially for QJ and BS coals, which have high extraction yields.