Five-stage sequential extraction of Hefeng coal and direct liquefaction performance of the extraction residue

Ultrasonic-assisted extraction (UAE) and direct liquefaction of the corresponding residue were conducted on an acid-washed ashless Hefeng bituminous coal (HBCAW). Petroleum ether (PE), carbon disulfide (CDS), methanol (MeOH), acetone and isometric CDS/acetone mixture (IMCDSAM) were used to sequentially extract HBCAW via UAE to obtain extracts (E-1-E-5). Gas chromatograph/mass spectrometer (GC-MS) analysis showed that most of the free molecules were extracted in the first two stages, and E-1 and E-2 were mainly consisted of aliphatics and alkanes. PE presented a strong extraction ability for the low polar compounds, and the average molecular weight of E-2 was higher than E-l. While E-3 is more varied containing more complex molecules than E-1 and E-2. IMCDSAM was more favorable for the diffusion and dissolution of alcohols. Direct liquefaction performance suggested that the oil yields of Hefeng bituminous coal (HBC), HBCAW and the fifth stage extraction raffinate (ER5 ) were 66.00%, 63.57% and 44.11%, respectively, and that the conversion of ER5 was only 69.67%, much lower than HBC and HBCAW (82.75% and 82.68%). Higher yield of gaseous product for ER5 might be resulted from the looser coal structure by UAE process, resulting in more small molecules during the direct coal liquefaction.