Extraction of intermediate and low-rank coals with supercritical toluene

Abstract

Supercritical gas extractions of 15 UK coal samples of rank CRC 500–900 were carried out in a 500 ml batch autoclave fitted with a condenser and a siphon arrangement. Preliminary experiments showed that extraction yields became constant above 400 °C, so the conditions used for the remaining extractions were 5 min contact time, 425 °C, 50 g of coal, 300 ml of toluene and 50 bar N 2 pressure. Extraction yields were calculated by a mass balance approach. The extracts, the hexane-soluble material and the asphaltenes were analysed for their C, H, N and S contents. Extraction yields showed some dependency on ash contents, volatile matter contents, H:C ratios, sulphur contents, oxygen contents and carbon contents. Often the coals were found to divide into two groups. The oxygen contents of the coals seemed to be the major factor influencing extraction yields. Generally, oxygen-containing fragments were the most difficult to solubilise and sulphur-containing fragments the easiest. The extracts were significantly richer in their hydrogen contents than the starting coals, indicating that lower molar mass compounds were solubilised.