Thermal extraction yields with N-methyl-2-pyrrolidinone for some acid-treated low rank coals were greatly increased compared to those for the raw coals. For example, the yield for Wyodak coal increased to 82.9% from 58.4% by the acid treatment. FT-IR measurements showed that carboxylate groups in raw coal were converted to carboxylic acid groups by the acid treatment. In contrast, the yield increase was not observed for extractions with 1-methyl-naphthalene, and the acid treatment effect was strongly coal-rank dependent Mechanism of the thermal extraction for the acid-treated coal is discussed.
