Formation of free radicals during drying and oxidation of a lignite and a bituminous coal

Abstract The changes in radical concentrations and species brought about by vacuum and gas flow drying of New Vale lignite and Stockton bituminous coal samples were studied. Vacuum drying produces a sudden increase in concentration, flow drying a more gradual but ultimately greater one. For both coals the changes depend on a balance between decarboxylation, which produces new radical sites, and removal of moisture from heteroatomic sites within the coal macrostructure. This balance is influenced by the drying method used. Exposure of dried samples to dry air produces changes in radical concentration consistent with the findings of previous oxidation studies of these coals.