Chemical changes accompanying oxygenation of coal by air and deoxygenation of oxidized coal by thermal treatment

This paper presents the results of progressive dry oxidation of coal by air at 170 ± 10 °C with reference to its acidity and alkali solubility. A mechanism is suggested for the oxidative degradation of coal, which results in the formation of smaller carboxylated species, soluble in alkali. This helps to reconcile the differences in acidity observed between the oxidized coal and the alkali-soluble and -insoluble fractions derived from it. The alkali solubility of oxidized coal diminishes, but not proportionately, with progressive elimination of COOH groups by thermal decarboxylation. Formation of linear anhydrides between the smaller alkali-soluble species (humic acids) by condensation of adjacent carboxyl groups is also indicated. The anhydride linkage may undergo thermal decomposition, giving rise to ether-type oxygen bridges. Different reactions that may occur during thermal decarboxylation of oxidized coal and their effects on its alkali solubility are discussed.