Chemical changes during natural oxidation of a high volatile bituminous coal

Core drilling of near surface seams near Wakefield NSW has provided high volatile bituminous coal samples that have been subject to varying degrees of natural in situ oxidation. Five levels of oxidation have been defined, mainly on visual evidence, ranging from unoxidised coal to highly oxidised material. Samples of each oxidation level have been investigated by chemical analysis, Fourier transform infrared spectroscopy (FTIR) and solid state carbon-13 nuclear magnetic resonance spectroscopy (NMR) with cross-polarization and magic angle spinning.The results show that mineral matter, moisture and oxygen content increase with oxidation, whereas carbon, hydrogen and specific energy decrease. Moisture determination is suggested as a quick, facile method for indicating the oxidation level for a coal sample from this seam. FTIR showed a marked loss of aliphatic C-H groups with increasing oxidation, and an increase in carbonyl and carboxylate groups. Aromatic C-H groups were much less affected by oxidation. Solid state carbon-13 NMR showed increasing carbon aromaticity with oxidation, but showed substantial carbonyl carbon only at the highest oxidation level. No evidence was found for an increase in aromatic substituents during oxidation; if anything, there is a small decrease in substituents.