THE EFFECTS OF OXIDATION ON THE MACROMOLECULAR STRUCTURE OF COAL

The effect of oxidation on the macromolecular structure of four coals of rank 902 to 204 in the British Coal classification scheme was investigated. Various particle size ranges of the coals were oxidized in air at 200 degrees C for 24 h. Oxidation led to an increase in the extent of swelling in pyridine, which indicates a decrease in the covalent cross-link densities of the coals. Unlike that of the initial coals, which can be described by Fickian diffusion and relaxation processes, the swelling of the oxidized coals occurred by a two-stage process. This behaviour was attributed to the differences in extent of oxidation with increasing depth in the coal particles. A two-stage swelling process was observed regardless of whether a well-defined oxidation front was observed by optical microscopy. FT-i.r. spectra indicated that oxidation reduced the aliphatic:aromatic ratio, increased the hydroxyl:aromatic ratio and increased the carbonyl functionality in the coals. The results are consistent with breaking of the cross-links in the coal by severe oxidation.