A PETROGRAPHIC EXAMINATION OF COAL OXIDATION

This laboratory-based study examines the effect of oxidation upon the optical properties of coal and coal macerals. All low rank coal macerals exhibit a differing rate of change in their optical properties with an increase in oxidation temperature (e.g. 70-210 degrees C). All macerals show an increase in reflectance, although their response differs. There is also a variation with the respect to the formation of ''oxidation rims'' in coal when oxidized at a fixed temperature (70 degrees C). When oxidized for durations ranging from 1 to 100 h at low temperature (70 degrees C), the character of the oxidation rim varies depending upon the rank of coal. The development of darkened rims around the outer edges of particles of low rank coal is contrasted against the development of bright particle-rims that exhibit a higher reflectance in the higher rank coals. An oxidative mechanism that involves physi-sorbed moisture and the macro-molecular structure of a lignite is proposed for the generation of humic acids that typify darkened rims in low temperature oxidative studies and naturally weathered coal. The oxidative quenching of huminite and vitrinite fluorescence is also examined and presented as a potential technique for the detection of coal oxidation, enumerated through a proposed oxidation quotient.