Changes associated with natural in situ weathering of a coking coal from southeastern British Columbia

A suite of naturally oxidized coals from a single coal seam from southeastern British Columbia was examined by means of pyrolysis gas chromatography together with ultimate and petrographic analysis. With increasing degree of oxidation (FSI decreasing from 7.5 to 0) a consistent decrease in fixed carbon and hydrogen and an increase in oxygen and moisture were found. Comparing the mildly oxidized coal (FSI 7.5 to 3) with the unweathered coal, there is very little detectable change in pyrolysis gas composition or chemistry. At more advanced stages of oxidation (FSI 3 to 0) the pyrolysis gas is markedly depleted in CO, CH4, C2H4 and C2H6 and correspondingly enriched in CO2. The carbon and hydrogen contents decrease and the oxygen content increases significantly. In the mildly oxidized coal, oxidation rinds develop on vitrinite particles, whereas in the more oxidized coal, microfissures and a darkish cast are evident.