Oxidatively and thermally altered high-volatile bituminous coals in high-temperature coal fire zone No. 8 of the Wuda Coalfield (North China)

The present paper demonstrates, by means of coal petrological methods, significant changes of microstructure, optical appearance, and optical properties of medium-rank (bituminous) coals affected by an underground coal fire. The studied changes are suggested to result from distinct oxidative and thermal alterations induced by a high-temperature regime (temperatures over 800 °C) encountered in the coal fire zone No. 8, Wuda Coalfield. The investigated coal seam No. 9 represents a high volatile A bituminous coal and is dominated by vitrinite, followed by inertinite with minor amounts of liptinite. The microstructure of the coal-fire affected coals facilitate a broad spectrum of characteristic features. The microscopic analyses revealed development of (1) non-tectonic fissures and non-tectonic fractures, (2) distinct micropores, (3) dark oxidations rims, and (4) bright spots interpreted as incipient mesophases. Optical properties were characterised for megasporinite and vitrinite. In the examined megasporinite a noticeable anisotropy and distinct granular appearance was observed. In the investigated coals both a dominant increase as well as a minor decrease in both random and maximum vitrinite reflectances were documented for (1) coal grains and (2) dark-rimmed vitrinite at non-tectonic fissures and non-tectonic fractures. In addition, also a characteristic bimodal vitrinite population was measured. The increase in bireflectance was clearly documented for two sub-zones within the coal fire zone No. 8. The applied Kilby's cross-plots suggested a large overlapping of the data sets of R'max and R'min, indicating marked changes in vitrinite structure. The applied coal petrological approach revealed distinct changes to the coal's microstructure, appearance, as well as to optical properties induced by the underground coal fire. The presented coal petrographic approach may help determine the extent of coal fires in coal mines and the direction of propagation of coal fire front.