Spontaneous combustion of some Permian coal seams of India: An explanation based on microscopic and physico-chemical properties

Abstract

Spontaneous combustion of coals is a well-known phenomenon in the coal mines of Raniganj and Jharia coalfields. Although several explanations have been given for this burning problem, yet, no general consensus has reached because of variable laboratory techniques. 1n the present paper, a viable explanation is given for auto-oxidation of coal integrating the microscopic and physico-chemical properties.

Organic petrological investigations of the Permian coal seams reveal vitric and fusovitric petrographic facies corresponding to the Late and Early Permian time, respectively. The characteristic micropetrographic, thermal and chemical properties of the Raniganj coal seams show high to moderate susceptibility to spontaneous combustion. The high amount of the reactive resin/hydrocarbon was found to be closely associated with these coal seams, apart from perhydrous Vitrinite having low rank values. In contrast, the Barakar coal seams show least susceptibility probably because of the absence of reactive resin/hydrocarbon with higher rank values.

Microspectrofluorimetric studies suggest green, yellow and orange varities of reactive resin, hydrocarbons in which green and yellow probably correspond to lipid and terpene fraction, of resins imparting more hydroaromatic and unsaturated aromatic structures to these coal in response to their coalification stage. The strong peak at 1600 ± 50 cm in infrared spectrographs of coal also suggests, the presence of resin content in Raniganj coals which are rich in C =C, C =0 structures. These unsaturated structures play a key role in oxygen-coal reaction leading to the development of an unstable peroxy complex which later through chain mechanism oxidizes the coal while Barakar coals show rigid structural configuration.

The chemical and thermal characteristic vis-a-vis. petrographic composition of these coal seams show conformity in the formation of peroxy complex and its relation with crossing point temperature values. Their crossing point temperature data also suggest conformity in trends with the quantitative distribution of the resin hydrocarbon contents on mmf basis. The endothermic and exothermic peaks and their breakdown steps revealed by differential thermal analysis and derivative thermogravimetric analysis indicate close correspondence with the coal seams of the Raniganj Formation.

Thus, it is presumed that the reactive resin content and expulged hydrocarbons from perhydrous vitrinite and organic matrix filled along the cracks, seem to be the main reactive sites for oxygen reaction in spontaneous combustion.