Coal weathering and laboratory (artificial) coal oxidation

Abstract

Coal weathering and laboratory (artificial) oxidation have been topics of much discussion and extensive research since over a century ago, as they cause a profound effect on the chemistry of coal and its technological properties and as they induce distinct and irreversible changes to coal macromolecular structure.This article focuses on the effect of coal weathering and laboratory (artificial) oxidation on the microscopic appearance of coal, its microstructure as well as coal optical properties. The coal petrographic effects of in situ weathering and laboratory oxidation have been a topic of research within the International Committee for Coal and Organic Petrology (ICCP) for years and involve discoloration of macerals, formation of oxidation rims, and changes in polishing relief with formation of micropores, microcracks, and microfissures. Regarding oxidation rims, their presence as a result of in situ weathering and laboratory oxidation is petrographically well documented. Vitrinite reflectance measured on weathered or artificially oxidised coals of different ranks exhibits a differential trend, depending on conditions such as temperature and duration of exposure. Also, the decline of fluorescence intensity has been reported for these coals.In addition to the overview of coal petrographic oxidation features of weathered and laboratory oxidised coals at ambient to low-temperature conditions, the influence of these processes on coal chemical, physical, and technological properties has been discussed. Previous investigations have shown that coal weathering and laboratory (artificial) oxidation affect the suitability of coal for technological uses and provide self-heating or spontaneous combustion risk to fossil resources in coal deposits, in open-air storage facilities and discard dumps, silos, and bunkers. Oxidation also affects coal transport facilities, preparation and processing techniques, as well as, the prediction of coal combustion and carbonisation behaviour, changing cokemaking requirements. It has been similarly shown in numerous research programs and investigations that it poses a detrimental effect on coal liquefaction, gasification and manufacturing of coal-based carbon materials. In addition, low-temperature oxidation is considered a primary source of heat, leading to self-heating and uncontrolled spontaneous combustion.Further, the review provides a broad overview of practical importance of coal weathering and coal oxidation research to coal conversion technologies and final coal utilization products, supported by numerous laboratory investigations. The article presents also a short summary of optical microscopy and non-optical microscopy methods applied to identify coal weathering and laboratory (artificial) oxidation.It is hoped that issues raised in the review article will enhance further debates and novel ideas in coal and coal-related research.