Spontaneous Combustion and Coal Petrology

This chapter presents the basic concepts of coal petrology and discusses coal parameters that have been noted as potential triggers for spontaneous combustion. Macerals, the microscopically identifiable organic constituents of coal, are one of three basic parameters that define coal. The other two parameters are the coal rank, the measure of metamorphism of the organic constituents, and the inorganic content of the coal, most visibly seen as the minerals associated with coal. Among many factors that trigger spontaneous combustion, oxidation of coal at ambient temperature is the major one. It is an exothermic reaction, the exact mechanisms of which are not fully understood. At very low temperatures, reaction between coal and oxygen is physical (adsorption), and it changes into chemisorptions starting at ambient temperature. Particle size and available surface area are important because adsorption is an important process in coal oxidation. Higher adsorption, and the monolayer capacity in particular, influences the amount of moisture that can be retained in the coal and moisture is frequently cited as the main control on spontaneous combustion. Fractured and faulted thick coal seams with pyrite and organic shale partings are particularly susceptible to spontaneous combustion.