A simplified model of spontaneous combustion in coal stockpiles

Abstract As part of a comprehensive study into the self heating of large coal stockpiles, a simplified one-dimensional model has been developed. The model consists of three differential equations describing the temperature, oxygen concentration and pressure variations in a coal bed. The model differs from previous treatments in that natural convection is taken into account as a mechanism for oxygen transport; this mechanism successfully explains the occurrence of spontaneous heating, without the need to assume the existence of a pressure gradient over the bed. The model is solved for the steady-state; these solutions give valuable insight into the nature of self heating. Coal dumps are classified as either safe, unsafe or conditionally safe. Safe coal dumps are either oxygen limited or heat transfer limited. The former are safe since all the oxygen transported into a dump of fine, reactive coal is consumed close to the surface where energy is easily dissipated. The latter are safe due to the low reaction rates of coarser, less reactive coals. The influence of coal particle size, voidage, coal reactivity and bed length is discussed, with the conclusion that particle size and voidage play a crucial role in determining the safety or otherwise of a coal dump.