The role of the diffusion of oxygen in the ignition of a coal stockpile in confined storage

This paper deals with the spontaneous combustibility and ignition behaviour of coal stored in confined spaces. The geometry, in which the stockpile is sealed on all sides except the top, which is exposed to the ambient conditions, is of interest in the transport of coals in barges or in rail cars. The role of oxygen diffusion as the cause of self-heating of coal is re-examined. In previous work, it was concluded that the diffusion in a one-dimensional stockpile can lead to a maximum temperature rise of similar to 80 K; this conclusion is shown to be a consequence of the unrealistic boundary conditions imposed on the oxygen and energy balance equations. Simple calculations, with more realistic boundary conditions which reflect that the resistance to heat transfer may lie at the interface between the pile and the ambient, lead to a maximum possible temperature rise of similar to 2150 K. Mathematical models are developed assuming that the pile is isothermal, to determine the influence of various parameters - coal reactivity, stockpile dimensions and ambient conditions - on ignition behaviour. In particular, it is shown that storage of coal in this configuration is, at best, conditionally safe. Based on the calculations, methods which may be implemented to suppress spontaneous combustibility in confined storage are discussed. Application of the model is illustrated by detailed calculations pertinent to the transport of coal in barges or in rail cars. (C) 1997 Elsevier Science Ltd.