A model for the spontaneous heating of coal

Abstract

As part of an investigation into the spontaneous heating of coal piles, a one-dimensional model has been developed to describe the spontaneous heating process at relatively low temperatures (< ≈ 100 °C). The ultimate unsteady-state model takes into account depletion of oxygen and production of heat by chemisorption of oxygen in the coal, transport of oxygen by diffusion and convection and transport of heat by conduction, convection and evaporation/condensation of coal moisture. It consists of four differential equations, for conservation of oxygen mass, of coal moisture and of heat and rate of reaction of oxygen with coal. Calculations using data from laboratory and field experiments give results that describe the process of spontaneous heating semiquantitatively. The most important parameters in the process of spontaneous heating, particularly for the time between stacking and spontaneous ignition, are the porosity of the pile (degree of compaction), the initial temperature of the coal and the evaporation and condensation of coal moisture. The influence of other parameters (e.g. reactivity of the coal, thermal conductivity) is much less pronounced.