Effect of natural convection on spontaneous combustion of coal stockpiles

Spontaneous combustion may occur in a coal stockpile when the heat generated within the pile cannot be dissipated at near ambient temperature. Under practical conditions, natural convection enhances the rate of heat removal from the bed and shifts the ignition to lower particle sizes (higher reactivities). Analysis of three limiting cases of a one-dimensional model yields criteria predicting the conditions under which ignition occurs as well as those for which a low-temperature (extinguished) state exists for all particle sizes. A simple asymptotic relation predicts the ignition point at large Rayleigh numbers.