Spontaneous combustion in beds of small fuel particles

The spontaneous combustion of beds of finely divided carbonaceopus material is of interest in several areas of power station operation and especially in coal milling, where the self heating of pulverised fuel deposits may lead to fires. This paper describes, an experimental technique for determining the susceptibility of various coals to spontaneous combustion and also shows that a simple approximation to the temperature profiles in, the bed can be used to extrapolate the results to plant-scale dimensions with sufficient accuracy for practical purposes. The simple model is first shown to agree adequately with the classical Frank-Kamenetskii, and more exact numerical, solutions for the cases of slab, cylindrical and spherical beds, and then with finite difference solutions for a rectangular prism. Measurements of ‘onset,’ where spontaneous heating is first detected, and ‘critical’ temperatures for, beds of pulverized coal heated in an oven, show the same variation with the bed depth as the model and can thus be fitted and extrapolated by suitable choice of activation energy. The neglect in deriving the model of any element of oxygen starvation or convective heat transfer within the bed is seen to be only partly justified under all sub-critical, conditions, but the effect at onset is not measurable. Wide variations in critical temperature have been observed over the range of bituminous coals so far examined.