The effect of moisture condensation on the spontaneous combustibility of coal

The reaction-diffusion approach developed in previous work is extended to consider the effects of moisture evaporation and condensation on the rate of oxidation of coal. For a single isothermal particle, pseudo-steady-state balances on moisture and oxygen permit calculation of the effect of different levels of coal-bound moisture on the rate of oxidation. These species balances are combined with an energy balance to determine the particle temperature history during evaporation and condensation. It is shown that partial wetting of coal or condensation of moisture exerts two competing influences on the overall rate of oxidation. On the one hand, a portion of the coal fills up with liquid moisture; in this region, the rate of oxidation becomes negligible, since the oxygen has to dissolve in the moisture before it can gain access to an active site on the coal surface and the solubility of oxygen in water is low. On the other hand, condensation also leads to the release of the latent heat of vaporization. This heat effect raises the temperature of the particle and increases the rate of oxidation in the dry region of the coal. The relative magnitude of the rates of these competing influences determines whether the potential for spontaneous combustion is abated or enhanced. Based on the insight provided by the model, a simple criterion is derived to determine conditions under which condensation of moisture on coal can lead to spontaneous combustion. Field observations have consistently indicated that moisture condensation can enhance the potential for spontaneous combustion. This model is however thought to be the first mathematical description which links the coal-moisture interaction parameters (which can be determined independently) to the potential for spontaneous combustion. Copyright (C) 1996 Elsevier Science Ltd.