THE EFFECT OF MOISTURE CONDENSATION ON THE SPONTANEOUS COMBUSTIBILITYOF COAL

The reaction-diffusion approach developed in previous work is extended to consider the effects of moisture evaporation and condensation on t he rate of oxidation of coal. For a single isothermal particle, pseudo -steady-state balances on moisture and oxygen permit calculation of th e effect of different levels of coal-bound moisture on the rate of oxi dation. These species balances are combined with an energy balance to determine the particle temperature history during evaporation and cond ensation. It is shown that partial wetting of coal or condensation of moisture exerts two competing influences on the overall rate of oxidat ion. On the one hand, a portion of the coal fills up with liquid moist ure; in this region, the rate of oxidation becomes negligible, since t he 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 wa ter is low. On the other hand, condensation also leads to the release of the latent heat of vaporization. This heat effect raises the temper ature of the particle and increases the rate of oxidation in the dry r egion of the coal. The relative magnitude of the rates of these compet ing influences determines whether the potential for spontaneous combus tion is abated or enhanced. Based on the insight provided by the model , a simple criterion is derived to determine conditions under which co ndensation of moisture on coal can lead to spontaneous combustion. Fie ld observations have consistently indicated that moisture condensation can enhance the potential for spontaneous combustion. This model is h owever thought to be the first mathematical description which links th e coal-moisture interaction parameters (which can be determined indepe ndently) to the potential for spontaneous combustion. Copyright (C) 19 96 Elsevier Science Ltd.