On the fundamentals of diffusive self-heating in water containing combustible materials

Self-heating is hazardous in chemical and agricultural industries. Most natural products, e.g. coal, wool, hay, bagasse etc. contain moisture and their self-ignition processes are influenced significantly by moisture and moisture transfer. For predictive purposes as well as for establishing fundamental understanding of the phenomena, it is important to formulate the basic governing equations of self-ignition of moist materials in the mathematical terms describing comprehensively and quantitatively the best known knowledge. So far, the models published, due to their limited or selected considerations, illustrate only one or several sections of the phenomena and the results can be misleading for practical purposes. In this paper, the shortcomings of the existing models have been analysed in detail. It is attempted to provide a fuller account of the problem in a more realistic manner. Firstly, the evaporation/condensation terms defined in previous literature are examined and they are shown to represent only a simplistic case of water-vapour equilibria in the presence of porous solids. Applying more realistic considerations, a variety of interesting observations has been predicted using the improved model for spatially uniform case. The effects of reactive surface reduction, moisture-dependent oxidation rate and the effect of the lowering humidity due to capillary effect have been explored.