A new concept of equivalent oxidation exposure-time for low temperature spontaneous combustion of coal

Numerical models of spontaneous combustion of coal have been studied for a long time. However, numerical modeling using with the Arrhenius equation applied to small amount of coal in low temperature range are not able to simulate actual heat and mass transfer phenomena. It still needs a new model to simulate heat generating rate. Some considerations of aging effect are required to express a ratio cumulative oxidation amount to oxidation capacity and elapsed time. In this paper a new concept of the equivalent oxidation exposure-time (EOE-time) has been presented for spontaneous combustion of coal in low temperature range less than 60°C. A calculation procedure of the heat generating rate from coal has been also proposed using a function of the EOE-time, coal temperature and oxygen concentration in the temperature range. The model follows some measurement results on coal heat generating rate against time, coal temperature and oxygen consumption. The present method using the EOE-time has been applied successfully to simulate temperature rising processes at coal seam and stockpile, which end faces and outer surface are exposed to the air, respectively. The simulations were carried out using with a finite difference method by solving equations of heat transfer, oxygen diffusion and natural convection flow. Furthermore, we have discussed a mechanism of spontaneous combustion of coal based on time delay of oxygen diffusion after thermal diffusion in a coal stockpile.