Equivalent oxidation exposure-time for low temperature spontaneous combustion of coal

Coal is a combustible material applicable to a variety of oxidation scenarios with conditions ranging from atmospheric temperature to ignition temperature. One of the most frequent and serious causes of coal fires is self-heating or spontaneous combustion. Opening an underground coal seam to mine ventilation air, such as long-wall gob and goaf areas and coal stockpiles, creates a risk of spontaneous combustion or self-heating. Careful management and handling of coal stocks are required to prevent fires. Furthermore, the spontaneous combustion of coal also creates a problem for transportations on sea or land. Generally, the self-heating of coal has been explained using the imbalance between the heat transfer rate from a boundary surface to the atmosphere and heat generation via oxidation reaction in the stock. The oxidation reaction depends on temperature and the concentrations of unreacted and reacted oxygen. When carbon monoxide exceeds a range of 100 to 200 ppm in the air around the coal and its temperature exceeds 50 to 55°C, the coal is in a pre-stage of spontaneous combustion. Thus, comprehensive studies of the mechanisms and processes of oxidation and temperature increase at low temperature (less than 50 to 55°C) have been investigated for long years.
In this chapter, a model is presented for spontaneous combustions of coal seam and coal stock. It is based on time difference between thermal diffusion and oxygen diffusion. Furthermore, the concept of “Equivalent Oxidation Exposure Time (EOE time)” is presented. Also, we compared the aging time to the oxidation quantity to verify the mechanism presented. Numerical simulations matching both the thermal behaviors of large stocks and small lumps of coal were performed.