Equivalent Oxidation Exposure - Time for Low Temperature Spontaneous Combustion of Coal

Abstract

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. Measurement of the heat generation rate using crushed coal samples versus constant temperature have been reported to evaluate its potential for spontaneous combustion. Miyakoshi et al.(1984) proposed an equation guiding heat generation in crushed coal via oxygen adsorption based on a micro calorimeter. Kaji et al. (1987) measured heat generation rate and oxygen consumption rate of three types of crushed coal at constant temperatures. They presented an equation to estimate heat generation rate against elapsed time. However, their time was defined under a constant temperature of coal, thus it is not able to be applied for the process with changing temperature of coal. According to our observations of surface coal mines, the spontaneous combustion of coal initiates at coal seam surfaces as "hot spots," which have temperatures ranging from around 400 to 600 °C. Generally, the hot spot has a root located at a deeper zone from the outside surface of the coal seam or stock that is exposed to air. When the hot spot is observed on the surface, it is smoldering because of the low oxygen concentration. The heat generation rate from coal in the high temperature range (over 60°C) follows the Arrhenius equation, which is based on a chemical reaction rate that accelerates self-heating. Brooks and Glasser (1986) presented a simplified model of the spontaneous combustion of coal stock using the Arrhenius equation to estimate heat generation rate. They used a natural convection model