Spontaneous combustion of carbonaceous stockpiles. Part II. Factors affecting the rate of the low-temperature oxidation reaction

In part 1 of this paper, the rate of the low-temperature oxidation reaction was found to be critical in determining the relative tendency of a carbonaceous material to self heat and hence undergo spontaneous combustion. A static isothermal apparatus was designed to directly measure rates of reaction at constant oxygen concentrations and at low temperatures. The results show the relative importance of extrinsic factors such as particle size, ambient humidity, oxygen concentration and concentration of reaction product. A distributed-rate model shows that the apparent fractional order of reaction with respect to oxygen can be explained by allowing for a distribution of first-order rate constants. The results are consistent with the porous structure of coal and with microscopic examination of oxidised coals. Sufficient samples were tested and analysed to perform statistical modelling, which relates the intrinsic properties measured by proximate, ultimate and petrographic analyses to the rate of oxidation of a coal. It was shown that the two factors that had the most statistical significance in determining the propensity for self-heating were the volatile content and the inherent moisture. It is suggested that the inherent moisture is related to the total surface area of the sample, and that the volatile matter component represents the reactive component. Measurement of these two parameters gives a reasonable prediction of a sub-bituminous coal's rate of reaction with oxygen at ambient temperature, and quickly identifies ' at risk ' samples for further testing. A separate set of experiments was conducted to examine the comparative importance of the sorption of moisture. It was shown that the rate of vapour phase adsorption and desorption of moisture is slow compared to the oxidation reaction. (c) 2005 Elsevier Ltd. All rights reserved.