Coal spontaneous combustion: Examples of the self-heating incubation process

Coal spontaneous combustion continues to pose a significant hazard to mining operations. It is a complex process that ranges from low-temperature oxidation in the normal mine environment to thermal runaway once temperatures exceed 120 degrees C. At the thermal runaway stage, the coal becomes dry locally after moisture liberation and evaporation and a well-defined hot spot forms. This process takes place over a period of time, which can be referred to as the incubation period. Assessing the spontaneous combustion hazard likelihood has normally relied on the use of laboratory testing to produce index parameters that give a propensity rating. These are often a single value on a relative rating scale, which gives no indication of the nature of the coal self-heating with respect to time as it would occur under various mine site conditions. To overcome this deficiency it is necessary to consider the incubation behaviour of the spontaneous combustion process, particularly as it relates to coal self-heating at initial mine ambient temperatures. This has recently been achieved using adiabatic oven testing, which shows the modifying influences of moisture content, initial start temperature, seam gas content and reactive pyrite content on the low temperature coal self-heating rate. The incubation testing procedure is able to determine whether self-heating can reach thermal runaway and if so in what timeframe this can take place for the environmental conditions present at the mine site.