Coal reacts with atmospheric oxygen even at ambient temperatures and this reaction is exothermic. If the heat liberated during the process is allowed to accumulate, the rate of the above reaction increases exponentially and there is a further rise in temperature. When this temperature reaches the ignition temperature of coal, the coal starts to burn and the phenomena is described as spontaneous combustion.
The temperature at which the coal oxidation reaction becomes self sustaining and at which spontaneous combustion occurs varies generally depending on the type (nature and rank) of coal and surrounding conditions of heat dissipation. In poor quality coal and where the heat retention is high the coal and carbonacous material may start burning at temperatures as low as 30-40° C. Excellent reviews on spontaneous combustion of coal are available in literature.
As is clear from the above discussion three factors viz. carbonaceous material (fuel), oxygen and heat are the three primary components for spontaneous combustion for any sustaining fire. All fire fighting and prevention plans are based on the elimination of one of more of these components in the fire triangle.
