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SPONTANEOUS COMBUSTION
Description
When exposed to air all coal will oxidize at some rate, as will timber, steel and some other materials, and oxidation is an exothermic process i.e. the process will produce heat.
The amount of heat produced will depend on a number of conditions, mainly the type of material involved - not all coals are the same e.g. some coals, in certain conditions, will produce enough heat for the coal to actually start burning. Strictly speaking, it is the oxidation process that is combustion and, to the extent that all coals oxidize, all coals are prone to spontaneous combustion. However in coal mines, coals spoken of as being liable to spontaneous combustion are those coals which will develop to an open fire under specific conditions. Many seams will not reach this point.
When spontaneous combustion occurs, there is an initial stage where the temperature is rising but no burning in the normal sense is taking place. This stage is known as a "heating". Apart from the increase in temperature, a heating can be detected by gases produced, notably carbon monoxide. This is an important indicator because a heating may be occurring in a remote location which cannot be inspected. Over time the heating will develop into a fire if conditions do not alter.
In most parts of a mine oxidation will be taking place however the air flow is sufficient to remove the heat produced and prevent development of a heating or worse. The danger lies in areas where there is sufficient air flow to provide a supply of oxygen but insufficient to provide enough cooling to prevent a heating. Such locations may be in areas of fractured coal with a pressure differential across them (e.g. between intakes and returns), in piles of cut or broken coal placed in unventilated locations and particularly in goaves.
The presence of water in sufficient quantities to exclude air (i.e. flooding) will prevent a heating taking place, but the presence of lesser quantities may increase the heating effects.