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Solid Fuel

Before solid fuel will burn, it must be changed to the vapour state. In a fire situation, this change usually results from the initial application of heat. The process is known as pyrolysis, which is generally defined as "chemical decomposition by the action of heat". In this case, the decomposition causes a change from the solid state to the vapour state. If the vapour mixes sufficiently with air and is heated to a high enough temperature (by a flame, spark, hot motor, etc.), combustion results.

The burning rate of a solid fuel depends on its configuration. Solid fuels in the form of dust or shavings will burn faster than bulky materials (that is, small wood chips will burn faster than a solid wooden beam). Finely divided fuels have much larger surface area exposed to the heat. Therefore, heat is absorbed much faster, and vaporization is more rapid. More vapour is available for ignition, so it burns with great intensity and the fuel is quickly consumed. On the other hand, a bulky fuel will burn longer than a finely divided fuel.

Dust clouds are made up of very small particles. When a cloud of flammable dust (such as grain dust) is mixed well with air and ignited, it burns extremely quickly, often with explosive force. Such explosions have occurred for instance on ships during the loading and discharging of grains and other finely divided materials.

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