Chemistry of fire

When investigating the origin and cause of a fire, one relies on fire chemistry and physics to trace the fire back to its origin and to identify the event that placed a suitable ignition source in contact with the first fuel ignited. As such, it is important for the fire investigator to fully comprehend the theory behind fire chemistry. This article briefly discusses these concepts.

Fire requires three conditions to interact together in order to occur: fuel, oxidizer, and a source of ignition. Fuel must be available in a sufficient amount, but not in excess, and lower and upper flammability limits are defined for given fuels. In addition, fuel must be in a suitable form, that is, in a gaseous phase. Liquids must evaporate and solids usually pyrolyze in order to produce smaller volatile molecules. Oxidizer, oxygen in most fires, must also be present in a concentration of at least 10% in order to have a flaming fire. Below that, only a smoldering fire can occur and only with suitable solids. Finally, the source of ignition must be suitable for the given fuel. Not all sources of ignition can ignite all fuels. Each type of fuel exhibits ignition characteristics, such as the flash point, fire point, and the autoignition temperature.