Spontaneous combustion is a type of combustion which occurs without an external ignition source.
How spontaneous combustion occurs
- A substance with a relatively low ignition temperature begins to release heat, which may occur in several ways, such as oxidation or fermentation.
- The heat is unable to escape, and the temperature of the material rises
- The temperature of the material rises above its ignition point
- Combustion begins, if sufficient oxygen is present.
The element sodium
is an example of a pyrophoric
material which can undergo a kind of spontaneous (and potentially very violent) explosion when exposed to oxygen, water, or moisture in the air. Pyrophoric substances have an autoignition temperature
below room temperature
and often require mere contact with air or water in order to spontaneously ignite. A characteristic of pyrophoric materials is also their large specific surface of contact with air. The Nickel
is pyrophoric because of the very fine size of its particules.
Some materials which can spontaneously combust
- Haystacks and unprocessed cotton may self-ignite because of heat produced by bacterial fermentation.
- Grain dust in a hot metal silo can explode violently, destroying the structure.
- Boiled linseed oil in a partially confined space (such as a pile of oil-soaked rags left out in an uncovered container) can oxidize leading to a buildup of heat and thus ignition.
- Coal can spontaneously ignite when exposure to oxygen causes it to react and heat up when there is insufficient ventilation for cooling.
- Pyrite oxidation is often the cause of coal spontaneous ignition in old mine tailing.
- Pistachio nuts are highly flammable when stored in large quantities, and are prone to self-heating and spontaneous combustion.
- People have also been reported as spontaneously combusting. However the phenomenon is not considered true spontaneous combustion, as it is largely attributed to the wick effect, whereby an external source of fire ignites nearby flammable materials and human fat.
In general practice, the threat of spontaneous combustion can be substantially reduced by placing the material in a tightly confined, self-closing disposal container (thus greatly limiting the supply of oxygen or water vapor), or submerging it in a fluid which smothers the reaction before it can begin. Depending on the hazard, this protective fluid may be water
(especially for a reactive metal such as sodium, which ignites upon contact with water) or an inert gas
such as nitrogen
- Backdraft: spontaneous ignition of hot gases produced by incomplete combustion of flamable materials in a poorly ventilated room (pyrolysis gases) when oxygen is suddently admitted into the system.
- Flashover: similar to backdraft but occurs without sudden ventilation.
- Spontaneous human combustion.