A large compost pile can spontaneously combust if not properly managed

Spontaneous combustion is a type of combustion which occurs by self heating (increase in temperature due to exothermic internal reactions), followed by thermal runaway (self heating which rapidly accelerates to high temperatures) and finally, ignition.^[1]

  Cause and ignition

1. A substance with a relatively low ignition temperature (hay, straw, peat, etc.) begins to release heat. This may occur in several ways, usually oxidation by a little moisture and air, bacterial fermentation generates heat.
2. The heat is unable to escape (hay, straw, peat, etc. are good thermal insulators), and the temperature of the material rises.
3. The temperature of the material rises above its ignition point (even though much of the bacteria are destroyed by ignition temperatures).
4. Combustion begins if sufficient oxidizer, such as oxygen, & fuel are present to maintain the reaction into thermal run-away.

  Affected materials

• Haystacks^[2] and compost piles^[3] may self-ignite because of heat produced by bacterial fermentation.
• 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.^[4][5]
• Coal can spontaneously ignite when exposed to oxygen which causes it to react and heat up when there is insufficient ventilation for cooling.^[6]
• Pyrite oxidation is often the cause of coal spontaneous ignition in old Mine tailings.
• Pistachio nuts are highly flammable when stored in large quantities, and are prone to self-heating and spontaneous combustion.^[7]
• Large cow manure piles can spontaneously combust during conditions of extreme heat.
• Cotton /Linen. when these materials come into contact with polyunsaturated vegetable oils(linseed, massage oils) bacteria slowly decompose the materials, producing heat. If these materials are stored in a way so the heat cannot escape, the heat build up increases the rate of decomposition and thus the rate of heat build up increases. Once ignition temperature is reached, combustion occurs with oxidizers present (oxygen).

In addition large areas of woodland, shrubland and grasslands during periods of dry hot weather are known to be capable of spontaneously combusting - though the mechanisms causing this are poorly understood. This has been proven to occur via the lifecycle of a variety of plants which require the area to be burned prior to germination.

People have also been reported as spontaneously combusting. This 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 or to other sources. ^[8]

  Hay

Hay is one of the more studied materials in spontaneous combustion. As all hay is not equal, varying by the type of grass and location grown utilized in its preparation, it is very hard to establish a unified theory of what occurs in hay self heating. It is anticipated that dangerous heating will occur in hay that contains more than 25% moisture content. The largest number of fires occurs within 2 to 6 weeks of storage, with the majority occurring at 4 to 5 weeks.

The process may begin with microbiological activity (bacteria or mold), but at some point, the process has to become chemical. Microbiological activity will also limit the amount of oxygen available in the hay. Moisture appears to be quite important, no matter what process. At 100^o C, wet hay absorbed twice the amount of oxygen of dry hay. There has been conjecture that the complex carbohydrates present in hay break down to simpler sugars, which are more readily oxidized.^[9]

  Charcoal

Charcoal, when freshly prepared, can self-heat and catch fire. This is separate from hot spots which may have developed from the preparation of charcoal. Charcoal, that has been exposed to air for a period of eight days, is not considered to be hazardous. There are many factors involved, two being the type of wood and the temperature at which the charcoal was prepared. ^[10]

  Coal

Self-heating in coal has been extensively studied. The tendency to self-heating decreases with increasing rank of the coal. Lignite coals are more active than bituminous coals, which are more active than anthracite coals. Freshly mined coal consumes oxygen more rapidly than weathered coal, as well as freshly mined coal self-heats to a greater extent than weathered coal. The presence of water vapor may also be important, as the rate of heat generation accompanying the absorption of water in dry coal from saturated air can be an order of magnitude or more than the same amount of dry air. ^[11]

  Oil Seeds and oil-seed products

Moisture is a critical factor in self-heating of these materials. Typically, storage at 9-14% moisture is commonly satisfactory, but limits are established for each individual variety of oil seed. In the presence of excess moisture, but at levels below the required amount for germinating the seed, the activity of mold fungi is a likely candidate. This has been established for flax and sunflower seeds, as well as soy beans. Many of the oil seeds generate oils that are self-heating. Palm kernels, rape seed and cotton seed have also been studied. ^[12]

  References

Notes

Bibliography

• Babrauskas, Vytenis (2003). Ignition Handbook. Boston: Society of Fire Protection Engineers. ISBN 9780972811132. 
• Bowes, P. C. (1984) Self-heating: Evaluating and Controlling the Hazards, London: Department of the Environment, Building Research Establishment. ISBN 011671364

  External links

• Article on the spontaneous combustion of coal
• Spontaneous combustion demonstration