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Spontaneous combustion or spontaneous ignition 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.
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.
There have been unconfirmed anecdotal reports of people spontaneously combusting. This alleged phenomenon is not considered true spontaneous combustion, as supposed cases have been largely attributed to the wick effect, whereby an external source of fire ignites nearby flammable materials and human fat or other sources.
Hay is one of the more studied materials in spontaneous combustion. As hay varies 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 °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.
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.
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, and 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.
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, canola, and cotton seed have also been studied.
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Instances of spontaneous combustion of large piles of festering bread have been recorded in warm climates. When bread is stored in atmospheric humidity of over 40%, and is at most 70% sodden with water, it has been known to combust.