Spontaneous combustion or spontaneous ignition is a type of combustion which occurs by self
heating, followed by thermal runaway and finally, ignition.
Cause and ignition A substance with a relatively low ignition
temperature begins to release heat. This may occur in several ways, usually oxidation by
a little moisture and air, bacterial fermentation generates heat.
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 oxidizer, such as oxygen, and fuel are present to maintain
the reaction into thermal run-away. Affected materials
Haypiles and compost piles may self-ignite because of heat produced by bacterial fermentation.
Linseed oil in a partially confined space can oxidize leading to a build-up of heat
and thus ignition. Coal can ignite spontaneously when exposed
to oxygen which 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 tailings. Pistachio nuts are highly flammable when stored
in large quantities, and are prone to self-heating and spontaneous combustion.
Large manure piles can spontaneously combust during conditions of extreme heat.
Cotton and linen. When these materials come into contact with polyunsaturated vegetable
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. 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
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, 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 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. 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, 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.
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, canola, and cotton seed have also been studied.
References Notes
Bibliography Babrauskas, Vytenis. Ignition Handbook. Boston:
Society of Fire Protection Engineers. ISBN 978-0-9728111-3-2. Bowes, P. C. Self-heating: Evaluating and
Controlling the Hazards, London: Department of the Environment, Building Research Establishment.
ISBN 0-11-671364-X External links
Article on the spontaneous combustion of coal Spontaneous combustion demonstration