The term "Spontaneous Combustion" refers to the occurrence of fire without the application of heat from an outside source. This combustion may take place when combustible matter, such as coal or hay, is stored in bulk. It begins with a slow oxidation process (as atmospheric oxidation or bacterial fermentation) under conditions not permitting ready dissipation of heat. This is the simple spontaneous combustion meaning or the spontaneous combustion definition.
Spontaneous Combustion Examples
Let us look at some of the Spontaneous Combustion examples. Examples are in the centre of a pile or a haystack of oily rags. Oxidation gradually increases the temperature inside the mass to the point where a fire starts. Typically, the crops are dried before storage or during storage using forced air ventilation to avoid spontaneous combustion and fermentation. For a similar reason, soft coal in small size is wetted to suppress aerial oxidation.
Cause and Ignition
Spontaneous combustion can take place when a substance having a relatively low ignition temperature (straw, peat, hay, and more) begins to release heat. This may take place in many ways, either by oxidation in the presence of air and moisture or bacterial fermentation that generates heat. The particular heat is unable to escape (straw, peat, hay, and more are good thermal insulators), and the temperature of the material increases. The temperature of the substance increases past its ignition point (even though most of the bacteria are destroyed by the temperature ignition). Combustion begins if sufficient oxidizers, such as fuel and oxygen, are present to maintain the reaction into a thermal runaway.
Hay is the most widely studied material in spontaneous combustions. It is also difficult to establish a unified theory of what takes place in hay self-heating due to the variation in the types of grass that are used in hay preparation and the different geographies where it is grown. It is also anticipated that the dangerous heating will take place in hay that has more than 25% moisture. The largest fire count takes place within either two to six weeks of storage, with the majority occurring in either the fourth or fifth week.
The process can begin with the microbiological activity (mold or bacteria), but at a point, the process has to change to the chemical. Also, the microbiological activity will limit the oxygen amount that is available in the hay. Moisture appears to be quite essential, no matter what the process is. At 100 °C, wet hay absorbed twice the oxygen amount of dry hay. There has been conjecture that complex carbohydrates available in hay break down to simpler sugars that are more readily oxidized.
There have been unconfirmed anecdotal people reports combusting spontaneously. The wick effect, in which an external fire ignites surrounding flammable materials and either human fat or other sources, has been linked to the alleged cases, and this alleged phenomenon is not considered true spontaneous combustion.
The compost piles and the hay piles may self-ignite due to the heat produced by bacterial fermentation. Danish oil and Linseed oil in a confined space (such as a pile of the oil-soaked rags left out in an uncovered container, especially if the rags are afterwards used with an anti-moisture solvent to clean up the oil) may oxidize, leading to a heat buildup and hence ignition. When exposed to oxygen, coal may spontaneously ignite, which causes it to react and heat up when there is insufficient ventilation for cooling.
Often, pyrite oxidation is the cause of spontaneous coal ignition in the old mine tailings. When stored in large quantities, pistachio nuts are highly flammable and are prone to self-heating with spontaneous combustion. Excess manure piles can spontaneously combust during and extreme heat conditions. Linen and Cotton can ignite when they come into contact with polyunsaturated vegetable oils (such as massage oils, linseed); bacteria decompose the materials slowly by producing heat. If these particular materials are stored in a way so heat cannot escape and the heat buildup increases the decomposition rate, and hence the rate of heat buildup also raises.
Coal self-heating has been extensively studied, and the tendency to self-heat decreases with an increased coal rank. Lignite coals are very active compared to bituminous coals that are more active than anthracite coals. At the same time, the freshly mined coal consumes oxygen more rapidly than weathered coal, and freshly mined coal self-heats to a greater extent than that of weathered coal. The water vapour presence can also be important, as the heat generation rate accompanying the water absorption in dry coal from saturated air may be an order of magnitude or more than a similar amount of dry air.
When freshly prepared, charcoal may self-heat and catch fire. This is separate from the hot spots that can have developed from the charcoal preparation. Charcoal, which has been exposed to air for eight days, is not considered to be hazardous. Also, there are several factors involved, two being the type of temperature and wood, where the charcoal was prepared.