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spontaneous combustion,phenomenon in which a substance unexpectedly bursts into flame without apparent cause. In ordinary combustioncombustion,
rapid chemical reaction of two or more substances with a characteristic liberation of heat and light; it is commonly called burning. The burning of a fuel (e.g., wood, coal, oil, or natural gas) in air is a familiar example of combustion.
..... Click the link for more information. , a substance is deliberately heated to its ignition point to make it burn. Many substances undergo a slow oxidation that, like the rapid oxidation of burning, releases heat. If the heat so released cannot escape the substance, the temperature of the substance rises until ignition takes place. Spontaneous combustion often occurs in piles of oily rags, green hay, leaves, or coal; it can constitute a serious fire hazard.
a rapid increase in the rate of chemical reactions leading under certain external conditions to the combustion of fuel-oxidizer mixtures in the absence of a flame or glowing object.
Spontaneous combustion can manifest itself either as a thermal or a chain process. The basic principles of thermal spontaneous combustion were explained in qualitative form by J. Van’t Hoff in 1883, and the theory was worked out more fully by N. N. Semenov in 1928. Heat liberated during slow exothermic reactions is normally dissipated in the surroundings. However, under certain conditions of temperature, pressure, and heat removal, this heat cannot be transferred to the surroundings, and the temperature in the reaction zone steadily rises. The reaction rate and the rate of heat production increase with the temperature; the rate of heat transfer also increases but more slowly than that of the reaction. The temperature at which the heat produced begins to exceed the heat dissipated is called the spontaneous combustion temperature. This temperature clearly depends both on the chemical composition of the fuel-oxidizer mixture and on the provisions for thermal transfer. Beginning at the spontaneous combustion temperature, heat is steadily added to the mixture and the reaction rate is steadily increased; the result is either thermal spontaneous combustion or an explosion. The theory of thermal spontaneous combustion permits a calculation of the temperature of spontaneous combustion if the thermal properties of the gaseous fuel-oxidizer mixture (heat of combustion, thermal conductivity) and kinetics of the combustion reaction (rate constant, activation energy) are known.
The theory of spontaneous combustion through chain processes was also worked out by Semenov (1928). Under certain external conditions, the rates of chain branching exceed the rates of the chain-terminating steps. As a result, a slow reaction may become autocatalytic. In these cases, the temperature of the mixture is almost immaterial; the reaction becomes autocatalytic and proceeds because of chain branching, even when the temperature of the mixture is held constant.
The most common form of spontaneous combustion is one involving a combination of chain and thermal process. Quantitative calculations based on the theories of thermal and chain spontaneous combustion have great practical importance in manufacturing processes involving oxidation, in particular, combustion.
REFERENCESSemenov, N. N. Tsepnye reaktsii. [Leningrad] 1934.
Semenov, N. N. “Teplovaia teoriia goreniia i vzryvov.” Uspekhi fizicheskikh nauk, 1940, vol. 23, issue 3.