Spontaneous Heating/Ignition of Hydrocarbons and Organics

SPONTANEOUS HEATING/IGNITION

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

OF HYDROCARBONS AND ORGANICS

SPONTANEOUS HEATING/IGNITION OF

HYDROCARBONS AND ORGANICS

Spontaneous Heating/Ignition of Hydrocarbons

Some hydrocarbons are capable of spontaneous heating and ignition under proper conditions.

Spontaneous heating of hydrocarbons usually involves a combustible liquid hydrocarbon in

contact with combustible materials. An example of this would be combustible rags

impregnated with oils or solvents. Some solid hydrocarbons, such as coal, can react directly

with atmospheric oxygen. Whether spontaneous heating leads to ignition depends on several

items:

The rate at which heat is generated and removed from the material being oxidized

The ignition temperature of the fibrous combustible material, hydrocarbon, or any gases

liberated by oxidation

The specific area (cm2 /g, defined below) of the hydrocarbon exposed to an oxidizer

The amount of moisture present in the atmosphere and the fibrous material.

For spontaneous ignition to occur, the rate of heat being generated through oxidation must

exceed the rate of heat removal by conduction, convection, and radiation (thermal). As the

temperature of the material begins to rise, the rate of heat generation will often increase.

The result is a "runaway" reaction which ultimately causes ignition. If the rate of heat

removal exceeds the rate of generation, the material will cool and will not ignite. The rate

of heat removal may be increased through physical contact with a thermally conductive

surface, by rotating piles of combustibles to cool hot spots, and by circulating inert gases

through the piles to cool hot spots and displace oxygen.

The ignition temperature of the materials is obviously of concern and varies widely among

materials. Much more stringent controls must be placed on materials which have lower

ignition temperatures and those which liberate explosive gases. Although most materials

with high ignition temperatures are of lesser concern, some are more explosive than those

with lower ignition temperatures. Material Safety Data Sheets (MSDSs) are a source for

information such as ignition temperature, vapor pressure, toxicity, and reactivity.

The specific area of a combustible substance is a measure of the surface area of the material

exposed to an oxidizing atmosphere per gram of material and is expressed in units of cm2/g.

Materials which have a high specific area are more prone to heat and ignite spontaneously.

For example, it was mentioned earlier that combustible liquids on fibrous material pose a

spontaneous fire hazard. This is because the fibers of the material allow the liquid to spread

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Pyrophoricity