Principles of Combustion

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

PRINCIPLES OF COMBUSTION

PRINCIPLES OF COMBUSTION

Combustion

Combustion (burning, or fire) falls into a class of chemical reactions called oxidation.

Oxidation may be defined as the chemical combination of a substance with oxygen or, more

generally, the removal of electrons from an atom or molecule. Oxidation reactions are

almost always exothermic, or release heat.

Many materials react with oxygen to some degree. However, the rates of reactions differ

between materials. The difference between slow and rapid oxidation reactions is that the

latter occurs so rapidly that heat is generated faster than it is dissipated, causing the material

being oxidized (fuel) to reach its ignition temperature. Once the ignition temperature of a

material is reached, it will continue to burn until the fuel or oxygen is consumed. The heat

release during combustion is usually accompanied by a visible flame. However, some

materials, such a charcoal, smolder rather than produce a flame.

A familiar slowly occurring oxidation reaction is the rusting of iron. Such a reaction

releases heat so slowly that the temperature hardly increases more than a few degrees above

the temperature of the surroundings. These slow reactions do not cause fires and are not

considered combustion.

The Fire Triangle

Generally, there are three items necessary to support combustion:

Oxidizing Agent

Fuel

Heat Source.

These are depicted pictorially in Figure 1, commonly called the Fire Triangle. The Fire

Triangle shows that for combustion to occur, fuel, an oxidizing agent, and a heat source

must all be present in the same place at the same time. If any one of the legs of the triangle

are removed, the fire will be extinguished.

Oxidizing Agents

An oxidizing agent (or oxidizer) is a chemical substance that gives up oxygen easily,

removes hydrogen from another substance, or attracts electrons. By far the most

common oxidizing agent is the oxygen in the earth's atmosphere. However, there are

many chemical compounds that also act as oxidizing agents. Some of these agents

react with fuels more readily and violently than oxygen. Further information on the

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Pyrophoricity