Combustion may be defined as the rapid chemical combination of oxygen
with carbon, hydrogen and sulphur, accompanied by the diffusion of heat
and light. That portion of the substance thus combined with the oxygen
is called combustible. As used in steam engineering practice, however,
the term combustible is applied to that portion of the fuel which is dry
and free from ash, thus including both oxygen and nitrogen which may be
constituents of the fuel, though not in the true sense of the term
combustible.
Combustion is perfect when the combustible unites with the greatest
possible amount of oxygen, as when one atom of carbon unites with two
atoms of oxygen to form carbon dioxide, CO_{2}. The combustion is
imperfect when complete oxidation of the combustible does not occur, or
where the combustible does not unite with the maximum amount of oxygen,
as when one atom of carbon unites with one atom of oxygen to form carbon
monoxide, CO, which may be further burned to carbon dioxide.
Kindling Point--Before a combustible can unite with oxygen and
combustion takes place, its temperature must first be raised to the
ignition or kindling point, and a sufficient time must be allowed for
the completion of the combustion before the temperature of the gases is
lowered below that point. Table 30, by Stromeyer, gives the approximate
kindling temperatures of different fuels.