First stage, application of existing heat to disengage the constituent gases of the fuel. In coals this is principally mixed carbon and hydrogen.
Second stage, application or employment of existing heat to separate the carbon from the hydrogen.
Third stage, further employment of existing heat to increase the temperature of the two combustibles, carbon and hydrogen, until they reach the heat necessary for combination with the air. If this heat is not obtained, chemical union does not take place and the combustion is imperfect.
Fourth and last stage, the union of the oxygen of the air with the carbon and hydrogen of the furnace in their proper proportions, when intense heat is generated and light is also given off from the ignited carbon. The temperature of the products of combustion at this final stage depend upon the quantity of air in dilution. Sir H. Davy estimates this heat as greater than the white heat of metals.
In the first stages heat is absorbed, but is given out in the last. When the chemical atoms of heat are not united in their proper proportions, then carbonic oxide, mixed carbon and hydrogen, and other combustible gases escape invisibly, with a corresponding loss of heat from the fuel.
When the proper union takes place, then only steam, carbonic acid and nitrogen, all of which are incombustible, escape.
The principal products, therefore, of perfect combustion are: steam, invisible and incombustible; carbonic acid, invisible and incombustible.
The products of imperfect combustion are: carbonic oxide, invisible but combustible; smoke, partly invisible and partly incombustible.
Steam is formed from the hydrogen gas given out by the coals combining with its equivalent of oxygen from the air. Smoke is formed from the hydrogen and carbon which have not received their respective equivalents of oxygen from the air, and thus pass off unconsumed. The color of the smoke depends upon the carbon passing off in its dark, powdery state.