The principal materials used for fuel are petroleum and coal. Ordinary hard coal is called anthracite coal, and the soft, lumpy kind that crumbles very easily is called bituminous coal. All fuels are composed of carbon, or compounds of carbon and hydrogen, called hydrocarbons, combined with such impurities as ash, sulphur, nitrogen, etc.
When fuel burns the chemical change which takes place is that the oxygen of the air combines with the hydrogen and carbon. The manner in which coal burns depends upon its composition, the nature of the fire, and the air supply.
If the draught of air is insufficient, the gases are only partly consumed. The oxygen then unites with the hydrogen and leaves the carbon in fine particles of soot or smoke, which float away with the draught or are deposited upon the surface of the boiler. Moreover, when the air is not sufficiently hot, partial combustion again results, changes the hydrogen to water vapor, and sets the carbon free as soot or smoke. If the gases become chilled, and pass off as a whole unburned, they thus carry away, not only their own heat of combustion, but also the heat which has been absorbed for their liberation. Smoke is therefore the sign of the imperfect combustion of hydrocarbons.