GAS MANUFACTURE. When coal is heated out of con tact with air it is resolved into a stable solid residue, known as coke, and volatile matter, the principal constituent of which is gas, known as coal gas. The coke residue may be gasified in steam to make water gas. Coal gas, either alone or mixed with water gas, is distributed in pipes for public supply.
Coal gas has an average heating value of about 500 British Thermal Units per cubic foot and water gas, unless enriched by carburetting (as see below), has a heating value of some 300 B.T.U. per cubic foot.
Another kind of gas can be made if air is blown either alone or mixed with steam through a deep hot bed of coal. From half to two-thirds of this gas by volume is made up of the nitrogen contained in the air used for gasification, and the heating value of such gas is of a much lower order than that of coal gas or water gas on that account, running about 12o-16o B.T.U. per cubic foot. This is the form of gaseous fuel mainly used for large industrial furnaces and is known as producer gas.