Gaseous fuels are limited to natural gas, blast furnace gas and coke oven gas, the first being a natural product and the two latter by-products from industrial processes. Though waste gases from certain processes may be considered as gaseous fuels, inasmuch as the question of combustion does not enter, the methods of utilizing them differ from that for combustible gaseous fuel, and the question will be dealt with separately. Since coal is by far the most generally used of all fuels, this chapter will be devoted entirely to the formation, composition and distribution of the various grades, from anthracite to peat. The other fuels will be discussed in succeeding chapters and their combustion dealt with in connection with their composition. Formation of Coal--All coals are of vegetable origin and are the remains of prehistoric forests. Destructive distillation due to great pressures and temperatures, has resolved the organic matter into its invariable ultimate constituents, carbon, hydrogen, oxygen and other substances, in varying proportions. The factors of time, depth of beds, disturbance of beds and the intrusion of mineral matter resulting from such disturbances have produced the variation in the degree of evolution from vegetable fiber to hard coal. This variation is shown chiefly in the content of carbon, and Table 35 shows the steps of such variation. TABLE 35 APPROXIMATE CHEMICAL CHANGES FROM WOOD FIBER TO ANTHRACITE COAL +----------------------+-------+--------+-------+ |Substance |Carbon |Hydrogen|Oxygen | +----------------------+-------+--------+-------+ |Wood Fiber | 52.65 | 5.25 | 42.10 | |Peat | 59.57 | 5.96 | 34.47 | |Lignite | 66.04 | 5.27 | 28.69 | |Earthy Brown Coal | 73.18 | 5.68 | 21.14 | |Bituminous Coal | 75.06 | 5.84 | 19.10 | |Semi-bituminous Coal | 89.29 | 5.05 | 5.66 | |Anthracite Coal | 91.58 | 3.96 | 4.46 | +----------------------+-------+--------+-------+ Composition of Coal--The uncombined carbon in coal is known as fixed carbon. Some of the carbon constituent is combined with hydrogen and this, together with other gaseous substances driven off by the application of heat, form that portion of the coal known as volatile matter. The fixed carbon and the volatile matter constitute the combustible. The oxygen and nitrogen contained in the volatile matter are not combustible, but custom has applied this term to that portion of the coal which is dry and free from ash, thus including the oxygen and nitrogen.