Coal is the most extensively used of all fuels. The different varieties of coal, with many different names, merge one into another, and the fundamental distinction between the several varieties is the difference in the amount of carbon contained. The several grades or qualities of each variety are due to the per cent of sulphur, ash and water contained. Coal usually contains some phosphorus.
A good practical classification of the varieties of coal is:
(1) Bituminous or soft coal.
(a) Non-caking varieties, long flame and gas coals, 40 to
70% fixed carbon.
(b) Caking, or coking varieties. Short flame coals, 55 to
80% fixed carbon.
(2) Anthracite or hard coal, 80 to 95% fixed carbon.
There are many grades of each kind, beginning with lignite, a brown soft coal low in carbon, and ranging gradually through the bituminous to the anthracite coals which are very rich in carbon and contain very little hydrogen. The flame from coal is due to the gases which are distilled off by the heat of the fire and burn after they are thus liberated. Fixed carbon is the solid carbon in coal as distinguished from the gaseous carbon, which is that combined with hydrogen.
Bituminous coals constitute most of the world's supply. They are fragile, and burn with more or less flame and smoke. From this variety of coal, coal gas and coke are made. Cannel coal is a variety of bituminous coal.
Anthracite coal has a lustrous black color, is hard, and does not readily pulverize in handling, hence is comparatively free from dust. The best varieties give off very little gas, hence they burn as incandescent coals with very short flame. The purer varieties are used in smelting furnaces occasionally, and more frequently to melt metal in crucibles.