Bituminous or soft coal
represents the chief fuel of commerce. The market prices of these coals are determined largely by reason of their reputation as desirable fuel. The variations in price depend on the physical qualities, rather than on the amount of heat evolved in combustion. The compositions of coals vary markedly in different localities and often in the same locality several grades are produced. It sometimes happens that from different parts of a mine the coal will differ very much in heat value.
Bituminous coals are roughly classified as coking and free-burning. The former is valuable for gas manufacture and for production of coke. The coking coals fuse on being heated, allowing the volatile portion to escape; and when the gas has been all distilled, the residue is coke. When used for gas making, the volatile portion forms the illuminating gas. When burned in a furnace, the gases from soft coal burn with a yellow flame and usually with considerable smoke. The classification of bituminous coals differ somewhat in the East from that of the West. Eastern bituminous coals are commonly graded:
A. Run-of-mine coal = unscreened coal as taken from the mine.
B. Lump coal = that which passes over a bar screen with 1¼-inch openings.
C. Nut coal = that which passes through a bar screen with 1¼-inch openings and over one with ¾-inch openings.
D. Slack = all that which passes through a ¾-inch bar screen.
Western bituminous coal:
E. Run-of-mine coal = the unscreened coal as taken from the mine.
F. Lump coal—divides as 6-inch, 3-inch and 1¼-inch according to the diameter of the mesh through which the pieces pass the screens.
G. Nut coal—varying from 1¼-inch size to ¾-inch in diameter.
H. Screening = all coal which passes a 1¼-inch screen including the dust.
Heat derived from coal—or any other fuel—in the process of combustion is due to oxidation. Combustion or burning is caused by rapid oxidation. When oxygen combines with carbon in sufficient quantities, carbon dioxide is formed and at the same time heat is liberated. In burning fuel, if the carbon is completely oxidized and changed into carbon dioxide, the greatest amount of heat is produced. The required oxygen is furnished by the air, which through the dampers of the furnace regulates the rate of combustion.