Caking Bituminous.This contains less carbon and more hydrocarbon than the former class. It is not so black; is more resinous; and, under intense heat, readily forms into a solid, pasty mass. Unless frequently broken up, this pasty mass forms a blanket over the grate, and checks the draft. Caking bituminous is a valuable coal for the manufacture of gas. It is mined chiefly in the Mississippi valley.
Cannel.Cannel or long-flame bituminous coal produces a considerable quantity of smoke. It is mined chiefly in Pennsylvania, Indiana, and Missouri; and is a free-burning coal, with a strong tendency to cake. It is largely used for open-grate purposes.
Lignite.Lignite, or brown coal is intermediate between coal and peat. It is made up mostly of carbon, with some moisture and mineral matter. Poor varieties are of little value. Good lignite kindles with ease, and burns freely, but is likely to contain a considerable amount of water, and unless kept in a dry place will absorb moisture. It is not a very good fuel, but is used in some localities where other varieties are more expensive. It comes largely from Colorado, Texas, and Washington.
Peat. This is a form of fuel consisting of decayed roots, treetrunks, etc., and earthy matter. It is found in swamps and bogs, and has been in process of decomposition a much shorter time than any of the coals. It is cut out in blocks and dried. Peat has a specific gravity of .4 to .5, but it can be compressed to a much greater density. It is necessary that peat should be kept in a dry place, for it will readily absorb moisture.
Coke. This is made by driving off by heat the hydrocarbon of bituminous or semi-bituminous coals. It may be made in gas retorts, as a by-product of gas production; or it may be made in coking ovens, the gas being the by-product. The latter form of coke is more valuable as a fuel. If the coal is very moist, or if steam is used in the coking process, as in the manufacture of water gas, the sulphur is burned out. Coke burns without flame; and, with a free supply of air, will make an intensely hot fire.
Charcoal. Charcoal is practically never used for steam fuel, its chief use being for household or manufacturing purposes. It is made
by evaporating the volatile matter from wood, either by partial combustion or by heating in retorts. About 50 bushels of charcoal can be obtained from a cord of wood.
Culm. This is a name given to refuse dust at the coal mines, sometimes called slack. It can be bought at the mines at a very low rate; but the cost of transportation prohibits its use except in the immediate vicinity of the mines. On account of its fineness, it cannot be burned in an ordinary grate, and is usually blown into the boiler with a sufficient quantity of air, where it bums somewhat like a gas. A grate beneath usually contains a moderate fire, which keeps the culm well ignited and prevents the loss of any particles that might otherwise drop out of the furnace.
Wood. There are two principal divisions of wood—hardwood, which is compact and comparatively heavy, such as oak, ash, and hickory; and soft wood, which is of soft and porous texture and of less specific gravity, such as pine, birch, and poplar. Wood contains considerable moisture, even if left to season in a dry place; and after being thoroughlfdried, it will absorb and retain from 10 to 20 per cent of moisture. Kiln-dried wood contains nearly 8,000 B.T.U. per pound, while the average wood, containing about 25 per cent of moisture, has a heating value of about 6,000 B.T.U.
The chemical composition of different woods is nearly the same, and pound for pound one class of wood contains about the same heating value as another. Pine weighs about half as much as oak per cubic foot, and a cord of such wood contains about half the heating value that a cord of oak would contain.
Sawdust and shavings are frequently used as fuel in sawmills and planing mills. This kind of fuel is blown into the furnace with air from a fan, and makes an intense heat. A fine grate at the bottom collects the burning embers, which might otherwise drop into the ashpan. In mills where sawdust and shavings are used, they are a byproduct.
Straw. Threshing machines through the West use straw almost entirely for fuel. It gives an intense heat, furnishing 5,000 to 6,000 heat units per pound ; and this is a quick and easy way to get rid of it.