Coke is a form of fuel obtained by the heating of coal in confined spaces whereby its more volatile constituents are driven off. Caking coal is the most suitable for the manufacture of coke, which, in its principal qualities, is approached by the hard anthracitic coals. The process of coking is conducted (1) in heaps or mounds, or (2) in ovens; but coke is also produced in gas retorts, where, however, it is merely a secondary product. Coking in mounds—a method now little practised—is done in a way similar to that employed for preparing charcoal from wood. The coal is built up into round stacks, around a wide open chimney or column, the larger masses being placed in the centre, and small broken coal outside, the whole being covered with wet coke-dust, except at certain air-holes. The mound of coal is ignited from above, and burns gradually downwards and outwards, giving off at first much smoke and vapour, which consist mainly of tar-water and coal-gas. When the fire ceases to be smoky, the process of coking is concluded, all openings are covered over to exclude air and extinguish combustion, and cooling of the mound is aided by drenching it with water. The principle of making coke in coke-ovens is the same, but the process is much quicker and more economical, and the resulting coke is better in quality than that made in mounds. Formerly the heat and volatile gases given off in coking were allowed freely to escape, but in modern coke-ovens arrangements are made for utilising the combustion in one retort to force the heat of another, by passing the hot gases in flues around contiguous ovens. The whole of the gases pass from these flues into a common flue, by which they are carried away to heat steam-boilers. In a similar way the coal-gas is, from certain kinds of ovens, collected and burned, and the tar-water evolved is condensed and used as a source of ammonia and gas-tar products.
Coke is a hard, brittle, porous solid, with a steel-gray, somewhat metallic lustre, and it does not soil the fingers when being handled. It is principally valued for the intense heat which it gives off in combustion, for its freedom from smoke in burning, and because it does not become pasty and adhesive in the fire. The process of coking also drives off a good deal of the sulphur which may be present in coal, and all these properties render coke a most valuable fuel for many metallurgical operations, and for use where intense smokeless combustion is a desideratum. The higher the temperature to which coal in coking is submitted, the harder and more valuable is the resulting coke, and the heat it gives off is relatively more intense. In chemical constitution coke consists of a modified form of graphite contaminated with earthy impurities from the ash present in coal. Coal yields by weight about 70 per cent. of coke, which, however, increases in volume in the process of coking by about one-fourth. Coke will absorb about 30 per cent. of moisture from the air, a circumstance which should be borne in mind in its purchase and its use, for such moisture in being driven off greatly reduces the calorific value of the substance.