Coke is a product of bituminous coal, bearing the same relation to coal which charcoal bears to wood. The better varieties, made from the softer bituminous coals, and used almost entirely in the blast furnace and for melting metals in the foundry, are primary products, while the poorer varieties are by-products from the manufacture of coal gas. A good grade of coke must be porous, yet not fragile, for it must not be crushed by the weight of the charge in a furnace; and it must contain little sulphur or ash, which requires that it be made from a good grade of coal.

79. Coke Making

Coke is made by subjecting coking coal to a high heat for about four days, without free access of air. This process drives off water and the hydrocarbon gases, leaving the free carbon and ash. Coal should be selected which contains little sulphur, as all of this is not driven off in coking. Fig. 12 shows a simple type of coke oven which is still used, though more elaborate and more economical ovens are displacing it. This type, known as the bee-hive oven, is shown here because of its simplicity. This form of oven is low in cost to build and to maintain, and is a good coke producer. A number of ovens are built together to economize space and heat. This oven is a chamber formed of brickwork, lined 5 with fire brick, and provided with an opening a in the top and a door at the side. Two styles of doors are here shown in two of the ovens. Doors are lined inside with fire brick, and are provided with dampers. The lower part of the chamber is circular, about 10 ft. in diameter, and is surmounted by a hemispherical dome.

Fig. 12. - Coke Oven.

A charge of coke having been removed through the door, the oven is left at a dull red heat. The door is closed and luted with clay, except that a few peep holes are left for watching the process and for admitting air for combustion. A charge of several tons of coal is dumped in at the top opening from a car which moves along rails laid over the furnaces. The furnace heat soon begins to distil off the gaseous parts of the coal, which are ignited by the admission of air at the dampers, and this combustion supplies heat which continues the coking process. It requires several hours for the charge to get thoroughly hot throughout, and the distillation of gases gradually extends into the mass, and when a maximum amount of gas is being evolved, the furnace lining and charge are at a red heat. After about thirty hours the evolution of gas begins to decrease, and a little air continues to be admitted until all flame ceases, when air is shut off entirely. The furnace is now nearly white hot, and combustion is stopped entirely, for the charge must cool down before it can be withdrawn. After about twelve hours the charge has cooled to a degree which will allow a limited amount of water to be introduced from a hose for quicker cooling, and after a short time the door can be opened wide and the charge raked out into iron barrows.

In this type of furnace the products of distillation escape partially burned through the top opening, and may be completely burned elsewhere, as they are rich in combustible gases. It is sought in coking to heat the mass of coal from above by burning some of the distilled gases before they escape from the oven, but the burning of some of the solid carbon is unavoidable, hence this furnace is not so economical as some of the later types.