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COAL

It has been proved that all coals had their origin in the vegetable matter of prehistoric forests. The woody fiber and other vegetable matter were transformed into peat by fermentation due to bacteria. During this process, a great part of the oxygen and hydrogen was eliminated, while the amount of carbon remained practically the same. Subsequently the peaty matter was changed into coal by a process of destructive distillation, which had been caused by great pressure and high temperature. The differences in types of coal can be easily explained by different conditions during this process of evolution. Among these variable conditions by which the formation of coal had been affected the following may be mentioned: time, depth of the bed below the surface of the earth, and amount of disturbance of the bed due to movements of the earth.

Foreign matter was introduced during this movement of the earth. Coal is composed of the following principal elements: carbon, oxygen, hydrogen, nitrogen, and sulphur. However, these are not present solely in their elementary state, but also in various combinations, principally moisture and volatile matter. Consequently, coal is classified into various types according to its constituents; some of these types will be mentioned below.

Peat is an intermediate condition between wood and coal. It is often used as fuel in the Temperate Zone, where it is found in large quantities in the swampy regions. It is commonly cut into blocks and dried in the air.

Bituminous coal is the name which is usually given to coal containing more than 20 per cent volatile matter. Bituminous coal is not susceptible to spontaneous combustion, although care must be exercised in its storage. Since there is a wide range of variation in the characteristics of the bituminous coals, they have been divided commercially into the following classes: coking, cannel, and non-coking. Coking coal, upon being burned gives off considerable gas and tends to fuse together in a pasty mass (coke). Cannel coal has a high percentage of volatile hydrocarbons, ignites easily and is so valuable as a gas-producing coal that it is rarely burned to produce steam.

Non-coking coal does not coke upon being burned and is very extensively used as a fuel to produce steam.