Destructive Distillation of Coal

The nature of the products of the destructive distillation of bituminous coal, in absence of air, depends largely upon the temperature at which the process is carried out. When such coal is heated to 450°-500° C. it yields gas and oily vapours consisting chiefly of paraffin hydrocarbons. Between 700° and 800° C. the residue evolves more gas consisting chiefly of hydrogen. Further heating of the paraffin hydrocarbons decomposes them into hydrogen and methane, together with some ethane and ethylene; whilst contact of these gases with the hot retort causes further splitting off of hydrogen and condensation of the nuclei into benzene, toluene, naphthalene, and anthracene, which are contained in the tar. Thus the raising of the temperature promotes more and more dissociation of hydrocarbons, the final result of which would be complete separation into carbon and hydrogen.

Some of the nitrogen of the coal remains in the coke, some appears as pyridine and quinoline in the tar, the rest as ammonia in ammoniacal liquor, as hydrocyanic acid and free nitrogen.

The oxygen of the coal is converted into steam, which condenses with ammonia as ammoniacal liquor or appears as CO or CO₂; some of the sulphur remains in the coke, the rest forms volatile compounds which condense in the tar or appear as CS₂ or H₂S, some of which combines with ammonia in the ammoniacal liquor and some remains in the gas.

The retorts in which the coal is heated were originally made of iron, but are now of fire-clay. When horizontal they are D-shaped, 20 ft. long and 21×16 in. in cross-section, and hold a charge of 11 cwt., which fills the retort to the depth of 14 in. The number of retorts in a furnace may vary from two to eighteen; they are heated by producer gas, and carbonisation takes about twelve hours, the temperature of the retort rising to 1100° C., though the interior of the charge does not reach this temperature. A ton of Newcastle coal yields 12,500 cubic feet of gas and 110 lb. of tar, which on distillation leaves 77 lb. of pitch, whilst the ammoniacal liquor yields 7 lb. of ammonia. The remaining coke is 65-70 per cent, of the coal.

The retorts may be closed at one end or open so as to admit of charging from both ends; sometimes they are inclined at an angle of 80° with the horizontal, so that coal is introduced at the top and coke removed at the bottom. Sometimes the retorts are vertical and are completely filled with coal, so that the hot surface to which the gas is exposed is reduced to a minimum, and the quality of the gas maintained by the prevention of an excessive production of tar.

With the continuous employment of cannel and richly bituminous coal the available supplies of these varieties gradually diminished, until, about 1890, it became customary to enrich the gas made from the poorer kinds of coal by adding carburetted water-gas, i.e. water-gas containing vaporised mineral oil, and also to increase the yield of gas at the expense of illuminating power by the employment of a higher temperature of carbonisation, and then to compensate by oil-enrichment.

About this time the incandescent gas mantle was introduced, and the need for gas of high illuminating power ceased. Consequently the addition of carburetted water-gas to coal-gas has largely been discontinued, and in cases where this gas is still employed it possesses approximately the same illuminating power as the coal-gas.

The volatile products from the coal pass from the retort by a wide pipe, bent twice and leading vertically downwards to the hydraulic main, where by partial cooling imperfect separation into the three following constituents take place:

1. impure coal-gas,
2. ammoniacal liquor,
3. tar.

The ammoniacal liquor forms an aqueous layer above the tar, and consists of an aqueous solution of ammonia, hydrogen sulphide, and carbon dioxide. From it most of the ammonia of commerce is prepared, whilst from the tar many important and complex "coal-tar products" are obtained.

The crude coal-gas still, however, contains ammonia, tar, sulphur compounds and other substances, from which it is purified by a series of operations.