CHAPTER XVI
THE PRELIMINARY PURIFICATION OF COAL GAS
'The purification of coal gas commences directly the gas reaches the hydraulic main, by which time it has undergone a considerable drop in temperature. At this point some of the impurities will be washed from the gas by the deposition of a portion of the aqueous vapour and by its passage through the hydraulic seal. The impurities in crude coal gas at this stage may be summarized as follows :—
(1) Suspended tarry matter and condensible hydrocarbons.
(2) Ammonia.
(3) Sulphuretted hydrogen.
(4) Carbon disulphide and otter sulphur compounds.
(5) Carbon dioxide.
(6) Hydrocyanic acid, usually spoken of as cyanogen.
(7) Naphthalene.
Of these, the last three can scarcely be classified as impurities, and there is no statutory obligation to remove either these or No. 4. Whilst it is in all cases to the engineer’s own interest to reduce the naphthalene content to such a quantity as can be carried by the gas without deposition, the removal of cyanogen and sulphur ■compounds is (owing to the very small quantities in which they are present) not ■so imperative, and is only carried out in rare instances. Carbon dioxide, though frequently classified as such., is in no sense an impurity, but may be looked upon ■as an inert diluent ha ving a deteriorating influence upon the illuminating and heating values of the gas (see page 582).
In the preliminary purification of coal gas by wet methpds it is the removal of a portion of the sulphuretted hydrogen and carbon dioxide with which we are chiefly concerned. At the same time the last traces of suspended tar must be eliminated. In order to understand the principles upon which the process is based, it is necessary to consider the nature of the various substances present in the crude gas, i.e. whether they are acid or alkaline. Thus we have :—
Ammonia—alkaline.
Carbon dioxide—acidic properties.
Sulphuretted hydrogen—acidic properties.
Hydrocyanic acid—acid.
In the first instance, advantage is taken of the powerful affinity possessed for ammonia by water which., at normal temperature and pressure, is capable of absorbing about 780 times its own. volume of ammonia gas. Thus, treatment of the crude gas with water, in special vessels known as washers and serubbers, affords a ready
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