CHÄPTER XVIII
THE DRY PURIFICATION OF COAL GAS
The theory and practice of the dry purification of coal gas, the chemical changes involved, and the construction and working of the necessary apparatus are subjects wliich, if dealt with in their entirety, would justly claim a complete volume to themselves. For this reason it is impossible, within the limits of a single cliapter, to touch upon more than the salient features of the process and upon those sections of it wliich are of direct interest to the gas engineer.
In some instances, nowadays, purification is carried to extreme lengths, in order that a gas of exceptional purity may be supplied to the consumer. In the majority of cases, however, the gas is treated in such a männer that it may satisfy the usual statutory lequirements, whilst no attempt is made to produce a super-quality.1
At this stage of the manufacturing process the impurities present in the gas. may be set down as follows :—
Ammonia . . . . . . . nil to 1’5 grains per 100 cubio feet.
Sulphuretted hydrogen .... 500 to 800 „ „ „ „
Sulphur compounds (other than H2S) . . 35 to 50 , „ „ „
Hydrocyanic acid . . . . . 50 to 70 „ „ „ „
Carbon dioxide _______________________..... 750 to 1,150 „ „ „ „
Of these, sulphuretted hydrogen must, in accordance with the requirements of the law, be removed completely. This, however, is not the case with the remaining four compounds.
Until the year 1905 a certain limit was placed (in London and many provincial towns) on the quantity of sulphur compounds present, with the result that special treatment was accorded the gas for the removal of these compounds before it left for the distribution area. The limits imposed at that time precluded, during the winter months, a greater quantity of sulphur compounds than 22 grains per 100 cubic feet of gas, whilst during the summer months the limit was reduced to 17 grains. The greater portion of the cyanogen is extracted during the passage of the gas through the oxide purifiers, unless, of course, a special plant for its recovery bas been previously interposed. Carbon dioxide, as previously explained, is not an impurity but a diluent. There are no statutory restrictions as to its presence in the finished gas,, and its effect is chiefly felt as a result of the reduction it causes in calorific value and. illuminating power.
1 Vide S. Metropolitan Gas Co.’s Bill (1920), Chap. I., p; 7.
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