After the coal-gas is produced it must be purified and the sulphureted hydrogen at least must be removed. One method of accomplishing this is by washing the gas with water and ammonia, which also removes some of the carbon dioxide and hydrocyanic acid. Various other undesirable constituents are removed by chemical means, depending upon the conditions. The purified gas is now delivered to the gas-holder; but, of course, all this time the pressure is governed, in order that the pressure in the mains will be maintained constant.
Much attention has been given to the enrichment of gas for illuminating purposes; that is, to produce a gas of high illuminating value from cheap fuel or by inexpensive processes. This has been done by decomposing the tar obtained during the distillation of coal and adding these gases to the coal-gas; by mixing carbureted water-gas with coal-gas; by carbureting inferior coal-gases; and by mixing oil-gas with inferior coal-gas.
Water-gas is of low illuminating value, but after it is carbureted it burns with a brilliant flame. The water-gas is made by raising the temperature of the fuel bed of hard coal or coke by forced air, which is then cut off, while steam is passed through the incandescent fuel. This yields hydrogen and carbon monoxide. To make carbureted water-gas, oil-gas is mixed with it, the latter being made by heating oil in retorts.
A great many kinds of gas are made which are determined by the requirements and the raw materials available. The amount of illuminating gas yielded by a ton of fuel, of course, varies with the method of manufacture, with the raw material, and with the use to which the fuel is to be put. The production of coal-gas per ton of coal is of the order of magnitude of 10,000 cubic feet. A typical yield by weight of a coal-gas retort is,
| 10,000 cubic feet of gas | 17 | per | cent. |
| coke | 70 | " | " |
| tar | 5 | " | " |
| ammoniacal liquid | 8 | " | " |
The coke is not pure carbon but contains the non-volatile minerals which will remain as ash when the coke is burned, just as if the original coal had been burned. On the crown of the retort used in coal-gas production, pure carbon is deposited. This is used for electric-arc carbons and for other purposes. From the tar many products are derived such as aniline dyes, benzene, carbolic acid, picric acid, napthalene, pitch, anthracene, and saccharin.
A typical analysis of the gas distilled from coal is very approximately as follows,
| Hydrocarbons | 40 | per | cent. |
| Hydrogen | 50 | " | " |
| Carbon monoxide | 4 | " | " |
| Nitrogen | 4 | " | " |
| Carbon dioxide | 1 | " | " |
| Various other gases | 1 | " | " |
It is seen that illuminating gas is not a definite compound but a mixture of a number of gases. The proportion of these is controlled in so far as possible in order to obtain illuminating value and some of them are reduced to very small percentages because they are valueless as illuminants or even harmful. The constituents are seen to consist of light-giving hydrocarbons, of gases which yield chiefly heat, and of impurities. The chief hydrocarbons found in illuminating gas are,
| ethylene | C2H4 | crotonylene | C4H6 |
| propylene | C3H6 | benzene | C6H6 |
| butylene | C4H8 | toluene | C7H8 |
| amylene | C5H10 | xylene | C8H10 |
| acetylene | C2H2 | methane | C H4 |
| allylene | C3H4 | ethane | C2H6 |