736 MODERN GASWORKS PRACTICE
cubic feet, or 5d. per therm. The above figures apply, of course, to the larger gas undertakings.
THE COMPOSITION OF WATER GAS
With the exception of the diluents nitrogen and carbon dioxide, straight or “ blue ” water gas consists almost solely of hydrogen and carbon monoxide. A small percentage of methane is usually, but not necessarily, present. The final composition of carburetted water gas necessarily depends to a large extent upon the proportion of oil used in its manufacture. Under any circumstances, the constituents difEer to some considerable extent from those of coal gas. As regards hydrocarbons, those of the unsaturated variety absorbed by bromine will be present to the extent of about 10 per cent, in a gas of 18 candle-power. The same quality gas will also contain about 13 per cent, of paraffins, the greater portion of which will consist of methane. The unsaturated hydrocarbons, with present-day heats, consist largely of the benzene series.
As regards the mixture of coal gas and carburetted water gas as supplied prior to 1914, with the usual proportion of carburetted gas, that is from 25 to 30 per cent, by volume, ha ving an illuminating power of from 17 to 18 candles, and admixed with coal gas of from 13 to 13| candles, the composition of the mixture approached the following :—
Composition of Mixed Gas, about 14| Candle-Power
Hydrogen ........ 44-0 per cent, by volume.
Methane ........ 22-0 „ „
Unsaturated hydrocarbons ..... 5-8 „ „
Carbon monoxide ....... 16-5 „ „
Carbon dioxide ....... 3-5 ,, „
Nitrogen ........ 8-0 „ „
Oxygen...............................0-2
To-day the high-grade carburetted gas has been displaced by “blue” water gasor a semi-carburetted in the manufacture of which from 0-75 to 1 gallon, of oil per 1,000 cubic feet is employed. In. the case of a mixture consisting of 70 to 75 per cent, coal gas and 25 to 30 per cent, semi-carburetted gas the mixture as distributed will approximate to the following composition3 :—
Composition of Coal Gas and Semi-cabburetted Watek Gas
Hydrogen ...... . 45-8 per oent. by volume.
Methane ...... . 21-5
Unsaturated hydrocarbons . 30 >>
Carbon monoxide ..... . 18-0 99 »
Carbon dioxide ..... . 30 >> 99
Nitrogen ...... . 8-5 99 99
Oxygen . 0-2 « • • .
Candle-power ..... 10 (approximately)
Calorific power (calculated) . 495 B.Th.U. per cubic foot, gross.
The composition of mixtures of coal gas and “ blue ” gas is given in Chap. XII, p. 421.