420 MODERN GASWORKS PRACTICE
imply that the heavy charge is more extravagant in this direction, this would by rtc means appear to be the rule. The “ make per mouthpiece ” per day shows some falling off, owing to the fact that with. the heavy charge the final hours of the period are largely yielding only the residual gases from the coke—comparatively small in volume, and which are not evolved to the same extent in the short carbonization periods. With the heavy charge, it is, of course, essential that the periods of charging of the various units should be so arranged that the resultant mixture of gas obtained is more or less constant throughout the twenty-four hours.
THE COMPOSITION OF TOWNS’ GAS
The composition of the gas supplied varies to some extent in different localities in accordance with the methods of manufacture adopted. The smaller and medium-sized undertakings for the most part distribute Straight coal gas only, although in some instances the character of this may be altered by some form of steaming the charge which bas recently become populär. The Chemical constituents of ordinary Straight coal gas are given, below, but it should be borne in mind that the proportions of the various constituents given are liable to variation, particularly so as far as the quantity of carbon dioxide, nitrogen, and oxygen is concerned.
Composition of Normal Straight Coal Gas (unstbipped).
Per cent, by volume.
Hydrogen . 47-0
Methane ...... . 27-5
Unsaturated hydrocarbons . 3-5
Carbon monoxide ..... . 7-75
Carbon dioxide ..... . 3-5
Nitrogen ...... . 10-5 '-14-25
Oxygen . 0-25)
Calculated calorific power . . . 518 B.Th.U. gross
Prior to 1914, when oil for the enrichrnent of water gas could be obtained comparatively cheaply, it was cus'omary to employ from 2 to 2| gallons of oil per 1,000 cubic feet of water gas made. Under such conditions the carburetted water gas produced had an illummating power of some 17 candles, and it was customary to distribute a gas consisting of from 70 to 75 per cent, of straight’ coal gas and 25 to 30 per cent, of carburetted water gas. In such cases the composition of the final gas would approximate to the following :—
Composition of Coal Gas and Carbttbetted Water Gas Mixture Per cent, by volume.
Hydrogen ..... , 44-0
Methane ..... . 22-0
Unsaturated hydrocarbons . 5-8
Carbon monoxide .... . 16-5
Carbon dioxide .... . 3-5 1
Nitrogen . . . . 8-0 -11-7
Oxygen . 0-2 J
Calculated calorific power . 545 B.Th.U. gross