768
MODERN GASWORKS PRACTICE
by-products is greater thaii if coal gas and water gas are produced separately. The
yield of tar is about 20 gallons per ton of coal, it has a specific gravity of 1’02, but
Fig. 462.—Strache’s “ Doppel-Gas ” Plant.
contains 33-7 per cent, of water. When distilled the tar gives the following fractions :—
Doppelgas Tar
Water. 33-70 per cent.
Paraffin 1'50 „
Light oils 19-40
Heavy oils . 24-30
Ammonia 0-91
Pitch . 18-90
Ash . 0-28
It should be understood that complete gasification may yield two distinet types of gas. The gas engineer is more directly concemed with those processes which give a gas having a calorific power of about 360 to 390 B.Th.U. per cubic foot, and is not directly interested in the alternative processes (such as Mond gas, semi-water gas, and producer gas) which yield a gas having a calorific power varying be-tween 125 and 175 B.Th.U. per cubic foot. Gases pro-duced by the latter processes have a nitrogen content of 50 per cent, or over, while in the case of the former the nitrogen content should not exceed 6 per cent.
The Tri-Gas Plant
Tri-gas is a mixture ' of coal gas, water gas, and producer gas, and has a lower calorific power thaa Doppelgas, and a higher nitrogen, content. The plant is made by the Dellwik-Fleischer Water Gas Company, who claim to have succeeded in perfecting th.e complete gasification of coal. An attempt was first made to decompose