THE COMPLETE GASIFICATION OF COAL
757
A. Dual Process.
1 ton of coal yields :—- B.Th.U.
13, 000 cubic feet of straight coal gas at 520 B.Th.U. = 6,760,000
and 11 cwt. surplus coke.
11 cwt. coke yields :—
11.500 cubic feet of “blue” gas at 300 B.Th.U. = 6,150,000
Total yield in gaseous form from 1 ton of coal = 12,910.000
8. Single Process.
1 ton of coal yields :— B.Th.U.
50, 000 cubic feet of mixed gas at 370 B.Th.U. = 18,500,000
In the second case the author prefers to assume that on an average no more than 50,000 cubic feet of gas will be obtained per ton of coal, although some experi-menters place the yield at 60,000 or even 70,000 cubic feet. In many cases a certain proportion of coke of a semi-burnt character is recovered when the single-stage plant is cleaned for the removal of ashes.
The composition of the two types of gases may be compared as follows :—
COMPOSITION OF COMPLETE ÖAS MlXTUBES
Two-Stage Process. Single-Stage Process.
Hydrogen 49-5 per cent. 51-0 per cent.
Carbon monoxide 27-0 ,, 30-0
Methane 10-5 8-0
CnHm 2-0 1-4
Carbon dioxide 4-0 4-6
Nitrogen 6-85 5-0
Oxygen 0-15 —
Yield per ton of coal (c. ft.) 33,500 50,000
Caloulated calorific power (gross B.Th.U.) . 385 370
Therms per ton of coal 129 185
It may be asked why coal, per unit of weight, should yield so much greater a number of gaseous thermal units when treated in the combined plant as compared with. the two-stage process. The main reason is to be found in the faet that the fuel deiiianded for the production of gas is veiy much less in the combined plant than when two plants are brought into use. For instance, two-stage manufacture demands some 3 cwt. of coke for heating the coal gas retorts, the incandescent coke is pushed out of these retorts at the conclusion of the distillation period, it is quenched with the destruction. of a large quantity of sensible heat, and it is then charged cold (and partly wet) into the water-gas generator where a considerable proportion of its own bulk is demanded for raising it again to the temperature at which it was discharged from the retorts. Obviously this is an uneconomical process,