402
MODERN GASWORKS PRACTICE
duets through the layers of red-hot coke. Chiefly, it is found that the heavy hydro-carbons, particularly the olefines, the acetylenes and the lower members of the paraffin series, are most prone to these influences, whilst the more stable methane is only aflected by diastic conditions. The small proportion of rieh gas present in the prima,ry products consists largely of ethane and its homologues, ethylene and acetylene, and Colman states that the condensible compounds are relatively rieh in hydrogen, and belong to those series in which the carbon atoms are united in open chains. If, however, they are present in closed chains, they are combined with much more hydrogen than is the case with the aromatic hydrocarbons, and are probably akin to the naphthencs and similar compounds. In their passage through the coke the primary gases ethane, ethylene, acetylene, and methane all tend to undergo decomposition, whilst the condensible products become partly gasified and altered in character to some extent. Accordingly, whilst a greater yield of permanent gas is obtained at higher temperatures, considerable deteriora-tion in quality follows the breaking down of the richer constituents into less valuable products.
The comparative value of the various constituents so far as the illuminating and calorific powers of the ultimate gas are concerned is emphasized in the following table:—
Calorific power, B.Th.U. per cubic feet at 15° C. and 30 inch bar. Candle-power. Candles per foot.
Gross. Net.
Benzene 3,738 3,587 165
Ethane 1,730 1,579 7
Ethylene 1,559 1,456 10
Propylene 2,301 2,151 —
Methane 992 890 0-9
Carbon monoxide 318 — nil
Hydrogen 320 269 nil
For the purpose of calculating the calorific power of coal gas the average calorific power of the unsaturated hydrocarbons may be taken as equivalent to the calorific power of propylene (i.e., 2,301 B.Tli.U. gross per cubic foot) when the gas has not been stripped for benzol recovery. When dealing with stripped gas, however, it should be remembered that as, from the thermal standpoint, the most valuable hydrocarbon has been removed, the average calorific power of those remaining -will be lowered. In such cases, the average figure for the unsaturated hydrocarbons may be taken as equivalent to the calorific power of ethylene, namely, 1,559 B.Th.U. gross per cubic foot.
With. regard to candle-power the effect of methane is questionable. When