Modern Gasworks Practice

416 MODERN GASWORKS PRACTICE The original volume of semi-primary gas obtained amounted to 4,500 cubic teet, and tlys was swelled by 495 cubic feet by the second treatment, or to 11,200 cubic feet m all when the additional gas obtained from further heating the coal residue (from which the primary gas was obtained) was taken into account. The most interesting feature of the experiment, however, is the männer in which the increased volume of gas is obtained. This is shown as follows :— Volume composed of :— Primary gas_________________________________________feet. Increase of volume due to degradation .... 495 Gas expelled from low temperature coke residue . . . 5,000 Gas due to gasification of tar . . . j 205 Total yield 11,200 cubic feet per ton. With regard to the final tar as yielded by Dessau vertical retorts, Davidson bas shown that 1 gallon when gasified will produce 68 cubic feet of gas having a candle-power of 17-35. In the ordinary way the yield of tar from coal carbonized tor prolonged penods at high temperatures varies from 9 to 12 gallons per ton whereas with a gas make approaching only 5,000 cubic feet per ton the tar recovered would be m the neighbouriiood of 20 to 22 gallons. The difference between these two figures represents the quantity of tar which under modern conditions is trans-tormed into permanent gas and free carbon. The character of the tar is, moreover, considerably affected. When the gas has been subjected to the prolonged infln-ence of secondary heat a tar of a distinctly benzenoid character results, whilst when the decomposition is limited—as with modern methods—the tar consists largely of derivatives comprised in the paraffin series. The primary objection to intermittent systems of carbonization is the nrnwr in which the quality of gas evolved undergoes deterioration as the period of dis-tillation proceeds. Thus during the first quarter of an hour there is a bürst of gas ° §°™ V<37 mUCh resembling the semi-primary gas given in the fable on page 415. Although carned out some years ago, the experiments of L. T. Wright °n tbe extent of deterioration still prove most instructive. The folio wing fieures obtained by Wright give an excellent idea of this effect Composition of Gas at Various Periods of Charge Hydrogen Methane Heavy hydrocarbons Carbon mon oxide . Carbon dioxide Sulphuretted hydrogen . Nitrogen Period from Commencement lO minutes. 1-i honrs. 31 hours • 20-10 .. 38-08 .. 50-68 . . 57-38 .. 44-03 .. 35-54 . • 10-62 .. 5-98 .. 3-04 . • 6’19 .. 5-98 .. 6-21 • 2-21 .. 2-09 .. 1-49 . • 1-30 .. 1-42 .. 0-49 . 2-20 .. 2-47 ■ .. 2-55 (Approximate). 5J hours. • 67-12 per cent. . 22-58 • 1’79 • 6-12 . 1-50 . 0-11 . 0-78 The results were, of 100-00 course, obtained at 100-00 100-00 lower temperatures 100-00 than those now