Modern Gasworks Practice

MODERN GASWORKS PRACTICE 414 Level-gauge in retort. Over-exhausting. Difference. Cubic feet per ton Cubic feet per ton of coal. of coal. Hydrogen 5,676 5,709 + 33 Methane 3,780 3,494 - 286 Carbon monoxide .... 1,164 1,254 + 90 Carbon dioxide 264 346 + 82 Heavy hydrocarbons . 408 422 + 14 Oxygen 60 90 + 30 Nitrogen 648 1,485 + 837 Total 12,000 cubic feet 12,800 cubic feet + 800 Candle-power 18 15 Calorific power 575 B.Th.U. 559 B.Th.U. The table is compiled from percentage composition figur es obtained by W. B. Davidson in distilling a standard second-class Midland, coal. The most noticeable results of the over-exhausting are a marked deficiency in the valuable heavy hydro-carbons and methane, whilst nitrogen has increased by 130 per cent. It cannot he too strongly emphasized, Jiowever, tliat the principle of over-exhausting, with. the consequent introduction of excessive quantities of waste furnace gases and air, is entirely wrong in principle. So long as gas continued to be sold on the volume basis the introduction. of large volumes of incombustible matter may certainly have appealed to the commercially minded ; but the position has beeil completely altered by the introduction of the heat-unit basis of selling. The result is that whereas carbon dioxide, nitrogen, and oxygen could at one time be sold for so many shillings per 1,000 cubic feet, to-day they yield nothing in th.e way of revenue, and merely detract from profits by reason of the faet that they consume power for their distribution, and appropriate valuable space in holders and distræt mains which would otherwise be available for revenue-producing constituents. On the other hånd, it must be borne in mind that if th.e conditions prevailing in the retort are such that no foreign constituents are drawn in, then. valuable products from the coal will assuredly be finding their way from the retort. They will be consumed. in the furnace and be wasted. Accordmgly, it is not economically possible to produce straight coal gas entirely free from foreign constituents, but th.e operations should be so controlled that incombustible matter is under normal circumstances never permitted to exceed from 12J to 15 per cent. It should be understood that this limit applies to the gas as produced, and not necessarily as supplied, for the author feels that, until further research, is completed and the results of prolonged practical experience can be turned to, it is not politic to discuss the relative merits of distri-buting gas of high or low thermal concentration. The erux of the whole problem is that the producer is paid solely for British Thermal Units ; accordingly the most effektive system of carbonization must inevitably be that which. produces the cheapest