Modern Gasworks Practice

GAS-MAKING AND OTHER COALS 365 (a) 12,000 cubic feet of gas per ton. Candle power, 15. (b) 13,000 „ „ „ ,, „ 14- Then. for (a) the gas multiple is 180,000, and for (b) it is 162,000. Coal (a) would tlien be adjudged. the most desirable from the point of view of gas yield. As candle power has now lost its significance the modem method of computing the “ gas multiple ” is to multiply the gas made per ton of coal by the average calorific power. In other words, this gives the yield of gaseous thermal units per ton of coal. The figure should normally vary, for good quality coals, between 6,500,000 and 7,000,000 B.Th.U., straight coal gas only being considered. CALORIFIC POWER RECOVERED Suggestions have frequently been made that calorific power should be made a basis on which to adjust the value of coal. Thus a certain price per ton would be charged for a definite calorific power, and a pro rata reduction made wheu the hcating value was below the standard. For those wlio employ coal as a fuel the system has many advantages. The gas engineer, howevcr, is concerned not so much with. the riumber of heat units in his original coal, but with. the number which. will be recover-able on distillation, and—more particularly—the form in which they will be recovered. The latter consideration is rarely accorded the share of attention it merits, but a simple illustration is sufficient to indicate its importance. Of the original heat units in a toa of coal about 95 per cent, are recovered in useful form in (a) the gas, (S) the coke, (c) the tar. Heat units sold in the gaseous form are by far the most rehiimera-tive ; therefore, the original calorific power of the coal should be recovered in the gas as far as possible. According to general experien.ce, coke accounts for from 60 to 65 per cent, of the original calorific power of the coal, tar from 6 to 8 per cent.,, and the gas for 25 to 35 per cent., while a loss approaching 5 per cent, is generally found, this being partly .accoun.ted for by retort carbon and sulphur impurities. Bedson1 has given figures representative of the heat distribution of the products of distillation in ordinary gas retorts. The heat in 100 Ib. of dry gas coal was dis-tributed as folio ws :— Per eent. of Original Calorific Power of Coal. In coke sold 47-8 In coke used for heating retorts . . . . ■ ■ • H’4 In gas (sold) 21-3 In tar In liquor, sulphur, and loss . . . . • • ■ • 13-4 Another example of the manner in which the original calorific power of the coal is distributed atter carbonization is given, by the folio wing table,2 which relates to statistics obtained from 8,500,000 (metric) tons of coal distilled in Germany. The figures are instructive in that they relate to results on a large scale. 1 Gas World (Coking Suppt.), Feb. 2, 1917, p. 16. 2 Die. Wirtschaft der Deutschen Gaswerke, 1914, p. 14.