Modern Gasworks Practice

THE HORIZONTAL RETORT BENCH 61 instances an additional quantity of fuel has had to be purchased from outside. About double the quantity of air enteis the furnace than is theoretically neces-sary, which. has the effect of retarding combustion and reducing the limit of temperature. This is chiefly due to the large volumes of nitrogen introduced with the air, and the defect has been overcome in some measure by employing the semi-gaseous system. The quantity of fuel necessary for the carbonization of coal has, by the curtail-ment of thermal losses in every direction, been reduced to such, fine limits that at the present day, under ideal conditions, the coke required is only about 11 per cent, of the original weight of coal. The fuel consumed under the various systems is normally as follows :— Direot-fired furnace ... 40 per cent, of coke made used as fuel. Generator furnace . . . 22 to 26 „ „ „ „ Regenerator furnace . . . 16 „ 20 „ „ „ „ Vertical retorts—■ (a) Intermittent type . . 23 „ „ „ „ (6) Continuous type . . . 16| „ 18| „ „ „ „ Outside producers . . . 18| „ 23 „ „ » „ In giving figures such, as the above the basis of coke and breeze made per ton of coal carbonized on whicli the figures have been calculated. must always be stated. In the present instance the yield has been taken as 14 cwts. per ton. It is owing to the widely varying yields of coke from coals that this method of gauging fuel results is somewhat unsatisfactory, and the most reliable basis is that of Ibs. of fuel used per 100 Ib. of coal carbonized,” such as given, in the table below. Chief importance, however, attaches to the coke actually sold per ton of coal, and so long as this amounts to more than 10 cwts. the economy of the system of firing is above question. Direct-fired furnace 28 per cent, of original weight of coal carbonized used as fuel. Generator furnace . 15 to 18 „ „ „ » » Regenerator furnace . . 11 „ 14 Vertical retorts— (a) Intermittent type 16 (6) Continuous type 111 „ Outside producers . 13 „ 16 The apparently low fuel figures for the continuous vertical system are chiefly accounted for by the faet tKat the h.eat is extracted from tlie coke before it leaves the retort, and is utilized. for heating up ei.th.er the primary or secondary air before combustion. It must be pointed out, however, that fuel fi.gu.res for verticals are at present more or less in the nature of experiments, as there has as yet been little opportunity of making comparisons over prolonged periods. The figures given above, in faet, may be looked upon as the most favourable obtained in operating vertical retorts, and for the sake of comparison the following results, obtained 1 Extracted from figures given by J. West, Proc. S. Association Gas Engs., November, 1917.