Modern Gasworks Practice

66 MODERN GASWORKS PRACTICE chamber. Hence, one of the golden rules of the retort house is the necessity for maintaining a deep fuel-bed (say, 6 feet) in the producer. Accordingly, in practical circles, it has generally come to be understood that in the ordinary process of the combustion of solid fuel, carbon dioxide is mainly formed under such conditions as prevail in the direct-fired furnace, that is to say, with a shallow layer of coke and an excess of air travelling at a relatively high velocity. The reaction would then merely be— 1 Ib. C + O2 = CO2 + 14,600 B.Th.U. When the fuel-bed is increased in depth, carbon dioxide gives way in great part to carbon monoxide, the general assumption being that the two following reactions occur at the base of the fire:— (1) C + O2 = CO2 + 14,600 B.Th.U. per Ib. of C. (6) 2C + O2 = 2CO + 4,400 Accordingly, in the lower layers of fuel there exist simultaneously hot carbon, carbon monoxide, and carbon dioxide, the last named reacting with. the upper layers of incandescent carbon according to th.e well-known endothermic reducing action :— CO2 + C = 2CO— 5,850 B.Th.U. per Ib. of C. This, of course, is the reversible reaction referred to above, and it must be borne in mind that high, temperature favours the formation of carbon monoxide, wh.ereas pressure tends in the reverse direction. As has been pointed out, however, this is a slow reaction, and as, moreover, the CO is being contimially witlidrawn, the effect of pressure in causing reversal is extremely small. The effect of temperature may be gauged from the results of Rhead and Wheeler, who found that at 850° C. (1,562° F.) the reduction of CO2 to CO proceeded 166 times more rapidly than the reverse reaction. The well-known experiments of Karl Wendt have established that, with a 7 feet 6 inches fuel-bed and air biast, equilibrium is attained at a height of from 20 to 30 inches above the plane of entry of the air, and that reversal of the reaction in the zones of lower temperature is shown by a slight increase in the CO2 content at higher levels of the fuel-bed. A series of tests carried out by the U.S. Bureau of Mines showed among other things that with a 6-in.ch bed of anthracite the oxygen is wholly consumed at a distance of from 3 to inches above the grate. The CO2 reached a maximum of 15 per cent, at a distance of 2 to 3 inches above the grate, and th.en. began to fall until, at the surface of the fuel-bed, it varied from 7 to 10 per cent. With a 6-incli bed of coke all oxygen disappeared at a height of inches above the grate. PRINCIPAL LOSSES OF HEAT Löss of heat, and, hence, waste of fuel in. retort benches results mainly from the following causes :— (®) Imperfect combustion of fuel. This may arise from imperfect regulation