DESTRUCTORS little more than the quantity theoretically necessary, while with chimney draught more than double the theoretical amount of air must be supplied. With forced draught, too, a much higher temperature is attained, and if it is properly worked, little or no cold air will enter the furnaces during stoking operations. As far as possible a balance of pressure in the cells during clinkering should be maintained just sufficient to prevent an inrush of cold air through the flues. The forced draught pressure should not •exceed 2 inches’ water-gauge. The efficiency of the combustion in the furnaces is conveniently measured by the “Econometer,” which registers continuously and automatically the proportion of COa passing away in the waste gases ; the higher the percentage of C02 the more efficient the furnace, provided there is no formation of CO, the presence of which would indicate incomplete combustion. The theoretical maximum of C02 for refuse burning is about 20 per cent. ; and, by maintaining an even clean fire, by admitting secondary air over the fire, and by regulating the dampers or the air-pressure in the ash-pit, an amount approximating to this percentage may be attained in a well-designed furnace if properly worked. If the proportion of free oxygen (i.e., excess ■of air) is large, more air is passed through the furnace than is required for complete combustion, and the heating of this excess is clearly a waste of heat. The position of the econometer in testing should be as near the furnace as possible, as there may be con.siderable air leakage through the brickwork of the flues. The modern high-temperature destructor, to render the refuse .and gases perfectly innocuous and harmless, is worked at a temperature varying fromtemperatures 1250° to 2000° F., naturally and the Ca or c ma intenance of such has very va ue ' suggested the possibility of utilizing this heat-energy for the production of steam - power. Successful steam-raising destructor stations have been in operation during recent years in England, and experience shows that a considerable amount of ■energy may be derived therefrom, amply justifying a reasonable increase of expenditure on plant and labour. The actual calorific value of the refuse material necessarily varies, but, as a general average, experience shows that, with suitably-designed and properly-managed plant, an evaporation of 1 lb of water per pound of refuse burned is a result which may be readily attained, and affords a basis of calculation which engineers may safely adopt in practice. Many destructor steam-raising plants, however, give considerably higher results, as will be seen from the following table :— Town.
Water eva' Average Water eva- porated porated lb of Re-per SteamType of per lb of fuse, from pressure Furnaces. Refuse square at 212° perinch. Actual. andFahr.
Rochdale. Meldrum Bros. (4 cells) Hereford . Meldrum Bros. (4 cells) Darwen . Meldrum Bros. (4 cells) Oldham . Horsfall (10 cells) Canterbury Beaman & Deas (2 cells)
1-64 lb
■LLUlDC-pi develope hour (has 20 lb of s per I.H.] hour
113 R>
350
1-51 „ 1-82 „ 70-92 ,,
236
1-48 „ 1-55 „
183 „
275
I’ll „ 1-33 „ 1-41 „ 1-59 „
128 „
200
132 ,,
236
1-97 lb
From actual experience it may be accepted, therefore, that the calorific value of unscreened house refuse varies from 1 to 2 lb of water evaporated per pound of refuse burned, the exact proportion depending upon the quality and condition of the material dealt with. Taking the evaporative power of coal at 10 lb of water per pound of coal, this gives for domestic house refuse a value of from TUT to 4- that of coal; or, with coal at 20s. per ton, refuse has a commercial value of from 2s. to 4s. per ton. In London the quantity of house refuse amounts to about 1J million tons per annum, which is equivalent to from 4 cwt. to 5 cwt. per head per annum. If it be burned in furnaces giving an evaporation of 1 lb of water per pound of refuse, it would yield a total power annually of about 138 million brake horse - power hours, and equivalent cost of coal at 20s. per ton for this amount of power, even when calculated upon the very low estimate of 2 lb1 of coal per brake horse-power hour, works out at over £123,000. On the same basis, the refuse of a medium-sized town, with, say, a population of 70,000 yielding refuse at the rate of 5 cwt. per head per annum, would afford 112 indicated horse-power 1 With medium-sized steam plant, a consumption of 4 lb of coal per brake horse-power per hour is a very usual performance.
429
per ton burned, and the total indicated horse-power hours per annum would be f^-QQ_0.^5 cwt- x 112 = 1,960,000 I.H.P. hours annually. If this were applied to the production of electric energy, the electrical horse-power hours would be (with a dynamo efficiency of 90 per cent.) 1,960,^000 xjO_-^yg^QQQ E.H.P. hours per annum ; and the watt-hours per annum at the central station would be 1,764,000 x 746 = 1,315,944,000. Allowing for a loss of 10 per cent, in distribution, this would give 1,184,349,600 watt - hours available in lamps, or with 8 candle-power lamps taking 30 watts of current per lamp, we should have 1,184,349,600 watt - hours = 39,478,320 8-c.p. lamp-hours per 30 watts 39 478 320 that is,’ 70,000’ population ’ , ,=563 8-c.p.h lamp-hours per annum per ea(i 0f population. Taking the loss due to the storage which would be necessary at 20 per cent, on three-quarters of the total or 15 per cent, upon the whole, there would be 478 8-c.p. lamp-hours per annum per head of the population ; i.e., if the power developed from the refuse were fulty utilized, it would supply electric light at the rate of one 8-c.p. lamp per head of the population for about 1J hours for every night of the year. In actual practice, when the electric energy is for the purposes of lighting only, difficulty has been experienced in fully utilizing the thermal energy from a destructor plant owing to D]ffjcujfjes the want of adequate means of storage either of the thermal or of the electric energy. A destructor station usually yields a fairly definite amount of thermal energy uniformly throughout the twenty-four hours, while the consumption of electric-lighting current is extremely irregular, the maximum demand being about four times the mean demand. The period during which the demand exceeds the mean is comparatively short, and does not exceed about six hours out of the twenty-four, while for a portion of the time the demand may not exceed ^jth of the maximum. This difficulty, at first regarded as somewhat grave, is now substantially minimized by the provision of ample boiler capacity, or by the introduction of feed thermal storage vessels in which hot feed-water may be stored during the hours of light load (say eighteen out of the twenty-four), so that at the time of maximum load the boilers may be filled directly from these vessels, which work at the same pressure and temperature as the boiler. Further, the difficulty above mentioned will disappear entirely at stations where there is a fair day load which practically ceases at about the hour when the illuminating load comes on, thus equalizing the demand upon both destructor and electric plant throughout the twenty-four hours. This arises in cases where current is consumed during the day for motors, fans, lifts, electric tramways, and other like purposes, and, as the employment of electric energy for these services is rapidly becoming general, no difficulty need be anticipated in the successful working of combined destructor and electric plants where these conditions prevail. The more uniform the electrical demand becomes, the more fully may the power from a destructor station be utilized. In the case above cited of a town of 70,000 population, the horsepower to be derived from the refuse, calculated upon the basis of 2 lb of coal per brake horse - power hour, which is the utmost efficiency practicable even for very good steam-engines, will cost £1750 per annum for fuel with coal at 20s. per ton, and, in practice, the actual cost would doubtless be nearly double. At Shoreditch during the year ending March 1899, a total of 1,031,348 Board of Trade units of electric energy was supplied to consumers; of this about seven-tenths were generated from the refuse of the district, and on many occasions a load of 400 kilowatts (i.e., 400 kilo, x 1000 j 1 x — =100 596 horse-power) has been, carried • by refuse fuel only. Some 200 municipalities in England have laid down destructor plants, but although the great majority are utilizing some of the surplus heat generated by the furnaces, at comparatively few stations is the full thermal energy of the refuse turned to commercial utility owing to the fact that the plants were installed before the value of refuse for steam-raising was properly understood. During recent years, however, new and improved plant has been introduced, and in the laying down of all new installations this phase of the question has been kept most prominently in view. For further information on the subject, reference should be made to William H. Maxwell, Assoc. M. Inst. C.E.,_ on the Removal and Disposal of Town Refuse, with an exhaustive treat-
