RUBBISH DESTRUCTION

New Zealand Times, Volume XXX, Issue 6492, 11 April 1908, Page 6

RUBBISH DESTRUCTION

TURNING REFUSE TO ACCOUNT. BY-PRODUCTS FROM THE FURNACE. In solving by hygienic methods tho problem of rubbish disposal, Wellington led the way in Australasia. Its new Keenan and Fronde destructor which is now running, is said to be ahead of anything south of the JUne, aud it certainly appears to be coping very eiiectively with the mass of miscellaneous and unpromising looking material deposited in its hoppers from the city .dust carts. Wo take from an article by Mr T. C. Onuiston Chant, M. Inst,, C.E., who superintended the erection of tho destructor, some details of the working process and what can be done with the by-products. THE COMBUSTION PROCESS.

The whole success of a destructor scheme as a steam-raising project depends upon the accuracy with which the average composition of the refuse can bo judged—not such an easy matter with a fuel like refuse, varying greatly in composition with every cartload, every shower of rain, and with each season of the year. Given tho composition, the engineer must modify the design of the fan, boiler, air heater, and furnace to obtain tho results, and as it is usually impossible to obtain useful analysis of the refuse to be dealt with, the reader will understand fiie great value of wide exiperieuco in dealing with tho question. 'Having estimated the amount of carbon and hydrogen in a given quantity of the garbage, it remains to be determined how much oxygen in exceed of the theoretical quantity will bo required to completed}' burn tho carbon, to form carbon dioxide- and tbe hydrogen, to form water. What chemical action really does take place is not definitely known, but that a water gas is formed in the lower layers of the burning refuse is highly piobable; however, there is no direct evidence of such action, and the resultant action is the formation of nitrogen, which are given off from the chimney as colourless, odourless gasses and steam, respectively. If tho design of the furnace is faulty or if the air supply is not properly controlled, there will be loss of temperature in the furnace, due citner to insufficient air and consequent incomplete combustion, or to excess, causing dilution of the hot furnace gasses, by tho comparatively cola air blast from the fans.

VALUABLE STEAM. After discussing the heat-giving value of refuse, Mr Ormiston Chant suggests that each pound of Vvellington refuse will probably evaporate l-Jlbs of water from and at boiling point. The present coal-fired boilers work with an evaporation of 81Bs of water per lb of coal, therefore, for every 5 l-3rd tons of reiFuse burnt in tho destructor, ctaough steam will bo raised to save I ton of coal, i. 0., if the quantity of refuse brought to the destructor every day he about 3D tons, it will he seen that, if all the x>ower available from, the plant bo used, tho price of 3420 tons of cool wjif bo saved per annum, without reckoning the money saved by using tho clinker for concrete material, instead of buying shingle. CHEAP CLLNKBE.

Shingle costs, say, Ss per yard delivered at Oriental Bay tramway terminus; crushed . clinker would cost about Is 4d per yard delivered from me destructor to the sanio place. Clinker delivered at Newtown would cost, say, about 2s Gd per yard, and shingle, at tho Lowest price, say, 6s 6d per yard. These figures ore approximate only, but serve to show the great saving'the use of properly crushed and screened clinker can effect.

The amount of real clinker in 50 tons ox Wellington refuse will be about 8 tons. Therefore, we see that a saving of 365 s 8 x (cost of shingle less cost of clinker) or about <£sß-1 per annum will be earned for the Corporation from tno clinker alone. -The maintenance, depreciation, and interest on loan for the clinker plant will be about £HQ per annum, so that this, clinker plant alono will make a clear profit of £Hi per year. The cost of burning the refuse and clinkering the furnace will be about Is 6d per ton. This cost saould bo shared by the sanitation department and by rill departlmemts njiltag the available steam- •

In any case the cost of destroying the refuse will not exceed tuat with the prosent destructor, and in addition there will be the assets of steam and crushed clmker, as detailed above. Some time, of course, will be needed for tho Council to determine exactly what uses can be made of the steam, over and above that required to work the plant and operate tno present drainage pumping station, but it is anticipated that it will be token for town electric supply purposes. i UNIQUE CHIMNEY.

An interesting feature of the plant is the ferro-concrote chimney; tho first reinforced concrete stack in tho Southern Hemisphere. Tho design is that of Mr W. E. Morton, A.M.1.C.E.. City Engineer,

The foimuations comprised 42 ferroconcrete piles supporting a bed of reinforced concrete 6ft thick. The method pi reinforcing was by vertical bars in lengths of about 30 feet, with a substantial overlap. These are bound together with rings of round steel bars at intervals of IS inches. The thickness of tho concrete varies from la inches at the. bottom to 6 inches at tno top. The deflection from the vertical wuen tho chimney was in a acavy wind was found by Mr Ormistou Chant to be only halt an inch.