Destructors
DESTRUCTORS. The name destructors is applied by English municipal engineers to furnaces, or combinations of furnaces, commonly called "garbage furnaces" in the United States, constructed for the purpose of disposing by burning of town refuse, which is a heterogeneous mass of material, including, besides general household and ash-bin refuse, small quantities of garden refuse, trade refuse, market refuse and often street sweepings. The mere disposal of this material is not, however, by any means the only consideration in dealing with it upon the destructor system. For many years past scientific experts, municipal engineers and public authorities have been directing careful attention to the utilization of refuse as fuel for steam production, and such progress in this direction has been made that in many towns its calorific value is now being utilized daily for motive-power purposes. On the other hand, that proper degree of caution which is obtained only by actual experience must be exercised in the application of refuse fuel to steam-raising. When its value as a low-class fuel was first recognized, the idea was disseminated that the refuse of a given population was of itself sufficient to develop the necessary steam-power for supplying that population with the electric light. The economical importance of a combined destructor and electric undertaking of this character naturally presented a somewhat fascinating stimulus to public authorities, and possibly had much to do with the development both of the adoption of the principle of dealing with refuse by fire, and of lighting towns by electricity. However true this phase of the question may be as the statement of a theoretical scientific fact, experience so far does not show it to be a basis upon which engineers may venture to calculate, although, as will be seen later, under certain circumstances of equalized load, which must be considered upon their merits in each case, a well-designed destructor plant can be made to perform valuable commercial service to an electric or other power-using undertaking. Further, when a system, thermal or otherwise, for the storage of energy can be introduced and applied in a trustworthy and economical manner, the degree of advantage to be derived from the utilization of the waste heat from destructors will be materially enhanced.
Composition and quantity of refuse
The composition of house refuse, which must obviously affect its calorific value, varies considerably in different localities, according to the condition, habits and pursuits of the people. Towns situated in coal-producing districts invariably yield a refuse richer in unconsumed carbon than those remote therefrom. It is also often found that the refuse from different parts of the same town varies considerably - that from the poorest quarters frequently proving of greater calorific value than that from those parts occupied by the rich and middle classes. This has been attributed to the more extravagant habits of the working classes in neglecting to sift the ashes from their fires before disposing of them in the ash-bin. In Bermondsey, for example, the refuse has been found to possess an unusually high calorific value, and this experience is confirmed in other parts of the metropolis. Average refuse consists of breeze (cinder and ashes), coal and coke, fine dust, vegetable and animal matters, straw, shavings, cardboard, bottles, tins, iron, bones, broken crockery and other matters in very variable proportions according to the character of the district from which it is collected. In London the quantity of house refuse amounts approximately to 1 million tons per annum, which is equivalent to from 4 cwt. to 5 cwt. per head per annum, or to from 200 to 250 tons per 1000 of the population per annum. Statistics, however, vary widely in different districts. In the vicinity of the metropolis the amount varies from 2.5 cwt. per head per annum at Leyton to 3.5 cwt. at Hornsey, and to as much as 7 cwt. at Ealing. In the north of England the total house refuse collected, exclusive of street sweepings, amounts on the average to 8 cwt. per head per annum. Speaking generally, throughout the country an amount of from 5 cwt. to 10 cwt. per head per annum should be allowed for. A cubic yard of ordinary house refuse weighs from 12 to 15 cwt. Shop refuse is lighter, frequently containing a large proportion of paper, straw and other light wastes. It sometimes weighs as little as 7 cwt. per cubic yard. A load, by which refuse is often estimated, varies in weight from 15 cwt. to 1 tons.
Refuse disposal
The question how a town's refuse shall be disposed of must be considered both from a commercial and a sanitary point of view. Various methods have been practised. Sometimes the household ashes, etc., are mixed with pail excreta, or with sludge from a sewage farm, or with lime, and disposed of for agricultural purposes, and sometimes they are conveyed in carts or by canal to outlying and country districts, where they are shot on waste ground or used to fill up hollows and raise the level of marshland. Such plans are economical when suitable outlets are available. To take the refuse out to sea in hopper barges and sink it in deep water is usually expensive and frequently unsatisfactory. At Bermondsey, for instance, the cost of barging is about 2s. 9d. a ton, while the material may be destroyed by fire at a cost of from 10d. to 1s. a ton, exclusive of interest and sinking fund on the cost of the works. In other cases, as at Chelsea and various dust contractors' yards, the refuse is sorted and its ingredients are sold; the fine dust may be utilized in connexion with manure manufactories, the pots and pans employed in forming the foundations of roads, and the cinders and vegetable refuse burnt to generate steam. In the Arnold system, carried out in Philadelphia and other American towns, the refuse is sterilized by steam under pressure, the grease and fertilizing substances being extracted at the same time; while in other systems, such as those of Weil and Porno, and of Defosse, distillation in closed vessels is practised. But the destructor system, in which the refuse is burned to an innocuous clinker in specially constructed furnaces, is that which must finally be resorted to, especially in districts which have become well built up and thickly populated.
Types of destructors
Various types of furnaces and apparatus have from time to time been designed, and the subject has been one of much experiment and many failures. The principal towns in England which took the lead in the adoption of the refuse destructor system were Manchester, Birmingham, Leeds, Heckmondwike, Warrington, Blackburn, Bradford, Bury, Bolton, Hull, Nottingham, Salford, Ealing and London. Ordinary furnaces, built mostly by dust contractors, began to come into use in London and in the north of England in the second half of the 19th century, but they were not scientifically adapted to the purpose, and necessitated the admixture of coal or other fuel with the refuse to ensure its cremation. The Manchester corporation erected a furnace of this description about the year 1873, and Messrs Mead & Co. made an unsatisfactory attempt in 1870 to burn house refuse in closed furnaces at Paddington. In 1876 Alfred Fryer erected his destructor at Manchester, and several other towns adopted this furnace shortly afterwards. Other furnaces were from time to time brought before the public, among which may be mentioned those of Pearce and Lupton, Pickard, Healey, Thwaite, Young, Wilkinson, Burton, Hardie, Jacobs and Odgen. In addition to these the "Beehive" and the "Nelson" destructors became well known. The former was introduced by Stafford and Pearson of Burnley, and one was erected in 1884 in the parish yard at Richmond, Surrey, but the results being unsatisfactory, it was closed during the following year. The "Nelson" furnace, patented in 1885 by Messrs Richmond and Birtwistle, was erected at Nelson-in-Marsden, Lancashire, but being very costly in working was abandoned. The principal types of destructors now in use are those of Fryer, Whiley, Horsfall, Warner, Meldrum, Beaman and Deas, Heenan and Froude, and the "Sterling" destructor erected by Messrs Hughes and Stirling.
The general arrangement of the destructor patented [1] by Alfred Fryer in 1876 is illustrated in fig. 1. An installation upon this principle consists of a number of furnaces or cells, usually arranged in pairs back to back, and enclosed in a rectangular block of brickwork having a flat top, upon which the house refuse is tipped from the carts.
A large main flue, which also forms the dust chamber, is placed underneath the furnace hearths. The Fryer furnace ordinarily burns from 4 to 6 tons of refuse per cell per 24 hours. It will be observed that the outlets for the products of combustion are placed at the back near the refuse feed opening, an arrangement which is imperfect in design, inasmuch as while a charge of refuse is burning upon the furnace bars the charge which is to follow lies on the dead hearth near the outlet flue. Here it undergoes drying and partial decomposition, giving off offensive empyreumatic vapours which pass into the flue without being exposed to sufficient heat to render them entirely inoffensive. The serious nuisances thus produced in some instances led to the introduction of a second furnace, or "cremator," patented by C. Jones of Ealing in 1885, which was placed in the main flue leading to the chimney-shaft, for the purpose of resolving the organic matters present in the vapour, but the greatly increased cost of burning due to this device led to its abandonment in many cases. This type of cell was largely used during the early period of the history of destructors, but has to a considerable extent given place to furnaces of more modern design.
A furnace [2] patented in 1891 by Mr Henry Whiley, superintendent of the scavenging department of the Manchester corporation, is automatic in its action and was designed primarily with a view to saving labour - the cells being fed, stoked and clinkered automatically. There is no drying hearth, and the refuse carts tip direct into a shoot or hopper at the back which conducts the material directly on to movable eccentric grate bars. These automatically traverse the material forward into the furnace, and finally push it against a flap-door which opens and allows it to fall out. This apparatus is adapted for dealing with screened rather than unscreened refuse, since it suffers from the objection that the motion of the bars tends to allow fine particles to drop through unburnt. Some difficulty has been experienced from the refuse sticking in the hopper, and exception may also be taken to the continual flapping of the door when the clinker passes out, as cold air is thereby admitted into the furnace. As in the Fryer cell, the outlet for the products of combustion into the main flue is close to the point where the crude refuse is fed into the furnace, and the escape of unburnt vapours is thus facilitated. Forced draught is applied by means of a Roots blower. The Manchester corporation has 28 cells of this type in use, and the approximate amount of refuse burnt per cell per 24 hours is from 6 to 8 tons at a cost per ton for labour of 3.47 pence.
Horsfall's destructor [3] (fig. 2) is a high-temperature furnace of modern type which has been adopted largely in Great Britain and on the continent of Europe. In it some of the general features of the Fryer cell are retained, but the details differ considerably from those of the furnaces already described. Important points in the design are the arrangement of the flues and flue outlets for the products of combustion, and the introduction of a blast duct through which air is forced into a closed ash-pit. The feeding-hole is situated at the back of and above the furnace, while the flue opening for the emission of the gaseous products is placed at the front of the furnace over the dead plate; thus the gases distilled from the raw refuse are caused to pass on their way to the main flue over the hottest part of the furnace and through the flue opening in the red-hot reverberatory arch. The steam jet, which plays an important part in the Horsfall furnace, forces air into the closed ash-pit at a pressure of about to 1 in. of water, and in this way a temperature varying from 1500° to 2000° F., as tested by a thermo-electric pyrometer, is maintained in the main flue. In a battery of cells the gases from each are delivered into one main flue, so that a uniform temperature is maintained therein sufficiently high to prevent noxious vapours from reaching the chimney. The cells being charged and clinkered in rotation, when the fire in one is green, in the others it is at its hottest, and the products of combustion do not reach the boiler surfaces until after they have been mixed in the main flue. The cast iron boxes which are provided at the sides of the furnaces, and through which the blast air is conveyed on its way to the grate, prevent the adhesion of clinker to the side walls of the cells, and very materially preserve the brickwork, which otherwise becomes damaged by the tools used to remove the clinker. The wide clinkering doors are suspended by counterbalance weights and open vertically. The rate of working of these cells varies from 8 tons per cell per 24 hours at Oldham to 10 tons per cell at Bradford, where the furnaces are of a later type. The cost of labour in stoking and clinkering is about 6d. per ton of the refuse treated at Bradford, and 9d. per ton at Oldham, where the rate of wages is higher. Well-constructed and properly-worked plants of this type should give rise to no nuisance, and may be located in populous neighbourhoods without danger to the public health or comfort. Installations were put down at Fulham (1901), Hammerton Street, Bradford (1900), West Hartlepool (1904), and other places, and the surplus power generated is employed in the production of electric energy.
Warner's destructor, [4] known as the "Perfectus," is, in general arrangement, similar to Fryer's, but differs in being provided with special charging hoppers, dampers in flues, dust-catching arrangements, rocking grate bars and other improvements. The refuse is tipped into feeding-hoppers, consisting of rectangular cast iron boxes over which plates are placed to prevent the escape of smoke and fumes. At the lower portion of the feeding-hopper is a flap-door working on an axis and controlled by an iron lever from the tipping platform. When refuse is to be fed into the furnace the lever is thrown over, the contents of the hopper drop on to the sloping firebrick hearth beneath, and the door is at once closed again. The door should be kept open as short a time as possible in order to prevent the admission of cold air into the furnace at the back end, since this leads to the lowering of the temperature of the cells and main flue, and also to paper and other light refuse being carried into the flues and chimney. The flues of each furnace are provided with dampers, which are closed during the process of clinkering in order to keep up the heat. The cells are each 5 ft. wide and 11 ft. deep, the rearmost portion consisting of a firebrick drying hearth, and the front of rocking grate bars upon which the combustion takes place. The crown of each cell is formed of a reverberatory firebrick arch having openings for the emission of the products of combustion. The flap dampers which are fitted to these openings are operated by horizontal spindles passing through the brickwork to the front of the cell, where they are provided with levers or handles; thus each cell can be worked independently of the others. With the view of increasing the steam-raising capabilities of the furnace, forced draught is sometimes applied and a tubular boiler is placed close to the cells. The amount of refuse consumed varies from 5 tons to 8 tons per cell per 24 hours. At Hornsey, where 12 cells of this type are in use, the cost of labour for burning the refuse is 9d. per ton.
Meldrum's
The Meldurm "Simplex" destructor (fig. 3), a type of furnace which yields good steam-raising results, is in successful operation at Rochdale, Hereford, Darwen, Nelson, Plumstead and Woolwich, at each of which towns the production of steam is an important consideration. Cells have also been laid down at Burton, Hunstanton, Blackburn and Shipley, and more recently at Burnley, Cleckheaton, Lancaster, Nelson, Sheerness and Weymouth. In general arrangement the destructor differs considerably from those previously described. The grates are placed side by side without separation except by dead plates, but, in order to localize the forced draught, the ash-pit is divided into parts corresponding with the different grate areas. Each ash-pit is closed airtight by a cast iron plate, and is provided with an air-tight door for removing the fine ash. Two patent Meldrum steam-jet blowers are provided for each furnace, supplying any required pressure of blast up to 6 in. water column, though that usually employed does not exceed 1 in. The furnaces are designed for hand-feeding from the front, but hopper-feeding can be applied if desirable. The products of combustion either pass away from the back of each fire-grate into a common flue leading to boilers and the chimney-shaft, or are conveyed sideways over the various grates and a common fire-bridge to the boilers or chimney. The heat in the gases, after passing the boilers, is still further utilized to heat the air supplied to the furnaces, the gases being passed through an air heater or continuous regenerator consisting of a number of cast iron pipes from which the air is delivered through the Meldrum "blowers" at a temperature of about 300° F. That a high percentage (15 to 18%) of CO2 is obtained in the furnaces proves a small excess of free oxygen, and no doubt explains the high fuel efficiency obtained by this type of destructor. High-pressure boilers of ample capacity are provided for the accumulation during periods of light load of a reserve of steam, the storage being obtained by utilizing the difference between the highest and lowest water-levels and the difference between the maximum and working steam-pressure. Patent locking fire-bars, to prevent lifting when clinkering, are used in the furnace and have a good life. At Rochdale the Meldrum furnaces consume from 53 lb to 66 lb of refuse per square foot of grate area per hour, as compared with 22.4 lb per square foot in a low-temperature destructor burning 6 tons per cell per 24 hours with a grate area of 25 sq. ft. The evaporative efficiency of the Rochdale furnaces varies from 1.39 lb to 1.87 lb of water (actual) per 1 lb of refuse burned, and an average steam-pressure of about 114 lb per square inch is maintained. The cost of labour and supervision amounts to 10d. per ton of refuse dealt with. A Lancashire boiler (22 ft. by 6 ft. 6 in.) at the Sewage Outfall Works, Hereford, evaporates with refuse fuel 2980 lb of water per hour, equal to 149 indicated horse-power. About 54 lb of refuse are burnt per square foot of grate area per hour with an evaporation of 1.82 lb of water per pound of refuse.
The Beaman and Deas destructor [5] (fig. 4) has attracted much attention from public authorities, and successful installations are in operation at Warrington, Dewsbury, Leyton, Canterbury, Llandudno, Colne, Streatham, Rotherhithe, Wimbledon, Bolton and elsewhere. Its essential features include a level-fire grate with ordinary type bars, a high-temperature combustion chamber at the back of the cells, a closed ash-pit with forced draught, provision for the admission of a secondary air-supply at the fire-bridge, and a firebrick hearth sloping at an angle of about 52°. From the refuse storage platform the material is fed into a hopper mouth about 18 in. square, and slides down the firebrick hearth, supported by T-irons, to the grate bars, over which it is raked and spread with the assistance of long rods manipulated through clinkering doors placed at the sides of the cells. A secondary door in the rear of the cell facilitates the operation. The fire-bars, spaced only 3⁄32 in. apart, are of the ordinary stationary type. Vertically, under the fire-bridge, is an air-conduit, from the top of which lead air blast pipes 12 in. in diameter discharging into a hermetically closed ash-pit under the grate area. The air is supplied from fans (Schiele's patent) at a pressure of from 1 to 2 in. of water, and is controlled by means of baffle valves worked by handles on either side of the furnace, conveniently placed for the attendant. The forced draught tends to keep the bars cool and lessen wear and tear. The fumes from the charge drying on the hearth pass through the fire and over the red-hot fire-bridge, which is perforated longitudinally with air-passages connected with a small flue leading from a grated opening on the face of the brickwork outside; in this way an auxiliary supply of heated oxygen is fed into the combustion chamber. This chamber, in which a temperature approaching 2000° F. is attained, is fitted with large iron doors, sliding with balance weights, which allow the introduction of infected articles, bad meat, etc., and also give access for the periodical removal of fine ash from the flues. The high temperatures attained are utilized by installing one boiler, preferably of the Babcock & Wilcox water-tube type, for each pair of cells, so that the gases, on their way from the combustion chamber to the main flue, pass three times between the boiler tubes. A secondary furnace is provided under the boiler for raising steam by coal, if required, when the cells are out of use. The grate area of each cell is 25 sq. ft., and the consumption varies from 16 up to 20 tons of refuse per cell per 24 hours. In a 24-hours' test made by the superintendent of the cleansing department, Leeds, at the Warrington installation, the quantity of water evaporated per pound of refuse was 1.14 lb, the average temperature in the combustion chamber 2000° F. by copper-wire test, and the average air pressure with forced draught 2 in. (water-gauge). At Leyton, which has a population of over 100,000, an 8-cell plant of this type is successfully dealing with house refuse and filter press cakes of sewage sludge from the sewage disposal works adjoining, and even with material of this low calorific value the total steam-power produced is considerable. Each cell burns about 16 tons of the mixture in 24 hours and develops about 35 indicated horse-power continuously, at an average steam-pressure in the boilers of 105 lb. The cost of labour at Leyton for burning the mixed refuse is about 1s. 7d. per ton; at Llandudno, where four cells were laid down in connexion with the electric-light station in 1898, it is 1s. 3d., and at Warrington 9d. per ton of refuse consumed. Combustion is complete, and the destructor may be installed in populous districts without nuisance to the inhabitants. Further patents (Wilkie's improvements) have been obtained by Meldrum Brothers (Manchester) in connexion with this destructor.
Heenan
The Heenan furnaces are in operation at Farnworth, Gloucester, Barrow-in-Furness, Northampton, Mansfield, Wakefield, Blackburn, Levenshulme, Kings Norton, Worthing, Birmingham and other places, and are now dealing with over 1200 tons of refuse per day. The general arrangement of this destructor somewhat resembles that of the Meldrum type. The cells intercommunicate, and the mechanical mixture of the gases arising from the furnace grates of the various cells is sought by the introduction of a special design of reverberatory arch overlying the grates. The standard arrangement of this destructor embodies all modern arrangements for high-temperature refuse destruction and steam-power generation.
Sterling
Destructors of the "Sterling" type, combined with electric-power generating stations, are installed at Hackney (1901), Bermondsey (1902) and Frederiksberg (1903) - the first-named plant being probably the most powerful combined destructor and electricity station yet erected. In these modern stations the recognized requirements of an up-to-date refuse-destruction plant have been well considered and good calorific results are also obtained.
In addition to the above-described destructors, other forms have been introduced from time to time, but adopted to a less degree; amongst these may be mentioned Baker's destructor, Willshear's, Hanson's Utilizer, Mason's Gasifier, the Bennett-Phythian, Cracknell's (Melbourne, Victoria), Coltman's (Loughborough), Willoughby's, and Healey's improved destructors. On the continent of Europe systems for the treatment of refuse have also been devised. Among these may be mentioned those of M. Defosse and M. Helouis. The former has endeavoured to burn the refuse in large quantities by using a forced draught and only washing the smoke. [6] Helouis has extended the operation by using the heat from the combustion of the refuse for drying and distilling the material which is brought gradually on to the grate.
Destructor accessories
Boulnois and Brodie's improved charging tank is a labour-saving apparatus consisting of a wrought iron truck, 5 ft. wide by 3 ft. deep, and of sufficient length to hold not less than 12 hours supply for the two cells which it serves. The truck, which moves along a pair of rails across the top of the destructor, may be worked by one man. It is divided into compartments holding a charge of refuse in each, and is provided with a pair of doors in the bottom, opening downwards, which are supported by a series of small wheels running on a central rail. A special feeding opening in the reverberatory arch of the cell of the width of the truck, situated over the drying hearth, is formed by a firebrick arch fitted into a frame capable of being moved backwards and forwards by means of a lever. The charging truck, when empty, is brought under the tipping platform, and the carts tip directly into it. When one of the cells has to be fed, the truck is moved along, so that one of the divisions is immediately over the feeding opening, and the wheel holding up the bottom doors rests upon the central rail, which is continued over the movable covering arch. Then the movable arch is rolled back, the doors are released, and the contents are discharged into the cell, so that no handling of the refuse is required from tipping to feeding. This apparatus is in operation at Liverpool, Shoreditch, Cambridge and elsewhere.
Various forms of patent movable fire-bars have been employed in destructor furnaces. Among these may be mentioned Settle's, [7] Vicar's, [8] Riddle's rocking bars, [9] Horsfall's self-feeding apparatus, [10] and Healey's movable bars; [11] but complicated movable arrangements are not to be recommended, and experience greatly favours the use of a simple stationary type of fire-bar.
A dust-catching apparatus has been designed and erected at Edinburgh, by the Horsfall Furnace Syndicate, in order to overcome difficulties in regard to the escape of flue dust, etc., from the destructor chimney. Externally, it appears a large circular block of brickwork, 18 ft. in diameter and 13 ft. 7 in. high, connected with the main flue, and situated between the destructor cells and the boiler. Internally it consists of a spiral flue traversing the entire circumference and winding upwards to the top of the chamber. There is an interior well or chamber 6 ft. diameter by 12 ft. high, having a domed top, and communicating with the outer spiral flue by four ports at the top of the chamber. Dust traps, baffle walls and cleaning doors are also provided for the retention and subsequent weekly removal of the flue dust. The apparatus forms a large reservoir of heat maintained at a steady temperature of from 1500º to 1800° F., and is useful in keeping up steam in the boiler at an equable pressure for a long period. It requires no attention, and has proved successful for its purpose.
Travelling cranes for transporting refuse and feeding cells are sometimes employed at destructor stations, as, for example, at Hamburg. Here the transportation of the refuse is effected by means of specially constructed water-tight iron wagons, containing detachable boxes provided with two double-flap doors at the top for loading, and one flap-door at the back for unloading. There are thirty-six furnaces of the Horsfall type placed in two ranks, each arranged in three blocks of six in the large furnace hall. An electric crane running above each rank lifts the boxes off the wagons and carries them to the feeding-hole of each well. Here the box is tipped up by an electric pulley and emptied on to the furnace platform. When the travelling crane is used, the carts (four-wheeled) bringing the refuse may be constructed so that the body of the carriage can be taken off the wheels, lifted up and tipped direct over the furnace as required, and returned again to its frame. The adoption of the travelling crane admits of the reduction in size of the main building, as less platform space for unloading refuse carts is required; the inclined roadway may also be dispensed with. Where a destructor site will not admit of an inclined roadway and platform, the refuse may be discharged from the collecting carts into a lift; and thence elevated into the feeding-bins.
Other accessory plant in use at most modern destructor stations includes machinery for the removal, crushing and various means of utilization of the residual clinker, stoking tools, air heaters or regenerators for the production of hot-air blast to the furnaces, superheaters and thermal storage arrangements for equalizing the output of power from the station during the 24-hours' day.
Working of destructors
The general arrangement of a battery of refuse cells at a destructor station is illustrated by fig. 5. The cells are arranged either side by side, with a common main flue in the rear, or back to back with the main flue placed in the centre and leading to a tall chimney-shaft. The heated gases on leaving the cells pass through the combustion chamber into the main flue, and thence go forward to the boilers, where their heat is absorbed and utilized. Forced draught, or in many cases, hot blast, is supplied from fans through a conduit commanding the whole of the cells. An inclined roadway, of as easy gradient as circumstances will admit, is provided for the conveyance of the refuse to the tipping platform, from which it is fed through feed-holes into the furnaces. In the installation of a destructor, the choice of suitable plant and the general design of the works must be largely dependent upon local requirements, and should be entrusted to an engineer experienced in these matters. The following primary considerations, however, may be enumerated as materially affecting the design of such works: -
(a) The plant must be simple, easily worked without stoppages, and without mechanical complications upon which stokers may lay the blame for bad results. (b) It must be strong, must withstand variations of temperature, must not be liable to get out of order, and should admit of being readily repaired. (c) It must be such as can be easily understood by stokers or firemen of average intelligence, so that the continuous working of the plant may not be disorganized by change of workmen. (d) A sufficiently high temperature must be attained in the cells to reduce the refuse to an entirely innocuous clinker, and all fumes or gases should pass either through an adjoining red-hot cell or through a chamber whose temperature is maintained by the ordinary working of the destructor itself at a degree sufficient to exclude the possibility of the escape of any unconsumed gases, vapours or particles. The temperature may vary between 1500° and 2000°. (e) The plant must be so worked that while some of the cells are being recharged, others are at a glowing red heat, in order that a high temperature may be uniformly maintained. (f) The design of the furnaces must admit of clinkering and recharging being easily and quickly performed, the furnace doors being open for a minimum of time so as to obviate the inrush of cold air to lower the temperature in main flues, etc. (g) The chimney draught must be assisted with forced draught from fans or steam jet to a pressure of 1½ in. to 2 in. under grates by water-gauge. (h) Where a destructor is required to work without risk of nuisance to the neighbouring inhabitants, its efficiency as a refuse destructor plant must be primarily kept in view in designing the works, steam-raising being regarded as a secondary consideration. Boilers should not be placed immediately over a furnace so as to present a large cooling surface, whereby the temperature of the gases is reduced before the organic matter has been thoroughly burned. (i) Where steam-power and a high fuel efficiency are desired a large percentage of CO2 should be sought in the furnaces with as little excess of air as possible, and the flue gases should be utilized in heating the air-supply to the grates, and the feed-water to the boilers. (j) Ample boiler capacity and hot-water storage feed-tanks should be included in the design where steam-power is required.
Cost
As to the initial cost of the erection of refuse destructors, few trustworthy data can be given. The outlay necessarily depends, amongst other things, upon the difficulty of preparing the site, upon the nature of the foundations required, the height of the chimney-shaft, the length of the inclined or approach roadway, and the varying prices of labour and materials in different localities. As an example may be mentioned the case of Bristol, where, in 1892, the total cost of constructing a 16-cell Fryer destructor was £11,418, of which £2909 was expended on foundations, and £1689 on the chimney-shaft; the cost of the destructor proper, buildings and approach road was therefore £6820, or about £426 per cell. The cost per ton of burning refuse in destructors depends mainly upon - (a) The price of labour in the locality, and the number of "shifts" or changes of workmen per day; (b) the type of furnace adopted; (c) the nature of the material to be consumed; (d) the interest on and repayment of capital outlay. The cost of burning ton for ton consumed, in high-temperature furnaces, including labour and repairs, is not greater than in slow-combustion destructors. The average cost of burning refuse at twenty-four different towns throughout England, exclusive of interest on the cost of the works, is 1s. 1½d. per ton burned; the minimum cost is 6d. per ton at Bradford, and the maximum cost 2s. 10d. per ton at Battersea. At Shoreditch the cost per ton for the year ending on the 25th of March 1899, including labour, supervision, stores, repairs, etc. (but exclusive of interest on cost of works), was 2s. 6.9d. The quantity of refuse burned per cell per day of 24 hours varies from about 4 tons up to 20 tons. The ordinary low-temperature destructor, with 25 sq. ft. grate area, burns about 20 lb. of refuse per square foot of grate area per hour, or between 5 and 6 tons per cell per 24 hours. The Meldrum destructor furnaces at Rochdale burn as much as 66 lb. per square foot of grate area per hour, and the Beaman and Deas destructor at Llandudno 71.7 lb. per square foot per hour. The amount, however, always depends materially on the care observed in stoking, the nature of the material, the frequency of removal of clinker, and on the question whether the whole of the refuse passed into the furnace is thoroughly cremated.
Residues:
The amount of residue in the shape of clinker and fine ash varies from 22 to 37% of the bulk dealt with. From 25 to 30% is a very usual amount. At Shoreditch, where the refuse consists of about 8% of straw, paper, shavings, etc., the residue contains about 29% clinker, 2.7% fine ash, .5% flue dust, and .6% old tins, making a total residue of 32.8%. As the residuum amounts to from one-fourth to one-third of the total bulk of the refuse dealt with, it is a question of the utmost importance that some profitable, or at least inexpensive, means should be devised for its regular disposal. Among other purposes, it has been used for bottoming for macadamized roads, for the manufacture of concrete, for making paving slabs, for forming suburban footpaths or cinder footwalks, and for the manufacture of mortar. The last is a very general, and in many places profitable, mode of disposal. An entirely new outlet has also arisen for the disposal of good well-vitrified destructor clinker in connexion with the construction of bacteria beds for sewage disposal, and in many districts its value has, by this means, become greatly enhanced.
Forced draught
Through defects in the design and management of many of the early destructors complaints of nuisance frequently arose, and these have, to some extent, brought destructor installations into disrepute. Although some of the older furnaces were decided offenders in this respect, that is by no means the case with the modern improved type of high-temperature furnace; and often, were it not for the great prominence in the landscape of a tall chimney-shaft, the existence of a refuse destructor in a neighbourhood would not be generally known to the inhabitants. A modern furnace, properly designed and worked, will give rise to no nuisance, and may be safely erected in the midst of a populous neighbourhood. To ensure the perfect cremation of the refuse and of the gases given off, forced draught is essential. This is supplied either as air draught delivered from a rapidly revolving fan, or as steam blast, as in the Horsfall steam jet or the Meldrum blower. With a forced blast less air is required to obtain complete combustion than by chimney draught. The forced draught grate requires 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 in. water-gauge. The efficiency of the combustion in the furnace is conveniently measured by the "Econometer," which registers continuously and automatically the proportion of CO2 passing away in the waste gases; the higher the percentage of CO2 the more efficient the furnace, provided there is no formation of CO, the presence of which would indicate incomplete combustion. The theoretical maximum of CO2 for refuse burning is about 20%; 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 considerable air leakage through the brickwork of the flues.
The air supply to modern furnaces is usually delivered hot, the inlet air being first passed through an air-heater the temperature of which is maintained by the waste gases in the main flue.
Calorific value
The modern high-temperature destructor, to render the refuse and gases perfectly innocuous and harmless, is worked at a temperature varying from 1250° to 2000° F., and the maintenance of such temperatures has very naturally suggested the possibility of utilizing this heat-energy for the production of steam-power. Experience shows that a considerable amount of energy may be derived from steam-raising destructor stations, 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, 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, evaporations approaching 2 lb. of water per pound of refuse being often met with under favourable conditions.
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 1⁄10 to 1⁄5 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 1 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 lb.[12] 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 per ton burned, and the total indicated horse-power hours per annum would be 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%) 1,764,000 E.H.P. hours per annum;
and the watt-hours per annum at the central station would be 1,315,944,000.
Difficulties
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 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 24 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 6 hours out of the 24, while for a portion of the time the demand may not exceed 1⁄ 20th of the maximum. This difficulty, at first regarded as somewhat grave, is 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 18 out of the 24), so that at the time of maximum load the boiler 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 24 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 addition to combination with electric-lighting works, refuse destructors are now very commonly installed in conjunction with various other classes of power-using undertakings, including tramways, water-works, sewage-pumping, artificial slab-making and clinker-crushing works and others; and the increasingly large sums which are being yearly expended in combined undertakings of this character is perhaps the strongest evidence of the practical value of such combinations where these several classes of work must be carried on.
For further information on the subject, reference should be made to William H. Maxwell, Removal and Disposal of Town Refuse, with an exhaustive treatment of Refuse Destructor Plants (London, 1899), with a special Supplement embodying later results (London, 1905).
See also the Proceedings of the Incorporated Association of Municipal and County Engineers, vols. xiii. p. 216, xxii. p. 211, xxiv. p. 214 and xxv. p. 138; also the Proceedings of the Institution of Civil Engineers, vols. cxxii. p. 443, cxxiv. p. 469, cxxxi. p. 413, cxxxviii. p. 508, cxxix. p. 434, cxxx. pp. 213 and 347, cxxiii. pp. 369 and 498, cxxviii. p. 293 and cxxxv. p. 300.
(W. H. Ma.)
[1] Patent No. 3125 (1876).
[2] Patent No. 8271 (1891).
[3] Patents No. 8999 (1887); No. 14,709 (1888); No. 22,531 (1891).
[4] Patent No. 18,719 (1888).
[5] Patents No. 15,598 (1893) and 23,712 (1893); also Beaman and Deas Sludge Furnace, Patent No. 13,029 (1894).
[6] Compte Rendu des Travaux de la Société des Ingénieurs Civils de France, folio 775 (June 1897).
[7] Patent No. 15,482 (1885).
[8] Patents No. 1955 (1867) and No. 378 (1879).
[9] Patent No. 4896 (1891).
[10] Patent No. 20,207 (1892).
[11] Patents No. 18,398 (1892) and No. 12,990 (1892).
[12] With medium-sized steam plants, a consumption of 4 lb. of coal per brake horse-power per hour is a very usual performance.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)
