Wonders of World Engineering

 © Wonders of World Engineering 2014-20 |  contents  |  site map  |  info@wondersofworldengineering.com

The disposal of the refuse of a large industrial town is a problem that has been treated in a variety of ways. In many place rubbish is now burnt in specially designed furnaces and used to generate electrical power


INCOMING REFUSE is brought to the destructor in special enclosed containers





































THE INCOMING REFUSE is brought to the destructor in special enclosed containers with a capacity of 3¾ cubic yards. Two or sometimes three containers are mounted on one lorry. They are lifted by a crane on to bogies to be parked by a system of rails and turntables. When the refuse is required for incineration, the containers are taken by a telpher for discharge into the destructor.

.




OLD methods of dealing with enormous quantities of refuse, such as dumping the material on waste ground and allowing it to smoulder or rot, are entirely out of place in a modern civilized community. The ordinary rubbish tip or dump shows no advance than that made uncounted centuries ago, when some progressive palaeolithic householder invented the kitchen midden and shot all his rubbish on to one pile instead of strewing it in insanitary profusion round the mouth of his domestic cave.


Rubbish should be destroyed, not dumped. We have not yet succeeded, however, in procuring complete destruction of rubbish; but modern engineering science has secured the partial destruction of refuse by fire. There are also methods whereby the matter is first screened and has certain of its ingredients extracted before it is passed into the great destructor furnaces which reduce it to an innocuous mass of ash and clinker. Other adjuncts have been adopted in various places during recent years, such as the installation of boilers in conjunction with the furnaces for the supply of steam to adjacent power plant. Yet even to this day some large cities retain the dump system to a great extent. Huge rubbish heaps, over 90 feet in height, cover some 150 acres of ground at a dump outside London, and these unpleasant ornaments to London’s outskirts are added to from day to day.


The modern refuse destructor, designed to eliminate such anachronisms as these, is an elaborate and fascinating installation. Types of destructor vary considerably. Different places produce different types of refuse. A typical large British town produces a high percentage of cinder waste and this increases in industrial areas adjacent to coalfields. An American town, where central heating is general, produces less cinder waste, and so does a German town in which the houses are heated by high stoves burning briquettes. The West End of London, with its huge hotels and department stores, produces an undue quantity of paper waste — the remains of cardboard boxes and wrappings. An Eastern city will be notable for the immense quantities of vegetable garbage it produces.


In the city of Birmingham, a typical large British industrial city, cinders form a little less than 20 per cent of the average type of refuse brought in. By far the largest proportion is that of fine dust, reminding us that the old-fashioned term “dust bin” is not nearly such a misnomer as one might be led to believe. At Birmingham this dust is removed before the garbage is fed to the incinerators. Nearly 7 per cent consists of glass and odd metal scrap. The glass comes from innumerable broken bottles and other vessels, and the metal scrap, though consisting principally of “tin” cans, may contain anything from the internal anatomy of a worn-out cheap clock to the dismembered remains of a bicycle.


Destruction of refuse








CENTRAL CONTROL PLATFORM of an inclined rotary refuse destructor. The controls can be seen at the left of the photograph, near the ignition grate in which the refuse is fired. On an illuminated instrument panel are recorded the carbon dioxide content of the waste gases and furnace temperatures at the combustion grate and at the flue leading to the boiler.



















POWER FROM RUBBISH. This condensing turbo-alternator is driven by steam produced by the burning of refuse at Gjentofe, a suburb of Copenhagen, Denmark. In this plant the output of steam ranges from 1 lb to 3 lb per pound of refuse burnt, when dealing with refuse of a net calorific value of from 2,000 to 5,000 British thermal units per lb.












An interesting method of dealing with the innumerable “tins” is that adopted at Huddersfield. The designers of the refuse destructor plant there have installed a magnetic separator which draws the broken cans out of the raw garbage before it is fed to the furnaces. The cans are placed in a hydraulic press which squeezes them into compact bundles. These are then sold at current market rates.


Apart from variations in the composition of refuse due to locality, there are also, in temperate regions at any rate, variations due to season. There are more cinders in winter than in summer. The destructors in such situations have therefore to be “flexible” in operation.


The designer of the modern destructor has certain essential requirements to fulfil. The calorific value of refuse is low, and cannot therefore be treated in the same way as coal, coke or other orthodox fuel. All solid fuels, it is true, give better results if they are fed continuously to the furnace, as by a mechanical stoker. With the refuse destructor, this is imperative if the apparatus is to serve its purpose with any degree of efficiency. Charging must be as nearly continuous as possible, and the continuity of discharging the clinker and unburnable remains must be maintained in harmony. Because of the moist nature of the fuel, and its tendency to slow down combustion, it is advisable for it to pass through some form of preheater which dries it before it passes into the incinerator, proper.


Totally Enclosed


Especially where the furnaces are in conjunction with boilers for the generation of steam, care must be taken to ensure an even rate of combustion and a consequently even temperature of the furnace gases. Where combustion is thoroughly efficient, much of the dust contained in the refuse will be burnt or fused with the clinker, but a proportion of it will eventually mix with the gases produced by combustion. Means must be provided for removing this before it passes into the smokestack, and also for ensuring that it has as low a carbon content as possible.


In foreign towns where the refuse is particularly low in calorific value, as happens in Eastern localities, it is sometimes necessary to mix it with certain solid or liquid fuels to ensure complete combustion. This is not necessary, on the other hand, in a typical large British town. At Birmingham, for instance, where all the destructor plants have been harnessed for the generation of power, no fuel is used apart from ordinary refuse.


Air for combustion purposes passes to the furnace through specially controlled inlets. Care must be taken to prevent infiltration of air apart from this regulated flow, as unregulated air destroys the efficiency of the incinerator. The fuel which passes into a destructor furnace is invariably offensive in character, and special precautions need to be taken to prevent it from becoming a nuisance.


IGNITION GRATE AND ROTARY KILN



IGNITION GRATE AND ROTARY KILN after having been in operation for eighteen months. The brick lining of the kiln is comparatively free from clinker, due to the cascading action of the material in the kiln.





As far as carriage is concerned, there is the modern closed refuse wagon, as opposed to the old-fashioned open dust cart; but strictly hygienic conditions are necessary where refuse is fed to the furnaces and where the residue is extracted. The total enclosure of the handling and discharging plant is therefore advisable, indeed essential, to promote the greatest possible cleanliness. In all branches of operation and in first cost the apparatus should be inexpensive and at the same time thoroughly efficient and robust in design. Destructors are of various types. There are the old destructor cells dating from the last century, which are incinerators pure and simple with no means for converting the energy generated by combustion into useful power. Then there are similar cells which have been converted and fitted with boilers for power generation. These may be seen in certain depots belonging to the Birmingham Salvage Department, to take one example. The most recent incineration power plants built at Huddersfield embody all the latest refinements of design and each is capable of dealing with from 5 tons to 6 tons of refuse an hour, the net calorific value of the material being some 3,500 British thermal units per pound.


The layout of one of these destructor plants is best understood from a description of the several stages through which the refuse passes. First of all the garbage is charged, almost continuously, though a vertical chute, into a pre-drying hearth, built in close conjunction with an ignition grate beyond. The radiation of heat from the grate effects the drying of the material in the hearth, which is rocked by machinery so as to effect a continual passage of the refuse into the ignition grate. The speed of the rocking can be varied at the discretion of the firemen. As the refuse enters the ignition grate from the lower end of the inclined pre-drying hearth, it is in a partly dried condition.


The ignition grate itself is situated at a lower level than the pre-drying hearth, and it is here that the material first comes into contact with the fire. In the pre-drying hearth, however, a certain amount of hot gases has been drawn from the refuse and carried forward to the combustion chamber beneath the boiler and beyond the intervening ignition grate. Alternate firebars beneath the ignition grate are mechanically rocked in the same way as the pre-drying hearth, causing the continual progress of the now burning refuse.


Exposure to Hot Gases


Air for combustion in the ignition grate is fed to it at low pressure through a series of air chambers situated underneath. At the base of the air chambers, in their turn, is a series of hoppers for the collection of dust which passes through the bars of the rocking grate. Each air chamber is equipped with a damper, so that the flow of air to any part of the grate can be regulated according to local requirements.


From the ignition grate the partly burnt refuse passes automatically into a rotary kiln. This is a huge revolving cylinder on an inclined axis. As it turns, every part of the charred mass of refuse inside is exposed to the full heat of the flames, through being constantly tumbled about, and it is in this kiln that the final stage in the fire-cleansing and purifying process takes place. From it, the hot gases pass forward into a combustion chamber, and to the boiler mounted in connexion with it, while the clinker falls in an even stream into a special hopper below. There is but little carbon in this clinker, because of the constant exposure of the refuse to the hot destroying gases during its passage through the rotary kiln. The clinker consists of a hard, slaggy material, for it contains the residue of broken glass which has been fused in the furnace.


THE INCINERATION PROCESS is shewn in this diagram of a Woodall-Duckham inclined rotary refuse destructor










THE INCINERATION PROCESS is shewn in this diagram of a Woodall-Duckham inclined rotary refuse destructor. Refuse entering the plant is first dried as it passes over a moving drying hearth. On the rocking ignition grate the refuse first starts to burn, before it is destroyed in the rotary kiln. Gases pass into the combustion chamber and generate steam in the boiler. Clinker, ash and dust are extracted at various points in the plant.












A rotary extractor effects the discharge of clinker from the hopper, and the clinker passes into steel tubs or wagons running on rails at right angles to the axis of the rotary kiln. Other wagons, running on rails concurrent with the centre line of the installation, at right angles, that is, to the clinker wagons, serve to carry off the dust residue from the other hopper outlets. No special type of boiler has to be provided in connexion with this type of furnace. Any normal type can be adapted to it.


The driving gear for operating the moving parts of a such destructor is quite simple. Two steel tyres, forming revolving tracks, surround the rotary kiln, and are supported in their turn on a set of wheels forming what amounts to a huge roller bearing. The cylinder also is encircled by a rack or gear-ring, to which the driving motor imparts its energy through a system of variable gears. At each end of the rotary kiln there is a system of airtight flexible joints, which allow the kiln to turn with perfect ease and at the same time prevent any possibility of dust escaping or of unregulated air finding its way in.


In the Woodall-Duckham destructor much of the dust in the refuse is destroyed with it. A certain proportion is drawn off through hoppers underneath and a final dust-extractor fan is provided between the combustion chamber surrounding the boiler and the base of the smokestack, to prevent any dust from finding its way into the atmosphere with the final gaseous products of combustion.


Destructors of this type have been installed in a number of Danish towns since 1931 and one was erected recently at Huddersfield. In this destructor a vertical four-drums boiler is used, delivering steam for use in an adjacent electricity generating station at a pressure of 200 lb. per sq. in. and at a final (superheat) temperature of 600 degrees Fahrenheit. The annual steam output serves to generate some 10,000,000 kilowatt-hours of electricity, after deduction of that supplied to the destructor plant. Apart from gases the only residue consists of the slag-like clinker, fine dust and pieces of metal.


Metals Extracted by Magnets


A magnetic separator, similar to that which treats the refuse before it is passed into the kiln, extracts odd pieces of metal from the crushed clinker. The remaining metal originates as nails, screws and similar articles which, initially embedded in broken pieces of wood, have thus escaped extraction in the first place.


Apart from the modern installations, power plant has been built into many existing destructor cells, and a lengthy programme of complete conversion has been carried out by the Birmingham Salvage Department during recent years. The system of maintaining ugly and noisome dumps in that city had been decided against some years before. Prior to the outbreak of war in 1914, tipping had supplemented the existing destructor cells to a certain extent, and the war itself held up the modernization of the destructors. The spring of 1924, however, saw the completion of the first modernized destructor plant at Brookvale Road. The works there, as they now stand, are capable of dealing with 40,000 tons of house refuse in the course of a year.


MODERN DESTRUCTOR PLANT









MODERN DESTRUCTOR PLANT, showing the ignition grate and (right) the top of the rotary kiln. During the whole process of handling and destroying the refuse it is completely enclosed, but instruments are provided and windows, so that the process can be watched at every stage. Dampers control the supply of air to the ignition grate.












The steam generating sets at Brookvale Road consist of two compound engines coupled to dynamos, each of which has a capacity of 100 kilowatts at a speed of 525 revolutions a minute. They supply direct current at 440 volts, which is used for charging the accumulators of the electric refuse-collecting vehicles, twenty-one of which are attached to the works. The current from the generators passes through a balancer which provides four circuits, each of 110 volts. The whole plant has an annual output of approximately 500,000 kilowatt hours. In addition to the two engines attached to the generators, an auxiliary engine is provided for driving the condenser air and circulating pumps, a common condenser serving the two main engines. For a little over two years the Brookvale Road depot remained unique as far as Birmingham was concerned, though it was the initial feature of a scheme for converting the whole of the Salvage Department’s equipment. Then the city’s engineers completed another plant, at Tyseley, the capacity of which allows for the destruction of 6,000 tons more rubbish in one year than is disposed of at Brookvale Road. Further installations and rebuilding then took place at Lifford, King’s Norton (completed in 1930) and Rotton Park Street (1932). The last-named installation has a considerably greater capacity than its precursors. Those responsible for its redesign installed two destructor units, each containing five cells and each having a grate area of 175 square feet. The combined “appetite” of these two units is 150 tons of garbage in one day, or 54,750 tons a year.


The method of feeding the refuse to the furnaces at Birmingham is as follows: The designers have arranged their furnace house on a three-storied plan. On the uppermost floor they have placed their refuse storage, this being completely screened from the open air. Thither the material is brought by conveyers after it has been screened and sorted in the usual way. In the middle of the floor they have placed hoppers to receive the material from the storage as it is fed into the furnaces.


350 Tons a Day


Beneath each hopper, and directly over the incinerators on the floor below, is a feeding door to regulate the amount of waste passed into the furnaces. Bach of these doors is mounted on flanged wheels running on horizontal rails laid at right angles to the main line of the building. The movement of each feeding door is controlled by a winch mounted on the floor above (the same as that accommodating the storage and hoppers). As the door is drawn aside by the winch, the accumulation of refuse in the hopper above it tumbles through and is shot into the furnace below.


The central part of the middle floor, below the hoppers, is taken up by the furnace unit itself, with its five separate cells. Beneath each of these cells the engineers have placed a movable grate, which can be withdrawn downwards by a hydraulic ram. At one side of this movable grate is another hopper, in this instance provided for the reception of the ashes and other non-combustible residue from each cell of the unit. As soon as a cell needs clearing of ash and clinker, the fireman in charge sets the hydraulic ram going, the grate drops away from the bottom of the cell, and the waste shoots down through the hopper which feeds it to a continuously running conveyer. This carries it away to the cooling bed and crushing equipment.


THE ROTARY KILN revolves on two steel tyres



THE ROTARY KILN revolves on two steel tyres fixed near the ends of the steel shell. Rotary motion is imparted from a variable-speed motor driving a series of gears, with a final pinion that engages with a large gear-ring encircling the steel shell, or cylinder.





The power generated by steam from the boilers in these big plants is sufficient for the complete electrification of the works. In addition to its most important task of charging the accumulators of the collecting lorries, the installation supplies current for lighting, for cooking and water-heating in the men’s canteens, and steam for the heating of the various offices and other buildings. Competent engineers have stated that current could be supplied, if necessary, for a number of external purposes. In normal modern conditions, however, there is no shortage of electricity from the existing Grid supply. The main duty of these destructor generators is to assist in the upkeep of the Salvage Department’s own equipment.


The last and greatest of the destructor power plant installations at Birmingham dates from 1934, and in this the city’s engineers have replaced an old depot dating from 1876. There are sixty-four collecting lorries attached to these works, which are at Montague Street, and the new equipment is capable of dealing with no less than 350 tons of refuse in the course of one day, or 127,750 tons in a year. Steam is generated in three large Babcock and Wilcox water tube boilers and two Lancashire boilers. These boilers are mounted in connexion with four destructor units, rebuilt from older units, and underground flues conduct the gaseous products of combustion to a single great smokestack situated in the middle of the triangular area covered by the works. The fumes are clarified by special soot-blowers before passing into the stack.


The boilers at Montague Street supply steam for several purposes. In the course of converting the depot, the engineers installed two new compound engines, each driving an electric generator working at a normal speed of 425 revolutions a minute. A third generator was installed in connexion with an older engine, which had been rebuilt as part of the conversion scheme.


Elimination of Waste


All three of these engines exhaust into a condenser with- a surface of 1,050 square feet. There is a source of condenser water conveniently close at hand, for the triangular site of the Montague Street Works is bounded on one of its three sides by a canal, and on another by the little River Rea.


Birmingham’s engineers completed the Montague Street Works early in 1934. They were officially opened on May 3 of that year. Thus in the course of ten years the refuse destructors of the Birmingham Salvage Department have undergone a complete conversion. The old wasteful and unhygienic methods have been eliminated and the tipping of offensive material on to waste ground has become a thing of the past. The total area covered by the new methods of disposal amounts to 51,147 acres, on to which is packed a population of over a million. The areas covered by the activities radiating from the different works are by no means equal. That of the Montague Street Works, for instance, is only 4,678 acres, as compared with the 13,255 acres served by the Tyseley Works, although the Montague Street depot is the biggest of them all. But whereas the 13,255 acres of Tyseley support a population of 230,000 inhabitants, no fewer than 234,000 persons live in the relatively small area served by Montague Street.


Methods similar to those now universal in Birmingham have been applied in various other progressive cities. Birmingham, however, is an excellent example as it provides an instance of the relatively rapid conversion of the whole of a large city’s existing equipment for disposing of the products of public scavenging. Glasgow is another city which has made tremendous strides in this direction during recent years.


In the early part of 1928, the engineers of the Glasgow Corporation Cleansing Department completed an enormous destructor power plant installation at their Govan depot. For this they installed six great Babcock and Wilcox marine type boilers, each having a total heating surface of 6,160 square feet, and two other boilers of equal power and capacity, the working pressure being, as at Huddersfield, 200 lb. per square inch. These boilers supply steam to turbo-altemators, the current being supplied to various users. The installation has almost halved Glasgow’s refuse destruction figures.


DISCHARGE OF CLINKER from the base of the combustion chamber






DISCHARGE OF CLINKER from the base of the combustion chamber. Clinker falls evenly from the kiln into a hopper, from which it is discharged by a rotary extractor into small wagons running on rails.











[From part 24, published 10 August 1937]



You can read more on “Britian’s Electric Power Supplies”, “Canning the Nation’s Food” and “The Story of Gas Production” on this website.

Destruction of Refuse