Title: FIRE OF LONDON (1971)
Pages: 74 - 81
Author: David Rowlands
FIRE OF LONDON (1971)
A massive incinerator plant, built at Edmonton by the GLC for eight London
boroughs, will be a much studied prototype. David Rowlands reports.
Rubbish has that timeless appeal to statisticians which promotes a peculiarly invidious example of the numbers game. Politicians, engineers and environmental lobbyists alike reveal with glee that London's annual rubbish turnout, all three million tons of it, would cover St James's Park to a height of 350ft, that in 20 years it will have doubled to occupy 50 million cubic yards, and that in a good year about 100 000 tons of waste glass, iron and paper will be recoverable for a revenue of £650 000. Ninety per cent of the refuse collected by the boroughs and the GLC finds its way to land reclamation sites and the rest comes to a fiery end in one of the 1 3 incinerators strung around London. The bill for disposing of this growing mountain tops £5 millions.
The GLC came into the refuse disposal business as recently as 1965 following the 1961 Royal Commission on local government in London which recommended a unified organisation to rationalise methods and costs of disposal throughout the area. The Council inherited obsolescent incinerators and inadequate road, barge and rail handling points. Worse, the supply of suitable tipping sites was rapidly drying up and fundamental changes in rubbish composition, principally the near absence of household cinders and ash, worried health officials and reclamation engineers. Large scale direct incineration was suggested as a long term solution and the site for the first plant would be in North London, which had more than its share of disposal problems. At Edmonton, where the GLC already operated the Deephams sewage treatment works on the North Circular Road, 272 acres of land was released.
The new plant consists of four sections. In the main building, occupying the centre of the site, is a vast tipping bay 33ft above ground level, its entrance and exit connected to two arms of elevated road which form a crescent around the western side of the plant. The central building also houses a bunker bay, capacity 3900 tons of refuse, the incinerators and boiler plant. Outside are a bank of precipitators to clean flue gases and, dominating everything, a 328ft high twin-flue chimney. A second rectangular block houses turbines which will feed 30MW of electricity onto the National Grid for a revenue of about £500 000 pa. Conveyor belts carry ash and clinkers from the incinerators, under an access road and up through a white grp clad housing to a separate plant for residuals handling. The fourth massing is an all wooden cooling tower block.
Built to handle up to 1800 tons of refuse a day, carried from eight London boroughs within a six-mile radius, the plant is a joint project of the GLC departments of Public Health Engineering and Mechanical & Electrical Services, is engineered by W S Atkins & Partners and constructed by Tarmac Construction Ltd. GLC architects, under Sir Hubert Bennett, have made extensive use of dark green plastics coated steel cladding, supplied by British Steel Corporation to erectors Boddy-Moir Ltd in the plant
houses, and white ceramic tiles (supplied by Shaw-Hathernware and fitted by Parkinsons Ltd) to face the administration section sited below the tipping apron. The grp conveyor housing is by AB Plastics. The overall effect on this giant plant is bright and crisp, well up with its bold concept combining striking external juxtaposition of plant units with stark functionality within. The incinerator is sited within the proposed Lee Valley Regional Park and extensive associated landscaping is intended to conceal both the plant and the motley assortment of neighbouring industry.
The £10 millions plant is a revolution in disposal techniques. Smell, dust and noise are minimal and although benefits are arguably smaller than they could be (for reasons given below) the plant itself causes few additional environmental problems. The speedy throughput of such vast volumes of offensive material at relatively low cost, offset by sales of ash, scrap and generated power is a model system that is one step nearer the eventual abolition of a foremost modern day fear - inundation in our own material decay. Edmonton is the forerunner of several stations that will ring London. Given the need for a more comprehensive design specification there is no reason why this solution should not largely solve disposal problems for the next 50 years - in combination with systems like vacuum collection from individual homes and techniques for total waste recovery.
Between 600 and 700 refuse wagons will be arriving at Edmonton every day, together with up to 100 contractors lorries for removing scrap iron, fly ash and clinkers - a sizable traffic problem. Fortunately the collection pattern in London boroughs is geared to an early morning start, so the first full loads do not reach the plant until 10 am, just after peak rush hour on the North Circular. From then on the wagons arrive at staggered intervals according to the distance they have to cover until work ends around 4 pm, shortly before the second rush hour snarl-up. The distances they travel are no more than when rubbish was taken to central loading and dispersal stations, and in many cases are less. There are few houses in the area to be disturbed by the traffic increase.
On site, unhindered flow-through is essential and a three-part traffic system is in operation. Arriving dustcarts enter from the main road (and from a northern entrance for Enfield traffic) and check in at a computerised double weighbridge. After the vehicle's identity has been established by a number, it is weighed and the computer works out the amount of refuse carried from a comprehensive record of unladen vehicle weights. Then the traffic system splits, with ash and scrap contractors' lorries (which have to be weighed separately) going to the right and refuse wagons sent straight up the first arm of the elevated road (heated to prevent wintertime ice and snow hindrance) to the enclosed tipping apron. The 312ft long reinforced concrete apron forms the roof to a large entrance foyer, first floor offices and ground floor maintenance workshops. Entering carts are directed to one of 23 tipping bays by backlit cues on a sign gantry worked from a control room high above the apron. After tipping its load into a hopper the cart is driven out of the bay, down the exit ramp and completes the full circle out to the main road. For conscientious drivers, two high pressure spray lorry cleaning machines are installed by the end of the exit ramp.
With the cart's departure from the tipping bay, sensed by an ultrasonic eye, a safety barrier automatically rises to stop further tipping and to prevent inadvertant falls into the hopper. Full hoppers discharge into one of five vast 80ft deep storage bunkers enclosed in their own cavern occupying the centre of the main building. The 3900 tons capacity is enough to ensure all day and every day incineration. With each avalanche of waste into the bunker a great cloud of dust arises to be swept away by giant 48 000 cubic ft per minute wet dust extractors, which return damped down soil into the bunker. Ducts also run through the wall to the tipping apron and catch the dust thrown up.
Like huge six-legged spiders, two grab cranes with a third spare, running on overhead rails, transfer refuse from bunker to furnace feed chute. The driver's underslung control cabin is air conditioned as, dust apart, the atmosphere of the bunker chamber is not good.
Hydraulic rams push the garbage from the chutes into the specially designed German VKW stepped roller grates, an inclined series of seven parallel rollers 5ft in diameter and 11 ft 6in long, driven at speeds between 1/2 - 5 mph. The refuse burning is controlled at between 1700 - 1900°F by a combination of roller speed and air flow adjustments, air being fed between the rollers from compressors. Combustion is claimed to be almost 99 per cent with only the occasional pile of telephone directories coming through partially unscathed; exhaust gases are fully burnt by extra air jets placed above the burning rubbish. The grates require constant adjustment to suit the varying composition of the refuse. Hot gases pass upward to the five Yarrow boilers which produce a maximum of 86 000lb of steam per hour at 625 psi and 850°F. By 1980 increases in plastics and paper content of the fuel will strengthen its calorific value by 450 BTU lb and the boilers have been designed with this in mind. However, although there is immediate operator access to the grates and boilers vertically, there is no horizontal connection.
Steam passes from the boiler house to a separate turbine house at right angles to the main building, to feed three of four AEI 12.5 MW generating sets and two 2.5 MW sets which supply in-house current needs. The turbine house is in the hands of ex-CEGB men who say that in terms of modern generating methods some of the power plant's maze of steam pipes and circuits is more complicated than the station demands. A curious anomaly surrounds the in-house sets which cannot yet be connected to supply the incinerator's major equipment - so whilst the plant is exporting cheap power to the grid it has to be bought back at ordinary, expensive prices. In the original conception of incinerators of this kind there was to be the dual benefit of power generation and cheap district heating. Apparently Edmonton could support both schemes were it not for the fact that the steam discharged by its turbines is at too low a temperature and pressure to be of any use. Turbines are now on the market that would have solved this problem.
The spent flue gases are cleaned of smoke particles by cyclonic and electrostatic precipitators, the only major plant that stands unclad outside the main buildings. The gases are discharged through the chimney at a temperature of 450°F - well above acidic gas vaporising temperatures - and the result looks remarkably clean from ground level. Fears that the gases themselves may cause pollution, especially from the rising volume of chlorinated plastics waste, are denied by the designers, although a flue gas analysis has not yet been performed. Sulphur levels in refuse are low but the high gas discharge temperature does reflect the fact that excess oxygen used in the incineration certainly fully oxidises a proportion of the sulphur dioxide produced to the more acidic trioxide - given the chance to condense out this would rapidly destroy the heart of the chimney. The GLC points out that the only other station of this kind in the world, run by Dusseldorf's municipal authority (from which valuable design data was obtained) suffered corrosion problems in the furnace refractory linings and flues. The cooling tower house has five counter-flow units which cool the water for the steam condensers. It's a closed circulation water system topped up from the Deephams sewage works effluent channel, ineptly named Salmon Brook. The quantity abstracted is about one million gallons per day.
From the incinerators the hot refuse remains pass through a cooling quench bath onto the twin 6ft wide conveyors running under the boiler bay and up through the grp housing to the top of the residuals plant. One not-to-be missed sight is the magnetic separator at work, picking off ferrous scrap and depositing it into a concrete hopper to be fed through a baler. This scrap emerges as innocuous cubes which are convenient for handling on magnetic cranes, transferring them to contractor's scrap lorries in one of three pull-in bays. The rest of the ash is screened - small clinkers to one bay and larger non-metallic residuals to another. The first makes excellent roadmaking material and the latter is tipped, but there's not enough of it to cause a problem in that respect. The residuals building is acoustically screened by brick walls under the steel cladding.
It would be nonsense to pretend that Edmonton has been constructed and commissioned without some delays, and there are several examples of lack of detailed design which must be rectified. The plant was designed for borough refuse vehicles alone whilst there is some demand for trade waste disposal. Trade wastes are brought in every kind of vehicle from a private car to a security wagon- HM Customs may use the place to cremate seditious literature, and only recently the Home Office burnt a load of police helmets. The admirable traffic flow system is not adaptable to the need for these vehicles to be weighed both in and out of the plant- any volume of this traffic going twice round the plant would cause delays at peak tipping times. In addition, many dustcarts pull paper trailers which have to be parked whilst the vehicle tips (entailing another return trip around the circuit to pick it up again). This fact alone may prompt local authorities to purchase a single standard collection wagon.
The tipping apron was originally designed for full remote control but now needs a superintendent to signal the control room if the rubbish clogs the hopper and does not fall into the bunker. His presence upsets the sonic warning device and causes delays until he moves, allowing hopper emptying to begin. On the other hand he could be there prodding the refuse pile and be hastened to a smelly fall by the rising of the safety barrier. Modifications to these barriers, which have sometimes failed because covered by refuse, are already in hand.
The signs are that it may take another two years to sort out problems which could have been foreseen at an earlier stage. Despite this the plant is operating, fulfilling the giant and exacting task for which it was designed: there's no doubt that the wealth of experience gained in its operation will enormously benefit the GLC's programme for the disposal of London's waste in the years to come and that it sets an impressive example to local authorities in similar predicaments.
Opposite: the 323ft twin-flue chimney of GLC s Edmonton refuse incinerator dwarfs the dark green plastics coated steel clad plant buildings. Below left: scale of the project can be judged from the aerial view taken during construction. Left arriving wagons are weighed on a computerised weighbridge. Below: they proceed up the ramp to the enclosed tipping
Above and left: the main entrance foyer of the administrative block is under the exit ramp from the tipping apron. Extensive use of white ceramic tiles offsets the tinted glass and contrasts well with the darker lines of the main plant. Steps lead to a balcony running the full length of the office block. Inside the foyer the same white tiles supplied by Shaw Hathernware. are used and an added touch is a large collage of ~ rubbish pinned together by council employees (cheaper than commissioning an artist).
Right: the vast 312ft concrete tipping apron has 23 bays for discharge of refuse - vehicles are directed to empty bays by the overhead sign gantry controlled from a room above the lorry entrance. Wagons will be entering the bay at the rate of one every 40 seconds between 10am and 4pm. Dust is extracted from the apron area through vents in the wall which lead to extraction plant in the bunker bay
Left: Edmonton's bunker bay has storage capacity for 3900 tons of refuse in five 80ft deep, 40ft square, bunkers; the whole bay is sealed to prevent escape of dust and smell. Dust extractors are mounted on gantries above the travelling grab cranes and return damped down dust from the tipping apron and the bunkers direct into the holds below. The crane operator's cabin is air conditioned. Below right: the rubbish burns, seen through a viewing panel at the bottom of one of the five VKW stepped roller grates fed from hoppers filled by the grab crane (immediately below). Refuse is pushed from the hoppers into the grates by hydraulic rams. Bottom: five electrostatic and cyclonic precipitators scrub the flue gases before they are discharged from the chimney
Opposite: twin steel conveyors run underground from the incinerators, carrying the burnt remains up through the grp-clad conveyor housing (top and bottom), to the top of the residuals plant
Top: Conveyors carry scrap-free ash to the hoppers to await collection by contractor's lorries. Left: lorries pull up under chutes in one of three bays at ground level in the residuals plant. Centre: a fourth bay is for the collection of bales of scrap metal picked off the conveyor by a magnetic separator and fed into a separate hopper. Conveyor belt running gear and the noisy separator and baler are acoustically screened by the double brick and steel wall of the plant housing
Top: The five vents atop the wooden cooling tower structure are fed with water from Salmon Brook. Each vent houses a large fan which draws in air and allows considerable size reduction without loss of capacity. Left: the control room of the turbine house (above) where one man watches the state of the turbines and switches Edmonton onto the National Grid - the
station supplies in-house needs and can contribute 30MW of electricity to the Grid for a revenue of £500 000