<Connah's Quay Power Station is located north of the A548 between Connah's Quay and Flint

The Central Electricity Generating Board

The Central  Electricity  Generating Board is responsible for the generation and bulk supply of electricity in England and Wales. It operates the largest  power  system  under  single  control anywhere in the world. This system is controlled by a central headquarters in London and five regions, who are charged with the operation of the power stations and transmission lines within their areas. Connah's Quay Power Station is under the control of the North Western Region which, with its headquarters in Manchester, stretches from Aberystwyth to Buxton and then to Carlisle.

The power generated in the CEGB's power stations is supplied at different high-duty voltages over the grid or supergrid transmission networks. Power is then tapped off at distribution points in order to supply local Area Boards who provide individual supplies to houses, factories and offices.

The grid and supergrid lines enable generating plant at power stations to be pooled so as to give both security of electricity supply and economy of operation. A Grid Control System ensures that power 15 available to meet fluctuations in demand as and when they occur. At the same time, the control system ensures that the most efficient stations meet the bulk of demand, with the older and less efficient stations used for standby duty. The costs of supplying the consumer are thus kept as low as is possible.

The CEGB also has two specialist construction and design divisions. One is concerned with new power stations and the other with transmission lines.

 

Connah's Quay power station in Clwyd is on the south shore of the Dee estuary. It is 15 kilometres from the City of Chester and 6 kilometres from the ancient town of Flint. It has a staff of about 250.

Reclamation of the 80 hectare site began on 22 March 1950 and was completed in 1958. The station and cooling towers are, in the main, supported on precast concrete piles, 90 centimetres square, of which over 5000 were driven into the earth.

The station's six boilers and turbo-alternators have a total generating capacity of 192 000 kilowatts. This capacity is equal to the average demand for electricity from a city the size of Cardiff. This means that over 400 000 tonnes of coal are burnt annually in the station and most of this is North Wales coal from the Flint and Wrexham areas.

The main building is 119m long, 72m wide and 40m high. The station's three cooling towers are 76m high and 72.6m in diameter at pond level. The two  chimneys  are  92m  high;  electrostatic precipitators remove most of the dust from the flue gases they discharge.

Pulverised Fuel Ash (PFA) - a by-product from the coal-burning process - has been used to reclaim 242 hectares of saltings and derelict land close to the station. The ash is either pumped into lagoons for settlement or passed through a pneumatic disposal system to provide moisture-controlled PFA for the construction industry.

The building of a power station on the Dee estuary was originally intended by Chester City Council in order to  replace  their existing station  at Queensferry. Following the establishment of the British Electricity Authority in 1948, the project was reviewed. It was decided that a large-scale station was required to meet the increasing electrical demand in the North Wales area.

 

Connah's Quay Power Station

< Aerial view of Connah's Quay Power Station

Site reclamation

A comprehensive aerial and ground survey of the district was undertaken. From the data obtained it

became clear that the present site was the most practicable.

The area mainly consisted of tidal saltings, which were totally covered at Spring tides with the exception of a small rock outcrop at the entrance to the site.

Reclamation on a major scale was necessary. It was undertaken by dredging and pumping sand from a suction dredger anchored in an inland lagoon on the opposite side of the river from the station site. From the pumps on this dredger a sand and water mixture was discharged through a pipe at a rate of 34 045 cubic metres an hour. The sand from the mixture settled and the water was drained away through pipes provided. A total of about 812 000 tonnes of sand were pumped in nine weeks. After the deposition of the sand the top soil was relaid. The result was a general site level of 6.7m above Ordnance Datum; the depth of the pumped sand being approximately 2.4m. The whole of the work was completed by 1 August 1950.

< Site plan of the station

One of six boilers in operation at Connah's Quay

Connah's Quay is a coal-fired station which works in accordance with the principles outlined in the preceding section How electricity is made. In the following pages details are given of the various parts of the station.

The station is installed with six ICL water tube type boilers each with a steaming capacity of 136 ,050 kilogrammes of steam per hour at a pressure of 43.1 bars and a temperature of 463 C at the boiler stop valve.  

The complete steam raising unit, including the economiser and air heater, is supported on structural steelwork. This rests upon the basement level and is independent from the building structure.

The boiler is of tri-drum type, each drum being of hollow forged steel construction. All boiler and furnace tubes are 82.Smm outside diameter. The finned tube construction consists of two 25.4mm wide fins welded diametrically opposite each other.

Each boiler is provided with two forced and two induced draught fans. These respectively handle air for combustion and withdraw gases from the boiler system.

From the boiler the flue gases pass to an electrostatic precipitator. This is arranged in two independent sections so that one half of the plant can be shut down at a time for maintenance. Each precipitator is equipped with its own high tension house and  rectifying equipment.  From the precipitator the gases pass to the induced draught fans and are then discharged to the main flue and chimney.

 

Plant

 

 

 

Boilers

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Electrostatic precipitator

 

 

 

Chimneys 

 

 

 

 

 

 

 

 

 

 

 

 

The diagram overleaf shows a cross-section of the power station in order to illustrate its operating cycle. From the coal store, fuel is carried along a conveyor system (1) and discharged by means of coal tippers (2) into the bunker (3). It then falls through weighers(4) into the coal pulverising mill (5) where it is ground to a powder as fine as flour. The mill consists of a round metal table on which large steel rollers or balls are positioned. The table revolves, forcing the coal under the rollers or balls which crush it.

Air is drawn from the top of the boiler house (6) by the forced draught fan (7) and passed through air pre-heaters (8) to the hot air duct (9) From here some of the air passes directly to the burners and the remainder is taken to the pulverising mill, where it is mixed with the powdered coal, blowing it along pipes to the burners (10) of the furnace (11) Here it mixes with the rest of the air and burns with great heat.

The boiler consists of a large number of tubes (12) extending almost the full height of the structure and the heat produced raises the temperature of the water circulating in them to create steam which passes to the steam drum (13) at very high pressure. The steam is then heated further in the superheater (14) and fed through the outlet valve (15) to the steam turbine (16)

From the turbine the steam passes into a condenser (17), to  be  turned  back  into  water called 'condensate'. This is pumped through feed heaters (18) (where it may be heated to about 250 C) to the economiser (19) where the temperature is raised sufficiently for the condensate to be returned to the lower half of the steam drum (13) of the boiler.

The flue gases leaving the boiler are used to reheat the condensate in the economiser (19) and pass through the air pre-heaters (8) to the electro-static precipitator (20). Finally they are drawn by the induced draught fan (21) into the main flue (22) and to the chimney (23).

The electro-static precipitator consists of metal plates which are electrically charged. Dust and grit in the flue gases are attracted on to these plates, so that they do not pass up the chimney to pollute the atmosphere. Regular mechanical hammer blows cause the accumulations of ash, dust and grit to fall to the bottom of the precipitator, where they collect in a hopper for disposaL Additional accumulations of ash also collect in hoppers beneath the furnace.

The ash is either sold for use in road and building constructions or piped as a slurry of ash and water to a settling lagoon, where the water drains off. Once this lagoon has been filled, it can be returned to agricultural use, or the ash removed for other purposes.

Oil-fired power stations have boilers that are very similar although there is no coal handling or pulverising plant

 

 

One of two induced draught fans associated with each precipitator.

< The turbine hall. Each of the six turbo-generators has a rating of 24,000 kilowatts

Turbo-alternator plant

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Coal handling plant

The turbine plant consists of six 32 000 kilowatt turbo-generators with an economic rating of 26 000 kilowatts. The turbines are twin-cylinder machines with a duplex exhaust and are reaction type running at 3000 revolutions per minute. The operating steam conditions are 41.4 bars pressure and a temperature of 412.8 C at the turbine stop valve.

Each high pressure rotor has 48 rows of blades and each low pressure rotor has twelve rows of blades (six rows of moving blades in each section). From the last stages of the low pressure cylinder the steam is exhausted to a three-pass-surface type condenser capable of maintaining a vacuum of 971 millibars of mercury when the turbine is operating at its economic continuous rating; the cooling water inlet temperature being 18 C and the outlet temperature being 28 C.

The  alternator  is  of  the  three-phase totally-enclosed ventilated type and is connected directly to the rotor of the turbine through a multiclaw semi-flexing coupling. It is excited by its own exciter, directly driven from the alternator shaft. The alternator is cooled by means of a closed-air system, the air being circulated by two motor-driven fans. These are each capable of providing the quantity of air necessary when the machine is operating at 60 per cent of its maximum continuous rating.

Some 10 000 tonnes of coal are consumed at Connah's Quay each week. Sidings deal with supplies that are delivered by rail wagons and they accommodate road borne coal also.

There are two sidings, each 1000m long for the reception of coal wagons from the British Rail main lines. The reception sidings feed a system of exchange sidings which join with two major tracks that lead to the two coal wagon tippler plants. Each of the wagon tipplers discharges at an average rate of 254 tonnes per hour when handling 20.3 tonnes capacity trucks. Wagons discharge into a common hopper, part of the tippler strricture, which has a capacity of 51 tonnes.

The coal from the hopper is discharged onto a short feeder conveyor which, in turn, feeds a main belt conveyor designed to deliver coal at the rate of 254 tonnes per hour to the coal junction house; there it can be fed either to the boiler house bunkers or to the coal storage area.

From the junction house coal can be conveyed by means of two belts into the boiler house. The belts are each capable of handling coal at the rate of 127 tonnes per hour as a continuous normal operation. Each  of  the  belts is capable of working independently of the other in case it should be necessary to close one down for repair.

 

One of several pulverised fuel mills which grind the coal to a consistency of face powder.

Inside the boiler house, coal is discharged into small hoppers, which feed two shuttle conveyors capable of serving the three hoppers of each boiler bunker. From the bunker outlets coal is fed through chutes down to the coal grinding mills of which there are three for each boiler. During the course of grinding the raw coal is dried by means of hot air which is also used as primary air to the burners. Each mill is provided with its own exhauster fan designed to deliver the coal laden alr to each of the burners via a system of fuel piping.

The ash from the pulverised coal fired boilers is in two forms. One form is the heavy 'bottom ash', or clinker, deposited in an ash hopper, which is an integral part of the boiler. This ash is then ejected by manual intervention into a sluiceway and conveyed in a stream of water, via ash pumps and an overhead launder into a lagoon.

The second form of ash is Pulverised Fuel Ash (PFA). This is carried away by the flue gases and is then removed from the gas stream by means of the electrostatic precipitators. The two precipitators per boiler have PFA hoppers into which the ash falls by means of gravity. The ash is then fed, intermittently, into a pressure vessel situated beneath and connected to the precipitator hopper. Assisted by a stream of air the PFA is then piped to one of two ash storage silos. Each silo can hold 100 tonnes of ash. From the silo the PFA can be discharged in a dry state into road tankers or, if it is required in a moist condition, then it can be discharged via a conditioner where a controlled amount of water is added according to the consistency required.

To condense the exhaust steam from the turbines each turbo-alternator required 88 725 litres of water per minute to pass through its condenser. The minimum flow of the River Dee is insufficient to meet the requirements of six machines, so it is necessary to have a cooling tower system.

Pulverised fuel mill

 

 

 

 

 

 

 

 

Ash and PFA handling plant

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Circulating water system 

< The coal handling plant. Approximately 10,000 tonnes of coal are burned at the station each week.
< The three cooling towers have become a familiar landmark along the Dee estuary

Cooling towers

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Make up water pump house

 

 

 

 

 

 

 

 

Water treatment plant

Water is drawn from the cooling tower ponds via twin open rectangular culverts, which connect to the circulating water pump house. There are six pumps, each having a capacity of 95 500 litres per minute which discharge to twin ducts running underneath the whole length of the turbine house. Each condenser has an inlet connection from each of the ducts and an outlet connection into each of the two discharge ducts, which return the water to the cooling towers.

There are three reinforced concrete cooling towers each capable of cooling 11.4 million litres of water per hour from 29.0 C to 21.0 C dependent upon the prevailing weather conditions. They stand upon a common pond which can be divided to enable operation of one half of any tower.

The towers are 76m high, 72m in diameter at the bottom and 34m diameter at the rim. The cooling tower pond, 2m in depth, holds about 38 million litres of water. The towers are fitted with spray eliminators and de-icing equipment. Each tower weighs approximately 4217 tonnes.

Supplies of make-up water for the cooling towers and for the ash and dust sluice water are drawn from the River Dee. A piled dolphin 11m from the bank of the river carries two 508mm pipes, through which water is drawn to make-up water pumps in the pump house on the river bank. The make-up water is drawn at periods of low ebb tides when the salt content of the main channel is at a minimum. This compensates for losses from the system and replaces water drawn from the ponds for sluicing purposes and returned to the river. The pump house contains three vertical pumps each having a capacity of 18 084 litres per minute.

The water to feed the boilers must be of a high degree of purity in order to prevent corrosion and the formation of scale and other matter. The station has a complete water treatment plant installed which receives water from the local authority town main. The treatment is carried out in a mixed bed Deminrolit plant.

 

Auxiliary switchgear house
Part of the 132,000 volt switchgear compound

Two compressors and air receivers are installed, each compressor being capable of discharging air at 5.51 bars pressure per cm2 with a capacity of 11.323m per minute.

The generators are connected to the 132kV system through 36 MVA 11.8/132kv generator transformers, which are situated in the 132kV switchgear compound. The transformers are of the oil immersed type with tan assisted cooling. Each transformer is fitted with 'on load' tap changing equipment.

The main 132kV switchgear is of the bulk oil type and has a breaking capacity rating of 3500 MVA. There are six generator units, two station transformer units and eight feeder units as well as busbar coupler and bus-section switch units. Six feeders, all of the overhead line type, are installed.

The station is connected to the 132kV switchgear compound by a cable tunnel. This conveys all maln and multicore cables running between the station and the compound.

In the administration block two adjacent rooms house the control panels and the panels mounting the station protective relays. A laylight and roof lighting provides normal lighting for the control room. Cold cathode lighting mounted above the laylight provides illumination during hours of darkness.

It was assumed at one time that nothing would grow on PF ash, so topsoil, sometimes as much as 0.90m deep was spread on the surface where ash had been lald. The cost of such an operation on large areas was enormous.

The CEGB initiated research at both Birmingham and Leeds Universities to study every aspect of the >>

 

Compressor

 

 

Generator transformers

 

 

 

 

 

 

 

 

 

 Cable Work

 

 

Control and relay rooms

 

 

Pulverised fuel ash

< Operation of the station can be continuously monitored from the control room

< Harvesting wheat on PFA at Connah's Quay Power Station

 

 

 

 

 

 

 

 

 

 

<<  growth of plants in ash. This research, together with practical field trials, showed that ash, without any soil covering whatever, can support the growth of certain crops. For satisfactory results the ash is lagooned, since it has been found that weathering improves it still further. When feasible, the incorporation with the ash surface of as little as 76mm of soil, subsoil or shale is beneficial. The lack of plant nutrients is serious though and has to be made good by the application of fertilizer.

Reclamation of 242 hectares of salt marsh land at Connah's Quay began in 1954, soon after the station was commissioned. The cultivation of reclaimed land started during 1956. The area reclaimed continued to increase as did the equivalent area under cultivation.

The cultivation experiments were carried out by members of Birmingham University, working in conjunction with the CEGB. The area involved amounts to approximately 65 hectares and something in excess of 500 000 tonnes of PFA Were used. No topsoil has been used; the PFA surface has had essential fertilizers and trace elements added according to the crop being grown.

The crops that have been sown include cereals such as wheat, oats and barley; various root crops like potatoes, beetroot and carrots and a selection of various  grasses,  (some  of  hay  producing characteristics), cabbages and cauliflowers. The results obtained could be described as varying from good to excellent.

Cultivation of the reclaimed areas is now restricted to grass. Consideration is being given to a long term experiment concerning the rearing of cattle, sustained solely on the grazing and produce from the  reclaimed  land.  The experiment would continue into the third and fourth generation of cattle.

 

Potatoes and mangelwurzels grown on PFA on reclaimed salt marshes adjacent to the station
Produced by Reprographic Services CEGB (North Western Region) 825 Wilmslow Road, Manchester M20 8RU
 

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