Introduction

AStation
A Station
B Station

B Station

Blyth Power Station was situated at Cambois in Northumberland on the northern bank of the River Blyth between the tidal estuary and the North Sea. The site comprised two stations, Blyth ‘A’ and Blyth ‘B’, with a combined generating capacity of 1180 megawatts. The 241 acre site was divided by the Bedlington-Cambois road. The coal handling installations lay to the north side of the road and the two main station buildings to the Southside. The increasing demand for power in the immediate post-war period led to the then existing North Eastern stations, of Dunston and North Tees, being extended and new stations being built on the Tyne at Stella North and South. The plant installed in these stations was of relatively small output but of well proven design, to enable the demand for power to be met quickly. Blyth saw the dawn of the new era in power station design and technology with the installation of larger and more efficient plant. It was initially planned to build a station containing 6 x 100MW turbo- generators. This was changed to suit new advances made in design, first to 6 x 120MW units and then finally to 4 x 120MW units with a ‘B’Station containing 2 x 275MW units and 2 x 350MW units. Ministerial consent was given for the building of the ‘A’ Station in February 1955 and all units were commissioned by June 1960. The four ‘B’ Station units were installed and commissioned by September 1966. At the times of their installation both ‘A’ Station Unit 1 and Station Unit 5 were the largest in ‘ the country. The 2x275MW units (5 & 6) on ‘B’ Station were decommissioned on grounds of economy in 1991.

Power Stations & The Grid

 
NGC Control

NGC Control Center

More than 50 years ago the interconnected grid system first linked power stations and local areas so that they could help each other over difficult periods and transfer energy when it was economical to do so.  During the 1939-45 War the role of the grid changed from an area to a national facility and electricity was first transmitted over long distances. The National Grid Control Center, works through four area grid control centers. It arranges for plant in power stations to be run in the most economic order possible to meet the constantly changing demand and is also responsible for the operation of the grid system, which is the largest in the world. National Power at present uses coal, oil, gas, water or wind power as fuel. Its power stations range in size from small hydro-electric units to the largest coal-fired plant in Western Europe. National Power competes with other generating companies and sells almost all of its output through the new wholesale market for electricity called ‘the pool’ It has contracts to supply its main customers, the twelve regional electricity companies of England and Wales with nearly 90% of its output. As well as its main customers, National Power has contracts to supply electricity directly to large consumers.




The Functions Of This Transmission Grid Are:

EM Centre

National Power’s Energy Management Centre bids into the electricity ‘pool





National Power’s Energy Management Centre bids into the electricity ‘pool To interconnect power stations and cut down the amount of reserve plant needed nationally. To make most use of stations with the lowest costs. To transfer power from one part of the country to the other. The National Grid Company is responsible for almost eleven thousand miles of overhead grid lines. Producing almost half of the electricity for England and Wales, National Power is one of the largest privately owned generating companies in the world. Its aim is to become the best.


Civil Engineering Work

Ash Plant

Ash Plant

A layer of strong boulder clay, about 70ft thick overlying sandstone and coal provides a bearing medium amply capable of supporting the station. The main foundations are spread to load the clay to about 2.3 tonne per square foot, adjustments having been made according to the depth, size and shape of individual foundations. The A Station turbine hall is 394ft long by 122ft wide by 85ft high and is constructed of reinforced concrete frames clad with brickwork. The boiler house (362ft long by 93ft wide by 157ft high) is of steel frame construction with aluminum cladding. The main buildings of the ‘B’ Station comprise a 675ft long by 166ft wide by 100ft high turbine hall and a boiler house, which is the same length but 105ft wide by 170ft high. Both are of steel framed construction, clad with aluminum and glazed. The roofs are of lightweight aluminium decking. The combined volumes of the main buildings represent 27cu ft/KW of installed capacity compared with 26.3 cu ft/KW for the A Station. There are two 450ft and two 550ft high chimneys serving the ‘A’ and ‘B’ boiler houses respectively.

Coal Handling

Coal Delivery by Trains

Coal Delivery by Trains

The coal handling equipment installed at Blyth consists of conveyor systems integrated to feed both stations as necessary. The consumption of coal averages 51,000 tonnes per week, rising to 70,000 tonnes per week during the winter. In the summer months, when the electricity demand is low, much of the coal supplied to the station is delivered to stock. This gives a suitable reserve supply which can be fed to the power station using the reclaim conveyors. The provision of new “Merry-Go-Round” (MGR) facilities became necessary in 1981 to accept the new high capacity rapid discharge (NBA) wagons adopted by British Rail for the North East rail transport system. The new coal handling equipment was designed to accept all coal deliveries by rail. Each train consists of thirty-six HBA wagons each of 45 tonnes gross weight. The system provides for each train, carrying approximately 1100 tonnes of coal, to be emptied within sixty minutes.

Coal stock handling equipment (Terex)

Coal stock handling equipment (Terex)

Limitations imposed by the site boundaries determined that the new facility could not be in the form of a conventional MGR loop which would have allowed the continuous movement of trains. Instead the train arrives on site and pulls onto a reception track. The locomotive uncouples, runs around the wagons, re-couples at the opposite end, then slowly moves over the unloading track hopper and discharges the coal before eventually leaving site.





‘A’ Station Boiler Plant

Coal is elevated from the coal handling plant by conveyor systems. A traveling distributor transfers the fuel from the last conveyor belt into the four station bunkers, each of which has a capacity of 2000 tonnes.
Coal pulverising mill

Coal pulverising mill

After pulverising, the coal is transferred by an air stream to the burners. This air is provided by the primary air fan associated with each mill. To permit the heat release from the fuel to follow the steam requirements of the generating sets, the raw coal is fed into the mills by rotary table feeders. These are driven by constant speed motors through variable gearboxes, the speeds of which are regulated by the automatic control system. Each of the four Babcock – Wilcox boilers has twenty, 26 inch diameter horizontal flame, circular type burners feeding pulverised fuel (P.F) into the furnace. In addition there are twelve pressure-atomised oil burners, with automatic propane ignition. These burners, used for starting up purposes and maintaining flame stability, are operated remotely from the unit control room. The oil is stored in two 10,000 tonne storage tanks, and is heated before being sprayed into the furnace at a pressure of about 550 p.s.i. Under full load conditions each boiler is capable of evaporating continuously 860,000 pounds of water per hour and converting it into steam at a pressure of 1,600 p.s.i. at a temperature of 543°C. The inlet water temperature under these conditions is 230°C, heat having been obtained from the feed water heating system associated with each turbine. Additional heat is taken from the flue gases by means of low temperature, high temperature, and topping economisers. The radiant furnace has a volume of 86,000 cubic feet and the steam raising area of the water walls and boiler tubes is 16,750 square feet. These tubes are 3 inches outside diameter with 0.348 inches wall thickness. Steam leaves the boiler drum and passes through three banks of super-heaters where its temperature is further increased.
Boiler Drum

Boiler Drum

The Primary Super-heater with a surface area of 18,875 square feet, has a horizontal inlet section and a pendant outlet section. It receives convected heat from the flue gases leaving the secondary super-heater zones. From the bunkers the coal descends to the pulverising mills where it is ground to a fine powder. There are five mills for each boiler, each of which is individually driven by a 160 h.p. induction motor. Each mill can handle up to 15 tonnes of coal per hour and four mills are sufficient to maintain the boiler at full output. It is therefore possible, under normal circumstances, to keep one mill in reserve or under maintenance. The Radiant Superheater, having a surface area of 3700 square feet, receives convected heat from gases leaving the furnace and radiant heat from the P.F. flames. The pendant Secondary Superheater has a surface area of 15,000 square feet. This receives convected heat from the furnace gases and raises the steam temperature to 543°C before it passes to the turbine. In the rear gas pass of the boiler, adjacent to the Primary Superheater, is the Reheater which has an effective heating area of 41.900 square feet. In conformity with modern practice a reheat cycle has been adopted, whereby the steam having expanded partially through the turbine, is returned to the boiler to receive more energy in the form of heat before being returned to the turbine for further expansion Development of the reheat cycle was pioneered in the North East of England at Blaydon Power Station in 1916 and at North Tees A Power Station in 1919. Operational experience was gained between 1933 and 1950 on the reheat plant installed at Dunston B Power Station . The reheat steam entering the boiler is at a pressure of 422 p.s.i. and is heated from 369°C to 541 °C before returning to the turbine. The hot gases can be prevented from passing through the reheater and the primary superheater by means of ganged dampers which divert the gases, so providing regulation of both superheat and reheat steam temperatures. Spray type desuperheaters give a fine adjustment of superheat temperature to within ± 8°C and spray type desuperheaters are also provided for emergency control of the reheat temperature. Each boiler is equipped with automatic boiler control. Variations in main steam pressure are detected and transmitted to the forced draught (FD) fans and pulverising mills causing the necessary change in fuel and combustion air to be made to maintain the boiler output. A change in the air condition alters the pressure in the combustion chamber; This pressure change is used to control the induced draught (ID) fans.
Coal Burners

Coal Burners

Each boiler has two FD fans, driven by 370 horsepower 3.3kV motors, operating at 730 rpm. These fans are mounted at ground level and take in warm air from the top of the boiler house at the rate of 145,000 cu ft/min. The air is passed through a horizontal tubular air heater, where it takes in additional heat from the flue gases before passing to the boiler. The two I.D. fans are driven by two-speed (730/585 rpm) motors, each of 800/435 hp and 3.3kV. Each .fan extracts gases at a maximum rate of 225,000 cu ft/min at 132°C. Both FD and ID fans have radial inlet vane control. Dust is extracted from the flue gases by cellular dust collectors and electrostatic precipitators, with a combined efficiency of 99.3%





‘A’ Station Turbo Generators

The four Metropolitan Vickers,120MW, 3,000 rpm turbo generators are operated on the unit principle of combining a boiler, turbo generator and their principal auxiliaries as a single operating unit. There are thus no connections between the separate units, the only common services being those of the station transformer supplies used for starting up purposes, circulating water, town water and treated make-up water. The turbines are of impulse design and have three, in- line, single casing cylinders. The high pressure (H.P.) and intermediate pressure (I.P.) cylinders are arranged in contra flow to balance out residual thrust and the low pressure (L.P.) cylinder is of double flow construction. To keep the length of the machine to a minimum, the ‘close coupled’ technique has been adopted and the three shafts, which are solidly connected, are supported on four bearings only. Steam at turbine stop valve conditions of 1500 p.s.i. and 538°C passes via loop pipes to the admission belt of the H.P. cylinder and expands towards the governor end through a velocity compounded stage and eight impulse stages before leaving the H.P. cylinder and returning to the boiler for reheating. Bled steam for No. 6 feed water heater is tapped from the H.P. turbine exhaust pipe work. After reheating, steam enters the I.P. cylinder through two interceptor valve chests and loop pipes, expanding towards the alternator through thirteen irrapuls® stages. Steam for Nos. 5 and 4 feed water headers i^bte* after four and eight re^ectWIywthstea’m for the No. 3 deaerator heater being taken from the I.P. exhaust. Steam entering the L.P. cylinder divides into two flows and exhausts to the condenser after expanding through six stages. The final two stages of each flow form ‘Baumann multi exhausts’ which give an increased exhaust annulus area for a given blade length. Bled steam for Nos. 2 and 1 feed water heaters is taken after stages 1 and 3 of each flow respectively. The guaranteed steam and heat consumptions of the turbo generators are 6.726 lb/kWh and 8.232 BTU/kWh, respectively. The hydrogen cooled alternators generate at 13.8kV and have an output of 120MW at 0.8 power factor with a hydrogen pressure of 30 p.s.i. Four coolers, embodied in the stator casing, transfer the heat in the hydrogen to a distilled water circuit, and the distilled water is in turn cooled by heat exchangers which transfer the heat to the main circulating water system. Excitation is provided by gear driven, air cooled, pilot and main exciters running at 991 rpm. To enable the exciter to have a short time response to voltage changes, an amplidyne system has been used which forces the field of the main exciter as required by the automatic voltage regulator. Manual control of excitation is provided by a field rheostat.

‘A’ Station Turbine Hall

A Station turbine hall

A Station Turbine Hall


Condensing & Feed Water Systems

The steam from the turbine exhaust is converted back into water in the condenser, and in so doing produces a vacuum which lowers the “back pressure” of the turbine thus increasing the efficiency and output of the machine. The condensers are of twin, two-pass design and with a total cooling surface of 70,000 square feet. They are designed to give a vacuum of 28.9 inches of mercury when supplied with 3,420,000 gallons of water per hour at a temperature of 11.7°C. Condensed water is extracted by two, 100 per cent duty pumps and air is removed by three Leblanc type motor-drive rotary air pumps. After extraction, the water passes through a drains cooler and then through the first low-pressure feed water heater, where steam bled off the low pressure stages of the turbine expansion adds more heat It then passes through a gland heater, where waste steam leaking along the labyrinth glands on the shaft gives up heat, through the second low pressure heater and finally enters the combined deaerator heater. Here, in addition to receiving more bled steam heat, the water is cleared of entrained air and gases. The outlet of the deaerator connects to the suction of two 100 per cent duty booster feed pumps which are driven by 850 horsepower, 1500 rpm, 3.3kV motors. The booster pump discharge is then passed through three high pressure feed water heaters to the two 100 per cent duty main feed pumps which are driven by 2650 horsepower, 300 rpm 3.3kV motors.
Main Feed Pumps

Main Feed Pumps

Although the booster and main feed pump motors are started together from one circuit breaker, the design is such that the booster pumps are up to speed before the main pumps and a positive water pressure is maintained at the main pump inlet. After leaving the main pumps the water passes into the boiler drum via the economiser. ../images/Main Feed Pumps2.jpg Main Feed Pumps Make-up for the boiler feed water system is obtained from the town’s mains through a demineralisation plant. In this plant, organic matter is first removed by flocculation and pressure filtration.
Condensers beneath turbine

Condensers beneath turbine

Condensers beneath turbine The filtered water passes through cation exchange units which convert the dissolved salts to acids. Carbon dioxide is removed in a scrubbing tower and the acids and most silica are removed in anion exchange units. Finally the water passes through the mixed bed exchange units which remove all the residual impurities to give a water of very high purity. The final treated water has a conductivity of less than 0.1 dionic units and a silica content less than 0.01 parts per million. This is essential to reduce the risk of boiler corrosion and of damage to the turbine plant.




Switchgear & Electrical

Indoor Substation

>br> Each machine has its own 13.8kV/3.3kV unit transformer, solidly connected to the alternator terminals and rated at 10MVA. The 3.3kV system is used to power the major auxiliaries. For starting there are two 10MVA station transformers drawing power from the 66kV system and reducing the voltage to 3.3kV. Lower voltage supplies are taken from the 3.3kV system through 3.3kV/415V auxiliary transformers. ../images/Indoor substation2.jpg Indoor substation The site is connected to both the 66kV and 275kV systems. All generator transformers are of 145MVA rating but the first three machines have a voltage ratio of 13.8/66 and feed into the 66kV system whereas generator transformer 4 has a 13.8/275 voltage ratio and feeds into the 275kV grid system. The 66kV indoor substation contains 24 small oil volume circuit breakers for switching :- Generators 1, 2 and 3 North Eastern Electricity Board Feeders Two 145MVA transformers connecting with the 275kV substation Bus couplers and Section switches.

Unit Control Rooms

A Station Unit control room

A Station Unit control room


There are two control rooms situated on the operating floor. Each serves two units and contains all the instrumentation and controls necessary to operate the boilers, turbo generators and auxiliary plant. These include the displaying and recording of levels, pressures and temperatures, associated with steam, water, flue gases and air, along with the control and operation of the pulverising mills, boiler draught plant and feed water systems, etc. Associated with each turbine, but outside the unit control rooms, are four consoles housing the valves, turbovisory gear and instrumentation used for starting up, running and closing down the turbines.

‘B’ Station Boiler Plant

The two assisted circulation, tangent tube boiler units have twin furnaces – superheater and reheater – connected to a common steam and water drum. The boilers for Units Nos. 7 and 8 were constructed by Clarke Chapman & Co. There are five 40 tonne/hr Babcock & Wilcox pressure type 10E pulverising fuel mills per boiler driven by 450 h.p. 3.3kV 950 rpm motors. Each mill has one primary air fan with an air capacity of 62,000 cu ft/min driven by a 540 h.p. 1480 rpm motor. Two mills feed each furnace; the centre mill can feed either furnace as required.
Babcox & Wilcox 10E pulverising mill

Babcox & Wilcox 10E pulverising mill

The two 350,000 cu ft/min F.D. fans per boiler are driven by 590 rpm, 1370 h.p. motor and supply combustion air to the burners via the two rotary air heaters which raise the air temperature to 271 °C. There are forty-eight 11,200 lb/hr p.f. burners per boiler, arranged in groups of six at each furnace corner. For lighting up there are 24 recirculating tip fixed oil burners, push button operated from the unit control panel. Each burner has a capacity of 3,300 Ibs/hr when operating in high mode and 2,200lbs/hr in low mode. The boiler gases leaving the economisers pass through rotary air heaters, mechanical cellular dust extractors and electrostatic precipitators to the I.D. fans and then to the flue. There are two 520,000 cu ft/min two speed 740/590 rpm I.D. fans driven by 2000/1400 h.p. motors. The guaranteed efficiency of these boilers is 89.95 per cent on the gross calorific value of the fuel and have a maximum continuous evaporative capacity of 2,350 lb/hr with a feed water temperature of 260°C. The superheater outlet steam conditions are 2400 p.s.i. at 269°C with reheating at 594p.s-i.from 372 to 569°C. Four circulating pumps are provided for each boiler, rated at 8.800 gal/min against a 136ft head with water at 0.595 specific gravity. The steam temperature at the superheater outlets is automatically controlled by the operation of tilting burners in conjunction with two spray type desuperheaters to regulate the final steam temperature at 569°C ± 8°C between 70 and 100 maximum continuous rating under specified operating conditions. The spray desuperheaters are arranged for automatic bias control and automatic operation at minimum burner angle. The reheat outlet temperature is similarly controlled using the reheat furnace tilting burners and emergency spray desuperheaters.

Units 7 and 8 Turbo Generators

  Manufactured by English Electric, are four cylinder, 3000 rpm, reheat machines each with a design rating of 350MW. The high pressure (H.P.) cylinder is of the reversed flow design, having a double casing at the higher pressure end. The intermediate (1.P.) cylinder has a partial double casing but is of straight flow construction. Two low pressure (L.P.) cylinders, each of double flow construction, exhaust to separate condensers. The turbine shafts are solidly connected to the hydrogen cooled alternator. The alternator stator conductors are directly cooled with water which is pumped around a closed system. Steam from the H.P. steam chests passes through the H.P. cylinder outer casing about half way along its length and enters the admission belts of the inner cylinder before expanding successively through eight stages of blading. The first five stages are housed within the inner casing, the steam expanding towards the governor end. The flow direction is then reversed and the steam passes over the inner casing. Further expansion takes place through the three remaining stages contained in the outer casing before the steam returns to the boiler for reheating. Bled steam for No. 7 feed water heater is taken from the H . P. exhaust piping. Steam enters the seven stage 1.P. cylinder from the reheater through two interceptor valve chests. Two loop pipes from each chest are connected to four steam admission pipes which pass through the outer casing to the inner casing admission belt. The steam expands successively through three stages of blading housed in the inner casing and then through four stages contained in the outer casing, before exhausting to the L.P. cylinders. Bled steam for No. 6 feed water heater is taken from after the third stage and passed over the inner cylinder before leaving the turbine. Nos. 5 and 4 feed water heaters receive their steam supply from after stage five and the 1. P. cylinder exhaust. Exhaust steam from the 1.P. cylinder enters the four flows of the L.P. cylinders and expands through two impulse and three reaction stages before exhausting to the condensers through 36in long last row blades. Steam for No. 3 feed water heater is bled from the LP.2 cylinder after the first stages, for No. 2 heater from L.P.I after the second stages, and for No. 1 heater from both cylinders after the third stages. The guaranteed steam and heat consumptions of the turbo generators are 6.3586 lb/kWh and 7,525 BTU per kWh respectively.

Unit 8 Turbo Generator

Unit 8 turbogenerator

Unit 8 turbogenerator

Excitation is provided by static rectifiers and an AC generator coupled to the main shaft. The main feed pump is separately powered by a bled steam turbine which forms an integral part of the feed heating system, providing bled steam for Nos. 5 and 4 feed water heaters. The introduction of 36in long last row blades in the ‘B’ Station turbo-generators was a significant advancement in turbine technology. They were the first turbines in the world, rotating at 3000 rpm, to have blades of this length. When the shaft is running at full speed the velocity of the blade tip is 1780 ft/sec with each blade exerting a centrifugal force of 112 tonnes at the root fastening . The 36in blades provide a much larger exhaust area than was previously attainable, enabling turbines to be designed for greater power outputs. This led directly to the present day single shaft, 3000 rpm turbines with power outputs of up to 660MW. Condensing & feed water systems Units 7 and 8 have twin-shell condensers with a total cooling surface of 210,000 sq ft. Two 100 per cent duty extraction pumps are used, each rated at 180,000 gal/hr and driven by 3.3kV, 800 h.p. motors at 740 rpm. Three 50 per cent duty air pumps can each deal with 204lb of dry air per hour. ../images/B Condensers2.jpg Condensers beneath turbine There are seven stages of feed heating consisting of three LP heaters, one deaerator heater and three HP heaters. Feed water is heated by bled steam from the turbine at a temperature of 252°C. The three 50 per cent duty booster feed pumps, each rated at 2440 gal/min and with a discharge pressure of 980 p.s.i., are driven by 2150 h.p. 1485 rpm motors. The two 50 per cent duty standby/starting boiler feed pumps, each rated at 276 / gal/min and having a discharge pressure of 2800 p.s.i., are driven by 4750 h.p. variable speed motors. The 100 per cent duty main feed pump is rated at 5,150 gal/min with a suction pressure of 900 p.s.i. and a discharge pressure of 2800 p.s.i. It is driven by an 8100 h.p. English Electric bled steam turbine. The 610 p.s.i. steam supply for this turbine is taken from the cold reheat line at the main H.P. turbine exhaust. After passing through the bled steam turbine it exhausts into the deaerator. A bleed point is also incorporated on the bled steam turbine, supplying steam for No. 5 h.p. feed water heater. Control of the boiler feed water flow is affected by speed variations of the standby/starting and main feed pumps. Feed regulating valves are provided only to control the low flows experienced during starting up and shutting down.

Circulating Water System

The arrangement of the ‘B’ Station cooling water system is similar to that of ‘A’ Station. Water is extracted from the head of the Blyth harbour tidal basin and discharged to the sea off Cambois beach below low tide level. The four vertical spindle, single entry, mixed flow C.W. pumps have a head of 57ft. They are driven through reduction gears by an 11 kV 3,250 h.p. 988 rpm motor to give an output, in each case, of 147,500gal/min at 185 rpm. The reinforced concrete pump volutes are cast integral with the pump foundations, which represented a new engineering development at the time of the station’s design. Cooling water is circulated by a ring main and flows through the condensers of each turbine at a rate of 155,000 gal/min.

Control Rooms

B station control room

B station control room



Control of the two Units in the ‘B’ Station is from one central control room. A central generator teaming desk is surrounded by a central console for the various electrical. systems, with the unit control and recorder panels-around the perimeter of the room. Unit loading instructions are received at the generator loading desk direct from the grid control centre. Synchronising, generator switching ..and main electrical switching are carried out from the electrical system control console. In each corner of the room there is a suite of panels for the control of a single unit comprising a control console and separate recorder panel. These panels carry the controls and instrumentation necessary to start up, run and close down the unit.


Switchgear & Electrical

Grid Transformer

Grid Transformer



Each machine has its own unit transformer connected solidly to the alternator terminals to provide 11kV supplies. These step-down transformers are rated at 30MVA foor Units 7 and 8.
The 11 kV system is used to power the major auxiliaries. For starting purposes there are two 30MVA station transformers drawing power from the 66kV substation and reducing the voltage to 11kV. Lower voltage supplies are provided by auxiliary transformers which reduce the voltage to 3.3kV and 415 volts. This substation contains eighteen air blast circuit breakers of 15.000MVA rating, used for switching the infeeds and the feeders connecting the station with the grid system. Lower voltage supplies are provided by auxiliary transformers which reduce the voltage to 3.3kV and 415 volts. The generator transformers are rated at 400MVA. They have a voltage ratio of 19.5/275kV and are connected, along with Unit 4 and the two 66/275kV grid transformers, as in feeds to the 275kV substation. This substation contains eighteen air blast circuit breakers of 15,000 MVA rating, used for switching the in feeds and the feeders connecting the station with the grid system.


Ash and Dust Disposal

Artists impression of the ash disposal scheme

Artists impression of the ash disposal scheme

Ash is removed by high pressure water jets from the boiler bottom ash hoppers and transported down sluiceways at high velocity, via ash crushers to the ash sump. Ash pumps then discharge the ash into settling ponds. Dust from the precipitators and mechanical grit arresters is conveyed by pneumatic gravity conveyors (air slides) to collector hoppers. Each hopper forms an integral part of a dust pump which conveys the dust by means of a scroll shaft to a mixing chamber.
Three-stage development

Three-stage development

Air from large capacity radial vane compressors enters the mixing chamber via nozzles and transports the dust down pipelines to silos adjacent to the coal stock. Dry dust from the silos, conditioned by adding water to avoid dust nuisance, is sent for sale or to the ash disposal scheme. This scheme is being developed in three stages. The dust is compacted, landscaped and covered with soil. Grass and other vegetation is then planted to form a natural landscape feature.