General Diagram Thermal Power Plant, Main Components

Diagram thermal power plant

It must have following principle items or equipment as
(i) Furnace – to burn fuel
(ii) Steam generator or boiler -in which heat generated in the furnace is utilized to convert water into steam
(iii) Prime mover (steam turbine) – to use the heat energy of steam and converts into mechanical work.
(iv) Piping system to convey steam and water.
(v) Electrical generator or alternator – to convert mechanical power into electricity.

Fig shows a schematic arrangement of equipments of a modern coal fired steam power plant. The entire arrangement for the sake of simplicity may be divided into four main circuit as (i) Coal and ash circuit, (ii) Air and flue gases circuit, (iii) Feed water and steam circuit and (iv) Cooling water circuit.

diagram layout of thermal power plant general
General Diagram or Layout of Steam Power Plant

(i) Coal and ash circuit

Coal received in coal storage yard of power station is transferred to the furnace by coal handling plant. After necessary preparation of the coal, from coal handling plant is taken into the boiler bunkers by means of bucket conveyers. Then the coal is stored in the bunkers. Coal from bunkers falls into the hoppers by gravity and finally the requisite quantity of coal either goes on falling directly on grate or where the coal spreaders are provided, coal is spread in the grate up to rear end with help of coal spreaders.
Ash is the product of coal combustion in the furnace, along with flue gases. About 20% of the ash falls in the bottom ash hopper of boiler and is periodically removed mechanically. The ash from hopper is removed to the ash storage by means of scrap conveyors (ash handling plant). Further ash disposed off in accordance with ash disposal arrangement.

(ii) Air and flue gases circuit

The air is taken from the atmosphere by the action of a forced draught fan and passed through an air pre-heater where it is preheated by the flue gases. The preheated air is supplied to the furnace in order to provide air for combustion of fuel. Due to combustion of fuel (coal), hot (flue gases) are produced. The flue gases from the furnace pass over boiler tubes and preheater tubes. The flue gases pass though economizer (in which water is heated) and air preheater (in which air is heated). Flue gases is then passed through dust and fly ash collecting device where suspended dust and fly ash are removed from it. The fly ash is separated from flue gases by means of ash precipitator. Finally, flue gases is exhausted to the atmosphere through chimney.

(iii) Feed water and steam circuit

The condensate (water which is converted from steam) is extracted from the condenser by the condensates extraction pump and passes through low pressure feed water heater. The temperature of water is raised in the feed water heater by the heat from steam (which is extracted from the lowest pressure extraction point of the turbine). The feed water, then passes through deaerator and high pressure feed water heater where it is heated by the heat from steam (which is extracted from the turbine at suitable point). The deaerator removes the dissolved 0 from the feed water (condensate). Then, feed water is heated in economiser and fed to the boiler with the help of the feed water pump.
In the boiler, water is converted into high pressure steam which may be wet. The wet steam gets superheated when it passes through the super heater. the superheated high pressure steam is Supplied to the steam turbine. The steam first expands in high pressure stage of turbine and then expands in low pressure stage after reheated in the reheater until it brought to original dryness or superheat. After the expansion, steam is passed through condenser where it rejects heat to cooling water and condensed. The condensed steam (condensate) again feed to the boiler. A part of steam and water is lost while passing through different components, in order to compensate mass of it, the water is added in the feed water system as make up water. In order to avoid scaling and deposition in the the boiler tubes, the feed water supplied from external source is passed through purifying (water treatment plant) to reduce the dissolved salts.

(iv) Cooling water circuit

The function of cooling water is to absorb heat from steam which is coming from turbine and to condense the steam. Cooling water is supplied from a natural sources such as river, canal, sea or lake. Cooling water is passed through the condenser and absorbs heat from steam hence it is heated. The heated cooling water is discharged to the lower side of the river. When the adequate fresh water is not available for condenser, then the hot water coming out from the condenser is cooled either in cooling pond or cooling tower. In the cooling tower the evaporating cooling takes place and there is loss of water. In order to compensate this loss, the water from the river is continuously supplied. When the cooling water coming out of the condenser is cooled again and supplied to the condenser then the system is called as closed system. When water coming out from the condenser is discharged to river downward side directly, the system is called as open system.

Main components of thermal power plant and their functions

(1) Boiler :

It is a closed pressure vessel in which the water is converted into steam with the help of heat produced by the burning of coal. The modern high pressure boilers are all water tube boilers in which water flows through the tubes and the combustion gases flow across the tubes. The high pressure boilers contains separate set of tubes which constitute heat exchange as follows

(a) Superheater : It is used to raise the temperature of steam above its saturation
temperature. The superheater is located at hottest part of the boiler (in the furnace) so that heat is transferred to steam by superheater from the hot gases.
(b) Reheater : It is used to raise the temperature of steam after it has expanded in the high pressure turbine. After reheating the steam supplied to the low pressure steam turbine.
(c) Economizer : The function of economizer is to increase temperature of feed water using waste heat of flue gases leaving the boiler through chimney. It is located before airpreheater and after superheater and reheater in the path of the flue gases.
(d) Air preheater : The function of air preheater is to increase the temperature of air before it supply to the furnace using heat from flue gases passing through chimney. 

(2) Steam turbine :

The function of steam turbine is to convert the heat energy of steam into rotational power at turbine shaft. In modem thermal power plant steam turbine generally has a three cylinder tendum compound machine consisting high pressure stage cylinder (I-IP), intermediate pressure stage (I.P.) and low pressure stage cylinder (LP.).

(3) Generator :

The function of electric generator is to convert mechanical (rotational power of turbine shaft into electric energy. The generator shaft is coupled to the turbine shaft.

(4) Condenser :

A condenser is a heat exchanger, consists of large cylindrical vessel containing large number of brass tubes through which cooling water circulated and steam flowing outside the surface of tubes. The function of condenser is to condense the steam which coming form the low pressure turbine.

(5) Cooling tower :

The function of cooling tower is to cool the hot water coming from the condenser. In the cooling tower the hot water is cooled with the help of the atmospheric air.

(6) Feed water pump :

It is used to supply feed water (coming from the condenser) to the boiler at a desired high pressure.

(7) Circulating water pump :

It is used to circulate cooling water in the condenser and cooling tower closed circuit.

(8) Coal handing plant

Wagon trippler : It is used to unload the coal when coal transported to the plant site by rail wagon.
Crusher house : It is used to crush coal to a size of 10 mm. Then coal is supplied to boiler raw coal bunkers.
Coal mill : It is used to pulverise the raw coal into a fine powder before it is burnt in the boiler furnace.

(9) Draught system :

The function of draught system is to create pressure difference which is required to introduce desired quantity of air to the boiler furnace and to carry away the burned product of combustion and discharge to atmosphere.

Forced fraught (F.D) fan : F.D. fan is placed in front of boiler and it is used to push the air through the air preheater and then the furnace of boiler.
Induced draught (I.D.) fan : I. D. fan is placed near the base of chimney and it draws flue gases through the interior of the boiler and dust extracting equipment and forces the flue gases to move through the chimney.

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