Working of a thermal power station can mainly be divided into four departments
1. Coal Handling Plant
2. Boiler
3. Turbine
4. Generator
Working of a Coal Handling Plant :
Coal is used as a main fuel in thermal power station. As the consumption of coal is huge, the layout of a coal handling plant should be simple,relable and low maintenance.
Coal is brought to power station by three means of coal transportation i.e. roadways,railways and ropeways.
Coal brought by railways is unloaded with the help of wagon tippler in a coal hopper. The movement of wagons are controlled by automatic in-haul and out-haul bettle chargers. This coal is then feeded on coal conveyor belt through vibrating feeder. These feeders are of elecro-magnetic type and controls the rate of feeding required for bunkering. By the various combinations of conveyor belts ,coal is conveyed to the surge hopper of a crusher house. Before the coal comes to the crusher house, the ferrous materials which comes along with the coal is taken out with the help of suspended and rotating type magnetic separators. Non-ferrous materials like stones.shells,wood etc. are removed manually. From surge hopper, coal is fed to the coal crusher through mechanical feeder. Here coal is crushed to the size of 20-25 mm.
Working of a Boiler :
Boiler is a device for generation of steam for power generation. In thermal power stations water tube boilers are used. Boiler is suspended from the top on four columns and kept free from the bottom side for free expansion on downward direction.
Type of a boiler: Natural circulation, natural draft, tangentially fired, radiant, reheat type, dry bottom and pulverised fuel fired.
Feed water is fed to the boiler drum through the economiser. Water then enters in bottom ring header through six numbers of down comers. In boiler furnace, coal is fired with fuel oil. The heat energy developed by combustion of coal in furnace is utilized for the evaporation of water in water walls. As the density of steam is lower than water ,this water steam mixture enters in boiler drum without help of any pump. This is called natural circulation. In boiler drum, steam is separated from the mixture in three steps i.e. cyclone separators (Primary separators) ,secondary separators and screen dryers (Final separators). Steam that comes out of boiler is called saturated steam. This saturated steam is then passed through number of superheaters i.e. primary, platen and final for superheating of steam to a temperature of 540oc . (Pressure being 138 Kg/cm2 ).
When coal is burned in the boiler furnace, hot flue gases passes through the first pass and then to the second pass to the exit of boiler. Economiser and primary superheaters are placed in second pass one above the other, economiser being placed at the exit. The temp. of the flue gases in the combustion zone is 1200-1400 oc and after furnace 1000-1000 oc . The temp. of flue gas gradually decreases to 400 oc when it leaves second pass.The flue gas then goes trough air pre heaters where its temp. drops down to 140oc. Primary and secondary air is passed trough the air heater to increase the temp. This hot secondary air is sent to the furnace through wind box and hot primary air is sent to coal mill for heating pulverised coal and to transfer it upto the furnace.
Boiler is sealed from the bottom by seal water arrangement to prevent any ingress of atmospheric air into the boiler. Bottom of the boiler is shaped like a hopper. Bottom ash falls in the bottom hopper and after crushing it is transferred to ash handling plant .Fly ash alongwith the flue gases goes through ESP where fine ash is taken out and send to the ash handling plant for further processing to the ash bunds.
Boiler drum, superheater and reheaters are fitted with safety valves for safety against the high pressures of the steam. Water attemperation system is provided for controlling the temp. of main and reheat steam. Burner tilting arrangement is also provided to control the temp. of steam.Soot blowers are provided at different location of boiler to clean the boiler tubes .
Working of a Turbine:
210 MW turbine is condensing, tandem compound, three cylinder, horizontal, disc and diaphram type with nozzle governing and regenerative feed water heating. The double flow L.P. Turbine (LPT) incorporates multi exhaust in each flow.
The complete turbine assembly is mounted on pedestals and sole plates which are designed to ensure that the components are free to expand whilst correct alignment is maintained under all conditions. Live steam from boiler enters two emergency stop valves (ESV) of high-pressure turbine (HPT). From ESV’s steam flows to the four-control valves (CV) mounted on the casing of HPT at the middle bearing side. Control valves in turn feed the steam to nozzle boxes located inside the HPT.
The HPT comprises of 12 stages, the first stage being governing stage. The steam flow in HPT being in reverse direction, the blades in HPT are designed for antoclockwise rotation, when viewed in the direction of steam flow.
After passing through HP turbine, steam flows to boiler for reheating and reheated steam comes to the intermediate pressure turbine (IPT) through two-interceptor valves (IV) and four control valves mounted on IPT itself.
The I.P.turbine has 11 stages. HP & IP rotors are connected by rigid coupling and have a common bearing.
After flowing through IPT, steam enters the middle part of low-pressure turbine through two crossover pipes. In LP turbine, steam flows in apposite paths having four stages in each path. After leaving the LP turbine the exhaust steam condenses in the surface condenser welded directly to the exhaust part of the LP turbine.
The rotors of IP & LP turbine are connected by a semiflexible coupling. The direction of rotor is clockwise when viewed from the front bearing and towards generator. The three rotors are supported on five bearings. The common bearing of HP & IP rotor is a combined journal and radial thrust bearing.
The anchor point of turbine is located at the middle foundation frame of the front exhaust part of low-pressure cylinder. The turbine expands towards the front bearing by nearly 32 mm and towards generator by 3 mm in steady state operation at full load with rated parameters.
GENERATOR :
As seen above, boiler produces super heated steam of pressure 138 Kg/cm2 & 540 oc temp. This steam enters in steam turbine and due to the heat energy of steam, turbine rotates at about 3000 rpm. The turbine is directly coupled to the generator rotor. Electricity is generated as per the “Faradays Law” in generator.
FARADAYS FIRST LAW : Whenever a conductor cuts magnetic flux,an emf is induced in that conductor.
FARADAYS SECOND LAW : The magnitude of the induced emf is equal to the rate of change of flux linkages.
In alternator time varying magnetic field is produced by rotating field winding with help of turbine. Field windings are wound over rotor of the alternator and rotor is coupled to the turbine. Field windings are connected to the excitation system through slip rings. From excitation circuit, D.C. current is allowed to pass through the field windings and produces a magnetic field. So when the rotor rotates, D.C. current carrying field windings also rotates and produces a time varying magnetic field. This time varying magnetic field is cut by the stator windings of the alternator and emf is induced in it of the order of 15.75 KV as per the “Faradays Law”.
Electricity produced in the stator is then passed though bus ducts to the generator transformer. GT increases the voltage level from 15.75 KV to 400 KV. This transformer is connected to 400 KV buses in switchyard through isolators and a circuit breaker. Various 400 KV lines are connected to this bus through isolators and breakers.
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