An electric power station is a factory in which energy is converted from one form to other i.e, from Mechanical into Electrical Energy in a conventional Thermal power station , the energy is first in the form of heat through the fuel which may be coal, gas or oil. This heat is liberated by combustion in boiler furnace and then is used to convert water into steam. The energy of steam is converted to the mechanical energy of a rotating shaft by some form of steam engine either reciprocating or turbine and its energy is converted into electrical energy by means of a generator. In a nuclear/atomic station the energy from the splitting of a atomic fuel is released in the form of heat in the reactor and heat in the reactor and heat is used to convert water into steam.
Steam/Thermal Power Station
Steam stations are vmainly used as central station to produce electric power for supply undertyakings. In these stations the A.C. generators are driven by turbines with condensing arrangements at high vacuums of about 736 water column . the size of the sets may be from 10mw to 500mw or even larger dependin on the system requirements. Generally all modern stations have 2 pole machine , the speed being 3000 rpm when the frequency is 50C/sec.
A large thermal station will be located at a site to which fuel can be easily transported by rail or ship and where enogh water is available for cooling the condensate the steam . station needs a large amount of water. The station needs a lot of space for storing fuel. Elaborate arrangements for fuel handling , layout of high pressure boiler , steam turbines , generators , control switch boards, cooling water arrangements etc. water treatment plant.
Unit Overview of the Plant, what I have visited
The circuit diagram of the unit overview is in the next figure which contains the parts and the elements of that power plant. Generally the main four system are:
- Boiler , its Mounting & Accessories
- Turbines & its parts
- Synchronous Generator
- Supporting systems such as fans, pumps, heaters etc.
Briefly the steam generated from boiler goes through main steam line to high pressure turbine where it performs its work & rotate the rotor of H.P. turbine & the exhaust of this turbine goes to reheater through C.R.H line (Cold Reheat Line) & again come through HRH line (Hot reheat line) & passes to I.P turbine &then from here it goes to L.P. turbine &finally the exhaust of L.P. turbine is cooled in condenser & then by condensate pump it (water ) goes to L.P heaters & then to Dearator from Dearator , Boler feed pumps takes water and passes it to the H.P heaters & then to feed regenerative system & then through Economizer it goes to drum & again steam is made in boiler.
The firing is done on grate &boiler & the different fans are used. Primary air fan give primary air/Force draft fan gives secondary air. The flue gas after heating all the parts through electrostatic precipitator it goes to chimney by induced draft fan Bunkers, mills are used to provide coal.
Preparation of coal for Mill
Coal through railways wagon come to the coal yard and then through machine it is lift on conveyor belts on which coal travels by passing through different conveyor belts it comes to primary crusher where its size is reduced to 20mm &then finally through conveyor belts it is passed to different mills bunkers.
In milling system raw coal from bunkers goes to a raw coal feeder where a motor is present to pass coal in the mill. In mill coal of 20mm size is powdered by boels which is rotated by rotors & motors and then by primary air from primary air from primary air fan ,coal powder is brought in furnace at that time coal powder temp is 80C due to presence of hot air in primary air. To prevent leakage of coal powder from the mill a sel air from seal air fan at higher pressure than mill pressure is given from outside towards inside.
The iron products coming in the mill with coal are separated in trump iron box, which contains two doors and at a time only one door can be open but now a days mill reject system is used.
Boiler feed Pump
Boiler feed pump act as hearty of plant so to get knowledge about plant its knowledge is necessary:
- It takes water from Dearator and discharges it to H.P. heaters.
- B.F.P. is main pump which must not be closed or open frequently according to need of plant of plant so to avoid this for continuous working of B.F.P a discharge line from B.F.P to Dearator is also provided that when drum is filled up or water is not required, water from B.F.P again goes to Dearator in this manner B.F.P runs continuously. As flow of water is continuous in B.F.P hence its metals parts temp remains constant. Imagine that when B.F.P is running at 3000 rpm and no water is flowing in B.F.P then its parts become heated and seized down.
- B.F.P is heated by providing heat through suction valve.
- The flow of the water along axis of B.F.P causes pressure and force which leaks out water so this water which exit i.e. leaks again goes to Dearator by blanking leak off line.
- Mechanical sealing is given of sealing of B.F.P & to cool it seal water is present & temp of seal water is maintained at seal water cooler.
- Bearings (Journal or Thrust ) of B.F.P are provided with the lubricating oil.
Flue Gases & Path
Flue gas is a bi product of combustion. It is that product which is responsible for heating all system of boiler. As we know when combustion of fuel takes place the heated air has same temperature as that of furnace i.e. about 1000C, furnace wall are surrounded by water tubes hence these are continuously heated. Flue gas first crosses platen heaters where superheated steam is passed & then after flue gas is passed to reheater where C R H line comes H R H line goes and then after it is passed to economizer to heat feed water before going to drum. When flue gases leave economizer its temp is little less than 500C & then it goes to air heater where primary and secondary air are heated by this flue gas & when it is weakened it is passed to electrostatic precipitator where the ash still present in flue gas separated electrostatically. The two plate one is grounded to earth where ash is to deposited& other plate made of wire nets, its electrode is held at negative charge whose magnitude is 30-40 KV. Thus ash particles become charged & accumulated to ground plate. From here the flue gas is allowed to pass through chimney by induced draft fan.
Steam in power plant is generated by thermo process. Feed water of drum come to the water tubes which surround the boiler from all sides by down come through header. This water passes to B-header then it become saturated steam, now to make it super heated steam it is passed to ;platen super heater first, yhen to primary super heater & then to final super heater & is accumulated in drum. Now main steam from super heater is passed towards turbine side I B D (intermittent blow down ) is present to blow down impurities present in main steam & from here is passed to P R D S ( Pressure Reducing and de Superheated steam) where the main steam is having high pressure is reduced to normal and its degree of superheat is also reduced after passing this it goes to U F T & then after to T F T & finally to high pressure turbine & its exhaust is again taken by C R H line.
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