Thursday, 21 July 2011

Schematic Arrangement of Steam Power Station

1. Coal and ash handling plant. The coal is transported to the power station by road or rail and

is stored in the coal storage plant. Storage of coal is primarily a matter of protection against coal

strikes, failure of transportation system and general coal shortages. From the coal storage plant, coal

is delivered to the coal handling plant where it is pulverised (i.e., crushed into small pieces) in order

to increase its surface exposure, thus promoting rapid combustion without using large quantity of

excess air. The pulverised coal is fed to the boiler by belt conveyors. The coal is burnt in the boiler

and the ash produced after the complete combustion of coal is removed to the ash handling plant and

then delivered to the ash storage plant for disposal. The removal of the ash from the boiler furnace is

necessary for proper burning of coal.

     It is worthwhile to give a passing reference to the amount of coal burnt and ash produced in a

modern thermal power station. A 100 MW station operating at 50% load factor may burn about

20,000 tons of coal per month and ash produced may be to the tune of 10% to 15% of coal fired i.e.,

2,000 to 3,000 tons. In fact, in a thermal station, about 50% to 60% of the total operating cost

consists of fuel purchasing and its handling.

2. Steam generating plant. The steam generating plant consists of a boiler for the production of

steam and other auxiliary equipment for the utilisation of flue gases.

       (i) Boiler. The heat of combustion of coal in the boiler is utilised to convert water into steam at

high temperature and pressure. The flue gases from the boiler make their journey through super-

heater, economiser, air pre-heater and are finally exhausted to atmosphere through the chimney.

      (ii) Superheater. The steam produced in the boiler is wet and is passed through a superheater

where it is dried and superheated (i.e., steam temperature increased above that of boiling point of

water) by the flue gases on their way to chimney. Superheating provides two principal benefits.

Firstly, the overall efficiency is increased. Secondly, too much condensation in the last stages of

turbine (which would cause blade corrosion) is avoided. The superheated steam from the superheater

is fed to steam turbine through the main valve.

    (iii) Economiser. An economiser is essentially a feed water heater and derives heat from the flue

gases for this purpose. The feed water is fed to the economiser before supplying to the boiler. The

economiser extracts a part of heat of flue gases to increase the feed water temperature.

     (iv) Air preheater. An air preheater increases the temperature of the air supplied for coal burn-

ing by deriving heat from flue gases. Air is drawn from the atmosphere by a forced draught fan and

is passed through air preheater before supplying to the boiler furnace. The air preheater extracts heat

from flue gases and increases the temperature of air used for coal combustion. The principal benefits

of preheating the air are : increased thermal efficiency and increased steam capacity per square metre

of boiler surface.

      3. Steam turbine. The dry and superheated steam from the superheater is fed to the steam

turbine through main valve. The heat energy of steam when passing over the blades of turbine is

converted into mechanical energy. After giving heat energy to the turbine, the steam is exhausted to

the condenser which condenses the exhausted steam by means of cold water circulation.

      4. Alternator. The steam turbine is coupled to an alternator. The alternator converts mechanical

energy of turbine into electrical energy. The electrical output from the alternator is delivered to the

bus bars through transformer, circuit breakers and isolators.

      5. Feed water. The condensate from the condenser is used as feed water to the boiler. Some

water may be lost in the cycle which is suitably made up from external source. The feed water on its

way to the boiler is heated by water heaters and economiser. This helps in raising the overall effi-

ciency of the plant.

      6. Cooling arrangement. In order to improve the efficiency of the plant, the steam exhausted

from the turbine is condensed* by means of a condenser. Water is drawn from a natural source of

supply such as a river, canal or lake and is circulated through the condenser. The circulating water

takes up the heat of the exhausted steam and itself becomes hot. This hot water coming out from the

condenser is discharged at a suitable location down the river. In case the availability of water from

the source of supply is not assured throughout the year, cooling towers are used. During the scarcity

of water in the river, hot water from the condenser is passed on to the cooling towers where it is

cooled. The cold water from the cooling tower is reused in the condenser.

1 comment:

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