What is a power station? What are the types of power station?
Ans.
A power station is an industrial facility for the generation of electrical energy or electric power. A power station is also called as a generating station, power plant, or power house.
At center of all power stations is a generator, or a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor (armature ).
A generating station essentially employs a prime mover coupled to an alternator for the production of electric power. The prime mover ( e.g., steam turbine, water turbine etc. ) converts energy from some other form into mechanical energy. The alternator converts mechanical energy of the prime mover into electrical energy. It may be emphasized here that apart from prime mover-alternator combination, a modern generating station employs several auxiliary equipment and instruments to ensure cheap, reliable and continuous service.
Depending upon the kind of energy converted into electrical energy, the generating stations are classified as under :
(a) STEAM POWER STATIONS (THERMAL STATION)
(b) HYDROELECTRIC POWER STATIONS
(c) DIESEL POWER STATIONS
(d) NUCLEAR POWER STATIONS
Describe the schematic arrangement of a steam power station with a neat schematic diagram.
Ans.
A thermal power plant or a steam power station basically works on RANKINE CYCLE.
The schematic arrangement of a modern steam power station is shown in the following figure.
The whole arrangement can be divided into the following stages……….
(a) Coal and ash handling arrangement
(b) Steam generating generating plant
(c) Steam turbine
(d) Alternator
(e) Feed water
(f) Cooling towers
(a) Coal and ash handling arrangement : 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 & general coal shortages. From the coal storage plant, coal is delivered to the coal handling plant where it is pulverized ( 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 pulverized 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.
(b) Steam generating generating plant : Steam generating generating plant consists of the following equipment…………….
(i) Boiler : Boiler is an enclosed vessel in which water is heated and circulated until the water is turned into steam at the required pressure. Coal is burnt inside the combustion chamber of boiler. The heat of combustion of coal in the boiler is used to convert water into steam at high pressure and temperature. The flue gases from the boiler make their journey through super heater, economizer, air preheater and finally to atmosphere through chimney.
The steam produced inside the boiler which is at high pressure and temperature is used as a heating medium or as the working fluid in a prime mover ( e.g., steam turbine ) to convert thermal energy to mechanical work, which in turn may be converted to electricity.
(ii) Superheater : Most of the modern boilers are having superheater and reheater arrangement. Superheater is a component of a steam generating unit in which steam, after it has left the boiler drum, is heated above its saturation temperature. In other words steam produced in the boiler is wet and is passed through a superheater where it is dried and superheated. Thus overall efficiency is increased.
(iii) Economiser : The feed water is fed to the economizer before supplying to the boiler. The economizer extracts a part of of flue gases ( the gases coming out from the superheater ) to increase the feed water temperature. The use of economizer results in saving in coal combustion, increase in steaming rate and high boiler efficiency.
(iv) Air preheater : The remaining heat of flue gases is utilized by air preheater. An air preheater increases the temperature of the air supplied for coal combustion or burning by deriving heat from flue gases. Thus it increases the thermal efficiency and steam capacity per square metre of boiler surface.
(c) Steam turbine : The dry and superheated steam from the superheater is fed to the steam turbine through the main valve. Here 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
(d) Alternator : The steam turbine is coupled to an alternator. The alternator is an electromechanical device which converts mechanical energy of turbine into electrical energy.
(e) Feed water : The condensate ( water ) from the condenser is used as feed water to the boiler. Some water may be lost in cycle which is suitably made up from external sources. The feed water on its way to the boiler is heated by water heater and economizer.
(f) Cooling towers : The condensate formed in the condenser after condensation is initially at high temperature. This hot water is passed to the cooling towers. It is a tower or building like device in which atmospheric air ( heat receiver ) circulate in direct or indirect contact with warm water and the water is thereby cooled.
