Introduction and details of working of typical thermal power plant

1. 12-Sep-17 1Prof. Niraj Solanki

2. ELECTRICAL ENGINEERING
12-Sep-17 2
By: Niraj .D.Solanki
Prof. Niraj Solanki

3. Introduction
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Steam Power Station (Thermal Station)
A generating station which converts heat energy of coal combustion into electrical energy is
known as a steam power station.
1. It is basically works on the Rankine cycle.
2. Steam is produced in the boiler by utilising the heat of coal combustion.
3. The steam is then expanded in the prime mover (i.e., steam turbine)
4. The steam turbine drives the alternator which converts mechanical energy of
the turbine into electrical energy.
5. This type of power station is suitable where coal and water are available in
abundance and a large amount of electric power is to be generated.
Prof. Niraj Solanki

4. Schematic arrangement
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1. The coal is transported to the power station by road or rail and is stored in
the coal storage plant.
2. Coal is delivered to the coal handling plant where it is pulverised (i.e.,
crushed into small pieces).
3. The pulverised coal is fed to the boiler by belt conveyors.
4. The coal is burnt in the boiler and the ash produced after the complete
combustion of coal is removed to the ash handling plant.
5. A 100 MW station 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
6. In a thermal station, about 50% to 60% of the total operating cost consists of
fuel purchasing and its handling.
Coal and ash handling plant.
Prof. Niraj Solanki

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Steam generating plant
Boiler: The heat of combustion of coal in the boiler is utilised to convert water
into steam at high temperature and pressure.
• Types : 1) Water Tube 2) Fire Tube
• In a water tube boiler, water flows through the tubes and the hot gases of
combustion flow over these tubes.
• in a fire tube boiler, the hot products of combustion pass through the tubes
surrounded by water.
• Water tube boilers have a number of advantages over fire tube boilers:
1. Its require less space,
2. smaller size of tubes and drum,
3. high working pressure,
4. less liable to explosion etc.
5. Therefore, the use of water tube boilers has become universal in large capacity steam
power stations.
Prof. Niraj Solanki

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Steam generating plant
Superheater : The steam produced in the boiler is wet and is passed through a
superheater where it is dried and superheated.
• Types: 1) Radiant superheater 2) Convection superheater
• The radiant superheater is placed in the furnace between the water walls and
receives heat from the burning fuel through radiation process.
• It has two main disadvantages. Firstly, due to high furnace temperature, it may get
overheated and, therefore, requires a careful design. Secondly, the temperature of
superheater falls with increase in steam output.
• Due to these limitations, radiant superheater is not finding favour these days.
• A convection superheater is placed in the boiler tube bank and receives heat from
flue gases entirely through the convection process.
• It has the advantage that temperature of superheater increases with the increase in
steam output. For this reason, this type of superheater is commonly used these days.
Prof. Niraj Solanki

8. 12-Sep-17 8
Steam generating plant
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.
Air preheater : An air preheater increases the temperature of the air supplied
for coal burning by deriving heat from flue gases.
Condensers : A condenser is a device which condenses the steam at the exhaust
of turbine.
Prof. Niraj Solanki

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Types of prime movers
• The prime mover converts steam energy into mechanical energy.
Types steam prime movers :
1. Steam engines . 2. Steam turbines.
Steam turbines are generally classified into two types according to the action of
steam on moving blades:
1. Impulse Turbine. 2. Reaction Turbine.
Prof. Niraj Solanki

10. 12-Sep-17 10
Steam generating plant
Electrical equipments:
Alternator: Each alternator is coupled to a steam turbine and converts
mechanical energy of the turbine into electrical energy. The alternator may be
hydrogen or air cooled. The necessary excitation is provided by means of main
and pilot exciters directly coupled to the alternator shaft.
Transformers : A generating station has different types of transformers,
(a) Main step-up transformers which step-up the generation voltage for
transmission of power.
(b) station transformers which are used for general service (e.g., lighting) in the
power station.
(c) auxiliary transformers which supply to individual unit-auxiliaries.
Prof. Niraj Solanki

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Steam generating plant
Switchgear: It houses such equipment which locates the fault on the system
and isolate the faulty part from the healthy section. It contains circuit breakers,
relays, switches and other control devices.
Cooling Tower: A cooling tower is a heat rejection system that rejects waste
heat to the atmosphere through the cooling of a water stream to a lower
temperature.
Prof. Niraj Solanki

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Efficiency of steam power station
The overall efficiency of a steam power station is quite low (about 29%) Due to a
huge amount of heat is lost in the condenser and secondly heat losses occur at
various stages of the plant.
Thermal efficiency : The ratio of heat equivalent of mechanical energy
transmitted to the turbine shaft to the heat of combustion of coal is known as
thermal efficiency of steam power station.
• The thermal efficiency of a modern steam power station is about 30%.
• more than 50% of total heat of combustion is lost in the condenser. The
other heat losses occur in flue gases, radiation, ash etc.
combustioncoalofHeat
shaftturbinetonsmittedenergy tramech.ofequivalentHeat
thermal
Prof. Niraj Solanki

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Efficiency of steam power station
Overall efficiency : The ratio of heat equivalent of electrical output to the heat
of combustion of coal is known as overall efficiency of steam power station.
The overall efficiency of a steam power station is about 29%. The following
relation exists among the various efficiencies.
coalofcombustionofHeat
ouputelectricalofequivalentHeat
overall
Overall efficiency = Thermal efficiency Χ Electrical efficiency
Prof. Niraj Solanki

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Sites Selection for thermal power stations :
1. Supply of fuel : It should be located near the coal mines so that
transportation cost of fuel is minimum.
2. Availability of water : A plant should be located at the bank of a river or
near a canal to ensure the continuous supply of water.
3. Transportation facilities : The plant should be well connected to other
parts of the country by rail, road. etc.
4. Cost and type of land : The steam power station should be located at a
place where land is cheap and further extension.
5. Nearness to load centres : In order to reduce the transmission cost, the
plant should be located near the centre of the load.
6. Distance from populated area : As huge amount of coal is burnt , smoke
and fumes pollute the surrounding area. So that the plant should be
located at a considerable distance from the populated areas.Prof. Niraj Solanki

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Advantages
1. The fuel (i.e., coal) used is quite cheap.
2. Less initial cost as compared to other generating stations.
3. It can be installed at any place irrespective of the existence of coal.
The coal can be transported to the site of the plant by rail or road.
4. It requires less space as compared to the hydroelectric power
station.
5. The cost of generation is lesser than that of the diesel power
station.
Disadvantages
1. It pollutes the atmosphere due to the production of large amount
of smoke and fumes.
2. It is costlier in running cost as compared to hydroelectric plant.
Prof. Niraj Solanki