A steam power plant / thermal power plant is using steam as working
A thermal power station is a power plant in which the prime mover is
Steam is produced in a boiler using coal as fuel and is used to drive
the prime mover, namely the steam turbine.
Water is heated, turns into steam & spins a steam turbine which
drives an electrical generator.
After steam passes through the turbine, it’s condensed in a condenser &
recycled to where it was heated; known as a Rankine cycle.
In the steam turbine, heat energy is converted into mechanical
energy which is used for generating electric power.
Generator is an electro-magnetic device which makes the power
available in the form of electrical energy.
4. General Layout
A schematic arrangement of equipment of a modern coal fired steam
power plant may be divided into four main circuit as
Coal – Ash circuit
Air – Flue Gases circuit
Feed Water – Steam circuit
Cooling Water circuit
6. Coal – Ash Circuit
The mixture (pulverized coal + preheated air) is taken into boiler, then
burnt in combustion zone. On ignition of fuel a fireball is formed at
the center of the boiler and large amount of heat energy is radiated from
The flue gases from the boiler make their way through superheater,
economizer, air preheater and finally get exhausted to the atmosphere
from the chimney.
Coal Handling : Coal from the storage yard is transferred to the boiler
furnace by means of coal handling equipment like belt conveyor,
bucket elevator, etc.,
Ash Disposal : Indian coal contains 30% to 40% ash. A power plant
of 100MW 20 to 25 tons/hour of ash.
Ash collects at the back of the boiler and is removed to the ash storage
yard through the ash handling equipment.
7. Air – Flue Gases Circuit
Air is taken from the atmosphere to the air preheater.
Air is heated in the air preheater by the heat of flue gas which is
passing to the chimney. The hot air is supplied to the furnace of the
The flue gases after combustion in the furnace, pass around the
The flue gases then passes through a dust collector, economizer and
pre-heater before being exhausted to the atmosphere through the
By this method the heat of the flue gases which would have been
wasted otherwise is used effectively.
Thus the overall efficiency of the plant is improved.
8. Feed Water – Steam Circuit
The steam generated in the boiler passes through super heater and is
supplied to the steam turbine.
Work is done by the expansion of steam in the turbine and the
pressure of steam is reduced.
The expanded steam passes to the condenser, where it’s condensed.
The condensate leaving the condenser is first heated in a water
heater using the low pressure extraction point steam of the turbine.
High pressure extraction point steam of turbine is used for heating
the feed water in the high pressure water heater.
The hot feed water is passing through the economizer, where it is
further heated by means of flue gases.
The feed water which is sufficiently heated by the feed water heaters
and economizer is then fed into the boiler.
9. Cooling Water Circuit
Abundant quantity of water is required for condensing the steam in the
Water circulating through the condenser may be taken from various
sources such as river or lake, provided adequate water supply is
available from the river or lake throughout the year.
If adequate quantity of water is not available at the plant site, the hot
water from the condenser is cooled in the cooling tower or cooling
ponds and circulated again.
10. Principle Components
The principle components of the modern thermal power plant are
Furnace : To burn fuel.
Steam Generator (Boiler) : In which heat generated in the
furnace is utilized to convert water into steam.
Prime Mover (Steam Turbine) : To use the heat energy of
steam & converts into mechanical work.
Piping System : To convey steam & water.
Electric Generator or Alternator : To convert mechanical
power into electricity.
11. Brief About Components
Boiler : The heat energy is utilized to convert the water into steam at
high temperature and pressure. Steel tubes run along the boiler walls
in which water is converted in steam.
Superheater : The saturated steam in the boiler tubes is superheated to
540 °C at high pressure.
Economizer : Essentially a feed water heater which heats the water before
supplying to the boiler.
Air pre-heater : The primary air fan takes air from the atmosphere & it’s
then warmed in the air pre-heater. Pre-heated air is injected with coal in
the boiler. Pre-heating the air improves the coal combustion.
Steam Turbine : High pressure super-heated steam is fed to the
steam turbine. The pressure and temperature of the steam falls to a
lower value and it expands in volume as it passes through the turbine.
The expanded low pressure steam is exhausted in the condenser.
13. Generator : To convert mechanical power of turbine shaft into
electric energy. The generator shaft is coupled to the turbine shaft.
Alternator : The steam turbine is coupled to an alternator. When the
turbine rotates the alternator, electrical energy is generated.
Condenser : Exhausted steam is condensed in the condenser by
means of cold water circulation. Condensing is essential compressing
a gases requires huge amount of energy with respect to energy required
in compressing liquid. Condensing increases efficiency of the cycle.
Cooling Tower : To cool the hot water coming from the condenser
with help of the atmospheric air.
Feed Water Pump : The condensed water is again fed to the boiler
by a feed water pump. Some water may be lost during the cycle,
supplied from an external water source.
Brief About Components
14. Circulating Water Pump : To circulate cooling water in the
condenser & cooling tower closed circuit.
Coal Handling Plant :
Wagon Tripler : To unload the coal when transported to the plant site.
Crusher House : To crush coal to a size of 10mm.
Coal mill : To pulverize the raw coal into a fine powder.
Ash Precipitators : To remove flyash & dust particles from the flue
gases before emitting into atmosphere to avoid air pollution.
Chimney : It’s Ferro-concrete structure lined with bricks used to
escape flue gases atmosphere at 30m above the ground level.
Water Treatment Plant : To remove impurities of water before water
supply to the boiler.
Brief About Components
15. Draft system : The circulation of air is caused by a difference in
pressure, known as Draft. There is a differential pressure b/w
atmosphere & inside the boiler. It is necessary to cause the flow of gases
through boiler setting
Mechanical draft :
Forced draught fan (FD)
Induced draught fan (ID)
Brief About Components
16. Thermal Efficiency : Thermal efficiency of modern thermal power
stations is about 30%.
Overall Efficiency : The overall efficiency of a thermal plant is about
29% (slightly less than the thermal efficiency).
Efficiency of Thermal Power Station
Initial cost is low compared to other generating stations.
It requires less land as compared to hydro power plant.
The fuel (i.e. coal) is cheaper.
Cost of generation is lesser than that of diesel power plants.
The power plant can be located near load center, so the
transmission losses are considerably reduced.
The generation of power is not dependent on the nature’s
mercy like hydro plant.
The construction and commissioning of thermal plant requires
less period of time than a hydro plant.
Air pollution : The pollution of the surrounding atmosphere is
caused by the emitting large amount of objective gases & dust
through the chimney.
The air pollution and smoke cause nuisance to people
surrounding the planet.
This is one of the causes of global warming.
The overall efficiency of a thermal power station is low (less
Major part of heat is lost in the condenser. That is why the
efficiency of thermal plants is quite low.