Working of thermal power plant
Layout of steam power plant:
is an important medium for producing mechanical energy. Steam is used to drive
steam engines and steam turbines. Steam has the following advantages.
can be raised quickly from water which is available in plenty.
does not react much with materials of the equipment used in power plants.
is stable at temperatures required in the plant.
Equipment of a Steam Power Plant:
A steam power plant must have the
furnace for burning the fuel.
steam generator or boiler for steam generation.
power unit like an engine or turbine to convert heat energy into mechanical
generator to convert mechanical energy into electrical energy.
system to carry steam and water.
shows a schematic layout of a steam power plant. The working of a steam power
plant can be explained in four circuits.
1. Fuel (coal) and ash circuit
2. Air and flue gas circuit
3. Feed water and steam flow circuit
4. Cooling water flow circuit
Coal and Ash circuit:
This includes coal delivery, preparation, coal handling,
boiler furnace, ash handling and ash storage. The coal from coal mines is
delivered by ships, rail or by trucks to the power station. This coal is sized
by crushers, breakers etc. The sized coal is then stored in coal storage (stock
yard). From the stock yard, the coal is transferred to the boiler furnace by
means of conveyors, elevators etc.
The coal is burnt in the boiler furnace and ash is formed
by burning of coal, Ash coming out of the furnace will be too hot, dusty and
accompanied by some poisonous gases. The ash is transferred to ash storage.
Usually, the ash is quenched to reduced temperature corrosion and dust content.
different methods employed for the disposal of ash. They are hydraulic system,
water jetting, ash sluice ways, pneumatic system etc. In large power plants
hydraulic system is used. In this system, ash falls from furnace grate into
high velocity water stream. It is then carried to the slumps. A line diagram of
coal and ash circuit is shown separately in figure.
Water and Steam circuit
consists of feed pump, economizer, boiler drum, super heater, turbine condenser
water is pumped to the economizer from the hot well. This water is preheated by
the flue gases in the economizer. This preheated water is then supplied to the
boiler drum. Heat is transferred to the water by the burning of coal. Due to
this, water is converted into steam.
raised in boiler is passed through a super heater. It is superheated by the
flue gases. The superheated steam is then expanded in a turbine to do work. The
turbine drives a generator to produce electric power. The expanded (exhaust)
steam is then passed through the condenser. In the condenser, the steam is
condensed into water and recirculated. A line diagram of water and steam
circuit is shown separately in figure.
Air and Flue gas circuit
It consists of forced draught fan, air pre heater, boiler
furnace, super heater, economizer, dust collector, induced draught fan, chimney
etc. Air is taken from the atmosphere by the action of a forced draught fan. It
is passed through an air pre-heater. The air is pre-heated by the flue gases in
the pre-heater. This pre-heated air is supplied to the furnace to aid the
combustion of fuel. Due to combustion of fuel, hot gases (flue gases) are
Figure: Air and flue gas circuit
The flue gases from the furnace pass over boiler tubes and
super heater tubes. (In boiler, wet steam is generated and in super heater the
wet steam is superheated by the flue gases.) Then the flue gases pass through
economizer to heat the feed water. After that, it passes through the air
pre-heater to pre-heat the incoming air. It is then passed through a dust
catching device (dust collector). Finally, it is exhausted to the atmosphere
through chimney. A line diagram of air and flue gas circuit is shown separately
Cooling water circuit:
The circuit includes a pump, condenser, cooling tower etc.
the exhaust steam from the turbine is condensed in condenser. In the condenser,
cold water is circulated to condense the steam into water. The steam is
condensed by losing its latent heat to the circulating cold water.
Figure: Cooling water current.
Thus the circulating water is heated. This hot water is
then taken to a cooling tower, In cooling tower, the water is sprayed in the
form of droplets through nozzles. The atmospheric air enters the cooling tower
from the openings provided at the bottom of the tower. This air removes heat
from water. Cooled water is collected in a pond (known as cooling pond). This
cold water is again circulated through the pump, condenser and cooling tower.
Thus the cycle is repeated again and again. Some amount of water may be lost
during the circulation due to vaporization etc. Hence, make up water is added to
the pond by means of a pump. This water is obtained from a river or lake. A
line diagram of cooling water circuit is shown in figure separately.
(Advantages) of a Thermal Power Plant
unit capacity of a thermal power plant is more. The cost of unit decreases with
the increase in unit capacity.
of the plant is more (25-30 years) as compared to diesel plant (2-5 years).
and maintenance cost is low when compared with diesel plant.
cost of the plant is less than nuclear plants.
for varying load conditions.
harmful radioactive wastes are produced as in the case of nuclear plant.
operators can operate the plant.
power generation does not depend on water storage.
are no transmission losses since they are located near load centres.
of thermal power plants
plant are less efficient than diesel plants
up the plant and bringing into service takes more time.
water required is more.
required is more
required for the fuel is more
handling is a big problem.
economical in areas which are remote from coal fields
transportation, handling and storage charges are more
of persons for operating the plant is more than that of nuclear plants. This
increases operation cost.
large units, the capital cost is more. Initial expenditure on structural
materials, piping, storage mechanisms is more.