ME6701 power plant engineering is the very
important subject for mechanical engineering.
This post covers the coal based thermal power
plant layout, working and various circuits
Coal Based
Thermal Power Plants
Introduction
Steam
is an important medium for producing mechanical energy. A steam power plant
continuously converts the energy stored in fossil fuels (Coal, oil and natural
gas) in the form of heat energy.
The
steam power plants are very much suitable where the coal is abundantly
available.
The
working principle of Thermal power plant is based on Rankine cycle.
Working Principle
In
steam or thermal power plants steam is used as the working fluid. Steam is
produced in a boiler using coal as fuel and it is used to drive the prime mover
(steam turbine).
The heat energy is converted into mechanical energy by the
steam turbine and the mechanical energy is used for generating power with the
help of a generator.
The
layout of the steam power plant consists of four main circuits. They are as
follows
(i)
Coal
and ash circuit
(ii)
Air
and flue gas circuit
(iii) Water and steam circuit
(iv) Cooling water circuit
Coal and ash circuit:
This circuit consists of coal storage, ash
storage, coal handling and ash handling systems. The handling system consists
of belt conveyors, screw conveyors, etc.
Coal from the storage yard is transferred to
the boiler furnace by means of coal handling equipments.
Ash resulting during combustion of coal in the boiler furnace is removed
and it is stored in ash storage devices through ash handling methods such as
hydraulic system, mechanical conveyors, steam jet system etc.
Air and flue gas circuit:
This circuit consists of air filter, air
preheater, dust collector and chimney.
Air is taken from the atmosphere and it is sent to an air preheater.
Then the dust from the air is removed by
means of using series of air filter.
Then the waste heat of flue gas is passed to
the chimney for preheating the air in the preheater.
After the combustion process, the flue gas
which has sufficient quantity of heat is passed around the boiler tubes, dust
collector, economizer and preheater before being exhausted to the atmosphere
through the chimney.
By passing the flue gas around the economizer
and air preheater, the water and air are preheated before going to the boiler.
Feed water and steam circuit:
This circuit consists of boiler feed pump,
boiler, turbine and feed water heaters.
The
steam generator in the boiler passes through super heater and it is supplied to
the steam turbine.
The
steam is expanded in the steam turbine and then it is passed to the condenser
where it is condensed.
Then
the feed water is passed through the economizer where it is further heated by
means of flue gases.
A
part of steam and water are lost while passing through different components of
the system.
So
the feed water is supplied from the external source to compensate losses.
Cooling water circuit:
This
circuit consists of condenser, cooling water pump and cooling tower.
Abundant
quantity of water is required for condensing the steam in the condenser.
Adequate
water supply is available from various sources such as river or lake.
If
adequate quantity of water is not available at plant sites, the warm water
coming out from the condenser is cooled in the cooling tower and it is
recirculated within the plant.
Advantages:
(i)
It
produces high efficiency
(ii)
Initial
investment is low
(iii)
The
power production does not depend on the nature mercy
(iv)
The
power plant can be located near load centre, so the transmission cost and
losses are considerably reduced
Disadvantages:
(i)
Transportation
of fuel is difficult
(ii)
Air
pollution occurs
(iii)
Power
generation cost is high when compared to hydro electric power plant
(iv)
It
cannot be used as a peak load plant.
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