A power plant which employs gas turbine as the prime mover to generate electrical energy is called as gas turbine power plant.
In
gas turbine plant, air is used as the working fluid. The air is
compressed by the compressor to increase the pressure of the air. The
pressurised air is then passed through the combustion chamber where the
air is heated to a high temperature. The heart is added to the air by
burning the oil in the combustion chamber or by the air heaters. The hot
and high pressure air is then expanded in the gas turbine which drives
the alternator to convert the mechanical energy in to electrical
energy. The exhaust gases are passed through the regenerator to heat
the compressed air and then they are released to the atmosphere.
The
Compressor, gas turbine and alternator are mounted on the same shaft,
so part of the driven power is used to drive the compressor once the
plant has been started. Gas power plants are used as standby plants for
hydro-electric power stations where these can be operated at peak loads.
Components of gas turbine power plant:
The schematic block diagram of gas power plant is shown in below figure.
The major components are
- Regenerator
- Combustion chamber
- Compressor
- Alternator
- Starting motor
- Gas turbine
Compressor:
The
compressor used in gas power plant is rotating type. The air at
atmosphere pressure is drawn by the compressor through a filter which
removes the dust. The rotary blades in the compressor push the air
through the stationary blades to raise its pressure. Thus air with high
pressure is available at compressor output.
Regenerator:
A
regenerator is a device which recovers the heat from the exhaust gases
to heat the air from the compressor. The exhaust is passes through
regenerator before releasing it to the atmosphere. Several numbers of
tubes are nested in a shell of the compressor. The compressed air passes
through these tubes and exhaust gases from the gas turbine passes
through the shell side which transfers heat to the compressed air. In
this way compressed air heated by the exhaust gases which is an
effective usage of waste gasses.
Combustion chamber:
This
is one of the important components of the gas power plant where the
high pressure air from the compressor is entered in it via regenerator.
The air from regenerator is quietly heated which is not adequate to
drive the gas turbine. Only hot air with high pressure can only drive
the gas turbine. So in combustion chamber the compressed air is heated
up to high temperature (3000 F). The heat is added to the air by burning
oil which is injected through a burner in to the chamber at high
pressure. The heated air with high pressure is then applied to gas
turbine after it attains suitable temperature.
Gas Turbine:
This
is heart component of the gas power plant. The hot air with high
pressure and temperature is passed through gas turbine. The gases are
expanded on the gas turbine blades which causes the rotation of blades
to the intended mechanical work. After expanding, the exhaust gases with
the temperature about 900 F are applied to the regenerator.
Alternator:
Alternator
is directly coupled with the gas turbine same as in the case of steam
power plant. Alternator converts the mechanical energy of the turbine in
to electrical energy. The output generated electrical energy is then
passed to the grid through a generator transformer, isolators and
circuit breakers.
Starting motor:
The
starting motor is placed to start the compressor before starting the
plant. This works as the initial driving component for the compressor.
The starting motor is coupled to the same shaft of the gas turbine for
this purpose. Once the gas turbine starts rotating, some part of the
mechanical energy is used to drive the compressor and the starting motor
is turned off. The starting motor is driven by the batteries.
Advantages of Gas power plant:
- The design of gas power plant is simple compared to steam power plant as that it does not require boiler and its auxiliaries.
- Occupies less space and size compared to the steam power plants where the boiler and feed water arrangement is not needed.
- Initial and Operation costs are lower compared to all other plants.
- Gas turbines are simple in construction compared to steam turbines and the maintenance of them is also less.
- It requires less water compared to steam power plants where the condenser is required.
- It can be started with less time from cold conditions.
Disadvantages of Gas turbine Power Plant:
- The efficiency and net output power is less because some amount of mechanical power is used to drive the compressor.
- Initial external power is needed to drive the compressor until the plant starts generating.
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