Gas turbine plant:
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
- Combustion chamber
- Starting motor
- Gas turbine
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.
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.
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.
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 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.
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.