Gas Turbine Power plant | Parts , Working , Advantages and Disadvantages
Gas Turbine Power plant
A gas turbine power plant may be defined as one “in which the principal prime mover is of the turbine type and the working medium is a permanent gas”.
A simple gas turbine plant consists of the following:
4. Combustion chamber
5. Gas turbine
6. Reheating unit
In gas turbine plant, the axial and centrifugal flow compressors are used. In most of the gas turbine power plant, two compressors are used. One is low pressure compressor and the other is high pressure compressor. In low pressure compressor, the atmospheric air is drawn into the compressor through the filter. The major part of the power developed by the turbine
(about 66%) is used to run the compressor. This low pressure air goes to the high pressure compressor through the intercooler. Then the high pressure air goes into the regenerator.
The intercooler is used to reduce the work of the compressor and it is placed in between the high pressure and low pressure compressor. Intercoolers are
generally used when the pressure ratio is very high. The energy required to compress the air is proportional to the air temperature at inlet. The cooling
of compressed air in intercooler is generally done by water.
Regenerators are used to preheat the air which is entering into the combustion chamber to reduce the fuel consumption and to increase the efficiency. His is done by the heat of the hot exhaust gases coming out of the turbine.
4. Combustion chambers
Hot air from regenerator flows to the combustion chambers and the fuel like coal, natural gas or kerosene are injected into the combustion chamber.
After the fuel injection, the combustion takes place. This high pressure, high temperature products of combustion are passed through the turbine.
5. Gas turbine
Two types of gas turbines are used in gas turbine plant.
1. High pressure turbine
2. Low pressure turbine
1. High Pressure turbine
In the beginning, the starting motor runs the compressor shaft. The burnt gases (product of combustion) expand through the high pressure turbine. It is important to note that when the turbine shaft rotates it infact drives the compressor shaft which is couples to it. Now, the high pressure turbine runs the compressor and the starting motor is stopped. About 66% of the power developed by the turbine is used to run the compressor and only
34% of the power developed is used to generate electric power.
2. Low pressure turbine
The purpose of the low pressure turbine is to produce electric power. The shaft of the LPT is coupled with the generator. The burnt fuel (gases) after leaving the HPT is again sent to a combustion chamber where it further undergoes combustion. Even if there is any left out unburnt fuel from the previous turbine it gets fully burnt in the combustion chamber. The burnt gases run the low pressure turbine (LPT). The shaft of the turbine is directly coupled with the generator for producing electricity.
The exhaust hot gases after leaving the LPT passes through the regenerator before exhausted through the chimney into the atmosphere. The heat from the hot gases is used to preheat the air leaving the HPC before it enters the combustion chamber. This preheating of the air improves the efficiency of the combustion chamber.
Working of Gas Turbine Power plant
The working of gas turbine plant is shown in fig. The atmosphere air is drawn into the low pressure compressor through the air filter and it is compressed.
The compressed low pressure air goes into the high pressure compressor through the intercooler. Here, the heat of the compressed air is
removed. Then the high pressure compressed air goes into the combustion chamber through the regenerator. In the combustion chamber, the fuel is added to the compressed air and the combustion of the fuel takes place.
The product of the combustion goes into the high pressure turbine. The exhaust of the high pressure turbine goes to another combustion chamber and the additional fuel is added and it goes to the low pressure turbine.
After the expansion in the low pressure turbine, the exhaust is used to heat the high pressure air coming to the combustion chamber through the regenerator. After that, the exhaust goes to the atmosphere.
Advantages of Gas turbine power plant
1. For a gas turbine plant, Natural gas is a very suitable fuel. It would be ideal to install gas turbine plants near the site where natural gas is readily available.
2. Gas turbine plants can work economically for short running hours.
3. Storage of fuel requires less area and handling is easy.
4. Gas turbine plant is small and compact in size as compared to steam power plants.
5. It can be started quickly and can be put on load in a very short time.
6. The cost of maintenance is less.
7. It is simple in construction. There is no need for boiler, condenser and other accessories as in the case of steam power plants.
8. The gas turbine can operate at high speed since there are no reciprocating parts
9. Cheaper fuel such as kerosene, paraffin, benzene and powdered coal (cheaper than petrol and diesel) can be used.
10. Gas turbine plants can be used in water scarcity areas
11. Less pollution and less water is required.
Disadvantages of gas turbine power plant
1. 66% of the power developed is used to drive the compressor; the gas turbine unit has a low thermal efficiency
2. The running speed of the gas turbine is in the range of (40, 000 to 1, 00,000 rpm) and the operating temperature is as high as 2000 0C, for
this reason special metals and alloys have to be used for the various parts of the turbine.
3. Special cooling methods are required for cooling the turbine blades.
4. It is difficult to start a gas turbine as compared to a diesel engine in a diesel power plant
5. The life of a gas turbine plant is upto 10 years, after which its efficiency decreases to less than 10 percent.
Applications of gas turbine power plant
- To drive generators and supply loads in steam, diesel or hydro plants
- To work as combination plants with conventional steam boilers
- Thermal process industries
- Petro chemical industries
- Power generation (used for peak load and as stand by unit)
- Aircraft and ships for their propulsion. They are not suitable for automobiles because of their very high speeds.
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