Gas turbine power plant

Application of Gas turbine power plant

1. Gas turbine power plants are used to supply peak loads in steam or hydro-plants.
2. They are used as standby plants for hydro-electric power plants.
3. Gas turbines are used in jet, aircraft and ships.

Elements of a Gas turbine power plant:

L.P air compressor:
Atmospheric air is drawn in and passed through the air filter. It then flows into the low pressure compressor. Major percentage of power developed (66%) by the turbine is used to run the compressor. The power required to run the compressor can be reduced by compressing the air in two stages, i.e., in low pressure and high pressure compressor and also by incorporating an intercooler between the two.

 Intercooler:
Intercooler is used to reduce work of the compressor and increase the efficiency. The energy required to compress air is proportional it the air temperature at inlet. Therefore if intercooling is carried out between the stages of compression, total work can be reduced.

H.P. Compressor:
From the intercooler, the compressed air enters the high pressure compressor, where it is further compressed to a high pressure. Then it is passed into the regenerator.

Regenerator:
In the simple open cycle system the heat of the turbine exhaust gases goes as waste. To make use of this heat a regenerator is provided. In the regenerator the heat of the hot exhaust gases from the turbine is used to preheat the air entering the combustion chamber.

Combustion chamber:
Hot air from regenerator flows to the combustion chamber. Fuel (natural gas or coal gas or kerosene or gasoline) is injected into the combustion chamber and burns in the steam of hot air. The products of combustion, comprising a mixture of gases as high temperature and pressure are passed to the turbine.

Gas turbines:
Products of combustion are expanded in high pressure turbine and then in low pressure turbine. The part of the work developed by the gases passing through the turbines is used to run the compressor and the remaining (about 34%) is used to generate electric power.

Open cycle and closed cycle system:
When the heat is given to the air by mixing and burning the fuel in the air and the gases coming out of the turbine are exhausted to the atmosphere, the cycle is known as “open cycle system”. If the heat to the working medium (air or any other suitable gas) is given without directly burning the fuel in the air and the same working medium is used again and again, the cycle is known as “closed cycle system”.

Reheating combustion chamber:
The output of the plant can be further improved by providing a reheating combustion chamber between high pressure and low pressure turbines. In this, fuel is added to reheat the exhaust gases of high pressure turbine. The addition of the regenerator, intercooler and reheating combustion chamber increases the overall efficiency of the plant.

Advantages of Gas turbine power plant

1. Natural gas is very suitable fuel and where this is available cheap, it is an ideal source of power in gas turbine.
2. Gas turbine plant is smaller in size and weight compared to an equivalent steam power plant. For smaller capacities the size of the gas turbine power plant is appreciably greater than a high speed diesel engine plant; but for larger capacities it is smaller in size than comparable diesel plant. If size and weight are main considerations such as in ships, aircraft engines and locomotives, gas turbines are more suitable.
3. The initial cost is lower than an equivalent steam plant.
4. It requires less water as compared to a steam plant.
5. It can be started quickly, and can be put on load in a very short time.
6. Maintenance cost is low.
7. It does not require heavy foundations and buildings.
8. Any poor quality and wide variety of fuels from natural gas to residual oil or powered coal can be used.
9. The running speed of the turbine (40,000 to 100,000 rpm) is considerably large compared with diesel engine (1000 to 2000 rpm).
10. The exhaust of the gas turbine is free from smoke.

 Disadvantage Gas turbine power plant

1. Major part of the work (66%) developed in the turbine is used to drive the compressor. Therefore net work output of the plant is low.
2. It requires special metals and alloys for different components because the operating temperature (2000’C) and speed (100,000 rpm) are very high.
3. Part load efficiency is poor compared to diesel plant.

The use of gas turbine in power generation industry is more recent in the last two decades than its use in other fields as early as in 1875. The major progress has been achieved in three directions: increase in capacities of gas turbine units (50-100 MW), increase in efficiency (38%) and drop in capital cost.

In India, Namrup gas turbine plant at Assam is the first plant of its type to be used as base load plant. Natural gas is the fuel used, which is available at Namrup. Uran gas turbine plant at Maharashtra is second power plant of base load type of 240 MW capacities. It is located close to ONGC (oil and natural gas commission) uran terminal.