Gas turbine power plant

GAS TURBINE POWER PLANT

When the gas turbine generator was introduced to the power generation industry in the late 1940s, it was a revolutionary self-contained fossil-fueled power plant. Twenty years later, gas turbine was established as an important means of meeting fast growing peak loads.

Gas Turbine

The working medium for transforming thermal energy into rotating mechanical energy is the hot combustion gas, hence the term 'Gas Turbine'. Gas turbines are also referred to as Combustion Turbines or Combustion Gas Turbines.

In Gas Turbine Power Plant, the gases are directly expanded in the gas turbine. The working medium is either a mixture of combustion products and air or heated air at a certain pressure and a higher initial temperature. With respect to design, a gas turbine is similar to steam turbine.

Fuels

Natural Gas is the ideal fuel for gas turbines. But, this is not available everywhere. Blast furnace and producer gas may also be used for these plants.

However, liquid fuels of petroleum origin such as distillate oils or residual oils are most commonly used for gas turbine plant. Gasoline and Kerosene Oil or blend of these two are commonly used. The essential qualities of these fuels include proper volatility, viscosity and calorific value. At the same time, it should be free from any contents of moisture and suspended impurities that would clog the small passages of the nozzles and damage the valves and plungers of the fuel pumps.

Use of solid fuels such as coal in pulverized form in gas turbines presents several difficulties, most of which have been only partially overcome.

Open Cycle and Closed Cycle Gas Turbines

In order to understand the working of the gas turbine power plant, it is necessary to study the working principles of Open Cycle Gas Turbine and Closed Cycle Gas Turbine in Chapter 8.

 

1. LAYOUT OF GAS TURBINE POWER PLANT (Fig. 8)

1. L.P. Air Compressor

Atmospheric air is drawn in and passed through the air filter. It then flows into the Low Pressure Compressor (LPC). Major percentage of power developed ( i.e., 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 compressors and also by incorporating an intercooler between the two.

2. Intercooler

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

3. H.P. Air Compressor

From the intercooler, the compressed air enters the High Pressure Compressor (HPC), where it is further compressed to high pressure. Then it is passed into the regenerator.

4. 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.

5. Combustion Chamber

Hot air from regenerator flows to the Combustion Chamber (CC). Fuel (Natural Gas/Coal Gas/KerosenelGasoline) is injected into the combustion chamber and burns in the stream of hot air. The products of combustion, comprising a mixture of gases at high temperature and pressure are passed to the turbine.

6. Gas Turbines

Products of combustion are expanded in High Pressure Turbine (HPT) and then in Low Pressure Turbine (LPT). A 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 Systems: 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. Refer Fig. 8.

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.

7. Reheating Combustion Chamber (R.C.C.)

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.

 

2. ADVANTAGES OF GAS TURBINE POWER PLANT

1. Clean Fuel Source: Natural gas is a very suitable fuel and where this is available cheap, it is an ideal source of power in gas turbine.

2. Size and Weight of the Plant: Gas turbine plant is smaller and compact 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 a comparable diesel plant. If size and weight are the main considerations such as in ships, aircraft engines and locomotives, gas turbines are more suitable.

3. Poor Quality of Fuel: Any poor quality and wide variety of fuels from natural gas to residual oil or powdered coal can be used.

4. Low Initial Cost and Operating Cost: The initial cost is lower than an equivalent steam power plant. A thermal plant of 250 MW capacity costs about Rs. 250 crores. However, a gas turbine plant of the same capacity costs Rs. 70 crores only.

5. Water Requirement: It requires only a fraction of water compared to a steam power plant.

6. Easy Installation: Gas turbine is easy to install and installation cost is low.

7. Quick Starting: It can be started quickly, and can be put on load in a very short time.

8. Maintenance Cost: The maintenance of the plant is easier and maintenance cost is low.

9. Foundation: It does not require heavy foundation and buildings.

10. Running Speed: The running speed of the turbine (40,000 to 100,000 rpm) is considerably e large compared with diesel engine (1000 to 2000 rpm).

11. No Pollution: The exhaust of the gas turbine is free from smoke.

12. No Standby Losses: There is no standby losses in the gas turbine power plant. But, in steam power plant, these losses occur because boiler is kept in operation even when the turbine is not supplying any load.

13. Reliability in Operation: Gas turbine plant has high reliability and flexibility in operation.

14. Lubrication: The lubrication of the plant is easy. In this plant, lubrication is needed mainly in the compressor and turbine main bearing.

15. Life: Gas turbine plants have long working life compared to diesel plant.

16. Light Weight: Weight of the gas turbine plant is relatively less.

 

3. DISADVANTAGES OF GAS TURBINE POWER PLANT

1. Higher Fuel Cost: Higher fuel cost due to the use of the natural gas or oil.

2. Low Overall Efficiency: Major part of the work (i.e., 66%) developed in the turbine is used to drive the compressor. Therefore net work output of the plant is low. Therefore, th overall efficiency is low.

3. Low Thermal Efficiency: Thermal efficiency is very low in the case of simple gas turbine due to high temperature of about 450°C in the waste exhaust gases.

4. High Operating Temperature: It requires special high temperature metals and alloys for different components, because of the higher operating temperature (2000°C) in the combustion chamber and turbine.

5. Poor Part Load Efficiency: Part load efficiency is poor compared to diesel engine power plant.

6. Noise Pollution: High pitch noise due to very high speed of the order of 50,000 rpm.

 

4. APPLICATIONS

1. Peak Load Plants: Gas turbine power plants are used to supply peak loads in steam or hydro-plants.

2. Standby Plants: They are used as standby plants for hydro-electric power plants.

3. Industrial Application: They are used in industries for driving compressors and electric generators.

 4. Jet Engine Technology: Gas turbines are used in jet planes, aircrafts and ships. They are not suitable for automobiles, because of their very high speeds.

Potential in India

The use of gas turbine in the power generation industry is more recent in the last two decades) than its use in other fields (as early as in 1875). A 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.

Generation of power using oil in India is neither desirable nor economical as 50% of the oil requirement of the country is imported. But large quantity of natural gas available from Bombay High and presently detected sizeable natural gas reserves in Western Region have created the hopes to install gas turbine units during VIII Five Year Plan Period. This will also help to meet the peak power demand which is the major problem in the national power development program.

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 the second power plant of base load type of 240 MW capacity. It is located close to ONGC (Oil and Natural Gas Commission) Uran terminal.