Gas Turbine Power Plants

GAS TURBINE POWER PLANTS

A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between.

Energy is added to the gas stream in the combustor, where fuel is mixed with air and ignited. In the high pressure environment of the combustor, combustion of the fuel increases the temperature. The products of the combustion are forced into the turbine section. There, the high velocity and volume of the gas flow is directed through a nozzle over the turbine's blades, spinning the turbine which powers the compressor and, for some turbines, drives their mechanical output. The energy given up to the turbine comes from the reduction in the temperature and pressure of the exhaust gas.

LAYOUT OF GAS TURBINE POWER PLANT

The gas turbine power plants which are used in electric power industry are classified into two groups as per the cycle of operation.

(1) Open cycle gas turbine.

(2) Closed cycle gas turbine.

Open cycle gas turbine

1-   Atmospheric Air

2-   Compressed Atmospheric Air

3-   Fuel air mixture after compression

4-   Exhaust gases.

The heated gases coming out of combustion chamber are then passed to the turbine where it expands doing mechanical work. Part of the power developed by the turbine is utilized in driving the compressor and other accessories and remaining is used for power generation.

Since ambient air enters into the compressor and gases coming out of turbine are exhausted into the atmosphere, the working medium must be replaced continuously. This type of cycle is known as open cycle gas turbine plant and is mainly used in majority of gas turbine power plants as it has many inherent advantages.

Advantages

1. Warm-up time is very less.

2. Low weight and size.

3. Almost any hydrocarbon fuels can be used.

4. Open cycle plants occupy comparatively little space.

6. Very economical when compared to other plants.

7. Independent of separate cooling medium.

Disadvantages

1.  The part load efficiency of the open cycle plant decreases rapidly as the considerable percentage of power developed by the turbine is used to drive the compressor.

2. The system is sensitive to the component efficiency; particularly that of compressor.

3. The open cycle plant is sensitive to changes in the atmospheric air temperature, pressure and humidity.

3. The open-cycle gas turbine plant has high air rate compared to the other cycles.

4. It is essential that the dust should be prevented from entering into the compressor.

5. The deposition of the carbon and ash on the turbine blades is not at all desirable as it also reduces the efficiency of the turbine.

Closed cycle gas turbine

1-   Low Pressure Working Fluid @ Low temperature

2-   High Pressure Working Fluid

3- Fuel + Working Fluid mixture @ High Pressure and Temperature 4- Low Pressure Working Fluid @ Temperature T4 < Temperature T3

In closed cycle gas turbine plant, the working fluid (air or any other suitable gas) coming out from compressor is heated in a heater by an external source at constant pressure.

The high temperature and high-pressure air coming out from the external heater is passed through the gas turbine. The fluid coming out from the turbine is cooled to its original temperature in the cooler using external cooling source before passing to the compressor.

The working fluid is continuously used in the system without its change of phase and the

required heat is given to the working fluid in the heat exchanger.

Advantages

1.The closed cycle plant is not sensitive to changes in the atmospheric air temperature, pressure and humidity.

2.  The closed cycle avoids erosion of the turbine blades due to the contaminated gases and fouling of compressor blades due to dust.

3. The need for filtration of the incoming air which is a severe problem in open cycle plant is completely eliminated.

4.  Load variation is usually obtained by varying the absolute pressure and mass flow of the circulating medium, while the pressure ratio, the temperatures and the air velocities remain almost constant.

5. The density of the working medium can be maintained high by increasing internal pressure range, therefore, the compressor and turbine are smaller for their rated output. The high density of the working fluid further increases the heat transfer properties in the heat exchanger.

6. As indirect heating is used in closed cycle plant, the inferior oil or solid fuel can be used in the furnace and these fuels can be used more economically because these are available in abundance.

8. The maintenance cost is low and reliability is high due to longer useful life.

Disadvantages

1. The system is dependent on external means as considerable quantity of cooling water is required in the pre-cooler.

2. Higher internal pressures involve complicated design of all components and high quality material is required which increases the cost of the plant.

3. The response to the load variations is poor compared to the open-cycle plant.

4. It requires very big heat-exchangers as the heating of workings fluid is done indirectly.