Gas Turbine Power Plant
● Gas turbines are increasingly being used in power plants both in the utility and power sectors for their compactness, tremendous energy producing capacity, inherent flexibility, operational reliability, high performance and multiple fuel capability.
● Today, gas turbines have found wide acceptance in a variety of applications ranging from the petrochemical industry to the sophisticated aircraft industry.
● The gas turbine has low weight to power ratio hence gas turbines are extensively used to drive aviation system of all kind in aircraft system. It is also being increasingly used in land vehicles such as bus and truck and also to drive locomotives and marineships. In oil and gas industries, the gas turbine is widely employed to drive auxiliarieslike compressor blower and pumps.
● The gas turbine obtain its power by utilization the energy of a jet of burnt gases and air, the velocity of the jet being absorbed as it flows over several rings of moving blades which are fixed to a common shaft. The gas turbine requires an air compressor which usually driven off its own shaft.
● This absorbs a considerable proportion of the power produced and thus lower the overall efficiency.
Fuel used to work gas turbine :
1) Coal gas
2) Producer gas
3) Blast furnace gas
4) Oil and pulverized coal5) Kerosene
6) Paraffin
7) Diesel
Classification of Gas Turbine :
a) Continuous combustion or constant pressure type
b) The explosion or constant volume type.
c) Open cycle and closed cycle gas turbine (cycle operation)
Constant Pressure Open Cycle Gas Turbine :
● The compressor takes in ambient (atmospheric) air and compressed it to raises its pressure and temperature and delivered to combustion chamber where fuel is automises through an injector and burned,
● This combustion occures nearly at constant pressure Due to the combustion heat is added to the working fluid and its temperature is raised.
● This product of combustion from the combustion chamber are passed through turbine where expand and pressure falls to atmospheric pressure and develop mechanical energy.
● The part of the power developed by the turbine must be utilized to drive engine accessories as well as compressor as turbine and compressor are mechanically coupled.
● So the network is equal to the difference between the workdone by the turbine and the work required to drive the compressor.
Closed Cycle Gas Turbine :
The idea of closed cycle gas turbine plant was originated and developed in Switzerland in the year 1935
Working :
● In closed cycle gas turbine fuel is not brought in direct contact with compressed gas, but the working gas coming out of compressor is indirectly heated at constant pressure in heat exchanger by an external heat Source.
● The external heat source may be gas cooled nuclear reactor or the gases resulting from the combustion chamber or furnace.
● First gas is compressed in compressor which increases temperature and prestere, this compressed gas is passed through the beat exchanger. where heat is added at constant pressure, the compressed gas leaving the heat exchanger is passed through the gas turbine, the expansion of gas takes place and reduce pressure and temperature, then exhaust gas is passed through pre cooler before it cutes the compressor to repeat cycle of operation.
● Due to the expansion of gas in turbine, a mechanical work is obtain (net power output) at turbine shaft.
● The net power output is the difference of turbine works and compressor work because of the turbine and compressor are mechanically coupled. To start the system operation, compressor is provided with starter and after starting the turbine starter is cutoff.
● The working gus may be air, helium, argon, carbon di exide and so on.
● The gases other than air. having more desirable property can be used to improve the performance Performance of gas turbine depends on the ratio of specific heat of working gas which in turn depends on the nature of the gas. The thermal efficiency of the of gas turbine power plant increases as pressure ratio increases. High pressure ratio is possible in closed cycle gas turbine plants for same maximum and minimum temperature limit.
Advantages of Closed Cycle Turbine Over Open Cycle Turbine :
1) The use of higher pressure through to the cycle results in reduced size of the plant for given output.
2) The closed cycle avoids erosion and corrosion of turbine blades
3) Solid fuel can be used more economically as indirect heating in use in closed cycle.
4) The maintenance cost is low and reliability is high due to longer useful life.
5) The filtration of the incoming air is eliminated in the closed cycle which is serve problem in open cycle.
6) By maintaining the lowest pressure of the eycle above atmosphere, the density of the working thuid improves the heat transfer in heat exchanger, therefore the size of the compressor, turbine and heat exchanger can reduced.
7) Loss due to fluid friction is less due to higher Reyholds numbers
8) The regulation is more simple.
9) Monoatomic gases such as helium may be used to increase efficiency.
Disadvantages of closed cycle :
1) The weight of the system per kW developed is high compared with open cycle, therefore it is not economical for moving vehicles.
2) A very large air heater is required compared with the combustion chamber used in open cycle because the heat transfer coefficient in exchanger is low,
3) It is necessary that the system must be air tight when the working fluid other than air is used in the system under initial high pressure. This increase the installation cost.
4) Large amount of cooling water is required in cooler, therefore, its use limited to the stationary installation or marine used.
