GAS TURBINE POWER PLANT

Initially steam engines were used for power generation because of their high efficiency as compared to steam turbines, but owing to the fixed volume of a stroke, their capacity is limited. Hence steam turbines were preferred for power generation. To derive higher efficiencies from the steam turbine the concepts like re-heating, regenerative feed heating, along with higher steam parameters became necessary. The idea to have a machine where combustion takes place inside it just like an internal combustion engine so as to avoid the complications of boiler plants, regenerative feed heating etc led to the development of gas turbines for power generation.

Gas turbine is the most satisfactory prime mover in many respects.  It has reliable working, no vibration and the ability to produce large power from units of comparatively smaller size and weight.  Although modern steam turbines work on higher efficiencies, the production of high pressure and high temperature steam requires installation of bulky and expensive boiler plants.  The gasses generated in the furnace of the boiler house are utilized to produce steam and then steam is expanded in the turbine.  If the water to steam step is avoided and the gasses themselves are expanded in the turbine it will be less wasteful and simpler in arrangement.

For producing an expansion in a turbine pressure ratio must be provided, it means the working fluid must be compressed in some machine known as a compressor.  If after compression the working fluid is expanded in the turbine, then the output from the turbine will be just sufficient enough to run the compressor, if the losses are neglected.  This type of working of the turbine compressor will not pay any response to our idea of power generation.  In order to increase the expansion (work) in the turbine, i.e. to increase the output of the turbine the volume of the working fluid should be increased at constant pressure or the pressure should be increased at constant volume.  These can be done by adding heat to the working fluid. In gas turbines, higher efficiencies can be obtained if the compressor works on higher efficiency and all accessories are used.

The addition of heat is possible in the heat exchanger or combustion chamber. By introducing the combustion chamber in between the compressor and the turbine, the work output of the turbine will be more than what it is required by the compressor and thus the plant will be able to run any other machine, say, generator.  The following section will give you a brief idea about the working principle of a simple gas turbine system.

Principle of Operation:

To know how a gas turbine works we need to clearly understand the basic steps in the cycle.   The image shows the simple gas turbine cycle. Simple gas-turbine plant consists of a compressor, a combustion chamber and a turbine.  The aim is to convert the heat energy of fuel into mechanical energy.  Rotating compressor sucks in atmospheric air, pressurizes it, and forces it into the combustor or furnace in a steady flow.  Fuel forced into the combustor burns with the air, raising the temperature of the mixture of air and combustion products.  

Thus, the compressor and combustion chamber produce a high-energy working fluid.  This high-energy mixture then flows through the turbine, where it drops pressure and temperature as it does work on the moving blades.  This develops mechanical energy just like expanding steam in the more familiar steam turbine. After doing work the exhaust leaves at atmospheric pressure and high temperature.  The turbine drives the compressor rotor through a shaft and also an external load through the load coupling.

Energy Flow Diagram:

The below image shows how fuel input divides between useful output and exhaust losses.  For a simple cycle plant the fuel represents net energy input.  Work output (net useful output of the cycle) plus exhaust energy equals the fuel energy input.

Starting with the compressor we can arbitrarily take the internal energy of the entering air as zero since it plays no part in energy conversion.  Shaft work into the compressor is transferred to the air as it is pressurized to give it an energy load entering the combustor.  

Here burning the fuel releases thermal energy, further raising the air temperature to develop the maximum energy load for the cycle.  In the turbine, part of this energy develops mechanical work.  Most of this energy goes to drive the compressor. (Of the total shaft work developed by the turbine, roughly two-thirds goes to drive the compressor and one third to drive the load).  Residual energy leaves with air as it exhausts.

Advantages and Disadvantages of Gas Turbine Plants :

Gas turbine plants possess certain advantages over steam turbine plants

• They are more compact, since fuel is burnt directly in the small combustion chamber in the gas turbine rather than in a bulky boiler. Besides, a gas turbine plant has no condenser.

• They can be started and take more load quickly (in 30 Seconds to 30 minutes).

• They are simpler in design and easy to maintain.

• They consume less metal and other materials for the same capacity.

• They cost less.

• Unlike steam turbines, they require very less water for cooling. So this is more suitable for power generation, where the water scarcity exists or the water is more precious.

Though the gas turbine posses many advantages over the steam turbine, they are also inferior to steam turbine in the following respects,

• They have a lower specific power.

• They have lower efficiency at the modern state of progress (However to increase the plant efficiency the gas turbines can be used in the combined cycle).

• They have a shorter service life.

• They are more sensitive to fuel quality.