Gas turbine power plants are classified into two types. These are
Open cycle gas turbine power plant
Closed cycle gas turbine power plant
Open cycle gas turbine power plant :
A simple open cycle gas turbine consists of a compressor, combustion chamber and a turbine.
The compressor takes in ambient air and raises its pressure. Heat is added to the air in combustion chamber by burning the fuel and raises its temperature.
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:
Warm-up time - Once the turbine is brought up to the rated speed by the starting motor and the fuel is ignited, the gas turbine will be accelerated from cold start to full load without warm-up time.
Low weight and size - The weight in kg per kW developed is less.
Disadvantages:
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.
The system is sensitive to the component efficiency; particularly that of compressor.
Closed cycle gas turbine power plant :
It used air as working medium and had a useful output of 2 MW. Since then, a number of closed cycle gas turbine plants have been built all over the world and largest of 17 MW capacity is at Gelsenkirchen, Germany and has been successfully operating since 1967.
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.
Advantages:
The need for filtration of the incoming air which is a severe problem in open cycle plant is completely eliminated.
The maintenance cost is low and reliability is high because of longer useful life.
The thermal efficiency increases as the pressure ratio (Rp) decreases.
Disadvantages:
The system is dependent on external means as considerable quantity of cooling water is required in the pre-cooler.
Higher internal pressures involve complicated design of all components and high quality material is required.
Components in gas turbine power plant :
The main components of gas turbine power plants are
Gas turbines
Compressors
Combustor
Intercoolers and regenerators
Gas turbines :
The common types of turbines, which are in use, are axial flow type. The basic requirements of the turbines are lightweight, high efficiency, reliability in operation and long working life.
Large work output can be obtained per stage with high blade speeds when the blades are designed to sustain higher stresses.
More stages of the turbine are always preferred in gas turbine power plant because it helps to reduce the stresses in the blades and increases the overall life of the turbine.
More stages are further preferred with stationary power plants because weight is not the major consideration in the design which is essential in aircraft turbine-plant.
The cooling of the gas turbine blades is essential for long life as it is continuously subjected to high temperature gases. There are different methods of cooling the blades. The common method used is the air-cooling. The air is passed through the holes provided through the blade.
Compressor :
The high flow rates of turbines and relatively moderate pressure ratios necessitate the use of rotary compressors. The most commonly used compressors are
Centrifugal
Axial flow
The centrifugal compressor consists of an impeller (rotating component) and a diffuser (stationary component). The impeller imparts the high kinetic energy to the air and diffuser converts the kinetic energy into the pressure energy.
The pressure ratio of 2 to 3 is possible with single stage compressor and it can be increased upto 20 with three-stage compressor. The compressors may have single or double inlet.
The efficiency of centrifugal compressor lies between 80 to 90%.
The axial flow compressor consists of a series of rotor and stator stages with decreasing diameters along the flow of air. The blades are fixed on the rotor and rotors are fixed on the shaft. The stator blades are fixed on the stator casing. The stator blades guide the air flow to the next rotor stage coming from the previous rotor stage. The air flows along the axis of the rotor.
The kinetic energy is given to the air as it passes through the rotor and part of it is converted into pressure.
The number of stages required for pressure ratio of 5 is as large as sixteen or more.
In both types of compressors, it has been found that lowering of the inlet air temperature by 15 to 20°C gives almost 25% greater output with an increase of 5% efficiency.
The advantages of axial flow compressor over centrifugal compressor are high isentropic efficiency (90-95%), high flow rate and small weight for the same flow quantity. The axial flow compressors are very sensitive to the changes in airflow and speed, which result in rapid drop in efficiency.
Combustor :
The main function of the combustor is to provide for the chemical reaction of the fuel and air being supplied by the compressor. Combustor should fulfill the following conditions
Pressure losses must be low.
Thorough mixing of fuel and air.
Carbon deposit must not be formed.
Combustion must take place at high efficiency.
Combustion design employs an outer cylindrical shell with a conical inner sleeve which is provided with ports or slots along the length.
At the cone apex is fitted a nozzle through which fuel is sprayed in a conical pattern into the sleeve.
The combustor is located between the compressor outlet and turbine inlet.
The nozzle sprays the fuel under the pressure in an atomized conical spray. The fuel is delivered to the nozzle through the fuel line and flows out through tangential slots in the nozzle.
Intercoolers :
Intercooler is used when the pressure ratio used is sufficiently large and the compression is completed with two or more stages.
The cooling of compressed air is generally done with the use of cooling water. Cross-flow type intercooler is mostly used for effective heat transfer.
Regenerators :
In regenerator , heat transfer takes place between the exhaust gases and cool air.
It usually made in shell and tube construction with gas flowing inside the tubes and air outside the tubes. These two fluids made flow in opposite directions.
The effect of counter flow is the highest average temperature difference between the heating and heated medium with consequent high heat transfer between the two fluids.
Advantages of gas turbine power plant over diesel power plant
Less capital cost
High mechanical efficiency
The work developed/kg of air is large
Less vibration
Simple ignition & lubrication system
Low maintenance cost
The running speed of the turbine is large
Less space is enough
Poor quality fuels can be used
Disadvantages of gas turbine power plant over diesel power plant
Special cooling method are required for cooling the turbine blades.
Poor part load efficiency
Short life
Special metals and alloys are required for different components.
Advantages of gas turbine power plant over steam power plant
No ash handling problem
Less auxiliaries used
Storage of fuel is smaller so easy to handle.
Gas turbines can be built quickly
Gas turbine plants can work economically
Gas turbine plant can be installed at selected load centre as space requirement
Above 550°C, gas turbine’s thermal efficiency increases three times.
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