Gas Turbine Power Plant - WordPress.com...Introduction A generating station which employs gas turbine as the prime mover for the generation of electrical energy is known as a gas turbine

GAS TURBINE POWER

PLANT

Dr. Sikder Sunbeam Islam,

Dept. of EEE, IIUC

Introduction

A generating station which employs gas turbine as the prime mover for the generation of electrical energy is known as a gas turbine power plant.

Working Principle

In a gas turbine power plant, air is used as the working fluid. The air is compressed by the compressor and is led to the combustion chamber where heat is added to air, thus raising its temperature.

Heat is added to the compressed air either by burning fuel in the chamber or by the use of air heaters.

The hot and high pressure air from the combustion chamber is then passed to the gas turbine where it expands and does the mechanical work.

The gas turbine drives the alternator which converts mechanical energy into electrical energy.

Advantages & Disadvantages

Advantages It is simple in design as compared to steam power station

since no boilers and their auxiliaries are required.

It is much smaller in size as compared to steam power station of the same capacity.

The initial and operating costs are much lower than that of equivalent steam power station.

It requires comparatively less water as no condenser is used. The maintenance charges are quite small.

Gas turbines are much simpler in construction and operation than steam turbines.

It can be started quickly form cold conditions.

Advantages & Disadvantages (Continues)

Disadvantages There is a problem for starting the unit. It is because

before starting the turbine, the compressor has to be operated for which power is required from some external source. However, once the unit starts, the external power is not needed as the turbine itself supplies the necessary power to the compressor.

Since a greater part of power developed by the turbine is used in driving the compressor, the net output is low.

The overall efficiency of such plants is low (about 20%) because the exhaust gases from the turbine contain sufficient heat.

The temperature of combustion chamber is quite high (3000oF) so that its life is comparatively reduced.

Schematic Arrangement of Gas

Turbine Power Plant (GTPP)

The main components of the plant are :

Compressor

Regenerator

Combustion chamber

Gas turbine

Alternator

Starting motor

Compressor The compressor used in the plant is generally of

rotatory type.

The air at atmospheric pressure is drawn by the compressor via the filter which removes the dust from air.

The rotatory blades of the compressor push the air between stationary blades to raise its pressure. Thus air at high pressure is available at the output of the compressor.

Two types: Centrifugal Compressor and Axial Compressor

Schematic Arrangement of GTPP (Continues)

Schematic Arrangement of GTPP:

Compressor

Fig.1.Centrifugal Compressor Fig.2. Axial Compressor

Impeller= Rotating part

Centrifugal Compressor:

The centrifugal compressor consists of an impeller (rotating component) and a diffuser (stationary casing).

When impeller rotates the pressure in the region R falls and therefore air enters through EYE. Then the air flows out radially through impeller and diffuser blades [In Fig.1].

The impeller imparts the high kinetic energy to the air and diffuser converts the kinetic energy into the pressure energy.

Axial flow Compressor

The axial flow compressor consists of a series of rotor and stator stages with decreasing diameters along the flow of air.

Rotors are fixed with rings of blades on the shaft and the stator blades are fixed on the stator casing. [In Fig.2, RB=Rotor blades, SB=Stator blades]

The stator blades guide the air flow to the next 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.

Schematic Arrangement of GTPP: Compressor(Continues)

Schematic Arrangement of GTPP: Compressor(Continues)

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.

Limitations:

The axial flow compressors are very sensitive to the changes in airflow and speed, which result in rapid drop in efficiency.

Regenerator:

A regenerator is a device which recovers heat from the exhaust gases of the turbine.

The exhaust is passed through the regenerator before wasting to atmosphere.

A regenerator consists of a nest of tubes contained in a shell.

The compressed air from the compressor passes through the tubes on its way to the combustion chamber.

In this way, compressed air is heated by the hot exhaust gases.

Schematic Arrangement of Gas

Turbine Power Plant (Continues)

Schematic Arrangement of Gas

Turbine Power Plant (Continues)

Fig.3. Schematic Arrangement of GTPP

Combustion Chamber:

The air at high pressure from the compressor is led to the combustion chamber via the regenerator.

In the combustion chamber, heat is added to the air by burning fuel. The oil is injected through the burner into the chamber at high pressure to ensure atomization of oil and its thorough mixing with air.

The result is that the chamber attains a very high temperature (about 3000oF). The combustion gases are suitably cooled to 1300oF to 1500oF and then delivered to the gas turbine.

Gas Turbine:

The products of combustion consisting of a mixture of gases at high temperature and pressure are passed to the gas turbine.

These gases in passing over the turbine blades expand and thus do the mechanical work. The temperature of the exhaust gases from the turbine is about 900oF.

Schematic Arrangement of Gas

Turbine Power Plant (Continues)

Alternator.

The gas turbine is coupled to the alternator. The alternator converts mechanical energy of the turbine into electrical energy.

The output from the alternator is given to the bus-bars through transformer, circuit breakers and isolators.

Starting motor.

Before starting the turbine, compressor has to be started. For this purpose, an electric motor is mounted on the same shaft as that of the turbine.

The motor is energized by the batteries. Once the unit starts, a part of mechanical power of the turbine drives the compressor and there is no need of motor now.

Schematic Arrangement of Gas

Turbine Power Plant (Continues)

OPEN CYCLE GTPP

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.

This type of cycle is known as open cycle gas turbine plant (in Fig.4) and is mainly used in majority of gas turbine power plants as it has many inherent advantages.

Fig.4.Open cycle gas turbine

plant

CLOSED CYCLE GTPP

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. The arrangement of the components of the closed cycle gas turbine plant is shown in Fig. 5.

Fig.5.Closed cycle gas turbine plant

Difference Between Open and Closed

Cycle GTPP

Open Cycle GTPP Closed Cycle GTPP

1. In this turbine the gases

exhausted into the atmosphere

from the turbine itself.

In this type of cycle the gases

from the turbine enters into the

cooling chamber and used again.

2. It has less thermal efficiency

than closed type.

It has more thermal efficiency.

3. Loss occurs in working fluid as

the cycle is open.

No loss occurs in working fluid.

4. Low cost of maintenance. It has high-cost efficiency.

5. It is easy to control the load. It is difficult to control the load.

6. This open cycle is operated at

lower pressures.

This is operated at higher

pressure.

Combined Cycle GTPP

In combined cycle gas turbine, a gas turbine generator generates electricity and waste heat is used to make steam to generate additional electricity via a steam turbine [Fig.6].

Gas turbine burns fuel

The gas turbine compresses air and mixes it with fuel that is heated to a very high temperature. The hot air-fuel mixture moves through the gas turbine blades, making them spin.

The fast-spinning turbine drives a generator that converts a portion of the spinning energy into electricity.

Heat recovery system captures exhaust.

A Heat Recovery Steam Generator (HRSG) captures exhaust heat from the gas turbine that would otherwise escape through the exhaust stack.

The HRSG creates steam from the gas turbine exhaust heat and delivers it to the steam turbine.

Steam turbine delivers additional electricity.

The steam turbine sends its energy to the generator drive shaft, where it is converted into additional electricity.

This combination cycles exhibit higher efficiency.

Combined Cycle GTPP(continues)

Fig.6.Combined Cycle GTPP

Few Terms and Definition

References

1. Principles of Power Systems,

V.K.Mehta & Rohit Mehta

2. Power Plant Engineering, G.R.Nagpal

3. Power Station Engineering

&Economy, William A Vopat