Operation Theory of Gas Turbine Power Plant

GAS TURBINE POWER PLANT

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.

Theory of operation

Gasses passing through an ideal a gas turbine undergo three thermodynamic processes. These are isentropic compression, isobaric (constant pressure) combustion and isentropic expansion. Together these make up the Brayton cycle.

In a practical gas turbine, gasses are first accelerated in either a centrifugal or radial compressor. These gasses are then slowed using a diverging nozzle known as a diffuser, these process increase the pressure and temperature of the flow. In an ideal system this is isentropic. However, in practice energy is lost to heat, due to friction and turbulence. Gasses then pass from the diffuser to a combustion chamber, or similar device, where heat is added. In an ideal system this occurs at constant pressure (isobaric heat addition). As there is no change in pressure the specific volume of the gasses increases.

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.

Closed cycle gas turbine power plant

In this, the compressed air from the compressor is heated in a heat exchanger (air heater) by some external source of heat (coal or oil) at constant pressure. Then the high pressure hot gases expand passing through the turbine and mechanical power is developed. The exhaust gas is then cooled to its original temperature in a cooler before passing into the compressor again.

The main difference between the open and closed cycles is that the working fluid is continuously replaced in open cycle whereas it is used again and again in a closed cycle. The open cycle plant is much lighter than the closed cycle. Hence it is widely used.

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