6. Gas Turbine Power Plant

7/30/2019 6. Gas Turbine Power Plant

1/23

Er. Khem Gyanwali

Lecturer

Chapter 6

Gas Turbine Power Plant


2/23

Open Cycle Gas Turbine A gas turbine, also called a combustion turbine, is a type of

internal combustion engine. It has an upstream rotatingcompressor coupled to a downstream turbine, and a combustionchamber in-between.

Energy is added to the gas stream in the combustor, where fuelis mixed with air and ignited. In the high pressure environment

of the combustor, combustion of the fuel increases thetemperature. The products of the combustion are forced into theturbine section. There, the high velocity and volume of the gasflow is directed through a nozzle over the turbine's blades,spinning the turbine which powers the compressor and, forsome turbines, drives their mechanical output. The energy given

up to the turbine comes from the reduction in the temperatureand pressure of the exhaust gas.

Energy can be extracted in the form of shaft power, compressedair or thrust or any combination of these and used to poweraircraft, trains, ships, generators, or even tanks.


3/23

The gas turbine has experienced phenomenal progress and growth since itsfirst successful development in the 1930s. The early gas turbines built inthe 1940s and even 1950s had simple-cycle efficiencies of about 17

percent because of the low compressor and turbine efficiencies and lowturbine inlet temperatures due to metallurgical limitations of those times.Therefore, gas turbines found only limited use despite their versatility andtheir ability to burn a variety of fuels.

Applications

Gas turbines can be used for large scale power generation. Examples areapplications delivering 600 MW or more from a 400 MW gas turbinecoupled to a 200 MW steam turbine in a co-generating installation. Suchinstallations are not normally used for base load electricity generation, butfor bringing power to remote sites such as oil and gas fields. They dohowever find use in the major electricity grids inpeak shaving applicationsto provide emergency peak power.

Low power gas turbine generating sets with capacities up to 5 MW can beaccommodated in transportation containers to provide mobile emergencyelectricity supplies which can delivered by truck to the point of need.
http://www.mpoweruk.com/electricity_demand.htmhttp://www.mpoweruk.com/electricity_demand.htmhttp://www.mpoweruk.com/electricity_demand.htmhttp://www.mpoweruk.com/electricity_demand.htm


4/23

Open Gas Turbine cycle


5/23


6/23

Open gas turbine cycle is the most basic gas turbine unit.The working fluid does not circulate through the system,

therefore it is not a true cycle. It consists of a compressor,a combustion chamber and a gas turbine. The compressorand the gas turbine are mounted on the same shaft. Thecompressor unit is either centrifugal or axial flow type.

The working fluid goes through the following processes:

1-2 irreversible but approximately adiabaticcompression

2-3 constant pressure heat supply in the combustionchamber

3-4 irreversible but approximately adiabatic expansionof combustion gases


7/23

In the simplified T-s diagram, shown above, pressure loss at

compressor inlet, in combustion chamber and at turbine outlet are

neglected. Thermal efficiency of the cycle is


8/23


9/23

Closed Cycle Gas Turbine

A closed-cycle gas turbine is a turbine that uses a gas(e.g. air, nitrogen, helium, argon etc.) for the working fluidas part of a closed thermodynamic system. Heat issupplied from an external source. Such re circulatingturbines follow the Brayton cycle.

A closed Brayton cycle recirculates the working fluid, theair expelled from the turbine is reintroduced into thecompressor, this cycle use a heat exchanger to heat the

working fluid instead of an internal combustion chamber.The closed Brayton cycle is used for example in closed-cycle gas turbine and space power generation.


10/23


11/23

Ideal Brayton cycle:

Isentropic process - ambient air is drawn into thecompressor, where it is pressurized.

Isobaric process - the compressed air then runs through a

combustion chamber, where fuel is burned, heating that

aira constant-pressure process, since the chamber isopen to flow in and out.

Isentropic process - the heated, pressurized air then gives

up its energy, expanding through a turbine (or series of

turbines). Some of the work extracted by the turbine isused to drive the compressor.

Isobaric process - heat rejection (in the atmosphere)
http://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isobaric_processhttp://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Isentropic_processhttp://en.wikipedia.org/wiki/Isentropic_process


12/23


13/23


14/23

Brayton Cycle with Reheat


15/23


16/23

Compression with Intercooling


17/23


18/23

Bryaton cycle with Regenerator


19/23


20/23


21/23

Starting of Gas TurbineStarting of a gas turbine power plant requires an

auxiliary power source, till the plants owncompressor inducts air and compresses it to a

pressure such that expansion from reasonable

temp. will develop enough power to sustain

operation. The starter may be i) an I.C. engine ii)a steam turbine iii) an auxiliary electric motor or

iv) another gas turbine. It must be coupled to the

turbo-compressor shaft with a disengaging or

over-running clutch. A main generator or its directconnected exciter may be pressed into temporary

service as a motor.


22/23

Starting Procedure

1. Run the unit and induct air.

2. Actuate the combustion ignition system and

inject the fuel. The fuel flow is controlled to obtain

the necessary warm up.

3. Adjust the speed and voltage and synchronize

the alternator.

4. Build up the load on the alternator by governorgear control.


23/23

Advantages of gas turbine engines

Very high power-to-weight ratio, compared to reciprocating engines;

Smaller than most reciprocating engines of the same power rating.

Moves in one direction only, with far less vibration than a reciprocating engine.

Fewer moving parts than reciprocating engines.

Low operating pressures.

High operation speeds.

Low lubricating oil cost and consumption.

Disadvantages of gas turbine engines

Cost is much greater than for a similar-sized reciprocating engine since the materials

must be stronger and more heat resistant. Machining operations are also more

complex;

Usually less efficient than reciprocating engines, especially at idle.

Delayed response to changes in power settings.

These disadvantages explain why road vehicles, which are smaller, cheaper and

follow a less regular pattern of use than tanks, helicopters, large boats and so on, do

not use gas turbine engines, regardless of the size and power advantages imminently

available.

Documents

6. Gas Turbine Power Plant