PART 1- Principle, configuration , fuels ,
applications.
PART 2- working of plant and parts
Blog on gas turbine power plant (Part 1- Principle,
configuration , fuels , applications).
Gas Turbine Working Principle
Gas
turbine engines derive their power from burning fuel in a combustion chamber
and using the fast flowing combustion gases to drive a turbine in much the same
way as the high pressure steam drives a steam turbine.
It is a positive feedback mechanism. As the gas turbine speeds up, it also causes the compressor to speed up forcing more air through the combustion chamber which in turn increases the burn rate of the fuel sending more high pressure hot gases into the gas turbine increasing its speed even more. Uncontrolled runaway is prevented by controls on the fuel supply line which limit the amount of fuel fed to the turbine thus limiting its speed.
The gas
turbine is comprised of three main components: a compressor, a combustor, and a
turbine. The working fluid, air, is compressed in the compressor (adiabatic
compression - no heat gain or loss), then mixed with fuel and burned by the
combustor under constant pressure conditions in the combustion chamber
(constant pressure heat addition). The resulting hot gas expands through the
turbine to perform work (adiabatic expansion). Much of the power produced in
the turbine is used to run the compressor and the rest is available to run
auxiliary equipment and do useful work. The system is an open system because
the air is not reused so that the fourth step in the cycle, cooling the working
fluid, is omitted.
Gas turbines
have a very high power to weight ratio and are lighter and smaller than
internal combustion engines of the same power. Though they are mechanically
simpler than reciprocating engines, their characteristics of high speed and
high temperature operation require high precision components and exotic
materials making them more expensive to manufacture.
Electrical Power Generation
In electricity generating applications the turbine is used to drive a synchronous generator which provides the electrical power output but because the turbine normally operates at very high rotational speeds of 12,000 r.p.m or more it must be connected to the generator through a high ratio reduction gear since the generators run at speeds of 1,000 or 1,200 r.p.m. depending on the AC frequency of the electricity grid.Turbine Configurations
Gas turbine power generators are used in two basic configurations
1. Simple Systems-
consisting of the gas turbine driving an electrical power generator.
2.Combined
Cycle Systems- which are designed for maximum efficiency in
which the hot exhaust gases from the gas turbine are used to raise steam to
power a steam turbine with both turbines being connected to electricity
generators
Turbine Performance
Turbine
Power Output
To
minimize the size and weight of the turbine for a given output power, the
output per pound of airflow should be maximized. This is obtained by maximizing
the air flow through the turbine which in turn depends on maximizing the
pressure ratio between the air inlet and exhaust outlet. The main factor
governing this is the pressure ratio across the compressor which can be as high
as 40:1 in modern gas turbines. In simple cycle applications, pressure ratio
increases translate into efficiency gains at a given firing temperature, but
there is a limit since increasing the pressure ratio means that more energy
will be consumed by the compressor.
Fuels
One further advantage of gas turbines is their fuel flexibility. They can be adapted to use almost any flammable gas or light distillate petroleum products such as gasoline (petrol), diesel and kerosene (paraffin) which happen to be available locally, though natural gas is the most commonly used fuel. Crude and other heavy oils and can also be used to fuel gas turbines if they are first heated to reduce their viscosity to a level suitable for burning in the turbine combustion chambers.Applications
Gas turbines can be used for large scale power generation. Examples are applications delivering 600 MW or more from a 400 MW gas turbine coupled to a 200 MW steam turbine in a co-generating installation. Such installations are not normally used for base load electricity generation, but for bringing power to remote sites such as oil and gas fields. They do however find use in the major electricity grids in peak shaving applications to provide emergency peak power.Low power gas turbine generating sets with capacities up to 5 MW can be accommodated in transportation containers to provide mobile emergency electricity supplies which can delivered by truck to the point of need.
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