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Thermal power plants
Transcript of Thermal power plants
Fuel burns releasing heat to obtain a gas stream to a temperature as much as possible uniform in condition requested form turbine.
This task must be carried out with the possible minimum of pressure leaks. What is Combustors ? Components The case is the outer shell of the combustor, and is a fairly simple structure.
The case is protected from thermal loads by the air flowing in it, so thermal performance is of limited concern.
Casing serves as a pressure vessel that must withstand the difference between the high pressures inside the combustor and the lower pressure outside. 1.Case 2.Diffuser The purpose of the diffuser is to slow the high speed, highly compressed, air from the compressor to a velocity optimal for the combustor. The liner contains the combustion process and introduces the various airflows (intermediate, dilution, and cooling, see Air flow paths below) into the combustion zone.
The liner must be designed and built to withstand extended high temperature cycles. For that reason liners tend to be made from superalloys like Hastelloy X.
liners must be cooled with air flow 3.Liner The snout is an extension of the dome.
Acts as an air splitter,separating the primary air from the secondary air flows (intermediate, dilution, and cooling air). 4.Snout The dome and swirler are the part of the combustor that the primary air (see Air flow paths below) flows through as it enters the combustion zone.
Their role is to generate turbulence in the flow to rapidly mix the air with fuel 5.Dome & Swirler The fuel injector is responsible for introducing fuel to the combustion zone and, along with the swirler.
Responsible for mixing the fuel and air.
There are four primary types of fuel injectors; pressure-atomizing, air blast, vaporizing, and premix/prevaporizing injectors 6.Fuel Injector Most igniters in gas turbine applications are electrical spark igniters.
similar to automotive spark plugs. The igniter needs to be in the combustion zone where the fuel and air are already mixed. 7.Ignitors
It is the main combustion air coming from the compressor
This air is mixed with fuel, and then combusted. Primary air
Intermediate air is the air injected into the combustion zone through the second set of liner holes (primary air goes through the first set).
This air completes the reaction processes, diluting the high concentrations of carbon monoxide (CO) and hydrogen as the results of combustion. Intermediate air
Dilution air is airflow injected through holes in the liner at the end of the combustion chamber to help cool the exhaust before it reaches the turbine stages.
The air is carefully used to produce the uniform temperature profile desired in the combustor and to avoid hotspots.
However, as turbine blade technology improves, allowing them to withstand higher temperatures, dilution air is used less, allowing the use of more combustion air. Dilution air
Cooling air is airflow that is injected through small holes in the liner to generate a layer (film) of cool air to protect the liner from the combustion temperatures.
The implementation of cooling air has to be carefully designed so it does not directly interact with the combustion air and process.
In some cases, as much as 50% of the inlet air is used as cooling air. Cooling air Types of Combustors Can Combustor Each "can" has its own fuel injector, igniter, liner, and casing
Multiple cans are arranged around the central axis of the engine, and their shared exhaust is fed to the turbine. Advantages.
1. Simple design, making it widely spread.
2. Easy to maintain, as only a single can needs to be removed, rather than the whole combustion section.
1.Weighs more than alternatives.
2.High pressure drop across the can comparing to other designs. Annular combustor Annular combustors don't use the separate combustion zones
simply have a continuous liner and casing in a ring (the annulus). Advantages
1. Including more uniform combustion.
2. Shorter size (therefore lighter).
3. Tend to have very uniform exit temperatures.
4. Lower pressure drop.
5. Most modern engines use annular combustors; likewise, most combustor research and development focuses on improving this type.
1. More Expensive.
2. Harder maintenance due to complex design Performance Requirements High combustion efficiency:
This is necessary for long range.
Combustion must be free from blowout at airflows ranging from idle to maximum power.
Low pressure loss:
High pressure losses will reduce thrust and increase specific fuel consumption. Uniform temperature distribution :
The average temperature of gases entering the turbine should be as close as possible to the temperature limit of the burner material to obtain maximum engine performance.
Avoiding hot spots:
High local temperatures or hot spots in the gas stream will reduce the allowable average turbine inlet temperature to protect the turbine. Emissions Standard oxygen is recommended in exhaust.
NO2 up to 150 ppm.
SO2 up to 150 ppm.
CO up to 500 ppm.
HC up to 75 ppm.15% Air Flow Combustor Design parameters Cross Sectional Area:
The combustor cross section is determined by a reference velocity appropriate for the particular turbine.
Combustor length must be sufficient to provide for flame stabilization.
The typical value of the length – to – diameter ratio for liner ranges from three to six.
The minimum pressure drop is up to 4%.
Volumetric Heat Release Rate:
The heat-release rate is proportional to combustion pressure. Modifications An afterburner is an additional component added to some jet engines, primarily those on military supersonic aircraft.
•Providing a temporary increase in thrust, both for supersonic flight and for takeoff
•On military aircraft the extra thrust is also useful for combat situations.
This is achieved by injecting additional fuel into the jet pipe downstream of the turbine and combusting it.
•Significant increase in thrust.
•Very high fuel consumption , though this is often regarded as acceptable for the short periods during which it is usually used. After Burner Improve gas turbine engine performance by using an interstage turbine burner (ITB), or as we call it reheat.
locate ITB in transition duct between high pressure turbine (HTP) and low pressure turbine (LPT)
Smaller and lighter.
Reduce CO2 emissions.
Reduce NO2 emissions by reducing peak flame temperature Intra Turbine Burner Combustor Maintenance