In GT what is the imp of Exh Back Pressure? Once Fuel gas burns in Combustor, How much energy of the FG in the form of pressure (temp very high) but how it impacts operation. What diff t makes if Simple Cycle and Combined Cycle.

 

Says that your combustion temperature is 1300 degC and your normal exhaust pressure and temp are 1bar deg 500C. Your rated generator output is 100MW.

Assuming thermal efficiency of Bryton cycle as 33%, then available energy across the high temp and low temp reservoirs, i.e. 800degC (1300-500degC) is approximately 305MJ/s. Additional 10MJ/s may account for electrical & mechanical losses.

Now you can approximate that total energy availability across the two temp reservoirs as <pre>
Available heat energy = 305MJ/s

Temp gradient = 800degC

Heat energy constant =305/800= 0.38MJ/Deg C</pre>
Using heat energy constant you can APPROXIMATE non convert heat energy. For example if your new GT exhaust temp increases by 50degC, then the non convert heat energy becomes<pre>
Non convert heat energy = 50*0.38= 15MJ/s </pre>
Total output reduction can be calculated by multiplying non convert heat energy with the expected thermal efficiency. Thus <pre>
Output reduction = 0.33* 15= 5MW</pre>
For CCGT you expect GT calculation remains the same but for each 3MW non convert GT heat, about 1MW will be recovered by your ST via HRSG.

All the calculations are close approximations only. Using the same calculations in fact you can determined the true values by using the actual enthalpies drops across your expansion turbine.

 

Calculation does not explain Back Pressure which I have no knowledge so Is it possible to explain in simple terms?

 

Imagine exhaling. Imagine exhaling with a dust mask over your nose and mouth.

Imagine exhaling with a tight-fitting dust mask over your nose and mouth.

Imagine exhaling with two tight-fitting dust masks over your nose and mouth.

Imagine exhaling with four tight-fitting dust masks over your nose and mouth.

The free flow of exhaust is just as important as the free flow of inlet air. A dirty inlet air filter will restrict air flow into the turbine's axial compressor, and high exhaust duct back pressure will also restrict air flow through the turbine.

The length of the exhaust duct, its overall area, its internal configuration, and the height of the stack all contribute to the amount of back pressure on the gas turbine exhaust.

When internal sheet metal in an HRSG (Heat Recovery Steam Generator, "waste heat boiler") comes loose in the exhaust duct, and insulation from behind the sheet metal comes loose it can get caught in the tubing/piping in the HRSG and increase the back pressure on the gas turbine exhaust.

Some gas turbine exhausts use ammonia injection to reduce emissions and excessive ammonia injection can increase the back pressure on the gas turbine exhaust.

Measuring exhaust duct back pressure can be problematic.

 

Thanks for simple and conceptual explanation CSA

I do appreciate.

 

> Calculation does not explain Back Pressure which I have no knowledge so Is
> it possible to explain in simple terms?

Let's try this way

Assuming you have a tank that is located 100m above its outlet that is connected to a 100m vertical pipe. The outlet is at the bottom of the pipe. Next, open the outlet tap and allows it pointing in a horizontal direction. Measure horizontally how far the water jet can reach. The is analogy to your normal back pressure of your GT.

Now, let us raise the the outlet tap by 10m. Open it again. Measure how far the water jet horizontal distance. You will find the distance will become shorter. This is analogy to increase back pressure of your GT.

Why? When you raise the tap elevation, part of available energy associated with a quantity of water that is placed at elevated position above the zero energy datum cannot be converted to kinetic energy. Therefore, exit velocity of the water jet will reduce. Thus, its horizontal reach will become shorter. Almost similar thing happens to your GT output when its back pressure increases. The turbine absorbs less energy although the total heat input remains the same.

Another way we can understand it is as follows. A fraction of heat energy that cannot be converted to useful power by the turbine will end up building up pressure of the GT exhaust diffuser.

 

Back Pressure at GT exhaust depends on resistance offered on flow by the HRSG (Tubes of different sections, Superheaters, Evaporators, Ecomoniser..& stack), To maintain the hot air flow, always the back pressure at Gas Turbine exhaust (Diffuser) will be more than resistance offered by HRSG. Normally there will be protection trip of Gas turbine on high back pressure as diffuser section of GT can not sustain high pressure (@45mbar).

Lower the back pressure higher the output of GTG.

In case of simple cycle operation, back pressure will be less so Power output will be more with same fuel input.

 

AntiHumming, an PLC-Idea, works like antiBlocking at your car break. You loose efficiency.

At present, the GuD-Burner is under research.

Pressure-Capability similar to rocket engines should be achieved by a sequence of gasflame plasma heater expandmedium f.i. water/steam.
mfg

VITUS