Difference between Premix Steady State and Lean Lean extended mode

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Thread Starter

DanVal

what is the difference between Premix Steady State and Lean Lean extended mode of a Frame 9E GE Gas Turbine?
 
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sherif elzoghby

hello friend

You Have two combustion zones in your combustor - primary zone(VGC1) and secondary zone(VGC2).

AT lean lean mode (from about 40MW to 80MW) the two zones have an actual flame (the primary and secondary flame detectors are reading actual values).

At premix steady state only the secondary zone has actual flame and the primary zone had it's flame off but only fuel gas is introduced into the primary zone to good mixing before it become an actual flame at secondary zone ( early at 1090C TTRF).

this action is to reduce the NOx emission from 24ppm to about 12 ppm.
 
sherif elzoghby,

I just want to clarify a couple of things. In Primary combustion mode, there is only fuel flowing into the primary combustion zone of the combustor where it is burned using a diffusion flame. Diffusion flame is the orange-red flame we are all accustomed to seeing; it also causes very high NOx emissions because it is very hot.

When the unit transitions from Primary to Lean-Lean combustion mode, fuel is flowing into both the Primary and Secondary combustion zones of the combustor and is being burned using diffusion flames in both zones.

When the unit transitions from Lean-Lean to Premix steady state about 80% of the fuel is being introduced into the Primary combustion zone of the combustor and the remaining fuel is going into the Secondary combustion zone where a portion of the fuel is being burned in a diffusion flame. The fuel in the Primary combustion zone is a very lean fuel-air mixture which is NOT burning with a diffusion flame; there is a temperature rise in the Primary combustion zone but there is no diffusion flame. While GE likes to say there is no "burning" going on in Premix mode, where there's a temperature rise there's combustion--but it's occurring at a much lower temperature which is what limits the formation of NOx.

A portion of the fuel going into the Secondary combustion zone is also premixed and is combusting at a lower temperature than if it were burning with a diffusion flame. A portion of the fuel going into the Secondary combustion zone is actually being burned in a diffusion flame in the combustor--and this helps to reduce CO emissions and helps to keep the temperature in the entire combustor high enough to combust the fuel to produce hot combustion gases that can be expanded in the turbine section.

When a primary zone re-ignition occurs and the unit transfers from Premix combustion mode to Extended Lean-Lean combustion mode diffusion flame has been detected in the Primary combustion zone and the fuel split is changed to about a 50-50 split between the Primary- and Secondary combustion zones where it is burned with diffusion flames.

This is not good--that so much fuel is being burned in the two combustion zones in diffusion flame. One hour of operation in Extended Lean-Lean mode is equivalent to 10 hours of Premix Steady-State operation. It's very hard on the combustion liner and hot gas path components and it is strongly recommended NOT to operate in Extended Lean-Lean for very long as it can cause serious problems with the combustion liner leading to catastrophic failure(s).

Extended Lean-Lean is a combustion mode that the unit transfers to when diffusion flame is detected in the Primary combustion zone when diffusion flame should NOT be present in the Primary combustion zone. Instead of tripping the turbine when diffusion flame is detected in the Primary combustion zone when it should not be present in the Primary combustion zone the unit transfers to Extended Lean-Lean to allow the operators to troubleshoot the problem and lower load to transfer back into Premix combustion mode.

If the problem can't be solved and the unit can't reliably transfer to and remain in Premix Steady-State combustion mode then the turbine should be stopped and the cause investigated and resolved. Again, long-term operation in Extended Lean-Lean combustion mode is not recommended, and was never intended to be permitted. It was just a temporary combustion mode that was intended to prevent tripping the turbine and allow some limited troubleshooting and attempts to return to Premix Steady-State combustion mode without stopping or tripping the turbine.

It is the operator's responsibility to take appropriate action if the unit cannot be returned to Premix Steady-State combustion mode--meaning, the Speedtonic will NOT shut down or trip the turbine if it is operated for extended periods of time in Extended Lean-Lean. (It should--but it doesn't, at this time.) It's almost NEVER a controls problem that causes primary zone re-ignitions and transfers to Extended Lean-Lean combustion mode. And, unfortunately, it almost always requires a shutdown to troubleshoot and resolve--even if it is a controls problem.

So, the major points are: Diffusion flame in the Primary combustion zone above about 40% load (above Primary combustion mode) causes high emissions and can lead to serious combustion liner problems and catastrophic failure. Extended Lean-Lean combustion mode is only intended to allow operators and technicians to attempt to troubleshoot and resolve the cause of primary zone re-ignition and return to Premix Steady-State operation without having to re-start the turbine after a trip caused by a primary zone re-ignition event or events. Long-term operation (more than a few hours) in Extended Lean-Lean combustion mode is NOT recommended. If the unit cannot be reliably returned to Premix Steady-state combustion mode it should be shut down and the cause investigated and resolved.

Hope this helps!
 
<b>!!!CORRECTION!!!</b>

>Diffusion flame in the Primary combustion zone
>above about 40% load (above Primary
>combustion mode) causes high emissions
>and can lead to serious combustion liner
>problems and catastrophic failure.

That should have read:

"Diffusion flame in the Primary combustion zone while operating in Extended Lean-Lean mode causes high emissions and can lead to serious combustion liner problems and catastrophic failure."

Whenever there is diffusion flame in the Primary combustion zone--either in Primary or Lean-Lean or Extended Lean-Lean mode--the emissions are high. But it's when there's diffusion flame in the Primary combustion zone during Extended Lean-Lean combustion mode that combustion liner problems and catastrophic failure can occur.
 
Comparison of Firing Temperature Vs NOx Emission in case of Natural Gas and HSD Fuel from Gas Turbine Operation, need to clarify that when firing temperature increases and reach at maximum level, NOx difference is also at the peak between RLNG and HSD Fuel
 
engr.salman58,

So, this thread is about GE DLN-I combustion principles.

As I know, HSD (High Speed Distillate; Diesel Fuel; #2 Distillate; etc) does not burn in Premix flame in a DLN-I combustor. It's pure diffusion flame. So, the hotter the calculated flame temperature (because that's what TTRF (or TTRF1) is: CALCULATED firing temperature, NOT actual firing temperature) the more NOx is going to be produced when burning HSD.

As far as the correlation between TTRF/TTRF1 and NOX when burning RLNG, it's accepted practice that NxX emissions will be highest when TTRF/TTRF1 is highest, since NOx formation is a function of flame temperature (flame temperature and firing temperature is not exactly the same temperature; one is the temperature of the "flame ball" (the mass of burning fuel) and the other is the temperature of the hot combustion gases leaving the first stage turbine nozzle).

So, in writing this I have come to the conclusion that NOx will be highest for either fuel when firing temperature is highest. Again--because NOx formation is a function of combustion temperature more than any other factor. (It's important to also remember that liquid fuels have fuel-bound nitrogen which also can contribute to NOx formation, as I'm told.)

NOx is reduced or limited when burning natural gas-based fuels by reducing the firing temperature (and the flame temperature)--neither of which is actually measured and reported. When burning liquid fuels, there is no Dry Low NOx combustion process for liquid fuel, and so water injection is usually used to reduce NOX formaton, and the amount of reduction is much less than with a DLN combustion process when burning gas fuels.

Hope this helps!
 
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