Hi,
By what means air flow increase inside the combustor to match the fuel increment when loading up after ttxm>371?

All bleed valves were already closed and IGV were fully opened.

 

Maythem,
Yes, the axial compressor is running at a constant speed when synchronized to a well-regulated grid. But, the IGVs on most GE-design Frame 9E heavy duty gas turbines do change with load--opening to increase air flow. (The compressor bleed valves are only open to prevent stalling/surging during acceleration to and deceleration from rated speed (some turbines equipped with DLN-I combustors don't close the IGVs until after breaker closure, but the basic concept is still the same).

The majority of air in a gas turbine with conventional, diffusion flame combustors is used for cooling and dilution of the combustion gases downstream of the flame ball--and in any heavy duty gas turbine there is a LOT of excess air (above that required for combustion). In any combustion process there is a range of fuel-air mixtures which will sustain good combustion, if not ideal combustion. It's worth noting that most heavy duty gas turbine performance is guaranteed only at Base Load--which is when the IGVs are fully open and the air-fuel mixture is at optimum for the optimum combustion efficiency. This is when the heat rate (BTU/MW) is best, meaning that combustion is ideal at this point.

At part load, heat rate and efficiency are not usually guaranteed probably just because the ideal fuel-air mixture can't be maintained at all loads. But, again--there is a range of acceptable fuel-air ratios for good, stable combustion and that's how combustion systems are designed: to maintain good, stable combustion during loading and unloading, and to maximize combustion efficiency (heat rate) at Base Load when the fuel-air mixture is best.

For units equipped with DLN combustors, it's similar--just that the IGVs are an integral part of trying to maintain the proper fuel-air mixtures at part load to maintain stable "combustion" (the OEM likes to say there is no "combustion" going on in Premix mode, but there is a temperature rise and in my book when there's a temperature increase when fuel is passing through a combustor there's combustion, albeit it might not occur with a diffusion flame). Part-load operation is also not occuring at ideal fuel-air mixtures in DLN-I combustors, and in fact, the heat rate is probably a little worse at part load for these machines.

Your reference to TTXM > 371 is not clear. I'm going to guess the turbine is equipped with conventional, diffusion flame combustors (one fuel nozzle per combustor), and is a simple-cycle application (meaning the gas turbine exhaust flows directly to atmosphere, not through a HRSG (Heat Recovery Steam Generator--a "boiler"). The last presumption (simple cycle application) is made because that's pretty early for the IGVs to be fully open for a combined cycle machine with IGV temperature control off. And, DLN-I machines would keep the IGVs closed until the exhaust temperature was much higher.

Again, while the fuel-air ratio is not ideal at part load, it is still sufficient to maintain stable combustion--and performance (heat rate) is not usually guaranteed at anything below Base Load.

Hope this helps!

 

Dear CSA,
Sorry for not being clear enough; and it was a good guess from your side, it is indeed a simple cycle/one nozzle per combustion chamber unit. You see what i meant was in our unit TTXM reached 371degC at around 50 MW (depending on atmospheric temp this value varies).

Anyhow after TTXM=371 is reached the IGV starts to move from 57 deg to 86 deg and remain at that angle whenever the ttxm is above 371 degC. Now, my question was: After the IGV were fully open and before that bleed vlvs were already close during 52G closure, and let's say we loaded up the unit from 60MW (already IGV=86 degC/fully open) to 100 MW thats means more fuel (achieved by bypass valve closing more) and consequently more air should be drawn to provide a proper mixture with the increment of liquid fuel(combustion air). So, how could that be done? providing that bleed valves were already closed, IGV were already fully open, and axial speed are fixed to ~3000 rpm.

thank you in advance,
Maythem

 

Maythem,

I believe you are suffering from a common belief that gas turbine control systems are always measuring and optimizing air/fuel (or fuel/air) ratios at all times during all phases of operation, and that's just not true.

Again, there are ranges of air/fuel (fuel/air) mixtures which result in perfectly acceptable combustion but which are not ideal (perfectly stoichiometric), and it's this fact of combustion stoichiometry which allows the IGVs to be fully open (and the compressor bleed valves to be closed) and fuel flow-rate to be varied over the load range of a gas turbine.

Even an automobile with a carburetor or some automobiles with fuel injection systems do not continuously monitor and adjust fuel/air (air/fuel) mixtures for all operating conditions. (Newer automobile fuel control systems do--but it requires a fair amount of compute horsepower and many sensors and variable control elements.)

Tuning a carbureted internal combustion engine (which a heavy duty gas turbine is--an internal combustion engine) is a highly subjective process, and has some variability. Without sophisticated sensors and equipment--which most internal combustion engines, including most heavy duty gas turbines, don't have--it's only possible to achieve a level of combustion that is acceptable given the adjustable parameters available.

So, this means that the fuel/air (air/fuel) mixture may be a little lean at times or that it may be a little rich at times--but when the unit is at Base Load is when all of the systems and components and the control system are designed to provide complete and optimal combustion and stoichiometry for optimal performance. That doesn't mean the unit won't operate at Part Load, it just means that it's likely not as efficient at Part Load because the combustion stoichiometry isn't optimized for Part Load operation--because the unit performance guarantee isn't valid at Part Load.

So, when the axial compressor speed is fixed and the IGVs are full open (at Maximum Operating Angle) the air flow doesn't change very much from Part Load to Base Load--you are correct. But, what does change is the air/fuel (fuel/air) mixture in the combustor and it's at optimum when the unit is at Base Load, and likely has some excess air when at Part Load which means that combustion isn't ideal but there is still combustion which is producing hot combustion gases which are being expanded through the turbine section to produce torque which is being applied to the generator which the generator is converting to amperes to supply load.

Is the combustion perfect at all operating conditions? No. Is it optimal for the machine at Base Load? Yes. Does the unit run when the combustion isn't ideal? Yes.

 

Thank so much CSA for your support, if you may i have a lot of questions to be asked, shall i feel free to ask them?