Hey folks,

We run 4 Frame 6's (Mark VI - base load is around 37MW) with 2 steam turbines in an islanded grid with an overall power demand of about 250 MW. Each GTG / HRSG also has auxiliary firing. GTGs run in droop control. The auxiliary firing / steam is then adjusted to try & maintain the GTGs about 5-7MW below base load to avoid significant load shedding on trips. We assume that we have about 5MW IRM per machine typically hence the 5MW below base load. I am wondering whether it is feasible to run closer to base load to minimize emissions (reduce auxiliary firing) and rely on overload / peak load on the GTGs during trip events. We probably have about 10% extra capacity in terms of MW with peak load.

Any thoughts or suggestions on things to consider and whether this is even possible

 

First of all, what is IRM?

Second, do you understand that Base Load and Peak Load are very different from Part Load (i.e., Droop Speed Control)? Both Base Load and Peak Load modes try to put as much fuel into the machine as possible without exceeding the maximum allowable exhaust temperature for the respective mode, whereas Part Load (Droop Speed Control mode) is only putting in a set amount of fuel based on the difference between the turbine speed reference (TNR) and actual turbine speed (TNH)? One can load a machine using Droop Speed Control until the machine automatically goes on Base Load (CPD- or CPR-biased exhaust temperature control) at which time Droop Speed Control basically become inactive and the Mark* is then just putting as much fuel into the combustors as possible without exceeding the maximum allowable exhaust temperature (TTRX) for Base Load/Exhaust Temperature Control mode.

USUALLY (but we don't know for sure without examining the application code running in the Mark* VI at your site) selecting Peak Load is done manually by a human operator. I guess it could be done automatically, based on some input from a external control system, but I, personally, have never seen that (but I haven't seen every machine every built, either--but it would be surprising).

Most combined cycle machines I have worked on which also had auxiliary firing (duct burners) DID NOT enable auxiliary firing (duct burners) unless the gas turbine was at Base Load and the IGVs were at maximum operating angle. In fact, I don't recall any combined cycle site I worked at which used auxiliary firing unless the machine was at Base Load with the IGVs at maximum operating angle. I was always under the impression that the duct burners were used to augment (increase) steam production when the machine was already at maximum power output. In most of these cases, the gas turbine emissions were sensed/monitored BEFORE the HRSG and duct burners and any increase in emissions because of the duct burners was dealt with by other means (such as ammonia injection). It was always understood that the HRSG manufacturer wanted maximum air flow (which occurs at maximum operating IGV angle) when the duct burners were being used.

Now, I can imagine some sites have different operating modes, but if you are running duct burners and the GT is running at rated power output (Base Load) or above (Peak Load) the GT emissions are going to be high (or higher with auxiliary firing), and I don't see how emissions (particularly NOx) is going to be reduced by running the GT at Base Load or Peak Load, if you're only running the duct burners at Part Load. Either I'm not understanding something or you haven't explained how your machines are operated very well, or some combination of the two.

It would seem you are confusing stack emissions and GT emissions, and if you're trying to use the GT to limit stack emissions by reducing auxiliary firing--well, that just doesn't compute for me. In true combined cycle mode for GE-design heavy duty gas turbines when the machine is at Part Load (on Droop Speed Control) the IGVs are actually closed below normal (for non-DLN machines!) below Base Load to try to maximize GT exhaust temperature and thereby steam temperature/production. When the machine reaches Base Load the GT exhaust temperature is high and the IGVs are at maximum operating angle. At this point the overall thermal cycle of the entire plant is at its maximum/most efficient.

If more steam production is required THEN the duct burners are used to increase the heat entering the HRSG to maximize steam production. And that's going to increase stack emissions, usually, even if Peak Load is selected and being used.

This really has my head screwed up. I really can't imagine a situation where it would be desirable to run the duct burners below Base Load. Unless there is some kind of really unusual emissions guarantee or the emissions permit that was originally issued wasn't properly written/agreed to when the plant was being built. Or maybe auxiliary firing was added AFTER initial commissioning and the emissions permit wasn't updated.?.?.?

Anyway, something isn't clear (to me, at least).

Best of luck!

 

Thanks WTF! for the response. More information below.

IRM = instantaneous reserve margin. Let's say base load is 37MW. If we are running at 32MW we say the IRM is 5MW. If we are running at 30MW we have 7MW of spinning reserve but only 5MW is quick enough for us to avoid load shedding so that's what we count on in our proactive load shedding system.

When I talk about emissions I am talking about CO2 emissions not NOx (units are DLN). The LHV efficiency of duct firing is lower than running the GTGs closer to base as we get the GTG power and the free steam power. So the closer we run the GTGs to base load the less CO2 emissions we have. CO2 emissions are taxed hence the desire (we would also use less fuel).

Our plant is definitely not the norm . Load shedding has big financial implications hence why we always keep 20MW (4 GTGs x 5MW) instantaneous reserve on the GTGs up our sleeve currently in case one of the GTGs or STGs trip to help avoid under frequency and mass load shedding of critical equipment. As a result of this we are duct firing continuously to meet the overall power demand.

What we want to do is say run at 35MW (instead of 32MW) and rely say on 2MW of instantaneous pick up (8MW in total - 4 GTGs x 2) to base load + say 3 MW of overfiring per GTG (12MW total) in a trip scenario purely on frequency response for 5-10 minutes while the auxiliary firing ramps up (this is roughly how long auxiliary firing takes to respond) to bring the machine out of overfiring and back below base load.

Hope that is clearer. If not let me know

 

Thanks for the feedback though it seems to have been removed.?.?.? There are emissions, and there are NOx emissions, and there are CO emissions.

I still don't think you can rely on any automatic enabling of and loading to Peak Lead as part of any scheme you might want to try to implement.

Best of luck with your planning and development of a scheme that might be useful for the not-so-common application at your site. From your description you are kind of stuck with what's already been developed and apparently works reasonably well, though not perfectly well.