Hi all,

We would like to achieve a maximum part load efficiency on our CCGT and at the same time maintain our maximum output to respectable values when needed. Of course, we also have to respect the allowed NOx emissions.

As our SCR is over-sized, we were wondering if we could reduce the water mass flow injected in the combustion chamber of our LM6000 PG Sprint so that we would increase our part load efficiency, for instance with a water to fuel (FOD) ratio (WFR) below 0.4.

However, when power output would be needed, we would revert to the original WFR above 1.

According to your experience, do you think GE would accept to implement in the DCS a WFR control optimizing the GT heat rate at part load and the power output at base load?

If so, what WFR ranges have you so far encountered? Is for instance a 0.4-1 range to large?

Kind regards,
Cedbont

 

Cedbont,

It would be very interesting know how the plant operating mission has changed since construction/commissioning. The thermal efficiency of aeroderivative gas turbines isn't really the reason one buys/operates them. Some LM6000s were sold as 40-40 machines--40 MW at 40$ efficiency, but that was the overall thermal efficiency of the plant which included inlet air chillers and other auxiliaries. And the 40-40 was only possible at Base Load.

There are many other considerations, when taken as a whole with efficiency, that make aeroderivative gas turbines a better choice for some plants. But, when plant operating parameters and objectives change, it's not so easy to adapt the main equipment to the new operational requirements, and that's just life. You are applauded for making the effort, but do remember to take into account emissions and equipment "ownership" and take all the factors into account.)

The answer really has to come from GE--especially if they own the engine(s)--meaning you lease them from GE; or the replacement unit is under warranty from GE.

Operating any kind of thermal power plant has inherent inefficiences at some operating point(s). In general, the control system tries as best as possible within the confines of the equipment--and any emissions requirements--to maximize efficiency at most load points.

Today, with so many power plants operating at part load--when they were originally designed and built to operate at Base Load--more and more people are looking for ways to squeeze a few more kw/BTU at part load. And, the machines just weren't designed to be operated like that, and it's very likely that emissions requirements may be violated.

Ammonia slip can and many times is part of the emissions requirement, and it costs money too (ammonia, that is). Most catalysts operate best at higher temperatures, so part load isn't always the best operating region for many SCRs (Selective Catalytic Reduction systems).

Your best possible answer is to work with GE to achieve the optimal conditions for emissions, parts life and plant efficiency. The plant designer (architect/engineer) may also be a source of assistance with your inquiry.

Please write back to let us know what you learn--and how you proceed.

 

Dear all,

Unfortunately, we have the experience that once the plant is commissioned, which is the case, GE is not that proactive in studying improvements or implementing them on under-warranty aero-derivative machines (the story may be different with heavy-duty). Still, the request was sent to GE offices but we do not expect a constructive answer for at least a few weeks.

Therefore, I was rather looking for someone who had the experience of tweaking the WFR control with the load and/or ambient conditions and if so up to what sort of range.

Lastly, you ARE right to point out emissions and life time of the machine (combustion system) as other aspects that also may set boundaries for that change, boundaries that can be investigated respectively with tests and with the OEM.

Kind regards,
Cedbont

 

Cedbont,

Do you have actionable data showing the actual, running part load efficiency of the plant versus the Base Load efficiency of the plant? We would be very interested to hear how much lower the efficiency of your machine is at Part Load versus Base Load, and how much water you believe it would take to increase the efficiency of your machine at Part Load to it's maximum (by which it's presumed you mean the same efficiency at Base Load).

But, good, hard ("actionable") data is required. And the spreadsheet being used to tally all of these numbers needs to take into account the cost of buying, storing, and treating the water to be injected (because it has to be boiler quality water--which isn't cheap to make!).

Have you done this test? Start and load the unit to, say, 30 MW, (approximately 75% load) WITHOUT Pre-Selected Load Control enabled and with water injection OFF and allow the machine to stabilize (warm up) for about 20-30 minutes or so. THEN watch the power output of the machine AND the fuel flow-rate as water injection is turned on and reaches the required injection flow-rate to meet emissions guarantee. You will see the power output of the machine decrease FOR THE SAME AMOUNT OF FUEL.

The effect of injecting a diluent (water or steam) into a combustion system to reduce the formation of NOx is to quench the hot combustion gas temperature--it's only by reducing the temperature inside the combustor that NOx formation can be reduced (on a combustion engine). This is done either with water or steam, or with LOTS of air (as in DLN (Dry Low NOx) combustion which premix lots of air with fuel to "burn" at low temperatures). Cold combustion gas temperatures entering the turbine section reduce the ability of the turbine to develop torque from the combustion gases--even though the mass flow rate through the machine increases due to the injection of water (or steam).

It's a common myth that because, if a machine operating stably at Base Load without water (or steam) injection and then water (or steam) injection (for NOx reduction) is started that the power output increases "for free." It doesn't. If you watch the fuel flow-rate before water (or steam) injection is started at Base Load versus the fuel flow-rate after water (or steam) injection is started at Base Load you will see the fuel flow-rate increases. Yes, the mass flow-rate at Base Load increases due to the water (or steam) injection, and yes the power output at Base Load increases, but that's because at Base Load the machine is trying to control temperature, not fuel flow-rate (as it is at Part Load).

This means that if you want to return to the same load (30 MW in our example) you would have to increase the fuel flow-rate to get back to 30 MW--no matter how much water (or steam) is being injected. Inject more water? Then to get back to 30 MW at Part Load (again--without Pre-Selected Load Control enabled) it would be necessary to inject more fuel.

There's no such thing as a free meal. I've seen the after-effects of sites turning on Off-Line Water Wash at Part- or Base Load for hours just to get a couple of extra MW--and it cost in the millions of USD to repair the damage that resulted. Sure, it was just water--but it's the effects of that water that weren't properly considered. (And the Plant Managers (I use that term loosely) at two of those plants were extremely angry the Speedtronic didn't block their technician from forcing the logic to run Off-Line Water Wash when the unit was running! Yes--it was the control system's fault for not preventing someone from forcing logic to start Off-Line Water Wash when the unit was on line <i>under the direction of the Plant Manager.</i>)

If you have concrete data for plant efficiency at Part Load vs. Base Load, and you can make the case that the cost of buying, storing and treating the additional water that will be injected into the turbine (and lost and gone forever up the stack and into the atmosphere) to increase the "efficiency" (by which you really mean power output for the given fuel flow-rate) then you should share that data with GE and ask them for their thoughts on this for your particular machine (combustors; exhaust components; fuel nozzles; etc.). Because if you have good data GE are always interested in ways to increase the power output of their machines--because that helps them sell machines.

Hope this helps! If you have good data, please write back with it; a lot of people reading these threads would be very interested to see and analyze it.