User shares best practice for reducing NOx in-house at CCUG

It’s only partly facetious to say that every molecule of NOx avoided in California means something. A representative of a plant with four LM6000 units reviewed its innovation program for reducing startup NOx emissions by 40%. That’s significant, because startup NOx mass can be three times that of a full operating hour at full load. Reducing NOx emissions has the welcome byproduct of increasing operating time for each unit, at least in that state.

“We developed this program ourselves because the OEM wasn’t interested in working with us,” he told attendees at the Combined Cycle Users Group’s (CCUG) 2017 Annual Conference, held in Phoenix the last week of August.

After implementing the obvious steps of tightening up maintenance, tuning up the system, and upgrading the NH3 curve, the key to the program was to focus on the many points in the startup sequence when you wait—such as warmup, breaker synch, SCR coming up to temperature, combustion entering the “zone,” and water injection coming on. The specific steps proved to be:

      • Reduce the SCR/NH3 injection interlock temperature.

      • Lower the SCR temperature NH3 block interlock.

      • Lower the NH3 injection heater temperature interlock.

      • Alter the NH3 motive heating scheme.

      • Add a megawatt-based feedforward capability to the normal NOx-ppm feedback loop and fuel feedforward process control loops.

As one example of the reasoning, the system had been programmed to wait for the SCR catalyst to reach 550F, before starting NH3 injection. Discussion with the supplier suggested 450F was acceptable. However, ammonia salts vaporize at 310F, so the owner’s team agreed that 350F would be acceptable. Essentially, the mods were based on rethinking the process control scheme to iron out much of the design margin so it more closely reflects the current operating mode of the units—that is, more time in startup mode than anticipated in the original design.

When the mods were concluded, operators measured the startup NOx for the other three units in a range of 6.3–7.2 lb/hr; for the modified unit, it was 4.0 lb/hr.

And they aren’t done. Coming mods include shortening the NH3 heater “clamp” by 50%, programming warm standby on the NH3 heater, reducing turbine warmup times and shortening breaker synch wait times and the turbine purge period. The mods can be extended to the other three units.

Posted in CCUG |

Comments are closed.