Nox level high


Thread Starter


Dear Gents,

We are having GE frame 7FA DLN 2.6 Gas turbines, 99% of the time we are running on base load

We are experiencing at typical problem, the scenario is like this:

Both GT were running on base load. in the morning shift we received a call from Fuel gas supplier (natural gas) that are performing fuel gas line scrapping activity. however they opened the blowdown at their end to remove any moisture and any contaminants in the fuel gas.

Scrapping activity was going on since morning shift.

in the evening shift suddenly we observed step load change from 140 MW to 155 MW in 10seconds!!!

we have observed increased in machine vibration, increased NOX values, change in fuel gas composition, flame intensities increased from 65% to 104 %, change in spread values

Please find the below parameters

Changes in fuel gas composition were noticed and major changes were occurred in Methane & Ethane composition. Both compositions were out of ECA limit (Methane- 76.4% & Ethane 11.6%)

Methane - 90% to 60%
Ethane - 3.8% to 33%
Changes in vibration profile and GT flame intensity were also observed Under normal condition
GT #1/2 vibration (max) - 2.4/3.2 mm/sec
During the sales gas disturbance
GT #1/2 vibration (max) - 7/11 mm/sec

Normal condition flame intensity
GT#1/2 flame intensity (max) –65 to 80%
During disturbance
GT#1/2 flame intensity (max)- 103-104%

Changes in GT Nox Emissions was noticed
Normal condition
GT1# 12 PPM, GT#02 15 PPM

during disturbance
GT#01 69.5 PPM, GT#02 74.8 PPM

1. we have moisture separator at battery limit to remove any moisture & dirt from fuel gas, moisture separator filter DP found increased from 0.4 Bard to 0.7 Bard

2. There was no change observed in fuel gas duplex strainer dp (upstream of SRV)

3. After the disturbance all parameters came to normal except the Nox values. it is still remaining on higher side, CO and O2 values also became normal (CO# 13 PPM, O2# 12.2 %)

4.GT#01 Spread values became normal (SP1# 35, SP2# 33, SP3# 40)

5. GT#02 spread in fact improved!!!! before the disturbance SP1# 52, SP2# 46, SP3# 45. after disturbance SP1# 43, SP2# 35, SP3# 35

6. Vibration also became normal

7. there was no black smoke observed from chimney,

8. Online NOx analyzer, CO analyzer & O2 analyzer values compared with portable analyzer and both values are matching

I have given suggestion to reduce the GT load below 90 M (where mode changes from 6Q to mode 5Q) and raise again and observed the Nox values whether there is a change or not.

Can you please give us your expert advice on above subject?

Appreciate your help in this regard


I presume "scrapping" is scraping, where the interior walls of pipes are cleaned using an abrasive method to remove deposits and such.

One would think scraping (or scrapping; it may be called different things in different parts of the world) would release bits of debris into the gas stream, some of which might not be caught by filters and or strainers.

Those bits might cause blockages in the smaller fuel nozzle orifices.

Flow imbalances caused by nozzle blockages can cause high NOx, increased dynamic pressure pulsations which can manifest themselves as increased vibrations if high enough.

GE have many diagrams and flow-charts for DLN combustion problems. Yours being an F-class unit it would be very unusual for the site not to have some kind of Control Services Agreement or Remote Monitoring & Diagnostics service which would allow you to avail yourself of the troubleshooting tools GE has for their F-class fleet DLN combustion systems.

Please write back to let us know what is the result of the troubleshooting.
Dear CSA,

We have contacted GE, but GE is advising us to replace few thermocouples, I wonder replacing the thermocuples will not help Nox level going down, for further trouble shooting we have done following things

1. Bring down the load of hte machine from base load to 75 MW (where DLN 2.6 mode changes from 6Q to 5Q diffusion mode), NOx level increased up to 118 PPM, O2 level no change observed (12.29%), CO level no significant change observed (19.5 PPM to 21.46 PPM)

2. after analyzing the data again we increased the machine load to base load, but still Nox value remaining on higher side (40 PPM)

We are planning to do borescope inspection end of this month to check the Combustion hardware

I will update you once the inspection done

Best regards

Dear CSA,

we did the boroscope inspection, so far we have inspected 5 combustion chambers and out of 5 we found 3 combustors having damaged nozzles ( very badly) and also one of the combustor liner top part found missing,

This concludes that we must trip teh machine if such situation occurs or we should shutdown immediately

The conclusion is change in fule gas composition (heavy hydro carbon like ethane) caused the combustor hard ware damage,

can you please explain why heavy hydrogen carbon damages the nozzles, are thy contain high energy? or do they easily convert in to liquid when there is a drop in pressure across control v/Vs?


Thanks for the reply. But, I'm not a combustion engineer. You should ask these questions of GE, and you would do well to have some samples of the gas fuel you were supplied after the "scrapping".

Something seems really wrong with the fuel you received for some period of time.

Please let us know what GE says.
Dear KKK

In combustion, there are 3 major role to have complete combustion. There are fuel, ignition and oxygen. I would to know either do you have any analyser before the combustion happen. It might be cause of over heated the nozzle.

Best Regard