NOx vs Excess Air (or excess oxygen)

[PeterPiper]

I am looking at the combustion in a reformer. I suspect that the excess air into the convective section is far too high as the excess oxygen in the stack is 5%. We also have high NOx which would seem to confirm this.

I have been looking for a plot of excess air vs NOx concentration in the stack to see if the ammount of excess air I have calculated would produce the corresponding amount of NOx. I realise that these will be fuel and probably system specific but at leat I will know I am in the right ball park. The system runs on natural gas,UK, and there is no fuel nitrogen.

Does anyone have a referance for such a plot or any good combustion books that might cover the subject.

Thanks,

Pete.

 

[Flareman]

The precise amount of NOx generated in any furnace tends to be secific to the burner and furnance specific so there may not be any curves of the type you want unless someone with a similar furnace has done the empirical work.
In generic terms, the NOx produced is generated predominantly in and around the flame itself and the localized temperature affects it greatly. The techniques of NOx reduction are mainly aimed at reducing these local temperatures. Obviously I don't know how you are making your measurements but I recommend that you make assessement based on mass rather than volume because any excess air in the volume calculation can mask the true characteristics of NOx generation.
If you are measuring high NOx in the convection bank do you know that the air is coming through the burner or could it be leakage. If you have cold air leakage which is inadvertently lowering a flue gas control temperature, it could be actually raising the temperature of the combustion zone, which would push up the NOx production.
What is happening with the combustion zone temperature and tube temperature? Have you done a heat balance to see whether the temperatures make sense?
You may find a guide to representative NOx values in EPA referenece AP-42, which you will be able to get from the web at

http://www.epa.gov/ttn/chief/ap42.

In general however, these regulatory numbers tend to have proportionality to the total heat release rather than the local temperature conditions.
Good luck
David

 

[25362]

Pete, there is an article in the Hydrocarbon Processing issue of November 1996: "Predict NOx from gas-fired furnaces" by Kunz, Smith and Adamo that may interest you and show to be applicable to your particular case.