Ammonia slip in SCR unit, any calculation possible?

[AnnoDomini2]

Hi everyone!
In the plant where I just moved as energy and utilities engineer, they asked me to try to assist with the boilers we use to produce steam. We have really low NOx limits, and we make us of a SCR unit. The catalyst has never been changed since 2016, and they would like to know if I can take a track of the activity of the catalyst, maybe in terms of ammonia slip.
I am a bit struggling with that, since we inject ammonia based on the NOx concentration in the gas inlet of the SCR unit, which of course depends on the type and amount of gas we are burning in the specific moment. We have measurement of how much ammonia we inject, NOx concentration in, NOx concentration out. So the idea would be to calculate the NH3 out of the stack and track it somehow over time in a comparable way ( similar gas burnt over time, similar steam production, etc..).
To do that, someone before me tried to do stoichiometrically, but not knowing exactly how much NO and how much NO2 is included in the NOx( we guess 95% NO) and also we do not know which of the several reactions possible occur in any moment, the results were not useful.
I tried to use a formula I found online, attached here.

But I came up with an ammonia slip of around 500 ppm when it should be lower than 5 ppm, so I might have done units mistakes or some mistake in assumptions..

Anyone with experience on something similar or who has any suggestion?

Thanks in advance!

 

[crshears]

Don't double post; please delete either this thread or the other one.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[AnnoDomini2]

Sure, I will keep this as I think is in a more appropriate section

 

[georgeverghese]

So, since you are operating boilers, this formula is relevant to an exit 3% dry basis of O2 in the stack gas? Is this what you had in the stack gas at the time of the test? Its not clear what this NH3@O2 means.