CFD Online Discussion Forums - How to calculate NH3 conversion during urea injection by FLUENT

Hello everyone,
For the first time, I am posting my query here. Kindly assist me by sharing your ideas.
My topic of research is on SCR aftertreatment system using urea injection technique.
Inlet -->Exhaust gases (673K)
Injection --> Urea (300K- wet combustion method)
The objective of my validation work lies in the analysis of urea injection flow behavior inside the uniform flow channel using wet combustion model. Due to temperature difference of exhaust gases and injected urea, urea evaporates to give NH3 and HNCO [CO(NH2)2 --> NH3+HNCO] . In order to eliminate the NOX emission from the exhaust gases of the diesel engine, NH3 is implemented inside the exhaust system of diesel engine. In my current project, NOx emission is ignored for the initial case. Thus, I am in need to calculate the evaporated NH3 from injected urea in the form of NH3 (species) conversion. NH3 conversion calculation is the key. There is no default tool in Fluent to calculate the species (NH3, Urea) conversion. Thus, the results for species conversion can be obtained through formula manipulation. So, I am getting bit difficult to calculate it during my simulation.
As per the reference of Choi et al. (2015), I tried implementing the formula,
NH3 conversion efficiency= (Amount of NH3 generated)/(Amount of NH3 when urea completely decomposes)

where, Amount of NH3 generated = Taken as mole fraction of NH3 obtained for every time step, and
Amount of Nh3 when urea completely decomposes = Maximum concentration of NH3 obtained from mole fractions.
With such approach, the results are not convincing (Image in the attachment).
Thus, my results for “AMMONIA CONVERSION” didn’t goes well with the experimental [Kim et al. (2004)] and numerical works [Birkhold et al. (2007)]. I executed my simulation without any external UDF. On the whole, I would like to know how the species conversion needs to be calculated by FLUENT during the flow simulation (through surface or volume monitor or custom field function or any other technique which I am not sure).

Kindly share your ideas with me.
Thank you

Ref : C. Choi, Y. Sung, G. M. Choi, and D. J. Kim, Numerical Analysis of NOx reduction for compact design in marine urea-SCR system, Int. J. Nav. Archit. Ocean Eng. (2015) 1020-1033.

Birkhold, F., Meingast, U., Wassermann, P., and Deutschmann, O., “Modeling and Simulation of the Injection of Urea-Water-Solution for Automotive SCR DeNOx-Systems,” Applied Catalysis B: Environmental, Vol. 70, pp. 119~127, 2007.

J.Y. Kim, S.H. Ryu, J.S. Ha, Numerical prediction on the characteristics of spray-induced mixing and thermal decomposition of urea solution in SCR system, in: Proc. 2004 Fall Technical Conference of the ASME Internal Combustion Engine Division, Long Beach, California USA, 2004[/SIZE]