Urea decomposition, oxidation, and SNCR: The effect of CO

Urea decomposition and oxidation at high temperatures, along with its interaction with NO in the SNCR process, have been investigated through a combination of flow reactor experiments and detailed chemical kinetic modeling. The experiments were performed at atmospheric pressure using CO(NH 2 ) 2 /H 2 O mixtures, investigating the effect of adding O 2 , NO, and/or CO. The experiments were highly diluted in N 2 and the temperature was varied from 973 to 1473 K. The reaction mechanism was based on the nitrogen chemistry review by Glarborg et al. (2018), with updates to the amine, HNCO, and N 2 O subsets based on more recent studies. Thermal urea decomposition and oxidation experiments were consistent with the theoretical rate constant for gas-phase urea dissociation proposed by Honorien et al. (2021), which deviates significantly from previously used values in SNCR modeling with urea. Adding CO has a similar effect on both urea oxidation and SNCR, shifting the chemistry towards lower temperatures. In the SNCR process, the temperature for maximum NO removal shifted by up to 200 K without compromising the overall reduction efficiency. A breakdown of the plug-flow approximation was observed at elevated temperatures and high CO concentrations, attributed to reactions occurring in the preheating zone, where strong radial temperature gradients are present. Under these conditions, cylindrical shear flow reactor models and full two-dimensional simulations provided more accurate predictions. Overall, the modeling predictions were in good agreement with both the experimental data from this study and literature data.