Interactions between nitric oxide and urea under flow reactor conditions

An experimental and theoretical study of the interactions between urea and NO under lean selective noncatalytic reduction conditions has been performed. The experiments were conducted in an isothermal quartz flow reactor at atmospheric pressure in the temperature range 700-1500 K. The influence of the temperature, oxygen concentration, and urea/NO ratio on the NO reduction has been analyzed. A reaction mechanism including literature NH3, HNCO, and moist CO oxidation subsets as well as their interactions with NO and reactions describing urea thermal decomposition have been used for calculations. The results show that urea is effective in reducing NO in a given temperature window, accompanied by the formation of an appreciable amount of N2O, which reaches its maximum value for the higher NO reduction. The impact of oxygen concentration in the 1-10% range is appreciable, and lower O-2 concentrations shift the reduction regime toward higher temperatures, the higher N2O formation being observed for the richer environment. Using urea, the onset of NO reduction is shifted to higher temperatures compared to the use of ammonia, even though the effective temperature window for NO reduction roughly coincides for both selective reduction agents. The efficiency of NO reduction and the NO to N2O conversion increase as the urea/NO ratio increases, even though at high temperatures the excess of urea can be oxidized to NO. Model predictions are in good agreement with the experimental results and indicate that the classical reaction pathway for urea decomposition (i.e., H2N-CO-NH2 --> NH3 + HNCO) is not able alone to reproduce the experimental findings obtained in the upper temperature range. This is attributed either to uncertainties of the HNCO oxidation mechanism or to the fact that other decomposition channels are likely to be produced. Results of this work as well as other literature data suggest that the method chosen for urea injection is important with respect to the N2O emissions attained. Adding urea at least partially decomposed prior to its interaction with NO results in similar NO reduction efficiencies but in considerably lower N2O concentrations.