Effect of NO ₂ on Gas-Phase Reactions in Lean NO _x /NH ₃ /O ₂ /H ₂ O Mixtures at Conditions Relevant for Exhaust Gas Aftertreatment

This combined numerical simulation and experimental study focuses on the effect of nitrogen dioxide (NO₂) in strong diluted oxygen-rich NO_x/NH₃/O₂/H₂O gas mixtures representative at exhaust gas aftertreatment in diesel engines. Conversion of nitrogen oxides (NO_x) and ammonia (NH₃) and the change of the NO/NO₂ and NH₃/NO_x ratios were analyzed by means of comparison between experimental measurements and kinetic simulations. Homogeneous gas-phase experiments have been carried out in a special countercurrent continuous flow reactor at intermediate temperature (473-923 K) and elevated pressure up to 5 bar. In the simulation with an ideal plug-flow model, the occurring reactions in the gas mixture were calculated. To explain the observations, a reactions flow analysis was also performed. The experimental and modeling results revealed that despite a high NH₃ conversion, the NO_x concentration stays nearly constant in the presence of NO₂. The NO₂ reduction by amino (NH₂) radicals producing NO was found to be the temperature-dependent chain-branching step between the competitive NO and NO₂ reduction reactions. The kinetic modeling results show that the experimentally observed higher NH₃ conversion at elevated pressure is a superposition of the direct effect of the elevated pressure and the effect of the pressure-caused longer residence time at a constant mass flow rate. The NO/NO₂ and NH₃/NO_x ratios are significantly changing with temperature increase by the elevated pressure. The observed changes of the gas mixture composition, especially of the NO/NO₂ and NH₃/NO_x ratios due to homogeneous gas-phase reactions, will consequently influence the efficiency of NO_x conversion catalysts in the exhaust gas aftertreatment systems. Therefore, a numerical prediction of the gas-phase reactions between the injection point of the urea-water solution and the inlet of the selective catalytic reduction (SCR) catalyst could contribute to the further development of the dosage strategy of the urea-water solution in SCR systems, when operated close to the engine or even upstream the turbocharger.