Effect of NO₂ on Gas-Phase Reactions in Lean NOₓ/NH₃/O₂/H₂O 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 NOx/NH3/O2/H2O gas
mixtures representative at exhaust gas aftertreatment in diesel
engines. Conversion of nitrogen oxides (NOx) and ammonia
(NH3) and the change of the NO/NO2 and NH3/NOx ratios
were analyzed by means of comparison between experimental
measurements and kinetic simulations. Homogeneous gasphase
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 NOx 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
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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/NOx 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/NOx ratios due to homogeneous
gas-phase reactions, will consequently influence the
efficiency of NOx conversion catalysts in the exhaust gas aftertreatment
systems. Therefore, a numerical prediction of the
gas-phase reactions between the injection point of the ureawater
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.