Experiments on a light duty SCR test exhaust system using ammonia gas to provide data for validation of a CFD model

摘要

Modelling of reactions in SCR catalysts is a challenging process, but kinetic schemes are available in the literature derived from reactor tests with specific catalysts. This paper reports experiments on a light duty common rail diesel engine and test exhaust fitted with copper zeolite SCR. The experiments aimed to provide data for CFD model validation. Four different lengths of SCR catalyst brick were investigated: 30, 45, 75 and 91 mm. The tests were undertaken for NO_2:NO_x ratios of 0.05, 0.46, 0.62 and 0.82 approximately. Three ammonia dosing levels were investigated. These were deficient ammonia, i.e. NH_3:NO_x ratio about 0.5, stoichiometric, where the ratio was near 1.0 and excess ammonia, ratio greater than 1.0. A 1D steady state CFD model using the porous medium approach was developed based on available kinetics. Most of the experiments were at low exhaust temperature around 220 °C but a case near 300 °C was also investigated. Comparison of CFD predictions with measurements showed that the kinetic scheme gave moderate predictions with a stoichiometric ammonia supply to the exhaust and an NO_2:NO_x ratio near 50% but that the model was inadequate for other conditions. There was notably more NO_x consumption than predicted for short bricks and for deficient ammonia levels. The data were obtained in a real engine exhaust at much higher space velocities than those used to derive the kinetics; this study is therefore a strong test of the kinetic model. Attempts to modify the CFD model to provide closer agreement with experimental observations are ongoing.