Numerical Optimization of a SCR System Based on the Injection of Pure Gaseous Ammonia for the NOx Reduction in Light-Duty Diesel Engines

Selective Catalytic Reduction (SCR) systems are nowadays widely applied for the reduction of NOx emitted from Diesel engines. The typical process is based on the injection of aqueous urea in the exhaust gases before the SCR catalyst, which determines the production of the ammonia needed for the catalytic reduction of NOx. However, this technology is affected by two main limitations: A) the evaporation of the urea water solution (UWS) requires a sufficiently high temperature of the exhaust gases and b) the formation of solid deposits during the UWS evaporation is a frequent phenomenon which compromise the correct operation of the system. In this context, to overcome these issues, a technology based on the injection of gaseous ammonia has been recently proposed: In this case, ammonia is stored at the solid state in a cartridge containing a Strontium Chloride salt and it is desorbed by means of electrical heating. In this work, an after-treatment system based on the injection of gaseous ammonia in the SCR system is considered. Numerical 1D and 3D CFD simulations are applied in order to optimize the NOx reduction process. In particular, CFD methodology is applied to study in details the process of injection of the gaseous ammonia in the main exhaust gas stream and the effectiveness of the mixing process. Different geometrical layouts are compared to evaluate their performances in terms of uniformity of the NH3 distribution across the inlet section of the catalyst and pressure drop introduced in the exhaust line. Moreover, a 1D simulation tool is applied to evaluate the performances of the entire exhaust after-treatment system. The 1D model is calibrated on the basis of the information coming from detailed CFD simulations, in particular for what concerns the modeling of the effects of the different mixer geometries in terms on NH3 distribution. In this case, a simplified 3D catalyst is simulated, to take into account the maldistribution of ammonia at the inlet cross section and to evaluate its impact on the global deNOx performance of the system for different dosing strategies and for different levels of the ammonia maldistribution.