Modelling and Optimization of SCR-Exhaust Aftertreatment Systems

This paper presents a modelling approach to the design optimization of Selective Catalytic Reduction (SCR) systems. The present study is concerned with ammonia slip and conversion efficiency of oxides of nitrogen (NOx), which are two major issues of SCR technologies.

A Computational Fluid Dynamics (CFD) code is employed to simulate the mixing characteristics with the purpose of optimization of the concentration distribution of the reducing agent. The physical processes including urea spray atomization, droplet evaporation, urea decomposition and turbulent mixing are accounted for in the modelling method. The Lagrangian discrete phase model is used to describe the urea spray, which contains sub-models for droplet breakup and evaporation. A reaction model of urea decomposition is proposed. The geometry of a specific example includes two air-assisted fluid nozzles, optimized mixing elements of the static mixer, and the SCR converter with two layers of substrates. The mal-distribution index of the ammonia at different cross sections is predicted and compared with that of a base case. In addition, the velocity distribution and pressure drop in the SCR converter are analyzed with the consideration of flow resistances of the catalyst substrates and perforated plates. The flow distribution is optimized using the concept of boundary layer control. Even though the scale of the SCR assembly considered here is one more suited to a stationary system, certain observations and conclusions made here such as spray modeling, urea decomposition modelling and flow modeling are equally applicable to a mobile SCR system and suggestions are made in the paper on how these observations could be applied to an automotive system.