Investigation on new structure of Selective Catalyst Reaction coated Diesel Particulate Filter for optimising DeNOx

Selective Catalyst Reduction (SCR) is the most promising technology to reduce NOx emissions from conventional diesel engines and other lean combustion engines. Traditional after treatment system in the majority of diesel vehicles includes a Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF) and a Selective Catalytic Reduction (SCR) catalyst. As emission control by regulations is tightening every year and the demanding for better efficiency, a combination of Selective Catalytic Reduction (SCR) catalyst and Diesel Particulate Filter (DPF) have been researched for several years. The combination catalyst is actually putting an SCR coating on and into DPF’s porous walls so that this combination catalyst (SCR coated DPF or SCR-DPF) can be used to filter diesel particulates and reduce NOx at the same time. The benefit of this combination catalyst system is obvious: the compactness of SCR-DPF reduces the weight, the cost and the complexity of the after treatment system; more importantly, it reduces the engine’s back pressure losses as one of the catalysts is removed from the after treatment system. Despite those benefits, SCR-DPF has some drawbacks. There are studies claiming that the NOx conversion efficiency of SCR-DPF is lower than traditional SCR catalyst and more challenging to control NOx conversion process during the regeneration process due to high temperature and so on. In this research, a new catalyst structure is proposed to solve one of the SCR-DPF limitations. This structure provides large extra surface catalytic area for NOx conversion and it is free from ash loading effect as the extra surface islocated in the outlet channel. And based on the results, for the same NOx conversion rate, the catalyst size can be reduced to half if using new structure. This is owning to the extra surface area in the new structure. In this thesis, 3 three-dimensional catalyst models on channel scale, a Flow-Through (FT) catalyst, a Wall-Flow (WF) catalyst and a Wall-Flow catalyst with Fin (WF-Fin) in outlet channel are proposed and built in order to investigate the cause of performance difference between traditional Flow-Through SCR, Wall-Flow SCR and Wall-Flow SCR with fins. The model of Flow-Through SCR is acted as a benchmarksince it is built and validated against Olsson’s experimental research work. A later model of Wall-Flow type catalyst is modified to Flow-Through SCR model by changing the geometry. The results from the Flow-Through SCR model follows Olsson’s experiment results closely for a wide temperature range under steady and transient conditions, that indicates the successful modelling of the base model. The comparison between WF and WF-Fin model has been made from many different aspects, such as velocity, species composition and reaction rates. The WF-Fin model is focused in the later investigation, the fins bring more active site surface for SCR reactions, but it also has adverse effects to the flow in porous walls if the fin is impermeable as a solid material. The effect of different performance factors requires more studies. But based on the current results, a conclusion can bedrawn for the difference between these three types of SCR catalyst, and the possible causes of performance difference are identified.