Modeling And Optimization Of Urea-SCR Exhaust Aftertreatment Systems

In the last few decades, the environmental effects of nitrogen oxides (NOx) from combustion sources have become increasingly serious, especially from motor vehicles. The diesel and lean-burn spark ignition gasoline operate with a high air-fuel ratio, thus allowing fuel-economy improvements. With the continued growth in motor vehicles and energy, the development of a DeNOx technology for vehicles equipped with lean-burn engines represents the main challenge of automotive emission control in the forthcoming years. The Selective Catalytic Reduction (SCR) of NOx by ammonia is currently considered the most effective technology for high NOx removal from lean-burn diesel engines. Ammonia in pressurized containers represents a safety risk when carried onboard a vehicle. A eutectic solution of 32.5% urea in water is used instead since they can be decomposed in the vehicle's exhaust gas system to form ammonia.Urea is a preferred reducing agent for automotive applications. Nevertheless, the essential thermal decomposition of urea into NH3 and carbon dioxide makes the deNOx process and design of automobile SCR system complicated. An integrated computational fluid dynamics (CFD) methodology could not only assist the design of efficient and optimized SCR systems, but also reduce the number of design loops and development expenses. In this paper, a modeling approach to design optimization of SCR system with the aid of CFD coupled with chemical reaction dynamics is present including evaporation of urea-water-solution, subsequent thermal decomposition and surface catalytic reactions in the monolith. The main contents of this paper are as follows.(1) The principle and time scales on working processes of the SCR system are studied. The relevant theory and mathematical models are summarized for modeling of automobile SCR system. In order to achieve simulation of complete processes of the mobile diesel SCR system, kinetic models of urea thermal decomposition and standard SCR reaction are coupled to a two-phase flow model.(2) A one-dimensional steady model is set up to describe the performance of an ideal SCR monolithic reactor such as NO conversion efficiency and NH₃ slip. According to analysis of experimental results obtained on the diesel test stand and calculated results, kinetic data of SCR reaction are chosen for the commercial catalyst. The time needed for SCR reactions at different temperatures are calculated when NH3 is abundant relative to NO_x (n(NH₃)/n(NO_x)≥1) . Dependence of NO conversion efficiency on n(NH₃)/n(NO_x) and space velocity at different temperatures are studied on the one-dimensional steady model. The conclusions from these researches can be used to optimize design of urea supply strategy and selection of catalytic converters.(3) A detailed three-dimensional numerical modeling of Urea-SCR systems, integrating the characteristics of urea spray, the mixture of chemical species, and the complex turbulence mixing flow pattern, is discussed in this study to predict the performance of mobile diesel SCR system. The Simulation result shows a good agreement with experimental data. The effects of urea thermal decomposition rate constant, initial particle diameter and exhaust flow rate on the calculation results are examined to further verify reasonableness of the model. The results illustrate the droplet propagation in the mobile diesel SCR system, evaporation, urea decomposition and SCR reaction profile with flow and concentration distribution in an injection cycle. The time scales of evaporation of urea-water-solution droplets, subsequent thermal decomposition and surface catalytic reactions in the monolith are analyzed. The results indicate evaporation of droplets and urea thermal decomposition can't be ignored in modeling of mobile SCR systems. A parameter to evaluate the effects of design changes on performance of SCR systems is proposed.(4) The effects of structures of exhaust pipes, mixer devices, injector location and configuration on the uniformity of the ammonia concentration distribution at the entrance of the SCR monolith as well as the concentration of reducing agent have been evaluated in the three-dimensional numerical simulation under various engine loads of a heave-duty diesel engine. The guideline for optimum design of mobile SCR systems is proposed. The calculated results show that evaporation of droplets, urea thermal decomposition and mixing process have strong impact on the performance of mobile SCR systems.