Numerical Simulation And Optimization Of Urea-SCR Catalytic Reduction On NO_x

The rapid development of the modern automobile industry has created great wealth,while brought much environmental problem. Among them, NO_x(nitrogen oxide)emissionof the heavy-duty diesel has become a major source of air pollutants. A variety oftechnology solutions have emerged at home and abroad to meet the increasingly stringentemission regulations about controlling the NO_x emission, and selective catalyticreduction(SCR) is the most effective control technology on diesel NO_x emission. Becausethere is a risk on the transport of the reduction NH3, it is replaced by use of the massfraction32.5%urea solution, which can generate ammonia in the exhaust pipe in thevehicle.Using urea solution to produced the reduction makes high design requirements aboutthe structure of the vehicles’ SCR system. Urea-SCR (Selective Catalytic Reduction ofurea system) for the diesel engine exhaust gas aftertreatment device was simulated andoptimized in this paper. On the basis of depth understanding of the response theory and thephysical and chemical processes of Urea-SCR system, the numerical model wasestablished for SCR system, and was verified the reliability of the model through the NO_xconversion rate and pressure drop of characteristics condition of the engine test. Simulationof the Urea-SCR system reveals the internal motion law, including the flow field, ureasolution evaporation, thermolysis, process of the mixture formation and selective catalyticreduction reaction. Specific contents are as follows:(1) Simulation process involving the theory and mathematical models have beensummed up basing on the understanding of the principle of SCR system and the time scaleof the physical and chemical processes. The urea thermal decomposition has beenconsidered on the basis of the computational fluid dynamics.(2) The numerical model has been verified through the conversion of nitrogen oxidesand exhaust pressure drop on the basis of experimental of emission in the engine; Sample test has made the NH3adsorption rate on the catalyst and law of temperature, concentrationof NH3and space velocity effecting on the adsorption rate has been studied from the pointof view of experiment. Conclusion provides a frame of reference for further study the ruleswithin the SCR system.(3) Three dimensional model of the Urea-SCR system was established and thesimulation results show velocity, temperature flow and low of each componentconcentration distribution. The results show that the time scale of urea droplet evaporation,thermolysis process is the same as SCR reaction, and simulation of the SCR system can notignore the droplet evaporation, thermolysis process.(4) Different spray nozzle holes and the position of nozzle effecting the catalyticconversion ratio have been studied on the basis of the three-dimensional model, andobtained some useful conclusions.