Numerical Simulation Of Reaction Mechanism For Diesel NO_x Reduction Over Zeolite SCR Catalyst

NOx emission from diesel engines is one of the contributors for air pollution. Vehicles emission regulations, which are becoming more and more stringent, have promoted the development and application of NOx emission control technologies for diesel engines. Many engine and aftertreatment technologies have been put forward to reduce the NOx emission of diesel engines, and one of the most promising aftertreatment technologies is Selective Catalytic Reduction (SCR). How to broaden the working temperature window of SCR and improve its NOx conversion at low and high temperature has always been concerned by researchers.Deeper understanding of NOx reduction process over catalyst is the precondition to improve the NOx conversion of SCR system. One dimensional global kinetics models were developed over Cu- and Fe-zeolite catalysts which have been widely used in urea-SCR aftertreatment technologies. NH3 adsorption/desorption, NH3 oxidation, NO oxidation and NOx conversion reactions were taken into account in these models. These models were calibrated with experiments, and results show that the developed models satisfactorily predict the trends of species concentrations and conversion. A comparative study of the performance of Cu- and Fe-zeolite catalysts was carried out based on the calibrated models and results show that Cu-zeolite catalyst has a higher NH3 storage capacity and is more active in NH3 oxidation reaction, while Cu- and Fe-zeolite catalysts have comparable NO oxidation capacities. When the temperature is below 400 ?, Cu-zeolite catalyst is more active in standard SCR reaction while the Fe-zeolite catalyst exhibits higher NOX conversion at higher temperatures. The influence of inlet NO2/NOX ratio on NOx conversion was investigated and found that Fe-zeolite catalyst is more sensitive to NO2/NOx feed ratio.Transient response runs of Cu- and Fe-zeolite catalysts reaction models were performed and a maximum of NO conversion was observed at the shut-off of inlet NH3 over Fe-zeolite catalyst which confirming the NH3 inhibition effect. At low temperature, with the increase of ammonia coverage, Fe-zeolite catalyst can achieve high steady-state NOX conversion sooner, at lower ammonia coverage. This hints that steady-state conversion does not accurately represent the performance of catalyst.Considering the differences in NO removal performance of Cu- and Fe-zeolite catalysts, the Cu-and Fe-zeolite monoliths were serially arranged to widen the operating temperature range of SCR system. In order to determine the optimal configuration, the influence of catalyst type and length on the NH3 oxidation and standard SCR reaction were analyzed, and it was found that the design of Cu-zeolite catalyst followed by Fe-zeolite catalyst does not improve the NOx reduction performance at high temperature. Then the sequence of two catalysts was changed and the rationality of the series combination of Fe-zeolite catalyst followed by Cu-zeolite catalyst was confirmed. The effect of monolith length on NO conversion was analyzed and the Fe (20%)+ Cu (80%) configuration, which showing higher NO conversion activity over a wider temperature range, was found to be the optimal configuration.