Numerical Simulation Of Reaction Mechanism For NO_x Reduction Over Fe-Cu Zeolite Combined SCR Catalyst

One major challenge for NO_x reduction aftertreatment technologies since several years is to be compliant with the increasingly stringent emission legislations.Selective Catalytic Reduction?SCR?technology is one of the most promising technologies to reduce NO_x emissions from diesel engines.Lots of researches have recently been concentrated on the optimization of the SCR systems for broadening the active temperature window as widely as possible and achieving the higher deNO_x efficiency.Global kinetic models were developed to study the NO_x reduction efficiency of Fe-ZSM-5and Cu-CHA catalysts.A dual-site model was developed to describe NH₃ adsorption,desorption and the process of SCR reaction carried out on Cu-CHA catalyst.The model accurately predicted the steady state NO_x and NH₃ conversions and the selectivity of the different products formed during these reactions.The results showed that Cu-CHA catalyst had a better performance on NH₃ storage and it was more active on NH₃ oxidation.Cu-CHA catalyst was more active than Fe-ZSM-5 catalyst at temperatures below 400?and it tended to selectively oxidize NH₃ to N₂ at temperatures above 400?,leading to poor NO_x conversion at elevated temperatures,while Fe-ZSM-5 catalyst had a better performance at temperatures above 400?.In view of the different performances of SCR reaction in the specific temperature range for both two catalysts,Fe-ZSM-5 and Cu-CHA catalysts were combined in the form of dual-brick catalyst to widen the temperature window of SCR catalyst.According to the optimization for the simulation results,it was confirmed that the optimal sequential arrangement is a short Fe-ZSM-5 brick?25%for length?followed by a longer Cu-CHA brick?75%for length?.Transient response was compared between Cu-CHA catalyst and Fe-Cu combined system at low temperatures based on the steady-state study.It was found that the Cu-CHA gave the better transient response than the combined catalyst at low temperatures.With the temperature increasing,the transient response of Fe-Cu combined catalyst was similar to that of Cu-CHA.Ammonia to NO_x ratio?ANR?plays an important role in NO_x conversion of SCR catalyst.The influences of varied ANR on steady state NO_x coinversion and NH₃ slip carried out on Fe-ZSM-5,Fe-Cu combined system and Cu-CHA catalysts were studied in this paper.The results showed that steady state NO_x conversion and NH₃ slip increased at the elevated temperatures with the increasing ANR.ANR was optimized to achieve high NO_x conversion and keep NH₃ slip below 10ppm.This was a simple optimization for the inlet gas component of Cu-CHA catalyst and Fe-Cu combined system.It was found that the high temperature performance of Cu-CHA catalyst and Fe-Cu combined system was improved by increasing ANR without substantial levels of NH₃ slip.