Experimental and Modelling Study of Cold Start Effects on a Cu-Zeolite NH_3 Selective Catalytic Reduction Catalyst

摘要

Microreactor, engine bench tests and modelling studies have been carried out to understand the influence of cold start (low temperatures) on the performance of NH_3/urea-SCR automotive exhaust aftertreatment systems. Water storage experiments using Simultaneous Thermal Analysis (STA) coupled with numerical modelling demonstrated that the exo/endo-therms associated with water adsorption and desorption at temperatures below 150°C strongly influence the catalyst temperature. Appreciable amounts of NO and NO_2 could be stored on the catalyst during reactor or engine testing in the absence of any NH_3 (blank tests). Modelling studies at different inlet NO_2/NO_x ratios demonstrated some of the influences of these surface adsorbed species on the performance of the SCR system during cold start. When the SCR inlet NO_2/NO_x ratio is greater than 0.5, reactor results using typical cold start steady state temperatures showed the possible formation of ammonium nitrate, which would then participate in further reactions at higher temperatures (>200°C). The developed mathematical model which included ammonium nitrate formation and further reaction kinetics gave good predictions of engine test results obtained during transient operation over the NEDC (New European Driving Cycle).