Impact of Hydrothermal Aging and Chemical Poisoning on NOx Reduction Performance of SCR – A Simulation Study

Diesel aftertreatment systems continue to play a critical role in compliance of tailpipe criteria pollutant compliance for commercial transportation applications. Quantification of performance of the aftertreatment system in particular Selective Catalytic Reduction component as a function of aging is critical in ensuring real world tailpipe NOx standard for aged systems. As part of A2CAT-II consortium at Southwest research Institute this aspect of the production AT system was studied for different aging conditions using a set of DAAAC (Diesel Aftertreatment Accelerated Aging Cycle) aged components. The performance of these aged components was quantified through a set of steady state reactor tests and transient ECTO burner lab tests that simulate on engine performance. The data was collected at 0, 33, 66 and 100% equivalent aging conditions and this data was used to develop a GT suite-based model with a set of inhibition factors to simulate the loss of Ammonia Storage Capacity and reduced SCR reaction rates caused by thermal load and chemical poisons. A quantitative and qualitative discussion of the degradation mechanism is presented based on the inhibition factors determined as part of the simulation study. Primary mechanism of performance degradation for fully aged components consisted of a 12% drop in ammonia storage capacity and a 10% and 12% drop in standard and fast SCR reaction rates leading to reduced NOx conversion rate. Due to catalyst degradation up to 11% reduced NH3 oxidation rate was observed which led to increased NOx conversion for temperatures above 350 C. These trends can have a significant impact of systems designed for ultra low NOx standards by EPA and CARB which require precise control over NOx conversion of the system.