Developing Design Guidelines for an SCR Assembly Equipped for RF Sensing of NH_3 Loading

摘要

The Cu-zeolite (CuZ) SCR catalyst enables higher NO_x conversion efficiency in part because it can store a significant amount of NH_3. “NH_3 storage control”, where diesel exhaust fluid (DEF) is dosed in accord with a target NH_3 loading, is widely used with CuZ catalysts to achieve very high efficiency. The NH_3 loading actually achieved on the catalyst is currently estimated through a stoichiometric calculation. With future high-capacity CuZ catalyst designs, it is likely that the accuracy of this NH_3 loading estimate will become limiting for NO_x conversion efficiency. Therefore, a direct measurement of NH_3 loading is needed; RF sensing enables this. Relative to RF sensing of soot in a DPF (which is in commercial production), RF sensing of NH_3 adsorbed on CuZ is more challenging. Therefore, more attention must be paid to the “microwave resonance cavity” created within the SCR assembly. The objective of this study was to develop design guidelines to enable and enhance RF sensing. Catalyst dimensions, cavity-defining design features, and antenna location and length were considered. RF measurements were made on a commercial SCR unit outfitted with antennas in various locations. In addition, a simulation tool was used to characterize the RF signals obtained with different design features within the context of an L12 screening DOE. Results of the simulations identified means for optimizing the cavity design and geometry, as well as antenna position, to enhance RF measurements of stored ammonia and enable direct feedback control based on the actual SCR ammonia storage state.