Ammonia Storage Measurements and Feedback Control of SCR Systems Using Radio Frequency Sensors

Paul  Ragaller, CTS Corporation

Achieving ultra-low NOx emissions levels requires precise information of the operation state of the SCR system.  Such information enables maximum NOx conversion over all operating conditions via direct feedback control.  Currently these systems rely on a variety of gas sensors, mounted upstream or downstream of the catalyst, which only provide an indirect inference of the operation state.  In this study, a radio frequency (RF) sensor was used to measure the ammonia loading state of catalyst systems.  The technology was applied to core samples on bench reactors and to a full-scale system evaluation on a heavy-duty engine application.


The evaluations utilized radio frequencies to monitor ammonia adsorption levels on the catalyst via changes in the bulk dielectric properties of the cavity within selected regions of the RF spectrum.  Sensor output was provided as feedback in the implementation of a simple closed loop SCR control strategy through the engine evaluations.  With the simple control strategies utilized in this study, improved performance with reduced NOx and ammonia slip was achieved.  The results indicate significant potential towards removing the control barriers typically associated with the implementation of SCR systems.