Modeling and Control of a Urea-SCR Aftertreatment System

A dynamic system model for simulating the transient performance of a NO x aftertreatment system using Selective Catalytic Reduction with urea as a reductant (urea-SCR) was developed, calibrated for a heavy-duty engine application, and used to develop a closed loop self-tuning control strategy. The closed loop controller was able to reduce the FTP cycle NO x emissions from a Cummins heavy-duty engine by 84% while maintaining the mean ammonia slip below 7 ppm and the peak ammonia slip below 55 ppm. The peak ammonia slip occurred during the LA Freeway phase of the FTP cycle. Components of the urea-SCR aftertreatment system model include a urea dosing system, an exhaust pipe and a fresh vanadia-based SCR catalyst. The urea dosing system model incorporates the evaporation, thermolysis and hydrolysis stages in the conversion of urea to ammonia in the exhaust pipe and on the catalyst. The catalyst model is a 2-dimensional model that incorporates the heat and mass transfer characteristics of a monolith channel, and the chemical kinetics of NO x conversion by ammonia. The Nusselt number, Sherwood number, and reaction probability are calculated as a function of axial position along the monolith channel. Results from a Cummins heavy-duty engine application were used to calibrate the dynamic system model and parametric studies were carried out to quantify the effect of ammonia storage capacity on NO x conversion and ammonia slip. A closed loop self-tuning control strategy with on-line adaptation of the controller gains was designed and implemented on a Cummins heavy-duty urea-SCR aftertreatment system with a rapid prototyping tool. The composite adaptive controller is based on a Model-Reference Adaptive Control (MRAC) system for a first-order plant with composite adaptation law.. The controller uses time varying input information for the desired NO x reduction rate, catalyst inlet exhaust gas temperature, catalyst outlet exhaust gas temperature, catalyst inlet NO x emissions rate, and catalyst outlet NO x emissions rate to determine the urea solution dosing rate.