In this paper we tackle issues relevant to model based control design for a Urea based Selective Catalytic Reduction (SCR) process relevant to automotive applications. A three state, control oriented, lumped parameter model of the system is used to investigate essential controllability and observability properties of the Urea-SCR plant. Results from the controllability and observability analysis of both nonlinear and linearized models are shown to have realistic implications. Observer design for predicting gas phase ammonia slip is outlined and results presented. An altered definition of the catalyst efficiency is used in control design. It is shown that this altered definition lends itself readily to control synthesis in the Sliding Mode framework while satisfying the dual control objectives of maximizing NOx reduction and minimizing ammonia slip.
A zero order lumped parameter control oriented model of a Selective Catalytic Reduction (SCR) catalyst is presented. The lumped parameter model is developed using two approaches. in the first approach it was assumed that the catalyst behaves as an Isothermal Continuously Stirred Tank Reactor (ICSTR). The second approach involved deriving the lumped parameter model from a 1D model using the method of weighted residuals. Both approaches led to a three state model, with the gas phase concentrations of NOx and NH3 and the surface coverage fraction as the states. The model depends on chemical properties specific to the SCR catalyst; consequently model validation requires knowledge of these parameters, either via laboratory-based experiments or as supplied by the catalyst supplier. We present an alternate approach that allows estimation of the essential parameters through a minimization of the l2 errors between measured and simulated results.
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