A State-Space Simplified SCR Catalyst Model for Real Time Applications 2008-01-0616
The use of Selective Catalytic Reduction (SCR) is becoming increasingly more popular as a way of reducing NOx emissions from heavy duty vehicles while maintaining competitive operating costs. In order to make efficient use of these systems, it's important to have a complete system approach when it comes to calibration of the engine and aftertreatment system. This paper presents a simplified model of a heavy duty SCR catalyst, primarily intended for use in combination with an engine-out emissions model to perform model based offline optimization of the complete system.
The traditional way of modelling catalysts using a dense discretization of the catalyst channels and non-linear differential equation solvers to solve the heat and mass balance equations, requires too much computational power in this application. The presented model is also useful in other applications such as model based control. The basic model structure is a series of continuously stirred tank reactors using discretized catalyst walls to describe the mass transport in the solid phase. The simplified model uses a state-space concept. At low temperatures the model uses an implicit method of calculating the coverage differential. At higher temperatures, the model is simplified to a first order system using an operating condition dependent characteristic time constant. These simple, yet robust methods allows for long step lengths in the process of solving the differential equations. This makes the model a useful improvement over current models.