Unsteady-state Models of the Monolith SCR Reactor

In recent years, several proposals have been made for the numerical simulation of SCR catalysts, primarily of coated monoliths [59, 60], and for control-oriented purposes [61]. 

Tronconi et al. [46] developed a fully transient two-phase 1D + 1D mathematical model of an SCR honeycomb monolith reactor, where the intrinsic kinetics determined over the powdered SCR catalyst were incorporated, and which also accounts for intra-porous diffusion within the catalyst substrate. Accordingly, the model is able to simulate both coated and bulk extruded catalysts. The model was validated successfully against laboratory data obtained over SCR monolith catalyst samples during transients associated with start-up (ammonia injection), shut-down (ammonia 

The same model was applied to the simulation of typical transients occurring during the operation of industrial SCR monolith reactors in large power plants. In all cases it was found that the change in NO outlet concentration is considerably delayed with respect to the variation of the inlet NH3 concentration. This is unfavorable for a feedback control system using the ammonia feed as the control variable and makes 

Recently, the dynamic model of ref. [46] was specifically adapted for mobile SCR applications by Tronconi and co-workers [62]. The model consists of unsteady mass and enthalpy balances in the gas phase and in the solid phase  

The strong intra-phase diffusion limitations are accounted for by the following equations for diffusion-reaction of the reactants in the catalytic monolith wall (Equation 13.21) with the appropriate boundary conditions (Equation 13.22)  

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