Modeling of the SCR reactor for coal-fired power plants: Impact of NH3 inhibition on Hg0 oxidation

Abstract

A 1D + 1D heterogeneous model of the SCR reactor with honeycomb monolith catalyst is developed; it solves the mass balances of the reacting species along the axial and radial direction, using a lumped approach to describe the gas–solid mass transfer and a differential approach to describe diffusion and reaction within the catalyst wall. The model describes the coexistence of the DeNOX reaction and the highly desired oxidation of Hg0. It is well known from the literature that the conversion of Hg0 is negatively affected by NH3, but the extent of such an inhibition is uncertain because of the inherent complexity of the mercury speciation measurement. We thus apply the model to examine the concentration and reaction rate profiles inside the monolith wall and comprehend the way and extent of the interference of NH3 on Hg0 oxidation. The estimate of the intrinsic kinetic parameters was obtained from lab scale and pilot scale tests over commercial V-based catalysts, which allowed to focus the analysis on realistic conditions. The DeNOX and the Hg oxidation processes are both affected by external and internal mass transfer limitations, but the kinetic control from diffusion is stronger in the case of the DeNOX reaction. Thus along the radial coordinate and along the axial coordinate, the decay of NH3 concentration is more pronounced than the decay of Hg0 concentration. In practice, the main operating parameter which determines the integral impact of NH3 inhibition is the area velocity, that is the sizing of the full scale reactor. Still, a parametric analysis provides useful elements for envisaging the desired catalyst characteristics which can promote Hg0 conversion and minimize NH3 inhibition.