Simulation of NOx Reduction in an Ammonia-SCR System With an Fe-Zeolite Catalyst and Calibration of Related Parameters

Abstract

NOx reduction in a single representative channel of an SCR converter is modeled accounting for convective mass transfer along the channel, diffusive mass transfer and chemical reactions within the catalytic layer, coupled via mass transfer boundary condition at the wall. The proposed 1D+1D code for the channel has different parameters involving rate expression constants for the surface chemistry and properties of the catalytic and washcoated layer. A comprehensive set of experimental data on the Fe-Zeolite washcoated monolith was available for different feed concentrations, temperatures and flow rates. A genetic algorithm was used as an optimization technique to calibrate the model parameters by minimizing the error between simulation results and transient responses of the real system. Dynamic tests showing the response of the system during fast transients were subsequently used to assess the predictive quality of the model with the calibrated parameters. In particular, different rate expressions for the surface reactions were compared to find the best fit for the tested catalyst, providing insight with respect to the importance of the different reaction pathways and the chemico-physical properties of the system. Based on the findings, an optimization strategy could be proposed both in terms of sequence as well as minimum required experimental tests for each sub-system according to the parameters which can be determined separately.

Calibration of a model for selective catalytic reduction with ammonia, including NO oxidation, and simulation of NOx reduction over an Fe–zeolite catalyst under highly transient conditions

“…To simulate the behaviour of the SCR process, one representative channel was selected. The model, which was described thoroughly by Sharifian et al, 1 accounts for the convective mass transfer through the channel, the diffusive mass transfer and the chemical reactions within the catalytic layer coupled together via the mass transfer boundary condition at the wall…”

“…Intra-porous diffusion limits ammonia adsorption on the catalytic surface within the washcoated layer and consequently influences NO x reduction, as was clearly demonstrated, for example, by Chatterjee et al 16 and Mladenov et al 16,18 Using a more simplified model neglecting intra-porous diffusion within the washcoat, such as a one-dimensional model, was seen to be insufficient also for the catalyst used in this study, as shown in by Sharifian. 19 The diffusion within the catalytic layer was described by…”

“…where C⌢i(x,y,t) denotes the concentration of the species inside the pores and r i shows the formation rate of species i due to the surface reactions. D eff denotes the effective diffusion coefficient within the washcoat, which was calibrated in a previous study, 1 starting from values proposed in the literature for similar Fe–zeolite catalysts. 3,16,20,21…”

“…The model includes several parameters describing the physical and chemical behaviour of the system. Properties of the catalyst layer, such as surface scaling factor, washcoat porosity and ammonia adsorption capacity, were determined beforehand, as documented by Sharifian et al 1 Since the present investigation employs more recently proposed surface kinetics 6 than the ones previously used in Sharifian et al, 1 all 26 parameters relating to the chemical reaction-rate expressions, equations (7) to (17), consisting of pre-exponential factors, activation energies and equilibrium constants, required an entirely new calibration.…”

“…A number of different ways to deal with simulations of fluid flow and surface kinetics have been developed in the past 20 years, and have been summarized in a previous publication. 1 To briefly mention the current state of the art, a 1D convection+1D diffusion (1D+1D) model approach is a good choice in most cases, when not only the fluid flow has to be considered, but also the convective mass transfer in the channel and the intra-porous diffusion within the washcoat layer on the channel walls. Simulations are most often performed for a single channel of a honeycomb SCR converter.…”