Petr Kočí

Current status of modeling lean exhaust gas aftertreatment catalysts

Decreasing emission limits lead to the development of combined aftertreatment systems, consisting of combinations of different catalyst technologies and particulate filters. Modeling such systems can contribute considerably in reducing development time and cost. The methodology for developing catalyst models is reviewed and models for the diesel oxidation catalyst (DOC) with hydrocarbon (HC) adsorption, the NOx storage and reduction catalyst (NSRC) and the urea–selective catalytic reduction system (urea–SCR) are developed. Applications for exhaust aftertreatment system modeling are shown.

Modelling of catalytic monolith converters with low- and high-temperature NOx storage compounds and differentiated washcoat

Two types of catalytic washcoats with different properties can be present simultaneously in a monolith channel, e.g. the flat foils and the corrugated ones in metallic monolith can be individually coated with specific type of the washcoat. The model of such multiphase, differentiated NOx storage and reduction (NSR) catalytic monolith converter has been developed. The important reactions as are the oxidation of carbon monoxide, hydrocarbons and hydrogen, the reduction of nitrogen oxides (NOx), the water gas shift and the steam reforming reactions, the NO/NO2 transformation, and the oxygen and NOx storage are considered. Unknown kinetic parameters of the NSR are evaluated from transient experiments with the samples of two different types of NSR catalysts. The first catalyst is of the Pt–γ-Al2O3–CeO2 type with alkali earth metals (e.g. barium) as NOx storage components (active at lower temperature). The second one is of the PtRh–γ-Al2O3–CeO2 type with both alkali earth metals and alkali metals (e.g. potassium) as NOx storage components (active at higher temperature). Simulations of periodic lean/rich operation of the low-temperature, the high-temperature and the differentiated (combined) NSR converters are performed. The results of the computations agree well with the experimental data. The dependences of integral NOx conversion on the lengths of the lean and rich phases and on the temperature of the inlet gas are discussed. The efficiencies of the low-temperature, the high-temperature and the differentiated NSR converters are compared at different operating conditions.

Effective Model for Prediction of N2O and NH3 Formation During the Regeneration of NOx Storage Catalyst

In this paper we propose an effective global kinetic model that allows prediction of N2O and NH3 formation during the reduction of stored NOx in dependence on the composition of the rich mixture (H2/CO/C3H6), actual operating temperature, and length of regeneration period. A bench flow reactor equipped with a high-speed FTIR was used to measure dynamic evolution of gas components during periodic lean/rich operation of a fully formulated NSRC catalyst (PtPdRh/Ba/Ce–Zr/Mg–Al/Al2O3).

Investigation of combined DOC and NSRC diesel car exhaust catalysts

Abstract The system of two monolithic catalytic reactors – diesel oxidation catalyst (DOC) and NOx storage and reduction catalyst (NSRC) – is studied. The catalysts are used in series for the conversion of exhaust gases from automobiles with Diesel engines (or other lean-burn engines). Several tens of catalytic reactions are considered in the description by a model based on a system of non-linear partial differential equations (mass and enthalpy ballances). A simulation study is performed to examine the effects of parameters determining the performance of the upstream-located DOC (size and effective heat capacity of the monolith, catalytic activity in individual reaction steps) on the performance of the NSRC in the European driving cycle (NEDC). Effective numerical methods used for the solution of the resulting non-linear PDEs are described. Results of the maximization of the overall NOx conversion over the NEDC for several DOC and NSRC reactor configurations and different regeneration strategies are presented. It is shown, that the DOC may have a positive effect on the overall NOx conversion over the NEDC. Regarding NOx conversion over the NEDC, the effect of the NOx storage capacity of the NSRC is significant.

Oscillations, period doublings, and chaos in CO oxidation and catalytic mufflers.

Early experimental observations of chaotic behavior arising via the period-doubling route for the CO catalytic oxidation both on Pt(110) and Ptgamma-Al(2)O(3) porous catalyst were reported more than 15 years ago. Recently, a detailed kinetic reaction scheme including over 20 reaction steps was proposed for the catalytic CO oxidation, NO(x) reduction, and hydrocarbon oxidation taking place in a three-way catalyst (TWC) converter, the most common reactor for detoxification of automobile exhaust gases. This reactor is typically operated with periodic variation of inlet oxygen concentration. For an unforced lumped model, we report results of the stoichiometric network analysis of a CO reaction subnetwork determining feedback loops, which cause the oscillations within certain regions of parameters in bifurcation diagrams constructed by numerical continuation techniques. For a forced system, numerical simulations of the CO oxidation reveal the existence of a period-doubling route to chaos. The dependence of the rotation number on the amplitude and period of forcing shows a typical bifurcation structure of Arnold tongues ordered according to Farey sequences, and positive Lyapunov exponents for sufficiently large forcing amplitudes indicate the presence of chaotic dynamics. Multiple periodic and aperiodic time courses of outlet concentrations were also found in simulations using the lumped model with the full TWC kinetics. Numerical solutions of the distributed model in two geometric coordinates with the CO oxidation subnetwork consisting of several tens of nonlinear partial differential equations show oscillations of the outlet reactor concentrations and, in the presence of forcing, multiple periodic and aperiodic oscillations. Spatiotemporal concentration patterns illustrate the complexity of processes within the reactor.

Modelling of wall-flow filters for diesel particulate removal

Abstract Diesel particulate filter (DPF) is regarded as the most useful technology to reduce particulate matter from exhaust gas of a diesel engine. Exhaust gas entering the channel is forced to flow through the ceramic porous walls into the adjoining cells and thus leaving the particulates behind. The collected particulate matter inside the trap has to be periodically oxidized to DPF regeneration. We have developed a nonstationary spatially 2D model of the filter, soot deposition and its regeneration with detailed kinetics of soot combustion. Optimized numerical methods and software for the solution of the models are described and results for various catalyst distributions along washcoat layer are presented and discussed.

Oscillatory behaviour in mathematical model of TWC with microkinetics and internal diffusion

Abstract Mathematical model of three-way catalytic converter (TWC) has been developed. It includes mass balances in the bulk gas, mass transfer to the porous catalyst, diffusion in the porous structure and simultaneous reactions described by a complex microkinetic scheme of 31 reaction steps for 8 gas components (CO, O 2 , C 2 H 4 , C 2 H 2 , NO, NO 2 , N 2 O and CO 2 ) and a number of surface reaction intermediates. Enthalpy balances for the gas and solid phase are also included. The method of lines has been used for the transformation of a set of partial differential equations (PDEs) to a large and stiff system of ordinary differential equations (ODEs). Multiple steady and oscillatory states (simple and doubly-periodic) and complex spatiotemporal patterns have been found for a certain range of operation parameters. The methodology of studies of such systems with complex dynamic patterns is briefly introduced and the undesired behaviour of the used integrator is discussed.

Spatially 3D simulation of a catalytic monolith by coupling of 1D channel model with CFD

Abstract Modern combustion engines are required to produce high power output while maintaining low specific fuel consumption and low emission level. Cars then need to be equipped with increasingly complex systems of catalytic converters, enabling oxidation of carbon monoxide, unburned hydrocarbons, filtration and oxidation of soot, and reduction of nitrogen oxides. Design of these complex systems includes optimization of pressure and heat losses. With increasing availability of processing power and effective 1D models it is possible to simulate the dynamic behavior of catalytic converters system on common desktop computers. In this work, a standard 1D-model of a monolith converter with optimized numerical solver is coupled to a commercial 3D CFD processing software via a newly developed interface. Simulation results obtained by the 1D-channel model alone and the coupled 3D-CFD with representative 1D-channels are compared.

Optimization of combined DOC and NSRC diesel car exhaust catalysts

The system of two monolithic catalytic reactors (Diesel Oxidation Catalyst, DOC, and NOx Storage Reduction Catalyst, NSRC) operating in series is studied. The catalysts are used for the conversion of exhaust gases from automobiles with Diesel engines. Several tens of catalytic reactions are considered in the description by a model based on a system of nonlinear partial differential equations (mass and enthalpy ballances). In the simulation study the effects of parameters determining the performance of the upstream-located DOC configuration (the size and effective heat capacity of the monolith, catalytic activity in individual reaction steps) on the performance of the NSRC in the European driving cycle (NEDC) are investigated. Numerical method used for solution of the resulting set of nonlinear PDEs is described. Results of the maximization of overall NOx conversion over the NEDC for several reactor configurations are presented.