Search Results

The removal of nitrogen oxides emitted by diesel and lean burn engines is one of the most important targets in catalytic exhaust aftertreatment research. Besides the selective catalytic reduction (SCR) reaction with ammonia the most promising approach is the NOx Storage and Reduction Catalyst (NSR) which utilizes the nitrogen oxides storage on barium sites to form nitrates during the lean phase and their reduction to nitrogen in a rich atmosphere. In this paper, we present a modeling approach for the description of the transient behavior of a NSR. The model applies a two-dimensional description of the flow field in the single channels coupled with detailed models for the chemical processes. The mechanism used for modeling the oxidation and reduction of carbon monoxide, hydrocarbons, and nitrogen oxides, respectively, is based on an elementary step reaction mechanism for platinum catalysts and a shrinking core model for the storage and reduction processes on the barium particles.

Numerical simulations are increasingly assisting research and development in the field of emission control of automotive vehicles. Our work focuses on the prediction of the tail-pipe emissions, based on a numerical simulation of the automotive catalytic converter. Besides the prediction of the tail-pipe emissions, an understanding of the processes occurring inside a monolithic catalytic converter implies new opportunities for the design of the optimum exhaust gas system. In this paper, we present a three-dimensional transient numerical study of the influence of the velocity distribution in front of the inlet face on the thermal behavior of the monolith during the light-off of a 3-way catalytic converter. The differences in the thermal and chemical behavior due to the shape of the velocity distribution are discussed. The recently developed code DETCHEMMONOLITH /1/ is used for the numerical simulation.

Monolithic three-way catalysts are applied to reduce the emission of combustion engines. The design of such a catalytic converter is a complex process involving the optimization of different physical and chemical parameters. Simple properties such as length, cell densities or metal coverage of the catalysts influence the catalytic performance of the converter. Numerical simulation is used as an effective tool for the investigation of the catalytic properties of a catalytic converter and for the prediction of the performance of the catalyst. To attain this goal, a two-dimensional flow field description is coupled with a detailed chemical reaction model. In this paper, results of the simulation of a monolithic single channel are shown. In a first step, the steady state flow distribution was calculated by a two dimensional simulation model. Subsequently, the reaction mechanism of the chemical species in the exhaust gas was added to the simulation process.

This combined numerical simulation and experimental study focuses on the effect of nitrogen dioxide (NO2) in strong diluted oxygen-rich NOx/NH3/O2/H2O gas mixtures representative at exhaust gas aftertreatment in diesel engines. Conversion of nitrogen oxides (NOx) and ammonia (NH3) and the change of the NO/NO2 and NH3/NOx ratios were analyzed by means of comparison between experimental measurements and kinetic simulations. Homogeneous gas-phase experiments have been carried out in a special countercurrent continuous flow reactor at intermediate temperature (473-923 K) and elevated pressure up to 5 bar. In the simulation with an ideal plug-flow model, the occurring reactions in the gas mixture were calculated. To explain the observations, a reactions flow analysis was also performed. The experimental and modeling results revealed that despite a high NH3 conversion, the NOx concentration stays nearly constant in the presence of NO2.

The selective catalytic reduction (SCR) based on urea-water-solution is an effective technique to reduce nitrogen oxides (NOx) emitted from diesel engines. A 3D numerical computer model of the injection of urea-water-solution and their interaction with the exhaust gas flow and exhaust tubing is developed to evaluate different configurations during the development process of such a DeNOx-system. The model accounts for all relevant processes appearing from the injection point to the entrance of the SCR-catalyst: momentum interaction between gas phase and droplets evaporation and thermolysis of droplets hydrolysis of isocyanic acid in gas phase heat transfer between wall and droplets spray/wall-interaction two-component wall film including interaction with gas phase and exhaust tube The single modeling steps are verified with visualizations, patternator measurements, phase-doppler-anemometer results and temperature measurements.

The ultimate goal in the numerical simulation of automotive catalytic converters is the prediction of exhaust gas emissions as function of time for varying inlet conditions, i.e. the simulation of a driving cycle. Such a simulation must include the calculation of the transient three-dimensional temperature-field of the monolithic solid structure of the converter, which results from a complex interaction between a variety of physical and chemical processes such as the gaseous flow field through the monolith channels, the catalytic reactions, gaseous and solid heat transport, and heat transfer to the ambience. This paper will discuss the application of the newly developed CFD-code DETCHEMMONOLITH for the numerical simulation of the transient behavior of three-way catalytic converters that have a monolithic structure.