Analysis of soot filter regeneration by combined numerical and experimental investigations

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In this project we aim to predict the morphology evolution of a porous solid due to chemical reactions between a solid and a penetrating gas phase. The oxidation of soot by diluted oxygen and NO2, passing through a porous soot layer, will serve as an example of practical relevance. It is well known that the overall oxidation rate during soot filter regeneration is a strong function of time, even under isothermal conditions. By combining experimental investigations and detailed numerical simulations we will try to show from first principles if (and to what extent) the change in the overall reaction rate primarily originates from morphological changes of the soot. Therefor the project aims to separate kinetics on the primary soot particles from morphological changes which affect the accessible surface area in the soot structure.Within the project a special reactor will be constructed that allows to oxidize a soot layer stepwise under isothermal conditions. From analysis of exhaust gas, the integral rate of soot conversion can be monitored. At discrete times the oxidation will be interrupted, and the overall BET surface of the partially oxidized soot layer will be measured without dismounting the sample. Thus, we can simultaneously obtain the effective reaction rates and changes of the overall surface.Detailed information of the porous soot structure has recently become assessable by Focused Ion Beam - Scanning Electron Microscope (FIB-SEM) images. This allows to reconstruct the porous medium in microscopic scale with high resolution and provides initial conditions for direct numerical simulations.A detailed 3D Lattice-Boltzmann (LB) model will be developed to simulate the soot morphology change during soot oxidation. While the transport of a multicomponent gas mixture inside pores will be simulated using the discrete form of the Boltzmann kinetic equation, the heterogeneous soot morphology change during soot oxidation will be modeled as new boundary condition. LB method can handle complex geometries efficiently, thus it is well suited to model solid-gas phase transition.We can calculate the effective oxidation rate and compare it to experimental values, and furthermore, FIB-SEM images obtained from soot samples after partial oxidation can be compared to simulation results.Once a detailed model is available, this will serve as a basis for the development of an extended application-oriented model. For deriving anisotropic macroscopic transport parameters asymptotic homogenization will be used. The derived macroscopic model comprises the classical mass and energy balances as well as balance equations for characteristic structural properties such as BET surface.