Ash transport and deposition, cake formation and segregation – a modeling study on the impact of ash on particulate filter performance 2019-01-0988
The application of particulate filters has been state-of-the art in the exhaust gas aftertreatment of diesel engine emissions for many years. Similar, the application of wall flow particulate filters is a promising technology to fulfil the upcoming legislation limits of gasoline engines. Wall flow filters feature high filtration efficiencies at acceptable levels of pressure loss. The increased pressure loss due to particle matter (PM) accumulation is handled by removing combustible particles by active and passive regeneration measures. Non-combustible particles, commonly summarized as ash, remain in the filter and deteriorate its lifetime performance. Considering that during a filter’s useful life there is more ash in the filter than soot, ash plays an important role in the design and control of filter components.
The contribution of this study is to bring ash modeling into a broader simulation context with the aim to investigate transient phenomena of coated filters. An existing transient 1D+1D wall flow filter model is extended in several modeling areas. The transport of ash through the gas phases—inlet channel, cake, wall, outlet channel—is handled by defining fractions of PM and ash for all transported particle classes. The cake in the filter is modeled in a discrete manner to describe changing PM and ash compositions over the cake height. The deposition of the transported particles in the discrete cake model is addressed by a simplifying phenomenological filtration model. The changing cake composition is reflected in the cake pressure drop model and in the evaluation of the local cake permeability and diffusion coefficients. A phenomenological model is introduced to capture the radial mobility within the cake. Depending on a mobility constant, the overall cake height changes when locally removing PM following regeneration reactions. Passive regeneration reactions are considered assuming catalytically supported oxidation of NO in the wall.
Results of three different types of simulations are shown. First, the various sub-models presented in this study are assessed in isolated simulation configurations. These simulations target to show the plausibility of principle correlations, their absolute value range and the impact of model parameters. The combination of these targets shall serve as theoretical model validation. The second type of simulations focuses on passive regeneration of a homogeneously initialized PM and ash cake. The aim of these simulations is to identify the impact of different cake migration constants on PM and ash segregation (i.e. ash accumulation) and on PM conversion performance. The third set of simulations investigates the impact of ash during a combined loading and regeneration cycle. The balance point of PM loadings is simulated for different steady-state feed gas specifications on mass flows, temperatures, PM and ash loadings.
Johann C. Wurzenberger, Susanne Kutschi, Astrid Nikodem