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Chemical Reactions and Soot Build-up in a Diesel Filter

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Chemical Reactions and Soot Build-up in a Diesel Filter

In this model, a filter system for a diesel engine is modelled, including where a soot layer builds up and is oxidized. The build-up of the layer is held in check by both catalytic and non-catalytic reactions, where carbon is oxidized to carbon monoxide and carbon dioxide, which in turn passes through the membrane.

A filter system's efficiency and durability is closely related to the manner in which it removes soot deposits from the porous filter walls. One approach involves introducing cerium additives to the fuel. Cerium oxide species are subsequently present in the soot layer, acting as a catalyst in carbon-oxidation reactions. Under these conditions, it is possible to remove carbon deposits in the filter without increasing the exhaust temperature. Five reactions and their accompanying kinetics are included in the mass balances.

A 2-step strategy is used to investigate this system using the COMSOL Reaction Engineering Lab: 1. Definition of a 1D tubular, isothermal reactor including the system kinetics for the system. 2. Definition of a perfectly stirred tank reactor including the system kinetics; then export to the Chemical Engineering Module in COMSOL Multiphysics to set up a time-dependent 2D model.

The second strategy is solved using four application modes in the Chemical Engineering Module: 1. Non-Isothermal Flow 2. Darcyís Law 3. Convection and Diffusion (material balance) 4. Convection and Conduction (energy balance)

Furthermore, the soot layer is modelled explicitly, its thickness affecting transport properties accordingly. This is modelled using the Moving Mesh application mode in COMSOL Multiphysics.

Results show that the catalytic reactions dominate the oxidation of carbon, which decreases with increasing temperature. An interesting effect arises at 505 K where the rate of CeO2 regeneration decreases substantially.

Further results from the 2D model show that although the oxidation of carbon is an exothermic reaction, the catalytic steps are endothermic, although the exothermic reactions dominate the overall energy balance.

Collage showing the chemistry, flow streamlines, concentration profile and chemical rate profile in a diesel filter

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