Empa Activities 09/10 Mobility, Energy and Environment
Ceramic foam substrates for automotive catalyst applications Ceramic foams are proposed as substrates for automotive catalysts in natural gas as well as diesel engines. Experimental fluid dynamic investigations have shown the homogenization impact of the foams on the exhaust flows. In parallel, exhaust measurements exhibited similar pollutant conversion performances as conventional honeycomb catalysts, although with substantial less surface area. Boundary layer dimensional arguments as well as numerical simulations ascribe the phenomena to enhanced diffusion mass transfer.
Panayotis Dimopoulos Eggenschwiler, Christian Bach
The homogeneity of flow distribution in automotive catalytic converters is a major parameter determining pollutant conversion efficiency, volume utilization and useful life. Current substrates for catalysts use a honeycomb structure with only a limited potential for gas-wall interactions due to the laminar channel flow. A substantial part of pollutant conversion occurs in the converter entrance section during boundary layer development with high species concentration gradients. At the same time, the entrance section is the most thermal and catalyst poisoncharged part of the catalytic converter. The remaining and largest part of the catalytic converter is characterized by a much lower heat and mass transfer due to laminar flow and therefore reduced specific conversion efficiency. Further downstream-positioned exhaust aftertreatment devices render the situation more complex. Homogeneous soot loading is known as the key factor for a reliable diesel particulate filter system. Empa and ETH Zurich have developed ceramic foam substrates for catalytic converters (patent pending) as an alternative to conventional honeycombs, since they can redistribute the flow of exhaust gases, enhancing turbulence and species mixing without increasing the flow resistance and pressure drop to prohibitive levels. Catalytically coated, they even can enhance pollutants conversion based on increased mass transfer induced by high turbulence inside the converter. In combination with low material and manufacturing costs, the potential of ceramic foams is promising.
The idea behind the development of ceramic foam as catalyst substrate is to “extend” the high efficiency of the entrance section of a conventional honeycomb to the whole catalytic converter volume. This would allow – as working hypothesis – to significantly reduce the precious metal content without a lack of conversion efficiency. a
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Mean velocity [m/s] 8.25 10.50 12.80
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Fig.1: Vector plot of the mean velocities measured by PIV downstream of the catalysts substrates, a) Honeycomb monolith, b) 8ppi ceramic foam c) 10ppi ceramic foam.
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