Metal substrate oxidation catalytic converters comprise a substrate, which resembles a honeycomb with many small channels, onto which a catalytic coating is applied. Metal substrates are made from extremely thin metal foil, typically with a thickness of just 0.05mm. By comparison the walls of a ceramic substrate may be four times thicker. For this reason, metal substrates offer less resistance to gas flow than the ceramic alternative which means that, if the size is kept constant, they are more active, or if the activity is kept constant, they can be made smaller.
Most metal substrates used in biomass combustion have between 25 and 40 cells per square inch (cpsi), whilst the most common cell densities used for cooking are between 50 and 100 cpsi.
Metal substrates can be made in a variety of profiles, with round, square and rectangular being most common.
There are various ways of making metal substrates but the top-quality versions utilize Fecralloy foil which consists of 74% iron, 21% chromium and 5% aluminium. The main advantage of Fecralloy is that, when heated in air, a layer of aluminium oxide forms on the surface which protects the iron from corrosion and also provides a good key for the catalytic coating. The foil is typically kept in place either by vacuum-brazing or by mechanical means.
A big advantage of metal substrates compared to the ceramic alternative is that they can be fixed directly to other metal components without the need for any gaskets.