As with all catalytic converters, the starting point for these is a substrate to provide lots of small channels, which is then coated with catalytic material.
Ceramic substrates are normally made from a special type of porous ceramic material known as ‘cordierite’. Liquid cordierite is extruded through a mould and then fired so that it dries out and becomes hard. By using different moulds, various size substrates can be extruded and the size of the channels (or cells) can also be varied.
Because tooling is required, ceramic honeycombs are best suited to medium or large volume requirements unless a standard size can be used, although standard sizes can sometimes be cut down. Ceramic honeycombs are generally the most economical option for consumer appliances such as biomass stoves and pyrolytic self-cleaning ovens.
The size of the cells is a key variable contributing to the performance of a catalytic converter. However, rather than referring directly to the size of the cells, it is normal to quantify how many there are per unit of area, for example cells per square inch (cpsi) or cells per square centimetre.
The greater the number of cells per unit of area, the greater the surface area will be and hence the greater the potential to reduce undesirable compounds. The downside is that more cells mean more walls between cells and hence more obstruction to gas flow. Therefore a higher pressure will be required to achieve a given flow rate, which may be referred to as a ‘pressure drop’ or ‘backpressure’ across the catalytic converter. Typical cell densities for ceramic honeycomb substrates used in cooking and heating appliances are 15, 18, 25 and 64 cpsi.
When choosing a cell density it is also important to consider the possibility of blockage by solids such as ash or foodstuff – this is less likely if the cells are bigger than the solid particles.
In order to facilitate the installation of ceramic substrates into appliances or housings which are largely made of metal, careful design of gaskets is essential to prevent problems caused by differing rates of thermal expansion. Gaskets are usually made of either vermiculite or knitted wire.