Where it is necessary or desirable to reduce emissions of diesel particulate matter ('DPM')by a substantial amount, the best available current technology is the diesel particle filter.
Types of filter and their effectiveness
The most popular filter in use is the so-called 'wall-flow' type. In this, narrow channels are blocked at alternate ends, forcing the exhaust gas to flow through the porous material before exiting the filter. These types of wall-flow filters can trap as much as 99% of the diesel particulates, including the ultra-fine particles which give the biggest health concerns (PM 2.5's). The most common materials used for wall-flow filters are ceramic (cordierite) and silicon carbide. Silicon carbide is the newer of the two and offers the following advantages:
- As the plugs at the end of each channel are themselves porous, they contribute to the filtration area and improve gas flow, thus permitting the length of the filter to be reduced
- Their more uniform pore size enables a given filtration efficiency to be achieved using a smaller filter and at a lower backpressure
- Silicon carbide has a higher melting point, higher thermal conductivity and higher heat capacity then cordierite, with the result that it is less susceptible to damage during regeneration
Filter elements are usually cylindrical with diameters ranging from 80 to 380 mm and lengths in the range 154 to 380 mm. Cell densities of 90, 150 or 200 per square inch are available.
Before a wall-flow filter can be incorporated into an exhaust section it needs to be 'canned', which involves wrapping it in a special cushioning material which expands when it is heated, inserting the wrapped filter into a sleeve made of low-expansion stainless steel, and then heating it in an oven.
