Nett diesel filters, available from Norman G Clark , utilise cordierite or silicon carbide wall-flowmonoliths to trap the soot produced by heavy-duty diesel engines. Thecylindrical filter element consists of many square parallel channels running inthe axial direction, separated by thin porous walls. The channels are open atone end, but plugged at the other. This way the particle laden exhaust gasesare forced to flow through the walls. Gas is able to escape through the poresin the wall material. Particulates, however, are too large to escape and aretrapped in the filter walls.
A proprietary catalyst is coated onto the inside surface of the filtermonolith. The catalyst lowers the soot combustion temperature allowing thefilter to regenerate. The accumulated soot is oxidized in the filter duringregular operation of the engine. Periods with exhaust temperatures of 325-400°C(615-750°F), which are necessary for proper filter regeneration, occur on mostheavy-duty diesel engine applications. The catalyzed filter monolith is wrappedin a fiber mat and packaged into a stainless steel housing, which is installedin the vehicle’s exhaust system.
The soot filtration efficiency of the Nett catalytic diesel filterincreases with the soot loading in the unit. Even at low soot loads the filterefficiency exceeds 90% (blue line in the graph). The visible smoke iscompletely eliminated by the filter. Due to the presence of the catalyst,reductions in carbon monoxide and hydrocarbon emissions are also observed.
Typical exhaust gas pressure drop on a properly regenerating filter (redline) is between 5 and 10 kPa (20-40" H2O). Applications with higherexhaust temperatures regenerate better, accumulate less soot in the filter, andexperience lower pressure drop.
Depending on the application, Nett particulate filters can be ordered withone of the following catalyst types
- Standard catalyst
- Sulphate-suppressed catalyst
The standard catalyst provides the best activity and the lowest regenerationtemperature requirements. However, if high sulfur fuels are used, the catalystmay generate sulfate particulates at elevated temperatures. As a generalguideline, the standard catalyst should be used in the following applications:
- All applications with ultra low sulfur fuel (S < 50ppm).
- All applications, regardless of fuel sulfur level,where sulfates are not considered a particulate emission and are not regulated.Examples of such applications include underground non-coal mining in the U.S.(particulates measured as total carbon) or occupational health environments inGermany (particulates measured as elemental carbon).
The sulfate-suppressed catalyst eliminates the sulfate make at the expenseof somewhat increased regeneration temperature requirement, typically by 10-15°C. This catalyst is used for applications with high sulfur fuel (S > 50ppm) which are sensitive to sulfate particulate emissions, such as most urbanbuses operated on high sulfur fuels.
Nett diesel particulate filters are available with two types of substrates:
- Cordierite substrates
- Silicon carbide (SiC) substrates