NETT TECHNOLOGIES - Catalytic Diesel Filters How the Filter Works Nett® diesel filters utilize cordierite or silicon carbide wall-flow monoliths to trap the soot produced by heavy-duty diesel engines. The cylindrical filter element consists of many square parallel channels running in the axial direction, separated by thin porous walls. The channels are open at one end, but plugged at the other. This way the particle laden exhaust gases are forced to flow through the walls. Gas is able to escape through the pores in the wall material. Particulates, however, are too large to escape and are trapped in the filter walls. A proprietary catalyst is coated onto the inside surface of the filter monolith. The catalyst lowers the soot combustion temperature allowing the filter to regenerate. The accumulated soot is oxidized in the filter during regular operation of the engine. Periods with exhaust temperatures of 325-400°C (615-750°F), which are necessary for proper filter regeneration, occur on most heavy-duty diesel engine applications. The catalyzed filter monolith is wrapped in a fiber mat and packaged into a stainless steel housing, which is installed in the vehicle’s exhaust system. Efficiency and Pressure Drop The soot filtration efficiency of the Nett® catalytic diesel filter increases with the soot loading in the unit. Even at low soot loads the filter efficiency exceeds 90% (blue line in the graph). The visible smoke is completely 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 (red line) is between 5 and 10 kPa (20-40" H2O). Applications with higher exhaust temperatures regenerate better, accumulate less soot in the filter, and experience lower pressure drop. Catalyst Types Depending on the application, Nett® particulate filters can be ordered with one of the following catalyst types Standard catalyst Sulphate-suppressed catalyst The standard catalyst provides the best activity and the lowest regeneration temperature requirements. However, if high sulfur fuels are used, the catalyst may generate sulfate particulates at elevated temperatures. As a general guideline, the standard catalyst should be used in the following applications: All applications with ultra low sulfur fuel (S < 50 ppm). 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 in Germany (particulates measured as elemental carbon). The sulfate-suppressed catalyst eliminates the sulfate make at the expense of somewhat increased regeneration temperature requirement, typically by 10-15 °C. This catalyst is used for applications with high sulfur fuel (S > 50 ppm) which are sensitive to sulfate particulate emissions, such as most urban buses operated on high sulfur fuels. Substrate Types Nett® diesel particulate filters are available with two types of substrates: Cordierite substrates Silicon carbide (SiC) substrates Cordierite substrates perform satisfactory in most heavy-duty applications with high exhaust temperature. However, in low-temperature applications which may experience “uncontrolled regenerations”, cordierite is more susceptible for thermal stress related damage. Silicon carbide has higher maximum operating temperature limits and better durability in high thermal stress applications. Disadvantages of SiC include higher weight and higher cost.