Diesel Particulate Filters (DPF)Retrofit Diesel Particulate Filters are used with diesel engines to remove diesel particulate matter (PM), also called soot. Based on engine technology and application specificities, different filter technologies may be used to reduce particles emissions.
In wall flow filters, particulate matter is removed from the exhaust by physical filtration using a honeycomb structure similar to an emissions catalyst substrate but with the channels blocked at alternate ends. The exhaust gas is thus forced to flow through the walls between the channels and the particulate matter is deposited as a soot cake on the walls. Such filters are made of ceramic (cordierite, silicon carbide or aluminium titanate) honeycomb materials.
Wall-flow filters trap most ultra-fine exhaust particles
Ceramic wall-flow filters remove almost completely the carbon particulates, including fine particulates of less than 100 nanometers (nm) diameter with an efficiency of >95% in mass and >99% in number of particles over a wide range of engine operating conditions. Since the continuous flow of soot into the filter would eventually block it, it is necessary to 'regenerate' the filtration properties of the filter by burning-off the collected particulate on a regular basis. Soot particulates burn-off forms water and CO2 in small quantity since it is less than 0.05% of the CO2 emitted by the engine. The most successful methods to achieve regeneration on retrofit applications include:
• Incorporating an oxidation catalyst upstream of the filter that, as well as operating as a conventional oxidation catalyst, also increases the ratio of NO2 to NO in the exhaust. Trapped particulate burns off at lower exhaust temperatures using the powerful oxidative properties of NO2 and oxygen.
• Incorporating a catalytic coating on the filter to lower the temperature at which particulate burns to normal exhaust temperatures.
• Using very small quantities of fuel-borne catalyst (FBC), such as iron or ceria additive compounds, added to the fuel thanks to an on board dosing system. The catalyst, when collected on the filter as an intimate mixture with the particulate, allows the particulate to burn at lower exhaust temperatures (around 350°C instead of 650°C) and increases the combustion kinetic (typically 2-3 minutes) while the solid residues of the catalyst are retained on the filter as ashes. Control over the combustion process is critically important in maintaining the long-term integrity of the filter. Its performances in terms of temperature and kinetic make the FBC technology particularly suitable for urban and stop-and-go cycle duty cycles with low fuel penalty.
Fuel-borne catalyst dosing unit
• External heating of the trap either on or off the vehicle or machinery with electrical heaters. It applies only to industrial applications and is usually only used when the engine is off.
• Fuel injector or burner placed in the exhaust line upstream of the DPF.
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Partial-flow filters are also available in various materials from fibre-based to metallic. The metallic partial flow filter uses a special perforated metal foil substrate with a metal 'fleece' layer so that the exhaust gas flow is diverted into adjacent channels and the particles are temporarily retained in the fleece before being burnt by a continuous reaction with the NO2 generated by an oxidation catalyst located upstream in the exhaust. It offers an option for reducing PM emissions by 30-60%.
Diesel partial-flow filter