Diesel Particulate Filters (DPFs)
Particulate filters are generally used with diesel engines to remove diesel particulate matter (PM), but in principle can be used with other types of engine/fuel combinations, although these produce less particulate matter. Based on engine technology and application specificities different filter technologies may be used to reduce particles emissions.
In the most common type, 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 particulate filters for large and small diesel engines.
Ceramic wall flow filters remove almost completely the carbon particulates, including fine particles of less than 100 nanometers (nm) diameter with an efficiency >95% in mass and >99% in number over a wide range of engine operating conditions. The latest European emissions limit values (i.e. Euro 5 and 6) are set on both mass and number to account for the number and size of particulates, which are thought to be more critical indicators of health impact. 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. The most successful methods to achieve regeneration include:
- Incorporating an oxidation catalyst upstream of the filter that, as well as operating as a conventional oxidation catalyst, also increases the ration of NO2 to NO in the exhaust
- Incorporating a catalytic coating on the filter to lower the temperature at which particulate burns
- Using very small quantities of fuel-borne catalyst, such as ceria or iron 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
- Place a fuel injector in the exhaust line upstream of the DPF
- Electrical heating of the trap either on or off the vehicle
Trapped particulate burns off at normal exhaust temperatures using the powerful oxidative properties of NO2 and can burn in oxygen when the temperature of the exhaust gas is periodically increased through post-combustion.
Wall-flow filters trap most ultra-fine exhaust particulate.
Diesel Partial-Flow filters are also available in various materials from fiber-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 temporarly retained in the fleece before being burnt by a continuous reaction with the NO2 generated by an oxydation catalyst located upstream.in the exhaust. It offers an option for reducing PM emissions by 30-60%.
Diesel partial-flow filter