Selective Catalytic Reduction of NOx (SCR deNOx) is a technology applied on diesel engines to reduce the NOx emissions, by injecting a urea-water solution (AdBlue) into the exhaust gas upstream of the SCR catalyst, generating ammonia (NH3). This is absorbed onto the catalyst, converting NOx in diatomic nitrogen (N2) and water (H2O).
A Diesel Particulate Filter reduces the particulate matter emissions from the engine exhaust gases. The most efficient DPF is the wall flow DPF, commonly made from ceramic materials with a honeycomb structure with alternate channels plugged at opposite ends. According to the Manufacturers of Emission Controls Association (MECA), particulate matter is captured by interception and impaction of the solid particles across the porous wall. The filter is designed to hold a certain quantity of soot. During the course of its operational hours, it gets loaded due to the high deposition of soot. This can result in increased back pressure on the engine and when not properly acted upon may lead to clogging of the filter. Therefore, it is important to maintain a sufficiently high average temperature such that the stored particle matter is regenerated (converted to CO2) and the filter is kept clean. This is needed to prevent it from becoming blocked and its function thereby being affected. Alternatively a special active regeneration system can be installed, which increases the filter temperature periodically to high temperature for fast filter regeneration.

SCR and DPF are often combined because then all gaseous as well as particulate emissions are reduced (by 70% or more) and usually the most stringent (future) emission legislation can be met. SCR and DPF often work together nicely leading to an increased SCR efficiency. One of the technical options is the “SCR on DPF technology”, where the DPF part acts as an SCR catalyst as well. This can lead to a more compact configuration.

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