Abstract
Keywords - SCR, SCRi®, AdBlue, Hydrolysis, PM-Reduction, NOx-Reduction, CO2- Emission
Due to the upcoming legislations for a further limitation of NOx emissions including the newly introduced air quality level for NO2 the automotive industry is developing new diesel engines with optimized exhaust gas recirculation and combustion processes. At the same time, continuous reduction of PM-mass limit in combination with PM-number limitations will lead to combined aftertreatment systems of particle filters and SCR catalysts. The proposed exhaust systems consist usually of an oxidation catalyst, a DPF followed by the unit for Urea injection, a mixing device and the SCR catalyst itself.
Today´s Diesel Particulate Filters can only be thermally regenerated by increasing the exhaust temperature to above 600°C, added by the exothermal heat caused by burning off the soot. Therefore new SCR-coatings with increased thermal aging stability had to be developed. These are nowadays typically Fe-Zeolith coatings, with the inherit disadvantage of decreased low-temperature efficiency. In addition, the need for two separate subsystems for NOx- and PM-reduction raises severe package constraints while the periodic pressure drop increase of the DPF in combination with the energy consumption for the regeneration increases the fuel consumption and thereby CO2-emission of the system.
The herewith described integrated SCR system (SCRi®) uses the continuously working PMMETALIT®, located not in front of the SCR part of the exhaust system, but integrated into it after the urea injection. At this position the filter not only oxidizes the particulate matters with the NO2 created by the DOC, but is also capable to mix the exhaust gas with the reduction agent due to his mixing structure. A thin Hydrolysis coating supports the building of Ammonia and avoids any deposits on the surface. Since the particles are reduced continuously without any thermally induced process the SCR coating can be optimised for low temperature levels.
The excellent efficiency of the SCRi®-system is supported by SCR catalysts which are designed with longitudinal and radial open structures to contribute to the design targets of such systems: efficiency, costs and packaging space.