When using DET’s Urea based SCR Elimin-NOx™ system, the NOx contained in the exhaust gas is reduced by 70 to 80% and converted to pure nitrogen. A downstream catalyst prevents a breakthrough of ammonia. Combining the longitudinal structure and perforated structures not only presents a large number of turbulence edges for improved mass transfer, and hence higher catalytic efficiency, but also gives them increased cross-sectional flow-through. Such metal substrates are significantly lighter and thus make it possible to reduce the overall size of the metal substrates, as well as coating mass and precious metal content. This development offers interesting options such as the turbulent mass transfer adapted coatings that are currently being developed and which will lead to a further reduction in design volume, weight, and system cost. For thorough mixing of exhaust gas with the injected urea solution, DET developed a twin-stage metal substrate resembling its Smartrap DPF, but which works using flat, perforated foils instead of metallic fleece. The initial section with MX structure of some 40 cpsi ensures optimum gas mixing with a urea injection, which has to be largely evaporated.  In the second stage, which consists of an LS/PE structure at 200/400 cpsi, intensive amalgamation is continued and hydrolysis of urea to ammonia takes place. Both stages force the gases into turbulent flows, so that they come into close contact with the catalytic coating and use the entire substrate cross section due to radial flow into the adjacent channels. The urea solution additive, called AdBlue, contains 68% mass of water, which by virtue of absorbing heat and the increased fluid flow does not facilitate hydrolysis of the urea to ammonia and catalytic conversion of ammonia with the NOx. This requires low heat inertia substrates and thus favors the LS/PE design accompanied with enhanced heat management. In addition to the design of such a system, in which the individual components are arranged in sequence, the parallel arrangement of oxidation and hydrolysis catalytic converter put forward by MAN is also possible, whereby PM-Metalit and SCR-catalyst follow downstream. However, more benefits can be offered, such as applications for trucks with larger cross sections or various types of non-round shapes. Ultimately, turbulent mass transfer results in a volume reduction of about 35% and permits a combination of diesel oxidation catalyst, DPF, and SCR converter within one single space-saving silencer compartment. The advantages of this combined system include minimal maintenance and the lowest possible fuel consumption, says DET. Also crucial for highest reduction of NOx and PM combined with long useful life is proper coating of all components, which must be precisely matched one to the other. |
