Selective Catalytic Reduction (SCR)

Unlike the NSC, the SCR is free of precious metals and is operated continuously under excess oxygen with ammonia used as reductant.

The ammonia is created as a decomposing product of a 32.5% aqueous urea solution (trade name "AdBlue" in Europe, "DEF" [Diesel Exhaust Fluid] in North America) dosed into the exhaust pipe upstream of the SCR catalyst.

The required dosing hardware basically consists of a heated tank, heated tubes, an injector nozzle, a mixing device to ensure homogeneous flow distribution and complete decomposition of the urea solution and an electronic control unit. For the effective dosing of aqueous urea solution a minimum exhaust temperature of 160°C is required.

In the presence of an SCR catalyst, the ammonia is able to react selectively with the nitrogen oxides in the exhaust gas to form nitrogen and water. For efficient NO_x reduction, adequate dosing of urea solution to form ammonia in the exhaust gas is necessary.

Depending on the NO/NO₂ content in the exhaust gas, two important main SCR reactions are distinguished:

• Without NO₂: Standard SCR: 
  
  4 NH₃ + 4 NO + O₂ → 4 N₂ + 6 H₂O
  
  
• With NO₂: Fast SCR: 
  
  4 NH₃ + 2 NO + 2 NO₂ → 4 N₂ + 6 H₂O

The Fast SCR reaction is important for effective NO_x control, especially at lower exhaust gas temperatures around the Urea dosing threshold. In order to increase the NO₂ content in the exhaust gas, either a DOC or a cDPF can be used upstream of the SCR catalyst.