SCR is short for ‘selective catalytic reduction’ and is a technology which is useful for breaking down oxides of nitrogen in an atmosphere which contains excess oxygen. It was originally designed for large-scale combustion situations such as coal-fired power stations, but the most common application these days is in purifying the exhaust gas from Diesel engines, where it can reduce oxides of nitrogen by in excess of 95%.
In chemistry, ‘reduction’ is the opposite of oxidation. Therefore, if you have a toxic gas such as nitrogen dioxide, and you want to separate the oxygen atoms from the nitrogen one, you need to reduce it. However, whenever you reduce a molecule in chemistry, you have to oxidise one too, in what is known as a ‘redox’ reaction. In the case of combustion gases where there is an excess of oxygen, there isn’t much present which can be oxidized, so it is necessary to introduce something for this purpose, and this is termed a ‘reductant’. The most common reductant used in SCR systems is ammonia. One of the paths by which ammonia (NH3) reacts with nitrogen oxides is as follows:
N0 + NO2 + 2NH3 → 2N2 + 3H2O
However, since ammonia is difficult to handle, it is standard practice to start off with a liquid called Adblue (or Diesel Exhaust Fluid – DEF for short), which breaks down to ammonia when heated but is much safer.
There are two main components in an SCR system. The first is a dosing system, the purpose of which is to inject exactly the right quantity of reductant into the exhaust. The second is a catalytic converter, which helps the necessary reactions to occur at the prevailing exhaust gas temperature.
As mentioned above, the most common application of SCR these days is on diesel engines, either alone or in combination with a diesel particulate filter (DPF). Combining SCR with a DPF introduces some extra complications.
SCR can also be used on lean-burn gas engines, and although the principles are the same as for Diesel engines, there are a few little differences.
Further information on the technology and our related products can be found by following the links on the right.