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 situations such as coal-fired power stations, but the most common application these days is in purifying the exhaust gas from Diesel engines.

There are two stages to an SCR system. The first is to inject a ‘reductant’ such as ammonia into the exhaust gas, and the second is to pass this mixture through a special type of catalytic converter.

Most trucks which meet the Euro 5 or 6 emissions standards rely on SCR systems to reduce their NOx emissions. These trucks can be identified by the presence of an extra tank near the fuel tank which contains ‘Adblue’ (or ‘Diesel Exhaust Fluid’ in the US). Adblue is a solution of urea in water which breaks down to produce ammonia under the right conditions. Its advantage over ammonia is that it is safe to handle and store, however converting Adblue reliably into the correct quantity of ammonia requires a sophisticated control system. It also necessitates some compromises, especially in conditions where the exhaust gas temperature is low, so it is worthwhile considering alternatives to Adblue. These are discussed further under the heading ‘reductants’.

As mentioned above, the most common application of SCR is on diesel engines, either alone or in combination with a diesel particulate filter.

It can also be used on lean-burn gas engines. In all cases the principles are the same, but the differences are the way that the technology is customized to suit particular operating conditions.