SCR stands for ‘selective catalytic reduction’ and is a technology that is used to reduce nitrogen oxide emissions in situations where there is excess oxygen in the exhaust gas, such as on diesel engines. The nitrogen oxides group comprises gases each containing one or more atoms of nitrogen and oxygen, such as NO, NO2, N20 etc. and is generally referred to as ‘Nox’. In keeping with this, SCR technology is often known as ‘SCR denox’.
The principal of SCR is that a reductant such as ammonia is mixed with the exhaust gas, and then this mixture is passed through a special type of catalytic converter which causes the ammonia to react with the Nox to give only harmless nitrogen and water. In practice, ammonia is difficult to source and handle, so it is more common to use a liquid called ‘Adblue’ for mobile applications since this is readily available and much more user-friendly than ammonia. Adblue is a solution of urea in water, which, when subjected to the appropriate temperature, converts into ammonia. Purchasing and storing ammonia gas remains a viable alternative for some large stationary engines.
Where Adblue is used, this is injected into the exhaust system using an electric pump, and this process needs to be carefully controlled so that the quantity is optimised. Injecting too little will mean that some Nox will remain unconverted, whilst injecting too much results in ammonia being emitted with the exhaust gas, and this has a very pungent smell. It is also important that the Adblue is only injected when the temperature of the exhaust gas is hot enough to convert it into ammonia.
Because of the importance of controlling the Adblue pump accurately, this is activated by an ECU which is also connected to several sensors, monitoring factors such as Nox levels, exhaust gas temperature etc. With a well-designed SCR system the Nox emissions can typically be reduced by between 85 and 99%. Aiming for the top of this range involves injecting more Adblue, and consequently there is a greater risk of excess ammonia being emitted, which is known as ‘ammonia slip’. There are two approaches to preventing ammonia slip. The first is to reduce the quantity of Adblue, which although it results in a slightly lower conversion of Nox, may nevertheless be a worthwhile trade-off. The alternative approach is to install an additional catalytic converter to oxidise any ammonia which remains in the exhaust gas after reacting with the available Nox, and this is referred to as an ‘ammonia slip catalytic converter’.