SCR systems require a ‘reductant’ to react with the NOx in the exhaust gas, and normally this reductant is ammonia. Unfortunately, ammonia is an unpleasant and dangerous gas, so storing it safely is a challenge. For this reason it is common practice to start with a urea solution, and then break this down into ammonia at the point where it is required. Urea solutions are much safer to store and transport than ammonia, and much more readily available.

The most common urea solution is known as ‘Adblue’ in Europe or ‘Diesel Exhaust Fluid’ in the US. This contains 32.5 % urea in water. An alternative which is popular for marine and industrial applications is AUS40, which contains 40% urea and therefore takes up less storage space.

When a urea solution is heated, the water evaporates and the urea converts to carbon dioxide and ammonia in processes known as hydrolysis and thermolysis. The heat required to facilitate these reactions normally comes from the hot exhaust gas, however this approach introduces a limiting factor i.e. if the exhaust is not hot enough the urea solution will not break down. It is also necessary to provide a lot of space between the injection point and the SCR catalyst so that there is sufficient time for these reactions to occur.

An alternative approach is to use a separate reactor to convert the urea solution into ammonia, and then inject the ammonia into the exhaust. This gives a benefit in that the SCR system can operate at lower exhaust temperatures and can be more compact.

The vast majority of problems experienced with SCR systems are due to the inherent characteristics of urea solution. If it gets too hot, it crystallises, which leads to blockages in the injector nozzles and catalytic converter. If it gets too cold, it freezes. Adblue remains liquid down to about minus 11°C but AUS40 freezes at zero which could necessitate a heated tank.

A promising development which offers the potential of overcoming the drawbacks of urea solution whilst reducing the dangers of storing ammonia gas is solid ammonia storage.

In one version of solid ammonia storage, salts capable of giving off ammonia when heated are placed into a cartridge. This cartridge is heated when ammonia is required, but at ambient temperature the ammonia remains trapped. When the cartridge becomes depleted, it is exchanged for a full one, and the depleted cartridge is recharged. This approach is a very promising alternative to the use of urea solution but until replacement cartridges become widely available it is most suited to either stationery applications or mobile ones where the vehicle returns to a depot at the end of each shift.