Most people associate SCR systems with Adblue (or Diesel Exhaust Fluid as it is known in the US). However, Adblue is just a convenient starting point to obtain the ammonia which is actually what is required.
Adblue is a solution of urea in water. When heated, the water evaporates and the urea converts to carbon dioxide and ammonia. This heating normally takes place due to the Adblue being injected into a hot exhaust, however this approach introduces a limiting factor i.e. if the exhaust is not hot enough the Adblue will not break down. An alternative approach is to use a separate reactor to convert the Adblue into ammonia, and then inject the ammonia into the exhaust. This gives a benefit in that the SCR system can operate at lower temperatures and also removes some of the reliability problems associated with Adblue.
The vast majority of problems experienced with SCR systems are due to the use of Adblue. It can crystallise, which leads to blockages in the injector nozzles and catalytic converter. It also freezes at minus 11 ℃ which often necessitates heated tanks. So why not just use ammonia instead?
The main reason is that ammonia is an unpleasant and dangerous gas, so storing it safely is a challenge. The second reason is that Adblue is readily available at petrol stations whereas ammonia is not.
However in the case of stationary engines requiring SCR it is well worth considering the use of ammonia as an alternative to Adblue. Two types of ammonia are commercially available:
- Anhydrous ammonia, which is a gas and needs to be stored under pressure.
- Ammonia solution, which is ammonia dissolved in water.
Solid ammonia storage is another alternative to Adblue which is also suitable for mobile applications. This is a very promising approach in which salts capable of giving off ammonia when heated are placed into a cartridge. This cartridge is heated when ammonia is required, and then re-filled once it is depleted. This approach removes the problems associated with Adblue and the safety issues related to the storage of ammonia. However due to the fact that replacement cartridges are not widely available it is most suited to either stationary applications or mobile ones where the vehicle returns to a depot at the end of each shift.