Commercial deployment of urea-SCR systems depended on the development of not only the catalyst, but also the urea dosing and injection system. The increase in NOx conversion efficiency of SCR systems that has been seen since the launch of SCR technology on diesel engines around 2005 is largely owed to advances in SCR control and urea injection. The main functions of the urea dosing and injection system include:
- Dosing of the precise amount of urea necessary for the SCR reactions with NOx, and
- Mixing urea and ammonia thoroughly with the exhaust gas.
The amount of injected urea must match the ammonia demand corresponding to the amount of NOx entering the catalyst and the NOx conversion efficiency at given operating conditions (catalyst temperature and space velocity). If the amount of ammonia is insufficient, a fraction of NOx that otherwise could be reduced will remain unconverted, resulting in a NOx conversion penalty. If the amount of injected ammonia is more than can be consumed in the SCR reactions, it will cause an unacceptably high ammonia slip. In systems that include an ammonia slip catalyst, some of the ammonia may be oxidized back to NO, thus decreasing the effective NOx conversion.