SCR Technology for Exhaust Aftertreatment

Limit values for nitrogen oxides (NOx) and particulate matters in exhaust emission standards worldwide become more and more stringent. With the treatment by the selective catalytic reduction (SCR) vehicle manufacturers have a solution to significantly reduce nitrogen oxides in exhaust gases without constraining the target of reduced soot emissions at the same time. Furthermore, the fuel consumption can be reduced by optimizing the combustion.

The SCR technology is regarded as essential to fulfill future limit values.

In an SCR system nitrogen oxides are converted to nitrogen and water with the aid of ammonia in an SCR catalytic converter. As ammonia is a toxic gas, a urea-water solution (brand name AdBlue^®) is supplied as a reducing agent from an additional tank in the vehicle equipped with SCR technology.

The AdBlue^® is injected into the exhaust gas flow of the engine and reaches the SCR catalytic converter with the exhaust gas. There — under the existing high temperatures — the urea-water solution is split into the desired ammonia and water vapor. Subsequently the ammonia reacts on the nitrogen oxides in the exhaust gas and reduces them to harmless nitrogen and water vapor.