Nitrogen oxides (mainly NO and NO2) are produced primarily in highly efficient combustion processes at high temperatures, such as those occurring in the gas engines of biogas plants. Selective catalytic reduction (SCR) is frequently used to reduce these pollutants. In contrast to the oxidation of hydrocarbons, the SCR process involves injecting a reducing agent, usually an aqueous urea solution, into the exhaust gas stream. On the SCR catalyst, the nitrogen oxides react with the ammonia formed from the urea to produce harmless nitrogen (N2) and water vapour. The technical complexity lies in dosing the reducing agent precisely according to engine load and exhaust gas temperature, in order to avoid “ammonia slip” (the release of unreacted ammonia). Modern cleaning systems often combine an oxidation stage for VOC reduction with a downstream SCR stage for nitrogen oxide reduction. This combination makes it possible to comply with comprehensive emission requirements such as the 44th BImSchV and contributes significantly to protecting the environment from acid rain and ozone formation.