Operating principle Selective catalytic reduction
The SCR usually operates within a temperature range of 300 to 350°C. If the exhaust air entering the system is not at the required temperature, it must be pre-heated using an air-to-air heat exchanger, for example. The air is then heated further by a burner or another type of heater until it reaches the catalyst operating temperature.
The process of reducing the nitrogen monoxide (NO) and nitrogen dioxide (NO2) takes place in the catalyst. The function of the catalyst is to lower the reaction temperature, which would otherwise be between 900 and 1000°C, to the required operating temperature of 300 to 350°C.
After the reaction temperature and the catalyst startup temperature have been reached, an ammonia and water solution is injected into the system. The ammonia is used as reducing agent and reacts with the nitrogen monoxide (NO) and nitrogen dioxide (NO2) in the exhaust air to form nitrogen and water. The chemical reaction in the catalyst is selective. This means that the priority is to reduce nitrogen oxide levels while as far as possible suppressing the unwanted reactions of other substances.
After the nitrogen oxides have been converted in the catalyst, the energy is extracted from the clean gas by an air-to-air heat exchanger and used to pre-heat the untreated exhaust gas.
