Selective Catalytic Reduction (SCR)
In contrast to the SNCR process, this process requires a catalyst. The flue gas flows through a reactor tower containing several levels with plate or honeycomb type catalysts, with ammonia solution fed through nozzles. If denitrogenation is carried out at the beginning of the flue gas cleaning process, plate-type catalysts are used since the flue gas could still contain dust particles. Honeycomb-type catalysts are mainly used for pure, dust-free gas at the end of the flue gas cleaning process.
The catalysts are installed in the reactor tower at several levels using a modular construction system. The ceramic structures are covered with catalytic materials such as titanium-vanadium or tungsten oxide. The degree of nitrogen reduction is influenced by the catalytic action of the substance as well as the catalyst volume. The reaction temperature, which is currently most favourable between 300°C and 400°C, is also significant.
Efforts are however being made to try to operate the catalyst at a flue gas temperature which is as low as possible (320°C), while achieving the same nitrogen oxide reduction result. Higher temperatures would require additional fuel to compensate the energy losses in the process chain.
At a temperature below 320°C, ammonia salts are formed by the nozzle fed ammonia solution. This could block the catalysts. These salts are however not formed above 320°C.