The SNCR process (Selective Non-Catalytic Reduction) is a DeNOx method (denitrification process, flue gas denitrification), which stands for reducing nitrogen oxides (NOx) in flue gases. Thermolysis acts to convert ammonia gas (NH3), ammonia water or urea solution with gaseous nitrogen oxides into water vapour and nitrogen.
At temperatures of 900 to 1,100 °C, both ammonia and urea transpose with the nitrogen oxides of the combustion gases in a radical reaction into nitrogen and water vapour in accordance with the simplified equations (1) and (2).
4 NO + 4 NH₃ + O₂ → 4 N₂ + 6 H₂O (1)
4 NO + 2 NH₂CONH₂ + O₂ → 4 N₂ + 4 H₂O + 2 CO₂ (2)
The formation of so-called NH₂* radicals, which are produced by the reaction of ammonia with oxygen or OH* radicals, or that are formed by thermal decomposition of urea, constitute the actual start reaction (3) to (5).
4 NH₃ + 2 O₂ → 4 NH₂* + 4 OH* (3)
NH₃ + OH* → NH₂* + H₂O (4)
NH₂CONH₂ → 2 NH₂* + CO (5)
In the subsequent reactions, these NH₂* radicals preferably transpose themselves with the nitrogen oxide molecules (6).
NH₂* + NO → N₂ + H₂O (6)
A prerequisite for the high selectivity of SNCR Denitrification is the compliance with the specified temperature range of 900 to 1,100 °C.
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