Selective non-catalytic reduction (SNCR) is a method to lessen nitrogen oxide emissions in conventional power plants that burn biomass, waste and coal.
The process involves injecting either ammonia or urea into the firebox of the boiler at a location where the flue gas is between 1,400 and 2,000 °F (760 and 1,090 °C) to react with the nitrogen oxides formed in the combustion process.
The resulting product of the chemical redox reaction is molecular nitrogen (N2), carbon dioxide (CO2), and water (H2O).
[1] The conversion of noxious NOx to innocuous N2 is described by the following simplified equation:[2] When urea is used, the pre-reaction occurs to first convert it to ammonia: Being a solid, urea is easier to handle and store than the more dangerous ammonia (NH3), so it is the reactant of choice.
Although in theory selective non-catalytic reduction can achieve the same efficiency of about 90% as selective catalytic reduction (SCR), the practical constraints of temperature, time, and mixing often lead to worse results in practice.