During the combustion process, the nitrogen present in the fuel and by the reactions of the atmospheric nitrogen forms NOx (nitrogen oxides, combination of NO and NO2). Formation depends on process parameters, such temperature, oxygen content, and burner type.
Selective Catalytic Reduction (SCR) is a post-combustion technology for NOx abatement.
SCR system is designed to carry out a chemical reduction to convert NOx to N2, which is harmless to the environment. NOx reacts with aqueous ammonia or urea on a catalyst surface at temperature between 200 and 500 ºC.
The system consists of the reagent injection, a mixing section and the catalytic reactor.
SCR systems have a reduction efficiency up to 90 %, compared to SNCR (Selective Non-Catalytic Reduction), whose maximum is around 60 %.
Each case has to be carefully assessed because there are a several features that affect the design, such as sulphur compounds presence, powders and catalyst poisons.