What is SCR?

SCR system (Stand for Selective Catalytic Reduction) is a means of converting nitrogen oxides, also referred to as NOx with the aid of a catalyst into diatomic nitrogen, N2, and water, H2O.
A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to a stream of flue or exhaust gas and is adsorbed[citation needed] onto a catalyst. Carbon dioxide, CO2 is a reaction product when urea is used as the reductant.