Selective Catalytic Reduction process includes the addition of a gaseous or liquid reductant to the flue or exhaust gas stream and latter is absorbed onto a catalyst. The reductant reacts with NOx in the exhaust gas to form H2O and N2. Specialty catalytic converters are required to make an SCR system work, the current options being a vanadium-based catalyst, or a catalyst with zeolites in the washcoat. The performance of Fe- and Cu-exchanged zeolite urea-SCR is approximately equal to that of vanadium urea-SCR if the fraction of the NO2 is 20% to 50% of the total Nox. Selective catalytic reduction (SCR) systems inject ammonia into boiler flue gas and pass it through a catalyst bed where the ammonia and nitrogen oxide gas (NOx) react to form nitrogen and water vapor. Selective catalytic reducers are a post-combustion technology used in coal burning and petrochemical processing applications to reduce NOx emissions. Selective catalytic reducers (SCR) work in a manner similar to the way a catalytic converter works to reduce automobile emissions. A gaseous or liquid reductant (generally ammonia or urea) is added to the exhaust gases before they exit a smokestack. The mixed gases travel through several catalytic layers, causing a reaction between the NOx emissions and the ammonia injection.