Process of removing nitrogen oxides from flue gases from a fluidized catalytic cracking unit

A process for removing nitrogen oxides from flue gas issuing from a regenerator of a fluidized catalytic cracking unit wherein the flue gas are cleaned of substantial amounts of dust in the regenerator, the process comprising the steps of:

• a) directing the flue gas into a tertiary cyclone and separating the solids therefrom so that not more than generally 250 mg/Nm3 of solids exit in an overflow from the tertiary cyclone and causing a minor percentage of flue gas entering the tertiary cyclone to exit as an underflow from the tertiary cyclone with solids separated in the tertiary cyclone;
• b) Conveying the overflow to a selective catalytic reduction unit and processing the to overflow in the selective catalytic reduction unit to form an outlet gas stream;
• and
• c) Introducing the outlet gas stream and the underflow into a wet scrubber and removing at least approximately 80 of particles of 3.0 microns size and larger and 30% of 1.5 micron size.