Reaction Mechanism for the Formation of Nitrogen Oxides (NO_x) During Coke Oxidation in Fluidized Catalytic Cracking Units

Fluidized catalytic cracking (FCC) units in refineries process heavy feedstock obtained from crude oil distillation. While cracking feed, catalysts get deactivated due to coke deposition. During catalyst regeneration by burning coke in air, nitrogen oxides (NO_x) are formed. The increase in nitrogen content in feed over time has resulted in increased NO_x emissions. To predict NO_x concentration in flue gas, a reliable model for FCC regenerators is needed that requires comprehensive understanding and accurate kinetics for NO_x formation. Based on the nitrogen-containing functional groups on coke, model molecules are selected to study reactions between coke-bound nitrogen and O₂ to form NO and NO₂ using density functional theory. The reaction kinetics for the proposed pathways are evaluated using transition state theory. It is observed that the addition of O₂ on coke is favored only when the free radical is present on the carbon atom instead of nitrogen atom. Thus, NO_x formation during coke oxidation does not result from the direct attack by O₂ on N atoms of coke, but from the transfer of an O atom to N from a neighboring site. The low activation energies required for NO formation indicate that it is more likely to form than NO₂ during coke oxidation. The favorable pathways for NO_x formation that can be used in FCC models are identified.