Elucidating the role of carbon dioxide (CO) in the transformation of hydrogen cyanide (HCN) during the regeneration of spent fluid catalytic cracking (FCC) catalysts.

The emission of hydrogen cyanide (HCN) from the industrial fluid catalytic cracking (FCC) catalyst regenerator is a concerning pollutant that is highly toxic. Yet, the underpinning rationale particularly the role of carbon dioxide (CO) and its competition with O, remains poorly understood. Through the tests of three industrial spent FCC catalysts via temperature-programmed oxidation and Chemkin simulation, this study revealed a dual role of CO in the transformation of HCN. At high temperatures (e.g. >700C), the presence of CO is in favor of promoting the thermal cracking of coke via the Boudourad reaction, which subsequently accelerates the cracking of the associated N-bearing species for an enhanced formation of HCN. Meanwhile, a high CO partial pressure >10 % was found to benefit the generation of OH and O radicals, which are the chain carriers for the oxidation of HCN into NO and/or N. This is distinct from an environment containing only O in N in which an optimum O partial pressure of ∼1 % maximises the HCN oxidation rate. Higher O partial pressure above 1 % leads to an early release of HCN before its ignition temperature, resulting in significant emission of unreacted HCN in the outlet gas. In an O-CO-N environment, where O and CO coexist, CO can promote additional coke conversion, leading to increased initial HCN formation when the available O is insufficient to fully oxidize the coke. During the subsequent gas-phase oxidation of HCN, CO competes with O for H radicals, reducing the production of OH and O which in turn diminishes the HCN oxidation rates. Additionally, heightened CO formation from the Bouoduard reaction reduced the NO formed into N. From a practical implication perspective, these findings underscore the importance of gas conditions and maintaining temperature uniformity across the regenerator to effectively manage the emissions of HCN and other pollutant gases.