Elucidating the role of carbon dioxide (CO2) 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 (CO₂) and its competition with O₂, 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 CO₂ in the transformation of HCN. At high temperatures (e.g. >700^oC), the presence of CO₂ 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 CO₂ 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_x and/or N₂. This is distinct from an environment containing only O₂ in N₂ in which an optimum O₂ partial pressure of ∼1 % maximises the HCN oxidation rate. Higher O₂ 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 O₂-CO₂-N₂ environment, where O₂ and CO₂ coexist, CO₂ can promote additional coke conversion, leading to increased initial HCN formation when the available O₂ is insufficient to fully oxidize the coke. During the subsequent gas-phase oxidation of HCN, CO₂ competes with O₂ 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_x formed into N₂. 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.