Pollutants Transformation During the Regeneration Process of Fluid Catalytic Cracking Catalysts

Fluid catalytic cracking (FCC) is the major process for heavy oil conversion in current refineries and is explored for the intake of renewable feedstocks, like biomass and plastic waste. Due to coke deposition, FCC catalysts undergo continuous reaction-regeneration cycles. However, many gas pollutants are generated in the FCC regeneration process, and their emission characteristics and formation mechanisms are poorly understood. Here, we conducted stack tests of three industrial FCC units to monitor pollutant emissions. The spent catalysts were characterized to identify the carbon deposits formed. We developed a method to correlate the decomposition of carbon deposits and the formation of gas pollutants in regeneration experiments using in situ Raman spectroscopy, operando FT-IR spectroscopy, and online gas-phase FT-IR spectroscopy. The evolution of coke species is significantly influenced by the oxygen content of the regeneration gas, leading to differences in emission concentration and formation temperature of various gas pollutants. The experimental results are compared with density functional theory (DFT) calculations to explain the formation of the major gas pollutants. This work is expected to advance pollutant emission prediction and control in FCC regeneration, thereby laying the foundation of future work in which different fossil-based and renewable feedstock compositions can be compared, including their effect on gas pollutant formation.