Non-Destructive characterisation of coke deposit on FCC catalyst and its transient evolution upon Air-Firing and Oxy-Fuel regeneration

In this paper, the regeneration and associated transient evolution of a commercial FCC spent catalyst in both air-firing and oxy-fuel combustion modes (i.e. 0–10% O₂ in N₂ or CO₂, 510–1000°C, and ∼ 4 sec) were studied in a lab-scale drop-tube furnace. A variety of non-destructive techniques including synchrotron NEXAFS, XPS and ATR-FTIR for both non-grounded and grounded spent catalyst and its combustion residues were employed to reveal the speciation, spatial distribution of coke within the catalyst matrix, as well as its evolution upon rapid heart-up and combustion. As have been confirmed, the coke species is highly broad in speciation, covering aliphatic and aromatic compounds as well as graphite that are both physically deposited and chemically adsorbed on acid sites on the catalyst's exterior surface. The majority of aliphatic hydrocarbons were also embedded deeply within the catalyst matrix, which were presumably caused by the penetration of precursor molecules that are small in size. Irrespective of the combustion mode, upon the initial rapid heat-up, the embedded light hydrocarbons were rapidly volatilised, and diffused outward toward the exterior surface, where it either underwent C-O₂/C-CO₂ reactions, or simply further recondensed into graphite in the case that oxygen is lean in the air-firing mode. In contrast, in the oxy-firing mode, the coke was confirmed highly reactive for reverse Boudouard reaction and methane drying reaction which were even triggered during the initial particle heat-up stage. Consequently, the initial graphitisation extent of light vapor was mitigated, and the micro-pores of catalysts were opened up to enable a continuous elution of the hydrocarbon vapors to boost the overall coke burn-off. Regarding the competition of the overall C-O₂ and C-CO₂ reactions on catalyst exterior surface, the former reaction is superior in the case where oxygen is lean (e.g. 3%). In contrast, when the oxygen content is higher (e.g. 10%), the latter one was intensified due to the strong heat feedback from the exothermic C-O₂ reactions. As a result, the overall coke including other two species, graphite and aliphatic hydrocarbons were consumed quickly in the O₂/CO₂ case.