NO_x Formation During The Fluid Catalytic Cracking (FCC) Regeneration

Regeneration of the coked catalyst is an important process of fluid catalytic cracking?FCC?in petroleum refining,however,this process will emit environmentally harmful gases such as nitrogen and carbon oxides.NO_x emission from FCC regeneration accounts for 50% of the refinery total NO_x emissions,and the primary component is NO?>95%?with small amounts of N2 O and NO₂.Therefore,intensive investigation of the N transformation chemistry of FCC coke species will be helpful in developing and improving FCC DeNO_x technology.Simulated FCC cokes prepared with nitrogen-containing precursors?like aniline,pyrrole and pyridine?have been exclusively examined.However,the simulated FCC cokes are deviate from the actual industrial ones due to the following reasons.?1?The simulated FCC coked catalysts contain much higher coke content than the industrial ones.?2?The simulated catalysts exhibit only one type of coke known as catalytic coke,however industrial catalysts exhibit up to three additional forms: Conradson coke,contaminate coke and catalyst-to-oil coke.In the present article,an industrial coked catalyst collected from a typical FCC unit is investigated.The gaseous products from coke decomposition are monitored during temperature-programmed processes via on-line mass spectrometry.The coked catalysts undergoing coke decomposition at different temperatures are analysed with NMR and XPS.Hence,exploration of the generated gaseous products,in conjunction with the evolution of the coke species,will provide further insight into the mechanism of real FCC coke transformation and yield further understanding of the formation of nitrogen and carbon based gases.The obtained results indicate that two distinct regions of gas evolution are observed during TPD for the first time,and they arise from decomposition of aliphatic carbons and aromatic carbons.Three types of N species,pyrrolic N,pyridinic N and quaternary N are identified in the FCC coke,the former one is unstable and tends to be decomposed into pyridinic and quaternary N.Mechanisms of NO,CO and CO₂ evolution during TPD are proposed and lattice oxygen is suggested to be an important oxygen resource.Regeneration process indicates that coke-C tends to preferentially oxidise compared with coke-N.Hence,new technology for promoting nitrogen-containing compounds conversion will benefit the in-situ reduction of NO by CO during FCC regeneration.Both TPD test of the empty pipe and FCC regenerated catalyst show that under the condition of FCC regeneration,nearly thermal NO_x are generated.Test in a simulated riser tube shows that under a continuous and stable FCC react-regenerating process,hardly thermal NO_x or prompt NO_x are detected.For the test in a simulated fixed FCC regenerator,which is an intermittent process,thermal NO_x can only be detected at the beginning stage but undetectable when the process is sustained and stable.Addition of the CO promoter can enhance oxidation,resulting in reduced CO levels and a subsequent increase in NO emission.It is found that the addition of steam is beneficial in reducing both CO and NO emissions,and increasing steam concentration promotes the conversion of CO into CO₂.Addition of CO promoter under 1 mol% steam concentration favours CO and HCN formation.5 mol% steam + CO promoter results in a synergistic effect,leading to decline of both CO and NO emission.Higher oxygen concentration accelerates the coke combustion and contributes to converting the reducing agents into oxides.