Abstract: Nitrogen oxides (NO_x) emission during the regeneration of coked fluid catalytic cracking (FCC) catalysts is an environmental issue. In order to identify the correlations between nitrogen species in coke and different nitrogencontaining products in tail gas, three coked catalystswithmultilayer structural cokemolecules were prepared in a fixed bed with model compounds (o-xylene and quinoline) at first. A series of characterization methods were used to analyze coke, including elemental analysis, FT-IR, XPS, and TG-MS. XPS characterization indicates all coked catalysts present two types of nitrogen species and the type with a higher binding energy is related with the inner part nitrogen atoms interacting with acid sites. Due to the stronger adsorption ability on acid sites for basic nitrogen compounds, the multilayer structural coke has unbalanced distribution of carbon and nitrogen atoms between the inner part and the outer edge, which strongly affects gas product formation. At the early stage of regeneration, oxidation starts from the outer edge and the product NO can be reduced to N₂ in high CO concentration. At the later stage, the inner part rich in nitrogen begins to be exposed to O₂. At this period, the formation of CO decreases due to lack of carbon atoms, which is not beneficial to the reduction of NO. Therefore, nitrogen species in the inner part of multilayer structural coke contributesmore to NO_X formation. Based on the multilayer structure model of coke molecule and its oxidation behavior, a possible strategy to control NO_X emission was discussed merely from concept.

Key words: NO_x, Basic nitrogen compounds, FCC catalyst, Coking, Reaction, Multilayer

摘要： Nitrogen oxides (NO_x) emission during the regeneration of coked fluid catalytic cracking (FCC) catalysts is an environmental issue. In order to identify the correlations between nitrogen species in coke and different nitrogencontaining products in tail gas, three coked catalystswithmultilayer structural cokemolecules were prepared in a fixed bed with model compounds (o-xylene and quinoline) at first. A series of characterization methods were used to analyze coke, including elemental analysis, FT-IR, XPS, and TG-MS. XPS characterization indicates all coked catalysts present two types of nitrogen species and the type with a higher binding energy is related with the inner part nitrogen atoms interacting with acid sites. Due to the stronger adsorption ability on acid sites for basic nitrogen compounds, the multilayer structural coke has unbalanced distribution of carbon and nitrogen atoms between the inner part and the outer edge, which strongly affects gas product formation. At the early stage of regeneration, oxidation starts from the outer edge and the product NO can be reduced to N₂ in high CO concentration. At the later stage, the inner part rich in nitrogen begins to be exposed to O₂. At this period, the formation of CO decreases due to lack of carbon atoms, which is not beneficial to the reduction of NO. Therefore, nitrogen species in the inner part of multilayer structural coke contributesmore to NO_X formation. Based on the multilayer structure model of coke molecule and its oxidation behavior, a possible strategy to control NO_X emission was discussed merely from concept.

关键词: NO_x, Basic nitrogen compounds, FCC catalyst, Coking, Reaction, Multilayer