The correlation between nitrogen species in coke and NOx formation during regeneration

Nitrogen oxides (NOx) 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 nitrogen-containing products in tail gas, three coked catalysts with multilayer structural coke molecules 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 contributes more to NOX formation. Based on the multilayer structure model of coke molecule and its oxidation behavior, a possible strategy to control NOX emission was discussed merely from concept.