Mechanisms of Nitrogen Transformation during Copyrolysis of Distiller’s Grains and Waste Plastics for Sustainable Jet Fuel Production

Copyrolysis of nitrogen-containing lignocellulosic biomass with waste plastics enables the production of low-nitrogen bio-oil rich in jet fuel components, yet nitrogen transformation mechanisms remain unclear. This study elucidated nitrogen evolution pathways by examining nitrogenous products during distiller’s grains (BDG) pyrolysis assisted by polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET). Protein-N in BDG underwent a rearrangement to form pyrrolic-N, pyridinic-N, and quaternary-N in char. Plastics enhanced protein decomposition, and PE/PP/PS promoted pyrrolic-N/pyridinic-N formation and pyrrolic-to-pyridinic-N conversion, while PET facilitated quaternary-N generation. In BDG pyrolysis oil, nitrogenous compounds formed via secondary reactions between protein derivatives and lignocellulose fragments, dominated by amines. Copyrolysis with PE/PP/PS reduced nitrogenous compounds by up to 97.1%, 98.3%, and 94.0%, respectively, via hydrogen radical-driven precursor depletion and direct denitrogenation, increasing gaseous NH3. Nevertheless, PET copyrolysis shifted the main nitrogen speciation to nitriles. Limited hydrogen radical generation contributed little to oil denitrogenation, while PET-derived carbon enhanced nitrogen retention in char, thereby reducing the level of gaseous nitrogen release. The strong denitrification effects induced by PE/PP/PS increased the jet fuel components by 108.73%, 111.69%, and 167.55%, respectively, versus theoretical values, whereas PET inhibited jet fuel formation. Comprehensive N-transformation mechanisms were proposed based on N-containing product distribution and evolution.