Nitrogen Migration Behavior and Formation Pathways of Nitrogen-Containing Products During Co-Pyrolysis of Waste Leather and Oil-Rich Coal Using Amino Acid Models

To investigate the synergistic effects between oil-rich coal and nitrogen-rich solid waste during co-pyrolysis and their influence on the formation of value-added nitrogen-containing (N-containing) compounds, oil-rich Hongliulin coal and waste leather were subjected to individual and co-pyrolysis at 300–600 °C. Waste leather was selected for its collagen-derived nitrogen-rich characteristics and potential for resource utilization. X-ray Photoelectron Spectroscopy (XPS) revealed distinct N 1s signals in the co-pyrolysis tar, indicating enhanced nitrogen retention in liquid-phase products. Pyrrolic-N identified as the dominant nitrogen species, accounting for 48.76% of the total fitted N 1s peak area, followed by amide/amine-N (29.07%) and pyridinic-N (22.17%). GC-MS analysis and co-pyrolysis resulted in the detection of 10 additional nitrogen-containing compounds that were absent from the individual pyrolysis products, including several high-value compounds. The greatest number of these newly detected nitrogen-containing compounds was observed at 500 °C, suggesting that nitrogen from leather was not entirely released directly but was also incorporated into liquid-phase organic products. To further evaluate the contribution of different nitrogen-containing structural units in leather, amino acids, the major nitrogen-containing constituents of leather, were employed as model compounds. Their structure significantly influenced nitrogen transformation and product distribution. Non-heterocyclic amino acids mainly generated simple N-containing compounds, such as nitriles and amides, whereas amino acids containing cyclic imine structures, particularly proline, preferentially formed aromatic nitrogen-containing heterocyclic compounds. The results suggest that amino acids and short peptide intermediates derived from collagen decomposition are important precursors for N-containing compounds within the co-pyrolytic tar, while coal-derived active hydrogen and aromatic fragments facilitate their stabilization and transformation. Our research provides a fundamental understanding of the synergistic conversion between coal-derived carbon and nitrogen-rich solid waste, offering a potential pathway for producing value-added nitrogen-containing heterocycles, which play important roles in pharmaceuticals and fine chemicals.