The structure-dependent regulation of CaO on NOx precursors during the pyrolysis of distinct amino acids

Abstract

Pyrolysis conversion of sewage sludge faces a technical bottleneck of NOx pollution caused by high ash content and rich protein components. Among them, the influence of CaO on nitrogen transformation during amino acid pyrolysis is significant, yet the underlying principles and mechanisms governing its effect across amino acid with different structures remain poorly understood. To address this issue, this study focused on four representative amino acids: leucine (Leu), aspartic acid (Asp), phenylalanine (Phe), and histidine (His). It systematically investigated the migration behavior of nitrogen during their pyrolysis, which was driven by the synergistic effects of temperature and CaO. The results demonstrated that CaO reconstructed the directional migration pathways of nitrogen into char, tar, and gas. This reconstruction is achieved through four key actions: complexation, alkaline deamination, hydrolysis, and CO₂ mineralization. This reconstruction exhibits a decisive structural dependency: In polar amino acids (Asp, His), CaO strongly complexes with carboxyl/imidazole groups at low temperatures (255 ℃ and 350 ℃). This interaction traps nitrogen intermediates and diverts them toward stable structures in char, such as nitrile-N and CaCxNy. Consequently, the formation of gas-phase NOx precursors is suppressed. In contrast, for non-polar amino acids (Leu, Phe), CaO’s alkaline effect dominates, enhancing early deamination to release NH₃ and promoting nitrogen migration via the isocyanic acid pathway, which increases nitrogen allocation to the gas phase. This study has revealed the pathway reconstruction mechanism and structural response patterns of nitrogen migration during pyrolysis under CaO influence. It further proposed design strategies for the directional regulation of nitrogen, thereby providing fundamental support and a mechanistic guide for achieving low-NOx pyrolysis and the conversion of N-rich biomass into the zero-carbon fuel, NH₃.