Unraveling an Unexpected Trade-Off: The NO x –N ₂ O Evolution and Chemical Mechanism for Co-Combustion of Coal and Biomass

Nitrogen oxides (NOx) reduction is of great significance for further reducing both PM2.5 and ground-level ozone concentrations. Controlling nitrogen oxide emissions from coal-biomass cocombustion is an urgent challenge. This study develops a novel standalone Chemical Reactor Network (CRN) model to mechanistically elucidate the distinct nitrogen transformation pathways in cocombustion environments. This work reveals an enhanced bidirectional NO-N₂O interconversion in the dilute phase, mediated through both direct NCO/NH radical routes and indirect pathways involving NO₂ intermediates. The key distinction arises from biomass-volatile induced pathway switching: biomass-derived HCN significantly elevates NCO radical pools, establishing parallel NO-to-N₂O conversion channels that create the characteristic emission profile of simultaneous NO reduction and N₂O accumulation. Temperature exerts precise regulation, with NO-N₂O conversion showing unidirectional characteristics in specific ranges, where higher temperatures promote NO formation via enhanced CH₃ radical chemistry. Furthermore, the proposed biomass-coal balancing theory reveals that moderate blending ratios favor HCN → N₂O conversion, reducing NO emissions, while excessive biomass shifts equilibrium toward HCN → NO pathways. This threshold behavior provides a theoretical foundation for targeted NOx control in multifuel combustion systems.