Investigation of NH* chemiluminescence and NO formation mechanisms in CH4/NH3 co-flow diffusion flames: A computational kinetic perspective

NH*, a characteristic radical in ammonia-blended flames, is a critical parameter for evaluating combustion efficiency and kinetic characteristics through its chemiluminescence properties. In this work, numerical investigations were conducted on NH* chemiluminescence and NO formation mechanisms in CH₄/NH₃ diffusion flames at different ammonia blending ratios using a modified Okafor 2018 reaction mechanism. A two-dimensional distribution of NH* chemiluminescence was obtained using a spectral detection platform with 337 nm and 355 nm filters for NH* background radiation subtraction. The NH* emission was mainly concentrated in the upstream region of the diffusion flame near the fuel outlet, and the peak intensity showed a non-monotonic variation with increasing ammonia blending ratio-initially rising and then decaying. It was found that the collisional quenching reactions NH* + M<=>NH + M and NH* + NH₃<=>NH + NH₃ were considered the main quenching pathways for NH* in CH₄/NH₃ flames. The generation reactions were N₂* + NH<=>N₂+ NH* and CH + NO<=>NH* + CO. HNO, NH and NH₂ were the key species influencing NO generation and consumption. The main generation reactions of HNO were NH + OH<=>HNO + H and NH₂ + O<=>HNO + H, which gradually increased with increasing ammonia blending ratio. In addition, the correlation between NH* and NO distribution was analyzed. NH* can characterize the distribution core of NO.

Novelty and significance statement: The first development of a refined kinetic mechanism integrating detailed NH*/N₂* mechanisms with the Okafor mechanism was presented, overcoming the critical limitation in existing models for predicting NH* chemiluminescence in the ammonia flame. This provided a validated tool for quantitative analysis of radicals in CH₄/NH₃ combustion systems. NH* chemiluminescence was identified as a novel optical marker for characterizing ammonia combustion dynamics in our study, with previously unreported correlations between NH* formation, quenching processes, and ammonia blending ratios being revealed. It established a new methodology for non-intrusive monitoring of ammonia-blended combustion. Furthermore, we elucidated of the dual-phase relationship between NH* evolution and NO formation pathways, uncovering the key mechanisms and two-dimensional distribution patterns of pollutant NO. These findings provided important insights for developing spectroscopy-based optimization strategies to enhance combustion efficiency and reduce NO emissions in ammonia-blended flames.