Kinetic modeling and experimental study of laminar burning velocities of CH4/NH3/N2O/Ar premixed flames

Abstract

Ammonia (NH₃) is regarded as a carbon-free alternative fuel in modern energy systems. Co-firing NH₃ with CH₄ and/or using N₂O as an oxidizer are promising strategies for overcoming the low reactivity of NH₃. An experimental and kinetic modeling study of laminar burning velocities of CH₄/NH₃/N₂O/Ar flames was first reported in this study. Experiments were performed using the spherical flame method, and the measured conditions covered a full range of CH₄ fractions and a large range of equivalence ratios at 1 atm and 298 K. Several literature mechanisms were tested, but none of them could accurately predict the laminar burning velocities for all the experimental conditions. A new mechanism with 72 species and 521 elementary reactions was proposed and validated. The new model performed well in predicting laminar burning velocity, ignition delay time, and species mole fraction profile (measured not only in this work but in the literature) for CH₄/NH₃/N₂O/Ar relevant flames, and the performance was better than the existing mechanisms. Detailed kinetic analyses using the present model were carried out to reveal the major reaction pathways based on N-atom and C-atom, the dominant elementary reactions, and the thermal and chemical kinetic effects. It was found that the dominant reactions with the two largest positive sensitivity coefficients, N₂O(+M)=N₂+O(+M) and N₂O+H=N₂+OH, were directly relevant to N₂O. Besides, most of the dominant elementary reactions influencing laminar burning velocities were relevant to the N-family, while few involved the C-family. The positive effect of CH₄ addition was mainly attributed to the enhancement of thermal and chemical kinetic effects. The present model provides insights into the chemical kinetics for CH₄/NH₃/N₂O/Ar flames, and can be considered as the foundation for developing larger fuel molecule mechanisms.