Methane/ammonia co-pyrolysis interaction and hydrogen generation mechanisms: A molecular dynamics study

Abstract

Ammonia replacing part of methane can significantly reduce CO₂ emission during their co-combustion. Pyrolysis is the vital stage of co-combustion, profoundly affects the combustion characteristics of methane/ammonia mixtures. In this study, the reactive molecular dynamics (ReaxFF MD) method was used to study methane pyrolysis, ammonia pyrolysis, and methane/ammonia co-pyrolysis. Firstly, by comparing the reaction rates of methane and ammonia at different temperatures and ammonia blending ratios, it was found that increasing the temperature and ammonia blending ratio both promote methane pyrolysis. In addition, analyzing the activation energies of methane/ammonia co-pyrolysis and methane and ammonia pyrolysis separately, it was found that ammonia has a promoting effect on methane pyrolysis. Then, by comparing the changes in the amount of hydrogen under different conditions, it was found that methane/ammonia co-pyrolysis generated more hydrogen. Furthermore, the hydrogen atoms in the hydrogen gas were traced using atomic labeling method. The results showed that the reason for the increase in hydrogen could be divided into two parts: the first 500ps were mainly attributed to the interaction between methane and ammonia, generating more coupled hydrogen, while the last 500ps were dominated by the promoting effect of ammonia gas on methane pyrolysis, and both reasons jointly promoted the generation of hydrogen. Finally, the pathways of hydrogen generation during methane/ammonia co-pyrolysis were analyzed, and it was found that NH₃ + H → NH₂ + H₂ and CH₄ + H → CH₃ + H₂ were the main pathways of hydrogen generation during methane/ammonia co-pyrolysis.