Investigation of NO emission characteristics from co-combustion of methane and ammonia at high-altitude areas

The high-altitude areas of China are abundant in renewable energy and have a natural advantage in ammonia production. Based on this advantage, this paper proposes a co-combustion strategy for methane and ammonia to reduce carbon emissions in these areas. However, the NO emission characteristics associated with this strategy remain uncertain. A custom-designed combustion system capable of simulating high-altitude environments was used to investigate the effect of ammonia mixing ratio, equivalence ratio, and pressure on NO emission in methane/ammonia/air flames. Additionally, chemical kinetic calculations were conducted to explore the mechanisms of how sub-atmospheric pressure influences NO emission. The results indicate that for stoichiometric flames, NO increases with the ammonia mixing ratio. In fuel-rich flames, NO remains nearly constant once the ammonia mixing ratio exceeds 10 %. Sub-atmospheric pressure leads to higher NO, particularly in fuel-rich flames, where the increase can reach up to 24.4 %. Analysis of nitrogen reaction pathways and key radical concentrations reveals that sub-atmospheric pressure has a minimal effect on nitrogen conversion pathways. The variation in NO is achieved by altering the pathway contributions and the concentrations of H, NH, and N. This work provides direction and guidance for improving the application of methane and ammonia co-combustion in high-altitude areas.