Numerical study on extinction characteristics of counterflow flame in premixed NH3/CH4/air mixtures under normal temperature and pressure

Use of ammonia fuel is of particular interest because it does not release carbon dioxide during combustion. However, ammonia flameout is easy to occur in practical applications due to its poor combustion intensity and instability. It has been suggested to mix methane with ammonia to enhance its combustion performance. In this study, the Okafor mechanism was selected as the most suitable detailed chemical reaction mechanism for numerical simulation at normal temperature and pressure (NTP) by comparing and analyzing the extinction stretch rate of pure ammonia with air flame under different reaction mechanisms. The NH3-CH4-air laminar premixed counterflow flame at NTP was numerically studied to reveal variations in flame extinction stretch rate over different conditions. It was obtained that the flame extinction stretch rate was increased by increasing CH4 mole fraction in NH3/CH4 mixed fuel, as expected, and that the flame extinction limit was the greatest under lean burn condition (phi = 0.9). When the extinction limit was achieved and then exceeded, the elementary reactions that significantly contributed to the flame heat release rate at different blending ratios were obtained. By comparing the pathway analysis of double ammonia blending ratios, the elementary reaction and radical associated with addition of methane to ammonia and its effects on the flame extinction characteristics were studied.