Extinction limits and structure of counterflow nonpremixed methane-ammonia/air flames

An experimental and computational investigation on the fundamental combustion characteristics of methane (CH₄)-ammonia (NH₃) blends is conducted to confirm their potential as a clean fuel with low carbon dioxide (CO₂) emissions and determine their reasonable burning conditions, considering counterflow nonpremixed CH₄–NH₃/air flames. Extinction limits and structure of the nonpremixed CH₄–NH₃/air flames are measured and predicted. Results show that flames gradually become orange and the flame thickness increases with NH₃ addition, compared with the pure CH₄/air flames. Also, flames can sustain less NH₃ at high strain rates. Compared with the pure CH₄/air flames, CH₄–NH₃/air flames exhibit remarkable reduction of CO₂ emissions with moderate reduction of combustion stability limits and no remarkable temperature drop in the flame, supporting the potential of CH₄–NH₃ blends as a clean fuel with low CO₂ emissions. However, additional investigations for reducing the enhanced NO_x emissions mainly via the fuel NO_x mechanism with NH₃ addition are needed. Finally, the quantitative discrepancy among the present measurements and predictions merits the development of a new reaction mechanism which is optimized for the reaction of CH₄–NH₃ fuel blends and air.