Effects of ammonia substitution on combustion stability limits and NO _x emissions of premixed hydrogen-air flames

The combustion stability limits and nitrogen oxide (NO_x) emissions of burner-stabilized premixed flames of ammonia (NH ₃)-substituted hydrogen (H₂)-air mixtures at normal temperature and pressure are studied to evaluate the potential of partial NH₃ substitution to improve the safety of H₂ use. The effects of NH₃ substitution, nitrogen (N₂) coflow and mixture injection velocity on the stability limits and NO_x emissions of NH₃-H₂-air flames are experimentally determined. Results show a reduction of stability limits with NH₃ substitution and coflow, supporting the potential of NH₃ as a carbon-free, green additive in H₂-air flames and indicating a different tendency from that for no coflow condition. The NO_x emission index is almost constant even with enhanced NH₃ substitution, though the absolute value of NO_x emissions increases in general. At fuel-rich conditions, the NO_x emission index decreases with increasing mixture injection velocity and the existence of coflow. The thermal deNO_x process in the post-flame region is involved in reducing NO_x emissions for the fuel-rich flames.