Effects of ammonia substitution on combustion stability limits and NOx emissions of premixed hydrogen–air flames

The combustion stability limits and nitrogen oxide (NOx) emissions of burner-stabilized premixed flames of ammonia (NH3)-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 NOx 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 NOx emission index is almost constant even with enhanced NH3 substitution, though the absolute value of NOx emissions increases in general. At fuel-rich conditions, the NOx emission index decreases with increasing mixture injection velocity and the existence of coflow. The thermal deNOx process in the post-flame region is involved in reducing NOx emissions for the fuel-rich flames.