Combustion stability limits and NOx emissions of nonpremixed ammonia-substituted hydrogen–air flames

The combustion stability (extinction) limits and nitrogen oxide (NOx) emissions of nonpremixed ammonia (NH3)–hydrogen (H2)–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH3 substitution for improving the safety of H2 use and to provide a database for the nonpremixed NH3-substituted H2–air flames. Considering coflow nonpremixed NH3–H2–air flames for a wide range of fuel and coflow air injection velocities (Vfuel and Vcoflow) and the extent of NH3 substitution, the effects of NH3 substitution on the stability limits and NOx emissions of the NH3–H2–air flames are experimentally determined, while the nonpremixed NH3–H2–air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NOx emission index for enhanced NH3 substitution, supporting the potential of NH3 as an effective, carbon-free additive in nonpremixed H2–air flames. With increasing Vcoflow the NOx emission index decreases, while with increasing Vfuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given Vcoflow/Vfuel the flame length increases with enhanced NH3 substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH3 is consumed more upstream than H2 due to the difference between their diffusivities in air.