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

The combustion stability (extinction) limits and nitrogen oxide (NO _x) emissions of nonpremixed ammonia (NH₃)-hydrogen (H ₂)-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 (V_fuel and V_coflow) and the extent of NH₃ substitution, the effects of NH₃ substitution on the stability limits and NO_x emissions of the NH ₃-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 NO _x emission index for enhanced NH₃ substitution, supporting the potential of NH₃ as an effective, carbon-free additive in nonpremixed H₂-air flames. With increasing V_coflow the NO_x emission index decreases, while with increasing V_fuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given V_coflow/V _fuel 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 H ₂ due to the difference between their diffusivities in air.