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

Abstract The combustion stability (extinction) limits and nitrogen oxide (NO x ) emissions of nonpremixed ammonia (NH 3 )–hydrogen (H 2 )–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH 3 substitution for improving the safety of H 2 use and to provide a database for the nonpremixed NH 3 -substituted H 2 –air flames. Considering coflow nonpremixed NH 3 –H 2 –air flames for a wide range of fuel and coflow air injection velocities ( V fuel and V coflow ) and the extent of NH 3 substitution, the effects of NH 3 substitution on the stability limits and NO x emissions of the NH 3 –H 2 –air flames are experimentally determined, while the nonpremixed NH 3 –H 2 –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 NH 3 substitution, supporting the potential of NH 3 as an effective, carbon-free additive in nonpremixed H 2 –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 NH 3 substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH 3 is consumed more upstream than H 2 due to the difference between their diffusivities in air.