hydrogen combustion, ammonia, nitrogen oxide, combustion instability
In order to evaluate the potential of partial ammonia (NH3) substitution to improve the safety of hydrogen (H2) use in general and the performance of internal combustion engines in particular, the propagation, development of surface cellular instability and nitrogen oxide (NOx) and nitrous oxide (N2O) emissions of laminar premixed H2/NH3/air flames were studied experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of ammonia substitution. Results show substantial reduction of laminar burning velocities with NH3 substitution in H2/air flames, similar to hydrocarbon substitution. The NH3 substitution enhances NOx and N2O formation for fuel-lean conditions. At fuel-rich conditions, however, NOx is less generated with the NH3 substitution and the extent of the N2O emission increase with the NH3 substitution is much lower than that under fuel-lean conditions. Ammonia substitution does not generate any carbon dioxide and carbon monoxide and compared with hydrocarbon substitution. These observations support the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use with low NOx and N2O emissions in fuel-rich H2/air flames.