The potential of partial ammonia (NH3) substitution to improve the safety of hydrogen (H2) use was evaluated, using premixed NH3/H2/air flames at normal temperature and pressure generated on a tube type burner. The effects of NH3 substitution on the stability limits and nitrogen oxide (NOx) emissions of premixed H2/air flames and the effects of inert (nitrogen, N2) coflows on the NH3-substituted H2/air flames were investigated. The lower stability limits due to heat losses are observed for all the present test conditions, while the upper stability limits due to insufficient residence times of injected mixture jet are observed for limited test conditions. Results also show reduction of the stability limits with NH3 substitution in H2/air flames, supporting the potential of NH3 as a carbon-free, clean additive for improving the safety of H2 use in premixed H2/air flames. Opposite tendencies of the upper stability limits with and without the coflow are observed since nonpremixed flames near the burner exit cannot be generated with the coflow. Ammonia substitution enhances the NOx formation in general; however, the NOx emission index that is defined as the fraction of the mass of the produced NOx per the mass of the provided NH3 and H2 is almost constant with the enhanced ammonia substitution. At fuel-rich conditions, the NOx emission index is reduced with increasing burner exit velocities of the injected mixtures.
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