Extinction limits and structure of counterflow nonpremixed hydrogen-doped ammonia/air flames at elevated temperatures

• Potential of H2-doped NH3 as a carbon-free fuel is found using nonpremixed flames.
• The high-stretch extinction limits are extended with H2 substitution.
• Flames can sustain more NH3 at low strain rates.
• The extinction limits are extended with elevated air temperature.

The present study demonstrates the potential of hydrogen (H2)-doped ammonia (NH3) as a carbon-free fuel. The extinction limits, flame temperature and morphology of the counterflow nonpremixed NH3–H2/air flames at elevated temperatures and normal pressure are experimentally determined. Also, the detailed flame structure and the extinction limits are computed using a detailed kinetic mechanism. Results show that the blow-off limits, the concentration of radicals H, OH and O and the maximum flame temperature are enhanced with H2 substitution in NH3/air flames. This supports the potential of H2 as an additive for improving the reactivity and ignition of nonpremixed NH3/air flames and thus the potential of H2-doped NH3 as a carbon-free fuel. Meanwhile, the extinction limits (in terms of the mole fraction of NH3 in the fuel gas) and the maximum flame temperature are reduced with increasing strain rates, indicating that flames can sustain more NH3 at low strain rates. Also, it is observed that the blow-off limits and the maximum flame temperature are enhanced with increasing air temperature. Measured and predicted tendencies of the extinction limits and temperature for various conditions show encouraging agreement, but quantitative discrepancies among the measurements and predictions merit additional consideration in boundary condition modeling and the reaction mechanism.