Macroscopic Kinetic Comparison of Hydrogen and Ammonia Oxidation at High Pressure and Temperature

This study investigates the macroscopic kinetics of hydrogen and ammonia oxidation under high-pressure conditions to compare their ignition characteristics, activation energies, and sensitivity to mixture composition. Experiments were conducted in constant-volume and flow reactors over a pressure range of 3–10. MPa and a temperature range of 550–850 K. Hydrogen exhibited significantly shorter ignition delays, reaching as low as 0.14 seconds at 800 K and 10 MPa, compared with 0.35 seconds for ammonia under the same conditions. The activation energy for hydrogen oxidation averaged 171,000 J/mol, whereas that for ammonia was approximately 209,000 J/mol, indicating a higher ignition threshold. The peak pressure during ignition for hydrogen mixtures exceeded 11.5 MPa, whereas that for ammonia mixtures peaked at 8.9 MPa. Hydrogen also exhibited higher concentrations of reactive radicals (H and OH), which explains its more intense chain reaction. Empirical global reaction equations were developed for both fuels, with deviations of up to 10% relative to experimental values. These findings provide a reliable basis for the kinetic modeling of combustion systems operating at high pressures with hydrogen, ammonia, or their mixtures.

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