Flow Reactor Oxidation of Ammonia–Hydrogen Fuel Mixtures

Hydrogen-assisted oxidation of ammonia under flow reactor conditions was investigated through experiments and chemical kinetic modeling. Novel experiments, conducted in a tubular laminar flow reactor as a function of the NH₃/H₂ ratio, stoichiometry, and temperature (725–1475 K), were analyzed along with literature results from tubular and jet-stirred flow reactors. Ignition and oxidation of NH₃ is strongly promoted by the presence of H₂ under all conditions investigated. In general, the behavior is captured well by the kinetic model. With an increasing fraction of H₂ in the fuel mixture, the generation of chain carriers gradually shifts from being controlled by the amine reaction subset to being dominated by the oxidation chemistry of H₂, which is known more accurately. However, under reducing conditions, the H₂ consumption rate is strongly underpredicted. This shortcoming suggests that the thermochemistry of amine radicals and/or the formation of higher amines need further assessment. The present analysis shows that for lean oxidation of NH₃/H₂ mixtures in tubular flow reactors, data obtained at higher temperatures, particularly for NO formation, may be strongly affected by the reaction during preheating or by mixing (dependent on reactor design) in the inlet section prior to the isothermal zone. Modeling predictions for the high pressure, medium-temperature ignition conditions in a large diesel engine indicate that NH₃/H₂ fuel mixtures may still require a cofuel to secure stable ignition.