When ammonia addition increases the burning velocity of a fuel blend with nitromethane

Combustion properties and combustion chemistry of ammonia (NH₃) are significantly different from those of hydrocarbons and thus require further investigation. NH₃ combustion with oxidizers different from the ambient air can reveal distinct chemistry of NOx formation, which, together with low reactivity, is one of the major obstacles in the direct deployment of ammonia as a practical fuel. In the present study, ammonia was blended with nitromethane (CH₃NO₂), which was used as a nitric oxide (NO) precursor. The laminar burning velocities (LBV) of (CH₃NO₂+NH₃)+air mixtures were investigated across a wide range of NH₃ mole fractions in the fuel blends, from 0% to 80%, spanning fuel-lean to fuel-rich conditions, at an initial temperature of 338 K and 1 atm. The results show that adding NH₃ enhances the reactivity of CH₃NO₂ when the NH₃ fraction in the fuel is below 70%. A kinetic model of the authors was updated, primarily on CH₃NO₂ chemistry, and shows very good agreement with the measurements without any rate constants tuning. Detailed kinetic analyses based on the present model reveal that the reaction NH₂+NONNH+OH significantly impacts the LBV even when a small portion of NH₃ is added to the fuel blend. NH₃ addition is found to increase adiabatic flame temperature and enrich the active radicals’ pools of H, OH, and O as well. The pathways of NH₃ and NO interaction in (CH₃NO₂+NH₃)+air flames are also analyzed, enlightening NO conversion into N₂ in the presence of ammonia.