Experimental and kinetic insight on auto-ignition process of ammonia/propane mixture: Focus on oxygen effect

Propane is a major component of liquefied petroleum gas. Ignition delay times of NH₃/C₃H₈ mixtures were measured in a rapid compression machine over 750–1100 K and 20–100 bar, with propane blending ratios from 10 % to 100 %. A new kinetic model, SJTU-2025, was developed based on POLIMI-2023 and Aramco 3.0 model. The model shows good agreement with IDTs, particularly at high dilution ratios. The model also improves speciation predictions for O₂, H₂, CO₂, C₂H₂, C₃H₆, and C₃H₈. A plateau in simulated mole fractions of O₂, NH₃, and N₂ is observed within 900–1000 K, partially consistent with experimental trends. The NTC trend of oxygen should result from the competition between NC₃H₇/IC₃H₇+O₂[dbnd]HO₂+C₃H₆ and NC₃H₇/IC₃H₇+O₂[dbnd]NC₃H₇O₂/IC₃H₇O₂. The effect of propane addition is pronounced at high ammonia proportions, and small propane fraction is suggested because HCN is prone to being produced at high propane content. A new parameter, the O₂-IDT ratio, reveals the oxygen effect on IDTs over wide conditions. The reason for the pronounced oxygen effect at low temperature is that oxygen participates actively in the R·→RO₂· and C₃H₇O₂=C₃H₆+HO₂ pathways, which are dominant at low temperatures. An important chain-terminating reaction, 2HO₂[dbnd]H₂O₂+O₂, gains importance at reduced oxygen concentration, further decreasing the reactivity of the fuel mixtures. The model considers reactions between peroxy radicals C₃H₇O₂ and NH₃/NH₂, which improve the model predictive ability in IDTs at NTC region and oxygen concentration profile. The source of these kinds of reactions comes from analogy to CH₃OCH₂O₂+NH₃/NH₂ reactions, and large uncertainty exists in the determination of the rate constants. More accurate kinetic parameters are imperative to improve model performance in the low-temperature region.