Ignition Delay Times and Chemical Reaction Kinetic Analysis for the Ammonia–Natural Gas Blends

The combustion characteristics of ammonia–natural gas (NH₃–NG) blends are usually studied using ammonia–methane (NH₃–CH₄) blends. However, the ignition characteristics of NH₃–NG and NH₃–CH₄ are different due to ethane (C₂H₆) and propane (C₃H₈) in NH₃–NG. In the present study, a natural gas fuel model (96.73% CH₄, 2.59% C₂H₆, and 0.68% C₃H₈ in molar fraction) was constructed using the real composition of China natural gas to investigate the ignition delay times (IDTs) of NH₃–NG. The IDTs of pure NH₃, 50% NH₃–50% CH₄, 50% NH₃–50% NG, and pure NG were measured experimentally using a high-pressure shock tube under an ignition pressure (P_i) of 10 bar, ignition temperatures (T_i) ranging from 1450 to 1900 K, and with 95% argon (Ar) dilution. The NUIG mechanism was selected for investigating chemical reaction kinetics. The IDTs for the fuels follow this order: 100% NH₃ > 50% NH₃–50% CH₄ > 50% NH₃–50% NG > 100% NG. At T_i = 1600 and 1800 K, the IDTs for NH₃–NG are 38.4 and 33.3% shorter than NH₃–CH₄, respectively. Adding C₂H₆ and C₃H₈ increases the CH₃ radical mole fraction during the first half of the ignition process (0–0.5 IDTs). During this stage, C₂H₆ participates in the NH₂ → NH₃ transition via reaction C₂H₆ + NH₂ ⇔ C₂H₅ + NH₃ (R11); in the meantime, the C₃H₈ is depleted through the reaction C₃H₈ (+M) ⇔ C₂H₅ + CH₃ (+M) (R9). During the second half of the ignition process (0.5–1.0 IDTs), the differences between NH₃–CH₄ and NH₃–NG become insignificant. C₂H₆ and C₃H₈ mainly affect the first half of the NH₃–NG ignition process, resulting in shortened IDTs.