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 (Pi) of 10 bar, ignition temperatures (Ti) 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 Ti = 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.