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 double left right arrow C2H5 + NH3 (R11); in the meantime, the C3H8 is depleted through the reaction C3H8 (+M) double left right arrow 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.