Ignition Delay Times and Chemical Reaction Kinetic Analysis for the Ammonia–Natural Gas Blends
Biao Liu, Mengni Zhou, Zunhua Zhang, Xiaoxiong Mi, Belal Y. Belal, Gesheng Li
Abstract
The combustion characteristics of ammonia–natural gas (NH 3 –NG) blends are usually studied using ammonia–methane (NH 3 –CH 4 ) blends. However, the ignition characteristics of NH 3 –NG and NH 3 –CH 4 are different due to ethane (C 2 H 6 ) and propane (C 3 H 8 ) in NH 3 –NG. In the present study, a natural gas fuel model (96.73% CH 4, 2.59% C 2 H 6, and 0.68% C 3 H 8 in molar fraction) was constructed using the real composition of China natural gas to investigate the ignition delay times (IDTs) of NH 3 –NG. The IDTs of pure NH 3, 50% NH 3 –50% CH 4, 50% NH 3 –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% NH 3 > 50% NH 3 –50% CH 4 > 50% NH 3 –50% NG > 100% NG. At T i = 1600 and 1800 K, the IDTs for NH 3 –NG are 38.4 and 33.3% shorter than NH 3 –CH 4, respectively. Adding C 2 H 6 and C 3 H 8 increases the CH 3 radical mole fraction during the first half of the ignition process (0–0.5 IDTs). During this stage, C 2 H 6 participates in the NH 2 → NH 3 transition via reaction C 2 H 6 + NH 2 ⇔ C 2 H 5 + NH 3 (R11); in the meantime, the C 3 H 8 is depleted through the reaction C 3 H 8 (+M) ⇔ C 2 H 5 + CH 3 (+M) (R9). During the second half of the ignition process (0.5–1.0 IDTs), the differences between NH 3 –CH 4 and NH 3 –NG become insignificant. C 2 H 6 and C 3 H 8 mainly affect the first half of the NH 3 –NG ignition process, resulting in shortened IDTs.