An experimental and detailed kinetic modeling study of the auto-ignition of NH3/diesel mixtures: Part 2- Wide pressures up to 120bar

Abstract

Ammonia (NH3) blending combustion strategy has been regarded as a high-efficient form of NH3 utilization. To understand the high-pressure gas-phase auto-ignition characteristics of NH3 blending with diesel, the ignition delay times of NH3/diesel mixtures with high NH3 energy fractions of 70%, 80% and 90% were measured in an ultrahigh-pressure rapid compression machine, at high pressures of 50–120 bar, temperatures of 755–996 K, and equivalence ratios of 0.5–1.5. As a consecutive study of Part 1 (Combust. Flame, 251 (2023), 112391), this study mainly focuses on the auto-ignition characteristics and oxidation mechanism of NH3/diesel mixtures at pressures higher than 50 bar. The ignition-promoting effect of increasing pressure from 50 to 120 bar was experimentally confirmed. Moreover, the increase of the diesel energy fraction, equivalence ratio, and oxygen concentration was found to decrease the ignition delay time of the NH3/diesel mixtures. The chemical reaction mechanism proposed in Part 1 was utilized to simulate the auto-ignition of the NH3/diesel mixtures. Simulation results show that the mechanism was able to quantitatively capture the ignition delay times of the NH3/diesel mixtures at the wide pressures. The cross-reactions between NH3 and diesel, especially reaction RH + NH2 = R + NH3, was found to play a key role in the correct prediction of auto-ignition at high pressure. In addition, it is revealed the effect of pressure is closely related to OH production reactions 2OH (+M) = H2O2 (+M) and NO + HO2 = NO2 + OH, the rate of which increase with rising pressure. The reaction H2NO + O2 = HNO + HO2 is an important pathway where oxygen concentration affects NH3/diesel mixture auto-ignition.