Experimental and kinetic studies on autoignition characteristics of ammonia/methyl 3-hexenoate mixture

Abstract

Biodiesel is a renewable and promising alternative to diesel with similar physicochemical properties. To explore the feasibility of biodiesel in improving the combustion performance of ammonia (NH${}_3$), this work investigates the autoignition characteristics of NH${}_3$ blended with a medium-size unsaturated biodiesel surrogate (trans-methyl-3-hexenoate, MHX3D), using a heated shock tube. The experiments are conducted at 1108–2097 K with different pressures (2.9–6.2 atm), equivalence ratios (0.5–2.0), and MHX3D blending ratios (0–100%). Ignition delay times of NH${}_3$/MHX3D mixtures decrease with increasing pressure and MHX3D blending ratio and decreasing equivalence ratio. A small addition of MHX3D dramatically reduces the ignition delay time and ignition temperature of NH${}_3$, and this promotion effect is slightly more significant than that of its saturated structure. Using the advanced kinetic theory, the rate constants of the important cross-coupling reactions between MHX3D and NH${}_2$ radicals are accurately determined, where the dual-level multi-structural torsional (MS-T) method is applied to characterize the MS-T anharmonicity. Based on our calculations and literature data, a detailed combustion model is proposed to reveal the combustion mechanism of NH${}_3$/MHX3D mixtures. The kinetic analyses demonstrate that the degeneration of MHX3D in the initial stage yields the reactive radicals that perturb the system to accelerate the consumption of NH${}_3$ by H-abstraction reactions. The cross-coupling reactions between NH${}_2$ radicals and C-containing species and the related subsequent reactions of produced cross-coupling intermediates are crucial in controlling the ignition process of NH${}_3$/MHX3D mixtures.

Novelty and Significance statement: This work investigates the autoignition characteristics of NH${}_3$ blended with a medium-size unsaturated biodiesel surrogate, trans-methyl-3-hexenoate (MHX3D), under a wide range of experimental conditions. The rate constants of important cross-coupling reactions between MHX3D and NH${}_2$ radical are calculated using the canonical variational transition-state theory and the small-curvature tunneling correction with the assistance of the dual-level multi-structural torsional method. Based on our calculations and literature data, a detailed combustion kinetic model is proposed to reveal the combustion mechanism of NH${}_3$/MHX3D mixture. The kinetic analyses demonstrate that the degeneration of MHX3D in the initial stage yields the reactive radicals because of its high reactivity, which perturb the system to accelerate the consumption of NH${}_3$ by H-abstraction reactions. The cross-coupling reactions between NH${}_2$ radicals and C-containing species and the related subsequent reactions of produced cross-coupling intermediates are crucial in controlling the ignition process of NH${}_3$/MHX3D mixture.