Shock tube and kinetic modeling study on high-temperature ignition of ammonia blended with methyl hexanoate

Abstract

Ammonia (NH${}_3$) is a promising carbon-free and alternative fuel, but its applications are hindered by high auto-ignition temperature and low burning velocity. A common approach to overcome such drawbacks is to blend NH${}_3$ with a high-reactivity fuel. In this study, a heated shock tube is employed to measure ignition delay time of NH${}_3$ blended with methyl hexanoate (MHX). The experiments are conducted at 6 atm, equivalence ratios of 0.5–2.0, temperatures of 1168–2115 K, and MHX blending ratios of 0, 20%, 50%, 70%, and 100%. Ignition delay time of the binary mixtures decreases monotonically with the addition of MHX. Compared with pure NH${}_3$, the reactivity of the binary mixtures increases significantly with the addition of only 20% MHX, leading to a 10 times faster ignition delay time at around 1500 K and 6 atm. The reactivity of the fuel-lean and stoichiometric ratio mixtures is similar, and higher than the fuel-rich mixtures. The promotion effect of ignition delay time decreases with increasing blending ratio and pressure, and decreasing temperature. The influence of equivalence ratio on the promotion effect of ignition delay time is less significant than that of blending ratio, temperature and pressure. A detailed NH${}_3$/MHX kinetic model is developed by updating the interaction reactions between MHX and NH${}_2$/NO${}_2$/NO radicals, and the NH${}_3$ and MHX sub-mechanism. The present kinetic model can reproduce satisfactorily the ignition delay time of pure MHX and NH${}_3$, and the NH${}_3$/MHX mixtures in the whole experimental conditions explored here. The kinetic analyses reveal that the interaction reactions between MHX and NH${}_2$ radical have a significant impact on the ignition of the binary mixtures. Moreover, the important intermediate N${}_2$H${}_2$ is more prone to forming N${}_2$H${}_3$ rather than NNH in the presence of MHX, different from the production of NNH in pure NH${}_3$ combustion. The H-atom abstraction reaction, NH${}_3$, H + NH${}_3$ = NH${}_2$ + H${}_2$, can proceed in the reverse direction with the addition of MHX, resulting in the production of more active H radicals that facilitate ignition.

Novelty and significance statement:

The practical utilization of pure ammonia (NH${}_3$) as a fuel still faces several challenges and an effective method is the dual-fuel combustion strategy which involves blending low-reactivity NH${}_3$ with a high-reactivity fuel. This work measures the new ignition delay time of methyl hexanoate (MHX) and NH${}_3$/MHX mixtures. A newly detailed NH${}_3$/MHX kinetic model is also developed by updating the interaction reactions between MHX and NH${}_2$/NO${}_2$/NO radicals, and the NH${}_3$ and MHX sub-mechanism. The kinetic analyses reveal that the interaction reactions between MHX and NH${}_2$ radical have a significant impact on the ignition of the binary mixtures and the important intermediate N${}_2$H${}_2$ is more likely to form N${}_2$H${}_3$ rather than NNH in the presence of MHX. To our best knowledge, this is the first study on the effect of methyl ester MHX addition on the ignition behavior of NH${}_3$.