Impacts of NO on low-temperature oxidation of n-heptane in a jet-stirred reactor

Low-temperature (low-T) oxidation experiments of n-heptane –with and without NO addition– were experimentally and numerically investigated at stoichiometric conditions in a jet-stirred reactor. Experiments were performed at atmospheric pressure over a temperature range of 500–800 K. Reactants, intermediates, and products, were measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed to gain insight into the chemical effect of NO on low-T oxidation chemistry of n-heptane. Taking 650 K as the transition temperature, the results revealed that NO addition exhibited an inhibiting effect on fuel reactivity below 650 K and a promoting effect above 650 K. The reactions of ROO + NO = RO + NO2 and HO2 + NO = OH + NO2 at different temperature regions were responsible for the inhibition and promotion effects, respectively. Evidence gathered from both experimental measurements and kinetic model predictions indicated that NO addition had a significant inhibitory effect on the formation of cool flame species during the low-T oxidation process. NO suppressed low-T oxidation via the reaction of ROO + NO = RO + NO2, which impeded the subsequent isomerization, O2 addition, OH-, and HO2-elimination reaction, and influenced product distribution of the cool flame species. The experimental observations provided detailed information about these reactive intermediates, which offered new insights into low-T oxidation phenomena and clarified the importance of NO reactions which prevent the formation of cool flame products during low-T oxidation.