Synchrotron vacuum ultraviolet photoionization mass spectrometry to examine low temperature oxidation chemistry of n-heptane under different fuel concentrations and pressures

Abstract

The study of low temperature oxidation provides valuable insight into the development of low temperature combustion (LTC) engines. Fuel concentration and pressure are the keys to controlling reaction activities, significantly influencing low temperature oxidation behavior. Understanding the effects of these parameters is important to develop and improve the kinetic models, however, the impact of fuel concentration and pressure is rarely examined in the low temperature oxidation process of hydrocarbons. In this work, n-heptane oxidation, with initial fuel mole fractions of 0.1 %, 0.25 % and 0.5 % and pressures of one, five and ten bar, was examined at 440–800 K. The goal was to investigate the influence of these key parameters on n-heptane low temperature oxidation. First, reactivity and formation of products was promoted by increasing the initial fuel concentration; there was a threshold for the initial fuel concentration, and reactions occurred only when it was higher than the threshold at a fixed pressure and residence time. However, the model in the literature was unable to capture this phenomenon. Species profiles were compared with the prediction of the kinetic model in the literature at three initial fuel concentrations and pressures; simulation results were verified, and the different pressure effects on product formation were observed. A preliminary analysis of the reaction mechanism was conducted using the kinetic model for clarification of the pressure effects. Finally, the selectivity of products under one and ten bar was revealed. In general, hydroperoxides and carboxylic acids, etc., displayed positive selectivity, while olefins and cyclic ethers, etc., showed negative selectivity at high pressures.