Studies of soot oxidative reactivity using a diffusion flame burner

Oxygen addition to a laminar diffusion flame burner was carried out by intake oxygen enrichment and by fuel oxygenation in order to study the relation between soot oxidative reactivity and the combustion process. For this work, oxidative reactivity of n-heptane-derived soot was analyzed with respect to adiabatic flame temperature at various oxygen concentrations of the oxidizer stream, and compared to those of ethylene-derived soot and soot derived from an oxygenated fuel (a mixture of 70 vol.% n-heptane and 30 vol.% monoglyme (C4H10O2), monoglyme mixture). There is a clear inverse relation observed between adiabatic flame temperature and soot oxidative reactivity. However, detailed experiments suggest that the primary factors impacting soot oxidative reactivity are the soot inception limit, which is closely related to soot formation rate, and the soot oxidation process: the earlier soot is formed, the less reactive it becomes. Consequently, ethylene-derived soot is shown to be less reactive than n-heptane-derived soot under the same carbon flow rate and the same adiabatic flame temperature, because soot is formed at an earlier stage with ethylene than with n-heptane. In addition, increasing oxygen concentration in the oxidizer stream made soot less reactive, because not only is the soot inception limit increased by the increase in the flame temperature, but also the soot oxidation process is enhanced by increased abundance of oxidizing gases during the combustion process. From these results, it is concluded that the reason why soot derived from the monoglyme mixture is more reactive than soot from n-heptane is related to the longer time for soot to be incepted, due to the reduced concentration of soot precursors with the oxygenated fuel.