Impact of PODE3 on soot oxidation reactivity at different stages in n-heptane/toluene diffusion flames

Abstract

PODE3, with the molecular formula of CH3O(CH2O)3CH3, is considered a highly competitive alternative fuel due to the absence of CC bond and its high oxygen content, and have been shown to significantly reduce soot emission when applied as a diesel fuel additive. However, the effect of PODE3 addition on the oxidation reactivity of soot generated in the flame remains unclear which is of great importance for the optimal design of the post-treatment devices during its application. In this work, the impact of PODE3 (0 %, 10 %, and 20 % by volumetric fraction) addition on the oxidation reactivity of soot at different maturity and oxidation degrees in n-heptane/toluene co-flow diffusion flames was specially investigated combining information on thermogravimetric analysis, high-resolution transmission electron microscope, and Raman spectroscopy. Results showed that PODE3 substitution significantly shrank soot generation region, and increased the flame temperatures before height above burner (HAB) < 45 mm. The addition of PODE3 improved the soot oxidation reactivity, which had a strong ability to hinder the growth of the microcrystalline carbon layers and resulted in a high content of amorphous carbon in soot particles. Specifically, it had more significant impacts on the soot in the oxidation stage than the surface growth stage. These can be explained that with PODE3 addition, on the one hand, the “reliable carbon atom” for soot formation is significantly reduced and the amount of oxidizing species is increased, resulting in a smaller particle size and a higher content of disordered and amorphous carbon in soot particles. On the other hand, the flame temperatures increased where HAB < 45 mm, which would promote the soot formation reactions. The combined effects of these factors make PODE3 have more significant inhibitory impacts in reactivity of soot sampled at oxidation stage than that at surface growth stage. Therefore, the impact of PODE3 addition on soot nanostructures and oxidation reactivity is not only reflected in its chemical effect but also in temperature effect.