Study of soot microscopic characteristics in hydrogen/methane/ethylene co-flow diffusion flame

Abstract

Hydrogen (H2) has been regarded as a highly competitive alternative fuel that does not produce CO2 and soot during combustion. However, the effect of hydrogen addition on soot formation and emission is questionable. The purpose of this paper is to study the influence of hydrogen/methane on the morphology of soot and the evolution of nanoparticle structure in laminar flame of ethylene. In this study, the soot morphology and nanostructure in H2/CH4/C2H4 laminar co-flow diffusion flames were investigated with thermophoretic sampling and high-resolution transmission electron microscopy (HRTEM). The results showed that the average primary particle diameters (Dp) of pure ethylene flame gradually decreased along the flame height direction, showing the competition between soot surface growth and soot oxidation. When the hydrogen is added into the fuel flow of ethylene, the Dp decrease at most sample points. Methane affects the inhibitory effect of hydrogen doping on soot, and with the increase of methane mixing ratio, the change trend of Dp of each hydrogen mixture with flame height is becoming more and more consistent. The nanostructure analysis showed that hydrogen doping made soot particles shorter, more tortuous and denser fringes. There is a synergistic effect to promote the growth of primary soot particles at low methane ratio, but it disappears after mixing 20 % hydrogen. The fringe length and tortuosity of soot particles gradually increase with the methane ratio.