Study of soot functional groups and morphological characteristics in laminar coflow methane and ethylene diffusion flames with hydrogen addition

Abstract

The soot in laminar coflow methane and ethylene diffusion flames with different hydrogen addition ratios is sampled by a capillary soot sampling system and analyzed by Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Soot functional groups and morphological characteristics are studied. The results show that with hydrogen addition, the relative contents of aromatic and aliphatic functional groups in the soot of methane flames increase, while the relative contents of the same functional groups in the soot of ethylene flames mostly show different trends, indicating that H2 has a chemical promotion and inhibition on soot in methane and ethylene flames, respectively. The relative content of oxygen-containing functional groups in the soot of both methane and ethylene flames with hydrogen addition increases. By adding H2, the coagulation and condensation of soot low in the methane flames are greatly enhanced, indicating that H2 promotes the formation and growth of PAH molecules. This is an important reason for the chemical promotion effect of H2 in methane flames. For the soot of C2H4 hydrogen-added flames, oxidation is relatively thorough such that a large number of soot branches are oxidized. This is an important reason for the chemical inhibition of H2.