Soot characteristics from diffusion flames coupled with plasma

Abstract

The study of soot characteristics in plasma-flame interaction was necessary to improve combustion efficiency and reduce pollutant emissions. This work experimentally investigated effects of plasma on soot formation, evolution and characteristics in ethylene diffusion flames by both optical diagnostic and sampling methods. The optical diagnostic method was employed to capture the two-dimensional distribution of soot temperatures and concentrations. Moreover, the nanostructure, crystallite properties and oxidation reactivity of soot from diffusion flames with plasma addition were obtained using transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray diffraction spectroscopy. The flame shapes and luminosity varied depending on the electrical parameters and oxygen concentration. It was found that the overall temperature increased somewhat with plasma activation, but remained roughly constant when the discharge frequency or applied voltage increased. The soot concentration immediately reduced with plasma on and then displayed different trends as the discharge frequency or applied voltage increased. The variation in soot concentration was caused by the combination of several parameters, including flame behavior, soot temperature and the effect of the electric field by energetic and chemically active species. The generated soot showed typical chain-like aggregates without plasma, but more film-like materials were presented with plasma acting on flames. The soot graphitization degree decreased notably with plasma generation resulting in higher oxidation reactivity because the shorter residence time and the plasma activation were simultaneously obtained, which both illustrated an inhibition effect of plasma on soot emissions. The disordered carbon content related to soot oxidation reactivity.