Insight of soot nanostructure and oxidation behavior in ammonia / ethylene coaxial diffusion flame

Abstract

In this work, the effects of ammonia addition on the morphology evolution, nanostructure and oxidation reactivity of soot particles in ethylene laminar diffusion flame were investigated. Soot sampling was performed using a thermophoresis probe and the morphological evolution of the particles was observed by transmission electron microscopy. In addition, high resolution transmission electron microscopy, Raman spectroscopy and thermogravimetric analysis were adopted to study the nanostructure and oxidation reactivity of soot particles sampled by quartz plate samples. The experimental results show that ammonia addition increases the flame height and compresses the soot formation region, delaying the evolution of each generation stage. For the primary soot particles, the surface growth is inhibited by ammonia doping, and the average particle size d¯p is reduced by about 33.13%. In addition, ammonia doping greatly inhibits the condensation between the primary particles of soot, and the parameters of the agglomerates such as the average area of the agglomerates A¯a, the number of primary particles N¯p and the fractal dimension Df are greatly reduced. Compared to the 100% argon doping ratio, ammonia doping reduced the average area of the agglomerates by approximately 76.27%, and the number of primary particles contained in the aggregate decreased by about 50%. The fringe length of soot particles decreases by about 15.38%, the fringe spacing increases by about 1.67% and the fringe tortuosity increases by about 4.23%, indicating that the degree of graphitization of soot decreases, the nanostructure becomes more disordered and the possibility of soot oxidation increases. The addition of ammonia results in the increase of disordered carbon in the soot of ethylene flame, and the activation energy of soot decreases significantly, so the soot has stronger oxidation reactivity.