Effects of ammonia addition on soot formation in ethylene laminar diffusion flames. Part 3. The morphology and nanostructure of soot particles

Ammonia (NH3) fuel has been widely considered a competitive alternative fuel and hydrogen carrier due to its "non-carbon and hydrogen-rich" nature. In this study, the thermophoretic sampling combined with transmission electron microscopy (TEM) analysis was conducted to investigate the effects of NH3 addition with varying ratios (between 0% and 60% by volume fraction while the remainder is argon) on the evolution of morphology and nanostructures of soot particles along the flame axis in the ethylene (40% by volume fraction) co-flow diffusion flames. Results showed that the flame visible height increased with the addition of NH3, and the flame tem-perature decreased at the height above burner (HAB) below 30 mm while increased at HAB > 30 mm. With NH3 addition, the size of soot (primary particles and aggregates) and the number of soot primary particles contained in the soot aggregates decreased significantly. In addition, the investigation of soot nanostructures showed that the carbon layers with shorter fringe length (Fl) and higher tortuosity (Ft) would be oxidized before it arrived to the flame tip in the NH3-doped flame, resulting in the longer Fl and higher Ft. In general, the inter-fringe spacing (Fd) increased with NH3 addition. The evolution of soot morphology and nanostructures indicated that NH3 would delay the process of particle inception and surface growth.