Effect of ammonia addition on the morphology and nanostructure evolution of soot particles in ethylene co-flow diffusion flames

Carbon-free ammonia (NH3) fuel blending hydrocarbon fuel is a viable option to overcome the drawbacks of pure NH3 combustion, but will lead to soot formation. In this study, the effects of NH3 addition (0-40 vol%) on soot morphology and nanostructure evolution in ethylene (C2H4) co-flow diffusion flames were investigated by using thermophoretic sampling particle diagnostic-transmission electron microscopy and lattice-fringe algorithm. The results showed that under the same carbon flow rate, NH3 addition has little effect on the flame height, but great effect on the flame structure and soot formation. With the increase of NH3 blending ratio, the soot volume fraction (SVF), the fractal dimension of the soot aggregates and the average primary particle size showed a gradual decrease. Among them, with 40% NH3 addition, the SVF decreased by 76.7% and the peak average particle size decreased by 34.3% at HAB = 30 mm. Moreover, many particles with multi-core core-shell structure appear in the A40 flame. In terms of nanostructure parameters, the mean fringe length of particle fringes shows a gradually increasing trend, while the mean tortuosity and mean inter-fringe spacing show a gradually decreasing trend with the increasing HAB. NH3 addition causes the fringe length to decrease, the tortuosity and inter-fringe spacing to increase. The soot particles formed in the doped NH3 flames have more disordered structure.