On the effect of ammonia addition on soot formation and nanostructure in laminar coflow flames of n-butane

The effect of ammonia addition on the formation and nanostructure of soot was experimentally investigated in laminar diffusion flames of n-butane. Soot volume fraction and flame temperature were measured in-situ by line-of-sight attenuation and spectral soot emission measurements respectively. Chemical, nanostructural, and electronic information of thermophoretically collected soot particles were obtained by ex-situ Raman and scanning tunneling spectroscopy measurements. The study highlights the influence of ammonia addition on soot temperature, concentration, and yield in n-butane flames. Soot levels dropped from 4.5 ppm in neat flames to below 2 ppm with a mole fraction , with n-butane flames showing less sensitivity to ammonia than ethylene flames investigated in earlier studies. Ammonia reduced soot yields, decreasing carbon-to-soot conversion from 12% to 8% at , and delayed the onset of the soot formation, shifting peak yields upward. Soot temperatures (1600–1900 K) increased with ammonia due to reduced radiative heat transfer from suppressed soot concentrations. These findings confirm the effectiveness of ammonia in suppressing and delaying soot formation in combustion. Finally, ammonia addition is shown to promote the formation of more graphitic soot particles, with larger aromatic domains, and the presence of nitrogen functionalities, as observed by both the Raman and photoluminescence spectra of the soot particles and by the local density of states of the carbon matrix, measured by scanning tunnelling spectroscopy.