Effect of ammonia on the soot surface characteristics in ammonia/ethylene co-flow diffusion flames

•Raman and XPS analysis were applied to study the soot surface functional groups.•Influences of ammonia on the soot graphitization degree were explored.•Effects of ammonia ratio on N-containing functional groups were investigated.•Transformation between N-content functional groups (N-5,N-6, N-Q) were revealed. As a carbon free and hydrogen rich fuel, ammonia is attracting more and more attention. The co-combustion of ammonia and hydrocarbon fuel can improve the combustion characteristics of ammonia and reduce CO2 emissions, and the influence of co-combustion of ammonia and hydrocarbon fuel on reducing soot emission characteristics has also aroused people's interest. In this study, the laminar co-flow ethylene diffusion flame with different ammonia addition ratios (0 %, 10 %, 20 %, and 30 %) was studied and the soot surface functional groups and graphitization structure characteristics were explored by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) respectively. The results showed that ammonia accelerated the soot graphitization process, improved the degree of graphitization, and increased the content of oxygen containing functional groups on the surface of soot particles. The XPS N1s analysis showed that, a certain amount of nitrogen-containing functional groups can be detected in ammonia/ethylene co-flow diffusion flames. The N-Ox content increased with the increase of ammonia ratio when the NHAB was>0.7. The N-5 content decreased linearly along the flame height for different the NH3 ratios, and the transforms between N-5, N-6 and N-Q were observed with the soot formation, growth and oxidation.