Synergistic effects of PODE3 and ammonia on soot formation in n-heptane diffusion flames

Soot formation in hydrocarbon diffusion flames remains a major challenge for clean combustion, and the comprehensive impact of oxygenated and carbon-free fuels on soot evolution is not fully understood. Experimental investigations were performed to evaluate the effect of ammonia (NH 3 ) addition on soot formation in polyoxymethylene dimethyl ether (PODE 3 )/ n -heptane blended diffusion flames. The particle size, nanostructure, and physicochemical properties of the soot samples were comprehensively analyzed. The results showed that, at both sampling heights, the primary soot particle diameter decreased as the NH 3 addition level increased. The reduction was only slight under the N20 condition, while the inhibitory effect became significant under the N40 and N60 conditions. The addition of NH 3 further decreased the graphitization degree of soot, thereby leading to a more disordered arrangement of microcrystalline layers. This structural regulation effect became more pronounced during the soot growth stage. The addition of PODE 3 introduced oxygen into the soot structure, while the blending of NH 3 modified the influence of PODE 3 , significantly altering the relative atomic ratio contents of N, O, and C on the soot surface. At low NH 3 blending levels, the (O + N)/C ratio slightly decreased, indicating that the functionalization of surface heteroatoms was somewhat inhibited. At high NH 3 blending levels, the (O + N)/C ratio significantly increased, indicating an increase in the surface defects and the degree of heteroatom functionalization of soot. FTIR analysis further confirmed that NH 3 blending resulted in a reduction in the absorption peak intensity of soot‑related characteristic functional groups. The results of this study provided experimental evidence that improved our understanding of the soot evolution mechanism during the combustion of n -heptane with PODE 3 /NH 3 .