Impact of ammonia addition on soot particle microstructure and formation in propane diffusion flames

Ammonia (NH₃), as a zero-carbon fuel, has drawbacks such as difficult ignition, slow combustion, and poor stability, limiting its application as a single fuel. The combined combustion of NH₃ and hydrocarbons is a feasible solution, but inevitably brings soot particle emission problem. This work investigated the effects of NH₃ addition to propane (C₃H₈) on soot particle microstructure and formation experimentally and numerically. The results showed that NH₃ addition shift the flame orange-yellow region downstream, reduces its area fraction, and delays the initial soot formation. The soot concentration decreases with increasing NH₃ blending ratio. Adding 20 %, 40 % and 60 % NH₃ decreases the peak mean size of primary particles by 3.78 %, 8.17 % and 23.61 %, respectively. At the nanoscale, NH₃-blended flames exhibit a reduced mean fringe length, increased fringe tortuosity and inter-fringe spacing compared to pure C₃H₈, indicating a lower graphitization degree and enhanced oxidation reactivity. Molecular dynamics indicated that addition of NH₃ delays initial carbon ring formation, reduces PAHs size, and forms N-PAHs hindering collision growth of PAHs. Kinetic analysis revealed that NH₃ addition triggers C-N cross-reactions to form CH₂NH₂ and CH₂NH, thereby reducing the key intermediate C₃H₃ for A1 formation, and inhibiting C₂H₂ formation, thus hindering both C₂H₂→C₃H₃→A1 pathway and HACA mechanism to inhibit large PAHs formation.