An experimental and molecular simulation study on the soot formation characteristics and mechanism in hydrogen-ammonia blended ethylene combustion

Extinction measurement and thermophoretic sampling techniques were employed to investigate the soot formation characteristics in H2-NH3 blended C2H4 combustion. And the formation mechanism of benzene (A1) was analyzed using reactive molecular dynamics simulation (ReaxFF MD), unveiling the chemical coupling effects on soot formation at the molecular level. As the hydrogen/ammonia (H2/NH3) ratio decreases, the flame height decreases gradually, the flame brightness noticeably darkens, and the dark non-sooting core continuously increases. The maximum of soot volume fraction (fvmax) gradually decreases and shifts from the flame wing towards the flame centerline. Addition of H2-NH3 leads to an increase in the proportion of small-sized fringes and fringe tortuosity, resulting in an increase in the oxidation activity and a decrease in the fv. The largest soot particle contains a small amount of nitrogen atom (N). Some of these N atoms replace hydrogen atom (H) to bond with carbon ring, while others substitute for carbon atom (C) to form nitrogen ring, ultimately resulting in nitrogen-containing polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing soot. The reduction in the H2/NH3 ratio leads to a reduction in C in the largest soot particle, which more effectively inhibits soot formation and graphitization. On the other hand, it increases the N and nitrogen ring in the largest soot particle. It is possible that the increase in NH3 proportion favors N in competition with H for active sites on soot particles. A1 - +H <-> A1 is the most crucial reaction pathway of A1 formation. As the H2/NH3 ratio increases, H2 concentration significantly rises, leading to an enhancement in the forward reaction rate. Furthermore, the competition for H between H2 and NH3 by NH3 +H <-> NH2 +H2 results in a chemical coupling effect on soot formation.