An experimental and molecular simulation study on the soot formation characteristics and mechanism in hydrogen-ammonia blended ethylene combustion
Yang Wang, Qianqian Liu, Jingyun Sun, Mingyan Gu, Dongming Liu, Xiangyong Huang
Abstract
Extinction measurement and thermophoretic sampling techniques were employed to investigate the soot formation characteristics in H 2 -NH 3 blended C 2 H 4 combustion. And the formation mechanism of benzene (A 1 ) 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 (H 2 /NH 3 ) 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 (fv max ) gradually decreases and shifts from the flame wing towards the flame centerline. Addition of H 2 -NH 3 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 H 2 /NH 3 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 NH 3 proportion favors N in competition with H for active sites on soot particles. A 1 - + H ↔ A 1 is the most crucial reaction pathway of A 1 formation. As the H 2 /NH 3 ratio increases, H 2 concentration significantly rises, leading to an enhancement in the forward reaction rate. Furthermore, the competition for H between H 2 and NH 3 by N H 3 + H ↔ N H 2 + H 2 results in a chemical coupling effect on soot formation.