Effects of hydrogen addition on soot formation and oxidation in laminar premixed C2H2/air flames

In an effort to elucidate the influence of hydrogen addition on soot formation and oxidation, a series of numerical investigations was performed for fuel rich laminar C2H2/air premixed flat flames using a modified CHEMKIN-II PREMIX code with a detailed soot chemistry mechanism. To clarify the influence of hydrogen addition, the hydrogen content (in volume %) in the fuel mixture was gradually increased from 10 to 50%. The hydrogen addition was found to slow the oxidation of C2H2 near the burner surface. The lowered rate of C2H2 oxidation coupled with lower C2H2 concentration near the burner surface impedes the formation of benzene. However, the formation of benzene was enhanced with the hydrogen addition as the height above burner (HAB) was increased. This was due to the increased reverse rate of the H abstraction reaction that prevents the radical formation process. Through the identical mechanism, the hydrogen addition slows further growth of benzene to larger polycyclic aromatic hydrocarbons (PAHs), eventually lowering the rate of particle inception. Numerical results also indicated that reductions in the soot emissions were mainly attributed to a significant reduction in the mass growth of soot particles. The abundance of hydrogen in the flames deactivated the surface site of soot particles covered with C–H bonds, lowering the surface growth rate (which leads to reductions in the mass growth of soot particles).

► The hydrogen addition to C2H2/air premixed flame suppressed the soot emission.
► The dilution effect of hydrogen was the most dominant mechanism.
► The hydrogen addition chemically decreased the formation of large PAH.
► Reductions in the rate of soot surface growth directly lowered the soot emissions.