Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Abstract

Previous studies showed that adding hydrogen (H2) can have an opposite chemical effect on soot formation: its chemical effect enhances and suppresses soot formation in methane (CH4) and ethylene (C2H4) diffusion flames, respectively. Such opposite chemical effect of H2 (CE-H2) remains unresolved. The different CE-H2 is studied numerically in the two laminar coflow diffusion flames. A detailed chemical mechanism with the addition of a chemically inert virtual species FH2 is used to model the gas-phase combustion chemistry in this study. Particularly, a reaction pathway analysis was performed based on the numerical results to gain insights into how H2 addition to fuel affects the pathways leading to the formation of benzene (A1) in CH4 and C2H4 flames. The numerical results show that the CE-H2 in CH4 diffusion flame to prompt soot formation is ascribed that the higher mole fraction of H atom promotes the formation of A1 and Acetylene (C2H2) and leads to higher nucleation rate and eventually higher soot surface growth rate. In contrast, adding H2 to C2H4 diffusion flames decreases soot nucleation and surface growth rate. The lower soot nucleation rate is due to the lower mole fractions of pyrene (A4), while the lower soot surface growth rate is due to the lower mole fractions of H atom and C2H2, higher mole fraction of H2 and lower soot nucleation rate. Furthermore, the CE-H2 in C2H4 diffusion flames promotes the formation of A1, but suppresses the formation of A4.