Numerical Simulation of the Effects of Hydrogen Addition to Fuel on the Structure and Soot Formation of a Laminar Axisymmetric Coflow C₂H₄/(O₂-CO₂) Diffusion Flame

ABSTRACT

Hydrogen is the cleanest energy carrier and has been extensively used as an additive to improve the combustion properties and lower pollutant formation of other hydrocarbons. This numerical study investigated the effects of H₂ addition to fuel on the structure and soot formation characteristics of laminar coflow C₂H₄ diffusion flames burning in an oxidizer stream consisting of 30% O₂-70% CO₂ at atmospheric pressure using the CoFlame code. Numerical calculations were conducted using a detailed gas-phase reaction mechanism and a soot model comprising the collision of two pyrene molecules as the soot nucleation step, the hydrogen abstraction acetylene addition (HACA) and pyrene condensation as the soot surface growth processes, and soot oxidation by oxygen and OH. Four H₂ addition levels of 0, 10, 30, and 50% on volumetric basis were considered. To demonstrate the chemical effects of H₂, additional calculations were performed with addition of fictitious hydrogen, FH₂. The results confirm the literature finding made in C₂H₄/air diffusion flames that hydrogen addition to ethylene suppresses soot formation significantly through not only the dilution but also the chemical effects. The effectiveness of H₂ on soot formation suppression on per unit percentage addition basis deteriorates as more hydrogen is added to ethylene. The fundamental cause for the reduced soot formation by H₂ addition is that the added hydrogen increases the mole fraction of H₂, resulting in decreased polycyclic aromatic hydrocarbons (PAH_S) concentrations, nucleation rates, and surface growth rates in the soot nucleation and early growth regions. The added H₂ to the fuel stream has no chemical interactions with CO₂ in the oxidizer stream.

KEYWORDS:

• Laminar coflow diffusion flame
• soot formation
• hydrogen addition
• O₂/CO₂ atmosphere

Acknowledgments

The authors would like to thank the Natural Science Foundation of China (grant no. 51776001) for financial support.

Additional information

Funding

The authors would like to thank the Natural Science Foundation of China [grant no. 51776001] for financial support.