Numerical analysis on the combined transport effects of H₂ and CO₂ on the laminar premixed flame characteristics of biogas-hydrogen blends

The combined and respective transport effects of H₂ and CO₂ on the flame structure, laminar flame speed and radical pool of the BG40H60 blends at different equivalence ratios are investigated quantitatively with the numerical simulation in this study. The results show that H₂ transport dominates the decrease and enhancement of HRR and mole fractions of minor species at the fuel-lean and fuel-rich conditions. However, H₂ or CO₂ transport hardly affects concentrations of major species expect for H₂ and CO₂. Besides, the dominated H₂ transport contributes to the decreased/increased laminar flame speed at the fuel-lean/fuel-rich condition, while the OH radical can reflect the laminar flame speed variation caused by the H₂ and CO₂ transport. Based on the rate-of-production (ROP) analysis of OH radical, the most sensitive reactions to H₂ and CO₂ transport are OH + H₂[dbnd]H₂O + H/H + O₂[dbnd]O + OH and OH + CH₂O[dbnd]HCO + H₂O at the fuel-lean and fuel-rich conditions respectively. The major production reactions (H + O₂[dbnd]O + OH, H + HO₂ = 2OH, O + H₂[dbnd]H + OH, 2OH = O + H₂O) of OH radical are suppressed or improved more significantly with the H₂ and CO₂ transport at the fuel-lean or fuel-rich condition, leading to the suppressed or improved OH radical pool and the flame propagation at the fuel-lean or fuel-rich condition. Furthermore, it is demonstrated that CO₂ transport suppresses the reaction of OH + H₂[dbnd]H₂O + H considerably to improve the OH radical pool at the fuel-rich condition and cannot be neglected when investigating the flame propagation of biogas-hydrogen blends.