Effects of diluents on laminar burning characteristics of bio-syngas at elevated pressure

A study on the effect of diluents (N₂/CO₂) on laminar flame speed and flame instability of H₂/CO/CH₄/air premixed flame was conducted using outwardly propagating spherical flame and CHEMKIN package. Experiments and numerical computation were conducted at an initial temperature of 303 ± 3 K, pressures of 0.1–0.5 MPa, equivalence ratios of 0.6–1.5 with N₂ or CO₂ dilution ratios from 0% to 45%, at an interval of 15%. The measured laminar flame speed was compared with values predicted using the Li mechanism. The thermal and chemical kinetic analyses were also presented. The results show that the Li mechanism can give a good predication on laminar flame speed of H₂/CO/CH₄/air premixed flames. But with N₂/CO₂ dilution, the Li mechanism gives slight underestimations at fuel-rich conditions. Laminar flame speed decreases with the increase of N₂/CO₂ dilution ratios. Results indicate that CO₂ dilution has a stronger dilution effect, thermal effect, and chemical effect than those of N₂ dilution, and thereby it can substantially decrease the laminar flame speed of H₂/CO/CH₄/air mixture. On the other hand, flame instability is slightly promoted with N₂/CO₂ dilution at atmospheric pressure, while at elevated pressure, it is suppressed with N₂/CO₂ dilution due to the significant decrease of thermal expansion ratio and increase of flame thickness. Compared with N₂ dilution, CO₂ dilution has a stronger inhibiting effect on hydrodynamic instability.