Variations in biomass-derived syngas composition can be a challenge to efficient combustion and low emissions. A study on the effects of CO2 dilution and CH4 addition on flame propagating characteristics of syngas at high pressure was conducted using the heat flux method and kinetic simulations. This paper presents the laminar burning velocity, S-L of H-2/CO/CO2/O-2/diluent mixtures and H-2/CO/CH4/O-2/diluent at phi 0.5-2.5, elevated pressures one-11 atm and H-2-CO mole fraction ratios 0.25:0.75 to 0.75:0.25. The effects of CO2 mole fraction dilution in the fuel (from 0.0 to 0.4), CH4/(CH4+CO) (from 0.0 to 1.0), and pressure dependence on S-L were experimentally investigated and compared to the simulated results from three kinetic mechanisms. The increase of the hydrogen mole fraction, or the decrease of the CO2 mole fraction in the fuel, both lead to an increase of S-L; the decreased mole fraction enhanced S-L significantly more in over-rich conditions. The dilution, thermal-diffusion, and chemical effects (including the direct reaction and three-body effects) of CO2 dilution were quantitively distinguished at different pressures and H-2 contents. The results showed that increasing the pressure and CO2 mole fraction in the fuel enhanced the competition of H consuming reactions, and the retarding effect of CO2 dilution was found to be favored at high hydrogen content syngas conditions. Both CO2 dilution and CH4 addition decreased the overall reaction order of the syngas flames by decreasing the adiabatic flame temperature. Increasing the pressure and H-2 content increased the syngas heat release rate by enhancing three-body collision reactions and enrichment of H radical, and their effects were reversed on syngas flame speed.
