Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

•Effect of CO2 and H2O dilution on syngas/air combustion was compared in detail.•Laminar burning velocities of CO/H2/air/CO2/H2O mixtures were measured and calculated.•Chemical kinetics analysis was conducted under a wide range of CO/H2 and dilution ratios.•Different effect of CO2 and H2O dilution on flame intrinsic instability was investigated at elevated pressures.

A study on the effect of CO2 and H2O dilution on the laminar burning characteristics of CO/H2/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO2 and H2O diluted CO/H2/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO2 or H2O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO2 and H2O diluted CO/H2/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO2 and H2O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H2 ratio of 20/80 changes from reaction R3 (OH + H2 = H + H2O) to R16 (HO2+H = OH + OH) when CO2 and H2O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO2 and H2O are added. Laminar burning velocities and kinetics analysis indicate that CO2 has a stronger chemical effect than H2O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO2 and H2O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.