Experimental and Numerical Investigation on the Laminar Flame Speed of CH4/O2 Mixtures Diluted With CO2 and H2O

Abstract

The effects of CO2 and H2O addition on premixed oxy-fuel combustion are investigated with experiments and numerical simulations on the laminar flame speed of CH4/O2/CO2/H2O(v) and CH4/O2/N2/H2O(v) mixtures, at atmospheric pressure and for a reactants inlet temperature Tu = 373 K. Experiments are conducted with steady laminar conical premixed flames over a range of operating conditions representative of oxy-fuel combustion with flue gas recirculation. The relative O2-to-CO2 and O2-to-N2 ratios, respectively defined as O2/(O2+CO2) (mol.) and O2/(O2+N2) (mol.), are varied from 0.21 to 1.0. The equivalence ratio of the mixtures ranges from 0.5 to 1.5, and the steam molar fraction in the reactive mixture is varied from 0 to 0.45. Laminar flame speeds are measured with the flame area method using a Schlieren apparatus. Experiments are completed by simulations with the PREMIX code using the detailed kinetic mechanism GRI-mech. 3.0. Numerical predictions are found in good agreement with experimental data for all cases explored. It is also shown that the laminar flame speed of CH4/O2/N2 mixtures diluted with steam H2O(v) features a quasi-linear decrease when increasing the diluent molar fraction, even at high dilution rates. Effects of N2 replacement by CO2 in wet reactive mixtures are then investigated. A similar quasi-linear decrease of the flame speed is observed for CH4/O2/CO2 H2O-diluted flames. For a similar flame speed in dry conditions, results show a larger reduction of the burning velocity for CH4/O2/N2/H2O mixtures than for CH4/O2/CO2/H2O mixtures, when the steam molar fraction is increased. Finally, it is observed that the laminar flame speed of weakly (CO2, H2O)-diluted CH4/O2 mixtures is underestimated by the GRI-mech 3.0 predictions.