Comparison of the characteristics and mechanism of CO formation in O2/N2, O2/CO2 and O2/H2O atmospheres

The characteristics and mechanisms of CO formation in O2/CO2 and O2/H2O atmospheres were investigated both experimentally and numerically. Comparison experiments in O2/N2, O2/CO2 and O2/H2O atmospheres were performed in a flow reactor at atmospheric pressure covering fuel-rich to fuel-lean equivalence ratios and temperatures from 973 K to 1773 K. Experimental results demonstrated that CO formation in an O2/CO2 atmosphere is the highest and that CO formation is the lowest under all fuel-rich, stoichiometric and fuel-lean conditions. The updated chemical kinetic mechanism satisfactorily reproduced the experimental results. For O2/CO2 atmospheres, the presence of a high CO2 concentration enhances CO2 + H = CO + OH and CH2(S) + CO2 = CH2O + CO dramatically, strengthens HCO + M = H + CO + M by the chaperone effect of CO2, and contributes exclusively to CH3OCO = CH3O + CO. The contribution of the pathway CO2 → CO is significant, and CH3 → CH3OCO → CH2O and CH3 → CH3OCO → CO are exclusive channels in O2/CO2 atmospheres. For O2/H2O atmospheres, although the high chaperone effect of H2O facilitates HCO + M = H + CO + M, CO + OH = CO2 + H is enhanced due to the abundant OH radicals and HCCO + H = CH2(S) + CO is suppressed due to the lack of H radicals. The pathway CO→CO2 is enhanced due to sufficient OH radicals, and CH3 → (CH2(S)) → CH3OH → CH3O → CH2O and CH3 → (CH2(S)) → CH3OH → CH2OH are exclusive channels. Moreover, the pathway CH2O → HCO → CO is amplified in both O2/CO2 and O2/H2O atmospheres.