Role of CO2 in the CH4 oxidation and H2 formation during fuel-rich combustion in O2/CO2 environments

Abstract

The effect of CO2 reactivity on CH4 oxidation and H2 formation in fuel-rich O2/CO2 combustion where the concentrations of reactants were high was studied by a CH4 flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH4 flat flame experiment, the residual CH4 and formed H2 in fuel-rich O2/CO2 combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O2/CO2 combustion was noticeably higher than that in air. In addition to this experiment, calculations were performed using CHEMKIN-PRO. They generally agreed with the experimental results and showed that CO2 reactivity, mainly expressed by the reaction CO2+H→CO+OH (R1), caused the differences between air and O2/CO2 combustion under fuel-rich condition. R1 was able to advance without oxygen. And, OH radicals were more active than H radicals in the hydrocarbon oxidation in the specific temperature range. It was shown that the role of CO2 was to advance CH4 oxidation during fuel-rich O2/CO2 combustion. Under fuel-rich combustion, H2 was mainly produced when the hydrocarbon reacted with H radicals. However, the hydrocarbon also reacted with the OH radicals, leading to H2O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H2 formed more easily; however, CO2 reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H2, whereas the H radicals did not reduce H2. It was shown that OH radicals formed by CO2 reactivity were not suitable for H2 formation. As for pulverized coal combustion, the tendencies of CH4, CO, and H2 formation in pulverized coal combustion were almost the same as those in the CH4 flat flame.