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

The effect of CO2 reactivity on CH4 oxidation and H-2 formation in fuel-rich O-2/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 H-2 in fuel-rich O-2/CO2 combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O-2/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 O-2/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 O-2/CO2 combustion. Under fuel-rich combustion, H-2 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, H-2 formed more easily; however, CO2 reactivity reduced the H/OH ratio by converting H to OH. Moreover, the OH radicals reacted with H-2, whereas the H radicals did not reduce H-2. It was shown that OH radicals formed by CO2 reactivity were not suitable for H-2 formation. As for pulverized coal combustion, the tendencies of CH4, CO, and H-2 formation in pulverized coal combustion were almost the same as those in the CH4 flat flame. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.