Role of CO_2 in the CH_4 oxidation and H_2 formation during fuel-rich combustion in O_2/CO_2 environments

摘要

The effect of CO_2 reactivity on CH_4 oxidation and H_2 formation in fuel-rich O_2/CO_2 combustion where the concentrations of reactants were high was studied by a CH_4 flat flame experiment, detailed chemical analysis, and a pulverized coal combustion experiment. In the CH_4 flat flame experiment, the residual CH_4 and formed H_2 in fuel-rich O_2/CO_2 combustion were significantly lower than those formed in air combustion, whereas the amount of CO formed in fuel-rich O_2/CO_2 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 CO_2 reactivity, mainly expressed by the reaction CO_2 + H → CO + OH (R1), caused the differences between air and O_2/CO_2 combustion under fuel-rich condition. Rl 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 CO_2 was to advance CH_4 oxidation during fuel-rich O_2/CO_2 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 H_2O production. In fact, these hydrocarbon reactions were competitive. With increasing H/OH ratio, H_2 formed more easily; however, CO_2 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 CO_2 reactivity were not suitable for H_2 formation. As for pulverized coal combustion, the tendencies of CH_4, CO, and H_2 formation in pulverized coal combustion were almost the same as those in the CH_4 flat flame.