NOx formation and reduction mechanisms in staged O2/CO2 combustion and air combustion was investigated. Primary O2/CH4 ratio, which yielded the minimum fixed nitrogen compounds in O2/CO2 combustion, was lower than in air combustion. The lowest NOx conversion ratio in O2/CO2 combustion was lower than it in air combustion by 40%. This could be explained by a high CO2 concentration, which was one of the most important features of O2/CO2 combustion. It was shown that abundant OH radicals were formed in O2/CO2 combustion through CO2 + H→CO + OH, both experimentally and numerically. OH radicals produced H and O radicals through H2 + OH → H + H2O and O2 + H→ OH + O because a large hydrogen source exists in the CH4 flame. O and OH radicals formed in the fuel-rich region enhanced the oxidation of NH3 and HCN. In staged combustion, NH3 and HCN are favorably converted to NO in the primary fuel-rich region where NO reduction activity is high because such intermediate nitrogen species are easily converted to NO by secondary O2 injection. Ultimately, a significant reduction in NOx emission could be achieved by staged combustion in O2/CO2 combustion at low O2/CH4 ratio.
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