The effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed CH4/NH3/air swirl flames in two-stage combustors at high pressure were studied. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH3 (ENH3) in CH4/NH3 mixtures. NO emissions at the combustor exit were significantly higher than those generated by CH4/air and NH3/air flames under both lean and stoichiometric primary zone conditions. These emissions were reduced to ~300 ppm by using far-rich equivalence ratios (Φ) of 1.3-1.4 in the primary zone. This effect was possibly due to the lower OH concentrations under far-rich conditions. An analysis of local flame characteristics using a newly developed mixture fraction equation for CH4/NH3/air flames indicated that the local temperature and NO and OH concentration distributions with local Φ were qualitatively similar to those in NH3/air flames. The maximum local NO and OH concentrations appeared at local Φ of 0.9, although the maximum temperature was observed at local Φ of 1.0. Both the temperature and OH concentration gradually decreased with the partial replacement of CH4 with NH3. Consequently, NO emissions from CH4/NH3 flames were maximized at ENH3 in the range of 20-30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH4/NH3 flames increased exponentially with temperature, which was not observed in NH3/air flames because of the lower flame temperatures in the latter. However, the maximum NO concentration in CH4/NH3 flames occurred at a temperature slightly below the maximum temperature, just as in NH3/air flames. The apparent exponential increase in NO emissions from CH4/NH3 flames was attributed to a similar trend in the OH concentration at high temperatures.
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