Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure.

This study examined the effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed methane (CH 4)/ammonia (NH 3)/air swirl flames in two-stage combustors at high pressure. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH 3 (E NH 3) in CH 4 /NH 3 mixtures. Although NO emissions at the combustor exit were found to be significantly higher than those generated by CH 4 /air and NH 3 /air flames under both lean and stoichiometric primary zone conditions, these emissions could be lowered to approximately 300 ppm by employing far-rich equivalence ratios (ϕ) of 1.3 to 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 CH 4 /NH 3 /air flames indicated that the local temperature and NO and OH concentration distributions with local ϕ were qualitatively similar to those in NH 3 /air flames. That is, 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 were found to gradually decrease with the partial replacement of CH 4 with NH 3. Consequently, NO emissions from CH 4 /NH 3 flames were maximized at E NH 3 in the range of 20% to 30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH 4 /NH 3 flames increased exponentially with temperature, which was not observed in NH 3 /air flames because of the lower flame temperatures in the latter. But, the maximum NO concentration in CH 4 /NH 3 flames was occurred at a temperature slightly below the maximum temperature, just as in NH 3 /air flames. The apparent exponential increase in NO emissions from CH 4 /NH 3 flames is attributed to a similar trend in the OH concentration at high temperatures. [ABSTRACT FROM AUTHOR]