Effect of Air/Fuel Burning Pattern on NO Emission Characteristics for NH3/CH4 Cofiring Flames

Ammonia (NH3) is a promising zero-carbon fuel, but it faces critical challenges in combustion utilization, especially NO emission. Intensive previous studies have been carried out to deepen the understanding towards NH3 combustion and NO emission characteristics but most of them focus on the premixed combustion mode. This work conducts both experiments and large-eddy simulations (LESs) for various NH3/CH4 mixtures, from pure methane to pure ammonia, under both premixed and non-premixed combustion modes, to gain a clear insight into the NO emission performance and formation mechanism of the two combustion modes. It is shown that the non-premixed combustion exhibits a stratified flame appearance, mainly due to the different reactivity between CH4 and NH3. Accordingly, the non-premixed combustion mode produces the lower NO emission across all NH3 blending ratios with respect to its premixed counterpart. Further, LES results show that the flame stratification is responsible for the lower NO emission by creating a strong fuel-rich region in the combustor center where a part of NH3 undergoes thermal cracking into H2 and N2. In addition, the performance of several existing NH3/CH4 mechanism models is estimated by comparing the predicted NO emissions against the present experimental measurement for both premixed and non-premixed mixing patterns, and the present proposed model shows the lowest error among the candidates.