Effects of air-staging and heat losses on NO emissions of NH3/CH4/air swirling flames

Ammonia (NH) is considered a promising carbon-free fuel in the context of carbon neutrality. However, the emission characteristics of NH swirling flames respond strongly to the influence of heat loss and fuel staging. This study investigated the NO emissions of NH/CH/air mixtures in an air-staging swirling combustor. The ammonia mole fractions range from = 0.3, 0.6, to 1.0, with the primary and overall equivalence ratios ranging from = 0.7 to 1.7 and = 0.5 to 0.7, respectively. Four different secondary air injection strategies and three heat losses rates were adopted to differentiate the thermal and chemical effects on NO emissions. O, NO, and CO emissions were measured using a flue gas analyzer. In-flame temperature was measured by fine-wire thermocouples, and chemiluminescence (OH*, NH*) was captured by an intensified CCD camera. The chemical reactor network approach was used to simulate the two-stage combustion. Secondary gas injection initially has a thermal effect by cooling the downstream flame temperature on the lean side. After = 1.3, the chemical effect dominated while the flame was challenged by lift-off instability. = 1.3 showed the minimum NO emission, while = 1.1, = 0.7 with cooling exhibited low NO levels and high flame stability. Flame at > 1.3 tended to lift-off and merge with secondary air, diminishing the air-staging effect. NH* and OH* indicate the flame lift-off and secondary flame formation. Simulation results show that NO removal is favored at cooling conditions in the primary stage and NO production is suppressed at thermal-insulation conditions in the secondary stage. To ensure low NO emissions, complete ammonia oxidation, and flame stability, stabilizing the primary flame zone close to = 1.0 and overall equivalence ratio close to = 0.7 in combination with the cooling strategy is effective.