Investigation on lean blow-off characteristics and stabilization mechanism of premixed hydrogen enhanced ammonia/air swirl flames in a gas turbine combustor

Ammonia (NH3), a suitable hydrogen carrier, is regarded as one of the most promising alternative energy sources due to its zero-carbon nature. However, the main issues of its application in power plants are resulted from the low reactivity, the high activation energy and high nitrogen content of the NH3 molecule. Among all the challenges of ammonia combustion in power plants, like gas turbines, flame stabilization is the first priority. NH3/H2 co-firing could be a potential strategy to improve the flame stabilization by enhancing the local combustion intensity, in which H2 can be obtained by cracking NH3. In this study, the co-firing effects of H2 on NH3/air swirl flame characteristics and stabilization mechanisms were investigated. The flame structures and the flow fields were measured by OH-PLIF and PIV techniques. LES with the dynamic thickened flame (DTF) model was performed on two experimental cases to further reveal the intrinsic mechanism of flame stabilization. Results show that the NH3 flame co-fired with 10% H2 by mole fraction is a promising strategy that enhances the flame stabilization without increasing the magnitude of NOx emissions in the present combustor. When comparing the flow structures, 10% H2 addition in the fuel leads to the flame being stabilized under higher positive axial velocity regions by increasing the flame speed. The 10%H2 flame shows stronger resistance to stretch, which means a stronger ability to stabilize the flame. Less large-scale and more small-scale wrinkles are found in the 10%H2 flame front, indicating combustion intensity improved, which contributes to the flame stabilization. The lower H, NH, and NH2 mass fractions at the flame root result in less reactivity and weaker flame stabilization ability in the NH3 flame. For the 10%H2 flame, these species are enhanced within the entire flame, especially at the flame root, indicating that the combustion activity enhancement comes from not only the hydrogen oxidation, but also the improvement of the ammonia oxidation. Moreover, when H2 is added, although the inner recirculation zone (IRZ) becomes thinner and lower, which is a disadvantage for flame stability, at the flame root the increase of these species mainly comes from the oxidation of fuels rather than the IRZ effect.