Combustion characteristics of ammonia-hydrogen/air swirl flames under secondary air injection in a two-stage premixed model combustor

To enable the practical industrial application of ammonia (NH3) as a carbon-free fuel based on its thermal cracking concept, the combustion characteristics of NH3-hydrogen (H2)(-nitrogen (N2))/air swirl flames under secondary air injection in a two-stage premixed model combustor are investigated for various conditions of H2 mole fraction (xh,f), primary and total fuel-equivalence ratios (ϕp and ϕt), air injection height (Hs) and pressure (P). Rich-quench-lean (RQL) combustion is achieved through the NH3-to-H2 conversion in the primary flames and H2-dominant combustion in the secondary flames. Key parameters such as xh,f, ϕp, and P significantly affect the low NOx (nitrogen oxides) and unburned fuel (LNF) emission condition via the primary flames, while ϕt influences unburned NH3 emissions via the post-flame zone of the secondary flames. With decreasing xh,f or increasing P, the LNF emission range narrows and the optimum ϕp for LNF shifts toward fuel-leaner condition. Hs has a notable impact on unburned NH3 emissions but does not have any significant effect on NOx emissions. The widest LNF emission range at normal temperature and P is found at xh,f = 0.50, ϕt = 0.9 and the optimum ϕp of 1.2. These results provide guidance for the design of low-emission, stably-operating NH3 combustion systems.