Towards efficient/low emission NH3 swirl combustion under axially staged regime through oxygen enrichment

Abstract

Aimed at controlling emissions and increasing efficiency in a gas turbine combustor, this work conducts the experimental and kinetic modelling investigation on oxygen-enriched ammonia swirl combustion under axially staged regime. Combustion and emission characteristics of ammonia/oxygen/nitrogen mixtures are explored using direct flame imaging, planar laser-induced fluorescence, and Fourier Transform Infrared spectroscopy. The results reveal that oxygen enrichment strengthens combustion intensity and flame stabilization, thus contributing to the expanded stability limit. Furthermore, oxygen enrichment can effectively widen the low NOx/NH₃ emission window which can reach six times in the primary stage from 21 %O₂ to 40 %O₂. Kinetic modelling is performed using the freely propagating laminar flame model and a hybrid chemical reactor network model. According to kinetic analysis, thermal effects play a dominant role due to the nearly 500 K increment in adiabatic flame temperature, which enhances self-promoted ammonia pyrolysis into nitrogen and hydrogen under rich conditions in the primary stage. The low NOx/NH₃ emission window is thus broadened and H₂ rather than NH₃ is released. Under axially staged regime, nearly 100 % combustion efficiency and zero H₂ emissions can be achieved under all conditions. Oxygen enrichment from 21 % to 40 % shifts the optimized primary equivalence ratio (ϕ_opt) from 1.05 to 1.32 at an overall equivalence ratio (ϕ_overall) of 0.6. Shifting the oxygen content and primary equivalence ratio from 21 % and 1.05 to 40 % and 1.32 can also halve the NOx emissions from 144 ppmv to 72 ppmv at ϕ_overall = 0.8. At 21 %O₂, the optimized NOx emissions can only be achieved through the thermal deNOx mechanism under near-stoichiometric conditions. This can be attributed to the narrow low NOx/NH₃ emission window in the primary stage leading to lean NH₃/H₂ combustion and NH₃-to-NO penalty in the secondary stage. Under oxygen enrichment, residual hydrogen rather than residual ammonia enters the ultra-lean secondary stage, avoiding the NH₃-to-NO penalty. This leads to a shift of ϕ_opt to richer conditions and a halving of the optimized NOx emissions. This study suggests that oxygen-enriched ammonia combustion under axially staged regime is a promising and efficient clean combustion technology for ammonia.