Combustion Characteristics and Emissions of 99% Cracked Ammonia Blends in a Gas Turbine Representative Swirl

Ammonia is increasingly seen as a promising fuel for combustion applications, including gas turbines, due to its hydrogen content and ease of storage, making it a potential method for storing renewable energy. However, using ammonia directly poses challenges in controlling NO _x emissions, especially for retrofitting existing gas turbines. Cracking ammonia to produce hydrogen and nitrogen could mitigate this issue, although uncracked ammonia traces may remain due to inefficiencies. Therefore, this paper evaluates the impacts on swirling flames, representative of gas turbine combustors, when highly cracked ammonia (17.5% H₂, 1.0% NH₃, and 81.5% N₂) is used. Experiments were conducted at pressures ranging from 1.1 to 6 bar absolute, with air preheated to 500 K and a constant power output of 22.7 kW maintained under lean conditions (equivalence ratio ∼ 0.545) throughout the tests. NH₂ chemiluminescence intensity increased monotonically with pressure from 1.1 to 6 bar, with a peak intensity observed at 6 bar due to enhanced radical formation at higher collision frequencies. NO _x emissions rose from 90 ppmv at 1.1 bar to 189 ppmv at 4 bar before stabilizing, indicating a balance between thermal NO _x formation and ammonia-mediated reduction pathways at higher pressures. NH* intensity decreased with increasing pressure, while OH* radicals remained relatively constant, providing insights into flame structure and reaction zone characteristics. A chemical reactor network model complemented the experimental findings, capturing flame zone dynamics and revealing consistent NO formation pathways through NH₃, NH₂, and NNH dissociation across different pressures. These findings demonstrate that pressure significantly influences radical distribution and NO _x formation mechanisms in highly cracked ammonia combustion, with implications for gas turbine combustor design and emission control strategies. To the authors' knowledge, this is the first systematic study of pressure-dependent chemiluminescence behavior in highly cracked ammonia swirl flames, providing critical insights for the development of low-emission gas turbine combustors using ammonia-derived fuels.