Evolution of N₂O production at lean combustion condition in NH₃/H₂/air premixed swirling flames

In the development of ammonia - hydrogen blends as potential substitutes for fossil fuels, the retrofitting of existing devices running at very lean condition is one of the promising solutions for decarbonisation of the power sector. However, little is known about the impact of these conditions on the production of NO_X, particularly N₂O that is a potent greenhouse gas. Therefore, the influence of varying thermal power and Reynolds numbers on the flame and emission characteristics, especially N₂O, of ammonia-hydrogen-air swirling flames has been evaluated for the first time through the use of spatially resolved OH*, NH* and NH₂* chemiluminescence, spectrometry analyses and advanced emissions characterisation at a fixed lean equivalence ratio, Φ = 0.65, representative of the Dry Low NO_X (DLN) approach in traditional stationary gas turbines. NO and NO₂ emissions were found to be decreasing (from ∼ 5000 ppmv to ∼ 1000 ppmv; NO and from ∼ 150 ppmv to ∼ 50 ppmv; NO₂) with increasing ammonia content (from 50% to 90%) in the fuel while N₂O followed reverse trends (from ∼ 50 ppmv to ∼ 200 ppmv). More than 80% ammonia content in the fuel blends exhibited high amounts of unreacted ammonia fractions (∼ 100 to ∼ 1200 ppmv), which can be potentially linked to flame instability and/or low temperatures. Furthermore, any increasing or decreasing trends in NO_X with ammonia fraction were made more extreme by increasing thermal power or Reynolds number due to the differences in relevant radicals (NH, OH, NH₂ etc.) formation in the flames. Experimental results suggest the unviability of these blends at the conventional lean conditions utilised at the DLN power applications due to excessive NO_X emissions. Detailed sensitivity analyses of N₂O concentration at the flame and post flame zone has been carried out utilising Ansys Chemkin-PRO to identify and investigate the reactions responsible for N₂O formation/consumption in the experimental flames. Results have identified the reaction NH + NO ↔ N₂O + H as the major source of N₂O production in the flame, while the reactions N₂O + H ↔ N₂ + OH and N₂O(+M) ↔ N₂ + O(+M) are responsible for N₂O consumption at the post flame zone, with higher reactivity for the latter reaction at longer residence time and relatively lower temperatures.