Reactive nitrogen emissions and combustion dynamics in an axially staged rich-premixed ammonia combustion system

The key technical problems associated with ammonia-fueled gas turbine combustion are well established in terms of flame-stabilization mechanisms for the less reactive ammonia and the overwhelming tendency to produce excessive nitrogen oxides through the fuel-NOx pathways. Recent studies suggest that these problems can be alleviated by chemical-kinetics-controlled fuel/air injection strategies, particularly those involving spatially separated reaction zones. The mechanistic role of axially staged secondary air and lean-premixed fuel/air jets injected into a high-temperature vitiated crossflow is, however, relatively underexplored; our knowledge of the effectiveness and relative importance of these strategies with regard to exhaust gas emissions and combustion dynamics is limited. Here, we undertake reduced-order kinetic modeling and comprehensive measurements of reactive nitrogen compounds (NOx, N₂O, NH₃) and unreacted hydrogen concentrations using an axial-staging-enhanced fuel-flexible test facility. We show that an increase in the primary equivalence ratio to 1.25 under non-staged rich-premixed conditions causes the total NOx emissions to be reduced substantially from 3030 to 57 ppmvd while producing unreacted ammonia and hydrogen emissions. When the rich-premixed ammonia condition is kept unchanged for the primary reaction zone, the second-stage reaction volume sequentially created by transverse air or lean-premixed H₂/air injection is revealed to mitigate unburned ammonia and hydrogen emissions effectively by reinitiating related elementary reactions. The existence of a second-stage reaction region, however, tends to augment NH-related NO production and thermal NO reactions, potentially nullifying or strongly reducing the effective gains. This apparent drawback is found to be much less pronounced in the H₂/air axial staging situations, as the production of the ammonia-derived fuel NO through N + OH → NO + H is relatively reduced, in close connection with the enhanced OH depletion reaction, H₂ + OH → H + H₂O. As well as achieving the minimum NOx concentration of about 367 ppmvd with no unreacted ammonia and nitrous oxide emissions, we demonstrate that high-amplitude low-frequency pressure fluctuations are largely suppressed under the investigated H₂/air-staged conditions.