Residual Ammonia Effects on NO Formation in Cracked Ammonia/Air Premixed Flames

Cracked ammonia is attracting attention as a carbon-free energy carrier, yet trace residual ammonia after reforming can significantly affect nitrogen oxide (NOx) emissions. This study quantifies how residual ammonia from 0 to 10,000 ppm affects NOx formation using one-dimensional premixed flame simulations under gas turbine-relevant conditions (ϕ = 0.5 and 2.0, 673 K, 20 atm). NO formation is evaluated using integrated rate of production (ROP) analysis, reaction pathway analysis, and A-factor sensitivity analysis (defined as sensitivity to the pre-exponential factor in the Arrhenius rate expression). Under lean conditions (ϕ = 0.5), NO increases approximately linearly with residual ammonia. Even at 100 ppm, the dominant NO formation route shifts rapidly from thermal and N2O mechanisms to fuel NO chemistry led by HNO. In contrast, under rich conditions (ϕ = 2.0), the final NO level remains below 10 ppm. Under rich conditions, residual ammonia and the higher flame temperature raise gross NO production in the reaction zone, yet strong DeNOx reactions in the post-flame region consume most of it, resulting in low net NO emissions. These mechanistic results inform cracking targets and the design of staged combustion strategies to minimize NOx formation when deploying cracked ammonia in practical gas turbine systems.