Multi-stage interactions on flame stability and NOx emissions in ammonia/methane co-combustion via fuel nitrogen-hydrocarbon separation

Abstract

Co-combustion of ammonia with fossil fuels is inherently constrained by the trade-off between flame stability and NO_x control. In our prior work, an innovative fuel nitrogen-hydrocarbon separation concept was proposed to broaden the low NO_x emission window by attenuating radical-driven reaction pathways. However, the influence of multi-stage flame interactions and mixing dynamics on the competing routes of nitrogen conversion remains insufficiently understood. In the present study, these effects are systematically investigated using a two-stage tangential swirl burner, with particular emphasis on their roles in governing flame stability and NO_x formation. Experimental investigations reveal three distinct flame stability regimes governed by ammonia blending ratio (E_NH3): (i) dual-flame stable, (ii) detached ammonia flame, and (iii) pulsating methane flame. Increasing E_NH3 enhances primary flame stability, but leads to a higher tendency for pulsation in the secondary methane flame due to attenuated secondary flow. NO_x emissions exhibit a non-monotonic dependence on the primary equivalence ratio (Φ_pri), with an optimal value around 1.05 achieving 134 to 298 ppm (@ 3.5 % O₂) NO_x emissions and NH₃ slip below 10 ppm. Notably, the influence of Φ_pri on NO_x emissions diminishes progressively as the overall equivalence ratio (Φ_ove) increases from 0.6 to 0.9, thereby remarkably broadening the low NO_x emission window. Spatially resolved NH₂* and OH*chemiluminescence analyses demonstrate that NO_x suppression correlates strongly with reduced overlap between primary and secondary reaction zones. Chemical reactor network analysis confirms that reduced multi-stage mixing suppresses HNO formation by impeding oxidative radical entrainment into ammonia-rich zones. This study fills a critical gap by ​quantifying how multi-stage mixing dynamics govern the trade-off between flame stability and NO_x emissions​ in ammonia/methane co-combustion.