Effects and Mechanisms of Injection Strategies on NH3/CH4 Swirl Co-Firing and Emission Characteristics

Developing green ammonia (NH3) as a carbon-free fuel faces challenges of flame instability and high NOx emissions, requiring optimized strategies for aerodynamic flow and fuel-air mixing. This study investigates four combustion organization modes, combining primary air routing (axial/tangential) and fuel-air mixing states (premixed/non-premixed) using a swirl burner. Experiments were conducted under lean conditions (φ = 0.9) across NH3 blending ratios (0~100%), alongside chemical reactor network (CRN) modeling. Increasing NH3 transitions flame topology from V-shape to M-shape, ultimately resulting in flame lift-off. Tangential injection broadens stability limits via strong shear vortices (sustaining combustion at 90%NH3), while its non-premixed mode reduces peak CO emissions by 36.5% versus the axial premixed baseline, and the peak weighted combined emission index (iCO + jNO) is reduced by approximately 18% on average. A favorable synergistic emission trade-off window of C-N exists around 17%NH3 based on iCO + jNO. CRN modeling confirms that the NH2 → HNO → NO pathway constitutes the primary backbone for nitrogen migration. At 30%NH3, increased HNO + H → H2 + NO reaction rates cause a rapid increase in fuel-NOx. Conversely, the non-premixed strategy creates local fuel-rich reduction zones at 70%NH3. Based on CRN kinetic inferences, these zones promote amine radical fluxes that effectively lead to a significant reduction in measured NO. These findings provide theoretical support and baseline data for optimizing low- or zero-carbon NH3 swirl burners on a certain scale.