NH 3 /CH 4 /Air Partial Premixed Flames: Flame Stability, Emissions, and Thermal Structure

Partially premixed combustion has proven effective in enhancing flame stabilization and reducing emissions in hydrocarbon systems. However, the widespread adoption of ammonia (NH 3 ) as a carbon-free fuel remains limited due to weak flame stabilization and high NO emissions. Therefore, this study investigates flame stability, NO emissions, flame appearance, and thermal structure in NH 3 /CH 4 /air flames under varying degrees of mixture inhomogeneity. Flames were stabilized using a conical nozzle fitted at the outlet of two concentric tubes: an NH 3 /air mixture was introduced through the inner tube, while a CH 4 /air mixture was flowed through the outer annular passage. The equivalence ratios and velocities of the inner (Φ in, Vm i ) and outer (Φ out = 0.9, 1.1, 1.3; Vm o ) streams were systematically varied. The mixture inhomogeneity of NH 3 /air and CH 4 /air streams was controlled by adjusting the axial recession distance (L) between the two tube exits, which was expressed as L/D (0–10), where D is the outer tube’s inner diameter. Flame stability was found to depend strongly on the interaction between L/D, Φ out, and NH 3 content. At lean Φ out = 0.9, increasing L/D (i.e., enhancing premixing) reduced flame stability, while at Φ out = 1.3, optimum stability occurred at L/D ≈5, with stability deteriorating at both lower and higher values. Partially premixed NH 3 /CH 4 flames exhibited improved NO emission performance and combustion efficiency relative to fully premixed NH 3 /CH 4 /air and pure NH 3 /air flames. Co-firing NH 3 in the inner stream with CH 4 in the outer stream enabled the use of richer NH 3 mixtures while limiting NH 3 slip. Although NO emissions generally increased with L/D, enrichment of Φ out or reduction of the velocity ratio of the inner to the outer stream ( V R ) mitigated this effect. Under lean Φ out, increasing L/D or V R enhanced the mixing degree, shifting the overall mixture toward leaner local conditions and promoting NO formation via enhanced radical activity. Conversely, under rich Φ out, these changes reduced flame temperature relative to those seen at lean Φ out and suppressed NO pathways by limiting radical production. These findings demonstrate that partially premixed NH 3 /CH 4 combustion─when optimized through control of L/D, Φ in, Φ out, and V R ─can achieve stable, efficient, and ultralow NO operation, supporting its viability in future hydrogen-based energy systems.