Trade-Off Between Soot Suppression and NOX Formation in Ammonia–Toluene Diffusion Flames: A Counterflow and Coflow Study

Abstract

Ammonia has gained increasing attention as a carbon-free energy carrier, and its blending with aromatic hydrocarbons represents a potential pathway for reducing carbon-related emissions in combustion systems. Despite this interest, the combined influence of ammonia on soot formation and nitrogen oxide emissions in aromatic flames remains insufficiently characterized. In particular, a direct comparative analysis between counterflow and co-flow ammonia–toluene diffusion flames, accounting for the coupled effects of strain rate and residence time, has not been systematically reported. In the present work, ammonia–toluene (NH₃–C₇H₈) diffusion flames are numerically examined under both counterflow and co-flow configurations using CHEMKIN-Pro coupled with ANSYS Fluent. A detailed chemical kinetic mechanism, consisting of approximately 150 species and 950 elementary reactions, is developed by extending the toluene ratio flame/ammonia (TRF/NH₃) framework with updated toluene sub-mechanisms and is validated against experimental data for flame temperature, major gas-phase species, and key soot precursors. Ammonia addition in the range of 0%–10% by volume leads to a measurable decrease in peak flame temperature (approximately 5%–10%) and a substantial reduction in major soot precursor species, including acetylene (C₂H₂), allene (C₃H₄), phenyl radical (C₆H₅), and benzene (C₆H₆). In counterflow flames, precursor concentrations are reduced by 30%–60%, while co-flow flames exhibit a more moderate reduction of 20%–40%. The stronger suppression observed in counterflow configurations is associated with elevated strain rates and reduced residence times, which hinder the progression of aromatic growth pathways. In contrast, ammonia enrichment markedly enhances nitrogen oxide (NO_X) formation, with nitric oxide (NO) levels increasing by a factor of 2–3, primarily through fuel-bound nitrogen routes involving amino groups (NH₂), (NH), and nitroxyl (HNO) intermediates. Sensitivity and rate-of-production analyses reveal that ammonia alters the radical pool, accelerating chain-branching reactions and carbon monoxide (CO) oxidation while simultaneously favoring nitrogen-driven NO formation pathways. Overall, the results indicate that ammonia blending can effectively suppress soot formation in aromatic diffusion flames, but at the expense of increased NO_X emissions. This study demonstrates that ammonia addition introduces a strong trade-off between soot suppression and nitrogen oxide (NO_X) formation. These findings provide new insight into the coupled role of strain rate and residence time in ammonia-aromatic fuel combustion, offering guidance for optimizing low-emission combustion systems.