Effect of hydrogen on soot formation in laminar ammonia-ethylene diffusion flames

Abstract

Hydrogen and ammonia, as carbon-free fuels, are widely recognized as alternative fuels for combustion systems. This study investigates the comparative impacts of hydrogen and ammonia on soot formation in ethylene-based and ethylene–ammonia diffusion flames via numerical simulations. In ethylene flames, hydrogen addition enhances soot formation by increasing H radical concentrations, which accelerate chain-initiating reactions and promote the production of reactive intermediates such as C₂H₃ and C₃H₃, thereby intensifying PAH growth and soot accumulation. Conversely, ammonia predominantly lowered the radical concentrations and suppressed precursor formation. In ethylene–ammonia flames, however, hydrogen addition reduces soot volume fraction primarily via dilution, with chemical effects contributing secondarily. Although hydrogen promotes ethylene decomposition, it simultaneously shifts key reaction pathways by diverting intermediates such as C₂H₂ toward oxidation rather than PAH synthesis, thus weakening soot growth. Overall, hydrogen plays a dual role in soot evolution: enhancing soot formation in hydrocarbon-rich environments while suppressing it in ammonia-containing systems through both dilution and radical-pathway modulation.