Study on interactions of nitrogen–hydrocarbon reaction pathways in soot formation of ammonia-doped flames

Due to ammonia’s low laminar flame speed and narrow flammability, co-firing it with hydrocarbon fuels is a promising strategy for reducing carbon emissions. Experimental studies show that ammonia significantly suppresses soot formation, but numerical simulations often fail to reproduce this effect accurately. This study employs reaction mechanisms with varying nitrogen–hydrocarbon interactions to examine their impact on soot in ammonia-doped ethylene flames. Results reveal that interactions between C1–C2 hydrocarbons and nitrogen-containing species significantly affect soot nucleation, surface growth, and condensation, particularly in counter-flow flames. However, these effects alone do not fully explain the observed soot suppression. Additional reactions between A1 and NH X 2 were tested but also showed limited influence on soot reduction. To further investigate the mechanisms of soot formation and suppression, Soot-based Global Pathway Analysis was performed. This analysis identified specific reaction pathways as dominant contributors to soot formation, with C X 9 H X 7 acting as a representative intermediate that facilitates the formation of A2R5, a key soot precursor. As a tentative approach, we introduced a reaction between C X 9 H X 7 and a nitrogen-containing species to explore the potential of disrupting this pathway. While other PAH intermediates could also be involved in similar interactions, this attempt led to a noticeable improvement in soot suppression predictions. In co-flow diffusion flames, surface growth remains the primary soot formation pathway, while C X 9 H X 7 -related reactions become increasingly significant at higher ammonia concentrations. Introducing the C X 9 H X 7 -related reaction to disrupt the reaction pathway leads to a noticeable reduction in the maximum soot volume fraction. These findings highlight the pivotal role of PAH-related pathways in ammonia-doped flames and emphasize the need for further investigation into reactions involving nitrogen-containing species and critical hydrocarbon intermediates. • The effects of ammonia doping on soot formation are investigated in both counter-flow and co-flow laminar diffusion flames. • Nitrogen interactions with hydrocarbons at different levels are examined, along with their influence on soot formation. • A soot-based global pathway analysis reveals that key intermediates such as C9H7 and A2R5 play important roles in soot formation routes. • Soot volume fraction, intermediate species, and dominant reaction pathways are analyzed. • A trial PAH– NH X 2 reaction is proposed, demonstrating its potential to suppress soot formation through nitrogen-induced chemical inhibition.