Study on the Synergistic Effect of H2 and CO2/H2O on the Characteristics of Soot Formation in Ethylene Laminar Diffusion Flame

To mitigate the adverse health and environmental impacts of soot emissions from combustion, this study combines experimental measurements with numerical simulations to investigate the effects of H2 and CO2/H2O addition on soot formation in ethylene laminar diffusion flames. Experimentally, a two-color pyrometry method was employed to map the two-dimensional distributions of temperature and soot volume fraction. Numerically, a detailed model incorporating PAH-based soot dynamics, gas-phase chemistry, and heat and mass transfer was applied. Results indicate that: The experimental results show good agreement with the simulations. Among the operating conditions examined, the combined addition of H2 and CO2 exerted the strongest inhibitory effect, reducing peak temperature and soot volume fraction by 16% and 58%, respectively. In contrast, simple dilution with hydrogen was considerably less effective than modifying the oxygen-enriched atmosphere. Notably, under identical oxygen-enriched conditions, the chemical effect of hydrogen reduced soot-related radiation and heat loss, leading to a slight temperature increase of 0.58%. Mechanistic analysis of intermediate species reveals that the H2+CO2 synergy broadly suppresses soot formation pathways. For the H2+H2O case, while nucleation and condensation rates are moderately enhanced, the OH oxidation rate is significantly promoted, resulting in net soot inhibition. Sensitivity analysis further identifies key reaction channels: H2+CO2 synergy inhibits the generation of A1 by consuming OH through R149 (upstream regulation), but also suppresses A1 formation by weakening R393 (i-C4H5 + C2H2 = A1 + H) (downstream regulation). Whereas R148 (H + H2O → H2+OH) dominates the inhibiting effect when H2O is present alongside H2. These findings provide valuable guidance for the development of carbon-free fuel combustion strategies aimed at reducing particulate emissions.