Soot formation characteristics during propane MILD combustion

TIAN Songjie; REN Hao; XU Shunta; XI Liyang; TU Yaojie; LIU Hao

doi:10.13226/j.issn.1006-6772.23122801

Abstract：Moderate or intense low-oxygen dilution(MILD)combustion is an advanced low-oxygen diluted combustion technology capable of achieving simultaneous reductions in NOx and soot emissions.This study employs numerical simulations based on the counter-flow flame model in the chemical kinetics analysis software CHEMKIN-PRO to investigate the soot formation pathways in propane MILD combustion and their distinctions from conventional combustion.Furthermore,the effects of strain rate(50-80 s-1)and CO₂ dilution(volume fraction:0-60%)on soot formation pathways under MILD conditions are systematically analyzed.The results reveal that the dominant soot formation pathways in MILD combustion are:2C3H3→A1,A1-+H(+M)⇌A1(+M),A1-+CH4⇌A1+CH3,A1-+C2H4⇌A1+C2H3,C6H5CH3+H=A1+CH3 andC4H-25+C2H2=A1+H;Compared to conventional combustion,the reaction rates of 2C₃H₃→A1 and A1-+H(+M)⇌ A1(+M)under MILD conditions are significantly reduced,leading to suppressed A1 formation and thus inhibiting soot nucleation.Consequently,the surface mass growth rate of soot decreases by 78.6%,and the peak soot volume fraction diminishes by 83.7%.Notably,the contribution of the 2C₃H₃→A1 pathway to soot formation decreases by 7.7%under MILD combustion,while the importance of the C₆H₅CH₃+H⇌A1+CH₃ andC4H-25+C₂H₂⇌A1+H pathways increases by 5.36%and 7.59%,respectively.Additionally,the peak soot volume fraction under MILD conditions exhibits a nonlinear dependence on strain rate,initially increasing and subsequently decreasing with rising strain rates.This behavior stems from the competitive interplay between the non-monotonic variation in nucleation rates and the continuous increase in surface growth rates.Both the physical and chemical effects of CO₂ dilution intensify with higher dilution ratios.At CO₂ dilution levels of 0-40%,the physical effect of CO₂ exerts minimal influence on peak soot volume fraction.However,CO₂ chemically promotes H consumption via the CO+OH⇌CO₂+H reaction,thereby weakening the H-abstraction-C₂H₂-addition(HACA)mechanism critical for polycyclic aromatic hydrocarbon(PAH)growth.This results in significant reductions in A1 and A4 concentrations.At 60%CO₂ dilution,the peak soot volume fraction further declines to 6.4×10-9,demonstrating enhanced suppression of soot formation in MILD combustion.