Numerical analysis of the effects of H₂O and CO₂ additions on the chemical reaction of soot formation

Abstract

The effects of H₂O and CO₂ additions to n-heptane/air mixture on soot formation studied using detailed soot formation model. The simulation model is comprised of gas-phase reaction model and particle formation/growth model. The gas-phase reaction model deals with polycyclic aromatic formation up to 6-ring via molecular growth processes following fuel oxidation and fragmentation. The particulate model is composed of nucleation, aggregation, surface reaction and PAH condensation mechanism. For the fuel-rich condition at the equivalence ratio 3, the numerical analysis was conducted assuming spatially uniform and temporally constant in temperature and pressure. The result showed an increase of soot formation yield ranging from 1700 to 2300K. Adding H₂O and CO₂ to the mixture reduced the soot yield similarly at the reaction time of 1ms. In the gas phase reaction, PAH formation was more suppressed by H₂O addition compared to CO₂ addition. Thus, soot yield at the initial stage of soot formation was smaller for H₂O addition than CO₂ addition. While in particle growth processes, the numerical analysis revealed that the surface growth rate influenced by C₂H₂ concentration was the most dominant and the C₂H₂ concentration for H₂O addition was higher than CO₂ addition. Thereby the particle growth for H₂O addition becomes relatively larger than CO₂ addition with the reaction progress. In other words, CO₂ addition suppresses particle growth due to lower C₂H₂ concentration. As a result, soot formation yields for both H₂O and CO₂ additions reach the same level at 1 msec due to the compensation of the above-mentioned effects of gas phase and particulate processes.