Comparative study of the soot formation process in a “nucleation” and a “sooting” low pressure premixed methane flame

The soot formation process was investigated in two low pressure premixed methane flames: a reference sooting flame with equivalence ratio 2.32 and a so called nucleation flame with equivalence ratio 1.95. In the nucleation flame no measureable soot growth after soot inception occurs, and the soot mass increases only by nucleation from the gas phase.Several experimental techniques were used to gain insights on the soot formation and growth processes, including (1) in situ laser induced incandescence (LII) to measure the soot volume fraction and the primary particle size, (2) online jet cooled laser induced fluorescence (JCLIF) to measure the mole fraction of pyrene and fluoranthene and (3) ex-situ secondary ion mass spectrometry (ToF-SIMS) to investigate the chemical composition of the gas phase. Particular attention is devoted to soot gaseous precursors like polycyclic aromatic hydrocarbons (PAHs).ToF-SIMS analyses provide evidences that large hydrocarbons are formed in both flames via hydrogen abstraction/acetylene addition (HACA) mechanism. However, significant differences in the soot volume fraction and concentration profiles of PAHs are observed in the nucleation flame with respect to the reference sooting flame. In particular, in the nucleation flame the average soot particle diameter and the concentration of the larger measured gaseous soot precursors show weak variation all along the flame reaction coordinate. Kinetic modeling was used to help understanding this steady state behavior which is well reproduced in simulations and is found to be related to nearly balanced soot mass growth and loss phenomena currently modeled only by acetylene addition and soot oxidation.