10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Soot Nucleation and Chemical Evolution during Combustion
K. Olof Johansson, Farid El Gabaly, Paul Schrader, Matthew Campbell, HOPE MICHELSEN, Sandia National Labs
Abstract Number: 290 Working Group: Combustion
Abstract There are substantial gaps in our understanding of the first steps in soot formation, growth mechanisms, and chemical evolution during combustion. The first steps in soot formation involve the transition of gas-phase hydrocarbon precursors to physically or covalently bound complexes that are sufficiently stable and long-lived to initiate rapid heterogeneous nucleation and particle growth. These complexes are known as “incipient particles”, and the search for their nucleation and growth mechanisms is a subject of active research. These incipient particles initiate further nucleation and particle growth by coalescence, generating liquid-like hydrocarbon particles, which eventually reach sizes in the range of 10-50 nm, known as “primary particles”. As these particles grow, they also lose hydrogen, solidify, and agglomerate into loosely bound clusters. Under high-temperature conditions, they become graphitic, covalently bound aggregates with a dendritic structure. Soot aggregate sizes, primary-particle sizes, and volume fractions grow as particles age in the flame. At high temperatures in the presence of oxygen, the aggregates fragment, and the primary-particle sizes and volume fractions decrease through oxidation. There is a poor understanding of the mechanisms by which particles undergo these transitions and the parameters that influence them.
We have coupled multiple in situ and ex situ particle diagnostics to study the evolution of soot composition and fine structure, i.e., maturity level, in atmospheric laminar premixed and diffusion flames. We have used aerosol mass spectrometry using single-photon vacuum ultraviolet photoionization (VUV-AMS) to provide information about gas-phase precursors involved in incipient particle nucleation and availability of gas-phase species to adsorb to the particle surface during particle growth. We have used laser-induced incandescence (LII), coupled with laser extinction, to provide information about soot volume fraction and maturity level of the bulk primary particle, and X-ray photoelectron spectroscopy (XPS) to provide complementary information about particle-surface maturity level 10. XPS is sensitive to the chemical environment of atoms in a sample, providing measures of atomic composition, surface functional groups, and electronic structure, including carbon hybridization. At a photon energy of 1,253.6 eV, the mean electron escape depth in graphite is ~1-2 nm, making XPS particularly surface sensitive. This surface sensitivity is a good complement to the bulk sensitivity of LII.
The results demonstrate that the bulk material and the particle surface evolve separately in the flame. Increased soot-maturity level is associated with increased long-range order of the particle fine structure. This increased order leads to an increase in the absorption cross section in the visible and near infrared and a shift of the absorption to longer wavelengths with increasing maturity level of the bulk particle. These trends result in a decrease in the dispersion exponent and increase in the absorption cross section scaling factor, as inferred from LII measurements. LII measurements demonstrate that bulk-maturity level increases with height-above-the-burner (HAB) until it reaches a plateau in the center of the flame at the maximum in the soot volume fraction. Bulk-maturity level only slightly decreases as soot is oxidized at larger HABs. Increased maturity level also leads to an increase in long-range sp2 hybridization. XPS measurements of the sp2/defect ratio demonstrate an increase in soot surface-maturity level with increasing HAB, but the surface-maturity level increases more gradually with HAB than the bulk-maturity level. Whereas the bulk-fine-structure order decreases slightly in the oxidation region, the surface order decreases dramatically, indicating that oxidation occurs preferentially at the surface under these conditions.