Synchrotron Radiation Studies of Additives in Combustion, II: Soot Agglomerate Microstructure Change by Alkali and Alkaline-Earth Metal Addition to a Partially Premixed Flame

Abstract

International audienceSmall-angle X-ray scattering (SAXS) has been used in a comparative study to characterize soot particle formation in a partially premixed ethylene−air flame into which plain water, and 0.1 mol/L solutions of potassium chloride, cesium chloride, barium chloride, and calcium chloride were aspirated. The plain water results are assumed to be the reference in this study. On the basis of previous findings, a two-level universal fitting function for the scattering intensity I(q), 0.04 nm−1 < q < 0.5 nm−1, is used to retrieve the size and concentration of both primary particles, which are the subunits of relatively large (100 nm) fractal aggregates, and smaller subprimary particles, which aggregate in turn to form primaries. It is found that CsCl addition produces 40% smaller primary particles at larger residence times, compared to plain water addition. CaCl2, BaCl2, and KCl addition have almost no effect on primary and subprimary sizes. Cesium chloride addition produces a dramatic reduction of the subprimary particle dimension, up to a factor of 2.2, while the concentration of primaries and subprimaries is increased by factors of up to 5 and 2, respectively, with respect to plain water. The contribution to the soot volume fraction (fv) from primary and subprimary particles is separately estimated for all the additives, with respect to the standard plain water addition. At larger residence times, CsCl addition leads to a reduction of the fv contribution from primaries, with larger rates compared to KCl. On the other hand, the effect of suppression by CsCl at smaller heights can be weaker than pure water, contrary to KCl, which is a better fv suppressor than water at all heights, even if a smaller rate than that for CsCl is in operation. The effect of CsCl and KCl is the same for the subprimary fv contribution. BaCl2 has the effect of yielding a primary fv contribution, which is a factor of 2 smaller than that for plain water, almost independently of the height in the flame and, moreover, it yields a large decrease (up to a factor of 4 at a height above the burner (HAB) of 28 mm) in the subprimary fv contribution, compared to plain water. CaCl2 addition has a negligible effect on both the primary and subprimary fv contribution, with respect to water

Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics

“…Corannulene has been found to be present in flames (Lafleur et al, 1993), however its clustering effectiveness is expected to be less significant than those for larger curved molecules prevalent in sooting environments due to its weaker interaction energy. Potassium is readily ionised in a flame and has been shown to impact soot nucleation through the production of higher particle numbers and smaller particle sizes (Haynes et al, 1979;Di Stasio et al, 2011;Simonsson et al, 2017). Due to this known influence and the straightforward incorporation of the spherical potassium cation within an atomic model, K + provides a representative cation useful for exploring the interactions of cPAHs and ions in a large dynamic system.…”

Flexoelectricity and the Formation of Carbon Nanoparticles in Flames

“…13 The concentration of ions in a flame can be artificially increased through the addition of alkali and other metals, which easily ionize at flame temperatures. Electron microscopy 15 and small angle X-ray diffraction 16 revealed that primary particles increased in number and decreased in size with these metal additives, with significant reductions in the mass of particulates produced (50-90% reduction being reported for polystyrene combustion 17 ).…”

Impacts of Organic Structures and Inherent Minerals of Coal on Soot Formation during Pyrolysis

“…Furthermore, the same as soot, the physical and chemical changes of gasified metal also occur near coal particles [25]. In the study of hydrocarbon flame and diesel engines, it is found that gaseous metals can affect the formation of soot and tar [26,27]. Therefore, these gasified metals, especially alkali metals and alkaline earth metals, may also affect the sooting process of coal tar.…”