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

Small-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 (similar to 100 nm) fractal aggregates, and smaller subprimary particles, which aggregate in turn to form primaries. It is found that CsCl addition produces similar to 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.