ARTICLE pubs.acs.org/EF
Synchrotron Radiation Studies of Additives in Combustion, II: Soot Agglomerate Microstructure Change by Alkali and Alkaline-Earth Metal Addition to a Partially Premixed Flame S. di Stasio,† J. L. LeGarrec,‡ and J. B. A. Mitchell*,‡ † ‡
Aerosol and Nanostructures Laboratory, Istituto Motori CNR National Research Council of Italy, Via Marconi 8, 80125 Naples, Italy Institut de Physique de Rennes, UMR No. 6251 du C.N.R.S., Universite de Rennes I, 35042 Rennes cedex, France ABSTRACT: 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 (∼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.
’ INTRODUCTION The study of how alkali-metal salts that are easily ionized can affect the development of soot particles in flames has been an interesting sideshow in combustion research that has highlighted our lack of hard knowledge concerning soot formation in general. However, such metals are found as impurities in biomass fuels and coal, so there is a practical interest in understanding the development of soot when these additives are present. Excellent reviews of the effects of additives in flames and the role of ionization in particular have been presented in refs 1 and 2. It has been found that introducing easily ionized alkali-metal salts— and, in particular, potassium and cesium salts—can either decrease or increase the quantity of soot detected. Although many reasons for these changes have been proposed, at this time, there is no definitive explanation for the phenomenon. In this paper, we have used X-ray scattering to directly characterize the changes in soot particle number density, size, and degree of aggregation induced by the aspiration of a cesium solution and a potassium solution into a partially premixed Bunsen ethyleneair flame, and we have compared these changes to that which is observed when barium chloride, calcium chloride, and plain water are aspirated into the flame. We can specifically classify several previous experiments on this subject according to the postulated explanations for the observed phenomena. Bulewicz et al.3 studied acetylene/oxygen and propane/oxygen diffusion flames and determined the soot r 2011 American Chemical Society
emission rate and particle sizes via the collection of the soot on a fiberglass filter and subsequent examination by electron microscopy. Metal salts in aqueous solution were aspirated into the flames via the fuel delivery system. They found that cesium exhibited enhanced soot formation when added in low concentrations and in fuel-rich acetylene-oxygen flames, while it had a strong soot-reducing action in oxygen-rich flames. Their explanation for the decrease in soot formation was that the charge transfer of electrons from the metal atoms M, MþRþTMþ þR led to the neutralization of natural flame ions Rþ that were believed to be the precursors for soot particle nucleation. The soot-enhancing effect was also explained by considering that the reverse of the above reaction would dominate at low metal concentrations or under fuel-rich conditions, thus providing more ion nuclei. This experiment was followed later by a mass spectrometric study by Hayhurst and Jones4 that showed that the addition of sodium to a premixed acetylene-oxygen flame did indeed lead to a reduction in the number of small (
