Formation and evolution of carbon particles in coflow diffusion air flames of vaporized biodiesel, diesel and biodiesel-diesel blends

Abstract

Evolution profiles on the formation of the carbon particulate (soot) in vaporized coflow diffusion flames of biodiesel (BD), No. 2 diesel and blended fuel mixtures were obtained. The evolution profiles contain the different stages of soot formation, including soot inception, particle growth and agglomeration, and oxidation. Carbon samples were collected directly from inside the flame’s yellow luminous zone along the axial direction at various heights above the burner (HAB) using a well-accepted approach. The studied BD flames were formed using canola methyl ester (CME), cotton methyl ester (COME) and soy methyl ester (SME) in their neat form (B100). The blends consisted of B80 (80% CME/20% No. 2 diesel), B50 (50% CME/50% No. 2 diesel) and B20 (20% CME/80% No. 2 diesel). The evolution profile of the No. 2 diesel flame was compared to the evolution profiles of the neat (B100) BD and blended fuel flames. Soot evolution profiles in the studied vaporized BD and diesel flames are consistent with the general trends of particle formation present during the combustion of diesel and gaseous fuels. That is, particle inception (singlet particles) present in a region at the lower part of the flame followed by particle growth and agglomeration, and the subsequent soot carbonization and oxidation in the upper regions of the flame. However, the soot evolution profiles also show that significant differences exist in the soot morphological properties of the tested BD and blended flames. The presence of “irregular-shaped” fragments such as the “liquid-like” droplets or “globules” at the different stages of soot formation is evident in these oxygenated flames. The “irregular-shaped” fragments resemble short chain-like aggregates that appear to be formed of fused particles having undefined shapes and boundaries. The “irregular-shaped” fragments resemble eutectic (solid/liquid) phases manifesting their viscous liquid nature. Some of these “irregular-shaped” structures contain embedded carbonized inclusions. By a small axial variation of the flame, the fragments are transformed into fully carbonized aggregates that are significantly larger and of complex fractal morphology (multi-branched). From the transmission electron microscopy analysis it can be suggested that the “liquid-like” droplets or “globule” structures serve as possible growth pathways for the formation of the fully carbonized aggregates composed of spherical particles in the upper part of the flame. The size and number density of the “irregular-shaped” fragments in the vaporized BD are much larger than those present in the flame formed from the vaporized diesel. It is also observed that as the percentage of BD is increased in the blended flames, these “irregular-shaped” structures become more pronounced. The evolution profiles also present other morphological properties of the carbon particulates (particle size and nanostructure) along the flame’s axial direction.