Biodiesel is a type of renewable fuel designed to mimic the properties and performance of conventional diesel. Thus, biodiesel can be used to partly replace conventional diesel without modification to the existing combustion devices. At present, biodiesel is widely used as a transportation fuel mostly by blending with fossil diesel. However, due to the diversity of feedstock used in biodiesel production, the physico-chemical properties of biodiesel may vary, which results in unexpected emissions and combustion performance. Driven by increasingly stringent environmental regulations, the research on particulate matter emissions from the combustion of biodiesel and its blends has attracted much attention. In the present work, the soot emission characteristics of different biodiesels produced from vegetable oils and animal fats are investigated. The chemical composition of the biodiesel is characterized before the biodiesel is burnt in a well-controlled flame environment, so as to examine the soot characteristics. In this study, we apply the laser-induced incandescence (LII) method calibrated by the extinction method to quantify the soot volume fraction produced by the neat oxygenated biodiesel and the blends and then assess the effect of the fuel chemistry on soot formation. Subsequently, the morphology and particle size of soot particulate matters produced from the fuels are compared.

An open pool flame combustion device is utilized to establish the laminar pool flame of the biodiesel and blends. The crucible used has a diameter and depth of 20 mm and a wall thickness of 2.5 mm. A co-flow of air is supplied at a constant speed of 18.2 cm/s to shroud the pool flame from air entrainment. At the bottom of the crucible, a ceramic heating plate is installed to maintain a constant heat supply to the liquid fuel and a constant evaporation rate. The fuel crucible is connected to a fuel tank to replenish the fuel, which thus enables the fuel to stay at a fixed level from the crucible rim and not be unaffected by the fuel consumption rate. In order to measure the soot volume fraction, the non-intrusive laser diagnostic method of planar two-dimensional (2D) LII is employed. The measured LII signal is quantitatively calibrated via absorption, and signal trapping is corrected. The dependence of the LII signal on the energy intensity per unit area of the laser sheet is also examined. The peak laser fluence (about 0.16 J/cm²) is used to conduct the LII measurement because the LII signal is less sensitive to the local laser energy fluctuations. The soot produced from the flames is collected using the thermophoretic deposition method. A quartz plate cooled to 0 ℃ is placed in the flames to collect the soot. The soot's morphology and size are examined via a scanning electron microscope. Five different types of biodiesel, produced from palm, waste cooking oil, duck fat, goose fat, and rice bran, respectively, are tested and compared against the baseline diesel.

Images of the pool flames show that the flame height decreases with the increase in biodiesel blends. The diesel pool flame appears to be the sootiest, but the tendency decreases with the increase in biodiesel fraction owing to the oxygen molecules assisting in soot oxidation. This implies that biodiesel, regardless of the feedstock type, is effective in suppressing the formation of soot. From the LII result, the peak value of the soot volume fraction of pure oxygenated biofuel is 7.1%-30.5% lower than that of conventional diesel. The soot formation decreases with the increase in the biodiesel blending ratio, which is similar to the trend exhibited by biodiesel/diesel blends. Oxygenated fuels with a high degree of unsaturation level tend to emit a higher amount of soot. Palm and rice bran biodiesels with the highest degree of unsaturation among all the biodiesels tend to emit a large amount of soot due to the presence of the double bond promoting the formation of soot. On the basis of Roper's model, the predicted diffusion flame height decreases with the diffusion flame temperature, with palm and duck biodiesel producing the tallest flames among all fuels. The soot particle morphology of the biodiesel and diesel is similar, which is spherical and clustered. Overall, the particle size of biodiesel is relatively 9.5%-41.3% smaller than that of traditional diesel. The soot particle size produced by highly unsaturated biodiesel is relatively larger in spite of lower particle number density.

In the present work, the soot volume fraction produced from five types of biodiesel, biodiesel blends, and conventional diesel is measured by using the LII technique calibrated by the extinction method. The pool flame height is not visibly different among the tested neat biodiesels, but the flame appearance varies with different biodiesel blend fractions in the diesel. The flame height reduces with the increase in biodiesel fraction, and the soot emission is reduced. The LII measurement shows that biodiesel with a higher degree of unsaturation is more prone to emit a large amount of soot. The emission of soot decreases linearly with the increase in biodiesel fraction in the diesel. The peak value of the soot volume fraction of the neat oxygenated biodiesel is 7.1-30.5% lower than that of the conventional diesel. Oxygenated fuels with a higher degree of unsaturation are inclined to emit more soot, which can be explained by the fact that unsaturated C-C double bond is more prone to generate acetylene or benzene during the oxidation process and thus provides precursors for the formation of soot. In general, biodiesel produces soot size that is about 9.5-41.3% smaller than that of diesel. The generated soot is clustered and spherical. Biodiesel with a higher degree of unsaturation tends to produce more fuels in spite of a lower particle number density.