Physical-chemical and aggregation properties of biodiesel soot particles prepared at normal temperature and atmospheric pressure

Abstract: As worldwide energy demands increase, renewable fuels are attracting plenty of attention as a pathway towards environmentally sustainable future and improved energy independence. Biodiesel is a renewable energy and is regarded as an excellent substitute for petro diesel. Biodiesel is a mixture of methyl esters with long-chain fatty acids and is typically made from nontoxic resources such as vegetable oils, animal fats or waste cooking oils. Engines powered by biodiesel can benefit from reduced particulate matter (PM) and greenhouse gas emission, however, the generation of soot is a by-product of the combustion process that cannot easily be eliminated. Most of the biodiesel soot (BDS) generated during the combustion process is exhausted, but some can contaminate the lubricating oil within the sump as a result of blow-by gasses. BDS agglomeration in the lubricating oil can lead to increase the viscosity of lubricating oil and may result in an increased wear of an engine's critical components, shortening of oil life, and the increased frequency of oil changes. Therefore, physical-chemical properties and aggregation of BDS must be studied. In this study, BDS and No.0 diesel soot (DS) were obtained by the combustion of biodiesel and No.0 diesel at normal temperature and atmospheric pressure. A rotating viscometer was used to investigate the effect of BDS and DS on the aggregation of liquid paraffin (LP). The morphology, composition, structure and aggregation mechanism of BDS and DS were investigated by means of field-emission transmission electron microscopy, X-ray diffraction, Raman spectrometer, Fourier transform infrared spectrometer, X-ray photoelectron spectroscopy, elemental analyzer, zeta potentiostat and optical contact angle/interface tension meter. The results showed that chain-like aggregation of BDS and DS were consisted of a large amount of near-spherical primary particles and the average primary particle diameter of BDS (35 nm) was smaller than the average primary particle diameter of DS (39 nm). BDS contained more carbon content and less oxygen, hydrogen, nitrogen and sulfur content than DS. The degree of graphitization disorder of BDS (ID/IG=2.937) was lower than that of DS (ID/IG=3.162). The groups (C-C, C-O-C and C-OH) were presented on the surfaces of BDS and DS. Moreover, only DS contained C=O group. The relative viscosity increased by exponential function with increasing BDS or DS content at 20 ℃. The relative viscosity of LP contaminated by BDS was higher than that of DS when the mass fraction of soot was higher than 1%. Both BDS and DS could agglomerate in to larger particles in LP, but the agglomerate dimension of BDS (227.8 nm) in LP was bigger than the agglomerate dimension of DS (194.5 nm) in LP. In terms of the aggregation mechanism, compared with DS, BDS possessed higher surface energy and less lipophilicity. BDS was apt to agglomerate in to larger aggregation particles in LP, which was the main reason for the effect of BDS on relative viscosity of LP was greater than DS. This study not only provides reference for the wide application of biodiesel in diesel engine, but also lays foundation of the dispersion of BDS in the lubricating oil.