Chapter 2 - Basic properties of biodiesel soot

Abstract

As worldwide energy demands increase, renewable fuels are attracting a great deal of attention as a pathway towards an environmentally sustainable future and improved energy independence. Biodiesel is a type of renewable engine fuel that is regarded as an excellent substitute for petrodiesel. 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 emissions; however, soot is generated as a combustion process by-product 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 gases. BDS agglomeration in the lubricating oil can lead to an increase in the viscosity of the oil and may result in increased wear of an engine's critical components, reduction in oil life, and increased frequency of oil changes. Therefore, more attention must be given to the physical-chemical properties and aggregation of BDS. In this study, cedar ash catalyst (green biomass ash catalyst) was prepared with a first calcination, hydration, and second calcination method using cedar ash as the raw material. The catalyst was characterized by inductively coupled plasma mass spectrometry, simultaneous thermal analysis, X-ray diffraction, field emission scanning electron microscopy, and the Hammett indicator method. On this foundation, BDS and No. 0 diesel soot (DS) were obtained from 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 spectrometry, Fourier transform infrared spectrometry, X-ray photoelectron spectroscopy, elemental analysis, Zeta potentiostat, and an optical contact angle/interface tension meter. The results showed that the active species of cedar ash catalyst is CaO and the base strength of cedar ash catalyst is 9.8 <H- < 15.0. Under the optimal conditions of first calcination temperature of 800°C, first calcination time of 2 h, second calcination temperature of 500°C, second calcination time of 2 h, catalyst mass fraction of 7%, catalytic time of 5 h, methanol/oil molar ratio of 14:1, and catalytic temperature of 65°C, the biodiesel yield reached 91.52%. Cedar ash catalyst has good reusability performance. After the four-time regeneration, the biodiesel yield still reached 82.03%. Chain-like aggregation of BDS and DS consisted of a large amount of near-spherical primary particles, with the average primary particle diameter of BDS (35 nm) being 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 (CC, COC, and COH) were presented on the surfaces of BDS and DS. Moreover, only DS contained the CO group. The relative viscosity increased exponentially with increasing BDS or DS content at 20°C. 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 into 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 into larger aggregation particles in LP, which was the main reason for the effect of BDS on relative viscosity of LP being greater than with DS. This study not only provides a reference for the wide application of biodiesel in diesel engines, but also lays a foundation for the understanding of dispersion of BDS in lubricating oil.