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Biodiesel is a renewable fuel which can be described chemically as a mixture of different alkyl esters. To predict its different thermophysical properties, critical properties (P c , T c , V c ) must be known or estimated. In this work, the use of group contribution and group interaction methods is compared regarding its goodness in the prediction of a widely measured property such as biodiesel density, as there is a lack of data of normal boiling point or densities of pure alkyl esters. The results show that a group interaction method, with the available data of 20 different biodiesels published, can predict better biodiesel density when methanol was used for the transesterification reaction. Data available for biodiesel produced with ethanol are scarcer; only six sets of biodiesel composition and density could be found, and in this case, both group interaction and group contribution methods predict density with a similar error, although more data are needed.

Unlike petroleum diesel, the chemical structure of biodiesel makes it prone to oxidation during long-term storage, thus involving fuel quality deterioration. Therefore, the addition of antioxidants is usually required to meet the quality standards for biodiesel commercialization. Synthetic sterically-hindered phenols have been usually employed for this purpose as free radical scavenging antioxidants. However, naturally occurring phenolics are also available, for example, in the bio-oil produced in the pyrolysis of lignocellulosic biomass. In this work, the antioxidant potential of extracted fractions of lignocellulosic bio-oil has been evaluated. Different organic solvents were tested as extraction agents, acetate esters being the best ones for incorporating bio-oil antioxidant compounds into biodiesel. In the best case, the incorporation of a small concentration of bio-oil compounds (< 4 wt. %) led to an improvement of the biodiesel oxidation stability of 475 % which, in our case, was enough to meet the European standard requirement.

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