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.

In the present work, several fatty acid methyl esters (FAME) have been synthesized from various fatty acid feedstocks: used frying olive oil, pork fat, soybean, rapeseed, sunflower, and coconut. The oxidation stabilities of the biodiesel samples and of several blends have been measured simultaneously by both the Rancimat method, accepted by EN14112 standard, and the PetroOXY method, prEN16091 standard, with the aim of finding a correlation between both methodologies. Other biodiesel properties such as composition, cold filter plugging point (CFPP), flash point (FP), and kinematic viscosity have also been analyzed using standard methods in order to further characterize the biodiesel produced. In addition, the effect on the biodiesel properties of using 4-allyl-2,6-dimethoxyphenol and catechol as additives in biodiesel blends with rapeseed and with soybean has also been analyzed. The use of both antioxidants results in a considerable improvement in the oxidation stability of both types of biodiesel, especially using catechol. Adding catechol loads as low as 0.05% (m/m) in blends with soybean biodiesel and as low as 0.10% (m/m) in blends with rapeseed biodiesel is sufficient for the oxidation stabilities to comply with the restrictions established by the European EN14214 standard. An empirical linear equation is proposed to correlate the oxidation stability by the two methods, PetroOXY and Rancimat. It has been found that the presence of either catechol or 4-allyl-2,6-dimethoxyphenol as additives affects the correlation observed.

In the present work, bio-oil derived from the catalytic pyrolysis of sewage sludge has been blended in small amounts with sunflower biodiesel with the aim of evaluating its potential as a novel, low-cost, and renewable biodiesel additive that could replace costly commercial biodiesel antioxidants normally used to date. The effect of blending small amounts of bio-oil with sunflower biodiesel on the biodiesel properties (oxidation stability, cold flow properties, flash point, and viscosity) has been analyzed. Furthermore, apart from studying the effect of adding low bio-oil concentrations (0.1, 1.8, and 3.5 mass %), the effect of other operating conditions, specifically the temperature (278–333 K) and mixing time (5–60 min), during the bio-oil and biodiesel blend preparations has also been analyzed. With regard the oxidation stability, blends prepared adding 3.5% mass fraction of bio-oil complied with the limits imposed by the ASTM D6751 and EN 14214 standards. Blending sewage sludge bio-oil and sunflower biodiesel did not result in an enhancement of the biodiesel cold flow properties in the studied range, while the flash point of these blends was lower than that of pure sunflower biodiesel. The viscosity was barely affected in all cases. The oxidation stability enhancement achieved by the addition of bio-oil obtained from sewage sludge catalytic pyrolysis was higher than the enhancement obtained with bio-oil from non-catalytic pyrolysis.

The major challenge for the pyrolytic conversion of lignocellulosic materials into crude bio-oil is the poor quality of the final product. Several strategies (addition of solvents, production of emulsions, and extraction with biodiesel) have been studied to improve its fuel properties. The extraction with biodiesel is an interesting solution because it allows direct utilization of some bio-oil fractions as fuels. However, fraction extracted with biodiesel is typically between 10 and 18 wt. %. In this paper we studied mild hydrotreatment of pyrolysis oil to enhance its solubility in biodiesel. The study was conducted with BTG and Amaron oils hydrotreated at temperatures between 200 and 325°C in the presence of Ru/C catalyst. Hydrotreated oils generated three phases: top oil (light hydrocarbons), middle aqueous phase and bottom heavy oil phase. Each of the phases was characterized and the content of acetic acid, phenols, aromatic compounds, and linear alkane hydrocarbons quantified. The upgraded bio-oils were more soluble in biodiesel than the crude bio-oils, obtaining blends with up to 48 and 38 wt. % for the BTG and Amaron bio-oil, respectively. Some of the fuel properties of the resulting blends are also reported here.

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