The use of biodiesel is being promoted by EU countries to partly replace petroleum diesel fuel consumption in order to reduce greenhouse effect and dependency on foreign oil. Meeting the targets established by the European Parliament for 2010 and 2020 would lead to a biofuel market share of 5.75% and 10%, respectively.
However, many voices have claimed that the associated agricultural development would bring considerable rise of food and water prices, unless biodiesel is made from waste materials or second-generation biofuels are developed. Waste cooking oil is one of the most promising feedstock in the Mediterranean countries, and in fact, many of the biodiesel production plants are currently using it. In a wide majority of cases these plants use methanol for their transesterification processes, which makes biodiesel
(mainly composed by methyl esters) only 90% renewable. By the contrary, the use of bioethanol in the production process would provide a fully renewable fuel (ethyl esters), which would further contribute to reduce life-cycle greenhouse emissions from vehicles. Different studies have shown that biodiesel from waste cooking oil can be used in different types of diesel engines with no loss of efficiency [1–5] and significant reductions in particulate matter –
PM– emissions [5–9], carbon monoxide –CO– emissions [3,6–9] and total hydrocarbon –THC– emissions [8–10] with respect to those obtained with conventional petroleum diesel fuel. Many of them have also found slight increases in nitrogen oxides –NOx– emissions [8,9] and a few others found also increases in aldehyde emissions [11,12]. Although the above mentioned are the most general tendencies, it is always possible to find opposite ones, such as that reported by Lin et al. [13], who found sharp increases in both NOx and particulate emissions when using cooking oil biodiesel fuel. Many of these results are described in recently published reviews about emissions from different biodiesel fuels [14] and about the potential of waste cooking oil as a source for biodiesel
[15].
However, the number of studies about the effect of ethanolderived biodiesel on diesel engine performance and emissions is still much lower than that of methanol-derived one. Al-Widyan et al. [16] compared the emissions from ethyl ester of waste cooking oil with those obtained from diesel fuel and with different blends and obtained some advantages in emissions when used ethyl esters, although their results were not consistent at all.