Storage stability of commercially available biodiesels and their blends under different storage conditions

The present study investigated the storage stability of two commercially available biodiesels and their blends with diesel spiked with different impurities, which were stored at two different temperatures (15 and 40 °C) with air tight and light screen. These samples were periodically monitored during the whole storage period by measuring a number of properties, such as acid value (AV), induction time (IT) and composition of fatty acid methyl esters (FAMEs). It was found that (1) acid values increased but induction time decreased with the extension of storage for all samples without copper; however, both IT and AV values kept nearly constant for all samples with copper; (2) the presence of water did not contribute significantly to the degradation of all tested samples over time; and (3) higher temperature (40 °C) was favorable to the degradation of unsaturated FAMEs, accompanying with the altered acid values and induction time in comparison with the same samples stored at 15 °C. Faster degradation of FAMEs in blended samples than those in pure biodiesels may be partially due to the diluting effects of antioxidants in biodiesel. However, the presence of any impurities did not affect the degradation rates of FAMEs, which was not in agreement with the above mentioned AV and IT time series. This suggested that the addition of copper affected the measurement of AV and IT. Therefore, for samples with copper, FAME profiles can represent their quality more appropriately than IT or AV.