Oxidative Degradation Behavior of Fatty Acid Methyl Ester in Fish Oil Biodiesel and Improvement of Oxidation Stability by Partial Hydrogenation

Waste cooking oil contaminated with fish oil has potential uses as a biodiesel fuel feedstock. The changes in properties of fish oil fatty acid methyl ester (FAME) caused by oxidation were first evaluated, and the oxidation stability of fish oil FAME was improved by partial hydrogenation using a noble metal catalyst. Oxidation of fish oil FAME resulted in many decomposition products such as aldehydes, carboxylic acids and ketones, and large amounts of sludge. The oxidation propensity of FAME was proportional to the degree of unsaturation, and polyunsaturated FAMEs with more than 4 double bonds, the main components of fish oil FAME, were almost completely oxidized. Elemental analysis and FT-IR analysis showed that the sludge contained large amounts of oxygen compounds assigned as ketones, esters and carboxylic acids. Gel permeation chromatography (GPC) indicated that the sludge seemed to be formed by polymerization of about ten molecules of FAME, especially polyunsaturated FAME molecules. To improve the oxidation stability of fish oil contaminated biodiesel fuel, partial hydrogenation of fish oil FAME was carried out over Pd-Pt/Yb-USY-Al2O3 catalyst, with the fish oil FAME mixed with rapeseed oil to simulate the waste cooking oil contaminated with fish oil. Unsaturated FAMEs with more than 2 double bonds were selectively hydrogenated to monounsaturated and saturated FAMEs. Hydrogenated mixed FAME did not form any sludge and the oxidation stability was significantly improved compared with the untreated FAME. The oxidation stability of hydrogenated FAME blended with petroleum diesel was almost equivalent to that of petroleum diesel. Reduction of polyunsaturated FAME by partial. hydrogenation is effective for improving the oxidation stability as well as suppression of sludge formation.