Improvement of the oxidation stability of biodiesel as prepared by supercritical methanol method with lignin

An antioxidation effect of lignin‐derived products in biodiesel prepared using supercritical methanol (300°C/20 MPa) with molar ratio between rapeseed oil and methanol of 1:42 was studied. It was found that lignin could be decomposed to low molecular compounds that have a free radical‐trapping effect after supercritical methanol treatment. However, longer treatment time decreased the antioxidation effect of the lignin‐derived compounds. Rapeseed biodiesel prepared by supercritical methanol method at 300°C/20 MPa for 20 min with a small amount of added lignin showed an induction period longer than 6 h at 110°C in a Rancimat test. In addition, it was found that lignin had a catalytic effect in biodiesel production using the supercritical methanol method without significantly affecting other fuel properties of the prepared biodiesel. Thus, the study proved that lignin addition provides an inexpensive and technically acceptable way to improve the oxidation stability of biodiesel prepared by the supercritical methanol method with satisfactory fuel properties.