Glycerol-free biodiesel production through transesterification: a review

Waste oils are becoming increasingly more important as feedstock for the production of fuels and chemicals. However, the high level of impurities in waste oils limits their use in transesterification reactions where methanol is used as acyl acceptor. A second consequence of increased biodiesel production is the oversupply of glycerol into the market that has caused a sharp decrease in glycerol prices. Novel production routes are thus necessary to limit glycerol formation while also allowing the use of crude or contaminated oil as feedstock. The aim of this work is to review the state of glycerol-free biodiesel synthesis routes with emphasis on routes using methyl acetate or dimethyl carbonate (DMC) as acyl acceptors. Dimethyl carbonate is favoured as acyl acceptor when using biocatalysts for synthesis, while methyl acetate is favoured as acyl acceptor in supercritical-assisted transesterification. Both dimethyl carbonate and methyl acetate as acyl acceptors are able to tolerate impurities such as free fatty acids, but methyl acetates has a higher tolerance for water in the feedstock than dimethyl carbonate. The performance of both acyl acceptors in the presence of used motor oils and industrial greases need to be investigated to assess the suitability for industrial application.