Biodiesel is a renewable fuel that is produced from oils derived from plants, animals, or microbes. It is nontoxic and biodegradable. Various methods such as transesterification, blending, cracking, microemulsification, and pyrolysis have been developed to convert oils from biological sources into biodiesel that is comparable to diesel fuel. Transesterification, by a number of consecutive reversible reactions, is the most common method: triglycerides are converted stepwise to diglycerides, monoglycerides, and finally glycerol, with the liberation of fatty acid methyl ester defined as biodiesel at each step. Generally, an alcohol and a catalyst are needed for transesterification of triglycerides.
Microalgae are microscopic (2-200 μm), autotrophic organisms that can grow by photosynthesis. Most are eukaryotes, but the prokaryotic cyanobacteria are often included. Some microalgae contain more than 70% (w/w, dry basis) lipids. Microalgae grow extremely rapidly under optimal conditions and can grow 100 times faster than terrestrial plants. The oil yield from microalgae containing 30% oil content is 58700 L ha^-1 year^-1, which is much higher than that from other crops (e.g., 636 L ha^-1 year^-1 for soybean and 5366 L ha^-1 year^-1 for oil palm). Importantly, microalgal cultivation need not encroach on arable land suitable for food production.
Currently, there are several areas of research on transesterification of microalgal oil. The conversion efficiency of biodiesel production via transesterification depends on microalgal characteristics, amount and type of alcohol used, catalyst, operating temperature, and reaction time. This study reviews the technologies that generate biodiesel from microalgae by transesterification. The relative performances of alkaline, acidic, and enzymatic catalysts are evaluated. New techniques used in biodiesel production, e.g., methods using microwaves and supercritical solutions, are discussed.