Algal Biofuels: Strain Selection, Cultivation Systems and Lipid Extraction

Abstract

Microalgae are attractive as a source of liquid fuel because some strains accumulate large amounts of neutral lipid, grow on land that is unsuitable for crops and can use waste nutrients and carbon dioxide. Yet a decade of enthusiasm has not produced a commercial fuel that competes on price. This narrative review follows the chain from strain choice, through cultivation in open ponds and closed photobioreactors, to harvesting and lipid extraction, and asks where the real constraints lie. It draws on published work on algal biology, bioprocess engineering, extraction chemistry, techno-economic analysis and life-cycle assessment, and it reports no new data. Lipid content measured in a flask under nutrient stress is a poor guide to lipid productivity outdoors, because the stress that raises lipid content also slows growth. Open ponds are cheap but vulnerable to contamination, whereas closed systems are controllable but costly. Dewatering and extraction contribute a large share of energy use and cost, and no single solvent or disruption method suits every cell wall. The review concludes that algal biofuels remain technically plausible and economically unproven, that coproducts and waste-stream integration offer the more realistic route, and that results should be reported on a productivity basis with the whole process counted.