Placing microalgae on the biofuels priority list: a review of the technological challenges

Abstract

Microalgae provide various potential advantages for biofuel production when compared with 'traditional' crops. Specifically, large-scale microalgal culture need not compete for arable land, while in theory their productivity is greater. In consequence, there has been resurgence in interest and a proliferation of algae fuel projects. However, while on a theoretical basis, microalgae may produce between 10- and 100-fold more oil per acre, such capacities have not been validated on a commercial scale. We critically review current designs of algal culture facilities, including photobioreactors and open ponds, with regards to photosynthetic productivity and associated biomass and oil production and include an analysis of alternative approaches using models, balancing space needs, productivity and biomass concentrations, together with nutrient requirements. In the light of the current interest in synthetic genomics and genetic modifications, we also evaluate the options for potential metabolic engineering of the lipid biosynthesis pathways of microalgae. We conclude that although significant literature exists on microalgal growth and biochemistry, significantly more work needs to be undertaken to understand and potentially manipulate algal lipid metabolism. Furthermore, with regards to chemical upgrading of algal lipids and biomass, we describe alternative fuel synthesis routes, and discuss and evaluate the application of catalysts traditionally used for plant oils. Simulations that incorporate financial elements, along with fluid dynamics and algae growth models, are likely to be increasingly useful for predicting reactor design efficiency and life cycle analysis to determine the viability of the various options for large-scale culture. The greatest potential for cost reduction and increased yields most probably lies within closed or hybrid closed-open production systems.

Figure 1.

Confocal microscope image of the microalgae species Tetraselmis suecica, provided courtesy of Dr Emily Roberts, Swansea University.

Figure 2.

Overview of the chemical structures of the most common representatives from seven lipid classes: (a) triacylglycerides; (b) diacylglycerides; (c) monoglycerides; (d,e) phospholipids; (f) sterols; (g) sulpholipids; (h) glycolipids; (i) carotenoids. Structures from www.LipidMAPS.org.

Figure 3.

Overview of the fatty acid synthesis pathway (Kennedy pathway) from acetyl-CoA via ACCase. Enzymatic reactions involved: (1) ACCase; (2) malonyl-CoA : ACP transferase; steps 3–5, subsequent condensation reactions catalysed by (3) 3-ketoacyl ACP reductase; (4) 3-hydroxyacyl ACP dehydrase and (5) enoyl ACP reductase. ACP, acyl carrier protein; FFA, free fatty acid; G3P, glycerol-3-phosphate; lyso-PA, lyso-phosphatidic acid; TAG, triacylglycerides; DAG, diacylglycerides. Adapted from Ohlrogge & Browse (1995) and Riekhof et al. (2005).

Figure 4.

Image of large-scale Seambiotic Nannochloropsis sp. culture ponds. Image courtesy of Nature Beta Technologies Ltd, Eilat, Israel, subsidiary of Nikken Sohonsha Co., Gifu, Japan.

Figure 5.

Tubular PBRs in operation. Such systems have a small path length ensuring high volumetric production coupled with a small footprint. Photograph courtesy of Varicon Aqua Solutions Ltd, UK.

Figure 6.

Schematic of a biorefinery concept based upon the production of several products using algae from waste materials allowing their complete utilization to products such as liquid fuels, commodity chemicals and materials for high-value formulated products. This concept has little or no waste products and allows for residual energy capture, recycling of unused nutrients, water purification and recycling. This system would mean low environmental impact and maximization of the value of products from the system.

Figure 7.

Schematic illustrating biofuel production routes from algae lipids. (a) Trans-esterification of a triglyceride. Methanol or ethanol is normally used, together with an acid or base catalyst. This is a three step reaction, proceeding via the tri-, di- and monoglycerides to the alkyl esters which can be used as biodiesel. (b) A decarboxylation process.