Mechanism and challenges in commercialisation of algal biofuels

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Abstract

Biofuels made from algal biomass are being considered as the most suitable alternative energy in current global and economical scenario. Microalgae are known to produce and accumulate lipids within their cell mass which is similar to those found in many vegetable oils. The efficient lipid producer algae cell mass has been reported to contain more than 30% of their cell weight as lipids. According to US DOE microalgae have the potential to produce 100 times more oil per acre land than any terrestrial plants. This article reviews up to date literature on the composition of algae, mechanism of oil droplets, triacylglycerol (TAG) production in algal biomass, research and development made in the cultivation of algal biomass, harvesting strategies, and recovery of lipids from algal mass. The economical challenges in the production of biofuels from algal biomass have been discussed in view of the future prospects in the commercialisation of algal fuels.

Introduction

Several biofuel candidates were proposed to displace fossil fuels in order to eliminate the vulnerability of energy sector (Korres et al., 2010, Prasad et al., 2007a, Prasad et al., 2007b, Singh et al., 2010a, Singh et al., 2010b, Singh et al., 2010c, Pant et al., 2010). The biofuels produced from crop seeds have come under major controversy as food vs. fuel competition (Nigam and Singh, 2010) as they require land for their production, whereas algae can be grown in the submerged area and also in the sea water (Singh et al., 2010c). The algal cultivation not only provides the biofuel but also provides greenhouse gas (GHG) saving as it utilized large amount of CO2 during the cultivation.

Algae range from small, single-celled organisms to multi-cellular organisms, some with fairly complex and differentiated form. Algae are usually found in damp places or bodies of water and thus are common in terrestrial as well as aquatic environments (Wagner, 2007). Algae include seaweeds (macroalgae) and phytoplanktons (microalgae). Many are eukaryotic organisms but the term is often used to also include cyanobacteria (blue-green algae), which are prokaryotic (Packer, 2009). Like plants, algae require primarily three components to produce biomass, i.e., sunlight, CO2 and water. The existing large-scale natural sources of algae include bogs, marshes, swamps, etc. (Wagner, 2007). Algae essentially harness energy via photosynthesis. They capture CO2 and transform it into organic biomass which can be converted to energy (Bruton et al., 2009). Algae can be either freshwater or marine, some grow optimally at intermediate saline levels and some in hypersaline conditions. Seaweeds are macroscopic multicellular algae that have defined tissues containing specialised cells. Many are unicellular and can be motile or non-motile depending on the presence of flagella. Where multi-cellular conglomerations exist, very little specialisation of cell types occurs, distinguishing them from seaweeds. There are a huge range of different types of microalgae including dinoflagellates, the green algae (chlorophyceae), the golden algae (chryosophyceae) and diatoms (bacillariophyceae) (Packer, 2009).

Algae contain complex long-chain sugars (polysaccharides) in their cell walls. These carbohydrate cell walls account for a large proportion of the carbon contained in these organisms (Packer, 2009), though many species contain quite high levels of various lipids and for some species under certain situations this has been quoted as up to 80% oil by wet weight (Singh et al., 2010c). Diatoms are a group including approximately 100,000 organisms many of which are marine and dominate the marine phytoplankton. They have silicate cell walls and have been of considerable interest in the biofuel production as they can accumulate very high levels of lipid. Diatoms, like many other organisms, use the triacylglycerol lipid molecules (TAGs) as energy storage molecules that can be easily transesterified to biodiesel, but a large percentage of the lipids contained in diatoms are phospholipids which are structurally dissimilar to TAGs and do not convert well to biodiesel using traditional transesterification procedures. Coccolithophores, that have calcareous external plates called coccoliths, also include some single celled flagellated algae and are also important in natural oceanic carbon capture (Packer, 2009). Keeping in view the above fact, this paper highlights the mechanism of biofuel production from algae and also summarizes the key points involved in the commercialisation of algal fuels.

Section snippets

Importance of algal fuel

The use of fossil fuels as energy is now widely accepted as unsustainable due to depleting resources and also due to the accumulation of GHGs in the environment. Renewable and carbon neutral biodiesel are necessary for environmental and economic sustainability. Biodiesel demand is constantly increasing as the reservoir of fossil fuel are depleting. Unfortunately biodiesel produced from oil crop, waste cooking oil and animal fats are not able to replace fossil fuel. The viability of the first

Composition and structure of algae

Microalgae biomass has a chemical composition which varies algal use (Table 1). Microalgae are being widely researched as a fuel due to their high photosynthetic efficiency and their ability to produce lipids (a biodiesel feedstock). Macroalgae do not generally contain lipids and are being considered for the natural sugars and other carbohydrates that can be fermented to produce either biogas or ethanol (Bruton et al., 2009). It can be rich in proteins or rich in lipids or have a balanced

Mechanism of triacylglycerol (TAG) production in algal biomass

Microalgae are eukaryotic cells that mean they contain a nucleus and other membrane-bound organelles and use sophisticated control mechanisms and post-translational biosynthetic processes. The flexible metabolic repertoire affords a greater choice and speed of response in metabolic approaches to different situations (Packer, 2009). Microalgae are able to survive heterotrophically, exogenous carbon sources offer prefabricated chemical energy, which the cells often store as lipid droplets (

Cultivation of algal biomass

There are two main cultivation systems, i.e., open pond and closed photobioreactor (PBR). Open pond refers to a simple open tank or natural ponds. Algae are grown in suspension with additional fertilizers. Gas exchange is via natural contact with the surrounding atmosphere and solar light. The highest productivity in open pond systems is obtained in raceway systems. A shallow depth pond with an elliptical shape (like a raceway) is mechanically mixed with a paddle wheel. This moves the water

Challenges in commercialisation of algal fuel

Microalgae are the untapped resource with more than 25,000 species of which only few are in use (Raja et al., 2008). The main genera cultivated include: Laminaria, Porphyra, Undaria, Gracilaria, Euchema, Ulva and Chondrus. From the vast number of known marine and freshwater species, only a handful are currently of commercial significance. These include Chlorella, Spirulina, Dunaliella and Haematococcus. Of these only Dunaliella is predominantly a marine species. These are generally cultivated

Future prospects

The costs in biofuel production from algal biomass amounts approximately 50 €/L that is very away to attract the commercial production of algal biofuels. An investigation of cost extensive approaches for the algal biofuel production is needed. One promising alternative seems to be the production of algal biomass in wastewater, providing a readily available medium for the production of algal biomass at almost no cost (Ahrens and Sander, 2010), and also the cultivation of algal biomass removed

Conclusions

The integration of microalgae cultivation with fish-farms, food processing facilities and waste water treatment plants etc., will offer the possibility for waste remediation through recycling of organic matter and at the same time low-cost nutrient supply required for the algal biomass cultivation. These options could all be explored as part of an integrated biorefinery concept. For regions at higher latitude, it may be possible to identify local strains of algae requiring low light intensities

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