Research review paperExtraction of oil from microalgae for biodiesel production: A review
Introduction
The search for sustainable and renewable fuels is becoming increasingly important as a direct result of climate change and rising fossil-fuel prices. Current commercial production of biodiesel or fatty acid methyl ester (FAME) involves alkaline-catalyzed transesterification of triglycerides found in oleaginous food crops with methanol. However, cultivation of these food crops for biodiesel (mainly rapeseed in Europe and soybean in the US) is no longer sustainable as it requires substantial arable land and consumes large amounts of freshwater (Chisti, 2007).
Microalgae are currently considered to be one of the most romising alternative sources for biodiesel (Sheehan et al., 1998). Since many microalgal strains can be cultivated on non-arable land in a saline water medium, their mass farming does not place additional strains on food production (Widjaja et al., 2009). Their high photosynthetic rates, often ascribed to their simplistic unicellular structures, enable microalgae not only to serve as an effective carbon sequestration platform but also to rapidly accumulate lipids in their biomass (up to 77% of dry cell mass). Even using a conservative scenario, microalgae are still predicted to produce about 10 times more biodiesel per unit area of land than a typical terrestrial oleaginous crop (Chisti, 2007, Rosenberg et al., 2008, Sheehan et al., 1998, Shenk et al., 2008).
There are, however, various technological and economic obstacles which have to be overcome before industrial-scale production of microalgal biodiesel can take place. The selection and successful outdoor large-scale cultivation of a robust microalgal strain, which has optimum neutral lipid content, possesses an elevated growth rate, and is immune towards invasion by local microbes, remain a major upstream challenge (Sheehan et al., 1998). On the other hand, the development of an effective and energetically efficient lipid extraction process from the microalgal cells is critical for the successful upscaling of the downstream processes. Despite the routine use of laboratory-scale extraction protocols to determine microalgal lipid contents, the variables affecting lipid extraction from microalgal cells are not well understood and no method for industrial-scale extraction is currently established (Halim et al., 2011).
This paper attempts to address the knowledge gap surrounding microalgal lipid extraction by summarizing and critiquing recent studies in the field. We report on the suitability of microalgal lipid compositions for biodiesel conversion and review the different conventional downstream bioprocessing steps required for microalgal biodiesel production. We then examine the technologies currently available for laboratory-scale microalgal lipid extraction, paying special attention to the use of organic solvent extraction and supercritical fluid extraction. We conclude with an assessment on how different cellular pre-treatment processes can effect microalgal lipid extraction as well as with an update on the recent advances in the field, such as the development of a simultaneous microalgal lipid extraction-methylation method and the establishment of a novel ‘single-step’ microalgal lipid extraction method by OriginOil, Inc.
Section snippets
Microalgal lipid composition
A fatty acid (FA) molecule consists of a hydrophilic carboxylate group attached to one end of a hydrophobic hydrocarbon chain (Fig. 1). Fatty acids are constituents of lipid molecules (both neutral and polar) and designated based on their two most important features ‘the total number of carbon atoms in the hydrocarbon chain: the number of double bonds along the hydrocarbon chain’. Saturated fatty acids have no double bond, while unsaturated fatty acids consist of at least one double bond (
Overview of downstream processes
Fig. 4 shows the downstream processing steps required to produce biodiesel from microalgal biomass (Halim et al., 2011). Table 1 lists the different laboratory-scale technological options currently available for each step. The table also examines the scale-up potential of each technology. After the microalgal culture is harvested from the bioreactor, it is concentrated in a dewatering step. The concentrated microalgal culture is then processed in a pre-treatment step to prepare it for lipid
Lipid extraction
Depending on its pre-treatment pathway, microalgal biomass to be submitted to lipid extraction can assume one of the following physical states: concentrate or disrupted concentrate or dried powder. During lipid extraction, the microalgal biomass is exposed to an eluting extraction solvent which extracts the lipids out of the cellular matrices. Once the crude lipids are separated from the cell debris, the extraction solvent, and water (only when extraction is performed on concentrate or
Effect of cellular pre-treatment on lipid extraction
The effects of cellular pre-treatment on microalgal lipid extraction have not been investigated extensively. As previously described, the pre-treatment process can take alternative pathways depending on the desired biomass alterations (Fig. 4). The process can be performed in a single step or multiple steps. It is noted that most of the pre-treatment steps (such as thermal drying for complete water removal or high-pressure homogenization for cell disruption) are energy intensive and should only
Simultaneous extraction and transesterification of microalgal lipids
Recent studies investigating biodiesel production from microalgae have focused their efforts on the development of an alternative downstream processing step termed simultaneous extraction and transesterification (Wahlen et al., 2011). This step, also known as direct transesterification or in-situ transesterification, combines lipid extraction and transesterification in a single step, thereby simplifying the downstream pathway required for biodiesel production from microalgal biomass (Fig. 19).
Microalgal biorefinery
The cost of producing microalgal biodiesel can theoretically be offset by revenues generated from other co-products of the microalgal biomass. Microalgae contain significant quantities of proteins and carbohydrates as well as smaller amounts of high-value functional ingredients (astaxanthin, canthaxanthin, carotenes, chlorophylls, Ω3 free fatty acids, and γ-linolenic acid). Each of these cell components can be appropriately utilized to co-generate a useable product in a biorefinery. Recent
OriginOil Single-Step Extraction of microalgal lipids
OriginOil, Inc has established a novel method for microalgal lipid extraction (OriginOil, 2010). Instead of following the traditional sequence-based downstream processing pathway outlined in Fig. 4, the method devised by OriginOil performs three simultaneous functions (dewatering, cell disruption, and lipid extraction) in a single downstream step (Fig. 20). This approach is referred to as OriginOil Single-Step Extraction™.
Within a single step (OriginOil, 2010), microalgal concentrate is exposed
Conclusions
The downstream technologies needed for industrial-scale production of microalgal biodiesel are still in the early stages of development. Lipid extraction from microalgal biomass has not received sufficient attention and represents one of the many bottlenecks hindering economic industrial-scale production of microalgal biodiesel. Future research on microalgal biodiesel should focus on developing an effective and energetically efficient lipid extraction process. A fundamental understanding of
Acknowledgments
This work was supported by an Australian Research Council (ARC) Linkage grant between the Department of Chemical Engineering in Monash University (Victoria, Australia) and Bio-Fuel Pty Ltd (Victoria, Australia).
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