Extraction techniques in sustainable biofuel production: A concise review

Fuel Processing Technology 193 (2019) 295–303

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Review paper

Extraction techniques in sustainable biofuel production: A concise review a,⁎

b

Peng Li , Kiyoshi Sakuragi , Hisao Makino a b

b

T

Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), 1646 Abiko, Chiba-ken, 270-1194, Japan Energy Engineering Research Laboratory, Central Research Institute of Electric Power Industry (CRIEPI), Yokosuka, Kanagawa-Ken 240-0196, Japan

ARTICLE INFO

ABSTRACT

Keywords: Extraction Sustainable energy Biofuel Biomass Algae biofuel

The concerns of fossil fuel depletion and environmental issues have increased general interest in studies on the development of sustainable biofuel. In the biofuel production process, the extraction technique is a key technology with respect to energy consumption and product value. In this review, recent advances related to the extraction process in biofuel production are discussed. The major extraction methods were classified as conventional solvent extraction (CSE), physical-supported solvent extraction (PSSE), supercritical fluid extraction (SFE), and novel extractions. We address each of these classified extraction methods in turn both in relation to the feedstock utilized in the different biofuel generation methods and to the targeted biofuels. Our objective is to provide a concise, timely, and comprehensive review on various extraction methods applied/studied regarding biofuel production, their principles and advantages/limitations, and recent case studies. In particular, the classified extraction methods in the production of third-generation algal biofuel were intensively evaluated. Consequently, the concept of free from drying and cell disruption (FDC) routes was advanced herein as it may possess the potential to reduce the requirement for input energy during the production of algae biofuels.

1. Introduction Fossil fuels still serve as the primary global energy resource and account for more than 88% of the primary energy consumption [1], although new sources of cheap fossil fuels are no longer available and experts have issued warnings regarding the possible depletion of the current sources in the near future [2]. In addition, greenhouse gas emissions such as CO2 caused by the combustion of fossil fuels also represent a serious concern. Therefore, the development of sustainable energy resources and reduction of the greenhouse gas substances from fossil fuels have become essential topics of interest worldwide [3]. The main current sustainable energy resources consist of the energy that can be derived from biomass and natural phenomena such as solar and wind (Fig. 1). The latter comprise the two largest sources of renewable electricity and have the major advantage that the source of energy is free albeit the major disadvantage of being variable and intermittent. In turn, biomasses constitute the third primary energy source overall after coal and oil [4]. Many technological options to convert biomass to biofuel have been and are being studied and implemented in practice with different degrees of success. Fig. 1 describes the general options for biofuel derivation from biomasses. Biomasses can be converted into fuel products by means of biological (e.g. fermentation, anaerobic digestion), physico-chemical (e.g. extraction, trans-

⁎

esterification), and thermochemical (e.g. gasification, liquefaction) approaches. In particular, thermochemical processes can be used to produce oil and gas [5,6] whereas biochemical processes yield products such as bioethanol. These conversion processes have been described in detail in several recent papers (e.g. [7–9]). Within the physico-chemical approach, extraction allows for recovery of the desired compositions or removal of undesired substances from target feedstock [10]. The goal of the current review was to summarize recent studies of extraction methods and classify these methods as well as to discuss their possible application in biofuel production in relation to the feedstocks utilized for the different biofuel generation strategies. Notably, classification of the extraction methods used in biofuel production is complex, especially considering the large volume of information available regarding this topic. This review was thus structured around the main theme “classification of existing extraction methods” and is organized as follows. Section 2 illustrates the classification and basic information regarding extraction methods. Section 3 presents the generation of biofuel and the extraction methods applied or studied with regard to the feedstocks of these different biofuels. Particularly, the extraction studies related to the new generation of algae biofuels are intensively reviewed. The meaningful considerations for selection of extraction methods based on algae biofuel production are presented in Section 4 as this is believed to likely comprise the biofuel of the future and to exhibit

Corresponding author. E-mail address: [email protected] (P. Li).

https://doi.org/10.1016/j.fuproc.2019.05.009 Received 1 February 2019; Received in revised form 1 May 2019; Accepted 8 May 2019 Available online 28 May 2019 0378-3820/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic diagram of sustainable energy options.

considerable potential to deliver clean energy. Finally, Section 5 provides the main conclusions and highlights of the current review.

substances such as lipids [18] and hydrocarbon from various biomasses [19]. Multifarious solvents can be selected with solvent selection usually based on the type of biomass and the specific components contained therein. Chemical and physical properties such as polarity comprise the main factors influencing the efficiency and selectivity of extraction solvents in addition to the extraction conditions [20]. In addition, economic and environmental concerns are also important issues that must be considered in the application of solvent extraction [21,22]. Some representative extractable feedstocks for the production of different generations of biofuels are listed in Table 1. The biofuel products acquired via extraction processes mainly comprise the precursors of bio-diesel and jet fuel. In some cases, bio-alcohols can also be acquired from the extracts of wood biomasses. Moreover, bio-solid fuel can be acquired after removal of the water and impurities from biomasses by using extraction (dewatering) technologies. Notably, a general search will yield hundreds of thousands of links involving information regarding either or both extraction methods or biofuels. However, even within academic-technical review papers that concentrate on biofuel studies, the content directly related to extraction only represents a very limited part of the article; moreover, recent

2. Basic information and classification of extraction methods 2.1. Basic information on extraction techniques Extraction normally constitutes a midstream step in the physicochemical process of biofuel production (Fig. 2) as some procedures such as cultivation and drying should be performed prior to extraction (upstream), whereas the extracts themselves need to be further refined by using procedures such as fractional distillation and trans-esterification (downstream). Extraction can be basically divided into mechanical, physical, and chemical approaches. The representative methods in the current classification are listed in Fig. 2; the respective theoretical backgrounds available in the cited references [11–17] and described as relevant hereafter. All three approaches are used in biofuel production although the latter two are considered the more important and are frequently applied. Solvents in different forms are widely used to extract fueling

Fig. 2. Schematic diagram of classification of extraction methods. SFE, supercritical fluid extraction. 296

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Table 1 List of extractable components for the production of different generations of biofuels. Generation

Extraction methods

Representative feedstocks

Extractable components

Targeted biofuels

Ref.

First Second

CSE Mechanical, CSE, PSSE, SFE, Novel

Energy crops, and animal fats Industrial, civil, and agricultural residues

Third

CSE, PSSE, SFE, Novel

Woody crops: oil and seeds Microalgae

Fatty acids Fatty acids Hydrocarbons Bio-solids Fatty acids Lipids Hydrocarbons

Diesel Diesel Jet fuel Solid fuel Diesel Diesel Jet fuel

[23] [22,24] [25] [26] [27] [28] [29]

CSE, conventional solvent extraction; PSSE, physical-supported solvent extraction; SFE, supercritical fluid extraction. Table 2 Case analysis of classified extraction methods in algae biofuel production. Extraction method

Representative methods

Necessity of drying and cell disruption

Energy demand: Comments: C

CSE

Polar solvent (e.g. Bligh-Dyer, Methanol-chloroform) Non-polar solvent (e.g. Soxhlet with hexane) Microwave supported

Both

E

Both

E

Drying

Sonication supported

Drying

SCCO2

Both combined by lyophilization

SuCH2O

Both combined by lyophilization

Switchable solvent extraction

Both combined by lyophilization

Liquefied gas

Neither

PSSE

SFE

Novel

E

:Energy intensive : Environmental issues etc. :Energy intensive C : Environmental issues etc. E : Energy intensive C : Easy to scale up L :Redundant energy E : Energy intensive C : Cells disrupted simultaneously E : High pressure high energy demand C : Good selectivity but safety issues E :Energy intensive C : Multiple products E :Low energy consuming C :Low extraction rate E : Low energy consuming C :Under development C

studies related to extraction have generally focused only on new technical developments. Therefore, a critical literature review devoted to the comprehensive application, recent research, and new methods of extraction in the biofuel production processes is appropriate.

Ref. [30] [30] [31] [31] [32] [35] [38] [18]

2.3. Conventional solvent extraction (CSE) CSE was the most frequently mentioned extraction method among the selected articles. This likely occurred because as a historical method, chemical solvent extraction has been applied in many fields such as the food and pharmaceutical industry, and many classical CSE methods have been verified. Chemical solvents used for CSE are normally in the liquid form at ambient pressure and room temperature and possess high selectivity and solubility for the desired compositions. Herein, some conventional and classical solvent extraction methods such as Soxhlet extraction and Bligh-Dyer methods, which are still frequently applied for the separation of oil from solid materials [16,17], are assigned as representing CSE. A wide range of solvents can be selected for the Soxhlet technique individually or as mixtures including hexane, chloroform, and methanol [16]. In the Bligh-Dyer method, the combination of utilized solvents can be changed based on the polarity of lipids present in the target materials [17], in accordance with the traditional classification of organic solvents as non-polar, aprotic polar, and protic polar based on their ability to form hydrogen bonds. Consistent with this, the use of various organic solvents has been reported for biofuel production. The principle of CSE in the extraction of biofuel is that upon contact of the organic solvent with the biomass, the target compositions are extracted into the organic solvent owing to the permeability effect of the solvent on the biomass. Therefore, a suitable organic solvent should possess biocompatibility and maximum solubility for the target compositions. However, a notable shortcoming of CSE is that most organic solvents cause health hazards and environmental pollution. Moreover, in CSE a pre-treatment is normally required for extracting the available components, thereby increasing the overall cost burden (Table 2).

2.2. Classification of extraction methods The general classification of extraction methods that are used in biofuel production is depicted in Fig. 2. Mechanical methods are conventional for first generation biofuel production, avoiding the use of chemicals while providing advantages such as the production of instantly consumable crude oil and low equipment cost [11]. In particular, the use of presses/expellers constitutes the most traditional mechanical oil extraction method applied for oleaginous material. Conversely, such mechanical extraction methods are rarely utilized for the production of second- and third-generation biofuels. Physical extraction methods include sonication, microwaving, homogenizing, and heating. Chemical extraction basically utilizes various solvents to extract the biofuel component from target feedstocks, comprising the well-utilized Soxhlet extractor with different solvents and the classical Bligh-Dyer method as well as traditional liquid-liquid extraction. Unlike mechanical methods, in many cases the application of physical and chemical extraction methods is combined, with the physical methods usually being applied to support chemical solvent extraction. Therefore, to provide a clear yet comprehensive assessment of these processes, physical and chemical methods were sub-classified in this review into conventional solvent extraction (CSE), physical-supported solvent extraction (PSSE), and supercritical fluid extraction (SFE). In addition, some new extraction methods are assigned as belonging to “novel” methods.

297

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Fig. 3. Relationships between feedstocks of different generation biofuels with the classified extraction methods along with targeted biofuels.

2.4. Physical-supported solvent extraction (PSSE)

to the high operational costs (Table 2) and safety-related issues; for example, the operating temperature and pressure of supercritical CO2 are above 304.25 K and 7.39 MPa, respectively. To reduce associated costs, recent studies have therefore attempted to instead use near- or sub-critical solvents such as water [35].

In this review, CSEs supported by physical methods, such as the most frequently used microwave and sonication methods, were assigned as PSSE. Microwaves, referring to a frequency of electromagnetic radiation around 0.3–300 GHz, are generally applied at a small scale for the disruption of plant cells [13]. In turn, the application of sonication for plant cell disruption in association with oil extraction has been studied for many years. Ultrasonic cavitation is significantly more intense at low frequency (around 18–50 kHz), with the type of plant cell wall and medium conditions such as viscosity and temperature affecting sonication efficiency [14]. Notably, PSSE represents a means to integrate pre-treatment processes into CSE. In general, the use of PSSE is dependent on the type of biomass, as some require pre-treatment prior to biofuel extraction. For example, microwaving generates high frequency waves, which may destroy the biomass through shock induction. Therefore, it was recently suggested to represent an efficient method for disrupting oil-containing plant cells [30]. Furthermore, sonication has been extensively used for microbial cells, as it disrupts both the cell wall and membrane through the cavitation effect. The characteristics of microwaving and sonication are shown in Table 2.

2.6. Novel methods Some new extraction methods that do not fit within the previously described categories were assigned as “novel” methods. For example, multiple ionic liquids were assessed for their ability to extract branched hydrocarbons from an aqueous medium [29,36]. Another study described a lipid extraction method using ozonation, which can produce saturated hydrocarbons from Dunaliella salina (green alga) [37]. Moreover, solvents with switchable hydrophilicity have been used for biofuel extraction from microalgae [38]. Finally, the simultaneous extraction and dewatering of biofuel from wet biomasses using liquefied gases such as dimethyl ether (DME) as the extraction solvent have also been reported [26]. 3. Extraction methods applied to different biofuel feedstocks 3.1. Extraction methods applied to the feedstocks in first-generation biofuels

2.5. Supercritical fluid extraction (SFE)

The figures of extractable feedstocks in different generations of biofuels are depicted in Table 1. First-generation biofuels are produced directly from arable food crops or animal fats. The biofuel is ultimately derived from the starch, sugar, animal fat, and vegetable oil that these crops or animals provide. Corn, wheat, sugar cane, and oily plants are the most commonly used first-generation biofuel feedstocks [39] with bioethanol and biodiesel being the two major types of derived firstgeneration biofuels. Biodiesel (fatty acid methyl ester) in particular is produced through the transesterification of oils extracted from oily crops. As shown in Fig. 3, the application of extraction methods for the feedstocks of first-generation biofuels mainly relies upon mechanical methods, because biodiesel derived from vegetable oil accounts for a

SFE was firstly developed for the extraction of oil in 1980 [15]. Although SFE also belongs to the categories of both physical and chemical extraction, owing to its importance, SFE is treated as independent from PSSE in the current review. SFE has a unique advantage compared to CSE as it shows high selectivity (Table 2). SFE involves the use of fluid in a supercritical state as an extracting solvent, the physical and thermal properties of which are between those of pure liquid and gas [33]. Carbon dioxide (CO2) and water are the most commonly used supercritical fluids, which could be potentially used for biofuel production. For example, supercritical CO2 has several advantages, especially for the extraction of low polar chemicals such as lipids from biomasses [34]. The disadvantages of current SFE methods are related 298

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large proportion of first-generation biofuel and the extraction of vegetable oil from seeds or plant parts is traditionally accomplished by mechanical pressing or expression [40,41]. First-generation biofuels have therefore been considered to exhibit economic and environmental limitations. Accordingly, fewer studies related to first-generation biofuels have been published in recent years, and very little information could be found in recent articles regarding the mechanical extraction methods applied in first-generation biofuels. For example, among the recent publications only one article was published concerning the synthesis of biodiesel from sunflower-extracted oil [42]. Therefore, considering the sustainability factor, successive generations of feedstocks are extensively analyzed in the current review with a particular focus on extraction in third-generation biofuel feedstocks.

in 24.0, 18.0, and 16.0 wt% total bio-oil extraction [27]. Hexane, acetone, and petroleum ether were also evaluated for the extraction of Bauhinia monandra (orchid tree) seed oil, with petroleum ether yielding 14.8 wt% oil [44]. Generally, the selection of extraction solvent should follow the premise of “similar dissolves similar”. Case studies between polarity and extractable compounds mediated by the three commonly used organic solvents hexane, chloroform, and acetone are described in Table 3. The compositions of the biofuels extracted by different solvents from same feedstock are normally dependent upon the polarities of the solvent used. As a PSSE method, ultrasonic associated extraction was used for liquid biofuel production from Jatropha seeds. This method potentially reduces the recalcitrance of lignocellulosic materials (lignin, cellulose, and hemicelluloses) and improves their suitability as bioethanol feedstocks [45]. Alternatively, the application of SFE to lignocellulosic biomasses was considered to be appropriate in terms of the high resolvability of this method [33]. The majority of studies regarding SFE for biofuels from lignocellulosic biomass have been carried out using either ethanol or methanol. Notably, the second-generation biofuels obtained from biomasses using SFE are significantly affected by extraction parameters such as biomass/solvent ratio, extraction temperature, extraction time, and operating pressure [33]. Moreover, as a novel technique, a highly cost-efficient and environmentally friendly method using liquefied gas operated under normal pressure and temperature has been reported with regard to the extraction of bio-crude fuel from vegetal biomass and industrial waste [26,46].

3.2. Extraction methods for the feedstocks in second-generation biofuels Second-generation biofuels comprise fuels that can be manufactured from various types of biomass, which are generally obtained from vegetal materials but can also include non-vegetal materials. Mechanical methods supported by chemical CSE are usually applied in first-generation biofuel production, which can effortlessly extract vegetable oils from arable crops as shown in Fig. 3. In comparison, second-generation biofuels are produced from lignocellulosic biomass, industrial, civil, and agricultural residues, or wastes from which extraction of the required fueling substances is more difficult. In particular, it is especially challenging to obtain the required fuel by means of only extraction methods from lignocellulosic wood biomasses. Accordingly, the extraction methods frequently mentioned in the recent articles for secondgeneration biofuels were instead those used for industrial, civil, and agricultural residues. In contrast to the food oil crops prevalent in firstgeneration feedstocks, the use of extractable woody crop oil for biodiesel fuel has been reported for second-generation feedstocks (Table 1) [27]. As shown in Fig. 3, all these classified extraction methods are used for the feedstocks in second-generation biofuels, from which diverse biofuel products can be produced. Few mechanical methods are used in the second generation because compared to the first and third generations, the oil content of second generation biomass is generally low leading to a poor recovery rate of oil if only mechanical methods are applied. Some articles described the derivation of fish oil-based biofuels from fish processing plant waste following application of a mechanical extraction method, whereupon the fish oil was subsequently recovered using carbon dioxide (SFE) [43] or Soxhlet extraction (CSE) [24]. Different conventional organic solvents have also been tested as extraction agents for second generation biofuel production; representative examples are listed in Table 3. Non- or aprotic polar solvents hexane, petroleum ether, and chloroform were used for the extraction of hydrocarbons from Nicotiana glauca (tree tobacco), yielding total hydrocarbons of 6.28, 5.69, and 6.20 μg mg−1 FW (fresh weight), respectively [25]. In another biodiesel production process, rubber seed oil was extracted by shaking with non-polar or polar solvents hexane, dichloromethane, or acetone at room temperature for 30 min, resulting

3.3. Extraction methods for the feedstocks of third-generation biofuels The main source of third-generation biofuels is microalgae. Therefore, third-generation biofuels are also termed algae-biofuels. This is presently regarded as a feasible alternative renewable energy resource for biofuel production, which overcomes the shortcomings of first and second-generation biofuels. Algae can provide several different types of renewable biofuels including biodiesel, hydrocarbons, and biohydrogen. There are many advantages for producing biofuel from algae as these organisms can yield 15 to 300-fold more biodiesel than traditional crops on an area basis [47]. In addition, the harvesting cycle of algae is very short and its growth rate is very high. Moreover, high quality agricultural land is not required for algae biomass production [48]. These undoubted advantages have led algae biofuels to receive increasing attention worldwide. Unlike for terrestrial oily crops, mechanical extraction such as via oil expellers/presses cannot be employed for extracting algae oil owing to their small cell size, complex cell membrane, and thick/rigid cell wall [49,50]. Accordingly, various alternative extraction methods have been and are being tested to effectively obtain fueling substances from fresh or pro-treated algae, as algae biofuel production is still in the developmental stage (Fig. 3). In contrast to those from second-generation feedstocks, the biofuels obtained from algae by means of extraction processes primarily comprise biodiesel and bio-jet fuel (Table 1), with this production depending on the different components biosynthesized by the various species of

Table 3 Case analysis of representative organic solvents applied in second generation feedstocks. Organic solvent (extractable compounds)

Feedstocks studied

Extracted biofuels and yields

Ref.

Hexane (Non-polar; hydrocarbons and triacylglycerols)

Tree tobacco Rubber seed Tree tobacco Orchid tree Tree tobacco Rubber seed Rubber seed

Hydrocarbons, 6.28 μg mg−1 FW Bio-oil, 20.4 wt% Hydrocarbons, 5.69 μg mg−1 FW Bio-oil, 14.8% wt Hydrocarbons, 6.20 μg mg−1 FW Bio-crude oil, 16.0 wt% Bio-crude oil, 18.0 wt%

[25] [27] [25] [43] [25] [27] [27]

Petroleum ether Chloroform (Aprotic polar; hydrocarbons, triacylglycerols, waxes, etc.) Dichloromethane Acetone (Polar; phospholipids, glycolipids, diacylglycerols, etc.)

FW, fresh weight; wt., weight. The polarity of solvents from hexane to acetone decreases in the order of non-polar to polar. 299

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Table 4 Case studies of representative extraction methods in algae biofuel production. Method

Species studied

Solvents and/or operating conditions

Compound, yield

Ref.

Soxhlet (CSE) Bligh-Dyer (CSE) Sonication (PSSE) Microwave (PSSE) SFE Liquefied gas (novel)

Nannochloropsis sp. Schizochytrium sp. Chlorella sp. Botryococcus sp. Tetraselmis sp. M. aeruginosa B. braunii

Hexane: methanol (3:2), 70 °C, NP Methanol-chloroform, 25 °C, NP 50 kHz, 15 min, NP 2450 MHz, 100 °C, 5 min, NP SCCO2, 40 °C, 15 Mpa Dimethyl ether, 25 °C, 0.51 Mpa

Lipid, 30.8% Lipid, 22.1% Lipid, 25.5% Lipid, 28.5% Lipid, 10.8% Bio-oil 40.1% Hydrocarbons 48.9%

[51] [23] [54] [55] [57] [59] [19]

Yields in dry weight base; NP, normal pressure.

microalgae. Case studies of representative extraction methods for algae biofuel production are listed in Table 4.

Tetraselmis sp. (Table 4). Pilot-scale SCCO2 analyses for the recovery of biofuels from microalgae were reported in 2012 [34]. In a more recent study, lipid yield and oil composition from Scenedesmus almeriensis extracted using SCCO2 were comparable to those from CSE via Soxhlet extraction and several organic solvents. In particular, results show SCCO2 to be the most efficient methodology to extract lipids from algae for further biodiesel production [32]. However, high infrastructure and operational costs associated with SCCO2 remain as primary disadvantages. Alternatively, subcritical water (SuCH2O) has been studied for generating liquid transportation fuels from algae, with the author claiming that this technology could in principle be used for the production of biofuels and bio-products from algae and other biomasses [35].

3.3.1. Studies of CSE in algae biofuel production Table 2 presents representative case analysis of classified extraction methods employed in algae biofuel production. As a widely applied standard method, various organic solvents have been tested for algae biomass extraction. Because different types of lipids comprise the main components in algae biomasses used for biofuel production, both polar and non-polar organic solvents have been utilized [30]. Polar organic solvents such as methanol disrupt hydrogen bonding between polar lipids whereas non-polar solvents such as hexane disrupt hydrophobic interactions between non-polar/neutral lipids. Soxhlet and Bligh-Dyer [17] extraction constitute the two typically applied methods for the extraction of lipids from algal biomass (Table 4), which utilize hexane (non-polar), chloroform (aprotic polar), and methanol (polar) as solvents, respectively. In a recent study, Soxhlet with hexane and methanol (3:2) yielded 30.8 wt% lipids from Nannochloropsis sp. [51]. The Bligh-Dyer method with methanol and chloroform (1,1) yielded 22.1 wt % lipids from Schizochytrium sp. [23]. Numerous other solvents have also been tested and mentioned in recent articles, such as butanol and diethyl ether [52]. The disadvantages of CSE for algae biofuel production are that pre-treatment processes such as drying and cell disruption are generally required prior to CSE to remove the water from algae cells and release the target components contained within the cells. Such processes are highly energy intensive; furthermore, the use of organic solvents also results in environmental damage.

3.3.4. Studies of novel extraction methods in algae biofuel production Several novel extraction methods have been recently studied for algae biofuel production. Switchable hydrophilicity solvents were used for lipid extraction from lyophilized microalgae (Botryococcus braunii) [38]. The application of lyophilization can simultaneously dry and disrupt algae cells. In addition, ionic liquids mixed with methanol at a volume ratio of 1:1 was used to dissolve algal biomass, leaving lipids insoluble [21]. This method allows the lipids to be easily recovered as undissolved lipids are lighter than the ionic liquid and methanol mixture. Ionic liquids were also reported to have the ability to extract branched, unsaturated hydrocarbons from the phototrophic microbes Synechocystis sp. and B. braunii [29]. Moreover, a new publication demonstrated photocatalysis to be an efficient method for bio-oil extraction from Nannochloropsis oculata [58]. Notably, liquefied gas as an organic solvent can not only extract the biofuel from second-generation biomasses [26,46] but has also been reported to facilitate the recovery of bio-oil from wet algae via a shaking extraction method [18]. Using liquefied dimethyl ether at room temperature (25 °C) and under 0.51 Mpa, 91.0 wt% of bio-crude oil and 48.9 wt% of hydrocarbons were obtained from Microcystis aeruginosa and B. braunii [19,59].

3.3.2. Studies of PSSE in algae biofuel production Physical methods are usually utilized to support CSE for the extraction of lipids from algae biomasses, such as application of microwaving and sonication, bead beating, autoclaving, grinding, or osmotic shock [53]. Sonication and microwaving are the most frequently used PSSEs (Table 4). Recent studies reported that the inclusion of sonication at 50 kH for 15 min yields 25.5 wt% of lipids from Chlorella sp. [54] whereas applying microwaves at 2450 MHz and 100 °C yields 28.5 wt% lipids from Botryococcus sp. in 5 min [55]. CSE assisted by sonication, bead beating, and heating was also applied for dry algae biomass [28]. A combined method has also been reported that resulted in a higher lipid extraction yield by first heating an algal paste in a continuous microwave system followed by CSE with hexane [56]. The limitation of these applied physical methods is that their energy demand is generally high; for example, the energy requirement of microwaving and sonication for 1 kg of biomass is 420 and 132 MJ kg−1, respectively (Table 2) [56].

4. Considerations for selection of extraction methods for algae biofuel production Fig. 4 depicts the concluding steps of the whole algae biofuel production process for each classified extraction method. Here, the energy demand for algae cultivation, harvesting, concentration, and biofuel refinery are assumed as a fixed constant because these processes are independent from the extraction process in relation to the total energy input of algae biofuel production. Conversely, the drying and cell disruption processes are treated as variable. The energy requirements (Units in MJ kg−1 for dried algae biomass) of each upstream process prior to extraction are assessed based on recently available literature [31,60–62]. For the CSE route, both drying and cell disruption are indispensable and have an energy demand that accounts for more than 80% of the input energy in the upstream process prior to extraction. Therefore, the selection of extraction methods for minimizing the use of drying and cell disruption processes is key for reducing the total input energy demand of algae biofuel production. Alternatively, only the

3.3.3. Studies of SFE in algae biofuel production SFE constitutes a promising green technology method that has the potential to displace CSE for algae biofuel production [32]. Supercritical carbon dioxide (SCCO2) represents a good option for SFE as it offers high solvating power and low toxicity [30]. For example, Li et al. [57] reported that at near room temperature (approximately 40 °C) and below 15 Mpa the SCCO2 method yields 10.8 wt% lipids from 300

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Cultivation (65.4 MJ kg-1 ) [60]

Harvesting and concentration (0.64 MJ kg-1 ) [60]

Drying (>25 MJ kg-1 ) [60], (13.8 MJ kg-1 ) [62]

Combining drying and cell disruption (CDC route)

Cell disruption

Cell disruption

• Sonication: 132 MJ kg-1 [31] • Microwave: 420 MJ kg-1 [31]

may not be needed

CSE

Free from drying and cell disruption (FDC route)

• Such as lyophilization 19 MJ kg-1 [61]

PSSE

SFE and Novel

Novel

Further processing for the production of biodiesel and bio-jet fuel Fig. 4. Diagram representing the concluding steps of the algae-to-biofuel flowchart for the classified extraction methods. The processes within the solid outline are assumed as a fixed constant and within the dashed outline are assumed as a variable. The energy requirement units in MJ kg−1 for dried algae biomass are assessed based on recent available literature.

drying process is needed for most PSSE methods studied in recent articles because physical and/or chemical cell disruption methods were integrated into the extraction process. However, as the representative physical cell disruption methods, microwaving and sonication impose a high energy burden and have a high maintenance cost [63]. The representative methods of SFE for algae biofuel production are SCCO2 and SuCH2O, both of which use lyophilization as a pre-treatment method. The advantage of lyophilization methods is that they combine both drying and cell disruption in a single process [64] and are therefore termed the combining drying and cell disruption (CDC) route. A recently developed switchable solvent extraction method also used lyophilization as a pre-treatment method [38]. In an experimental small-scale model, microalgae were pre-treated by lyophilization with the reported energy demand being 19 MJ for 1 kg of dried algal biomass [61]. Considering that in the case of a large-scale model the energy demand of lyophilization should be lower than 19 MJ kg−1, it might be expected that the CDC route would reduce the energy burden in comparison with CSE and PSSE routes. However, SFE itself may be more energy-intensive than CSE and PSSE owing to the use of high pressure and temperature (Table 2) although it offers advantages such as high selectivity. Alternatively, some recently developed methods have been reported that could be free from drying and cell disruption pre-treatment processes (known as the FDC route). An example of such a route is the use of liquefied dimethyl ether, which serves as a novel solvent that can directly extract lipids and hydrocarbons from undamaged wet algae cells [18,19]. Numerous possible extraction methods are currently being studied based on various ideas with several solvents having been tested for these processes [65,66]. In an updated study of high-quality aviation biofuel production, crude algae lipids were extracted comparably from wet or dry Nannochloropsis with a ratio of algae:hexane:ethyl alcohol = 1:6.7:3.3 in association with ultrasound [65]; the final biofuel yield from dry and wet algae lipids was 73.02% and 76.47%, respectively. Another study on an extraction program within the recent 3 year National Alliance for Advanced Biofuel and Bio-

products algal consortium project (NAABB) indicated that an emerging extraction technology (wet solvent extraction) may be selectable to provide scale-up data and have sufficient capabilities to produce lipids for the NAABB program [66]. Nevertheless, the assessment of input energy demands for these possible methods is a large and complex task owing to the differences in the physical and chemical properties of each applied solvent and method. Therefore, the logical outcome of the present review is that under the current scenario, a potential extraction method should minimize the use of pre-treatment processes to the greatest extent possible. 5. Summary This review was intended to fill the gap created by the lack of current review papers discussing recently published academic articles concerning studies of extraction processes in biofuel production. To clarify the multiple existing and emerging extraction techniques, predominantly utilized physical and chemical methods were classified as CSE, PSSE, SFE, and novel methods. The relationship of feedstocks of different generation biofuels with particular classified extraction techniques was confirmed. The extraction techniques already applied in the first-generation biofuel industry mainly relied on the mechanical method supported by CSE [67] whereas those for second and thirdgeneration biofuels are still under development, although most of the classified techniques might be employed if the energy demand could be ignored. Theoretically, SFE has numerous advantages over CSE and PSSE including the elimination of organic solvents; i.e., reducing the risk of storage, along with rapid extraction speed. However, to date SFE is still predominantly applied only in the food and pharmaceutical industries. At its current stage, SFE retains a technical bottleneck for large scale application in the biofuel production field owing to its inefficient cost. As a new generation biofuel source, the extraction techniques used and/or studied in algae biofuel production were intensively reviewed. Currently, various methods, ideas, and theories regarding extraction 301

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methods have been and are being tested for algae biofuel production, with each extraction method exhibiting specific advantages and disadvantages. However, it is still difficult to extend the existing oil extraction techniques to the industrial scale as it remains necessary to dry the algal paste to a solid content of 90% prior to processing with conventional extraction methods [68]. Therefore, a logical criterion for the selection of extraction method for algae biofuel should be based on the ability of the potential extraction technique to minimize the use of pretreatment processes as a means to reduce total energy demand in algae biofuel production.

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Abbreviations CDC CSE FDC PSSE SCCO2 SuCH2O SFE

combining drying and cell disruption conventional solvent extraction Free from drying and cell disruption physical-supported solvent extraction supercritical carbon dioxide subcritical water supercritical fluid extraction

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