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Sustainable production of liquid biofuels from renewable microalgae biomass
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JIEC-2491; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
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Review
Sustainable production of liquid biofuels from renewable microalgae biomass Ok Kyung Lee a, Dong Ho Seong b, Choul Gyun Lee b, Eun Yeol Lee a,* a b
Department of Chemical Engineering, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea Marine Bioenergy Research Center, Department of Biological Engineering, Inha University, Incheon 402-751, Korea
A R T I C L E I N F O
Article history: Received 15 February 2015 Received in revised form 13 April 2015 Accepted 14 April 2015 Available online xxx Keywords: Microalgae Biodiesel Bioethanol Bio-oil Thermochemical treatment
A B S T R A C T
Microalgae are important as feedstock in production of liquid biofuels such as biodiesel, bioethanol and bio-oil. Biodiesel and bioethanol can be produced from lipids and carbohydrates of microalgae biomass, respectively. Bio-oil and bio-char are prepared using thermochemical treatment of microalgae biomass or residual biomass after lipid extraction and/or saccharification of cellular carbohydrates. Recent advances in biorefinery present opportunities to develop sustainable and integrated productions of various liquid fuels from microalgae biomass in economical way within the next decades. This review examines the recent progress of microalgae-based liquid biofuel production with regard to characteristics and applicability of microalgae as feedstock. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saccharification and bioethanol fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermochemical microalgae conversion for bio-oil production . . . . . . . . . . . . . . . . . . . . . Biorefinery approach for integrated production of liquid fuels from microalgae biomass . Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction Biofuel production from biomass is a promising alternative to petroleum-based fuels [1]. Among available biomass sources, microalgae have been frequently considered and investigated as a third-generation biomass [2–5]. Microalgae are photoautotrophic microorganisms that use carbon dioxide, water and sunlight to grow. Microalgae have several distinct advantages over other biomass sources [6]. First, microalgae can be cultivated using carbon dioxide [7], thus providing greenhouse gas mitigation
* Corresponding author. Tel.: +82 31 201 3839; fax: +82 31 204 8114. E-mail address: [email protected] (E.Y. Lee).
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benefits. Second, microalgae have high growth rates of 1–3 doublings/day, which is at least five to ten times higher than those of plants. Third, microalgae can use growth nutrients such as nitrogen and phosphorus from waste streams. Additionally, microalgae can be cultivated from non-productive, non-arable lands such as deserts, coasts and offshore marine environments. Large-scale photobioreactors can also be used for microalgae cultivation [8–11]. Thus, large amounts of biomass can be obtained as non-food-based sustainable feedstock from cheap substrates. Production of liquid fuels from microalgae requires many downstream processing steps (Fig. 1). Harvesting is the subsequent step right after cultivation of microalgae and potentially contributes up to 20–30% of the total production cost of microalgae biomass. Generally, after harvesting step such as centrifugation
http://dx.doi.org/10.1016/j.jiec.2015.04.016 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Fig. 1. Downstream processing of biomass harvesting, disruption, and integrated production of biofuels from the major components of microalgae.
[12], filtration [13] and flocculation [14], microalgae consist of 30% solid with 70% moisture content [15]. Drying or dewatering of microalgae biomass is generally used prior to various conversion steps. The energy required for dewatering is known to constitute up to 84.9% of total energy consumption [16]. Extraction of lipid is required for biodiesel production. The main components of microalgae for liquid biofuel production are lipids, carbohydrates and others such as proteins. Various conversion methods including transesterification, fermentation, pyrolysis, liquefaction and anaerobic digestion are used for production of biodiesel, bioethanol, bio-oil and methane (Fig. 2). The lipid is extracted using organic solvent and then transesterified to biodiesel in the presence of base or acid catalysts. Bioethanol is produced from microalgae fermentation, while methane can be produced through anaerobic digestion. Thermochemical processes such as hydrothermal liquefaction and pyrolysis can be applied for bio-oil production. Currently, no liquid biofuel production from microalgae is commercially implemented because ‘‘liquid fuel production only’’ is not economically feasible. Thus, the whole components of microalgae biomass should be used to produce multiple products including various liquid biofuels for economically viable option [17]. In this review paper, recent progress on the production of liquid biofuels, including biodiesel, bioethanol and bio-oil, are addressed, and some concluding remarks are also discussed for a successful zero-waste microalgae biorefinery. Biodiesel production Biodiesel is a mixture of fatty acid methyl esters (FAMEs) produced by transesterification of triglycerides with alkyl acceptors such as methanol in the presence of a catalyst, usually NaOH, KOH or lipase. Since methanolysis of triglyceride is an equilibrium reaction, a high excess of methanol, generally six moles of methanol per mole of triglyceride, is applied [6]. At present,
biodiesel is mainly produced from rapeseed and palm oils. Recently, microalgae are being considered as feedstock for biodiesel production [16,18,19]. The lipid content of some microalgae species such as Botryococcus braunii exceeds 80% of the dry weight [20]. Chlorella and Dunaliella are known to have lipid contents as great as 50% of the dry weight. Thus, considerable amounts of lipid for biodiesel production can be obtained from large-scale cultivation of microalgae. As indicated in some reports, the yield of microalgae lipid per hectare is around 58,700 l/ha, almost ten times greater than those from palm crops [21]. Biodiesel can be produced using a two-step method of oil extraction-transesterification or one-step transesterification (so called ‘in site or direct transesterification’). The common method for extracting oil from microalgae is a solvent extraction using hexane, ethanol, methanol and methanol–chloroform mixture (2:1 v/v) [22]. The extraction efficiency of microalgae oil using n-hexane is rather low, although it is widely used for oil extraction from seed crops [23]. Ultrasonic-assisted extraction, microwaveassisted extraction and supercritical fluid extraction can be an alternative to organic solvent extraction. After lipid extraction, alkali, acid catalysts and lipase are used for transesterification. One important thing is microalgae oil generally contains a certain amount of free fatty acid. Thus, an acid-catalyzed conversion can be an efficient method with high conversion, though its reaction rate is approximately 4000 times slower than base-catalyzed processes [24]. Recent advances in biodiesel production from microalgae are summarized in Table 1. In two-step transesterification, microalgae lipids were extracted using different solvents and extraction methods from various microalgae strains [25–32]. The extracted lipids were transesterified to FAMEs using H2SO4, HCl and lipase. An acid catalyst was used because microalgae biomass generally contains large amounts of free fatty acids that cause the formation of soaps in the presence of alkali catalysts [25]. Lam et al. [26]
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Fig. 2. Various biochemical and thermochemical approaches for production of liquid biofuels from microalgae.
Table 1 Biodiesel production from various microalgae. Method
Microalgae species (dry or wet)
Two-step
Nannochloropsis gaditana (dry biomass) Phaeodactylum Tricornutum (dry biomass) Chlorella vulgaris (dry biomass) Chlorella protothecoides (dry biomass) Chlorella vulgaris (dry biomass) Chlorella vulgaris ESP-31 (dry biomass) Chlorella sp. KR-1 (dry biomass) Tribonema minus (wet biomass) Nannochloropsis sp. (80% moisture)
One step
Chlorella sp. KR-1 (dry biomass) Chlorella protothecoides (dry biomass)
Tetraselmis suecica (dry biomass) Chlorella sorokiniana (dry biomass) Nannochloropsis oceanica (65% moisture) Chlorella pyrenoidosa (80% moisture) a b
Biodiesel production process Stirring extraction Chloroform–methanol Ultrasonic assisted extraction Chloroform–methanol Stirring extraction Chloroform–methanol Soxhlet extraction Hexane Soxhlet extraction Hexane Ultrasonic assisted extraction Chloroform–methanol Stirring extraction DMC–methanol Subcritical ethanol extraction Microwave assisted extraction Hexane– methanol DMC based transesterification Non-catalytic transesterification in supercritical methanol (ethanol) Microwave assisted transesterification
In situ transesterification in water bath Microwave assisted transesterification
Catalyst
Lipid content (%)a
Biodiesel conversion (%)b
Reference
Transesterification solvent Chloroform–methanol Transesterification solvent Chloroform–methanol
HCl
13.1
41.8
[25]
HCl
24.5
37.5
Transesterification Methanol-THF Transesterification Hexane Transesterification ([BMIm][PF6]) Transesterification Hexane
solvent
H2SO4
–
95.0
[26]
solvent
lipase (Candidia sp.) lipase (P. expansum) lipase (Burkholderia sp.)
44–48.7
98.1
[27]
40.7
90.7
[28]
63.2
72.1
[29]
Transesterification solvent DMC Transesterification solvent Methanol Transesterification solvent Methanol–hexane
lipase (Novozyme 435) H2SO4
38.9
75.5
[30]
20.2
96.5
[31]
NaOH
38.3
86.4
[32]
lipase (Novozyme 435)
40.9
89.7
[33]
–
90.8 (87.8)
[34]
H2SO4
23.0
78.0
[35]
H2SO4
23.5 19.1
77.0 90.6
[36]
H2SO4
19.0
55.2
[37]
solvent IL solvent
Lipid content = (total lipid/total biomass) 100. Biodiesel conversion = (weight of FAMEs/total lipid) 100.
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reported that more than 95% biodiesel yield from highly viscous lipids (extracted from Chlorella vulgaris) was achieved by introducing tetrahydrofuran as co-solvent. The lipids extracted from Chlorella protothecoides using hexane were converted to FAMEs by lipase, with biodiesel conversion as high as 98.15% [27], because lipase generally showed greater activity toward hexaneextracted lipids consisting mainly of triglycerides with very small amounts of free fatty acids. Ionic liquid was successfully used for lipid extraction from C. vulgaris, with a 40.7% yield based on dry cell weight [28]. Lee et al. [30] reported a method for the highly efficient lipid extraction and lipase-catalyzed transesterification of lipids from Chlorella sp. KR-1 using dimethyl carbonate (DMC). With a mixture of DMC and methanol at a ratio of 7:3, total lipids were almost completely extracted under simple stirring at mild conditions of 50 8C. Biodiesel conversion of 75.5% was obtained using lipase. Recently, direct lipids extractions from wet microalgae were conducted to minimize the energy required for microalgae biomass drying. Lipids were extracted from wet paste of Tribonema minus using subcritical ethanol extraction, and then converted to biodiesel through acid-alkaline transesterification [31]. Washidin et al. [32] reported that 38.3% (w/w) of lipids in wet microalgae was extracted by microwave irradiation. The subsequent biodiesel conversion using microwave irradiation was 86.4% based on the extracted lipid. In case of one step transesterification, Jo et al. [33] developed a DMC-based direct transesterification for Chlorella sp. KR-1 biomass in the presence of lipase. A 367.31 mg of biodiesel was obtained from 1 g of Chlorella sp. KR-1 containing 40% (w/w) lipid. Interestingly, the biodiesel yield of direct transesterification was increased 1.25-fold, compare with two step transesterification (293.82 mg of biodiesel) [30,33]. To enhance the biodiesel conversion, supercritical fluid was applied. Nan et al. [34] reported a production of biodiesel with 90.8% yield through non-catalytic transesterification of microalgae lipid in supercritical methanol. For direct transesterification of wet microalgae, Im et al. [36] reported a direct transesterification of wet Nannochloropsis oceanica (65% moisture) with 91.1% biodiesel conversion in the presence of 0.3 ml H2SO4, 2 ml chloroform, and 1 ml methanol at 95 8C for 90 min. Microwave has been employed for an one-step transesterification of Chlorella pyrenoidosa (80% moisture) in the presence of chloroform, methanol and sulfuric acid as the catalyst. The biodiesel conversion with the aid of microwave was 1.3-fold higher than that of conventional two-step heating method [37]. The fuel properties of microalgae biomass-based biodiesel such as density, viscosity, flash point, cold filter plugging point, heating value and solidifying point have been well characterized [38]. The heating value of microalgae-based biodiesel was approximately 31–41 MJ/kg, a little bit low compared to that of petroleum-based diesel (46 MJ/kg). One interesting characteristics of microalgaebased biodiesel is it’s lower cold filter plugging point of 11 8C, more suitable fuel for aviation than petroleum-based diesel. On the other hand, viscosity of microalgae biodiesel is high, approximately 1.2–2.6 times more than that of petroleum-based diesel. The composition of biodiesel from microalgae depends on the species, which also strongly affects fuel properties like viscosity. Thus, selection and genetic improvement of microalgae species containing an appropriate fatty acid composition needs to be conducted to lower the fatty acid composition for lower viscosity. Although the fuel properties of biodiesel is similar to petro-diesel, biodiesel production from microalgae is still too expensive to be commercialized based on a commercial feasibility study. Davis et al. reported that the production of microalgae biodiesel could cost from $9.84 (open pond) to $20.53 (photobioreactor) per gallon, much higher than $2.60 per gallon for petroleum diesel production [39].
Saccharification and bioethanol fermentation Microalgae contain large amounts of various carbohydrates such as starch, cellulose and hemicellulose, depending on the species [40]. Microalgae such as Chlorella, Dunaliella, Spirulina and others are known to contain carbohydrates in the form of starch and glycogen in amounts as great as 50% of the dry weight [41,42]. Carbohydrates can be used for bioethanol fermentation [43]. Microalgae biomass has several advantages over other biomasses such as lignocellulose. The most advantageous feature of microalgae is that they contain little or no lignin [42], which mitigates cost-intensive biomass pretreatment. Additionally, carbon dioxide produced as a byproduct of bioethanol fermentation can be used to cultivate microalgae. Saccharification is one of the most critical steps for economically viable bioethanol production. Acid hydrolysis, the most commonly applied method, is conducted at 121 8C in the presence of H2SO4 and HCl (1–5%) [44,45]. Acid hydrolysis provides high sugar yields up to 100% for short time. However, acid hydrolysate needs to be neutralized before fermentation. On the contrary, enzymatic saccharification is conducted at mild condition and can be liked to fermentation, simultaneous saccharification and fermentation (SSF). Each species of microalgae has different polysaccharide composition, thus suitable enzymes such as cellulase, glucanase, xylanase and etc. should be employed. Various microalgae have been used for saccharification and bioethanol fermentation (Table 2) [45–53]. One advantage in a saccharification of microalgae biomass is that intensive pretreatment is not required. Various chemical saccharifications have been employed for microalgae. Harun et al. [46] used alkali saccharification for bioethanol production from Chlorococcum infusionum. The highest glucose yield was 0.35 g glucose/g microalgae using 0.75% (w/v) of NaOH and 120 8C for 30 min, and the maximum bioethanol yield was 0.26 g ethanol/g microalgae. Harun and Danquah [47] investigated the influence of acid saccharification on Chlorococcum humicola for bioethanol production. A 7.2 g/L bioethanol was obtained after 15 g/L biomass was treated using 1% (v/v) of sulfuric acid at 140 8C for 30 min. A 40 g/L Scenedesmus obliquus CNW-N containing 51.8% (w/w) carbohydrate was hydrolyzed using 2% (v/v) H2SO4. The resulting broth was fermented by Zymomonas mobilis, and an 8.6 g/L bioethanol was produced at a 21.3% yield (g ethanol/g biomass) [48]. A 14.6 g/L bioethanol was produced from 50 g/L biomass of Chlamydomonas reinhardtii UTEX 90 by Saccharomyces cerevisiae with 29.2% yield after the biomass was saccharified using 3% (v/v) H2SO4 at 110 8C for 30 min [49]. Lee et al. [50] hydrolyzed C. vulgaris using 5% (v/v) H2SO4 at 110 8C for 120 min. The maximum ethanol yield of 0.4 g ethanol/g biomass was achieved using ethanolic E. coli SJL2526. Carbohydrate-rich C. vulgaris FSP-E containing 51% carbohydrate per dry weight was used as feedstock for bioethanol fermentation [51]. Dilute acidic (1% H2SO4) and enzymatic saccharification of C. vulgaris FSP-E biomass produced glucose yields of 93.6 and 90.4%, respectively. A 11.7 g/L bioethanol with 87.6% theoretical yield was obtained from the acidic hydrolysate of a 50 g/L biomass using a separate hydrolysis and fermentation (SHF) process. The enzymatic hydrolysate was converted into bioethanol by both SHF and SSF processes, producing 79.9 and 92.3% theoretical yields, respectively. Choi et al. [52] used enzyme saccharification using two different commercial enzymes (including amylase from Bacillus licheniformis and glucoamylase from Aspergillus niger) for bioethanol production from C. reinhardtii UTEX 90. A sugar conversion of 0.5 g sugar/g microalgae was obtained and 0.23 g ethanol was produced from 1.0 g microalgae by SHF method. Likewise, many investigations have been successfully conducted in order to commercialize bioethanol production from microalgae. However, the commercial production of bioethanol from microalgae
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Table 2 Saccharification and bioethanol fermentation of microalgae biomass. Microalgae species
Hydrolysis Method
Biomass concentration (g/L)
Fermentation type
Carbohydrates (%) [glucose (g/L)]
Yeast strain
Ethanol (g/L)
Ethanol yield (%, g ethanol/g biomass)
Reference
Chlorococcum infusionum Chlorococcum humicola Scenedesmus obliquus CNW-N Chlamydomonas reinhardtii UTEX 90 Chlorella vulgaris
Chemical 0.75%(w/v) NaOH Chemical 1% (v/v) H2SO4 Chemical 2% (v/v) H2SO4
50
SHF
43.9 [35]
S. cerevisiae
N.D
26
[46]
15
SHF
32.5
S. cerevisiae
7.2
52
[47]
40
SHF
51.8
Z. mobilis
8.6
21
[48]
Chemical 3% (v/v) H2SO4
50
SHF
60 [28.5]
S. cerevisiae
14.6
29
[49]
Chemical 5% (v/v) H2SO4 Chemical 1% (v/v) H2SO4 Enzymatic Cellulase + Amylases Enzymatic Cellulase + Amylases Enzymatic Termamyl 120L +AMG 300L Enzymatic AMG 300L
N.D
SHF
N.D
E.coli
N.D
40
[50]
50
SHF
47–48 [23.6]
Z. mobilis
11.7
23
[51]
20
SHF
43.5–46.8 [7.78]
Z. mobilis
3.6
18
20
SSF
N.D
Z. mobilis
4.3
21
50
SHF
59.7 [25.21]
S. cerevisiae
11.7
23
[52]
50
SHF
51.9 [42.0]
S. cerevisiae
7.2
14
[45]
10
SHF
N.D
S. bayanus
3.8
38
[53]
Chlorella vulgaris FSP-E
Chlamydomonas reinhardtii UTEX 90 Dunaliella tertiolecta (lipid-extracted microalgae) Chlorococcum sp. (lipid-extracted microalgae)
Physical Supercritical CO2
is still in the research stages, similar to biodiesel production from microalgae. Thermochemical microalgae conversion for bio-oil production Representative two thermochemical processes for bio-oil production using microalgae biomass are hydrothermal liquefaction (HTL) and pyrolysis process (Fig. 3). The HTL is conducted at subcritical conditions in water, for obtaining liquid fuels from biomass. During the HTL reaction, water is acted as both a reaction solvent and reagent for hydrolysis of biomass. The HTL have relatively good efficiency of energy recovery on energy investment because its feedstock is wet biomass itself instead of dry matters. The drying step of biomass is one of the most energy consuming
processes in fuels production process from microalgae. The main product of the HTL is bio-oil namely ‘biocrude’, a liquid crude fuel composed of many hydrocarbon compounds. Bio-oil is organic solvent extractable black liquid oil with a high heating value and high viscosity. The reported high bio-oil production yield of microalgae HTL is 50–60% based on dry biomass [54]. The microalgae bio-oil is typically composed of 70–75% carbon, 10–16% oxygen, 4–6% nitrogen, and 0.5–1% sulfur. It has higher heating value (HHV) of 33.4–39.9 MJ/kg. The major components of bio-oil are aromatic hydrocarbons, long chain fatty acids, nitrogen heterocyclic compounds, alcohols, organic acid, aldehydes and etc.[55]. The HHV of microalgae bio-oil is relatively high because of low content of oxygen. The characteristics of microalgae bio-oil as a fuel and production yield are closely affected by the microalgae
Fig. 3. Production of bio-oil from thermochemical treatment of microalgae.
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composition as a feedstock, reaction temperature, time, catalyst and extraction method of bio-oil from reactant. When the same microalgae biomass is used as a HTL feedstock, the temperature is the most critical factor among the influencing conditions for production of microalgae bio-oil. The typical reaction temperature for microalgae HTL is 300– 374 8C, a subcritical temperature, and as the reaction temperature is elevated within that subcritical water temperature, the bio-oil yield and nitrogen content in bio-oil are increased and the oxygen content is decreased [56,57]. When the reaction temperature exceeds the critical point, the bio-oil is produced less and gas takes place more. The microalgae bio-oil produced through HTL is more suitable liquid fuel than that produced by pyrolysis method because not only it contains a less oxygen and water in bio-oil but also it has higher storage stability than that of pyrolysis oil [58]. Although bio-oil from HTL contains less nitrogen and sulfur than the feed microalgae biomass, nitrogen and sulfur contents in biooil are still in a high level. Bio-oil contains a large quantity of double bonds, so upgrading needs to be conducted for practical use as liquid fuel. Hydrotreatment, a hydrogen addition process, is being used for upgrading bio-oil. Generally, although HTL and pyrolysis have their own advantages, HTL is expected to be more cost-effective for wet microalgae biomass due to the omitting of energy-intensive drying process in the thermal conversion. Pyrolysis is a thermochemical conversion of biomass to bio-oil, bio-char and gas at medium to high temperatures (generally 450– 500 8C) with short residence time (order of seconds to minutes) in the absence of oxygen [59,60]. Contrary to HTL, biomass drying is necessary (Fig. 3). Pyrolysis of microalgae can produce a wide range of products including liquid oil, solid char and gas, depending on the reaction parameters such as heating rate, residence time, and pyrolysis temperature. In fast pyrolysis, according to the vapor residence time, finely ground biomass is treated for short residence times and fast heating rates at 350– 500 8C [61]. One major advantage of pyrolysis over other conversion processes is fast biomass conversion on the order of seconds to minutes. Fast pyrolysis with short residence times and fast heating rates tends to favor formation of bio-oil as the main product. In contrast, slow pyrolysis produces more bio-char [62]. One disadvantage of the pyrolysis process is the high capital cost of
separation equipment for various fractions. Another significant hurdle in using pyrolysis for microalgae is the high moisture content of biomass feedstock, which requires dewatering to control moisture content of the bio-oil. Development of an inexpensive dewatering or extraction process is a key factor for cost-competitive pyrolysis. Reports on pyrolysis of microalgae for bio-oil production are less available compared to those for lignocellulosic biomass. Pyrolysis converts triglycerides to fatty acid alkyl esters as bio-oil [63]. Microalgae carbohydrate and protein components as well as fat are converted to bio-oil by pyrolysis [64]. Bio-oil was obtained at a 57.9% yield from fast pyrolysis of C. protothecoides at a heating value of 41 MJ/kg [65]. Bio-oil from HTL and pyrolysis can be used as drop-in fuel in industrial boilers for heat production [66,67]. To use bio-oil directly as transportation fuel, catalytic hydrogenation can be applied to upgrade the bio-oil [68]. Lower oxygen content and a higher heating value were obtained using a catalyst [69,70]. Carbon yield of aromatic hydrocarbons increased from 0.9 to 25.8% in the pyrolysis of C. vulgaris using zeolite HZSM-5 [71]. When ZSM-5 catalyst was used with an increased catalyst to biomass ratio, oxygen content in the bio-oil decreased from 30.1 to 19.5 wt%, and the heating value of the bio-oil improved to 24.4–32.2 MJ/kg [72]. Biorefinery approach for integrated production of liquid fuels from microalgae biomass Biorefinery is a process of integrated utilization of every component of the biomass in order to enhance the economics of the process [73,74]. In a biorefinery process, major components of microalgae are subsequently used to produce biofuels, chemicals and feeds [75,76]. Some biologically active materials such as astaxanthin or tocopherol that are used as additives for nutraceuticals and cosmetics are produced from microalgae. However, the amount of those biomaterials to meet market demand does not require large scale production using biorefinery process for biofuels and chemicals. In case of feed production, there might be some problem in safety and approval when animal feed production is integrated with the production of biofuels and chemicals. Thus, an integrated production of various liquid
Fig. 4. Sequential production of biodiesel, bioethanol and bio-oil from microalgae biomass.
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biofuels in large scale based on biorefinery concept needs to be developed from the industrial point of view. All cellular components in microalgae biomass should be utilized to produce valuable liquid fuels in order to reduce overall production costs. Each cellular component of microalgae has own advantage for different liquid fuels. Lipid is suitable for production of biodiesel and bio-oil, whereas polysaccharide is appropriate for bioethanol. Recently, sequential production of biodiesel, bioethanol and bio-oil were successfully conducted for Dunaliella tertiolecta biomass (Fig. 4) [45,77]. The lipids of D. tertiolecta were extracted using a chloroform and methanol mixture and then converted to biodiesel. The residual biomass after lipid extraction has been successfully used as raw material for saccharification and subsequent bioethanol fermentation due to the large amount of polysaccharides present in the residual biomass. The residual biomass was composed of 51.9 wt% carbohydrate, 35.0 wt% protein, 13.1 wt% ash and no trace of lipids. The residual biomass was saccharified using an AMG 300 L enzyme with a saccharification yield of 80.9% (w/w) based on the total amount of carbohydrates [45]. Without any pretreatment, bioethanol was directly produced from the enzymatic saccharification products with a fermentation yield of 0.14 g ethanol/g residual biomass, which was 82.0% of the theoretical fermentation yield. For complete utilization of D. tertiolecta biomass, the residual biomass after the conversion of lipids and carbohydrates into biodiesel and bioethanol was further converted into bio-oils by pyrolysis. The predominant pyrolysis reaction pathway was from D. tertiolecta residual biomass to bio-oil rather than to gas and/or bio-oil to gas based on the proposed lumped kinetic model. These study demonstrated microalgae biomass could be completely used for an integrated production of various liquid biofuels. Conclusions and future prospects Microalgae are promising feedstock to replace petroleum-based fuels due to their high biomass productivity, high oil content and easy biomass processing due to the absence of lignin. Although microalgae biomass has great potential, biofuel production from microalgae is not economically feasible due to high production cost at present. Thus, cost reduction is the top priority for commercial implementation of biofuel production. The development of a costeffective algal cultivation process is the key factor for successful cost reduction [78]. Culture costs are expected to decrease with the development of a novel photobioreactor system together with use of a microalgae species with high photosynthetic efficiency [79,80]. The cost of nutrients to cultivate microalgae can be decreased when wastewater is used. In addition to microalgae culture, genetic and metabolic modifications of microalgae will improve cost-effectiveness of microalgae-based biofuel production [81]. Microalgae oil content can be enhanced and its composition can be manipulated by the search for new algae species and improvement of existing algae strains by molecular breeding. The development of genetically modified microalgae is still in a premature stage for commercial application. Metabolic modifications to enhance lipid content in microalgae are being intensively investigated. For example, genetic deletion or transcriptional deactivation using regulatory small RNA as an antisenser was used for down regulation of fadD or phosphoenolpyruvate carboxylase to prevent degradation of free fatty acids and enhance lipid biosynthesis [21,79–85]. Uncertainty regarding the use of genetically modified microalgae as biofuel production strains in real culture environments should be addressed together with extensive metabolic engineering of microalgae. Integrated biorefinery is an appealing way to lower production costs for microalgae-based biofuels. In biorefinery, destruction of
7
microalgae biomass, extraction and separation of cell components and conversion of the components to biofuels need to be integrated. Less energy-intensive pretreatment processes such as cost-effective harvesting and dewatering prior to extraction and conversion need to be developed. A cost-effective extraction method is still required even though microalgae biomass is ligninfree. Recruit of new methods for extraction of oil directly from wet microalgae biomass without energy-intensive processing are important. Along with oil for biofuel, other substances such as carbohydrates and proteins should be used to improve biorefinery process economics. Fully integrated production of biodiesel, bioethanol, bio-oil and bio-char can be a key solution for zerowaste biorefinery of microalgae. In the near future, biorefinery for biofuel production from microalgae is expected to play an important role in the bio-based economy. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2013R1A2A2A01068863). This work was supported by a grant from the Marine Biotechnology Program funded by the Ministry of Oceans and Fisheries of the Korean Government. References [1] S.N. Naik, V.V. Goud, P.K. Rout, A.K. Dalai, Renewable Sustainable Energy Rev. 14 (2010) 578. [2] V. Patil, K.Q. Tran, H.R. Giselroed, Int. J. Mol. Sci. 9 (2008) 1188. [3] I. Rawat, R.R. Kumar, T. Mutanda, F. Bux, Appl. Energy 88 (2011) 3411. [4] L. Reijnders, Trends Biotech. 26 (2007) 349. [5] J. Singh, S. Gu, Renewable Sustainable Energy Rev. 14 (2010) 2596. [6] Y. Chisti, Biotech. Adv. 25 (2007) 294. [7] S. Sawayama, S. Inoue, Y. Dote, S.Y. Yokoyama, Energy Convers. Manage. 36 (1995) 729. [8] M. Cuaresma, M. Janssen, C. Vilchez, R.H. Wijffels, Biotechnol. Bioeng. 104 (2009) 352. [9] M. Cuaresma, M. Janssen, C. Vilchez, R.H. Wijffels, Bioresour. Technol. 102 (2011) 5129. [10] J.W.F. Zijffers, M. Janssen, J. Tramper, R.H. Wijffels, Mar. Biotechnol. 10 (2008) 404. [11] J.W.F. Zijffers, K.J. Schippers, K. Zheng, M. Janssen, R.H. Wijffels, Mar. Biotechnol. 12 (2010) 708. [12] M. Haesman, J. Diemar, W. O’Connor, T. Soushames, L. Foulkes, Aquacult. Res. 31 (2000) 637. [13] N. Rossignol, T. Lebeau, P. Jaouen, J.M. Robert, Bioprocess Eng. 23 (2000) 495. [14] R.M. Knuckey, M.R. Brown, R. Robert, D.M.F. Frampton, Aquacult. Eng. 35 (2006) 300. [15] T.M. Mata, A.A. Martins, N.S. Caetano, Renewable Sustainable Energy Rev. 14 (2010) 217. [16] L. Lardon, A. Helias, B. Sialve, J.P. Stayer, O. Bernard, Environ. Sci. Technol. 43 (2009) 6475. [17] B. Subhadra, G. George, J. Sci. Food Agric. 91 (2010) 2. [18] A. Demirbas, Energy Sources, A: Recov. Util. Environ. Effects 31 (2009) 163. [19] H. Xu, X. Miao, Q. Wu, J. Biotechnol. 126 (2006) 499. [20] Q. Hu, M. Sommerfeld, E. Jarvis, M. Ghirardi, M. Posewitz, M. Seibert, A. Darzins, Plant J. 54 (2008) 621. [21] Y. Chisti, Trends Biotechnol. 26 (2008) 126. [22] M.K. Lam, K.T. Lee, Biotechnol. Adv. 30 (2012) 673. [23] A. Ranjan, C. Patil, V.S. Moholkar, Ind. Eng. Chem. Res. 49 (2010) 2979. [24] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin, Ind. Eng. Chem. Res. 44 (2005) 5353. [25] S.D. Rı´os, J. Castan˜eda, C. Torras, X. Farriol, J. Salvado´, Bioresour. Technol. 133 (2013) 378. [26] M.K. Lam, K.T. Lee, Fuel Process. Technol. 110 (2013) 242. [27] X. Li, H. Xu, Q. Wu, Biotechnol. Bioeng. 98 (2007) 764. [28] J.Q. Lai, Z.L. Hu, P.W. Wang, Z. Yang, Fuel 95 (2012) 329. [29] D.T. Tran, K.L. Yeh, C.L. Chen, J.S. Chang, Bioresour. Technol. 108 (2012) 119. [30] O.K. Lee, Y.H. Kim, J.G. Na, Y.K. Oh, E.Y. Lee, Bioresour. Technol. 147 (2013) 240. [31] H. Wang, L. Gao, L. Chen, F. Guo, T. Liu, Bioresour. Technol. 142 (2013) 39. [32] S. Wahidin, A. Idris, S.R.M. Shaleh, Energy Convers. Manage. 84 (2014) 227. [33] Y.J. Jo, O.K. Lee, E.Y. Lee, Bioresour. Technol. 158 (2014) 105. [34] Y. Nan, J. Liu, R. Lin, L.L. Tavlarides, J. Supercrit. Fluids 97 (2015) 174. [35] B.D. Wahlen, R.M. Willis, L.C. Seefeldt, Bioresour. Technol. 102 (2011) 2724. [36] H. Im, H.S. Lee, M.S. Park, J.W. Yang, J.W. Lee, Bioresour. Technol. 152 (2014) 534. [37] J. Cheng, T. Yu, T. Li, J. Zhou, K. Cen, Bioresour. Technol. 131 (2013) 531. [38] A. Bahadar, M. BilalKh, Renewable Sustainable Energy Rev. 27 (2013) 128. [39] R. Davis, A. Aden, P.T. Pienkos, Appl. Energy 88 (2011) 3524.
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JIEC-2491; No. of Pages 8 Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Review
Sustainable production of liquid biofuels from renewable microalgae biomass Ok Kyung Lee a, Dong Ho Seong b, Choul Gyun Lee b, Eun Yeol Lee a,* a b
Department of Chemical Engineering, Kyung Hee University, Gyeonggi-do 446-701, Republic of Korea Marine Bioenergy Research Center, Department of Biological Engineering, Inha University, Incheon 402-751, Korea
A R T I C L E I N F O
Article history: Received 15 February 2015 Received in revised form 13 April 2015 Accepted 14 April 2015 Available online xxx Keywords: Microalgae Biodiesel Bioethanol Bio-oil Thermochemical treatment
A B S T R A C T
Microalgae are important as feedstock in production of liquid biofuels such as biodiesel, bioethanol and bio-oil. Biodiesel and bioethanol can be produced from lipids and carbohydrates of microalgae biomass, respectively. Bio-oil and bio-char are prepared using thermochemical treatment of microalgae biomass or residual biomass after lipid extraction and/or saccharification of cellular carbohydrates. Recent advances in biorefinery present opportunities to develop sustainable and integrated productions of various liquid fuels from microalgae biomass in economical way within the next decades. This review examines the recent progress of microalgae-based liquid biofuel production with regard to characteristics and applicability of microalgae as feedstock. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saccharification and bioethanol fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermochemical microalgae conversion for bio-oil production . . . . . . . . . . . . . . . . . . . . . Biorefinery approach for integrated production of liquid fuels from microalgae biomass . Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction Biofuel production from biomass is a promising alternative to petroleum-based fuels [1]. Among available biomass sources, microalgae have been frequently considered and investigated as a third-generation biomass [2–5]. Microalgae are photoautotrophic microorganisms that use carbon dioxide, water and sunlight to grow. Microalgae have several distinct advantages over other biomass sources [6]. First, microalgae can be cultivated using carbon dioxide [7], thus providing greenhouse gas mitigation
* Corresponding author. Tel.: +82 31 201 3839; fax: +82 31 204 8114. E-mail address: [email protected] (E.Y. Lee).
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benefits. Second, microalgae have high growth rates of 1–3 doublings/day, which is at least five to ten times higher than those of plants. Third, microalgae can use growth nutrients such as nitrogen and phosphorus from waste streams. Additionally, microalgae can be cultivated from non-productive, non-arable lands such as deserts, coasts and offshore marine environments. Large-scale photobioreactors can also be used for microalgae cultivation [8–11]. Thus, large amounts of biomass can be obtained as non-food-based sustainable feedstock from cheap substrates. Production of liquid fuels from microalgae requires many downstream processing steps (Fig. 1). Harvesting is the subsequent step right after cultivation of microalgae and potentially contributes up to 20–30% of the total production cost of microalgae biomass. Generally, after harvesting step such as centrifugation
http://dx.doi.org/10.1016/j.jiec.2015.04.016 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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Fig. 1. Downstream processing of biomass harvesting, disruption, and integrated production of biofuels from the major components of microalgae.
[12], filtration [13] and flocculation [14], microalgae consist of 30% solid with 70% moisture content [15]. Drying or dewatering of microalgae biomass is generally used prior to various conversion steps. The energy required for dewatering is known to constitute up to 84.9% of total energy consumption [16]. Extraction of lipid is required for biodiesel production. The main components of microalgae for liquid biofuel production are lipids, carbohydrates and others such as proteins. Various conversion methods including transesterification, fermentation, pyrolysis, liquefaction and anaerobic digestion are used for production of biodiesel, bioethanol, bio-oil and methane (Fig. 2). The lipid is extracted using organic solvent and then transesterified to biodiesel in the presence of base or acid catalysts. Bioethanol is produced from microalgae fermentation, while methane can be produced through anaerobic digestion. Thermochemical processes such as hydrothermal liquefaction and pyrolysis can be applied for bio-oil production. Currently, no liquid biofuel production from microalgae is commercially implemented because ‘‘liquid fuel production only’’ is not economically feasible. Thus, the whole components of microalgae biomass should be used to produce multiple products including various liquid biofuels for economically viable option [17]. In this review paper, recent progress on the production of liquid biofuels, including biodiesel, bioethanol and bio-oil, are addressed, and some concluding remarks are also discussed for a successful zero-waste microalgae biorefinery. Biodiesel production Biodiesel is a mixture of fatty acid methyl esters (FAMEs) produced by transesterification of triglycerides with alkyl acceptors such as methanol in the presence of a catalyst, usually NaOH, KOH or lipase. Since methanolysis of triglyceride is an equilibrium reaction, a high excess of methanol, generally six moles of methanol per mole of triglyceride, is applied [6]. At present,
biodiesel is mainly produced from rapeseed and palm oils. Recently, microalgae are being considered as feedstock for biodiesel production [16,18,19]. The lipid content of some microalgae species such as Botryococcus braunii exceeds 80% of the dry weight [20]. Chlorella and Dunaliella are known to have lipid contents as great as 50% of the dry weight. Thus, considerable amounts of lipid for biodiesel production can be obtained from large-scale cultivation of microalgae. As indicated in some reports, the yield of microalgae lipid per hectare is around 58,700 l/ha, almost ten times greater than those from palm crops [21]. Biodiesel can be produced using a two-step method of oil extraction-transesterification or one-step transesterification (so called ‘in site or direct transesterification’). The common method for extracting oil from microalgae is a solvent extraction using hexane, ethanol, methanol and methanol–chloroform mixture (2:1 v/v) [22]. The extraction efficiency of microalgae oil using n-hexane is rather low, although it is widely used for oil extraction from seed crops [23]. Ultrasonic-assisted extraction, microwaveassisted extraction and supercritical fluid extraction can be an alternative to organic solvent extraction. After lipid extraction, alkali, acid catalysts and lipase are used for transesterification. One important thing is microalgae oil generally contains a certain amount of free fatty acid. Thus, an acid-catalyzed conversion can be an efficient method with high conversion, though its reaction rate is approximately 4000 times slower than base-catalyzed processes [24]. Recent advances in biodiesel production from microalgae are summarized in Table 1. In two-step transesterification, microalgae lipids were extracted using different solvents and extraction methods from various microalgae strains [25–32]. The extracted lipids were transesterified to FAMEs using H2SO4, HCl and lipase. An acid catalyst was used because microalgae biomass generally contains large amounts of free fatty acids that cause the formation of soaps in the presence of alkali catalysts [25]. Lam et al. [26]
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Fig. 2. Various biochemical and thermochemical approaches for production of liquid biofuels from microalgae.
Table 1 Biodiesel production from various microalgae. Method
Microalgae species (dry or wet)
Two-step
Nannochloropsis gaditana (dry biomass) Phaeodactylum Tricornutum (dry biomass) Chlorella vulgaris (dry biomass) Chlorella protothecoides (dry biomass) Chlorella vulgaris (dry biomass) Chlorella vulgaris ESP-31 (dry biomass) Chlorella sp. KR-1 (dry biomass) Tribonema minus (wet biomass) Nannochloropsis sp. (80% moisture)
One step
Chlorella sp. KR-1 (dry biomass) Chlorella protothecoides (dry biomass)
Tetraselmis suecica (dry biomass) Chlorella sorokiniana (dry biomass) Nannochloropsis oceanica (65% moisture) Chlorella pyrenoidosa (80% moisture) a b
Biodiesel production process Stirring extraction Chloroform–methanol Ultrasonic assisted extraction Chloroform–methanol Stirring extraction Chloroform–methanol Soxhlet extraction Hexane Soxhlet extraction Hexane Ultrasonic assisted extraction Chloroform–methanol Stirring extraction DMC–methanol Subcritical ethanol extraction Microwave assisted extraction Hexane– methanol DMC based transesterification Non-catalytic transesterification in supercritical methanol (ethanol) Microwave assisted transesterification
In situ transesterification in water bath Microwave assisted transesterification
Catalyst
Lipid content (%)a
Biodiesel conversion (%)b
Reference
Transesterification solvent Chloroform–methanol Transesterification solvent Chloroform–methanol
HCl
13.1
41.8
[25]
HCl
24.5
37.5
Transesterification Methanol-THF Transesterification Hexane Transesterification ([BMIm][PF6]) Transesterification Hexane
solvent
H2SO4
–
95.0
[26]
solvent
lipase (Candidia sp.) lipase (P. expansum) lipase (Burkholderia sp.)
44–48.7
98.1
[27]
40.7
90.7
[28]
63.2
72.1
[29]
Transesterification solvent DMC Transesterification solvent Methanol Transesterification solvent Methanol–hexane
lipase (Novozyme 435) H2SO4
38.9
75.5
[30]
20.2
96.5
[31]
NaOH
38.3
86.4
[32]
lipase (Novozyme 435)
40.9
89.7
[33]
–
90.8 (87.8)
[34]
H2SO4
23.0
78.0
[35]
H2SO4
23.5 19.1
77.0 90.6
[36]
H2SO4
19.0
55.2
[37]
solvent IL solvent
Lipid content = (total lipid/total biomass) 100. Biodiesel conversion = (weight of FAMEs/total lipid) 100.
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reported that more than 95% biodiesel yield from highly viscous lipids (extracted from Chlorella vulgaris) was achieved by introducing tetrahydrofuran as co-solvent. The lipids extracted from Chlorella protothecoides using hexane were converted to FAMEs by lipase, with biodiesel conversion as high as 98.15% [27], because lipase generally showed greater activity toward hexaneextracted lipids consisting mainly of triglycerides with very small amounts of free fatty acids. Ionic liquid was successfully used for lipid extraction from C. vulgaris, with a 40.7% yield based on dry cell weight [28]. Lee et al. [30] reported a method for the highly efficient lipid extraction and lipase-catalyzed transesterification of lipids from Chlorella sp. KR-1 using dimethyl carbonate (DMC). With a mixture of DMC and methanol at a ratio of 7:3, total lipids were almost completely extracted under simple stirring at mild conditions of 50 8C. Biodiesel conversion of 75.5% was obtained using lipase. Recently, direct lipids extractions from wet microalgae were conducted to minimize the energy required for microalgae biomass drying. Lipids were extracted from wet paste of Tribonema minus using subcritical ethanol extraction, and then converted to biodiesel through acid-alkaline transesterification [31]. Washidin et al. [32] reported that 38.3% (w/w) of lipids in wet microalgae was extracted by microwave irradiation. The subsequent biodiesel conversion using microwave irradiation was 86.4% based on the extracted lipid. In case of one step transesterification, Jo et al. [33] developed a DMC-based direct transesterification for Chlorella sp. KR-1 biomass in the presence of lipase. A 367.31 mg of biodiesel was obtained from 1 g of Chlorella sp. KR-1 containing 40% (w/w) lipid. Interestingly, the biodiesel yield of direct transesterification was increased 1.25-fold, compare with two step transesterification (293.82 mg of biodiesel) [30,33]. To enhance the biodiesel conversion, supercritical fluid was applied. Nan et al. [34] reported a production of biodiesel with 90.8% yield through non-catalytic transesterification of microalgae lipid in supercritical methanol. For direct transesterification of wet microalgae, Im et al. [36] reported a direct transesterification of wet Nannochloropsis oceanica (65% moisture) with 91.1% biodiesel conversion in the presence of 0.3 ml H2SO4, 2 ml chloroform, and 1 ml methanol at 95 8C for 90 min. Microwave has been employed for an one-step transesterification of Chlorella pyrenoidosa (80% moisture) in the presence of chloroform, methanol and sulfuric acid as the catalyst. The biodiesel conversion with the aid of microwave was 1.3-fold higher than that of conventional two-step heating method [37]. The fuel properties of microalgae biomass-based biodiesel such as density, viscosity, flash point, cold filter plugging point, heating value and solidifying point have been well characterized [38]. The heating value of microalgae-based biodiesel was approximately 31–41 MJ/kg, a little bit low compared to that of petroleum-based diesel (46 MJ/kg). One interesting characteristics of microalgaebased biodiesel is it’s lower cold filter plugging point of 11 8C, more suitable fuel for aviation than petroleum-based diesel. On the other hand, viscosity of microalgae biodiesel is high, approximately 1.2–2.6 times more than that of petroleum-based diesel. The composition of biodiesel from microalgae depends on the species, which also strongly affects fuel properties like viscosity. Thus, selection and genetic improvement of microalgae species containing an appropriate fatty acid composition needs to be conducted to lower the fatty acid composition for lower viscosity. Although the fuel properties of biodiesel is similar to petro-diesel, biodiesel production from microalgae is still too expensive to be commercialized based on a commercial feasibility study. Davis et al. reported that the production of microalgae biodiesel could cost from $9.84 (open pond) to $20.53 (photobioreactor) per gallon, much higher than $2.60 per gallon for petroleum diesel production [39].
Saccharification and bioethanol fermentation Microalgae contain large amounts of various carbohydrates such as starch, cellulose and hemicellulose, depending on the species [40]. Microalgae such as Chlorella, Dunaliella, Spirulina and others are known to contain carbohydrates in the form of starch and glycogen in amounts as great as 50% of the dry weight [41,42]. Carbohydrates can be used for bioethanol fermentation [43]. Microalgae biomass has several advantages over other biomasses such as lignocellulose. The most advantageous feature of microalgae is that they contain little or no lignin [42], which mitigates cost-intensive biomass pretreatment. Additionally, carbon dioxide produced as a byproduct of bioethanol fermentation can be used to cultivate microalgae. Saccharification is one of the most critical steps for economically viable bioethanol production. Acid hydrolysis, the most commonly applied method, is conducted at 121 8C in the presence of H2SO4 and HCl (1–5%) [44,45]. Acid hydrolysis provides high sugar yields up to 100% for short time. However, acid hydrolysate needs to be neutralized before fermentation. On the contrary, enzymatic saccharification is conducted at mild condition and can be liked to fermentation, simultaneous saccharification and fermentation (SSF). Each species of microalgae has different polysaccharide composition, thus suitable enzymes such as cellulase, glucanase, xylanase and etc. should be employed. Various microalgae have been used for saccharification and bioethanol fermentation (Table 2) [45–53]. One advantage in a saccharification of microalgae biomass is that intensive pretreatment is not required. Various chemical saccharifications have been employed for microalgae. Harun et al. [46] used alkali saccharification for bioethanol production from Chlorococcum infusionum. The highest glucose yield was 0.35 g glucose/g microalgae using 0.75% (w/v) of NaOH and 120 8C for 30 min, and the maximum bioethanol yield was 0.26 g ethanol/g microalgae. Harun and Danquah [47] investigated the influence of acid saccharification on Chlorococcum humicola for bioethanol production. A 7.2 g/L bioethanol was obtained after 15 g/L biomass was treated using 1% (v/v) of sulfuric acid at 140 8C for 30 min. A 40 g/L Scenedesmus obliquus CNW-N containing 51.8% (w/w) carbohydrate was hydrolyzed using 2% (v/v) H2SO4. The resulting broth was fermented by Zymomonas mobilis, and an 8.6 g/L bioethanol was produced at a 21.3% yield (g ethanol/g biomass) [48]. A 14.6 g/L bioethanol was produced from 50 g/L biomass of Chlamydomonas reinhardtii UTEX 90 by Saccharomyces cerevisiae with 29.2% yield after the biomass was saccharified using 3% (v/v) H2SO4 at 110 8C for 30 min [49]. Lee et al. [50] hydrolyzed C. vulgaris using 5% (v/v) H2SO4 at 110 8C for 120 min. The maximum ethanol yield of 0.4 g ethanol/g biomass was achieved using ethanolic E. coli SJL2526. Carbohydrate-rich C. vulgaris FSP-E containing 51% carbohydrate per dry weight was used as feedstock for bioethanol fermentation [51]. Dilute acidic (1% H2SO4) and enzymatic saccharification of C. vulgaris FSP-E biomass produced glucose yields of 93.6 and 90.4%, respectively. A 11.7 g/L bioethanol with 87.6% theoretical yield was obtained from the acidic hydrolysate of a 50 g/L biomass using a separate hydrolysis and fermentation (SHF) process. The enzymatic hydrolysate was converted into bioethanol by both SHF and SSF processes, producing 79.9 and 92.3% theoretical yields, respectively. Choi et al. [52] used enzyme saccharification using two different commercial enzymes (including amylase from Bacillus licheniformis and glucoamylase from Aspergillus niger) for bioethanol production from C. reinhardtii UTEX 90. A sugar conversion of 0.5 g sugar/g microalgae was obtained and 0.23 g ethanol was produced from 1.0 g microalgae by SHF method. Likewise, many investigations have been successfully conducted in order to commercialize bioethanol production from microalgae. However, the commercial production of bioethanol from microalgae
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Table 2 Saccharification and bioethanol fermentation of microalgae biomass. Microalgae species
Hydrolysis Method
Biomass concentration (g/L)
Fermentation type
Carbohydrates (%) [glucose (g/L)]
Yeast strain
Ethanol (g/L)
Ethanol yield (%, g ethanol/g biomass)
Reference
Chlorococcum infusionum Chlorococcum humicola Scenedesmus obliquus CNW-N Chlamydomonas reinhardtii UTEX 90 Chlorella vulgaris
Chemical 0.75%(w/v) NaOH Chemical 1% (v/v) H2SO4 Chemical 2% (v/v) H2SO4
50
SHF
43.9 [35]
S. cerevisiae
N.D
26
[46]
15
SHF
32.5
S. cerevisiae
7.2
52
[47]
40
SHF
51.8
Z. mobilis
8.6
21
[48]
Chemical 3% (v/v) H2SO4
50
SHF
60 [28.5]
S. cerevisiae
14.6
29
[49]
Chemical 5% (v/v) H2SO4 Chemical 1% (v/v) H2SO4 Enzymatic Cellulase + Amylases Enzymatic Cellulase + Amylases Enzymatic Termamyl 120L +AMG 300L Enzymatic AMG 300L
N.D
SHF
N.D
E.coli
N.D
40
[50]
50
SHF
47–48 [23.6]
Z. mobilis
11.7
23
[51]
20
SHF
43.5–46.8 [7.78]
Z. mobilis
3.6
18
20
SSF
N.D
Z. mobilis
4.3
21
50
SHF
59.7 [25.21]
S. cerevisiae
11.7
23
[52]
50
SHF
51.9 [42.0]
S. cerevisiae
7.2
14
[45]
10
SHF
N.D
S. bayanus
3.8
38
[53]
Chlorella vulgaris FSP-E
Chlamydomonas reinhardtii UTEX 90 Dunaliella tertiolecta (lipid-extracted microalgae) Chlorococcum sp. (lipid-extracted microalgae)
Physical Supercritical CO2
is still in the research stages, similar to biodiesel production from microalgae. Thermochemical microalgae conversion for bio-oil production Representative two thermochemical processes for bio-oil production using microalgae biomass are hydrothermal liquefaction (HTL) and pyrolysis process (Fig. 3). The HTL is conducted at subcritical conditions in water, for obtaining liquid fuels from biomass. During the HTL reaction, water is acted as both a reaction solvent and reagent for hydrolysis of biomass. The HTL have relatively good efficiency of energy recovery on energy investment because its feedstock is wet biomass itself instead of dry matters. The drying step of biomass is one of the most energy consuming
processes in fuels production process from microalgae. The main product of the HTL is bio-oil namely ‘biocrude’, a liquid crude fuel composed of many hydrocarbon compounds. Bio-oil is organic solvent extractable black liquid oil with a high heating value and high viscosity. The reported high bio-oil production yield of microalgae HTL is 50–60% based on dry biomass [54]. The microalgae bio-oil is typically composed of 70–75% carbon, 10–16% oxygen, 4–6% nitrogen, and 0.5–1% sulfur. It has higher heating value (HHV) of 33.4–39.9 MJ/kg. The major components of bio-oil are aromatic hydrocarbons, long chain fatty acids, nitrogen heterocyclic compounds, alcohols, organic acid, aldehydes and etc.[55]. The HHV of microalgae bio-oil is relatively high because of low content of oxygen. The characteristics of microalgae bio-oil as a fuel and production yield are closely affected by the microalgae
Fig. 3. Production of bio-oil from thermochemical treatment of microalgae.
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composition as a feedstock, reaction temperature, time, catalyst and extraction method of bio-oil from reactant. When the same microalgae biomass is used as a HTL feedstock, the temperature is the most critical factor among the influencing conditions for production of microalgae bio-oil. The typical reaction temperature for microalgae HTL is 300– 374 8C, a subcritical temperature, and as the reaction temperature is elevated within that subcritical water temperature, the bio-oil yield and nitrogen content in bio-oil are increased and the oxygen content is decreased [56,57]. When the reaction temperature exceeds the critical point, the bio-oil is produced less and gas takes place more. The microalgae bio-oil produced through HTL is more suitable liquid fuel than that produced by pyrolysis method because not only it contains a less oxygen and water in bio-oil but also it has higher storage stability than that of pyrolysis oil [58]. Although bio-oil from HTL contains less nitrogen and sulfur than the feed microalgae biomass, nitrogen and sulfur contents in biooil are still in a high level. Bio-oil contains a large quantity of double bonds, so upgrading needs to be conducted for practical use as liquid fuel. Hydrotreatment, a hydrogen addition process, is being used for upgrading bio-oil. Generally, although HTL and pyrolysis have their own advantages, HTL is expected to be more cost-effective for wet microalgae biomass due to the omitting of energy-intensive drying process in the thermal conversion. Pyrolysis is a thermochemical conversion of biomass to bio-oil, bio-char and gas at medium to high temperatures (generally 450– 500 8C) with short residence time (order of seconds to minutes) in the absence of oxygen [59,60]. Contrary to HTL, biomass drying is necessary (Fig. 3). Pyrolysis of microalgae can produce a wide range of products including liquid oil, solid char and gas, depending on the reaction parameters such as heating rate, residence time, and pyrolysis temperature. In fast pyrolysis, according to the vapor residence time, finely ground biomass is treated for short residence times and fast heating rates at 350– 500 8C [61]. One major advantage of pyrolysis over other conversion processes is fast biomass conversion on the order of seconds to minutes. Fast pyrolysis with short residence times and fast heating rates tends to favor formation of bio-oil as the main product. In contrast, slow pyrolysis produces more bio-char [62]. One disadvantage of the pyrolysis process is the high capital cost of
separation equipment for various fractions. Another significant hurdle in using pyrolysis for microalgae is the high moisture content of biomass feedstock, which requires dewatering to control moisture content of the bio-oil. Development of an inexpensive dewatering or extraction process is a key factor for cost-competitive pyrolysis. Reports on pyrolysis of microalgae for bio-oil production are less available compared to those for lignocellulosic biomass. Pyrolysis converts triglycerides to fatty acid alkyl esters as bio-oil [63]. Microalgae carbohydrate and protein components as well as fat are converted to bio-oil by pyrolysis [64]. Bio-oil was obtained at a 57.9% yield from fast pyrolysis of C. protothecoides at a heating value of 41 MJ/kg [65]. Bio-oil from HTL and pyrolysis can be used as drop-in fuel in industrial boilers for heat production [66,67]. To use bio-oil directly as transportation fuel, catalytic hydrogenation can be applied to upgrade the bio-oil [68]. Lower oxygen content and a higher heating value were obtained using a catalyst [69,70]. Carbon yield of aromatic hydrocarbons increased from 0.9 to 25.8% in the pyrolysis of C. vulgaris using zeolite HZSM-5 [71]. When ZSM-5 catalyst was used with an increased catalyst to biomass ratio, oxygen content in the bio-oil decreased from 30.1 to 19.5 wt%, and the heating value of the bio-oil improved to 24.4–32.2 MJ/kg [72]. Biorefinery approach for integrated production of liquid fuels from microalgae biomass Biorefinery is a process of integrated utilization of every component of the biomass in order to enhance the economics of the process [73,74]. In a biorefinery process, major components of microalgae are subsequently used to produce biofuels, chemicals and feeds [75,76]. Some biologically active materials such as astaxanthin or tocopherol that are used as additives for nutraceuticals and cosmetics are produced from microalgae. However, the amount of those biomaterials to meet market demand does not require large scale production using biorefinery process for biofuels and chemicals. In case of feed production, there might be some problem in safety and approval when animal feed production is integrated with the production of biofuels and chemicals. Thus, an integrated production of various liquid
Fig. 4. Sequential production of biodiesel, bioethanol and bio-oil from microalgae biomass.
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biofuels in large scale based on biorefinery concept needs to be developed from the industrial point of view. All cellular components in microalgae biomass should be utilized to produce valuable liquid fuels in order to reduce overall production costs. Each cellular component of microalgae has own advantage for different liquid fuels. Lipid is suitable for production of biodiesel and bio-oil, whereas polysaccharide is appropriate for bioethanol. Recently, sequential production of biodiesel, bioethanol and bio-oil were successfully conducted for Dunaliella tertiolecta biomass (Fig. 4) [45,77]. The lipids of D. tertiolecta were extracted using a chloroform and methanol mixture and then converted to biodiesel. The residual biomass after lipid extraction has been successfully used as raw material for saccharification and subsequent bioethanol fermentation due to the large amount of polysaccharides present in the residual biomass. The residual biomass was composed of 51.9 wt% carbohydrate, 35.0 wt% protein, 13.1 wt% ash and no trace of lipids. The residual biomass was saccharified using an AMG 300 L enzyme with a saccharification yield of 80.9% (w/w) based on the total amount of carbohydrates [45]. Without any pretreatment, bioethanol was directly produced from the enzymatic saccharification products with a fermentation yield of 0.14 g ethanol/g residual biomass, which was 82.0% of the theoretical fermentation yield. For complete utilization of D. tertiolecta biomass, the residual biomass after the conversion of lipids and carbohydrates into biodiesel and bioethanol was further converted into bio-oils by pyrolysis. The predominant pyrolysis reaction pathway was from D. tertiolecta residual biomass to bio-oil rather than to gas and/or bio-oil to gas based on the proposed lumped kinetic model. These study demonstrated microalgae biomass could be completely used for an integrated production of various liquid biofuels. Conclusions and future prospects Microalgae are promising feedstock to replace petroleum-based fuels due to their high biomass productivity, high oil content and easy biomass processing due to the absence of lignin. Although microalgae biomass has great potential, biofuel production from microalgae is not economically feasible due to high production cost at present. Thus, cost reduction is the top priority for commercial implementation of biofuel production. The development of a costeffective algal cultivation process is the key factor for successful cost reduction [78]. Culture costs are expected to decrease with the development of a novel photobioreactor system together with use of a microalgae species with high photosynthetic efficiency [79,80]. The cost of nutrients to cultivate microalgae can be decreased when wastewater is used. In addition to microalgae culture, genetic and metabolic modifications of microalgae will improve cost-effectiveness of microalgae-based biofuel production [81]. Microalgae oil content can be enhanced and its composition can be manipulated by the search for new algae species and improvement of existing algae strains by molecular breeding. The development of genetically modified microalgae is still in a premature stage for commercial application. Metabolic modifications to enhance lipid content in microalgae are being intensively investigated. For example, genetic deletion or transcriptional deactivation using regulatory small RNA as an antisenser was used for down regulation of fadD or phosphoenolpyruvate carboxylase to prevent degradation of free fatty acids and enhance lipid biosynthesis [21,79–85]. Uncertainty regarding the use of genetically modified microalgae as biofuel production strains in real culture environments should be addressed together with extensive metabolic engineering of microalgae. Integrated biorefinery is an appealing way to lower production costs for microalgae-based biofuels. In biorefinery, destruction of
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microalgae biomass, extraction and separation of cell components and conversion of the components to biofuels need to be integrated. Less energy-intensive pretreatment processes such as cost-effective harvesting and dewatering prior to extraction and conversion need to be developed. A cost-effective extraction method is still required even though microalgae biomass is ligninfree. Recruit of new methods for extraction of oil directly from wet microalgae biomass without energy-intensive processing are important. Along with oil for biofuel, other substances such as carbohydrates and proteins should be used to improve biorefinery process economics. Fully integrated production of biodiesel, bioethanol, bio-oil and bio-char can be a key solution for zerowaste biorefinery of microalgae. In the near future, biorefinery for biofuel production from microalgae is expected to play an important role in the bio-based economy. Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF2013R1A2A2A01068863). This work was supported by a grant from the Marine Biotechnology Program funded by the Ministry of Oceans and Fisheries of the Korean Government. References [1] S.N. Naik, V.V. Goud, P.K. Rout, A.K. Dalai, Renewable Sustainable Energy Rev. 14 (2010) 578. [2] V. Patil, K.Q. Tran, H.R. Giselroed, Int. J. Mol. Sci. 9 (2008) 1188. [3] I. Rawat, R.R. Kumar, T. Mutanda, F. Bux, Appl. Energy 88 (2011) 3411. [4] L. Reijnders, Trends Biotech. 26 (2007) 349. [5] J. Singh, S. Gu, Renewable Sustainable Energy Rev. 14 (2010) 2596. [6] Y. Chisti, Biotech. Adv. 25 (2007) 294. [7] S. Sawayama, S. Inoue, Y. Dote, S.Y. Yokoyama, Energy Convers. Manage. 36 (1995) 729. [8] M. Cuaresma, M. Janssen, C. Vilchez, R.H. Wijffels, Biotechnol. Bioeng. 104 (2009) 352. [9] M. Cuaresma, M. Janssen, C. Vilchez, R.H. Wijffels, Bioresour. Technol. 102 (2011) 5129. [10] J.W.F. Zijffers, M. Janssen, J. Tramper, R.H. Wijffels, Mar. Biotechnol. 10 (2008) 404. [11] J.W.F. Zijffers, K.J. Schippers, K. Zheng, M. Janssen, R.H. Wijffels, Mar. Biotechnol. 12 (2010) 708. [12] M. Haesman, J. Diemar, W. O’Connor, T. Soushames, L. Foulkes, Aquacult. Res. 31 (2000) 637. [13] N. Rossignol, T. Lebeau, P. Jaouen, J.M. Robert, Bioprocess Eng. 23 (2000) 495. [14] R.M. Knuckey, M.R. Brown, R. Robert, D.M.F. Frampton, Aquacult. Eng. 35 (2006) 300. [15] T.M. Mata, A.A. Martins, N.S. Caetano, Renewable Sustainable Energy Rev. 14 (2010) 217. [16] L. Lardon, A. Helias, B. Sialve, J.P. Stayer, O. Bernard, Environ. Sci. Technol. 43 (2009) 6475. [17] B. Subhadra, G. George, J. Sci. Food Agric. 91 (2010) 2. [18] A. Demirbas, Energy Sources, A: Recov. Util. Environ. Effects 31 (2009) 163. [19] H. Xu, X. Miao, Q. Wu, J. Biotechnol. 126 (2006) 499. [20] Q. Hu, M. Sommerfeld, E. Jarvis, M. Ghirardi, M. Posewitz, M. Seibert, A. Darzins, Plant J. 54 (2008) 621. [21] Y. Chisti, Trends Biotechnol. 26 (2008) 126. [22] M.K. Lam, K.T. Lee, Biotechnol. Adv. 30 (2012) 673. [23] A. Ranjan, C. Patil, V.S. Moholkar, Ind. Eng. Chem. Res. 49 (2010) 2979. [24] E. Lotero, Y. Liu, D.E. Lopez, K. Suwannakarn, D.A. Bruce, J.G. Goodwin, Ind. Eng. Chem. Res. 44 (2005) 5353. [25] S.D. Rı´os, J. Castan˜eda, C. Torras, X. Farriol, J. Salvado´, Bioresour. Technol. 133 (2013) 378. [26] M.K. Lam, K.T. Lee, Fuel Process. Technol. 110 (2013) 242. [27] X. Li, H. Xu, Q. Wu, Biotechnol. Bioeng. 98 (2007) 764. [28] J.Q. Lai, Z.L. Hu, P.W. Wang, Z. Yang, Fuel 95 (2012) 329. [29] D.T. Tran, K.L. Yeh, C.L. Chen, J.S. Chang, Bioresour. Technol. 108 (2012) 119. [30] O.K. Lee, Y.H. Kim, J.G. Na, Y.K. Oh, E.Y. Lee, Bioresour. Technol. 147 (2013) 240. [31] H. Wang, L. Gao, L. Chen, F. Guo, T. Liu, Bioresour. Technol. 142 (2013) 39. [32] S. Wahidin, A. Idris, S.R.M. Shaleh, Energy Convers. Manage. 84 (2014) 227. [33] Y.J. Jo, O.K. Lee, E.Y. Lee, Bioresour. Technol. 158 (2014) 105. [34] Y. Nan, J. Liu, R. Lin, L.L. Tavlarides, J. Supercrit. Fluids 97 (2015) 174. [35] B.D. Wahlen, R.M. Willis, L.C. Seefeldt, Bioresour. Technol. 102 (2011) 2724. [36] H. Im, H.S. Lee, M.S. Park, J.W. Yang, J.W. Lee, Bioresour. Technol. 152 (2014) 534. [37] J. Cheng, T. Yu, T. Li, J. Zhou, K. Cen, Bioresour. Technol. 131 (2013) 531. [38] A. Bahadar, M. BilalKh, Renewable Sustainable Energy Rev. 27 (2013) 128. [39] R. Davis, A. Aden, P.T. Pienkos, Appl. Energy 88 (2011) 3524.
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