Liquid Biofuels From Microalgae: Recent Trends

Chapter 18

Liquid Biofuels From Microalgae: Recent Trends Michele Greque de Morais1, Ba´rbara Catarina Bastos de Freitas2, Luiza Moraes2, Aline Massia Pereira2 and Jorge Alberto Vieira Costa2 1

Laboratory of Microbiology and Biochemistry, College of Chemistry and Food Engineering, Federal University of Rio Grande, Rio Grande, Brazil, 2Laboratory of Biochemical Engineering, College of Chemistry and Food Engineering, Federal University of Rio Grande, Rio Grande, Brazil

18.1 INTRODUCTION The global demand for energy is increasing due to high global population growth. At present, about 90% of the energy produced is from fossil fuels and about 10% is from renewable sources [1 3]. However, the continued use of fossil fuels is commonly considered unsustainable. This is due to the depletion of energy reserves and environmental problems caused by pollutant emissions released from burning these fuels [4]. Given the increasing demand for energy as well as global warming and environmental impacts caused by the use of fossil fuels, biofuel production becomes a promising alternative [5 7]. Biofuels may offer new opportunities for the diversification of income sources, promotion long-term fossil fuels replacement, reduction of emissions of greenhouse gases (GHGs), and increasing the security of the energy supply [7]. Biofuels are produced from biomass and have several advantages compared to fossil fuels, such as nontoxicity, biodegradability, and low carbon dioxide emissions into the atmosphere [8,9]. Microalgae are unicellular or multicellular photosynthetic microorganisms that are able to convert CO2, water, and light through photosynthesis into oxygen and biomass [10 12]. The microalgae biomass produced is composed of macromolecules such as lipids, carbohydrates, and proteins and can be employed in producing liquid biofuels [13]. Thus, biofuels from microalgae are known as “third generation,” representing a new alternative to the bioenergy field [14]. Furthermore, these microorganisms can produce

Advanced Bioprocessing for Alternative Fuels, Biobased Chemicals, and Bioproducts. DOI: https://doi.org/10.1016/B978-0-12-817941-3.00018-8 © 2019 Elsevier Inc. All rights reserved.

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bioactive molecules such as polyunsaturated fatty acids (PUFAs) [15], pigments, polysaccharides [16], and biopeptides [17]. These compounds can be used in human and animal feeding [18], in the development of biomaterials [19], cosmetics, pharmaceuticals [20], and nanostructures [21]. However, the high cost of production has made the large-scale production of biofuels from microalgae infeasible. Several studies have shown that one of the alternatives to enable largescale production of microalgae biofuels would be applying the concept of a microalgae biorefinery [20,22]. The application of this strategy encourages process integration, which aims to produce biofuels in parallel with high value-added coproducts, thus promoting major use of the biocompounds present in microalgae biomass. This chapter aims to address the key aspects of the production of liquid biofuels from microalgae, as well as the present integration of processes via the concept of biorefineries. Furthermore, the potential for industrial application and scaling limitations when microalgae biomass is used as a raw material will be discussed.

18.2 BIOFUELS Biofuels can be produced from several renewable raw materials and can be classified as first, second, or third generation. The first uses edible sources such as corn, soybeans, sugar cane, and rapeseed. Due to the use of these food resources, this option has been blamed for the increase in food prices [23]. Second-generation biofuels are produced from lignocellulosic biomass. This biomass consists predominantly of agricultural residues, forestry harvest (e.g., branches, leaves, straw, and wood chips), wood treatment remains (sawdust), and the nonedible components (e.g., corn, sugar, cane, and beets). Nevertheless, the pretreatment of biomass to obtain fermentable carbohydrates requires sophisticated and expensive technologies, which is an economically unprofitable form of generation for commercial production [5,24]. The third-generation biofuels consolidate its biomass from microalgae which have shown several potential advantages over the first and secondgeneration biofuels [25]. Microalgae biomass can be used to produce several biofuels, such as biodiesel, bioethanol, biomethane, biohydrogen, and biooil [12] (Table 18.1). The production of biofuels from microalgae sources can result in high yields in relation to lignocellulosic biomass, being capable of producing 30 100 times more energy per hectare compared with ground crops [43]. Microalgae have many desirable characteristics such as rapid growth, higher photosynthetic efficiency, and high content of lipids as well as carbohydrates in the biomass [44]. Microalgae cultivation can be performed on infertile land, integrated with wastewater treatment processes, and CO2 fixing from flue gas, producing biomass rich in biocompounds with commercial

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TABLE 18.1 Microalgae Application in the Production of Biofuels Biofuels

Microalgae

Reference

Bioethanol

Chlorella vulgaris FSP-E

[26]

Scenedesmus obliquus CNW-N

[27]

Chlorococcum humicola

[28]

Chlorella vulgaris

[29]

Tribonema sp.

[30]

Scenedesmus dimorphus

[31]

Spirulina platensis

[32]

Chlamydomonas reinhardtii CC1010

[33]

Scenedesmus sp.

[34]

Chlorella vulgaris

[35]

Nannochloropsis oculata

[36]

Chlorella minutissima

[37]

Nannochloropsis oculata

[38]

Chlamydomonas reinhardtii

[39]

Chlorella pyrenoidosa

[40]

Spirulina maxima

[41]

Spirulina platensis

[41]

Scenedesmus dimorphus

[42]

Biodiesel

Biooil

applications [5,45]. The potential lipid productivities of some species of microalgae can be at least 60, 15, and 5 times higher than those from soya, Jatropha, and palm oil production per acre of land per year, respectively [46]. The microalgae-based biofuels do not compete with the food supply and therefore cause no concerns about food security [47]. Regarding GHG emissions to the atmosphere, biofuels from microalgae can be considered neutral in carbon emissions. This neutrality is possible because during photosynthesis, the microalgae absorb the CO2 emitted when the biofuel is burned [48]. In spite of being considered promising, the biofuels production process is not economically viable due to the high cost of production [20]. According to the survey data conducted by the US Department of Energy, the price of biodiesel from microalgae produced on a large scale was estimated to reach up to $8 per gallon, while those made from a traditional soya culture are sold on the market for $4 per gallon [20,49]. According to Molina Grima et al. [50], much of the total costs are concentrated in the production of

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microalgae biomass (50%), while the other expenses (20% 30% of the total costs) are allocated to the biomass recovery process (harvesting and drying). In order to facilitate the use of microalgae biomass for biofuel production, it is necessary to make technological advances and/or process integration, which promote the development of profitable cultivation and the increased use of biomass biocompounds. This integration may occur through the application of the concept of biorefineries, which promote the conversion of biomass into biofuels and high value-added products [43]. The implementation of the concept of microalgae biorefineries in addition to improving the prospects of biofuel production contributes to fill the economic gap between the cost of production of microalgae biomass and fossil fuels [20].

18.3 LIQUID BIOFUELS FROM MICROALGAE Microalgae are mainly composed of macromolecules such as lipids, carbohydrates, and proteins [51], allowing for the wide use of their biomass. In human and animal feeding, the extractions of bioactive compounds as well as the production of biofertilizers and biofuels are among the main applications [5,12]. Biodiesel, bioethanol, and biooil stand out from the many liquid biofuels produced from microalgae (Fig. 18.1). According to Lee et al. [13], the production of biofuels from microalgae demands several steps, which include growing, separation, and drying of biomass, cell disruption for the extraction of biomolecules, and at long last, the application of conversion processes for the production of biofuels.

FIGURE 18.1 Schematic diagram of the cultivation, production of liquid biofuels and coproducts from microalgae.

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The most widely used methods for converting biomass into biofuel components can be classified into three categories: chemical (e.g., transesterification of lipids for conversion into biodiesel) [52], biochemical (e.g., fermentation of carbohydrates for processing into bioethanol) [53], and thermochemical (e.g., pyrolysis and liquefaction in the production of biooil) [54,55].

18.3.1 Biodiesel The lipid content of the microalgae may be influenced by the species and cultivation conditions. Microalgae species such as Chlorella, Dunaliella, Isochrysis, Nannochloris, Scenedesmus, Nannochloropsis, and Tetraselmis have high lipid content varying on average from about 20% to 50% [7]. In this context, microalgae are considered promising raw materials for biodiesel production. Most microalgae species direct their metabolism to the production of nonpolar lipids, particularly triacylglycerols (TAGs) which are used as storage molecules. When great cultivation conditions are reestablished, the TAGs are rapidly degraded to produce energy. In biodiesel production, microalgae stand out because they can produce high levels of TAGs. These can be accumulated in plastids or in the form of lipid bodies in the cytoplasm [56]. Several studies evaluated cultivation conditions that promote stressful situations in order to stimulate lipid biosynthesis in microalgae metabolism and thus enable the production of biodiesel. The conditions that were studied include nitrogen limitation [57,58], high carbon dioxide concentrations [59], light intensity [60], temperature [61], and salinity [62]. Damiani et al. [63] have found that the neutral lipid fraction of Haematococcus pluvialis biomass doubled when stressful conditions were applied (i.e., increased light intensity and nitrogen restriction). The authors confirmed the potential of the microalga H. pluvialis as a raw material for biodiesel production, but suggested that more research needed to be done in order to achieve an adequate energy balance for mass cultivation. It is reported that factors promoting stress to cells can result in a decline in growth, thus not allowing the correlation between productivity and biomass accumulation of lipids. Lipid productivity becomes a more useful way to indicate the potential costs of production of microalgae liquid biofuels [46]. As a strategy to increase the lipid productivity of microalgae, studies suggest employing two-stage cultivation. In this process, the first stage provides great conditions for growth of microalgae and the second provides stress to promote the accumulation of lipid into biomass [46,64]. According to Pancha et al. [62], the addition of 400 mM sodium chloride in Scenedesmus sp. NMSC 1077 cultivation promoted greater accumulation of lipid in the biomass (33.1%) and the lowest value of cell concentration in a single stage of cultivation. When two-stage cultivation was performed, a lipid content of

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24.8% was observed together with increased cell growth compared to single stage cultivation. Therefore, this strategy is effective in increasing the production potential of biofuels such as biodiesel. Biodiesel is a mixture of alkyl esters of long-chain fatty acids, obtained by the transesterification process of renewable raw material such as vegetable oils and animal fats [65]. The use of microalgae as a raw material for the production of biodiesel has advantages such as high growth rate, high lipid productivity, tolerance to environmental conditions; low competition for arable land, cultivation can be carried out in brackish water, and no seasonal crop restrictions [14,44]. The biodiesel production process from microalgae include steps such as selecting the suitable strain, cultivation, separation, biomass processing, lipids extraction (cell rupture and extraction), and their conversion into biodiesel [7]. The microalgae lipids can be converted into biodiesel by different methods, including microemulsification, pyrolysis, or catalytic cracking, which are high cost processes and produce low-grade biodiesel. Transesterification is a commonly used method to convert lipids (TAG) to fatty acid alkyl esters of low molecular weight (i.e., biodiesel) [66]. The transesterification process is affected by several parameters including alcohol/oil molar ratio, type of alcohol, and catalyst employed as well as the amount, reaction time, temperature, and purity of reagents. An alkali catalyst can be problematic when the content of free fatty acids of unrefined lipids is above 1%, making it unsuitable for biodiesel production. In addition to the use of alkali catalyzed transesterification in biodiesel production, this transesterification reaction can also be performed by using enzyme catalysts [44,67]. Moreover, the transesterification reaction can be performed via a one-step acid catalyzed reaction or sometimes two-step acid base catalyzed transesterification reaction. The choice of the specific catalyst and the type of the reaction mainly depends on the preliminary properties of the feedstock (e.g., free fatty acid content of the feedstock) chosen for the biodiesel production [68]. As a byproduct of the transesterification process, crude glycerol is obtained which can be converted into high value-added products (e.g., organic acid, mannitol, etc.) or be used as a carbon source for some eukaryotic microorganisms such as yeast and microalgae [44,66]. Mata et al. [7] have carried out a comparative study of biodiesel productivity from microalgae with terrestrial cultures. It showed that although the lipid contents are similar, biodiesel productivity from microalgae biomass is high and has advantages over the tested cultures. Biodiesel from cultures such as corn and soybean reach yields of 152 562 kg of biodiesel per hectare per year and requires larger areas of land (66 and 31 m2 year per kg of biodiesel produced), respectively. However, high lipid content microalgae species may produce up to 121,104 kg of biodiesel per hectare per year (using only 0.1 m2 lands per year per kg of biodiesel produced).

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18.3.2 Bioethanol In a scenario where industrial and social awareness of the environmental impacts associated with the use of fossil fuels is increasing, bioethanol produced from renewable biomass becomes a prominent option in the search for further sustainable fuels. Bioethanol can be produced from several species of microalgae, converting their polysaccharides into simple sugars by using appropriate methodologies. Some of the species used for the production of bioethanol are Cholorococcum, Chlamydomonas, and Chlorella, which have better bioethanol conversion rates [66,69]. Microalgae can be considered as potential substrates for bioethanol production because several microalgae species have high carbohydrate content [70], varying between 4% and 64% (w/w) [51]. Carbohydrate contents of microalgae species are mainly present in the form of starch and cellulose (absence of lignin), which can be converted to monosaccharides. Among the advantages of producing this biofuel from microalgae biomass, one can highlight rapid growth and the ability to fix CO2 at higher rates than terrestrial plants [29,71]. In order to produce bioethanol from microalgae biomass, it is desirable that microalgae biomass have a high carbohydrate content. As lipids, carbohydrates are reserve macromolecules and are accumulated in conditions that promote cultivation stress. Factors such as light intensity, temperature, and nutrient deprivation (e.g., nitrogen, sulfur, and phosphate) are known to interfere with metabolic strategies and change the composition of the microalgae biomass and should be considered when maximizing the carbohydrate content during cultivation [29,72]. Margarites and Costa [73] found that a high content of carbohydrates in a biomass (69.2% w/w) of Chlorella minutissima was produced when cultivated in nitrogen and phosphorus deprived conditions. Salla et al. [74] observed an increase in the productivity of carbohydrates (60 mg/L/day) of Spirulina platensis LEB 52 when cultivated with a diluted Zarrouk medium (20% v/v) added with 2.5% whey alongside ultrafiltration and nanofiltration processes. Bioethanol production from microalgae involves a series of process steps including pretreatment of biomass, saccharification, fermentation, and product recovery. Microalgae bioethanol results from the fermentation of biomass. In this process, biomass derived from cultivations is typically separated from the liquid medium and then undergoes a treatment in order for biomass carbohydrates to be converted into simple sugars, as yeast added, and is kept in a suitable temperature in fermenter. Afterwards, the end product will be purified. As it was mentioned previously, microalgae carbohydrates consist mainly of starch (produced in chloroplasts) and polysaccharides (predominantly cell walls) [75]. However, these carbohydrates are not readily fermentable. Thus, before fermentation, polysaccharides must be hydrolyzed to fermentable sugars. This step is known as

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saccharification and transforms the polysaccharides into assimilable forms by microorganisms (yeasts) through the hydrolysis process. The most common methods of hydrolysis of polysaccharides are chemical (acid or alkali) and enzymatic. Acid hydrolysis is considered fast, easy, and inexpensive way to convert polysaccharides into simple sugars [26]. However, it is found that acidic conditions can lead to the decomposition of sugars in unwanted compounds, which inhibits the fermentation process [76]. In contrast, enzymatic hydrolysis processes are slow, have a higher cost and require physical or chemical pretreatments for better efficiency. One of the advantages of using enzymes for saccharification is that higher glucose contents are obtained without the production of by-products that inhibit fermentation [28,29]. Lee et al. [77] evaluated several saccharification methods to obtain ethanol from microalgae biomass. The authors found that the best ethanol yield (0.4 g ethanol/g biomass) was obtained when Chlorella vulgaris biomass was used with pretreatment (acid and enzyme hydrolysis). Similarly, Nguyen et al. [78] observed a yield of 0.292 g ethanol/g biomass when Chlamydomonas reinhardtii UTEX 90 biomass was subjected to acid hydrolysis. Choi et al. [69] found that the enzymatic pretreatment of C. reinhardtii UTEX 90 biomass promoted a yield of 0.235 g ethanol/g biomass. After the saccharification of sugar, the anaerobic fermentation with Saccharomyces cerevisiae (biological process for obtaining ethanol) will start. S. cerevisiae is generally considered as the most efficient yeast to produce ethanol [72]. Some microalgae have also shown the ability to produce ethanol by intracellular anaerobic fermentation [79]. This self-fermentation method is simpler and has less energy expenditure than conventional routes. With self-fermentation, the stored carbohydrates are degraded and bioethanol is produced [80]. After the last fermentation, the application of a distillation (purification) process is required to remove water and other impurities (10% 15% of crude product) from the obtained ethanol. Immediately after this step, the purified ethanol is removed, condensed into liquid form, and can then be used as pure fuel or as a gasoline supplement [5,81].

18.3.3 Biooil Biooil is a product obtained by the thermochemical treatment of the biomass or its residue after lipid extraction and/or the saccharification of carbohydrates [13]. Its production is based on breaking down biomolecules (i.e., carbohydrates, proteins, and lipids), and transforming them into an organic liquid phase called biooil. This bioproducts are considered promising alternatives to oil for the production of liquid fuels and further chemical extraction [82]. Among the diverse biological resources of biomass, microalgae are the most promising raw materials for obtaining liquid fuels, such as biooil to

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replace fuels that are petroleum-based [5]. Microalgae biooils are less polluting in relation to fossil sources because they are neutral since the amount of CO2 assimilated from the atmosphere by photosynthesis is equivalent to the quantity generated during the combustion of biooil for energy production [82,83]. The biomass of microalgae can be converted into biooil through thermochemical processes such as pyrolysis and hydrothermal liquefaction [84]. The most cultivated microalgae species currently used for the conversion of biomass into biooil are Scenedesmus dimorphus [42], C. reinhardtii [39], Chlorella protothecoides [85], Nannochloropsis sp. [86], S. platensis [87], and Dunaliella tertiolecta [88]. Pyrolysis is generally defined as the decomposition of biomass at high temperature (350 700 C) in the absence of oxygen, producing products that can be used as an energy source. Based on the parameters of temperature, residence time, and heating rate, pyrolysis can be classified as either slow, fast, or flash pyrolysis [89]. The fast and flash pyrolysis processes are frequently applied to microalgae because they provide a higher biooil yield [39]. When producing biooil through the pyrolysis of microalgae, the biomass should be dry; otherwise, during the process, a large amount of energy is required to remove moisture from the biomass. In this context, hydrothermal liquefaction presents some advantages as compared to pyrolysis when generating biooil from biomass with high humidity (i.e., microalgae), thus avoiding unnecessary expenditure of energy during the thermochemical conversion process [90]. Hydrothermal liquefaction constitutes physicochemical conversion amongst biomass and water at temperatures above 250 C under high pressure (5 40 MPa), with or without a catalyst. Under these conditions, the degradation of macromolecules within the biomass creates small reactive fragments that make products, including biooil, as they repolymerize [91,92]. The biooil obtained from microalgae is a dark oil that is soluble in organic solvents and has a high viscosity and boiling point [93]. The quality of this biooil is superior to the biooil created from lignocellulosic biomass, due to its lower oxygen content and higher calorific value thus providing better application as a liquids fuel [94]. The chemical composition of the biooil depends on the composition of the biomass, the thermochemical treatment type used, and its operating conditions [95]. Among the main compounds of the microalgae biooils are aromatic hydrocarbons, long-chain fatty acids, nitrogen heterocyclic compounds, alcohols, organic acids, and aldehydes, some of which may negatively impact its application as a transportation fuel [82]. Thus, an additional treatment is typically required that is referred to as upgrading of microalgae biooil [96]. Several techniques can be applied to improve the applicability of the biooil produced by microalgae, including the addition of solvents,

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emulsifying, esterification, use of supercritical fluids, cracking, distilling, adsorption, and many others [38,95]. The choice of additional treatments is mainly dependent upon the physicochemical property that is desired to be improved (e.g., to reduce viscosity, density, water content, and increase the heat capacity) [95]. Advancement and challenges for the production of microalgae biooils includes the study of processes to obtain desirable and high-quality end products. However, economic viability is also required for this process to be developed on a large scale.

18.4 COPROCESSES APPLIED IN THE PRODUCTION OF MICROALGAE BIOFUELS WITH A FOCUS ON THE CONCEPT OF BIOREFINERIES Environmental and social aspects of the production of liquid biofuels from microalgae have not been enough to justify the large-scale production due to high production costs. In this context, there is the idea of using a microalgae biorefinery. The biorefinery concept is similar to a petroleum refinery, where multiple fuels and chemicals are derived from the same starting material, in this case, the biomass of microalgae. It has been recognized as the most promising way to establish a liquid fuel production industry with the biomass of microalgae as the main raw material [66]. As production of biofuels from microalgae becomes competitive, researchers should consider the production of high value compounds, such as biopolymers, pigments, antioxidants, nutraceuticals, pharmaceuticals, and chemicals alongside the production of biofuels [97]. In addition, more than one biofuel can be obtained from the same microalgae, depending on the process applied and the by-products generated [98]. The main macromolecules present in the biomass of microalgae are lipids, carbohydrates, and proteins, wherein the concentration of those depends on the species used and the employed cultivations conditions [99]. Bioethanol and biodiesel are obtained from the processing of carbohydrates and lipids, respectively, whereas biooil is obtained from biomass or its residues. Consequently, each of the processes generates value-added products [88]. In the biodiesel production process, the main by-product of transesterification is glycerol. It is estimated that over 400,000 t of glycerol can be produced for every 1 billion gallons of biodiesel produced [20]. To enhance the generated bioproducts, coprocesses can be applied such as growing eukaryotic microorganisms that use glycerol as the carbon source, thus obtaining high value biocompounds (e.g., PUFAs, organic acids, microbial biomass, mannitol) [66,100]. Glycerol can also be converted into 1 3-propanediol, a valuable product that can be used as a food additive or as raw material for pharmaceutical and cosmetics industries [13].

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Another coprocess which can be applied to the production of biodiesel is the fractionation of the microalgae lipids, since most microalgae lipids are rich sources of PUFAs that can potentially be used as nutritional supplement for human or / and as animal feed additives but are undesirable for transesterification reaction [20,101]. The extraction, recovery, and purification of PUFAs is typically challenging from the perspective of biorefinery operation and biodiesel production. Recent studies encompass the use of supercritical fluid for PUFAs extraction [102] but are more focused on methods that extract the saponifiable fraction mainly for biodiesel production [103], or use strains with a low content of PUFAs [104]. After the extraction of the lipid fraction of microalgae, many high-value compounds are still present in the residual biomass. For example, proteins can be used for different applications including animal feeding, fertilizers, industrial enzymes, surfactants, and bioplastics [66]. Microalgae proteins are known to improve the functional properties of food [105] and can also be modified chemically or enzymatically for a particular purpose, such as obtaining bioactive peptides [17,106]. When producing biooils from microalgae via pyrolysis, a solid carbonaceous product called biochar is produced. This biochar can be used in a variety of applications such as in fertilizers or soil agents, due to its high content of nitrogen and minerals (i.e., P, Fe, Ca, Mg) [107]. Due to the high porosity of the material, it can also be used as an adsorbent material [42]. While pyrolysis generates biochar, hydrothermal liquefaction generates liquid and gaseous wastes which can be recycled back to the microalgae culture medium itself or it may be used as a nutrient source for other microorganisms [39]. There are many studies that characterize the biochar generated from microalgae pyrolysis; however, there are few works of literature that study its application. The potential of microalgae for the production of liquid biofuels is incontestable, but alternatives should be developed that reduce production costs. The fractionation of the main macromolecules is interesting as it produces more than one biofuel and obtains high value-added compounds according to the biorefinery concept of microalgae that promotes large-scale production.

18.5 POTENTIAL INDUSTRIAL APPLICATIONS/LIMITATIONS AND CHALLENGES FOR SCALE-UP Microalgae have numerous biotechnological applications and microalgae biomass products range from food and drinks to solvents, organic acids, esters, amino acids, polysaccharides, enzymes, vitamins, antibiotics, hormones and biofuels [12]. Therefore, microalgae have become an emerging source of raw materials for biofuel, biochemicals, and biopharmaceuticals production as well as for food products [108,109]. Due to the various potential applications of microalgae, several species have been studied and commercially used for

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many types of industries [110]. Some species of microalgae (e.g., Dunaliella, Spirulina, and Chlorella) were reported to have applications in pharmaceutical and cosmetic industries, or they were reported as a feed additives and food sources [111]. In addition to its application as a food supplements, Chlorella sp. was also reported to have medicinal values as well [112]. Moreover, microalgae present a great potential as renewable fuel sources, but the processes for growth and bioprocessing still require high levels of academic and industrial research. Identifying strains that efficiently utilize CO2 and industrial waste as sources of nutrients, suitable extraction methods and designs on an industrial scale for the production of biofuels are critical issues that must be solved in order to ensure long-term sustainable energy. One of the biggest challenges is to integrate microalgae cell engineering with bioprocess engineering in order to ensure the economic and environmental viability of large-scale trials. This integration is very important since the transition from pilot studies to industrial operations typically exposes the cells to harsh environments, which can reduce biomass productivity [113]. One of the main advantages associated with the use of microalgae is the possibility of cultivation being carried out in nonarable land that is not suitable for livestock, such as desert areas. Furthermore, microalgae have high biomass productivity, and it is possible to manipulate their cultivation conditions, through genetic modifications or metabolic engineering. When compared to the cultivation of land plants that double in a period of months or years, some microalgae species can double their biomass in a span of few hours and some in 3 4 days [12,13,44,68,113]. This feature of microalgae enables increased biomass productivity and avoids specific harvest periods, depending on the reactor and the cultivation conditions applied. The scale-up of microalgae cultivation involves a number of factors which include the selection of growth systems, biomass applications, metabolic nature of microalgae, cost, sources of nutrients required by the microorganism, and CO2 sequestration capacity. Heterotrophic cultures can be conducted in conventional reactors with deprivation of light. However, for autotrophic growth, light availability and the period of light exposure are the major challenges since these factors greatly influence the biomass productivity and coproducts. Autotrophic growth requires higher light intensity making tubular photobioreactors the most suitable reactors, while for mixotrophic growth, raceway reactors type are considered suitable [4,114,115]. For large-scale production, a number of systems are used including different types of closed photobioreactors systems (e.g., tubular, flat-plate, and column) and open (e.g., open pounds and raceway). Closed systems can generate high biomass production, with estimated yields between 40 and 80 t of dry biomass per hectare per year [109,116]. Studies concerning potential applications and the scaling-up of microalgae cultivation have increasingly highlighted. In Brazil, the Laboratory of Biochemical Engineering (LEB) of

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the Federal University of Rio Grande has developed technologies since 1996 for microalgae cultivation. In an attempt to scale-up the cultivation of microalgae, the LEB has two pilot plants for Spirulina cultivation (Fig. 18.2). One of the ways to overcome the limitations and challenges of microalgae production on a large-scale is the incorporation of a biorefineries microalgae system. The biorefinery concept is able to make the production of microalgae biofuels economically viable via a parallel production strategy of high-value coproducts. This strategy involves cultivation using wastewater and CO2 absorption (atmospheric, as well as flue gas), to extract high-value products while transforming biomass into biofuels [117].

FIGURE 18.2 (A) Pilot pant of Spirulina biomass production (Santa Vito´ria do Palmar, RS, Brazil) and (B) pilot plant of CO2 biofixation by microalgae in Thermal Power Plant Presidente Me´dici—UTPM (Candiota, RS, Brazil).

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18.6 CONCLUSION Liquid microalgae biofuels are considered as a sustainable alternative to fossil fuels which can potentially supply global energy demand in future with a lower environmental impact. The biggest challenge of the industry is to make the production of biofuels using microalgae biomass as a raw material more economically viable. The limitations of scaling-up might be softened with the integration of processes proposed by the concept of biorefineries. In a refinery based on microalgae, biodiesel and ethanol are obtained by the transesterification of lipids and fermentation of carbohydrates, respectively, while biooil can be produced by the thermochemical conversion of biomass or biomass waste. This approach can potentially make the large-scale production of liquid biofuels possible while also obtaining high-value compounds.

18.7 FUTURE OUTLOOK The most promising way to establish a liquid fuel production industry with the biomass of microalgae as the main raw material is using the biorefineries concept. It leads to the interest of making a sustainable cultivation of microalgae (using atmospheric CO2 or from flue gases) alongside the production of more than one biofuel from the same microalgae biomass, as well as the extraction of high value compounds. The transition to all this integrated process, from pilot studies to industrial operations still needs to be very well studied and designed.

ACKNOWLEDGMENTS The authors would like to thank the Brazilian development agencies CAPES (Coordination for the Improvement of Higher Education Personnel), R & D Program ANEEL-Eletrobra´s CGTEE (National Agency of Electric Energy-Company of Thermal Generation of Electric Power), CNPq (National Council of Technological and Scientific Development), and MCTIC (Ministry of Science, Technology, Innovations and Communication).

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FURTHER READING O. Pulz, W. Gross, Valuable products from biotechnology of microalgae, Appl. Microbiol. Biotechnol. 65 (2004) 635 648. Available from: https://doi.org/10.1007/s00253-004-1647-x.