Figure 12.2 shows the projected techno-economic performance of algal biofuels at the cur­rent phase as well as the projected outcome for the future. From the figure, it can be observed that current algal biofuel production is still in its infancy stage for commercialization, predominantly due to the high investment cost and premature technology. However, with emerging technologies and innovation through progressive R&D efforts, the way will be paved to develop truly sustainable algal biofuels with affordable production costs in the near future. All the technical problems surrounding the supply chain of algal biofuels, such as nutrient source, cultivation system, harvesting and drying processes, and biofuel-conversion technology, must be resolved to attain its commercialization threshold and economic viability. Thus, current techno-economic assessments of algal biofuels should be highly

interdisciplinary, with a flexible modeling and analysis framework that can completely address the multiple pathways, coupled with various integration systems for algal biofuel production.

The production cost of algal biofuels is defined as the sum of capital and operating costs minus the revenues derived from all coproduct, as shown in Eq. 12.1 (Sun et al., 2011). Capital cost is usually related to one-time expenses such as the cost of land, buildings (e. g., indoor cultivation systems, offices, and laboratories), equipment (photobioreactors, dryer, filter press), and infrastructure (piping and pumps), whereas operating cost is associated with day-to-day expenses such as power (e. g., to operate photobioreactors), raw materials (nutrients and water), labor costs, and maintenance fees (Sun et al., 2011).

The coproduct that could possibly be derived from algal biomass residue after lipid extraction is carbohydrate, which can be an excellent substrate for anaerobic digestion or fermentation process to produce biomethane or bioethanol, respectively. The biomethane and/or bioethanol produced can be diverted to use at an algal cultivation farm to reduce the associated energy burden, or it can be sold as energy fuel. However, additional capital and operating cost will be required to build the facilities to convert carbohydrates to biomethane/bioethanol. In the worst-case scenario, the coproduct processing facilities may consume substantial amounts of cost and energy due to the low conversion efficiency of biomethane or bioethanol, making it commercially impractical to integrate the coproducts into the supply chain of algal biodiesel. On the other hand, coproducts that have high market value, such as phytonutrients and proteins, could be the alternative options instead of carbohydrates.

Certain strains of microalgae biomass are accepted as functional food for human and an­imals. This is because they contain active compounds such as omega-3 fatty acids (eicosapentanoic acid-EPA, decosahexaenoic acid-DHA), chlorophylls, carotenoids, phycobiliproteins, and the like. Although the production of strains containing these com­pounds can be performed in open reactors, it is preferable to carry out the process in closed photobioreactors to ensure good manufacturing practices, as imposed by the food and phar­maceutical industry.

Marine strains such as Pavlova viridis (Hu et al., 2008), Nannochloropsis sp. (Chini Zittelli et al., 1999) and Phaeodactylum tricornutum (Acien et al., 2000) have proven useful for the out­door production of omega-3 fatty acid-rich biomass in closed photobioreactors. However, its production cost is higher than omega-3 produced heterotrophically or obtained from fish oil.

For this reason, a cost-effective system based on autotrophic growth is not yet available. Concerning freshwater strains, Chlorella biomass is accepted for human consumption and is produced by more than 70 companies. The Taiwan Chlorella Manufacturing Co. Ltd. (Taipei, Taiwan) is the largest producer, with 400 t/year, although there is also significant production in Klotze, Germany, with 130 t/year using tubular photobioreactors. Annual world sales of Chlorella are in excess of $38,000 million (Spolaore et al., 2006).

Chlorella biomass has related health benefits, such as being an active inmunostimulator and reducer of blood lipids, among other things, in addition to its taste, flavor, and coloring prop­erties. There is also a demand for microalgae biomass production of monoalgal strains by the feed market. Thus, microalgae can be incorporated into fish, pet, and farm animal feed. In 1999, the production of microalgae for aquaculture reached 1,000 t/year (62% for molluscs, 21% for shrimps, and 16% for fish) (Spolaore et al., 2006). The importance of algae in this do­main is not surprising, given that microalgae are the natural food source for these animals. The main microalgae applications in aquaculture are associated with nutrition, either being used fresh (as the sole component or as a basic nutrient food additive) or for coloring the flesh of salmonids and for inducing other biological activities.

The production cost of monoalgal microalgae biomass in closed photobioreactors has re­cently been analyzed (Acien et al., 2012a) (see Figure 14.5); a pilot-scale facility of 0.04 Ha consisting of 10 tubular photobioreactors, each one 3 m3, operated in continuous mode at an average dilution rate of 0.34 L/day year round. Fertilizers are used to prepare the culture medium, which is filtered and ozonized to sterilize it. The strain Scenedesmus almeriensis is cultivated under controlled pH (by injecting pure CO2) and temperature excess is avoided by passing cool water from heat exchangers located inside the reactors. S. almeriensis has proven to be a source of lutein, with an average percentage of this carotenoid in the biomass of 1% (dw) all through the year (Sanchez et al., 2008).

Biomass productivity throughout the year ranges from 0.3 to 0.7 g/L. The biomass is harvested and concentrated by centrifugation daily in continuous mode, then is freeze-dried to obtain dry biomass; a production capacity of 3.8 t/year has been reported. Analysis of the facility’s production cost shows depreciation (42.6%) and labor (51.6%) as being the main fac­tors contributing to the final biomass cost of up to $89/kg (see Figure 14.6). As expected for the production of microalgae biomass using closed photobioreactors for the production step, in addition to centrifugation for harvesting and freeze drying for stabilization, these are the

FIGURE 14.5 Block diagram of the process for the production of Scenedesmus almeriensis dry biomass in tubu­lar photobioreactors. (Adapted from Acien et al., 2012a.)

FIGURE 14.6 Analysis of production costs related to the production of Scenedesmus almeriensis using tubular reactors scaled up to 3.8 t/year. (Adapted from Acien et al., 2012a.)

major contributions to the depreciation cost. Closed photobioreactors make up 47% of the de­preciation cost, whereas harvesting represents 45%. Although raw material and utility costs are much lower than depreciation and labor costs, the contributions of CO2 and power costs are highly relevant. The cost of CO2 represents 71% of raw material cost, whereas power rep­resents 98% of the utility cost. These reported data have been obtained and verified over two years of operation, making their robustness higher than other cost analyses performed using laboratory data.

From these results it can concluded that, to reduce the production cost, it is necessary to reduce labor by implementing extensive automation in addition to reducing the depreciation cost by simplifying the equipment used and increasing production capacity. Indeed, by in­creasing the production capacity up to 200 t/year, by adequate scale-up of the process, by reducing manpower to 1 person/ha, and by avoiding the use of expensive equipment such as freeze dryers and sterilization units, the production cost can be reduced to $16/kg versus $89/kg at small scale (Acien et al., 2012a). This reduction to less than 20% of the initial value demonstrates that the production capacity increase has a great effect on the reduction of the production cost. Therefore, the obtained value is similar to that reported (Norsker et al., 2010), indicating a production cost of $5.4/kg when producing microalgae biomass in tubular photobioreactors at scales up to 100 ha.

It has also been reported that the production cost in tubular photobioreactors is lower than that obtained using flat panels or open raceways. This is due to the lower productivity and
higher harvesting costs in these reactors compared to tubular photobioreactors (Posten, 2009), production costs being as high as $6.4/kg and $7.7/kg when using flat panels and open race­ways, respectively (Norsker et al., 2010). These studies demonstrate that biomass productivity is a key factor determining the total production cost, in addition to the unitary cost of the re­actor and unitary harvesting cost. Moreover, production costs can be lower using tubular photobioreactors instead of open raceways in spite of lower costs for the latter; this is because of the higher productivity achieved in tubular photobioreactors and lower volume to be processed in the harvesting step.

The green halophilic alga Dunaliella is the best natural source of p-carotene. This microalgae is marketed in several countries, such as the United States, Australia, and Israel. The biopigment p-carotene is extracted from microalgal biomass and used as a food supple­ment or a natural pigment added to foods, or the dry biomass is marketed in tablets (Wood, 1998).

The biomass of the microalga Dunaliella has demonstrated several biological activities, such as being antihypertensive, bronchodilator, analgesic, muscle relaxant, and anti-edema. The natural p-carotene contains many essential nutrients that are not present in the same pigment produced synthetically (Yousry, 2002). The human body converts p-carotene to vitamin A without forming toxins in the liver. p-carotene has antioxidant activity while avoiding the effects of free radicals.

This microalga is grown in high salinity, with optimal growth in 22% of NaCl. Under these cultivation conditions the microalga culture is axenic and thus poses no problems of contam­ination when kept in open ponds (Wood, 1998).

The concentration of p-carotene accumulated in the cells of Dunaliella overcomes the traditional source of this pigment, and about 14% of the compound may be extracted.

Dunaliella is a eukaryotic green algae that grows in saline sites. Halophilic representatives of microalga have an osmotic mechanism that is different from halophilic bacteria. Dunaliella, which has no cell wall, can be developed with high salt concentration in the cytoplasm by the synthesis of glycerol. This microalga also responds to osmotic stress with the synthesis of glycerol if the high salinity is caused artificially by polyols.

The amount of glycerol produced by the microalga when exposed to saline stress is pro­portional to the concentration of NaCl in the culture.

Open systems can be simply categorized into natural waters (lakes, lagoons, ponds) and artificial ponds or containers. The most commonly used systems include shallow big ponds, tanks, circular ponds, and raceway ponds (Suh and Lee, 2003). Open ponds are much easier to construct and operate than most closed systems. However, major limitations in open ponds include poor light utilization by the cells, evaporative water losses, diffusion of CO2 to the atmosphere, and the requirement of large areas of land. The ponds are usually kept shallow to ensure sufficient light exposure for the microalgae because sunlight can penetrate the pond water to only a very limited depth. Furthermore, contamination by predators, alien microalgae species, and other fast-growing microorganisms restrict the commercial produc­tion of algae in open culture systems. In addition, due to inefficient stirring mechanisms in open cultivation systems, their gas transfer rates are relatively poorer than those of closed systems. All these limitations lead to lower biomass productivities for open systems com­pared with those of closed systems. Nevertheless, the simple operation and easy scale-up for mass cultivation make open systems the first-choice option for microalgae cultivation in industrial applications.

The characteristics of microalgae and the state in which they thrive can greatly affect the choice of algae-harvesting technology and its performance (Cooney et al., 2009). Separation of tiny and loosely suspended algal particles from the broth can be cumbersome because algal

cells normally carry negative charge and excess extracellular polymeric substances (EPS) to maintain algal stability in a dispersed state (Gudin and Therpenier, 1986). The stability of microalgae in the growth medium is mainly associated with algal surface charge, size, and density of the algal cells, which in turn influence their separability from aqueous suspensions. Both the electric interactions between algal cells and cell interactions with the surrounding culture broth contribute to the stability of the algal suspension (Tenney et al., 1969), whereas size and density of algal cells dictate their settling rate, which is an important consideration for sedimentation process design. Harvesting cost can be high since the mass fractions in culture broth are generally low.

Studies of the effect on surface charge of particles by various treatment methods have been extensively documented. Farvardin and Collins (1989) noted that pre-ozonation increases surface charge of humic substances. In another study, Chheda et al. (1992) noted an increase in stability of suspension of Na-montmorillonite particles at increased ozone dose, attributed to the increase in surface charge as a result of disruption of metal-oxygen bonds in crystal lattice. These studies, however, reported the existence of an optimal ozone dose whereby the coagulation of particles can be improved.

Conversely, Chheda and Grasso (1994) revealed that ozonation reduced stability of the Na-montmorillonite particles coated with natural organic matters (NOM) in river waters. It was postulated that the adsorption of NOM on Na-montmorillonite particles per se would render the particles more hydrophilic. Subsequent ozonation, however, turns the particle surface less negatively charged. This, in turn, resulted in partial dealuminization of Na-montmorillonite and transformation of coated NOM to increase the hydrophobicity of the particle surface, hence destabilizing the particles. It can be deduced from these studies that appropriate treatment of ozonation would help destabilize particles, leading to improved separation from the medium. Possible mechanisms for enhanced coagulation of suspended particles caused by ozonation were proposed (Reckhow et al., 1986; Plummer and Edzwald, 2002). These mechanisms include:

1. Increase in carboxylic content to enhance adsorption to alum floc and calcium and

magnesium precipitates

2. Reduction in molecular weight of adsorbed organics to reduce steric hindrance of

particles

3. Breakdown of organometallic bonds to release ions such as Fe3+ for organics precipitation

4. Rupture and lyses of algal cells to release biopolymers for coagulation

5. Polymeriazation to large particles for sedimentation

Henderson et al. (2009) noted that bubbles with surfaces modified using chemicals of both a hydrophobic long tail and a hydrophilic high charge head can yield sufficient algal removal without upstream coagulation and flocculation. In an earlier study by Henderson et al. (2008), it was reported that the algogenic organic matters (AOM) extracted from four algal species (Chlorella vulgaris, Microcystis aeruginosa, Asterionella formosa, and Melosira sp.) were domi­nated by hydrophilic polysaccharides and hydrophobic proteins of low specific UV absor­bance and negative zeta potentials. The hydrophobicity of AOM was attributable to the hydrophobic proteins of molecular mass greater than 500 kDa. Additionally, the charge density for the AOM, being attributable to hydrophilic and acidic carbohydrates and not hydrophobic humic acids, decreases inversely with hydrophobicity. On the other hand, inhibition on ferric chloride coagulation of algae by isolated AOM secreted by cyanobacgterium Aphanothece halophytica was reported (Chen et al., 2009). It was hypothe­sized that the AOM can form complex compounds with ferrum thereby inhibits the coagu­lation. As discussed earlier, ozonation is able to increase hydrophobicity of NOM, thereby enhancing its coagulation. The impact of ozonation on AOM, however, is unclear and yet to be investigated.

Just like planktonic cells, algal cells normally carry negative surface charge. Whereas algal surface charges are derived from ionization of ionogenic functional groups at the algal cell wall (Golueke and Oswald 1970) and selective adsorption of ions from the culture medium, the intensity of the charge is influenced by algal species, ionic concentration of medium, pH, and other environmental conditions.

Based on the principles of the Deyaguin-Landau-Verwey-Overbeek (DLVO) theory of col­loid stability, the interactions between colloidal particles are influenced by various interacting forces such as electrostatic double-layer repulsion, van der Waals attraction, and steric inter­action. There is a potential energy barrier to be overcome if coagulation of the minute charge particles is to be attained. It can be exceeded by the kinetic energy of the particles or, alter­natively, by the reduction of the energetic barrier. This is done by compressing the double layer through either by increasing the counter-ion concentration or by using counter-ions of higher valency (Ives, 1959). Although the double-layer theory is of great theoretical impor­tance, its use is restricted to cases in which specific chemical interactions do not play a role in colloid stability.

Destabilization of colloidal suspension such as that in algal culture as a result of specific chemical interactions is attainable by the presence of organic polymers (Shelef et al., 1984). Commercial polymers, usually those of high molecular weights such as polyelectrolytes or polyhydroxyl complexes, are considered superior coagulants or flocculants. The polymeric coagulation-flocculation is explained by the bridging model, postulating that a polymer can attach itself to the surface of an algal particle by several segments with remainder segments extended into solutions. These segments are then able to attach to vacant sites of other algal particles, forming a three-dimensional floc network (Gregory, 1977).

A planktonic algal cell can be considered a very minute spherical object that falls in a continuous viscous fluid medium at velocities governed by gravity’s downward force and the upward drag (or frictional) and buoyancy forces. If the algal particle is falling in the vis­cous fluid by its own weight due to gravity, then a terminal velocity, also known as the settling velocity, is reached when this frictional force, combined with the buoyant force, exactly bal­ances the gravitational force, as described by Stokes’ law. In actual fact, the settling velocity of planktonic algae in natural habitat is dictated by a variety of complex factors, which include cell mobility, water turbulence and flow, and upwelling caused by winds and temperature stratification (Hutchinson, 1967). The settling velocity of planktonic algae can be reduced in an ecosystem by the following:

1. Motility

2. Reducing cell dimensions

3. Increment of the drag forces as in the Scenedesmus species, which contain seta (Conway

and Trainer, 1972)

4. Reducing cell density, as in many blue-green algae, which contain gas vacuoles (Fogg, 1975;

Paerl and Ustach, 1982)

Hence, the settling velocity of an algal cell can be increased by increasing cell dimensions,

i. e., by cell aggregation into a larger body. This principle is applied in algal separation pro­cesses where chemical coagulants are added to form large algal flocs which settle rapidly to the reactor or tank bottom. Conversely, air bubbles, which may attach to the already formed algal flocs, will reduce drastically the floc density, causing the floc to float atop the vessel. Increasing the gravity force will increase the settling velocity of algal cells, which is attainable by applying centrifugal forces on algal suspensions.

In summary, destabilization and flocculation of algal suspension are important consider­ations in most of the various algal separation and harvesting processes, which are described separately in the following section.

Chlorella has long been used as human health food. Under certain stress conditions, Chlorella species are capable of accumulating as high as 60% (w/w, on dry-weight basis) oil within cells (Table 6.4). Together with the characteristics of high growth rate and ease of culture and scale-up in bioreactors, Chlorella has attracted unprecedented interest as a feedstock for biofuels, in particular biodiesel (Xu et al 2006; Li et al 2007a; Xiong et al 2008; Hsieh and Wu 2009; Gao et al 2010; Liu et al 2010, 2012a). The synthesized fatty acids in Chlorella are mainly of medium length, ranging from 16 to 18 carbons, despite the great variation in fatty acid composition (Table 6.5). Generally, saturated fatty esters possess high cetane numbers and superior oxidative stability, whereas unsaturated, especially poly­unsaturated, fatty esters have improved low-temperature properties (Knothe, 2008). It is suggested that the modification of fatty esters—for example, enhancing the proportion of oleic acid (C18:1) ester—can provide a compromise solution between oxidative stability and low-temperature properties and therefore promote the quality of biodiesel (Knothe,

2009) . In this regard, C. protothecoides, with the highest proportion of oleic acid (71.6%), may be better than other Chlorella species as biodiesel feedstock (Cheng et al., 2009). The properties of C. protothecoides-derived biodiesel were assessed, and most of them proved to comply with the limits established by the American Society for Testing and Materials (Xu et al., 2006).

There are increasing reports of using heterotrophic C. protothecoides cultures for oil produc­tion, from laboratory scale to large scale of 11,000-L of culture volume (Table 6.4). The scale-up from 5 to 11,000 L just caused a slight decrease in productivities, suggesting the C. protothecoides may represent a potential producer of oils for commercially large-scale production (Li et al., 2007a). In a nonoptimized fed-batch culture of C. protothecoides, the record cell density, biomass productivity, and oil productivity were achieved by Yan et al (2011), namely, 97.1 g L-1, 12.8 g L-1 day-1, and 7.3 g L-1 day-1, respectively. Later, using a nonlinear-mode-based optimi­zation approach, De la Hoz Siegler et al. (2012) maximized the cell density and oil productivity of fed-batch culture of C. protothecoides to 144 g L-1 and 20.2 g L-1 day-1.

Selection of appropriate inoculum and mode of cultivation are the key aspects involved in microalgae cultivation, which comes under preharvesting. Followed by preharvesting, the

FIGURE 8.9 Schematic view of the processes involved in microalgae processing, from algae biomass cultiva­tion to biodiesel production

Extraction

processes for converting the algae biomass to biodiesel are crucial and involve a series of se­quentially integrated post-harvesting steps: harvesting, drying, cell disruption, extraction, and transesterification, followed by the characterization of the fuel (Figure 8.9). These post-harvesting steps can be performed in different ways depending on the strain, substrate, and extraction method employed. Harvesting algal biomass could be the most energy­demanding process due to its concentration, smaller size, and surface charge, especially when the cultures are operated in open pond systems (Singh et al., 2011). Flocculation, sedimenta­tion, and filtration are the common harvesting techniques that are widely used (Harun et al.,

2010) . Drying the biomass prior to extraction is a prerequisite so as to avoid moisture inter­ference with the solvents. Drying can be performed using dryers or by exposing the biomass to diffused solar drying. Exposure to solar drying minimizes the production cost as well as power consumption. Subsequent to drying, cell disruption, oil extraction, transesterification of oil to fuel, and characterization of the fuel are explained in the following sections.

Agar is a mixture of polysaccharides, typically extracted from the cell walls of red algae; it is composed of agarose and agropectin and exhibits structural and functional properties sim­ilar to those of carrageenans. Like carrageenans, agar is also extracted with hot water. The genera Gelidium and Gracilaria are the major commercial sources of agar (Carlsson, 2007). Like carrageenans, agar is used as stabilizers for emulsions and suspensions and as gelling agents. Approximately 90% of all agar produced worldwide is intended for food applications; the remainder is used in the manufacture of capsules for medical applications and as a medium for cell cultures (Carlsson, 2007). Agar affects absorption of ultraviolet radiation (Murata and Nakazoe, 2001) and exhibits a few bioactivities as well (see Table 10.2).

Algal biomass is attractive for renewable liquid fuels, but much water accompanies this aquatic biomass feedstock. The energy requirements for drying algae are very high, which militates against a large-scale fuel production process employing this step. Thus, there is a need for processes that convert wet algal biomass directly and therefore operate in the aque­ous phase. Two major considerations of the emerging algal biofuel industry are the energy­intensive dewatering of the algae slurry and nutrient management. The process is suitable for high-moisture aquatic biomass such as defatted microalgae and macroalgae because the bio­mass is processed as slurry in hot compressed water.

CO2 must be supplied to the growth medium to reach high algal productivities. It has been shown that, provided that pH is regulated, the microalgae can be very tolerant to the source of CO2 (Doucha et al., 2005). However, the dissolution efficiency, together with the ability of microalgae to consume this CO2, are very dependent on the cultivation system. The supply rate in the LCA studies ranges from 0.51 to 2.36 kgCO2 kgDM-1. Depending on the studies, CO2 is supplied from compressed and purified gas or from the flue gas of a local power plant, either after capture or directly (Table 13.6). The percentages of CO2 in the flue gas vary from 5% (Stephenson et al., 2010) to 15% (Brentner et al., 2011; Campbell et al., 2011). It is common to point out the lack of knowledge of the long-term consequences on algae and on culture facility due to the use of flue gas. However, Yoo et al. (2010) demonstrated that Botryococcus braunii and Scenedesmus sp. could grow using flue gas as a source of carbon. Energy costs of

injection and head losses are always taken into account. The injection of flue gas in the growth medium without prior enrichment or compression requires compressing higher volumes of gas and reduces the efficiency of the gas-injection system. Hence there is a clear trade-off in terms of energy consumption between prior purification and gas injections. Some authors (Kadam, 2002; Brentner et al., 2011; Clarens et al., 2011) include in their study the costs of purification and transport.