Employing relatively moderate conditions of temperature (ca. 200°C), time (<1 h), and pressure (<2 MPa), microalgae can be converted in an energy-efficient manner into an algal char product that is of bituminous coal quality. Potential uses for the product include creation of synthesis gas and conversion into industrial chemicals and gasoline; application as a soil nutrient amendment; and as a carbon-neutral supplement to natural coal for generation of electrical power. Some strains of cyanobacteria also provided high-quality chars, but yields were only half those obtained with green microalgae (Heilmann et al., 2010).

The inventory phase allows the estimation of all resources, products, and emissions re­quired for the production of one unit of the FU. This inventory phase will be used to deter­mine potential environmental impacts, including global-warming potential, and the energy balance. In addition to the variability stemming from different process designs or parameter assumptions, the way of handling coproducts and the actual method chosen to assess energy balance or environmental impacts will strongly affect the conclusions.

Microalgae grow in soil unfit for agriculture and livestock and in lakes or ponds located in inhospitable lands, such as deserts, which are usually unsuitable for generating any kind of food. Microalgae can double their biomass in a period of 3.5 days, achieving high yields (Chisti, 2007). After harvesting and drying of the biomass, the final state of the product is a powder. According to the chemical composition of microalgae, the biomass may have several applications.

Studies on microalgae are preferably done under controlled conditions. Microalgae biore­actors are often designed differently from bioreactors used to grow other microorganisms. Two parameters are the most important in algae cultivation: efficiency of light utilization and availability of dissolved CO2.

Like any organism, microalgae have nutritional requirements: carbon sources, energy, water, and inorganic nutrients. In the case of microalgae, the carbon source can be CO2 and the energy comes from sunlight. As microalgae grow in aqueous suspension, the manip­ulation and control of culture conditions makes their cultivation feasible, thus the productiv­ity is limited mostly by the available of light. Responses by algal cells to nutrients and cultivation environments can be used to manipulate the processes to favor the production of algal biomass (Benemann et al., 2002).

The development of media for microalgae cultivation involves a sufficient carbon source (carbon is a part of all the organic molecules in the cell, making up as much as 50% of the algal biomass); salt concentration (depending on the original biotope of the alga); nitrogen (represents about 5-10% of microalgae dry weight); phosphorus (part of DNA, RNA, ATP, cell membrane); sulfur (constituent of amino acids, vitamins, sulfolipids and is involved in protein biosynthesis); potassium (cofactor for several enzymes and involved in protein syn­thesis and osmotic regulation); magnesium (the central atom of the chlorophyll molecule); iron (constituent of cytrochromes and important in nitrogen assimilation); pH of the medium; temperature; trace elements, and addition of organic compounds and growth promoters.

Carbon is important because it is the source of energy for many cellular events (such as metabolites production) and reproduction and is part of the physical structure of the cell. In conditions of low dissolved inorganic carbon (DIC), a DIC transport is induced in most microalgae (Matsuda and Colman, 1995), allowing normal cell growth.

Depending on the material used in cultivation of microalgae and the utilization of biomass, three different systems can be distinguished (Becker, 1994):

1. Systems in which a selected algal strain is grown in a so-called clean process, using fresh water, mineral nutrients, and carbon sources. The algae in such systems are intended to be utilized mainly as food supplements.

2. Systems using sewage or industrial wastewater as the culture medium. The cultivation of the microalgae involves secondary (BOD removal) and tertiary (nutrient removal) treatments and production of biomass-based products.

3. Cultivation of algae in enclosed systems under sunlight or artificial light, with cells preferably being grown in autotrophic media.

Microalgae are microorganisms that are capable of producing many different compounds of industrial interest, some with high and some with low aggregated value. The final value of the product and its destination directly influence the conditions of cultivation. Therapeutical compounds produced by microalgae, for example, must be produced through a totally con­trolled and clean process, whereas for the fuel industry residues can be used and the control of the process can be less accurate. The low culture concentration and the corresponding high downstream costs define production trends.

The utilization of complex media (those of which the composition is not determined, such as industrial residues) in the cultivation of microalgae is one alternative to make the produc­tion of some microalgal metabolites economically feasible. Associated with residue compo­sition and microalgae metabolism, knowledge of the needs of the microalgae might save time (and money) in the development of a process. It is very important to supply all microalgae chemical needs because it is known that variations in the chemical composition of phytoplankton are also tightly coupled to changes in growth rate (Goldman et al., 1979).

The solid-bowl decanter centrifuge is characterized by a horizontal conical bowl containing a screw conveyor that rotates in the same direction. Feed slurry enters at the center and is spun against the bowl wall. Settled solids are moved by the screw conveyor to one end of the bowl and out of the liquid for drainage before discharge, while separated water forms a concentric inner layer and overflows an adjustable dam plate. The helical screw conveyor pushing the deposited slurry operates at a higher rotational speed than the bowl.

A solid-bowl screw centrifuge was used to separate various types of algae (Mohn, 1980). Fed with an algal suspension of 2% solids, the separated algal slurry was able to attain 22% solids concentrations. Although the reliability of this centrifuge seems to be excellent, the energy con­sumption is too high. An attempt to concentrate an algae feed of 5.5% solids derived from a flotation process by a co-current solid-bowl decanter centrifuge was not successful (Shelef et al., 1984). Subsequently, algae slurry concentration was improved to 21% TSS by reducing the scroll speed to 5 rpm (Shelef et al., 1984). The solid-bowl decanter centrifuge was recommended for use concurrently with polyelectrolyte coagulant to increase the efficiency.

The most common procedure for cultivation of microalgae is autotrophic mode. Microalgae in photoautotrophic nutrition mode use sunlight as the energy source and inor­ganic carbon (CO2) as the carbon source to form biochemical energy through photosynthesis (Huang et al., 2010). This is one of the most prevailing environmental conditions for the usual growth of microalgae (Chen et al., 2011). In photoautotrophic nutritional mode, photosynthet­ically fixed CO2 in the form of glucose serves as a sole energy source for all metabolic activities (Figure 8.6). The simpler form of photosynthate, such as simpler carbohydrates, serves as sole energy source for carrying out the metabolic activities of the algal cells (Chang et al., 2011). These carbohydrates, under nutrient-limiting and stress conditions, will favor the lipid biosynthesis, which also helps to cope — up with the stress (Gouveia and Oliveira, 2009). Lipid productivity greatly depends on the photosynthetic activity in terms of atmospheric CO2 fix­ation and microalgae species. Large variations in lipid productivity, ranging from 5% to 68%, were reported under varying operating conditions and species diversity (Murata and Siegenthaler, 2004; Ohlroggeav and Browseb, 1995; Chen et al., 2011; Mata et al., 2010). A major advantage of the autotrophic nutritional mode is the algal oil production at the expense of atmospheric CO2. Large scale microalgae cultivation systems (such as open/raceway ponds) are usually operated under photoautotrophic conditions (Mata et al., 2010). Autotro­phic nutritional mode also has fewer contamination problems compared with other

Autotrophic Nutrition

Calvin

Cycle

FIGURE 8.6 Autotrophic mode of nutrition in microalgal cells towards CO2 fixation and lipid biosynthesis

nutritional modes. Under autotrophic nutrition, the photosynthates also get consumed dur­ing respiration associated with the biomass growth, and hence the lipid productivity repre­sents the combined effects of oil content and biomass production (Chiu et al., 2008).

It has been established that electrons are derived from water upon photochemical oxidation by PSII or so-called water plastoquinone oxidoreductase (PQOR) and are trans­ferred to the [Fe]-hydrogenase, leading to the photosynthetic hydrogen production in the direct photolysis process. Apart from the previously described PSII-dependent hydrogen production, catabolism of endogenous substrate and the associated oxidative carbon metab­olism in green algae may generate electrons for the photosynthetic systems (Gfeller and Gibbs, 1984; Melis, 2002).

Electrons generated from such an endogenous substrate catabolism flow into the PQ pool between photosystems PSI and PSII (Stuart and GaDron, 1972; Godde and Trebst, 1980). An NADPH-PQOR that has been ascertained in vascular plant chloroplasts supplies elec­trons to the PQ pool (Shinozaki et al., 1986; Kubicki et al., 1996; Neyland and Urbatsch, 1996; Endo et al., 1998; Field et al., 1998; Sazanov et al., 1998). Daylight assimilation by PSI and the associated electron transfer raise the redox potential of these electrons to the equivalent level of ferredoxin and the [Fe]-hydrogenase. Functioning as the terminal electron acceptor, the hydrogen ions (protons) would lead to the production of molecular hydrogen (Gfeller and Gibbs, 1984; Bennoun, 2001; Gibbs et al., 1986). It has been found that in the presence of the PSII inhibitors 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), which impede photosynthetic electron flow from PSII to the PQ pool, the process generates molecular hydrogen and carbon dioxide in a stoichiometric ratio of 2 to 1 (Bamberger et al., 1982). Thus, following a dark incubation of the culture under anaerobic conditions and the ensu­ing induction of the [Fe]-hydrogenase, considerable rates of hydrogen generation can be captured upon illumination of the algae in the presence of DCMU (Happe et al., 1994; Florin et al., 2001).

Due to the dramatic increase in primary energy consumption and the increasingly strict environmental issues triggered by fossil-fuel sources, it is our firm belief that development of algal biofuel is urged. As discussed in this chapter, the main challenges pertaining to algal biofuel viability entail lower environmental impact beyond a number of associated benefits (namely, CO2 reduction and wastewater treatment), which may contribute to ensuring eco­nomic competitiveness.

However, associated with biofuel production is the spent biomass that is produced, with huge potential in terms of applications—from secondary biofuels through feed formulations and fine chemicals to bioremediation purposes. Therefore, biofuel production using spent biomass entails a strong economic interest, as thoroughly discussed in this chapter.

For competitiveness in this algae-based scenario, industry should follow an integral upgrade approach via implementation of an algal-based biorefinery, thus maximizing the economic return on all components of algal biomass, aiming at the point of zero residues.

A careful analysis of the current state of the art indicates that it is difficult to develop algal biofuel to the point where it can fully replace fossil fuels, in either developing or developed economies. Governments should indeed adopt an affirmative action by enforcing carbon taxes to limit use of fossil fuels as well as subsidizing investment, funding R&D efforts, and promoting consumption of renewable energies. Multilateral alternative energy develop­ments will probably be necessary to fully address the CO2 emission objectives of the Copenhagen Agreement and the Kyoto Protocol—and extensive cultivation of algae could play a central role in that process.

Acknowledgments

This work received partial funding from project MICROPHYTE (ref. PTDC/EBB-EBI/102728/2008), coordinated by author F. Xavier Malcata and under the auspices of ESF (III Quadro Comunitario de Apoio) and the Portuguese State.

A postdoctoral fellowship (ref. SFRH/BPD/72777/2010), supervised by author F. Xavier Malcata and cosupervised by author Isabel Sousa-Pinto, was granted to author A. Catarina Guedes, also under the auspices of ESF. A Ph. D. fellowship (ref. SFRH/BD/62121/2009), further supervised by author F. Xavier Malcata and cosupervised by author Isabel Sousa-Pinto, was granted to author Helena M. Amaro, again under the auspices of ESF.

In situ transesterification, better known as reactive extraction, has been developed with the purpose of simplifying the biodiesel production process by allowing extraction and transesterification to occur in a single step, in which oil-bearing seeds or algal biomass are in direct contact with the chemical solvent in the presence of a catalyst (acid or base). Through intensive research in recent years, the optimum conditions for in situ transesterification have become well established for different edible and nonedible oil feedstock, such as jatropha (Shuit et al., 2010), soybeans (Haas and Scott, 2007), and castor (Hincapie et al., 2011). However, the main constraint in commercializing this technology is the requirement of a high volume of chemical solvent, and the process is limited to homogeneous catalyst usage only.

In situ transesterification of algal biomass has been explored to attain high biodiesel con­version, including optimization of the alcohol-to-lipid molar ratio, reaction temperature, catalyst loading, and the effect of the use of a cosolvent, microwave, and ultrasonication. In a study performed by Ehimen et al. (2010), dried Chlorella biomass was subjected to in situ transesterification, attaining 90% of biodiesel yield at a reaction temperature of 60 °C, a methanol-to-lipid molar ratio of 315:1, a H2SO4 concentration of 0.04 mol, and a reaction time of 4 h.

To further reduce methanol consumption for the in situ transesterification, adding a cosolvent to the reaction mixture is suggested to increase the solubility of the algal lipids in methanol, creating a single phase reaction that could subsequently improve the reaction mass transfer rate. A yield of approximately 95% Chlorella pyrenoidosa biodiesel was attained when hexane was used as a cosolvent (hexane-to-lipid molar ratio of 76:1). The methanol-to — lipid molar ratio was significantly reduced to 165:1 and the total reaction time was shortened to 2 h at a reaction temperature of 90 °C and a catalyst loading of 0.5 M H2SO4. Nevertheless, the presence of water in the reaction media could impede the in situ transesterification and cause negligible biodiesel conversion (Ehimen et al., 2010). Thus, extensive drying of algal biomass is absolutely necessary to facilitate biodiesel conversion by avoiding the occurrence of any side reactions and to simplify the subsequent refining processes (Lam and Lee, 2012).

Other technologies that could further improve the reaction conditions for in situ transes­terification of algal biomass are microwave irradiation (Patil et al., 2011a; Patil et al., 2012), ultrasonication (Koberg et al., 2011), and supercritical alcohol (Levine et al., 2010; Patil et al., 2011b). However, these technologies are still far from commercialization due to safety — and health-related problems.

F. G. Aden, J. M. Fernandez, E. Molina-Grima

Department of Chemical Engineering, University of Almeria, Almeria, Spain

13.2 INTRODUCTION

Microalgae have been proposed as the potential source for a wide range of products, rang­ing from fine chemicals and pharmaceuticals to nutraceuticals and additives, foods, and feeds and as a biofuel source as well as playing a role in wastewater treatment (Borowitzka, 1999; Richmond, 2000). However, of all these products and roles, only a few are performed on an industrial scale. Microalgae are produced as a source of certain carotenoids, such as p-carotene and astaxanthin; microalgae biomass is also produced as food in nutraceutial applications and as feed for aquaculture. The amount of microalgae produced worldwide for these markets is around 5 kt/year. The price of microalgae biomass ranges from €10-300/kg, and the size of these markets is from 10-50 kt/year (Pulz and Gross, 2004). The development of new applications for microalgae biomass can increase the present pro­duction capacity. Thus, large-scale markets such as energy or commodities have the potential to absorb enormous amounts of microalgae biomass—up to 104 kt/year—but the price of biomass in these markets is far lower, from €0.01-0.50/kg. For this reason, microalgae bio­mass production costs must likewise be reduced to comply with these markets (Chisti, 2007).

Even though biomass production is normally performed under continuous operation in order to maximize the system yield, some products can be produced by varying operation modes from discontinuous to continuous-discontinuous combinations, as is the case with astaxanthin. Whatever the final use of the microalgae biomass and whichever production mode is used, the steps required to produce it are the same. The culture medium has to be prepared and introduced into the photobioreactors, where the biomass is produced, then it has to be harvested and stabilized. Alternatively, it can be processed to create products according to adequate downstream schemes (see Figure 14.1). Each one of these steps requires

FIGURE 14.1 General scheme of microalgae biomass production systems. Major inputs are nutrients, water, and CO2 in addition to energy. Processes can be built to produce stabilized biomass or final products according to an adequate downstream process.

materials and energy input. In addition, waste released in each step has to be treated. Differ­ent possibilities exist for each of the necessary steps, the overall yield and cost of the finished product being a function of the final scheme used. For example, a culture medium might be prepared using fine chemicals, fertilizers, or wastes—the resultant costs using wastes being less but the final biomass quality produced significantly diminished.

In this chapter, the cost of producing microalgae biomass is reviewed for various applica­tions using various schemes. Analysis is performed based on (1) the product obtained, (2) the overall scheme of the process, and (3) the production capacity. In each case, the major factors determining total production costs are identified and strategies are discussed to reduce those costs.