Biodiesel typically consists of a mixture of fatty acid alkyl esters obtained by transesteri­fication of oils or fats, which are normally composed of 90-98% triglycerides, much smaller levels of mono — and diglycerides and free fatty acids, and residual amounts of phospholipids, phosphatides, carotenes, tocopherols, sulphur compounds, and water (Bozbas, 2008).

The biomass left after biodiesel production takes the form of an oilcake containing glycerol as a byproduct of transesterification. This compound may then be refined and sold to the pharmaceutical industry or else used livestock feed (Shirvani, Yan et al., 2011). The oilcake stores 35-73% of its total energy as carbohydrates and proteins (Hu, Sommerfeld et al., 2008), and three distinct options may be considered: (1) an adjacent coal-fired power system that co-fires the left biomass (Xu et al., 2006); (2) a direct combus­tion of the oilcake in an integrated biomass-heating system (Amaro et al., 2011); or (3) a bio­mass combined heat and power unit (Huang and Wang, 2004) for energy cogeneration (and, indirectly, electricity production) (Bozbas, 2008; Mata, Martins et al., 2010; Yang, Guo et al., 2011).

Despite these possibilities, most oilcakes from microalgal origin are fermented into ethanol or methane or else to H2 via anaerobic digestion. Alternatively, they may be incorporated in livestock feed or simply used as organic fertilizer, owing to a particularly high N:P ratio.

In this section, some aspects of algal biology and biochemistry are introduced in view of their relevance to the underlying economics. The composition of algal biomass in terms of polysaccharides, proteins, lipids, pigments, iodine, phenols, and halogenated compounds is expected to critically determine its overall value.

10.3.1 Polysaccharide Material

Algae contain large contents of polysaccharides, notably as contributors to cell-wall structure but also as storage polysaccharides (Holdt and Kraan, 2011). Polysaccharides are polymers of simple sugars (monosaccharides) linked by glycosidic bonds. They entertain nu­merous commercial applications as stabilizers, thickeners, and emulsifiers in food (including beverages) and feed (Tseng, 2001).

Different groups of algae produce specific types of polysaccharides; for example, green algae produce starch for energy storage, which consists of both amylose and amylopectin, in a way similar to higher plants (Williams and Laurens, 2010). Their total concentrations range from 4-76% of dry weight (Holdt and Kraan, 2011).

On the other hand, macroalgae have a low lipid content, and even though their carbohy­drate content is normally high, most of it is accounted for by dietary fibers that are not taken up by the human body but are rather utilized as a bulking agent (Holdt and Kraan, 2011).

The polysaccharides consist mainly of cellulose and hemicelluloses as well as neutral polysaccharides, yet cell-wall and storage polysaccharides are species-specific: green algae may contain sulphated galactans and xylans, whereas brown algae may have alginic acid, fucoidan (sulphated fucose), laminarin (p-1,3 glucan), and sargassan, and red algae may contains agars, carrageenans, xylans, floridean starch (amylopectin-like glucan), and water- soluble sulphated galactan as well as porphyran (Chandini, Ganesan et al., 2008). Cyanobacteria can produce cyanophycin and multi-L-arginyl-poly-L-aspartic acid (Williams and Laurens,

2010) , but the contents of both common and species-specific polysaccharides undergo seasonal variations (Holdt and Kraan, 2011).

Algal polysaccharides can be classified as dietary fibers and hydrocolloids, as is done in the following sections, but they usually possess more than just one type of functional group.

10.3.1.1 Dietary Fibers

These kinds of polysaccharides are very diverse in chemical structure and in composition in the algal biomass. Edible marine macroalgae contain 33-62% total fibers (on a dry-weight basis), quite a bit higher than in higher plants, and such fibers are rich in soluble fractions (Dawczynski, Schubert et al., 2007). Recall that dietary fibers maybe insoluble (e. g., cellulose, mannans, and xylan) or water-soluble (e. g., agars, alginic acid, furonan, laminaran, and porphyran, addressed in further detail in the next subsection).

These algal fibers are commonly extracted by precipitation, as described by Venugopal (2008), and may be used as nutraceuticals for functional food formulation (Holdt and Kraan,

2011) . Examples of polysaccharides bearing antitumor and antiherpetitic bioactivity (among others) are tabulated in Table 10.2.

Water at high temperatures becomes a good solvent for hydrocarbons that are typically nonpolar hydrophobic under standard environmental conditions. Ionic reactions of organics should be favored by increased solubility in water. The enhancement of this solubility of hydrocarbons in water will further enhance the possibilities of contact of dissociated H+ with hydrocarbons and hence accelerates the activities of hydrolysis. Water has the ability to carry out condensation, cleavage, and hydrolysis reactions and to affect selective ionic chemistry. This is largely due to changes in its chemical and physical properties, which become more compatible with the reactions of organics as the temperature is increased.

Hot water as a reactant and catalyst likely creates a second pathway for the cascade of molecular transformations that leads to oil. In this pathway, water causes organic material to disintegrate and reform (by adding H+ to an open carbon bond) into fragments, which then transform into hydrocarbons. This implies that hot water becomes a catalyst for a series of ionic reactions. The acidic and basic nature of hot water—rather than heat—drives this cas­cade. For example, water may function first as a base, nibbling away at certain linkages in the organic material. As new molecular fragments build up and modify the reaction environ­ment, water can change its catalytic nature. It can then act as an acid, accelerating different reactions. The resulting products attack parts of the remaining molecules, further speeding the breakdown (Siskin and Katritzky, 1991).

The exact pathways of HTU to produce crude oil from biomass remain unclear, and addi­tional research is needed. The following examples may give some hints of possible pathways of HTU of waste biomass feedstock. The basic reaction mechanism can be described as depoly­merization of the biomass; decomposition of biomass monomers by cleavage, dehydration, de­carboxylation; and deammination and recombination of reactive fragments (Toor et al., 2011).

In a study by Appell et al. (Appell et al., 1975), one of the mechanisms for the conversion of carbohydrates into oil that was consistent with the results is as follows.

Sodium carbonate reacts with carbon monoxide and water to yield sodium formate:

Na2CO3 + 2CO + H2O! 2HCO2Na + CO2

Vicinal hydroxy groups in the carbohydrates undergo dehydration to form an enol followed by isomerization to a ketone. The following reaction will be initiated with the attack of H + on the compound with vicinal hydroxyl groups; the water molecule will be eliminated to form carbocation, and further rearrangement is:

H H H H H H

—— C—C——— ► ———- C—— C C^=C——— ——— ► ——— C— C——

-H2O + I — H+

OH OH OH OH HO

The hydroxyl ion then reacts with additional carbon monoxide to regenerate the formate ion:

OH~ + CO! HCO-

A variety of side reactions may occur, and the final product is a complex mixture of com­pounds. One of the beneficial side reactions occurring in alkaline conditions is that the car­bonyl groups tend to migrate along the carbon backbone. When two carbonyl groups become vicinal, a benzylic type of rearrangement occurs, yielding a hydroxy acid. The hydroxy acid readily decarboxylates, causing a net effect of reducing the remainder of the carbohydrate — derived molecule.

This type of reaction is beneficial to HTU because it leads to the formation of paraffin-type structures, which have less oxygen than the original compounds. In addition, the reaction happens by disproportionation and does not require any additional reducing agent. Aldol condensation may also be part of the reaction process. Aldol condensation occurs between a carbonyl group on one molecule and two hydrogens on another molecule with the elimi­nation of water. The condensation product is a high-molecular-weight compound, typically with high viscosity. Condensation reactions become a major pathway in the absence of reduc­ing agents such as carbon monoxide and hydrogen. Reducing agents keep the carbonyl con­tent of the reactant system sufficiently low so that liquid instead of solid products are formed.

In a study by Appell et al. (Appell et al., 1980), the authors believed that the free hydrogen radical (H*), not the hydrogen molecule (H2), participates in the chemical conversion reac­tions. Thus, they concluded that the addition of carbon monoxide (CO) to the process was more efficient than the addition of hydrogen gas. Based on the water-gas shift reaction, carbon monoxide reacts with water to form carbon dioxide and two hydrogen radicals:

C=O + H-O-H O=C=O + 2H-

In the presence of the hydrogen radicals, the oxygen is removed from the compounds containing carbonyl and hydroxyl groups, then forms paraffin and water. A possible pathway is described in the following four reactions (He, 2000):

O

I II II

— C — C + 2H’ —- ► —C = C— + H2O

Keto group

Hydroxyl group

Table 13.2 specifies the electricity and heat sources used in the publications in our study. The electricity mix is determined by the country where the production is supposed to take place; in some publications, electricity and heat consumption are totally or partially covered by internal production from the microalgae, either by anaerobic digestion of the oilcakes

NC = Not communicated.

(Stephenson et al., 2010; Brentner et al., 2011; Campbell et al., 2011; Clarens et al., 2011) or of the algal biomass (Clarens et al., 2011; Collet et al., 2011) or by direct combustion of microalgal biomass or extraction residue (Clarens et al., 2011).

Most of the authors (Lardon et al., 2009; Baliga et Powers, 2010; Sander and Murthy, 2010; Stephenson et al., 2010; Khoo et al., 2011) underlined the important contribution of energy consumption to the global-warming potential of algal energy productions. The sensitivity of this choice has been assessed with inventories from the EcoInvent database and the ReCiPe impact assessment method (Goedkoop et al., 2009) in a hierarchical perspective. With this perspective, characterization factors of the global-warming potential are the ones defined by the IPCC (IPCC, 2006). As shown in Figure 13.3, climate change impact can vary by a factor of two according to the chosen electric mix. Consequently, the potential reduction of green­house gases by producing bioenergy from microalgae is strongly correlated with the origin of the electricity. It is important to note that the variations of endpoint impacts (i. e., human

100

90

80

70

60

50

40

30

20

10

0

health, ecosystems, and resources) are almost identical to that of the climate change impact. This underlines the strong dependence of all the impacts on the energy mix composition.

There is limited literature on algae sedimentation in ponds without any flocculation process. Isolation of facultative oxidation pond from inflow feed to promote water clarifica­tion was investigated (Koopman et al., 1978). Operations involving fill-and-draw cycles for secondary ponds gave rise to significant removal of algae from facultative oxidation pond effluent (Benemann et al., 1980).

Similar secondary ponds were used for algae settling from high rate oxidation pond effluent (Adan and Lee, 1980; Benemann et al., 1980). Well-clarified effluent and algae slurry of up to 3% solids content were achieved at the secondary ponds attributable to algae autoflocculation, which enhanced the settling. The autoflocculation phenomenon is distinctly different from the coprecipitative autoflocculation suggested by Sukenik and Shelef (1984), as discussed earlier. The autoflocculation mechanism involved remained unclear (Eisenberg et al., 1981).

Coagulant dosing to a settling tube to promote algae sedimentation was looked into by Mohn (1980). The batched operation achieved an algal concentration of 1.5% solids content. Algae separation by sedimentation tanks or tubes is considered a simple and inexpensive process. Its concentrating reliability is low without coagulant dosing. Algae autoflocculation may be used as an inexpensive reliable algae separation method. However, the natural flocculation processes should be closely studied and well understood before it can be incor­porated for primary concentration.

Pyrolysis is a physical-chemical process in which biomass is heated to between 400°C and 800°C, resulting in the production of a solid phase rich in carbon and a volatile phase

composed of gases and condensable organic vapors (Mesa-Perez et al., 2005). These organic vapors condensate in two different phases: bio-oil and acid extract (Beenackers and Bridgwater, 1989).

Through pyrolysis, carbon-carbon bonds are broken, forming carbon-oxygen bonds. It is a redox process in which part of the biomass is reduced to carbon (coal) while the other part is oxidized and hydrolyzed yielding phenols, carbohydrate, aldehydes, ketones, and carboxylic acids, which combine to form more complex molecules such as esters and polymers (Rocha et al., 2004).

Due to the extreme conditions to which biomass is submitted, many simultaneous reac­tions occur, resulting in gaseous, liquid, and solid products:

1. Gas phase. Consists primarily of low-weight products that have moderate vapor pressure at room temperature and do not vaporize at pyrolysis temperature.

2. Liquid phase. Further subdivided into two other phases determined by density differences:

• Bio-oil, which is a mixture of many compounds with high molecular weight that became vapors at pyrolysis temperature but condense at room temperature.

• Acid extract (or aqueous extract), which consists of an aqueous phase with numerous soluble and/or suspended substances.

3. Solid phase. Also known as biochar, the solid phase is composed of an extremely porous matrix, very similar to charcoal (DalmasNeto, 2012).

Pyrolysis conditions can be manipulated to produce preferably one phase or the other. Residence time is one of the factors that most influence the final result. To produce incondensable gases, high residence time at high temperature is generally used; higher yields of solids are generally achieved by very high residence time at low temper­atures (allowing polymerization reactions) (Sanchez, 2003). For preferential production of the liquid phase, fast pyrolysis is often chosen. Table 7.1 summarizes the conditions and main effects of residence time and temperature in gaseous, liquid, and solid product generation. Other pyrolysis technologies and their characteristics are presented in Table 7.2.

Due to its tendency to preferentially form bio-oil, coupled with high-speed reaction and greater productivity, fast pyrolysis is the best model for the production of biofuels from algae.

Pulsed electric field (PEF) processing is a method for processing cells by means of brief pulses of a strong electric field (Guderjan et al., 2007). Algal biomass is placed between two electrodes and the pulsed electric field is applied. The electric field enlarges the pores of the cell membranes and expels its contents (Guderjan et al., 2004).

8.Є.2.8 Enzymatic Treatment

Enzymatic extraction uses enzymes to degrade the cell walls, with water acting as the solvent (Mercer and Armenta, 2011). This makes the fraction of oil much easier. The combi­nation of "sono-enzymatic treatment" causes faster extraction and higher oil yields compared to individual ultrasonication and enzymatic extractions alone (Fajardo et al., 2007). The draw­backs associated with the process are lack of commercial feasibility and inapplicability for mass cultures (Halim et al., 2011).

Glycolipids are carbohydrate-attached lipids that can be extracted from algal biomass. Their role is to provide energy and to serve as markers for cellular recognition owing to their association with cell membranes.

Red algae contain monoglycosyldiacylglycerol (MGDG), diglycosyldiacylglycerol (DGDG), and sulphaquinovosyldiacyl-glycerol at essentially similar levels. Conversely, MGDG and DGDG are the chief glycolipids in green algae. On the other hand, the MGDG content of brown algae varies from 26-47%, the DGDG content from 20-44%, and the ulphaquinovosylglycerol content from 18-52% of the total glycolipids (Dembitsky and Rozentsvet, 1990).

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.

Continuous discharge of solids as a slurry is possible with the nozzle-type disc centrifuge. The shape of the bowl is modified so that the slurry space has a conical section that provides sufficient storage volume and affords a good flow profile for the ejected cake (Shelef et al., 1984). The bowl walls slope toward a peripheral zone containing evenly spaced nozzles. The number and size of the nozzles are optimized to avoid cake buildup and to obtain reasonable concentrated algal biomass.

The application of a nozzle-type disc centrifuge for algae harvesting was suggested by Golueke and Oswald (1965). The influence of nozzle diameter on flow rate, algae removal efficiency, and resultant slurry concentration was looked into. Through comparison with other algae harvesting methods, it was concluded that the nozzle-type centrifuge seemed promising, albeit it is less attractive because of power requirements and capitalization costs. In other studies, the centrifuge appeared to be more effective to harvest Scenedesmus than Coelastrum (Mohn and Soeder, 1978; Mohn, 1980). By returning the centrifuge underflow to the feed, the solids content of the algae suspension (0.1%) can be concentrated by a factor of 15-150%. The reliability of this device can be ensured as long as the clogging of the nozzles is avoided.