Bubble column PBRs are more widely used than other reactors. In them, mixing and CO2 mass transfer are carried out through spargers with an external light supply (Nigar et al., 2005; Doran, 1995). Photosynthetic efficiency depends on gas flow rate, which further de­pends on the light and dark cycle as the liquid is circulated regularly from central dark zone to external photic zone at higher gas flow rate (Janssen et al., 2003). Photosynthetic efficiency can be increased by increasing the gas flow rate (>0.05 m/s), leading to shorter light and dark cycles. Degen et al., 2001 used a bubble column photobioreactor to improve light utilization efficiency of the strain Chlorella vulgaris through a flashing-light effect in batch mode opera­tion and achieved 1.7 times higher productivity of biomass (Degen et al., 2001).

Biochar is a solid material obtained as the product of carbonization of biomass. This ma­terial can adsorb fatty acids, thus unfolding a potential application as hydrophobic adsorbent for use in water and air purification systems. Despite biochars possessing a relatively hydro­phobic core, they are wetted by water due to such hydrophilic functional groups as carboxylic acids, aldehyde, and hydroxyl on the surface (Mursito, Hirajima et al., 2010), so these biochars may be useful as reenforcing additives in cement and organic polymers. The low ash content of carbonized char (Heilmann, Jader et al., 2011) also points to potential application as a car­bon source for production of synthesis gas or as an alternative to coke in steel manufacture. This material may be easily stored in subterranean locations, thus entailing a form of carbon sequestration; it may also be applied in soil amendment, since it is rapidly attacked by soil microorganisms (Heilmann, Jader et al., 2011).

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.

Microalgal cultures are susceptible to contamination by different species of microalgae, bacteria, viruses, fungi, protozoa, and rotifers. The contamination by other microorganisms can cause changes in the cell structure and reduce the concentration and microalgal yield in just a few days (Park et al., 2011). These are controlled in open ponds by effectively operating the system as a batch culture and restarting the culture at regular intervals with fresh water and unialgal inoculum. Other contaminants include insects, leaves, and airborne material. It is essential to control this contaminants within acceptable limits. In open ponds, large con­taminants can be removed regularly by placing a suitably sized screen in the water flow. This can be done manually or it can be automated.

Some characteristics can make cultures more susceptible to contamination, such as cultures in continuous mode. According to the characteristics of the microalgal species used, one can apply techniques to maintain an axenic culture. Some of these techniques are maintaining the process of cultivation at an alkaline pH (9.0 to 11.0), using high concentrations of nutrients or salinity, and using antibiotics. The photobioreactor must be periodically cleaned to minimize the chances of contamination (Wang et al., 2012).

If the microalgal biomass is applied to products such as biofuels, waste treatment, biofertilizers, or biofixation of CO2, impurities are acceptable in the microalgal cultivation. However, for bioproducts such as drugs and food, crops must be kept in axenic cultures (Wang et al., 2012).

Despite the fact that great progress has been made in developing photobioreactors for mass production of microalgal cells, more efforts are still required for further improvement, espe­cially regarding the cost reduction of bioreactor design. For large-scale outdoor microalgae cultivation, large amounts of required land space are still the critical issue. In addition, since outdoor photobioreactors usually utilize natural solar light and without additional temper­ature control, the growth of biomass would greatly depend on weather conditions and am­bient temperature. Due to these limitations, in most regions of the world it is not feasible to have stable microalgal biomass production through outdoor mass cultivation. In addition, the potential contamination is also a serious threat to the operational success of outdoor open ponds or raceways. In contrast, closed system photobioreactors have the advantages of better operational stability and condition control. However, the high equipment cost and process cost of closed photobioreactors are still barriers impeding the mass cultivation of microalgae. Finding more rigid, reliable, and transparent materials with lower costs for the design of closed photobioreactors is crucial to enhance cultivation efficiency and to reduce the cost of photobioreactors for the development of closed systems for the autotrophic cultivation of microalgae.

In deep-bed filtration, algae particles are harvested in a depth filter. Smaller than the medium openings, algal particles flow into the medium and are retained within the filter bed. Deep-bed filtration is most often operated as a batch process. When the pressure drop reaches the maximum available, the filter must be taken out of service for backwashing.

Harris et al. (1978) and Reinolds et al. (1974) reported successful separation of algal cells from pond effluent with average solids concentration of 30 mg/L by intermediate sand filtration. The filtration systems, however, rapidly experienced a severe clogging problem and filtration flux dropped drastically.

Intermittent sand filtration was also investigated in a wastewater treatment plant upgrading (Middlebrooks and Marshall, 1974; Marshall and Middlebooks, 1973). The inves­tigation revealed that only large algal particles can be harvested by separating the dried cake from the surface of the bed. Fine algal particles infiltrated and trapped within the bed could not be efficiently harvested.

To successfully make the transition from fossil fuels to biofuels, it is necessary to achieve a similar or better quality product (chemical and physical characteristics) for at least the same price. This shift toward biofuels will take place if petroleum prices increase so much that the prices of petroleum-derived fuels become greater than those of biofuels. Unfortunately, the eco-friendly characteristics of biofuels (renewable sources and less pol­luting gas emissions) are not sufficient to lead the transition if no economic benefit is generated.

If we examine gasoline prices since 1997, the strong price increase becomes clear (Figure 7.1). It is accepted that prices of petroleum-based fuels will keep increasing, a situa­tion that forces humankind to search for new sources of energy.

Carbon dioxide usually behaves as a gas in air at standard temperature and pressure (STP) or as a solid called dry ice when frozen (Sahena et al., 2009; Mendiola et al., 2007). If the tem­perature and pressure are both increased from STP to at or above the critical point for carbon dioxide, CO2 can adopt properties midway between a gas and a liquid and behave as a su­percritical fluid, expanding like a gas but with a density like that of a liquid. Supercritical CO2 is becoming an important commercial and industrial solvent due to its role in chemical extraction in addition to its low toxicity and environmental impact (Cooney et al., 2009). The relatively low temperature of the process and the stability of CO2 also allow most com­pounds to be extracted with little damage or denaturing. The main drawbacks of this method include high power consumption and expense and difficulty involved in scaling up at this time (Eller, 1999).

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).