12 1 Pierre Collet1, Daniele Spinelli2, Laurent Lardon1, Arnaud

Helias1’3 , Jean-Philippe Steyer1, Olivier Bernard4

1INRA UR0050, Laboratoire de Biotechnologie de l’Environnement,

Avenue des Etangs, Narbonne, France
2Department of Chemistry and Center for Complex System Investigation,
University of Siena, Siena, Italy
3Montpellier SupAgro, Montpellier, France
4INRIA BIOCORE, Sophia Antipolis Cedex, France

13.1 INTRODUCTION

Environmental impacts and depletion of fossil energies have promoted the development of alternative and renewable sources of energy. Nonetheless, it is clear now that the replacement of current fossil energy will require both the development of new strategies to reduce our global energy consumption and the development of a panel of renewable energy sources. Re­newable energy can be extracted from solar, wind, or geothermal energy. However, these en­ergy forms are globally hard to store and hence cannot yet replace our consumption of fossil fuel for some important functions, such as powering cars and planes.

So far, several paths have been explored to produce fuels from renewable sources, the most developed strategies leading to the production of so-called first — and second-generation biofuels. First-generation biofuels are based on fuel production (ethanol or methylester) from a currently cultivated and harvested biomass (e. g., corn, rapeseed). Second-generation biofuels correspond to the development of new energy production pathways from usual feed­stock not reclaimed by food production (e. g., straw or wood). The development of first- generation biofuels has been criticized—first, because of the direct competition they create with food crops in a context where food security is a raising concern, and second, because of their actual poor environmental performance. Indeed, inputs to production (e. g., fertilizer or pesticides), feedstock culture, harvest, and transformation imply fuel consumption and lead to new pollutant emissions (Borjesson and Tufvesson, 2011), especially emissions of greenhouse gases to the environment (Searchinger et al., 2008). Consequently, energy and en­vironmental benefits of these biofuels are limited. Second-generation biofuels improve envi­ronmental performance but are not free of criticisms.

These observations advocate for the necessity of systematically assessing new energy pro­duction paths with a life-cycle assessment (LCA) perspective, which means, first, the adoption of a cradle-to-grave perspective, that is, looking at resource depletion, energy consumption, and substance emissions of all the processes required to achieve the production and use of the fuel, and second, the assessment of several environmental impacts, not only global warming potential or net energy production. LCA is an ISO method developed with this aim: It allows the detection of pollution transfer from one step to another or from one kind of environmental impact to another. The European Directive on Renewable Energy (European Union, 2009), adopted in 2009, embraces LCA as a reference method to assess environmental impacts of biofuel and to meet greenhouse gas reduction objectives of 50% in 2017.

Third-generation biofuels correspond to the development of bioenergy productions based on new feedstock reputed to have lower land competition. Microalgae belong to this third category. Their very high actual photosynthetic yield and their ability to accumulate lipids, or, for some species, starch, added to the possibility that they can be cultivated in controlled environments, promise the potential of biofuel that has a low competition with food crops (Chisti, 2007) and limited environmental impacts. In addition, the ability to use CO2 directly from industrial emissions as a source of carbon for the growth of microalgae is a promising feature for flue-gas mitigation (Huntley and Redalje, 2007; Chisti, 2007). However, this promise should be challenged. So far, microalgae industrial production has been developed only for the production of high-value molecules (such as beta-carotenes) or dietary supplements (Spirulina or Chlorella can be found as pills in health shops); hence, energy or environmental performance has never been a concern. Moreover, the scale of the existing facilities is far smaller than that required for fuel pro­duction (at least several hundred ha). It is necessary to assess the expected environmental performance of these potential production systems in order to detect technological bottle­necks and to determine which processes should be optimized in priority. This approach is now necessary to design a sound, energetically efficient, and environmentally friendly bio­fuel production system.

Since the new focus of international scientific and economic communities on microalgae — based biofuel, many environmental, energy, or economical assessments have been published, with different final energy carriers or different production assumptions. Here we propose to review a set of publications, all of them published in peer-reviewed scientific journals, using the LCA method to assess the environmental impacts linked to microalgae-based biofuel. The lack of real industrial facilities dedicated to energy production from microalgae imposes the use of models and extrapolations to describe the production systems. In addition, system frontiers and coproduct management differ among the studies. Altogether, this leads to di­vergences between publication results. This review aims to identify and explain this variabil­ity and then to propose guidelines to improve future LCAs of algal-based bioenergy production systems. This work is a mirror of this diversity and underlines the difficulty in comparing different studies without common assumptions.

The driving force for vacuum filtration results from the application of suction on the filtrate side of the medium. Although the theoretical pressure drop for vacuum filtration is 100 kPa, it is normally limited to 70 or 80 kPa in actual operation (Shelef et al., 1984). Vacuum filtration can yield algal harvests with moisture contents comparable to those of pressure filtration at lower operating cost if the content of large algal cells in the feed is high.

Five different vacuum filters—vacuum drum filter (not precoated), vacuum drum filter precoated with potato starch, suction filter, belt filter, and filter thickener—have been tested for the harvesting of Coelastrum (Mohn, 1980). Suspended-solids content of the harvested al­gae was in the range of 5-37%. Based on energy consideration, reliability, and dewatering capability, the precoated vacuum drum filter, the suction filter, and the belt filter were recommended. The precoated filter can also be used to harvest tiny microalgae such as Scenedesmus (Shelef et al., 1984). The nonprecoated vacuum drum filter was ineffective and not reliable due to clogging problems. The filter thickeners were not recommended because of low solids content (3-7%) of the algal cake, low filtration velocity, high energy demand, and poor reliability.

Dodd and Anderson (1977) were the first to harvest microalgae by a belt filter precoated with eucalyptus and pine-crafts fibers. The use of a precoat was found to cause undesirable operational complexity and increased costs. In another study, fine-weave cloth rather than the precoated filter was investigated (Dodd, 1980). This method required a relatively low energy input and no chemicals were added. It was found to be efficient in harvesting larger species of algae such as Micractinium, but it had problems with fouling in smaller algal species such as Chlorella. Its capital costs are higher than dissolved-air floatation, but the operating expendi­tures are the lowest among all harvesting methods with the exception of natural settling (Dodd, 1980).

Over the last hundred years, world energy consumption has increased greatly. In just the last 38 years, energy demand increased 99%; between 1973 and 2011, consumption went from 6.111 to 12.150 million of tons of petroleum (International Agency of Energy, 2011). According to the same study, 81% of that energy came from fossil-dependent sources such as petroleum, coal, and natural gas.

The scientific community continues to discuss whether global warming is caused by the excessive increase of carbon dioxide in the atmosphere, but this idea is generally accepted. This situation has caused a rush to development of economically feasible and sustainable technologies, those independent of fossil sources. Among these new technolo­gies, microalgal technologies have gained importance and are being widely explored due to their capacity to absorb carbon dioxide from atmosphere via photosynthesis and their high capacity to accumulate lipids, which can in turn be transformed into different forms of energy.

The independence of organic carbon sources for growth opens the possibility to develop technologies using wastewater that are unfeasible for heterotrophic microorganisms. At the same time, microalgae have many advantages compared to vascular plants (Benemann and Oswald, 1996): All physiological functions are carried out in a single cell, they don’t differ­entiate into specialized cells and they multiply much faster, they carry low costs for harvest and transportation (Miyamoto, 1997), they consume less water (Sheehan et al., 1998), and they have the possibility to be cultured under conditions (such as infertile land) not suitable for the production of conventional crops (Miyamoto, 1997).

The Soxhlet extraction procedure is also used commonly for oil extraction. The goldfish extraction procedure may also be employed for this purpose. The Soxhlet extraction proce­dure is a semicontinuous process that allows the buildup of a solvent in the extraction chamber for 5 to 20 minutes (Additions and Revisions, 2002). The solvent surrounding the sample is siphoned back into the boiling flask. The procedure provides a soaking effect and does not permit channeling. Polar and bound lipids are not recovered from this method.

8.6.2.2 Wet Lipid Extraction

The wet lipid extraction process uses wet algae biomass by using solvent proportionately (Sathish and Sims, 2012). This method resembles the solvent extraction process but varies with the nature of biomass (wet). The advantage of the process includes the elimination of a drying step, the interference of moisture content with the extraction solvents and lack of wide applicability to all kinds of solvents are the major limitations of this extraction procedure.

Bioactive peptides usually contain 3-20 amino acid residues, and their activities stem from both their amino acid composition and sequence (Pihlanto-Leppala, 2000). Usually such short chains of amino acids are inactive within the sequence of the parent protein, but they become active upon release during gastrointestinal digestion or during food processing, including

fermentation. Examples of bioactive peptides obtained by enzymatic hydrolysis of algal proteins (Kim and Wijesekara, 2010) are shown in Table 10.3 together with their characteristic physiological roles.

Savage et al. demonstrated hydrothermal liquefaction to produce a crude bio-oil from wet algae paste and then hydrothermal catalytic upgrading of the biocrude to produce hydro­carbon product in high yield. This work provides new results on the liquefaction pathways and kinetics and on the roles and effectiveness of different upgrading catalysts for removing heteroatoms from algae and reducing the viscosity of the biocrude (Savage et al., 2012b).

Duan et al. reported the catalytic hydrotreatment of crude bio-oil produced from the hy­drothermal liquefaction of microalgae (Nannochloropsis sp.) over Pd on C (5% Pd/C) in super­critical H2O (SCW) at 400°C and 3.4 MPa high-pressure H2. Longer reaction times and higher catalyst loadings did not favor the treated oil yield due to the increasing amount of gas and coke products formation but did lead to treated bio-oil with higher HHV (41-44 MJ kg-1) than that of the crude feed. Highest HHV of treated oil (ca.44 MJ kg-1) was obtained after 4 h using an 80% intake of catalyst on crude bio-oil. The product oil produced at longer reaction times and higher catalyst loadings, which was a freely flowing liquid as opposed to being the vis­cous, sticky, tar-like crude bio-oil material, was higher in H and lower in O and N than the crude feed, and it was essentially free of S (below detection limits). Typical H/C and O/C molar ratio ranges for the bio-oils treated at different reaction times and catalyst loadings were 1.65-1.79 and 0.028-0.067, respectively. The main gas-phase products were unreacted H2, CH4, CO2, C2H6, C3H8, and C4H10. Overall, many of the properties of the treated oil obtained from catalytic hydrotreatment in SCW in the presence of Pd/C are very similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011a).

Duan and Savage determined the influence of a Pt/C catalyst, high-pressure H2, and pH on the upgrading of a crude algal bio-oil in supercritical water (SCW). The SCW treatment led to product oil with a higher heating value (ca.42 MJ kg-1) and lower acid number than the crude bio-oil. The product oil was also lower in O and N and essentially free of sulfur. Including the Pt/C catalyst in the reactor led to freely flowing liquid product oil with a high abundance of hydrocarbons. Overall, many of the properties of the upgraded oil obtained from catalytic treatment in SCW are similar to those of hydrocarbon fuels derived from fossil-fuel resources (Duan and Savage, 2011b).

Studied LCAs use three different kinds of energy carriers: electricity obtained by direct combustion of the biomass, biodiesel by sequential or direct triglycerides esterification, and biogas by anaerobic digestion.

An injected air stream containing ozone gas was used in separating microalgae from high rate oxidation pond effluent by ozone flotation. Use of ozone-induced flotation for algae recovery and effluent treatment was studied (Betzer et al., 1980). The ozone gas promotes cell flotation by modification of algae cell wall surface and by releasing some surface active agents from algae cells. The ozone-flotation process has been studied in numerous applications (Jin et al., 2006; Benoufella et al., 1994).

Elimination of a Microcystis strain of cyanobacteria (blue-green algae) by the use of ozone flotation was investigated in a pilot study (Benoufella et al., 1994). The oxidizing properties of ozone and the physical aspects of flotation were exploited in the flotation process. A specific ozone utilization rate of Microcystis was calculated, and ozone concentration and contact time curves were plotted versus algal removal. The study found that use of ozone in pretreatment leads to an inactivation of the algal cells. A prior coagulation stage was necessary for satis­factory cyanobacteria removal, and use of ferric chloride as a coagulant produced the best performance. Preozonation was also of influence on enhancement of the coagulation efficiency. Coupling ozone flotation with filtration can improve water quality, with treated effluent indicating low turbidity and low organic content.

After the formation of seven malonyl-CoA molecules, a four-step repeating cycle (exten­sion by two carbons/cycle), i. e., condensation, reduction, dehydration, and reduction, takes place for seven cycles and forms the principal product of the fatty acid synthase systems, i. e., palmitic acid, which is the precursor of other long-chain fatty acids (Fan et al., 2011;

Alban et al., 1994). With each course of the cycle, the fatty acyl chain is extended by two carbons. Figures 8.2 and 8.3 illustrate the palmitic acid formation and chain elongation. When the chain length reaches 16 carbons, the product (palmitate) leaves the cycle (Liu and Benning, 2012). All the reactions in the synthetic process are catalyzed by a multienzyme com­plex, i. e., fatty acid synthase (FAS).

Biohydrogen can be generated by microorganisms such as microalgae and cyanobacteria through biophotolysis and catabolism of endogenous substrate. Biophotolysis occurs due to the effect of light on the microbial systems, resulting in dissociation of water into molecular hydrogen and oxygen. The light-dependent biophotolysis metabolic pathways can be differ­entiated into two distinct categories: direct photolysis and indirect photolysis. Whereas elec­trons derived from water lead to photosynthetic hydrogen production in biophotolysis, electrons from catabolism of endogenous substrate would result in hydrogen production in a distinct mechanism.