Pyrolysis forms the base of thermochemical conversion in most cases. The products of con­version include biocrude, tars, charcoal (carbonaceous solid), and permanent gases, including methane, hydrogen, carbon monoxide, and carbon dioxide. The products and ratios in which they are formed vary depending on the reaction parameters, such as environment, reactors used, final temperature, rate of heating, and source of heat. Pyrolysis is the fundamental chem­ical reaction process and is simply defined as the chemical change that occurs when heat is applied to a material in the absence of oxygen. Hydrothermal upgradation (HTU) is one of the processes of a general term of thermochemical conversion (TCC), which includes gasification, liquefaction, and pyrolysis. Various conversion processes for the production of a wide range of products from algal biomass are provided in Figure 11.2.

The hydrothermal upgradation process is a promising liquefaction process because it can be used for the conversion of a broad range of biomass feedstock. The process is especially best suited to wet materials; the drying of feedstock is not necessary because the water is used as one of the reactants. This thermochemical means of reforming biomass may have energetic advantages since, when water is heated at high pressures, a phase change to steam is avoided, which in turn avoids large enthalpic energy penalties. Superior to pyrolysis technology, high — pressure direct liquefaction technology has the potential for producing liquid oils with much higher caloric values and a range of chemicals, including vanillin, phenols, aldehydes, and organic acids (Appell et al., 1971). The advantage of liquefaction is that the bio-oil produced is not miscible with water and has a lower oxygen content, and therefore higher energy con­tent, than pyrolysis-derived oils (Goudriaan et al., 2001; Huber et al., 2006). Oxygen hetero­atom removal occurs most readily by dehydration, which removes oxygen in the form of water, and by decarboxylation, which removes oxygen in the form of carbon dioxide (Peterson et al., 2008). The changes and optimization of reaction parameters and catalysts can produce the functional hydrocarbons/specialty chemicals in a single step. In the follow­ing sections the process of hydrothermal upgradation is explained in detail and its use for the valorization of algae is discussed.

FIGURE 11.2 Product profile from algae by various processes.

The main selection criterion has been a clear definition of a functional unit. The concept of

functional unit (FU) is the main characteristic of the LCA (Udo de Haes et al., 2006) and allows

relevant and fair comparisons between studies or between different technological options.

Here the studies are briefly described:

• Kadam (2002) (Kad). Comparative LCA of electricity production from coal only or from coal and microalgal biomass. Half of the CO2 emitted from the power plant is assumed to be captured by a monoethanolamine (MEA) process.

• Lardon et al. (2009) (Lar). LCA of biodiesel production in open raceways with or without nitrogen stress and with wet or dry extraction of the lipids.

• Baliga and Powers (2010) (Bal). LCA of biodiesel production in photobioreactors located in cold climates. Cultivation is realized under greenhouses; heat losses from a local power plant are used as the heat source.

• Batan et al. (2010) (Bat). LCA of biodiesel production in photobioreactors based on the Greenhouse Gases, Regulated Emissions, and Energy use in Transportation (GREET) model.

• Clarens et al. (2010) (Cla10). Comparative LCA of the energy content of microalgae with terrestrial crops used as biofuel feedstock. Microalgae are cultivated in open raceways using chemical fertilizers.

• Jorquera et al. (2010) (Jor). Comparative LCA of microalgal biomass production in open raceways, tubular photobioreactors, and flat plate photobioreactors.

• Sander and Murthy (2010) (San). LCA of biodiesel production in open raceways based on the GREET model with a culture in two stages (first photobioreactors, then open raceways).

• Stephenson et al. (2010) (Ste). Comparative LCA of biodiesel production in open raceways and photobioreactors. Oil extraction residues are treated by anaerobic digestion; the digestates are used as fertilizers.

• Brentner et al. (2011) (Bre). Combinatorial LCA of industrial production of microalgal biodiesel. The base configuration consists of cultivation in open raceways, hexane extraction of dry algae, and methanol transesterification. Oilcakes are considered as a waste; the optimized configuration is composed of cultivation in PBR, extraction with in situ esterification by supercritical methanol, anaerobic digestion of oilcakes, and use of the digestates as fertilizers.

• Campbell et al. (2011) (Cam). LCA and economic analysis of biodiesel production in open ponds. Pure CO2 produced during the synthesis of nitrogen fertilizer is used as a source of carbon.

• Clarens et al. (2011) (Cla11). LCA of algae-derived biodiesel and bioelectricity for transportation. Four types of bioenergy production are compared: (1) anaerobic digestion of bulk microalgae for bioelectricity production, (2) biodiesel production with anaerobic digestion of oilcakes to produce bioelectricity, (3) biodiesel production with combustion of oilcakes to produce bioelectricity, and (4) direct combustion of microalgae biomass to produce bioelectricity. Four ways to supply nutrients are compared: (1) pure CO2, (2) CO2 captured from a local coal power plant, (3) CO2 in flue gas, (4) CO2 in flue gas and nutrients in wastewater.

• Collet et al. (2011) (Col). LCA of biogas production from anaerobic digestion of bulk microalgae. Biomass is grown in open raceways; digestates are used as fertilizers.

• Hou et al. (2011) (Hou). LCA of biodiesel from microalgae and comparison with soybean and jatropha.

• Khoo et al. (2011) (Kho). LCA of biodiesel from microalgae. Cultivation is carried out in two phases: first in photobioreactors, then in open raceway.

• Yang et al. (2011) (Yan). LCA of biodiesel production limited to water and nutrient consumption.

Among the 15 selected papers, two functions are assessed: either biomass production (two publications) or bioenergy production (14 publications). Three final vectors for the bioenergy are considered: methylester (11 publications), methane (2 publications), and electricity (2 publications). It is worth noting that these different energy carriers have different charac­teristics. Methane and methylester are easily storable, unlike electricity. There is also an im­portant diversity of FUs. Most of the studies focus on the production of biodiesel as the main energy output from microalgae. The amount of biodiesel produced is described in different units: volume (Baliga and Powers, 2010), mass (Stephenson et al., 2010), or energy content (Lardon et al., 2009). Unfortunately, there is no consensus on the values of energy content or on the mass density of algal oil and algal methylester; in addition, the description of the energy content is not harmonized and can be based either on the lower heating value (LHV) or the high heating value (HHV). Finally, among the studies dedicated to biodiesel production, six are well-to-pump studies, which means that the use of the fuel is not included in the perimeter (Baliga et Powers, 2010; Batan et al., 2010; Sander and Murthy, 2010; Brentner et al., 2011; Khoo et al., 2011; Yang et al., 2011), and five are well-to-wheel studies, where the use of the fuel is included (Lardon et al., 2009; Stephenson et al., 2010; Campbell et al., 2011; Clarens et al., 2011; 2011; Hou et al., 2011).

This diversity of FUs leads to a diversity of perimeters for the inventory. Table 13.1 sum­marizes the assessed systems. The different steps potentially included in the perimeter of the study can be classified among five categories: production of the inputs required for the cul­tivation (I), cultivation (C), harvesting and conditioning of microalgae (H), transformation into different types of energy carrier (T), and, eventually, use of the produced energy (U).

Figures 13.1 and 13.2 illustrate the various options met in the selected LCAs. The culture phase is the more consensual, with two options: open raceways or photobioreactors. The transformation phase is the one with the largest number of alternatives, including the final energy carrier or the fate of the coproducts.

The biological fixation of CO2 can be carried out by higher plants or microalgae. The sources of CO2 for microalgal cultivation are atmospheric CO2; CO2 from industrial flue gas; and chemically fixed CO2 in the form of soluble carbonates (Kumar et al., 2010). One ki­logram of algal dry cell weight utilizes around 1.83 kg of CO2. Annually, around 54.9-67.7 tonnes of CO2 can be sequestered from raceway ponds, corresponding to an annual dry weight biomass production rate of 30-37 tonnes per hectare (Brennan and Owende, 2010).

The CO2 can be the limiting nutrient in microalgal cultivation if it is available in low con­centration in the feed gas (when air is used as a source of CO2) or when mixing is not suffi­cient. However, the high CO2 concentration causes a reduction in pH, which can inhibit the growth of some microalgae (Wang et al., 2012).

Open tanks may have a limited carbon source due to the low transfer of mass. The simple bubbling of CO2 in the cultures may not be sufficiently effective, because the residence time of the bubble can be very short and is lost to the atmosphere. In these cases a high concentration of free CO2 must be maintained through direct injection of the flue gas from power plants, cement, and petrochemical factories during cultivation.

The biofixation of CO2 can be increased while maintaining an alkaline pH, because this will accelerate the absorption of gas through two reactions: CO2 hydration and subsequent acid — base reaction to form HCOg and direct reaction of CO2 with OH — to form HCOg (Amaro et al. 2011). The most common system employed for pH control is the on-off type system in which CO2 is injected into culture when the pH exceeds a predefined set point (Kumar et al., 2010).

The engineering of a photobioreactor must also be designed to add gas transfer equipment, which will increase the gas distribution and the contact of the gas with the liquid. Some of these items include mechanical systems (propellers, blades, and brushes), coarse and fine bubble diffusers (perforated piping, slotted tubes, discs, or domes), jet aerators, aspirators, U-tubes, and hollow fiber membrane modules (Kumar et al., 2010).

In vertical tubular, horizontal, or airlift photobioreactors, the biofixation of the CO2 is increased by the route of the gas along the tube as well as by the use of sprinklers that release small bubbles, increasing the contact surface between the gas and the liquid.

The fixation of CO2 by microalgae has received attention due to the production of biomass with potential application in the production of biofuels, reducing the emission of greenhouse gases and participating in the treatment of effluents (Kumar et al., 2010).

A vertical column photobioreactor is made up of vertical tubing (glass or acrylic) that is trans­parent to allow the penetration of light for the autotrophic cultivation of microalgae. A gas sparger system is installed at the bottom of the reactor; it converts the inlet gas into tiny bub­bles, which provide the driving force for mixing, mass transfer of CO2, and removing O2 pro­duced during photosynthesis (Figure 2.2). Normally, no physical agitation system is implemented in the design of a vertical column photobioreactor. Vertical tubular

photobioreactors can be categorized as bubble column or airlift reactors based on their liquid flow patterns inside the photobioreactor.

Bubble column reactors are cylindrical vessels with height greater than twice their diameter. They are characterized by low capital cost, high surface-area-to-volume ratio, lack of moving parts, satisfactory heat and mass transfer, relatively homogenous culture environment, and efficient release of O2 and residual gas mixture (Loubiere et al., 2009). The gas bubbling up­ward from the sparger provides the required mixing and gas transfer. Therefore, the sparger’s design is critical to the performance of a bubble column. In scale-up of the photobioreactor, perforated plates are adopted as the sparger used in tall bubble columns to break up and re­distribute coalesced bubbles (Janssen et al., 2000). Light supply for autotrophic cultivation often comes from outside the column. Nevertheless, an inner-illumination design is gradually becoming acceptable due to higher light-penetration efficiency and more uniform light dis­tribution (Loubiere et al., 2009). Photosynthetic efficiency greatly depends on gas flow rate as well as the light and dark cycle created when the liquid is circulated regularly from central dark zone to external zone at a higher gas flow rate.

Airlift reactors, common in traditional bioreactor designs, are made of a vessel with two interconnecting zones. One of the tubes, called a gas riser, is where the gas mixture flows

Air

compressor

upward to the surface from the sparger. The other region, called the downcomer, does not re­ceive the gas, but the medium flows down toward the bottom and circulates within the riser and the downcomer. Based on the circulation mode, the design of an airlift reactor can be fur­ther classified into one of two forms: internal loop or external loop (Loubiere et al., 2009). The riser is similar to that designed for a bubble column, where the gas moves upward randomly and haphazardly. An airlift reactor has the advantage of creating flow circulation where liquid culture passes continuously through dark and light phases, giving a flashing-light effect to the microalgal cells. Residence time of gas in various zones controls performance, affecting parameters such as gas-liquid mass transfer, heat transfer, mixing, and turbulence. A rectangular airlift photobioreactor is also suggested to have better mixing characteristics and high photosynthetic efficiency, but the design complexity and difficulty in scale-up both are disadvantages.

Use of chemicals to induce coagulation-flocculation of algal cells is a routine upstream treat­ment in various algae-harvesting technologies such as sedimentation (Friedman et al., 1977; Mohn, 1980), flotation (Moraine et al., 1980), filtration (Danquah et al., 2009) and centrifuga­tion (Golueke and Oswald, 1965; Moraine et al., 1980). Coagulation-flocculation causes algal cells to become aggregated into larger clumps, which are more easily filtered and/or settle more rapidly to facilitate harvesting. Chemicals that were used as algal coagulants can be broadly grouped into two categories: inorganic and long-chain organic coagulants.

Inorganic coagulants include metal ions as Al+3 and Fe+3, which form polyhydroxy com­plexes at appropriate pH. Hydrated lime is a common coagulant inducer used in water and wastewater treatment. Its use would raise the pH to the point at which a milk-like inor­ganic compound, magnesium hydroxide, is formed and acts as a coagulant (Folkman and Wachs, 1973; Friedman et al., 1977). Aluminum sulphate (commonly called alum, with the chemical formula Al2(SO4)3 • 18 H2O) or other salts of aluminum, common coagulants used in water treatment, have also been used as coagulants in algae harvesting (Golueke and Oswald, 1965; McGarry, 1970; Moraine et al., 1980). Ferric sulfate was found to be inferior in comparison with alum with respect to the optimal dose, pH, and the quality of the harvested algal paste (Bare et al., 1975; Moraine et al., 1980).

Satisfactory treatment of algal pond effluent has been achieved by lime addition (Folkman and Wachs, 1973; Friedman et al., 1977). However, satisfactory lime treatment was limited to algal cultures containing magnesium above 10 mg/L, and the quality of the harvested prod­uct was significantly affected due to excessive calcium content of up to 25% by weight.

Common flocculation theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. Based on such electrostatic

flocculation theory, the more cells to be flocculated, the more coagulant would be needed in a linear stoichiometric fashion, rendering flocculation overly expensive. Contrary to this theory of electrostatic flocculation, a study found that the amount of alkaline coagulant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH (Schlesinger et al., 2012). Various other theories abound that flocculation can be due to multivalent cross-linking or coprecipitation with phosphate or with magnesium and calcium. However, the study revealed that monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories and leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used.

Long-chain organic coagulants or polyelectrolytes could exist as anionic, cationic, and non­ionic synthetic or natural polymeric substances (Stumm and Morgan, 1981). In examining various organic polymers as algal coagulants, it was reported that only the cationic polyelec­trolytes were found to be efficient coagulants (Tenney et al., 1969; Tilton et al., 1972; Moraine et al., 1980). Organic cationic polyelectrolytes at low dosages (1-10 mg/L) can induce efficient flocculation of freshwater microalgae (Bilanovic et al., 1988). Effective flocculation was attained at salinity levels lower than 5 g/L. However, the high salinity of the marine environ­ment was found to inhibit flocculation with polyelectrolytes. The reduced effectiveness of cationic polymers to induce microalgae flocculation in high-salinity medium is primarily attributed to the effect of medium ionic strength on the configuration and dimension of the polymer, as indicated by changes in the intrinsic viscosity. At high ionic strength, the polymer shrinks to its smallest dimensions and fails to bridge between algal cells.

Studies also revealed that while anionic polyelectrolytes enhanced lime flocculation, most polyelectrolytes can be used in conjunction with alum or ferric sulfate as coagulant aids to strengthen the flocs, thus enhancing algae harvesting (Friedman et al., 1977). When used as coagulant aids, the polyelectrolytes can be applied at reduced dosages than they would have been used alone. This helps save chemical costs.

Algal coagulation-flocculation mechanisms based on the use of polymeric coagulants were postulated (Tenney et al., 1969; Tilton et al., 1972). Adsorption and the bridging model were hypothesized, and parameters affecting the process were investigated. It was reported that higher molecular weight cationic polyelectrolytes are superior in flocculating algal particles than their lower molecular weight counterparts. Optimal dose decreased with increasing mo­lecular weight. However, very high molecular weight polymers may reverse the algal surface charge, thus stabilizing the suspension (Tilton et al., 1972). The study also pointed out that for a given level of algal flocculation, variations in algal concentrations would affect the polyelec­trolyte dosage needed, and the relationship between algal concentration and polyelectrolyte dosage can be established based on stoichiometry (Tenney et al., 1969).

A commercial product called chitosan, commonly used for water purification, can also be used as a coagulant but is far more expensive. To create chitosan, the shells of crustaceans are ground into powder and processed to acquire chitin, a polysaccharide found in the shells, from which chitosan is derived via deacetylation. Flocculation of three freshwater algae, Spirulina, Oscillatoria, and Chlorella, and one brackish alga, Synechocystis, using chitosan was examined (Divakaran and Pillai, 2002). With suspension in the pH range of 4 to 9 and chlorophyll-a concentrations in the range of 80 to 800 mg/m3, the chitosan-aided flocculation achieved a clarified water turbidity of 10 to 100 NTU units. The chitosan was found to be effective in separating the algae by flocculation and settling. It was found that the flocculation efficiency is very sensitive to pH, with optimal pH 7.0 for maximum flocculation of fresh­water algal species. The optimal chitosan concentration for maximum flocculation depended on the concentration of algae. Flocculation and settling rates were faster when higher than optimal concentrations of chitosan were used. The settled algal cells were intact and live and could not be redispersed by mechanical agitation. The clarified water may be recycled for fresh cultivation of algae. Studies of harvesting microalgae with chitosan flocculation were also reported (Lavoie and de la Noue, 1983; Morales et al., 1985).

In addition to the type of coagulant, the composition of the algal medium can also influence the optimum flocculation dosage. For lime treatment whereby magnesium hydroxide pre­cipitate is functioning as a coagulant, as discussed earlier, it was found that the higher the dissolved organic substances in the algal suspension, the higher was the concentration of magnesium hydroxide required for good algal flocculation (Folkman and Wachs, 1973). In­hibition of flocculation caused by the presence of dissolved organic matter was also observed in other investigations (Hoyer and Bernhardt, 1980; Narkis and Rebhun, 1981). Conversely, it was found in another study that algal exocellular organic substances reduced the optimal coagulant dose during the early declining growth phase of algal culture but increased the dose during the late growth stages (Tenney et al., 1969). The authors attributed the increased optimal dose to the development of the organic substances into protective colloid.

There are many variables that could affect algal coagulation-flocculation in a collective and complicated manner, rendering predictions for operational conditions almost impos­sible. Other than algal type, the optimal coagulant dosages can be dictated by the concen­trations of phosphate, alkalinity, ammonia, dissolved organic matter, and temperature of the algal medium (Moraine et al., 1980). In practice, optimal coagulant dosages are determined using bench-scale jar tests to simulate the complex coagulation-flocculation process.

Harvesting by chemical flocculation is a method that is often too expensive for large operations. The main disadvantage of this separation method is that the additional chemicals are difficult to remove from the separated algae, probably making it inefficient and uneco­nomical for commercial use, though it may be practical for personal use. The cost to remove these chemicals may be too expensive to be commercially viable. One way to solve this prob­lem is to interrupt the carbon dioxide supply to the algal system, which would cause algae to flocculate on its own—namely, autoflocculation. In some cases this phenomenon is associated with elevated pH due to photosynthetic carbon dioxide consumption corresponding to pre­cipitation of inorganic precipitates (mainly calcium phosphate), which cause the flocculation (Sukenik and Shelef, 1984). In addition to this coprecipitative autoflocculation, the formation of algal aggregates can also be due to excreted organic macromolecules (Benemann et al., 1980), inhibited release of microalgae daughter cells (Malis-Arad et al., 1980), and aggregation between microalgae and bacteria (Kogura et al., 1981).

A fungi pelletization-assisted bioflocculation process for algae harvesting and wastewater treatment was developed (Zhou et al., 2012). Microalga Chlorella vulgaris UMN235 and two locally isolated fungal species, Aspergillus sp. UMN F01 and UMN F02, were used to study the effect of various cultural conditions on pelletization for fungi-algae complex. The results showed that pH was the key factor affecting formation of fungi-algae pellets, and pH could be controlled by adjusting glucose concentration and the number of added fungal spores.

The best pelletization occurred when adding 20 g/L glucose and approximately 1.2 x 108/L spores in BG-11 medium, under which almost all of algal cells were captured onto the pellets with shorter retention time. The fungi-algae pellets can be easily harvested by simple filtration due to their large size (2-5 mm). The filtered fungi-algae pellets were reused as immobilized cells for wastewater treatment. It was claimed that the technology developed is highly promising compared with current algae harvesting and biological wastewater treat­ment technologies in the literature.

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.

The free amino-acid fraction of macroalgae is composed chiefly of alanine, taurine, omithine, citrulline, and hydroxyproline (Holdt and Kraan, 2011). In the case of microalgae, high quantities of lysine, methionine, cysteine, and threonine have been reported (Morist, Montesinos et al., 2001), which differ among species (McHugh, Food et al., 2003).

All essential amino acids were reported in brown and red seaweed species, whereas red species feature unusually high concentrations of taurine compared to their brown counter­parts (Dawczynski, Schubert et al., 2007). Taurine is not a true amino acid due to the lack of a carboxyl group, but it contains a sulfonated acid group instead. It is found in fish and shellfish in addition to macroalgae. A number of health-promoting properties of algal amino acids are depicted in Table 10.3.

In addition to taurine, other unusual (but bioactive) amino acids, such as laminine, kainoids, and mycosporine-like amino acids, have been found in marine macroalgae. Kainoids are a unique group of amino acids that are structurally and functionally related to aspartic and glutamic acids; they have attracted an interest due to their strong insecticidal, anthelmintic, and neuroexcitatory properties (Parsons, 1996). Algal extracts containing domoic and kainic acid have indeed been used as anthelmintic agents in Japan for centuries for treatment of ascariasis caused by the parasitic roundworm (Parsons, 1996; Smit, 2004). Such compounds are currently being tested against neurophysiological disorders such as Alzheimer’s and Parkinson’s diseases and epilepsy (Smit, 2004).

Ross et al. studied the preliminary classification of five macroalgae from the British Isles: Fucus vesiculosus, Chorda filum, Laminaria digitata, Fucus serratus, and Laminaria hyperborea, and Macrocystis pyrifera from South America, using a Van Krevelen diagram. The macroalgae have been characterized for proximate and ultimate analysis, inorganic content, and calorific value. The different options for thermal conversion and behavior under combustion and pyrolysis have been evaluated and compared to several types of terrestrial biomass, includ­ing miscanthus, short rotation willow coppice, and oat straw. Thermal treatment of the macroalgae has been investigated using thermogravimetry (TGA) and pyrolysis-GC-MS. Combustion behavior is investigated using TGA in an oxidizing atmosphere. The suitability of macroalgae for the different thermal processing routes is discussed. Ash chemistry restricts the use of macroalgae for direct combustion and gasification. Pyrolysis produces a range of pentosans and a significant proportion of nitrogen-containing compounds. High char yields are produced. Significant differences in fuel properties exist between kelps and terrestrial bio­mass. The heating value is lower than that of the terrestrial energy crops (cf. 14-16 MJ kg-1 to 17-20 MJ kg-1) since, in general, the ash content is higher. Consequently, the metal contents (especially alkali metals) are, for the most part, higher in the seaweeds studied here compared to the terrestrial biomass. Total halogen content is in the range 0.5-11% in kelps, which is also significantly higher than the terrestrial biomass (1-1.5%). Thus it is clear that unless washing were utilized to reduce the alkali levels, these macroalgae could not be used in dedicated sys­tems without encountering problems in component failure (Ross et al., 2008).

The liquefaction of "green tide" macroalgae Enteromorpha prolifera in sub — or supercritical alcohols in a batch reactor has been investigated. Under the conditions of the reaction time of 15 min and algae/solvent ratio set at 1:10, the macroalgae in methanol at 280°C produced a bio-oil yield at 31.1 wt% of dry wt, and the ethanol at 300°C yielded bio-oil at 35.3 wt%. The bio-oils obtained by liquefaction of macroalgae in alcohols are mainly composed of ester com­pounds. A variety of fatty acid (C3-C22) esters (Me or ethyl) in the bio-oils obtained in meth­anol and ethanol, respectively, were qualified by GC-MS, and their relative contents are above 60% of the total area for each bio-oil. Overall, bio-oils obtained in two alcohols are very similar to biodiesel in composition. The elemental analysis of bio-oils indicated that bio-oils still have high oxygen content (Zhou et al., 2012).

The marine brown algae, Sargassum patens C. Agardh, floating on the Yellow Sea, was col­lected and converted to bio-oil through hydrothermal liquefaction with a modified reactor. A maximum yield of 32.1 ± 0.2 wt% bio-oil was obtained after 15 min at 340°C at a feedstock concentration of 15 g biomass/150 mL water, without using a catalyst. The bio-oil had a heating value of 27.1 MJ kg-1 and contained water, lipid, alcohol, phenol, esters, ethers, and aromatic compounds. The solid residue obtained had a high ash and oxygen content. The results suggest that Sargassum patens C. Agardh have potential as biomass feedstock for fuel and chemical products (Li et al., 2012).

The brown macroalga Laminaria saccharina was converted into biocrude by hydrothermal liquefaction. A maximum biocrude yield of 19.3 wt% was obtained with a 1:10 biomass — to-water ratio at 350°C and a residence time of 15 min without the presence of the catalyst. The biocrude had an HHV of 36.5 MJ kg-1 and is similar in nature to a heavy crude oil or bitumen. The solid residue has high ash content and contains a large proportion of calcium and magnesium. The aqueous phase is rich in sugars and ammonium and contains a large proportion of potassium and sodium (Anastasakis and Ross, 2011).

Marine macroalgae Enteromorpha prolifera, one of the main algae genera for green tide, was converted to bio-oil by hydrothermal liquefaction in a batch reactor at temperatures of 220-320°C. The liquefaction products were separated into a dichloromethane-soluble fraction (bio-oil), water-soluble fraction, solid residue, and gaseous fraction. A moderate temperature of 300°C with 5 wt% Na2CO3 and reaction time of 30 min led to the highest bio-oil yield of 23.0 wt%. The HHV of bio-oils obtained at 300°C were around 28-30 MJ kg-1. Acetic acid was the main component of the water-soluble products (Zhou et al., 2010).

Electricity production is the easiest process to develop. Biomass is first dried up to 50% or 98% DM and then burned in co-combustion with coal. According to Clarens et al. (2011), this transformation path has the lowest impacts on the environment. However, it is important to note that in this study, the heat needed to dry the biomass comes from the recovery of flue gas and hence is not accounted for in the environmental balance nor the energy balance.