The major cost attributed to the production of biodiesel is the dewatering and drying step, which consumes 9 to 16 GJ of energy per ton of biodiesel produced (Chowdhury et al., 2012). The dewatering and drying step can be negated if the microalgal oil is subjected to pyrolysis for the synthesis of bio-oil as a biofuel. The thermochemical method adopted for the preparation of fuel from microalgae is through pyrolysis, where the organic compound is thermally decomposed at a high temperature in the absence of oxygen. Zou et al. (2009) produced bio-oil by thermochemical catalytic liquefaction of Dunaliella tertiolecta. A yield of 97.05% was obtained. The reaction conditions were optimized and found to be H2SO4 (2.4 wt%); reaction temperature, 170°C; and reaction time, 33 min. A high-quality bio-oil was produced that possessed significant ester content. The bio-oil also possessed a low ash content of 0.4% to 0.7%. However, the product had a low pH value (3.8 to 4.0) and thus necessitates storage in acid-resistant bottles (e. g., polypropylene or stainless steel). Thermochemical treat­ment resulted in a high calorific value of 28.42 MJ kg-1. The bio-oil also had a low nitrogen content compared to bio-oils produced by methods such as pyrolysis or direct liquefaction. A high oxygen content was observed, thus providing the requirement for deoxygenation of the bio-oil. The composition of microalgae bio-oil obtained through thermochemical catalytic liquefaction consists of several methyl and ethyl esters, which result from esterification between organic acids and the glycol solvent, and is similar to that of biodiesel. Campanella et al. (2012) performed thermolysis of microalgae (consisting of mixed wild culture with Scenedesmus sp. as the principal constituent) and duckweed (primarily Wolffia and Spirodela species) in a fixed-bed reactor using CO2 as a sweep gas for the synthesis of bio-oil and called it “bioleum.” The thermolysis of microalgae gave a higher bioleum yield in comparison to that from the duckweed. This is attributed to the difference in composition of the two feed­stocks. The fuel properties of the bioleum were found comparable to heavy petroleum crude oil. The use of microalgal oil over lignocellulosic materials for pyrolysis has advantages in the form of lower oxygen concentration and a higher heating value for the former. The heating rate during the thermolysis of microalgae was found to be an important parameter in the formation of the bioleum, where the slow ther­molysis did not produce a liquid fuel that could be used as fuel. Maddi et al. (2011) reported that the pyrolysis products of algal (primarily consisting of Lyngbya sp. and Cladophora sp.) and lignocellulosic biomass (corncobs, woodchips, and rice husk) gave a similar yield of bio-oil. Other compounds that formed along with the bio-oil were bio-char, gases, and ash. The calorific value of lignocellulosic bio-char (except rice husk) was higher than that of algae-derived bio-char. This has been attributed to the higher carbon content in the lignocellulosic biomass. The difference in composi­tion of the bio-oil in the two feedstocks was the presence of nitrogenous compounds in the algal bio-oils. This is assumed to have occurred through degradation of the proteins present in the algae and may decrease its fuel value.

Chakraborty et al. (2012) synthesized bio-oil from Chlorella sorokiniana in a two — step sequential hydrothermal liquefaction technique to produce bio-oil and valuable co-products. The bio-oil consisted of 76% carbon, 12% hydrogen, 11% oxygen, 0.78% nitrogen, and 0.16% sulfur. The low nitrogen content avoids the denitrogenation step involved in the production of bio-oil. High oxygen content in the bio-oil necessi­tates further processing viz. hydrogenation to improve its quality. The yield of the bio-oil obtained by this method consisted of 24% of the dry weight, and optimum polysaccharide extraction occurred at 160°C. The advantage of the two-step sequen­tial hydrothermal liquefaction technique over the direct hydrothermal liquefaction technique was a low formation of bio-char in the former (i. e., 7.6% in comparison to 20.8% in the latter). Li et al. (2012) synthesized bio-oil from the marine brown microalgae, Sargassum patens C. Agardh, via hydrothermal liquefaction within a modified reactor. A comparatively moderate yield of 32.1 ± 0.2 wt% bio-oil was obtained in 15 min at 340°C. The feedstock used had a concentration of 15 g biomass per 150 mL water. The bio-oil obtained had a heating value of 27.1 MJ kg-1. The major constituent of the bio-oil was carbon (64.64%), followed by oxygen (22.04%), hydrogen (7.35%), nitrogen (2.45%), and sulfur (0.67%). The characterization of the bio-oil by infrared spectroscopy showed a diverse group of compounds consisting of fats, alkanes, alkenes, alcohols, ketones, aldehydes, carboxylic acids, phenol, esters, ethers, aromatic compounds, nitrogenous compounds, and water. A high concentra­tion of water may be the reason for the low calorific value of the bio-oil produced from the microalgae. Pie et al. (2012) carried out the co-liquefaction of a Spirulina and high-density polyethylene (HDPE) mixture in sub — and super-critical ethanol at a reaction temperature of 340°C to obtain bio-oil. The bio-oil thus produced was similar to that obtained from the pure HDPE derived bio-oil. The benefit of the co­liquefaction process of Spirulina and HDPE was the synthesis of bio-oil that pos­sessed a high calorific value (48.35 MJ kg-1) due to higher “carbon” and “hydrogen” contents and a lower oxygen content. The samples analyzed by gas chromatograph — mass spectroscopy (GC-MS) showed different compositions for bio-oil derived from Spirulina, HDPE, and Spirulina-HDPE mixture. While the bio-oil derived from Spirulina consisted of oxygen-containing compounds along with fatty acids, fatty acid esters, and ketones as prominent compounds, the bio-oil derived from pure

HDPE consisted of a wide spectrum of hydrocarbons, including saturated and unsat­urated aliphatic hydrocarbons. The bio-oil component obtained from the mixture of Spirulina and HDPE possessed more hydrocarbons and less oxygen-containing com­pounds. Hence, the product of the co-liquefaction of Spirulina and HDPE was similar in nature to that of pure HDPE liquefaction with a lower reaction temperature needed for thermal degradation of the feedstock. Hu et al. (2012) utilized the microwave — assisted pyrolysis of Chlorella vulgaris for the production of bio-oil with a yield of 35.83 and 74.93 wt% using microwave powers of 1,500 and 2,250 W, respectively. It was found that using activated carbon as a catalyst could enhance the bio-fuel yield to 87.47%. The calorific value of the microalgae was determined to be low (21.88 MJ kg-1).

Tabernero et al. (2012) evaluated the industrial potential for production of bio­diesel from Chlorella protothecoides. It has been estimated that supercritical fluid extraction (supercritical CO2) for biomass covering a surface area of 7,500 m2 could generate 10,000 tonnes biodiesel per year in a 150-m3 bioreactor. Lohrey and Kochergin (2012), in an attempt to minimize the energy consumption of algal bio­fuels, suggested locating a biodiesel plant close to a sugar mill plant to complement one another. It has been estimated that a cane sugar mill that discards 15% excess bagasse of 10,000 tonnes-per-day capacity can support a 530-ha algae farm to pro­duce 5.8 million L biodiesel per year and will also reduce CO2 emissions from the mills by 15%. The input in parameters of CO2, energy, and water are estimated at 2.5 kg kg-1, 3.4 kW-h kg-1, and 1.9 L kg-1, respectively, of algae dry weight.

The fatty acid composition of feedstock plays a significant role in the quality of the biodiesel produced. The European Standard (EN 14214) has limited the linolenic acid (C18:3) content, to not more than 12%. Wu et al. (2012) studied Chlamydomonas sp. as a potential feedstock for the synthesis of biodiesel. It was found that Chlamydomonas sp. possessed linolenic acid less than 12% and an oleic acid (a monounsaturated fatty acid) constituent of 31.6%. The almost equal compositions of saturated and unsaturated fatty acids in Chlamydomonas sp. are desirable for a trade-off between the oxidation stability and low-temperature prop­erty of the biodiesel. The FAME (fatty acid methyl ester) content in biodiesel was found to be 25% of total volatile suspended solids from microalgae cultivated using municipal wastewater (Li et al., 2011). Although the ester content in the biodiesel was low, the utilization of microalgae for the production of lipids coupled with wastewater treatment has environmental and economic significance. Upon increas­ing the ester content in the biodiesel by improving the technology, the process will become far more attractive. Lam and Lee (2012) are of the opinion that biodiesel production will be the ideal product with microalgae as feedstock. To ensure cost effectiveness, the residual biomass after lipid extraction can contain high concentra­tions of carbohydrates, which should be further utilized for bio-oil and bio-ethanol production. Table 8.1 depicts the ester content and calorific value of the biofuels (biodiesel and bio-oil).

A unique method of thermal analysis to differentiate the oleaginous and non­oleaginous microorganisms (fungi, algae, and yeasts) was developed by Kang et al. (2011). Along with the synthesis of biodiesel, algal biomass residue can be used for other purposes. A linear relationship was observed between exothermic heat and

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TABLE 8.1 Comparison of the FAME Ester Content and Calorific Value of the Biodiesel and Bio-oil from Microalgae Microalgae Method for Extraction of Oil Process Product FAME Content HHV/Calorific Value Ref. Unidentified Using hexane as solvent Transesteri fication Biodiesel Not reported 40 MJ kg-1 11 Chlorella vulgaris (ESP 31) Sonication to disrupt the cell wall Transesteri fication Biodiesel 72.1% (from extracted — 13 Chlorella sorokiniaria of microalgae and then vigorously mixing the disrupted cell with biphasic solvent of chloroform & methanol Two-step sequential hydrothermal Bio-oil microalgal oil) 97.25% (upon direct disruption of algal biomass) 40.8 MJ kg-1 22 Sargassum patens C. Agardh liquefaction Hydrothermal liquefaction _ Bio-oil _ 27.1 MJ kg-1 23 Sp і m І і nafH D P E Co-liquefaction — Bio-oil — 48.35 MJ kg-1 24 Chlorella vulgaris Microwave-assisted pyrolysis — Bio-oil — 21.88 MJkg-la 25 Low calorific value reported that of microalgae Chlorella vulgaris.

the total lipid content in the tested microorganisms. The exothermic heat per dry sample mass (kJ g-1) in the temperature range from 280°C to 360°C differentiated the oleaginous from the non-oleaginous microorganisms. It was found that the heat evolved from the oleaginous microorganisms was larger than that from the non­oleaginous microorganisms in the specified temperature range. The sharpness of the exothermic peak was also more distinct in the oleaginous microorganisms. Kim et al. (2011) utilized the residual biomass of Nannochloris oculata as a biosorbent for the removal of chromium from aqueous solutions. The biological route can also be adopted for biodiesel synthesis. It is anticipated that biodiesel production will increase in the coming years and there will be large amounts of residual biomass that can be used for the treatment of wastewater. The process for the synthesis of biodiesel and bio-oil from microalgae can be depicted through a flowchart (Figure 8.1).

Microalgae

/

Cultivation
k

Harvesting

М/

De-watering and concentrating microalgae

Extraction of oil/lipids
l/

Crude lipids (Neutral lipids & Pigments)
k

Neutral lipids (triglycerides, free fatty acids, hydrocarbons,
sterols, wax and sterol esters, and free alcohols)

Separation

Triglycerides and free fatty acids

Pyrolysis/thermochemical catalytic liquefaction

4/

Biodiesel

FIGURE 8.1 Steps in the production of bio-oil and biodiesel from microalgae.