Batan et al. (2010) reported that the net energy ratio of biodiesel produced from microalgae (Nannochloropsis species) was found to be 0.93 MJ (MJ = megajoule) of energy needed to produce 1 MJ of energy. The major advantage of using microalgae-derived biodiesel is the reduction in CO2 equivalent emissions amounting to 75 g MJ-1 of energy produced. Liu et al. (2012) also report that biodiesel from algae has a positive energy impact and estimate 1.4 MJ of energy production per megajoule consumption of energy. In addition, there will be reduction of 0.19 kg CO2-equivalents per kilometer of travel by transport.

Johnson and Wen (2009) adopted two methods for the production of biodiesel from a heterotrophic microalga, Schizochytrium limacinum. The first method adopted was direct transesterification of algal biomass using wet and dry biomass separately. The second method consisted of two steps involving extraction of oil from algae, followed by transesterification of wet and dry biomass. When the direct transesterification method was used, the yield of biodiesel obtained was greater than 66% for wet as well as dry biomass. However, the fatty acid methyl ester (FAME) content in the wet biomass was found to be very low (7.76%) in comparison with that from dry biomass (63.47%). A 57% crude biodiesel yield was obtained through the two-step method with a FAME content of 66.37%. Using the wet biomass, the FAME content of biodiesel was 52.66%. The one-stage direct transesterification method used various solvents (e. g., chloroform, hexane, and petroleum ether) to treat the algal biomass. However, a comparatively higher content of FAME was observed when only chloroform was used as the solvent. It has been found that direct transesterification is preferable instead of the conventional steps involved in the production of biodiesel from microalgae (i. e., extraction of oil from microalgae and transesterification of the expelled oil) as the production cost of the fuel will be reduced. However, the drying of algal biomass was found to be a prerequisite in order to obtain a high yield and conversion of biodiesel when using direct transesterification of the microalgae. To prevent oxidation of the unsaturated FAME in biodiesel, Johnson and Wen (2009) added 100 ppm butylated hydroxytoluene to the biodiesel. However, the fuel did not meet the European standards (EN 14103) specifications, which specify that the ester content in biodiesel must be at least 96.5% (Sarin et al., 2009).

Vijayaraghavan and Hemanathan (2009) reported on the production of biodiesel from freshwater algae. A high lipid content of 45 ± 4% was obtained from the micro­algae and was used for the synthesis of biodiesel by transesterification using metha­nol as a reactant and KOH as a catalyst. Upon transesterification, the fuel properties of biodiesel were determined. The acid value of the biodiesel (0.40 mg KOH g-1) was within ASTM (American Society for Testing & Materials) D 6751 specifications for biodiesel. The values of the other important parameters (i. e., density, ash, flash point, pour point, calorific value, cetane number, water content, and copper strip corro­sion) were characterized. It was found that some of the parameters (i. e., density, ash, flash point, and water content) did not meet the ASTM specifications. The density of biodiesel was found to be low (801 kg m-3), whereas the Indian specifications have a range of 860 to 900 kg m-3. The ash content of the biodiesel was 0.21 mass%, in contrast to the 0.01% specified by Indian standards. Similarly, the flash point also had a low value of 98°C instead of the minimum value of 120°C. The water content (<0.02 vol.%) was also slightly higher than specifications (<0.03 vol.%). However, other parameters (i. e., pour point, calorific value, and cetane number) were found to be within Indian specifications.

An in-situ transesterification method has been adopted by Velasquez-Orta et al. (2012) for the transesterification of microalgae, Chlorella vulgaris. The in situ transesterification of this microalga was performed by combining the two steps of lipid extraction and transesterification into a single step. Although the reaction ran to com­pletion in less time (75 min) using NaOH as the catalyst, a low conversion of FAME (77.6 ± 2.3 wt%) was obtained that does not meet the specifications of the European Union (EN 14103), which specifies that the ester content must be at least 96.5% (Sarin et al., 2009). However, when an acid catalyst (sulfuric acid) was used, a high FAME yield of 96.8 ± 6.3 wt% was obtained although a longer reaction time (20 h) was required. Also, a high methanol ratio (600:1) was employed, which will escalate the production cost of biodiesel. Tran et al. (2012) produced biodiesel from Chlorella vul­garis (ESP-31) using an enzyme (Burkholderia lipase) as a heterogeneous catalyst. The biodiesel was synthesized in two ways: (1) transesterification of microalgal oil, and (2) direct transesterification of the microalgae after disruption of its cells by sonica- tion. A moderate conversion (72.1%) of the microalgae to biodiesel was obtained with the first method, whereas a high conversion (97.25%) was obtained using the second method. The immobilized enzyme was reused for six runs without any significant loss in catalytic activity. Being catalyzed by an enzyme, the catalyst was found to func­tion even in the presence of water (>71.39 wt%). However, a higher molar ratio (67.93, methanol to oil) was needed to achieve an ester conversion of greater than 96 wt% of oil that will escalate the production cost of the biodiesel.

The fuel properties of the biodiesel synthesized from the microalgal oil derived from Chlorella protothecoides has been investigated by Chen et al. (2012). The microalgal oil methyl ester (MOME) with a high ester content (97.7%) demonstrated development of biodiesel of high fuel quality with a cold filter plugging point of -13°C, which is an indication that the fuel may be used even under extremely cold conditions. The vis­cosity of the MOME was 4.43 mm2 s-1 at 40°C, which is within the specifications for biodiesel specified by the ASTM. However, the oxidation stability of the fuel was low (4.5 h), which was due to high amounts of unsaturated fatty acid content in the MOME (i. e., 90.7 wt%). The induction time as per the Indian and European specifications is at least 6 h (Sarin et al., 2009). Hence, it had been recommended that the MOME should use up to 20 vol% blended with mineral diesel. A higher blend of biodiesel in mineral diesel will require the addition of antioxidants so that the fuel does not get oxidized rapidly and remains within specifications. Siegler et al. (2012) extracted oil from the microalgae Auxenochlorella protothecoides and found it to be a potential source for biodiesel production. The degree of unsaturation (DU) in the microalgal oil, which is a measure of the unsaturated fatty acid content, was determined to be 137. Using the DU, the cold filter plugging point value of the biodiesel was expected to be -12°C, which can support the use of fuel even in cold climatic conditions.

Lardon et al. (2009) observed that despite biodiesel derived from microalgae having immense potential to provide an alternative source of fuel, energy and fertilizer consumption should be reduced for its economic viability. Using Chlorella vulgaris as a model species, it has been found that a substantial portion of energy consumption amounting to 70% and 90% of the total energy is used for lipid extrac­tion when using wet and dry biomass, respectively. Hence, technologies must be developed for economical extraction of oil from microalgal cells. Rosch et al. (2012) advocate the reuse of residual algal biomass after oil extraction for the supply of nutrients, which according to estimates may vary from 0.23 to 1.55 kg nitrogen and 29 to 145 g phosphorous (depending on the cultivation conditions of microalgae) for the production of 1 L biodiesel.