ALGAL FUEL PROPERTIES

The characterization of the algal oil derived after transesterification showed the possibility of using it as biodiesel. The properties of the microalgae oil are mostly dependent on the feed­stock and the conversion method used. Key aspects to evaluate the properties of microalgae oil are acid number, iodine number, specific gravity, density, kinematic viscosity, flash point, pour point, heating value, and cetane number. Table 8.1 illustrates the properties of algal fuel compared to conventional fuel.

Physical properties of microalgae oil show its efficiency to use as biodiesel. Of the prop­erties derived, acid number (AN) indicates the corrosiveness of the oil; iodine values (IV) refer to the degree of unsaturation. The AN and IV recorded within the limits indicate the less cor­rosiveness and higher saturation of the algae fuel. Similarly, the specific gravity and density enumerated its energy efficiency as fuel. Flash point expresses the lowest temperature at which the oil vaporizes to form an ignitable mixture. The temperature of the flash point recorded for microalgae oil determined the potential of the oil to form ignitable mixtures at relatively lower temperatures over conventional diesel fuel. Pour point is the lowest tem­perature at which the oil becomes semisolid and loses its flow properties. It is also an impor­tant diesel quality parameter in tropical countries like India. The solidifying temperature of the microalgae oil shows its application as diesel. Similar to pour point, viscosity defines the fluids’ resistance to flow; heating value is the energy released as heat when a compound

178

8.

ALGAE OILS AS FUELS

TABLE 8.1 Characterization of Microalgae Biodiesel

S. No.

Fuel Property

Biodiesel Standards (ASTM*)

Microalgae

Biodiesel

Diesel Fuel (ASTM*)

1

Acid number (mg KOH/g of oil)

<0.5

0.42a

0.7-1.0

2

Iodine value (g I/100 g of oil)

<25 (efficient fuel)

19.0a

120

3

Specific gravity (g/cm3)

0.85-0.90

0.85c

0.82-0.90

4

Density (g/cm3)

0.88

0.85a

0.86-0.90

5

Kinematic viscosity (mm2/s)

1.9-6.0

2.0-4.5b

3.5

6

Heating value (MJ/kg)

44

Ik

CO

42.2

7

Flash point (°C)

130

>130c

>62

8

Pour point (°C)

-11.6

-6c

-16

9

Cetane number

47

46c

60

a Venkata Mohan et al., 2011 b Demirbas, 2008

c U. S. Department of Energy, 2006 * ASTM = American Society for Testing and Materials

undergoes combustion. The less viscosity and higher energy values recorded for the algae oil denote its comparable features with standard norms and conventional fuel (Demirbas, 2008). Cetane number refers to the ignition quality of the diesel engines where it can be operated efficiently. The relative cetane number of microalgae oil with standard fuel indicates ignition and operational quality of algae fuel. Fatty composition of the microalgae oil (after transester­ification) showed diverse fatty acid profiles over the other biological feedstocks (Table 8.2). The microalgae oil profile depicted a higher degree of saturation with wide fuel and food characteristics, whereas the rest of the feedstock documented higher degrees of unsaturation. Algal lipids contain a substantial quantity of long-chain polyunsaturated fatty acids (LC — PUFA), including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (Chisti 2007). The EPA fatty acid has a carbon chain length of 20 with five double bonds (C20:5), and the DHA fatty acid has a carbon chain length of 22 with six double bonds (C22:6). The algal lipids have greater quantities of LC-PUFA compared to typical feedstock associated with higher quantities of fully saturated fatty acids (C14:0, C16:0 and C18:0), which have im­plications in terms of fuel properties (Harrison et al., 2012). The fatty acid composition (carbon chain length and degree of unsaturation) of FAME has a major effect on fuel properties. The most important characteristics affected by the level of unsaturation are oxidative stability, ignition quality (i. e., cetane number), and cold flow properties (Graboski and McCormick, 1998; Knothe et al., 1997; Ramos et al., 2009). Fully saturated methyl esters have high oxidative stability and a high cetane number but suffer from poor cold flow properties (Harrison et al.,

2012) . Conversely, methyl esters with a higher degree of unsaturation have better cold flow properties but decreased oxidative stability and decreased cetane number. Higher concentra­tions of some of the significant fatty acids, such as palmitic acid (C16:0) and oleic acids (C18:1), in microalgae oil are also a positive feature supporting the biofuel applications.

Fatty Acid

Lipid

Number

Jatropha

curcasa

Fungal

Oilb

Rapeseed

Oilb

Sunflower

Oilb

Palm

Oilb

Soy

Oilb

Scenedesmus

obliquusc

Microalgae Oil

Chlorella

vulgarisc

Mixed

Cultured

Lauric acid

12:0

Nd*

Nd

Nd

Nd

Nd

Nd

Nd

0.10

0.9

Myristic acid

14:0

0.1

1.6

0.1

Nd

0.7

Nd

Nd

Nd

6.7

Myristoleic acid

14:1

Nd

0.6

Nd

Nd

Nd

Nd

0.66

0.63

Nd

Penta decanoic acid

15:0

Nd

2.5

Nd

Nd

Nd

Nd

Nd

0.44

4.3

Palmitic acid

16:0

14.2

20.7

5.0

6.3

36.7

11.3

52.07

40.31

33.9

Palmitoleic acid

16:1

0.7

1.1

Nd

0.2

0.1

0.1

Nd

3.16

5.5

Stearic acid

18:0

7.0

7.0

1.6

2.2

6.6

3.6

7.48

8.01

15.9

Oleic acid

18:1

44.7

28.0

36.3

20.6

46.1

24.9

21.46

29.29

2.8

Linoleic acid

18:2

32.8

12.7

19.8

52.8

8.6

53.0

4.60

8.54

3.5

Linolenic acid

18:3

0.2

22.5

7.8

3.5

0.3

6.1

2.83

Nd

3.4

Arachidic acid

20:0

0.2

0.3

0.1

1.6

0.4

0.3

Nd

Nd

2.0

Gadoleic acid

20:1

Nd

Nd

9.1

0.3

0.2

0.3

Nd

Nd

Nd

Behenic acid

22:0

Nd

0.4

Nd

7.2

0.1

Nd

Nd

Nd

0.0

Erucic acid

22:1

Nd

0.07

20.2

5.1

Nd

0.3

Nd

Nd

1.3

Lignoceric acid

24:0

Nd

1.2

Nd

0.2

0.1

0.1

Nd

Nd

3.8

Nervonic acid

24:1

Nd

Nd

Nd

Nd

Nd

Nd

Nd

Nd

Nd

Others

Nd

1.3

Nd

Nd

Nd

Nd

Nd

Nd

0.1

TABLE 8.2 Comparative Evaluation of Fatty Acid Composition of Microalgae with Other Biological Feedstocks

a Akbar et al, 2009 b Vicente et al., 2010 c Nascimento et al., 2012 d Venkata Mohan et al., 2011 * Nd = Not detected.

8.8 ALGAL FUEL PROPERTIES 179

8.2 CONCLUDING REMARKS

Commercialization of algal oil production needs to overcome several obstacles. Space, water availability, efficient light utilization, cultivation system design, productivity of algal culture, algal growth and nutrient uptake, gas transfer and mixing, requirement of cooling, dissolved oxygen degassing, dewatering, oil extraction, and so on are some of the key issues that require considerable attention. Cost-cutting research with a multidisciplinary ap­proach will help resolve some of the inherent limitations prior to up-scaling. Conjunction of the algal fuel production process with waste gas, wastewater, and water reclamation is a promising strategy to be considered for economic viability. Integration of algal fuel with simultaneous production of valuable byproducts will also have a positive impact on the overall process economics. At present, considerable interest in algal-based fuel in conjunc­tion with intensified research makes a testimony that the process of algal biofuels will be economically viable and will be able to replace some proportion of fossil-fuel usage in the near future.

Acknowledgments

The authors want to thank Director, CSIR-IICT, Hyderabad, for his encouragement. Grant from CSIR in the form of the 12th plan task force project "BioEn" (CSC-0116) project is gratefully acknowledged.

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