Review of Literature
2.3 Microalgal biodiesel and characteristics
During the period 1978 to 1996, US Department of energy (DOE) funded an Aquatic Species Program (ASP) which targeted the process design for sustainable generation of algal
biodiesel. The program analyzed various aspects of algal biodiesel generation including strain isolation, screening, characterization, understanding the biology and production biochemistry followed by process development and cost analysis. Detailed and sophisticated cost analysis revealed that production cost of biodiesel was two times higher than the production cost for petroleum diesel (Sheehan et al., 1998). Later in the year 1996 the project was withdrawn attributed to increased biodiesel prices, reduced government funding and lowered petroleum costs. However, renewed interest on microalgae based biodiesel production has been observed recently among the researchers and investors, attributed to increased petroleum diesel prices and limiting conventional fuel resources (Brennan and Owende, 2010). Several countries including India have initiated prominent research in the area of algal biotechnology for sustainable biofuel production.
Microalgae are unique organisms that possess several inherent properties such as:
(i) high biomass and oil productivity than other plant crops with short doubling time less than 4 h, (ii) capability to grow in non-arable lands utilizing fresh, brackish, marine and waste waters, (iii) ability to utilize flue gas as a source of CO2, (iv) minimum land usage and environmental impacts and (v) the multiproduct paradigm ranging from simple to complex bioorganics, biofuels and therapeutics which make them a potential cell factory for biodiesel production. The most important is that, huge amount of lipid rich biomass can be generated with exceeding levels of oil productivity in comparison to other energy crops characterized so far (Fig. 2.4). The oil productivity of microalgae is 101 % higher than the productivity obtained from palm plant which is the largest producer among the energy plant crops (Fig. 2.4). Table 2.2 provides the range of lipid content observed in different microalgal strains. The lipid content of algae vary significantly with various species and based on the cultivation conditions it may vary within the species. Therefore, as observed
from different literatures the oil productivity from microalgae may vary from 12,000 to 136886 L ha-1 year-1 (Singh et al., 2011a).
Fig. 2.4 Biodiesel productivity shown by various crops and microalgae in L ha-1 year-1 (obtained from Chisti, 2007 and Schenk et al., 2008). The * represents the oil productivity from microalgae calculated at 30% (w/w, DCW) lipid content with biomass productivity 10 g m-2 day-1 and it may vary depending upon productivity of the strains and growth conditions It is also important to note that along with the lipid content, the type of lipids generated in the strains may differ depending upon the species and cultivation conditions.
For instance, Botryococcus braunii has been known for the production and secretion of hydrocarbons while, other species like Nannochloropsis sp., Neochloris oleabundans and Chlorella sp. were found to accumulate huge amounts of neutral lipids which are further converted in to biodiesel via transesterification as shown above in Fig. 2.3. It is also worth noting that these reported lipid content may not be suitable completely for fuel as many unwanted lipids and chlorophyll remains unsuitable to use as biodiesel (Sathish and Sims, 2012). In general biodiesel contains heterogeneous mixture of fatty acids with variations in carbon chain length (C16-C24) and degrees of unsaturation. With differences in these fatty acid compositions, the biodiesel will show variable fuel properties that can substantially influence the fuel performance in engines. Therefore, the biodiesel generated from these
algal systems has to be in accordance with the fuel standards as framed by ASTM (American Society for Testing and Materials).
Table 2.2. Oil content obtained from various microalgal strains measured in weight percentage of the dry cell weight (source: Mutanda et al., 2011; Singh et al., 2011a)
Algal strain Lipid Content (%, w/w, DCW)
Anabaena cylindrical 4–7
Botryococcus braunii 25–75
Chaetoceros muelleri F&M-M43 33.6
Chaetoceros calcitrans CS178 39.8
Chlamydomonas rheinhardii 21
Chlorella vulgaris 14-22
Chlorella sp. 28–32
Chlorococcum sp. UMACC112 19.3
Crypthecodinium cohnii 20
Cylindrotheca sp. 16–37
Dunaliella bioculata 8
Dunaliella salina 6
Dunaliella primolecta 23
Euglena gracilis 14-20
Isochrysis sp. 25–33
Monallanthus salina >20
Monodus subterraneus UTEX151 16.1
Nannochloris sp. 20–35
Nannochloropsis sp. 31–68
Neochloris oleoabundans 35–54
Nitzschia sp. 45–47
Pavlova salina CS49 30.9
Pavlova lutheri CS182 35.5
Phaeodactylum tricornutum 20–30
Porphyridium cruentum 9–14
Prymnesium parvum 22-38
Scenedesmus obliquus 12-14
Scenedesmus dimorphus 16-40
Scenedesmus sp. F&M-M19 19.6
Scenedesmus sp. DM 21.1
Schizochytrium sp. 50–77
Skeletonema sp. CS252 31.8
Spirogyra sp. 11-21
Spirulina platensis 4–9
Spirulina maxima 6–7
Synechoccus sp. 11
Tetraselmis maculate 3
Tetraselmis sueica 15–23
The neutral lipid or triacyl glycerol (TAG) content of algal strains typically contains C16 to C18 and their polyunsaturations. They differ from plant oils with higher degree of polyunsaturations which mandates necessary modifications (transesterification to reduce the viscosity) to meet the international fuel standards. Testing of transesterified algal biodiesel properties and engine performance showed in high compliance with international standards (Table 2.3) and better engine performance (Demirbas, 2011).
Table 2.3 Selected properties of plant biodiesel, microalgal biodiesel, petroleum diesel and corresponding ASTM standards
Properties Units ASTM Standards
Diesel# Plant Biodiesel*
Microalgal Biodiesel**
Kinematic viscosity
mm2 s-1 1.9 to 6.0 1.2 to 3.5
3.6 to 4.9 4.18 to 5.07 Cetane number nd 47 (min) 51 45 to 70 45 to 61.4 Heating value MJ kg-1 45.9
(max)
31.8 to 43.2 38 to 41.5
Cloud point °C nd -15 to 5 1 to 13 -2.34 to 17.02
Pour point °C nd -35 to -
15
-16 to 9 nd Flash Point °C 130 (min) 60 to 80 76 to 183 nd
Density Kg L-1 0.83 to
0.84
0.85 to 0.88 0.85 to 0.89
# represents the data obtained from Brennan and Owende, 2010
*represents the data obtained from Singh et al., 2010
** represents the data obtained from Song et al., 2013 nd – not defined
Other than biodiesel (neutral lipids), microalgae are also a source of several bio- products (Fig. 2.5). They accumulate carotenoids and polysaccharides which are used as therapeutics and bioplastics respectively (Hempel et al., 2011). Moreover, the biomass rich in carbohydrates are further fermented for production of biofuels such as bioethanol (Parmar et al., 2011), biobutanol (Ellis et al., 2012) and biomethane (Parmar et al., 2011). Several cyanobacteria and certain algal species were also reported to produce hydrogen in significant amounts via chloroplastic hydrogenase activity. The other advantage is that the algae sequestrate CO2 in the atmosphere as the preferable carbon source and harvest light from the solar system as the energy source for growth at a greater efficiency than higher
plant systems. Thus, the net generation of CO2 will equal the net accumulation of CO2 in algae leading to a carbon neutral process and at higher efficiencies of CO2 sequestration the process is expected to be carbon negative in which the net generation from burning of fuel will be lesser than the accumulation (Taylor et al., 2013). These properties of algae are providing chances for the algal biomass to be a viable third generation feedstock for liquid transportation fuels (Subhadra, 2010). Therefore, microalgae are believed to be the only potential feedstock that can fulfil the current energy requirements (Chisti, 2007; Mata et al., 2010). But several bottlenecks remains ahead to develop a sustainable process for feasible biodiesel production in large quantities and there is still a lot to learn about these primitive and diverse group of species.
Fig. 2.5 Various value added products and biofuels (shown in red) generated from microalgal biomass