Environmental Engineering Reference
In-Depth Information
26.5 lIPIds and BIodIesel
Microalgae are seen as an important future source of renewable biodiesel. The potential advantages
of microalgae as sources of liquid biofuels compared to other oleaginous or sugar producing plants
such as canola, oil palms, jatropha, sugarcane, and corn is that they
• Have markedly higher annual productivities than land plants per unit land area when
grown in intensive culture,
• Can be grown using saline water (at salinities up to NaCl saturation), thus not competing
with food crops for scarce freshwater resources, and
• Can be grown on land unsuitable for agriculture.
Compared with fossil diesel, algae-derived biodiesel has also been shown to have a substantial
positive greenhouse gas and energy balance in preliminary modeling (Campbell et al. 2009).
The recognition that algae are potential sources of lipids for the production of biodiesel is not
new. Many species of microalgae contain high levels of lipids in the range of 20-60% of dry weight,
although contents greater than about 30% are generally only found in nutrient-depleted stationary-
phase cultures (Borowitzka 1988; Griffiths and Harrison 2009; Huerliman et al. 2010) (Table 26.1).
The lipid composition (i.e., fatty acid composition, saturated/polyunsaturated fatty acid ratio,
proportion of phospholipids, etc.) of algae varies between taxa and, to some degree, with growth
conditions.
The bulk of microalgal lipids are C
14
to C
22
chain length esters of glycerol and fatty acids.
Triglycerides are the most common storage lipids, and these may constitute up to approximately
80% of the total lipids in nutrient-starved nongrowing cells (Tornabene et al. 1983). These
storage lipids are usually located as droplets in the cytoplasm. The other major algal lipids are
sulphoquinovosyl diglyceride, monogalactosyl diglyceride, digalactosyl diglyceride, lecithin,
phosphatidyl glycerol, and phosphatidyl inositol as the main membrane lipids (Guschina and
Harwood 2006).
The lipids of the different algal taxa vary in the composition of the main fatty acids (Table 26.2),
and these differences in the composition of the lipids affect the efficiency of the conversion
process to biodiesel and the properties of the biodiesel. Biodiesel is produced from algal lipids by
esterifying free fatty acids or transesterifying triacylglycerol fatty acids using an alcohol, usually
taBle 26.1
range of lipid contents reported for microalgae
Percent of total lipid
total lipids
(% dry
weight)
hydrocarbons
(% dry
weight)
neutral
lipids
algal class
Glycolipids
Phospholipids
Cyanophyceae
2-23
11-68
12-41
16-50
0.005-0.6
Rhodophyceae
41-58
42-59
Cryptophyceae
3-17
0.004
Dinophyceae
5-36
0.004-0.2
Bacillariophyceae
1-39
14-60
13-44
10-47
0.2-0.7
Heterokontophyta
12-72
0.2-70
b
Chlorophyceae
1-70
21.66
6.62
17-53
0.03-1.0 (39.0)
a
a
High value for
B. braunii.
b
High value for the coccolithophorid algae (Prymnesiophyceae).
