Environmental Engineering Reference
In-Depth Information
Lipid extracts of many algae have been reported to contain relatively high levels of free fatty acids.
These can lead to the formation of soaps in alkali-based transesterification, which reduces yield and
increases the level of downstream processing and water use required to remove these soaps. Acid-
based catalysis simultaneously esterifies the free fatty acids and transesterifies the triacylglycerols.
This is well demonstrated in a study comparing acid- and alkali-catalyzed transesterification of
lipids from the diatom Chaetoceros muelleri (Nagle and Lemke 1990). Using hydrochloric acid-
methanol they achieved a maximum 4% FAME yield, whereas using NaOH as catalyst the yield
was only 1.65%. Robles-Medina et al. (2009) have proposed a two-stage process to overcome this
problem. In the first stage, acid catalysts are used to convert the free fatty acids to methyl esters, and
in the second stage, an alkali-catalyzed process is used to convert the remaining triacylglycerols
to methyl esters. However, the high free fatty acid content may be an artifact due to the activity
of endogenous lipases during the lipid extraction process. If, when harvested, cells of the diatom
Skeletonema costatum were immediately treated with boiling water to inactivate the lipases before
lipid extraction by the Bligh and Dyer (1959) method; no free fatty acids could be detected (Berge
et al. 1995).
Combined extraction and esterification is also possible. Belarbi et al. (2000) used a slurry (82%
water by weight) of either the diatom Phaeodactylum tricornutum or the green alga Monodus
subterraneus and transesterified these with methanol and acetyl chloride by heating in a boiling
water bath for 120 min at 2.5 atm. They obtained a yield of 77.5% FAMEs.
The properties of the biodiesel are mainly determined by the component fatty acids of the algal
lipids used to produce them (Knothe 2005). Of particular interest are the cloud point (the temperature
at which the fuel becomes cloudy because of solidification), the pour point (the temperature at which
the fuel stops flowing), and the cetane index (related to the ignition delay time and combustion
quality of the fuel). Oils with a high content of unsaturated fatty acids result in a biodiesel that is
less viscous and has a greater cloud point and pour point, making it more suitable for use in colder
climates. However, this biodiesel is more prone to oxidation and has a lower cetane index. Oils with
a high proportion of long-chain fatty acids (>C 18 ) have a higher cetane index. The oxidative stability
of the biodiesel is strongly affected by the position of double bonds in the saturated fatty acids. For
example, esters of linoleic acid (double bonds at Δ9 and Δ12) oxidize more slowly than esters of
linolenic acid (double bonds at Δ9, Δ12, and Δ15) (Frankel 1998). Unsaturation may also decrease
the lubricity of the fuel and may contribute to gum formation in the engine.
The European standards for biodiesel for vehicle use (EN14214) and for heating oil (EN 14213)
limit the content of FAMEs with four or more double bonds to a maximum of 1 mol % (Knothe
2006). Many oleaginous microalgae, especially the diatoms, cryptomonads, haptophytes, and
eustigmatophytes (Brown et al. 1997) have a high content of highly unsaturated fatty acids such
as eicosapentaenoic acid (C20:5 n -3) and docosahexaenoic acid (C22:6 n -3), which means that their
lipids are likely to meet the European standards without further treatment such as partial catalytic
hydrogenation of the oils (Dijkstra 2006). Much of the research to date has focused on microalgae
species with a high content of long-chain polyunsaturated fatty acids for pharmaceutical and
nutritional applications (Molina Grima et al. 1999; Kawachi et al. 2002; Tonon et al. 2002), but the
great and still largely unexplored diversity of the microalgae does provide the opportunity to seek
species and strains with reduced levels of polyunsaturated fatty acids as sources of lipids for the
production of biodiesel.
The lipid content of microalgae also varies between species as well as with the growth stage
and growth conditions (Borowitzka 1988). Many microalgae increase their lipid content when
nutrient limited, especially nitrogen limited (Griffiths and Harrison 2009; Rodolfi et al. 2009).
This increased lipid content is mainly due to an increase in triacylglycerols (TAGs), which act as
storage lipids (Borowitzka 1988; Roessler 1990). Diatoms also increase their lipid content under
silicon limitation (Coombs et al. 1967; Taguchi et al. 1987). In some microalgae species such as
Monodus subterraneus , Isochrysis galbana , Pavlova lutheri , P. tricornutum , and Chaetoceros spp.,
phosphate limitation also leads to an increase in the TAG content (Khozin-Goldberg and Cohen
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