Microalgae as an Attractive Source for Biofuel Production - Environmental Sustainability: Role of Green Technologies - page 145

13.4 Physico-Chemical Extraction

14 Conclusion

The physico-chemical technique can be used for
the disruption of cells in order to extract lipids
from microalgae (Cooney et al. 2009 ; Lee et al.
2010 ). Some of the physico-chemical tech-
niques are microwave, autoclaving, osmotic
shock, bead-beating, homogenization, freeze-
drying, French press, grinding, and sonication.
Microwave and bead-beating methods yield a
higher lipid content from microalgal cells . In
Botryococcus sp., using 5 min microwave pre-
treatment increased the lipid extraction yield
from 7.7 to 28.6 % g lipid/g dry weight (Lee
et al. 2010 ).

Increases in atmospheric CO 2 , and depletion of
petro-diesel and mineral oil reserves requires
alternative environmentally friendly energy
sources. To overcome this problem, production
of biodiesel from microalgal biomass might be a
solution to reduce CO 2 emissions from industry
and could also meet the global demand for trans-
port fuels. Production of biodiesel from microal-
gae is an emerging technology and an economical
choice because of its availability and low cost.
Microalgae have several advantages over conven-
tional crops in that less land is required and non-
arable land can be cultivated; microalgae double
their weight with respect to biomass within 24 h;
they require low pesticide application and have
no impact on food security; and microalgae bio-
mass can also be used in other energy-generation
processes after the oil has been extracted. Many
companies are already achieving commercial-
scale production of microalgal biofuels. Large-
scale cultivation and harvesting systems are
needed for the production of algal biodiesel to
reduce the cost per unit area. Large-scale micro-
algal biomass can be achieved using open pond
or PBRs, and each system has its own advantages
and disadvantages. The open pond system is
widely used for large-scale production due to its
advantages and the potential for CO 2 sequestra-
tion when compared with PBRs. Some essential
factors need to be optimized for large-scale appli-
cation, including strain selection, seed culture
preparation, medium composition, biomass and
lipid yield optimization, harvesting, and extrac-
tion of lipids from biomass. Certain other impor-
tant factors include providing optimum conditions
of certain parameters such as light, nutrients,
temperature, and CO 2 and O 2 levels. However,
the above parameters cannot be controlled in an
open pond system. Due to the high costs involved
in the harvesting and extracting of lipids from the
microalgal biomass, more effort must be made to
reduce process costs and to increase the quality
of biodiesel.

13.5 Biochemical Extraction

Only limited studies have used biochemical extrac-
tion to extract lipids from microalgae. For example,
after 72 h cellulase hydrolysis pre-treatment, 70 %
of sugar was obtained, whereas the lipid content
was increased slightly from 52 to 54 % g lipid/g
dry weight in Chlorella sp. (Fu et al . 2010 ).

13.6 Direct Transesterifi cation

Direct transesterifi cation is a process that mixes
alcohol and a catalyst with microalgae without
prior extraction. In microalgae, many catalysts
have been examined, including hydrochloric
(HCl) or sulphuric acid (H 2 SO 4 ), but acetyl chlo-
ride (CH 3 COCl) produces a higher FAME yield
of 56 % g FAME/g dry weight (Cooney et al .
2009 ). This process can be enhanced by coupling
it with microwave heating. The heterogeneous
catalyst (SrO) together with heating with micro-
waves increased the FAME yield from 7 to
37 % g FAME/g dry weight in Nannochloropsis
(Koberg et al . 2011 ). The disadvantage of this
technique is the high harvesting cost due to the
necessity for dry microalgal biomass. The overall
fl ow chart of the processing of algal biomass to
biodiesel is shown in the Fig. 3 .