Environmental Engineering Reference
In-Depth Information
Table 8.3
Microalgae grown using flue gases and their biomass and lipid yields
Biomass (gL
−1
D
−1
)
Microalgae
CO
2
concentration (%)
Lipid yield (%)
Reference
Chlorella sp.
6-8
19.4-22.8
a
-
(Doucha et al.
2005
)
Chlorella sp.
MTF7
25
0.48
25.2
(Chiu et al.
2011
)
Scenedesmus obliquus
SJTU-3
10
0.155
19.25
(Tang et al.
2011
)
Chlorella pyrenoidosa
SJTU-2
10
0.144
24.25
(Tang et al.
2011
)
a
Biomass in g m
−2
D
−1
membranes, and cryogenic distillation. Cap-
tured CO
2
is transported to storage locations, and
stored in geological or ocean storage or mineral-
ized. However, with this approach of capture and
storage several technological, economical, and
environmental issues are related. Biological CO
2
capture is a sustainable approach and provides al-
ternatives to conventional CO
2
capture methods.
Microalgae have the ability to fix atmospher-
ic CO
2
through photosynthesis, with ten times
greater efficiency than terrestrial plants (Pires
et al.
2012
). Carbon is the key component of mi-
croalgal cell which constitutes 36-56 % of dry
matter. 1.3-2.4 kg CO
2
is fixed by microalgae for
per kg of dry biomass generation (Van Den Hende
et al.
2012
). Microalgae cultivated by supplying
CO
2
from flue gases produce biomass which can
be utilized for biofuels, value added products,
and animal feed. Microalgae can be cultivated
in open or closed system for biomass generation.
For microalgae cultivation either CO
2
is separat-
ed from flue gases and used or directly flue gases
are applied. Direct use of flue gases is beneficial
in terms of energy and cost saving; however,
microalgal strain should be resistant toward the
high percentage of CO
2
(15 %) and presence of
SO
X
and NO
X
(Maeda et al.
1995
). Maeda et al.
(
1995
) screened several microalgal species, and
found a
Chlorella sp.
strain with high growth rate
at a temperature of 35 ﾰC and 15 % CO
2
concen-
tration. Table
8.3
depicts the studies using flue
gases for microalgae cultivation. CO
2
can direct-
ly diffuse through the microalgal plasma mem-
brane. CO
2
is assimilated to 3-phosphoglycerate
by enzyme rubisco (ribulose 1,5-bisphosphate
carboxylase/oxygenase) in the Calvin cycle.
Fixed CO
2
through various metabolic pathways
assimilated into carbohydrates, proteins, and
lipids in microalgae. CO
2
has low mass transfer
coefficient, and thus mass transfer from gaseous
phase to liquid phase could be a limiting step in
application of this technology. High flow rate in
closed system or proper mixing in open cultiva-
tion could be the possible solutions to overcome
mass transfer limitations (Pires et al.
2012
). Flue
gases contain many compounds like SO
X
, NO
X
,
CO, C
x
H
y
, halogen acids, and particulate matter
apart from CO
2
. Direct utilization of flue gases
can pose problems for microalgae cultivation as
some of these compounds could have toxic ef-
fect on microalgae (Van Den Hende et al.
2012
).
Thus, a better understanding of the effect of these
individual compounds and their concentration
limits on microalgal physiology is needed. Ef-
fective utilization of flue gases for microalgae
cultivation can reduce environmental concerns
as well as earn carbon credits. Application of in-
novative scientific technologies for utilization of
flue gases by microalgae, can aid in improving
economics of biomass production.
8.5
Conclusion
Microalgae have shown a promising future with
simultaneous production of more than one biofu-
els viz., biodiesel, biomethane, biohydrogen, and
bioethanol to cater the energy demands. High
energy input and the cost associated with pro-
duction of microalgal biofuels are still a bottle-
neck which has to be overcome for its industrial
viability. The biorefinary concept could be the
possible answer to this problem, where with the
production of biofuels, emphasis is also given on
the coproduction of value added products and
utilization of refusals. For commercial feasibil-
ity of microalgal biofuel production, it is neces-
sary to produce other valuable products along
Search WWH ::
Custom Search