Biomedical Engineering Reference
In-Depth Information
available to microalgae for uptake and intracellular conversion to CO
2
by intracellular
carbonic anhydrases. CO
2
is then made available to Ribulose-1,5-bisphosphate
carboxylase/oxygenase (RuBisCO) for its fixation into energy compounds (Kaplan
et al., 1991). Microalgae may provide a better tool for simultaneous CO
2
sequestra-
tion and biofuel generation. Current CO
2
levels (0.0387% (v/v)) in the atmosphere are
inefficient in supporting the high microalgal growth rates and biomass productivities
needed for full-scale biofuel production (Kumar et al., 2010). Flue gases from various
industries typically contain CO
2
in the concentration range around 15% (v/v), which
will provide sufficient amounts of CO
2
for large-scale microalgae biomass production
(Kumar et al., 2010). Owing to the cost of upstream separation of CO
2
gas, direct utili-
zation of power plant flue gas would be advantageous in microalgal biofuel production
systems. Flue gases that contain CO
2
concentrations ranging from 5% to 15% (v/v)
have been scrubbed for direct use in microalgal culture systems for biomass growth
(Kumar et al., 2010). This approach is believed to be pragmatic, more eco-friendly, and
technologically feasible for bio-mitigation of CO
2
as compared to physicochemical
adsorbents or deep-ocean injections. This is a win-win scenario wherein combating
air pollution through microalgal cultivation is possible while simultaneous microalgal
biomass generation can be exploited to produce biofuel and other VAPs.
A comparative evaluation of CO
2
sequestration potential of various microalgal species
is presented in Table 11.1. Some microalgal species such as
Chlorella
,
Scenedesmus
,
and
Botryococcus
are among the microalgae that have been studied for CO
2
consump-
tion and are promising for bio-mitigation of CO
2
(Griffith and Harrison, 2009; Fulke
et al, 2010).
Scenedesmus obliquus
was found to tolerate high CO
2
concentrations (up to
12% v/v) with optimal removal efficiency of 67%, when grown at pilot scale using
industrial flue gas as a carbon source (Li et al., 2011). Biomass generation through CO
2
sequestration and exploitation of biomass for biodiesel precursor formation has been
studied by Fulke et al. (2010).
Chlorella
sp. was found to have biomass productivity of
0.322 g L
−1
d
−1
with lipid productivity of 0.161 g d
−1
at 3% CO
2
as feed gas.
The presence of FAMEs (fatty acid methyl esters) suitable for biodiesel (e.g., palmitic
acid (C 16:0), docosapentaenoic acid (C 22:5), and docosahexaenoic acid (C 22:6))
have been confirmed. The calcite produced was characterized by Fourier transform
infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and x-ray dif-
fraction (XRD) (Fulke et al., 2010). The ability to tolerate CO
2
concentration during
growth is confined to the individual specie's characteristics. However, when exposed,
the CO
2
concentration in the gaseous phase does not provide a true reflection of the
actual concentration of CO
2
in the flue gas to which the microalgal specie is exposed
during dynamic liquid suspension. It depends on the alkalinity (pH) and the CO
2
concentration gradient created by the resistance to mass transfer (Kumar et al., 2010).
11.3 MICROALGAE: VALUE-ADDED PRODUCTS (VAPS)—
FUEL-BASED
Microalgae appear to be the only source of biodiesel that have the potential to com-
pletely replace fossil diesel (Table 11.2). Unlike other oil crops, microalgae grow
rapidly and many of them are exceedingly rich in oil (Griffith and Harrison, 2009).
Microalgae commonly double their biomass within 18 to 24 h (Sheehan et al., 1998).
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