Fuel 311 (2022) 122549
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Full Length Article
Bioconversion of chemically captured carbon dioxide into microalgal lipids,
a potential source of biodiesel: An integrated technique
Anoar Ali Khan a, b, *, Madhumanti Mondal b, G.N. Halder b, A.K. Saha c
a
Department of Chemical Engineering, Vignan’s Foundation for Science, Technology & Research, Vadlamudi, Guntur, India
Department of Chemical Engineering, National Institute of Technology Durgapur, Durgapur, India
c
Department of Chemical Engineering, Haldia Institute of Technology, Haldia, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Carbon dioxide
Absorption
(MDEA+PZ+H2O) blend
Chlorella sorokiniana BTA 9031
Biodiesel
The rise of atmospheric carbon dioxide (CO2) concentration due to extensive anthropogenic activity besides
rapid exhaustion of non-renewable energy sources demands evolution of clean and ecofriendly alternative fuel
source. Recently, lipid rich microalgal biomass is being considerably researched for generation of biodiesel
conversely, the expenses incurred on production of microalgal biomass is a substantial obstacle. Almost 80 % of
the production cost is generated from the cultivation medium which majorly consists of carbon, nitrogen and
phosphate. CO2 absorption by means of aqueous amine solvents is known to be a mature technology and could be
integrated with microalgal cultivation unit for efficient utilization of the captured CO2. In this current investi­
gation, piperazine (PZ) promoted aqueous blend of methyldiethanolamine (MDEA) having mass percentage ratio
of (8/22 wt%) was used a CO2 capturing agent and then the captured CO2 was utilized as an inorganic carbon
source for growing Chlorella sorokiniana BTA 9031 for biodiesel production. The CO2 absorption rate was gov­
erned by series of process variables namely, absorption temperature, initial CO2 concentration, solvent flow rate,
gas flow rate and the optimal conditions were 315 K, 15 kPa, 3.6 × 10− 4 m3 min− 1 and 4 × 10− 3 m3 min− 1
respectively. The highest biomass strength of Chlorella sorokiniana BTA 9031 was observed to be 1.20 ± 0.028 g
L− 1. The fatty acid methyl esters (FAME) profile was determined after acid transesterification of the extracted
microalgal lipids. It was observed to contain fatty acids suitable for biodiesel production.
1. Introduction
The continuous escalation of greenhouse gases (GHGs) in the at­
mosphere caused by numerous anthropogenic activities directing to
worldwide climate change has been a subject of global consideration
and significant research over the past decades [1]. Scientists and re­
searchers around the world are working towards the mitigation of this
global threat. Among the other GHGs, 60 % of the global warming is
caused only due to the carbon dioxide (CO2) because of its huge emission
rate [2]. The CO2 concentration in atmosphere increased from 300 ppm
in the pre-industrial period to 417.07 ppm in the present time, May 2020
[3]. This gradual escalation of CO2 in the present day’s atmosphere
stresses on dedicated studies and research towards the development of
suitable capturing methods for reducing the influence of global warming
effect due to CO2 emission.
Various methods of carbon capture pathways are available like,
conventional chemical and physical sorption, membrane based capture,
gas liquefaction means and CO2 capture by biological fixation. Physical
adsorption and chemical absorption techniques are considered to be the
extremely encouraging process towards post combustion CO2 capture
among all other separation processes since, membrane separation is still
at their nascent stage and capture of CO2 via cryogenic routes is a greatly
energy demanding method and is not cost effective [4,5]. Biological CO2
fixation has attracted many researchers but it requires longer time and
energy [6–7]. Chemical solvent (as absorbent) fascinated researchers
since it possesses suitable potential for CO2 removal through absorption
process and is also considered to be the most technologically advanced
capture technique [8–10]. Alkanolamine solvent poses a greater affinity
towards CO2 capture which is acidic in nature through carbamate for­
mation. The motivation towards blended amine solvent instead of any
single amine provides the significant improvement for the CO2 capture
technology through liquid amine solvent. The exact amalgamation
(mass percentage ratio combination) of various primary, secondary and
tertiary amines offers distinctive single amines beneficial features, such
* Corresponding author at: Department of Chemical Engineering, Vignan’s Foundation for Science, Technology & Research, Vadlamudi, Guntur, India.
E-mail address: anoaralikhan@gmail.com (A.A. Khan).
https://doi.org/10.1016/j.fuel.2021.122549
Received 12 May 2021; Received in revised form 23 July 2021; Accepted 7 November 2021
Available online 14 November 2021
0016-2361/© 2021 Elsevier Ltd. All rights reserved.
A.A. Khan et al.
Fuel 311 (2022) 122549
Fig. 1. Schematic of an integrated carbon dioxide capture unit along with microalgal cultivation unit.
as, the superior CO2 loading capability of tertiary amines along with
reasonably minimized energy requisite for regeneration activity in
addition to rapid reaction kinetics of primary or/and secondary amines
[11]. In recent years, attention moved towards the development of
activated aqueous solutions of tertiary or hindered amines like meth­
yldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP)
with polyamines like piperazine (PZ).
There are several literatures available which suggest an advantage of
(MDEA + PZ) blend solvent for CO2 capture through absorp­
tion–desorption process.
According to Closman et al. [12] the solvent blend methyl­
diethanolamine/piperazine (MDEA/PZ) has been investigated as an
alternative for CO2 capture from coal-fired power plants. MDEA/PZ
offers advantages over monoethanolamine (MEA) and MDEA alone
because of its resistance to thermal and oxidative degradation at typical
absorption/stripping conditions. The MDEA/PZ solvent blend provides
greater stability than conventional MEA (30 to 50 wt%) when tested at
conditions pertinent to CO2 scrubbing in flue gas. The presence of PZ in
the MDEA/PZ solvent blend may inhibit the thermal degradation of
MDEA. Samanta and Bandyopadhyay [13] reported that the addition of
small amounts of PZ to an aqueous solution of MDEA significantly en­
hances the rate of absorption and enhancement factor. Khan et al. [14]
conveyed the loading capacity of the aqueous solution of PZ activated
MDEA solvent was higher at same temperature and pressure conditions.
The high loading capacity of the investigated solvents makes it as good
potential solvent to capture CO2 in absorption process. Thus it can be
utilized as an effective solvent for CO2 capture at high pressure. Khan
et al. [15] investigated a piperazine (PZ)-promoted methyldiethanol­
amine (MDEA) solution for a carbon dioxide (CO2) removal process from
the flue gas of a large-scale coal power plant. They reported reboiler
duty and the total equivalent work were reduced by about 24.6 and
16.2%, respectively, as compared to the reference case. Dubois and
Thomas [16] reported that MDEA + PZ blend leads to a regeneration
energy of 2.19 GJ/tCO2 (35% energy savings in comparison with MEA
30% conventional configuration), the utilities costs being also lower
(24.5% savings) in comparison with the same reference case. The spe­
cific reason behind the selection of this particular blend of amine i.e.
(MDEA + PZ) is their outstanding characteristics for the CO2 capture
through absorption–desorption process.
In this current study, PZ activated aqueous blend of MDEA having
respective mass percentage ratio of (8/22 wt%) was used as a CO2
capturing agent to absorb CO2 from a self-modified coal-fired flue gas
generator unit. The selected blending combination offers an improved
absorption rate for CO2 capture in a packed absorber. The enhanced
reaction rate is attained because of the PZ characteristics which includes
higher reaction rate along with faster reaction kinetics with CO2.
However, relatively reduced regeneration energy requisite in the strip­
per section is attained since the reaction occurs through MDEA and CO2
forms only bicarbonate which could dissociate readily with temperature
effect. The polymeric cyclic diamine configuration allows one mole of
PZ to absorb theoretically two moles of CO2 and accelerate the carba­
mate formation [17–22]. The CO2 captured from exhaust gas by the
aqueous amine blend was transformed into clean fuel source-biodiesel
through microalgae. Numerous studies have been reported on the
growth of microalgae using flue gases rich in CO2 or pure CO2 but,
microalgal cultivation using the CO2 obtained from a desorption unit of
CO2-amine capture system is rarely stated. The CO2 recovered after
desorption was fed into photobioreactors for growing microalgae.
Microalgae are unicellular photosynthetic microorganisms which
use CO2 for growing photoautotrophically [22]. They have been
recognized as new biofuel feedstock however, the production cost of
microalgal biomass is a serious difficulty since, the cost incurred on
cultivation medium (mainly carbon) is considerably higher than other
essentials. Therefore, if the CO2 capture unit is integrated with the
microalgal cultivation unit then the cost of production of microalgal
biomass could also be dropped to a large extent. The CO2 consumed by
microalgae for growth is assimilated in them as lipids, which can be
extracted and transesterified into biodiesel. Biodiesel from microalgae
appears to be a suitable worldwide solution towards the replacement of
conventional fossil fuels. Biodiesel is an immediate option as a renew­
able fuel source as it contains no sulphur or aromatics and the burning of
biodiesel results in substantial reduction of emission of unburned hy­
drocarbons, carbon monoxide and particulate matter [23]. Therefore,
this integrated approach (as portrayed in Fig. 1) serves three purposes;
mitigation of GHG (through CO2 capture), energy crisis management as
well as the reduction in the cost of production of microalgal biomass for
2
A.A. Khan et al.
Fuel 311 (2022) 122549
culture of the Chlorella sp. was deposited in the National Repository for
Cyanobacteria and Microgreen algae (Fresh water) in Department of
Biotechnology, Government of India funded autonomous institute
named Institute of Bioresources and Sustainable Development, situated
in Imphal, Manipur, India. The submitted strain was confirmed to be in
pure form and devoid of bacterial contamination therefore, an accession
id ‘BTA 9031′ s was allotted to the strain. The species was identified to be
Chlorella sorokiniana by DNA extraction, PCR amplification and 18S
rDNA sequencing in our previous study [24]. The microalgae was
cultured and maintained in a broth named Blue Green-11 (BG-11) me­
dium retaining pH- 7.4. The components of the media were according to
Rippka et al., 1979 [25]. The culture was preserved at the light con­
centration under 80 μmol m− 2 s− 1 with photoperiod of 12 h: 12 h (light:
dark cycle) at 25 οC. At a precise time period of 2–3 days cultures were
agitated physically to prevent settling of microalgal cells at the bottom
of the conical flasks.
Table 1
Generated flue gas composition from customized coal-fired
boiler unit.
Gas composition
% or ppm
CO2
O2
N2
CO
NOx
SOx
10–15%
8–10%
74–76%
850–1090 ppm
180–380 ppm
420–660 ppm
biodiesel production.
In the current study, the effect of various parameters like, absorption
temperature, initial CO2 concentration, solvent flow rate, gas flow rate
on the capturing performance of PZ activated aqueous blend of MDEA
was observed. The flue gas was generated from self-modified coal-fired
boiler for the study. Further, the stripped off CO2 stored in a storage
container was utilized for cultivation of microalgal biomass for biodiesel
production.
2.3. Biomass productivity and growth rate determination
Microalgal growth was determined by spectrophotometric analysis.
The absorbance of the microalgal cells was recorded at 540 nm every
day through a spectrophotometer (Shimadzu spectrophotometer, UV1800, Japan). The dry cell weight (DCW) of the microalgae (g L− 1)
was detected by means of weighing the microalgal cells after drying
them at 60 οC in an air oven. Linear regression equations were used to
determine the association between DCW and optical density.
The yield of biomass, P (g L− 1 d− 1) for a time interval (cultivation
time) was determined by calculating the variation in the amount of total
biomass observed. The biomass productivity was measured according to
the following Eq. (1):
2. Materials and methodology
2.1. Absorption and desorption of CO2 from generated flue gas
In this current research, the exhaust flue gas (source of CO2) was
produced from a customized coal-fired boiler. The working gas or the
element of concern in the flue gas was CO2. The gas generated from the
boiler unit was analyzed by means of flue gas analyser (TESTO 350-S,
Germany). Table 1 represent the generated flue gas composition from
customized unit. The exhaust gas which liberated from the boiler stack
was sucked through a water scrubber unit and finally collected in a flue
gas storage bag. The absorption–desorption experiment of CO2 from flue
gas was performed by means of aqueous amine blend (PZ 8 wt% +
MDEA 22 wt%) as an absorbent in a packed absorber. Material of con­
struction of both absorber and stripper are made-up of glass. Entire
length of the absorber and stripper was 1.3 m with effective packing
length of 0.84 m and having the inner diameter of 0.04 m. A randomly
distributed metal HELI-PAK was introduced as a packing substance for
the absorber and stripping unit. A liquid distributor was fixed on top of
the column for the proper dispersal of solvent throughout the absorber.
The pre-determined process variables such as liquid absorbent flow rate
ranges (1.2, 2, 2.8 and 3.6) × 10− 4 m3 min− 1; absorption temperature
(300, 305, 310 and 315) K; concentration of CO2 (8 to 15) kPa and
absorbate (gas) flow rate (4 to 7) × 10− 3 m3 min− 1 were maintained
throughout the experiment. A flue gas analyser was employed to esti­
mate the concentration of CO2 before and after absorption all through
the experimental run. Stripping operation was performed after absorp­
tion process in a stripping column using CO2 enriched blends of amine.
The regeneration activity of working solution was carried out in a
stripping unit maintaining the temperatures of (373, 380, 385, and 390
K) with process duration of 30 to 120 min. The stripping performance
was conducted at 50–55 kPa (vacuum) and controlled by a needle valve
functioning in the bypass route of water ring vacuum pump. Detailed
experimental procedure along with physicochemical properties (i.e.
density, viscosity and surface tension) of different mass ratio combina­
tion of (MDEA + PZ) was reported in author’s earlier research [4]. After
regeneration, pure CO2 (99 %) was stored in a CO2 storage cylinder from
the top section of the stripper unit (i.e. condenser section). The pure CO2
obtained via the above mentioned process was utilized as a source of
microalgal growth towards biodiesel (clean fuel) production.
P = (X1 − X0 )/t1 − t0
(1)
Specific growth rate μ (d− 1) was calculated from the following Eq.
(2):
μ = ln(X1 /X0 )/t1 − t0
(2)
where X1 and X0 signifies the total amount of biomass produced (g L− 1)
on t1 day and t0 day correspondingly [26].
2.4. Microalgal cultivation using the CO2 obtained after stripping process
The microalgae was cultivated in a photobioreactor (capacity 1 L)
fabricated with perspex sheet which was filled with 0.5 L of sterile BG-11
(pH 7.4) medium without sodium carbonate since, 15 % CO2 was fed
into the photobioreactor which served as the carbon source for the
microalgae. The pure CO2 (from the CO2 storage cylinder after stripping
operation) was connected to a mixer vessel. In the mixer vessel, air was
mixed along with pure CO2 for achieving the exact CO2 concentration
required for cultivation. A CO2 analyser was interconnected with the
mixer vessel for continuous measurement of the CO2 percentage. The
CO2 analyser accurately delivered the exact CO2 concentration in the
mixture gas. The pre-determined mixer gas concentration (15 % CO2)
was provided and flowing at 250 mL min− 1 continually for 20 days from
the lower section of photobioreactor. As an experimental control, a
similar photobioreactor with same species was also maintained and fed
with only air (containing approximately 0.04 % CO2). The photo­
bioreactors were illuminated with 70 μmol m− 2 s− 1 light intensity and
photoperiod of 12 h: 12 h (light: dark cycle) at 25 ◦ C. For all the
experimental runs, 0.05 (g L− 1) cell concentrations were used as the
initial biomass. The axenic environments of cultures were conserved by
frequent monitoring of the samples of microalgal culture under light
microscope.
2.2. Microalgal strain and cultivation condition
In the current study, Chlorella sorokiniana BTA 9031 was used as the
microalgal species. It was isolated from a coalmine named Mahavir
(23ο37′ 44′ ’N 87ο06′ 54′ ’E) in Raniganj, West Bengal, India. The pure
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Fuel 311 (2022) 122549
2.5. Total lipid content determination
The total lipid content of microalgae was determined using modified
protocol of Folch. Concisely, 10 mL of the microalgal culture was har­
vested by centrifugation at 4000 rpm for 10 min and the pellet was dried
overnight at 65 ◦ C. Afterwards, 4 mL of methanol was then added to the
pellet and incubated at 160 rpm for 1 h followed by addition of 8 mL
chloroform and incubation at 160 rpm for 2 h. Centrifugation was done
at 4000 rpm for 10 min and the supernatant containing lipids was
transferred into another tube. The residue was extracted second time for
30 min with 3 mL of a mixture of methanol/chloroform (1/2) followed
by centrifugation at 4000 rpm for 10 min. The supernatant was then
pooled with the first one and washed with 4.5 mL of a 0.88% KCl so­
lution. Separation in two phases was accelerated by centrifugation and
the lower chloroform layer was transferred into another tube. The
chloroform layer was further centrifuged at 4000 rpm for 10 min for the
complete elimination of water and any trace solids. Solvent was evap­
orated at 50 ◦ C, the extracted lipid was re-dissolved in 5 mL of chloro­
form and transferred into a pre weighed tube and dried until constant
weight was achieved. Thereafter, the weight of the crude lipid obtained
was measured gravimetrically [27]. Finally, the crude lipid was
measured and the total lipid content was signified as a percentage of dry
cell weight (DCW).
Fig. 2. Specific absorption rate of CO2 using aqueous (22 wt% MDEA + 8 wt%
PZ) blend.
2.6. Preparation and profiling of FAME
The lipids extracted by the above mentioned Folch protocol from the
microalgal cells are not appropriate through gas chromatography (direct
injection) analysis since, structurally they are extremely polar. Conse­
quently, the conversion of it towards the methyl ester derivatives is an
essential step for analysis. Briefly, 15 mL of 2 % H2SO4 solution was
added to 500 mg of dried biomass in a round bottomed (RB) flask. RB
flask containing the mixture was refluxed by placing it in a heating
mantle for 4 h at 60 οC. The FAME solution obtained after reflux was
poured into a separating funnel and thoroughly mixed with ethyl acetate
and distilled water. Two separate aqueous phase layers were obtained
among which the lower layer was discarded keeping the upper layer
undisturbed. The upper layer was retained and poured into a fresh
separating funnel where it was washed with distilled water until pH 7.0
was obtained. The extract was then separated out into a RB flask and
Na2SO4 was added to it and kept for 20 min. The RB flask containing the
extract was rota-evaporated at 65 οC. Finally, the FAME solution was
rinsed by adding 50 µL dichloromethane and preserved in a fresh vial for
analysis [28,29].
The composition of the FAME solution was detected by using gas
chromatography with FID (Thermo scientific Chemito ceres 800 plus).
The sample was passed through capillary column (BPX70) using nitro­
gen as carrier gas flowing at a rate of 1.8 mL min− 1. The temperature of
the injector was 240 οC while temperature of the detector was main­
tained at 250 οC. The standard FAME mix SUPELCOTM 37 was used to
compare the composition of the FAME solution. The concentration of
each of the different FAMEs was calculated by the percentage area
method. The peak area of each fatty acid was compared with their
corresponding concentrations of standard using Chemito Chrom-card
software version 2.6.
Fig. 3. Percentage of CO2 absorption through (22 wt% MDEA + 8 wt%
PZ) blend.
3. Results and discussion
3.1. Absorption and desorption of CO2
The sorption phenomena of CO2 using predetermined blended
combination of (PZ 8 wt% + MDEA 22 wt%) was addressed under
consideration of specific rate of absorption, absorbed CO2 percentage
and regeneration efficiency of solvent blend through stripping opera­
tion. Fig. 2 revealed that CO2 absorption rate steadily rises with growing
solvent flow rate from (1.2 to 3.6) × 10− 4 m3 min− 1 and the value ranges
from (18.8–26.6) × 10− 6 kmol m− 2 s− 1 for the gradual rise of CO2
concentration. The enhanced rate of absorption is fundamentally due to
the increase in mass transfer coefficient but meanwhile specific rate of
absorption has been observed to increase as mass transfer coefficient
increases. This is because with the increase of liquid flow rate, the
droplets flow rate increases and the boundary layer of liquid phase de­
creases. So the resistance for gas diffusion to the liquid phase decreased
and the mass transfer performance is enhanced. Conversely, mass
transfer coefficient also increases with increasing interfacial area per
unit volume through higher interaction area accessible for mass transfer
which concludes the increasing rate of specific absorption with increase
in liquid flow rate [29].
The effect of CO2 concentration associated with absorption rate as
displayed in Fig. 2 states that the rate steadily escalates from 8 to 15 kPa
with a highest rate of 26.6 × 10− 6 kmol m− 2 s− 1. Samanta and
2.7. Statistical analysis
All experiments were performed in triplicates and results are
expressed as mean values ± standard deviation. The results of micro­
algal growth and lipid production were analysed by ANOVA single
factor and Student’s t test, applying a significance level of α = 0.05.
4
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Fuel 311 (2022) 122549
Fig. 6. Effect of number of cycle performance on regeneration efficiency.
Fig. 4. Influence of gas flow rate on CO2 removal efficiency (η).
Fig. 5. Regeneration efficiency of aqueous (22 wt% MDEA + 8 wt% PZ) blend.
Fig. 7. Influence of number of cycle performance on CO2 loading.
Bandyopadhyay [12] also reported the specific absorption rate of 28.8
× 10− 6 kmol m− 2 s− 1 with CO2 concentration of 14 kPa using (MDEA +
PZ) as a solvent blend. CO2 concentration is one of the substantial
considerations for CO2 gas capture by means of absorption (chemi­
sorption) technique in a packed column. The increased absorption rate
of CO2 was noticed because of more and more CO2 molecular trans­
formation from bulk gas stream to solvent-gas interface since there was a
steady increase of CO2 concentration in inlet gas stream. According to
two film theory, the gas phase driving force and the gas phase mass
transfer coefficient will increase with the increasing CO2 partial pres­
sure, which is beneficial to enhance absorption rate. Logically, an in­
crease in the CO2 partial pressure allows more CO2 molecules to travel
from gas bulk to the gas–liquid interface, which would result in higher
removal efficiency [30].
Fig. 3 represents the percentage absorption of CO2 and it gradually
rises with rise in CO2 concentration and blended solvent flow rate. It is
directly proportional to the absorbent flow rate and the highest CO2
absorption (95.6%) is attained when all the respective process param­
eter possesses their higher value. The percentage of CO2 absorbed is
directly proportional to the solvent flow rate and it indicates that
increasing liquid flow rate increases the CO2 absorbed.
CO2 removal efficiency (η) is a critical measure for CO2 gas capture
through absorption process. It is a collective outcome of gas flow rate
and absorbent concentration. Fig. 4 represents the effect of gas flow rate
on CO2 removal efficiency and it displays that the efficiency steadily
declines with rising gas flow rate from (4 to 7) × 10− 3 m3 min− 1 holding
the constant parameter of 315 K absorption temperature and liquid flow
rate of 3.6 × 10− 4 m3 min− 1 respectively. The obtaining CO2 removal
efficiency detected for the (22 wt% MDEA + 8 wt% PZ) amine blend is in
a declining trend of 84.8–72.4% with an increasing range of gas flow
rate from (4 to 7) × 10− 3 m3 min− 1. Increase in the gas flow rate leads to
an increase of volumetric overall mass transfer coefficients which gives
rise to the absorption rate increasing. However, the mole ratios of amine
to carbon dioxide decreases as the total gas flow rate increasing from (4
to 7) × 10− 3 m3 min− 1, this is the main reason of the reduction of
removal rate [31].
Regeneration temperature and requisite time for regeneration of CO2
enriched amine blends are the important parametric conditions to be
considered towards regeneration performance. Fig. 5 depicts that the
regeneration efficiency of aqueous (MDEA 22 wt% + PZ 8 wt%) blended
solvent. It displays that the efficacy steadily rises with rise in tempera­
ture from (373 to 390) K and regeneration time from 30 to 120 min with
the resultant value ranges of 80.9 to 91.7 %. The optimal regeneration
parameter was obtained at regeneration temperature of 390 K and
regeneration time of 120 min. Fig. 5 illustrates that the regeneration
efficiency increases with increase in temperature for aqueous (MDEA +
PZ) blend because the temperature effect delivers the necessary heat for
thermal breakdown of all the carbamate, bicarbonate and dicarbamate
formation throughout the reaction with CO2-amine and transform to
CO2 and free amine [4]. The regeneration temperature provides the
enthalpy of dissociation for the CO2 gas released from the CO2 rich
amine and it is the matter of carbamate stability. After completion of
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Fuel 311 (2022) 122549
1.4
A
The CO2 obtained after desorption process is continuously being
stored in the storage container of the CO2 capture unit. This pure CO2
(after stripping operation) is connected to the microalgae cultivation
unit, where the CO2 serves as an inorganic carbon resource for photo­
trophic microalgae growth. The pure CO2 is mixed with air with the help
of a mass flow controller and predetermined 15 % CO2 is supplied into
the photobioreactor continuously throughout the cultivation period.
B
Biomass concentration (g L-1)
1.2
1
0.8
0.6
3.2. Effect of 15 % CO2 on the production of biomass in Chlorella
sorokiniana BTA 9031
0.4
0.2
0
0
3
6
9
12
15
18
The growth of microalgae depends upon the three basic things- light
intensity, carbon source and water availability [32]. The carbon source
can be inorganic or organic according to the availability. The microalgal
isolate used in the study Chlorella sorokiniana BTA 9031 is capable of
consuming both organic and inorganic carbon. Depending upon the
accessibility of carbon it is decided whether it will grow photoautotro­
phically, heterotrophically or mixotrophically. In the present study, the
microalgal culture is fed with 15 % CO2 which serves as an inorganic
carbon source and directs the microalgae to grow photoautotrophically.
The CO2 obtained after desorption process is continually stored in the
storage tank of the CO2 sorption unit and from there it is fed into the
bottom of the photobioreactor after passing through a mixer vessel
where the appropriate percentage of CO2 is obtained by mixing the pure
CO2 with air.
The microalgal isolate cultivated in 15 % CO2 was observed to pro­
duce higher amount of biomass when compared to the control culture
cultivated in atmospheric CO2. At the end of cultivation, the final
amount of biomass was observed to 0.98 ± 0.01 g L− 1 for Chlorella
sorokiniana BTA 9031 grown in 15 % CO2 whereas, 0.45 ± 0.01 g L− 1 for
Chlorella sorokiniana BTA 9031 grown in air as depicted in Fig. 8. The
amount of biomass on the 15th day of cultivation was assessed to be
1.20 ± 0.028 g L− 1 while the amount of biomass in the control photo­
bioreactor fed with air was detected to be 0.547 ± 0.035 g L− 1. The
biomass achieved on the 15th day of cultivation was also the highest
amount of biomass recorded over the cultivation period of 20 days. The
biomass concentration enhanced from 0.061 ± 0.032 g L− 1 to 1.20 ±
0.028 g L− 1 over a period of 15 days with air enriched with 15 % CO2.
The biomass productivity was noticed to be 0.045 ± 0.005 g L− 1 d− 1 for
culture grown in air enriched with 15 % CO2 and 0.023 ± 0.0007 g L− 1
d− 1 for the control culture. The µ of the culture grown in 15 % CO2 was
determined to be 0.14 ± 0.012 d− 1, which is 1.4 times higher than the
control culture. Yee-Keung et al. 2013 [33] also reported µ of Chlorella
vulgaris grown in 15 % CO2 concentration as 0.148 d− 1 which is similar
with the µ obtained by the microalgae under study. The higher biomass
concentration can be attributed to the fact that Chlorella sorokiniana BTA
9031 was isolated from a coalmine in West Bengal. The microalgal
species isolated from coalmines and regions around the coalmines or
thermoelectric power plants tends to possess the capability of growing
under circumstances dominant in such areas like, occurrence of excess
carbon quantity in soil and combustible gas-air mixture produced by the
power stations. The results also corroborated with Sankar et al. 2014
[34] which conveyed a final biomass concentration 0.8824 g L− 1 of
Chlorella minutissima grown in 15 % CO2 in a stirred tank reactor. Basu
et al. 2014 [35] also reported that Scendesmus obliquss SA1 generated a
maximum amount of biomass of 1.1 g L− 1 at 15 % CO2.
Usually microalgal species utilizes CO2 for carrying out photosyn­
thesis and biomass production. The ability of microalgae to endure CO2
concentrations can be congregated into CO2 sensitive (2–5 %) or CO2
tolerant (5–20 %) and this capability is highly species specific [36]. The
results of the current study indicate that Chlorella sorokiniana BTA 9031
could be considered CO2 tolerant microalgae. Numerous studies have
been performed and reported which have explored the possibility of
growing Chlorella sp. in higher percentages of CO2 like, 15 %, 20 %, 30
%, 50 % or 100% but, not all higher percentages of CO2 assisted in
producing higher quantity of biomass concentration and productivity
20
Days
Fig. 8. Biomass concentration of Chlorella sorokiniana BTA 9031 observed
when grown A. In air enriched with 15 % CO2 and B. In air over the cultiva­
tion period.
regeneration of CO2 rich (22 wt% MDEA + 8 wt% PZ) blend, the
stripped off CO2 gas (99% pure) was stored in a CO2 cylinder. The exact
concentration of the stripped off CO2 was confirmed through CO2 ana­
lyser and desorption cell study.
The cyclic capacity of aqueous amine blends have been estimated
from the collective information of regeneration efficiency and CO2
loading capacity and it offers a better indication of CO2 removal per­
formance during absorption-stripping route. So as to find out the cyclic
capacity of aqueous blend of (22 wt% MDEA + 8 wt% PZ) regeneration
study has been carried out using the absorbed solvent of highest CO2
partial pressure of 15 kPa which has the maximum absorption capacity
at different regeneration temperature of 373, 380, 385, and 390 K. This
investigation has been performed with the CO2 rich amine blend having
the maximum loading capacity and the experiment is performed for five
cycles through absorption- stripping method. Figs. 6 and 7 illustrates the
cyclic capacity of (22 wt% MDEA + 8 wt% PZ) aqueous blend. Fig. 6
depicts that the regeneration efficiency decreases steadily after the 1st
cycle through 2nd, 3rd, 4th and 5th but it has a decreasing tendency
which is flat in nature because of the tertiary amine characteristics of
MDEA and strong carbamate and dicarbate formation of PZ with CO2.
Fig. 7 shows the CO2 cyclic capacity of aqueous blend of (22 wt% MDEA
+ 8 wt% PZ) and superior result obtained when the regeneration tem­
perature was kept constant at 390 K for cycle run 1 to cycle run 5 after
absorption and within a range of (0.788–0.737) moles of CO2 per mole
of amine through five absorption–stripping cycle.
Fig. 9. Total lipid content of Chlorella sorokiniana BTA 9031 obtained when
grown A. In Air enriched with 15 % CO2 and B. In air over the cultiva­
tion period.
6
A.A. Khan et al.
Fuel 311 (2022) 122549
Table 2
Quantitative determination of fatty acid composition of Chlorella sorokiniana BTA 9031 whilst grownup in 15 % CO2.
Polyunsaturated fatty acid (PUFAs) constituent
Monounsaturated fatty acid (MUFAs) constituent
Saturated fatty acid (SFAs) constituent
Lipid number
% fatty acid composition
Lipid number
Lipid number
% fatty acid composition
C18:2n6t
C18:2n6c
C18:3n6
C18:3n3
C20:3n6
C20:3n3
C20:4n6
C20:5n3
0.524
0.355
1.39
0.140
0.125
2.15
1.093
0.183
C14:1
C15:1
C16:1
C17:1
C18:1n9t
C18:1n9c
C20:1
C22:1
C24:1
Percentage total of PUFAs
Grand Total = 99.198
5.96
Percentage total of MUFAs
C10:0
C12:0
C13:0
C14:0
C15:0
C16:0
C17:0
C18:0
C20:0
C10:0
Percentage total of SFAs
0.021
0.360
0.661
1.233
52.18
10.30
4.20
0.77
8.24
0.021
77.965
% fatty acid composition
0.304
2.08
0.289
0.709
2.121
0.24
9.15
0.07
0.31
15.273
[37]. Maeda et al. 1995 [38] reported that Chlorella sp. T-1 was cultured
in air, 10 %, 30 %, 50 %, 80 % and 100 % CO2 concentrations. The
microalgal species survived in all the high CO2 concentrations but
showed minimal growth. Highest growing rate and biomass production
was observed only at 10 % CO2 in their study. Therefore, the species
might be tolerant to higher percentages of CO2 but it is also important to
know whether it is able to grow and reproduce in that high percentage of
CO2, only then it will be useful in CO2 capture studies [39,40].
to have capability to adapt to higher CO2 concentration (i.e. 15 % CO2)
and considered it as a CO2 resource for biomass generation as well as
lipid formation. The highest biomass strength of Chlorella sorokiniana
BTA 9031 was observed to be 1.20 ± 0.028 g L− 1. The fatty acid methyl
esters (FAME) profile determined the presence of fatty acids suitable for
biodiesel production. Hence, the integrated technology proved fruitful
in utilization of the chemically captured CO2, reduction in microalgal
cultivation cost and production of biodiesel.
CRediT authorship contribution statement
3.3. Effect of 15 % CO2 on production of lipid in Chlorella sorokiniana
BTA 9031
Anoar Ali Khan: Conceptualization, Data curation, Formal analysis,
Writing – original draft, Writing – review & editing. Madhumanti
Mondal: Conceptualization, Data curation, Formal analysis, Writing –
original draft. G.N. Halder: Conceptualization, Supervision. A.K. Saha:
Conceptualization, Supervision.
The total lipid content in the microalgal cells grown in 15 % CO2 was
observed to follow the similar trend as the concentration of biomass. The
total lipid content increased from 8 ± 0.005 % to 22 ± 0.016 % of DCW
over the cultivation period. The highest total lipid content in the
microalgal cells was found to be 23 ± 0.013 % of DCW and 14.50 ±
0.026 % of DCW in the 15 % CO2 treated culture and control culture
respectively on the 15th day of the cultivation as represented in Fig. 9. It
could be suggested that the air enriched with 15 % CO2 helped towards
the enhancement of the total lipid content in the microalgal cells from
14.50 ± 0.026 to 23 ± 0.013 % of DCW. The total lipid content in
Chlorella sorokiniana BTA 9031 was noticed to be higher when compared
to Chlorella kessleri (13.4 % DCW) grown in air enriched with 15 % CO2
[33].
The composition of fatty acid in the microalgae grown in 15 % CO2
has been conveyed in Table 2. Fatty acids from C-10:0 to C-24:0 was
found in the FAME solution. Along with ten saturated fatty acids (SFAs)
and nine mono unsaturated fatty acids (MUFAs), eight poly unsaturated
fatty acids (PUFAs) were also found. SFAs like Pentadecanoic acid
(C15:0), Arachidic acid (C20:0) and Palmitic acid (C16:0) were observed
to be in abundance while Elaidic acid (C18:1n9t), Cis-11-Eicosenoic acid
(C20:1) and C15:1 were observed to be the leading MUFAs. PUFAs,
MUFAs and SFAs represented 5.96 %, 15.273 % and 77.965 % respec­
tively of the total FAME esters. Since, the FAME profile shows fatty acids
required for biodiesel production, it could be suggested that Chlorella
sorokiniana BTA 9031 might be stated as a potential biodiesel feedstock.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
The authors might want to thankfully recognize National Institute of
Technology Durgapur and Vignan’s Foundation for Science, Technology
& Research, Vadlamudi for providing the facilities to execute the
research experiment.
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