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Philip ManninoTingyu WenKaixuan Xu
Atmospheric CO2 Capture and Biofuel Production of Microalgae
Purpose, Identity of Microalgae
The increasing amount of carbon dioxide produced from burning fossil fuels has
led to a climate change that could have serious and catastrophic consequences.
Therefore, any method that can capture the carbon dioxide from flue gas and prevent
it from entering the atmosphere is of the utmost importance. Microalgae can be used
to not only capture CO2 from flue gas, but the captured CO2 can be utilized as
biofuels.1 Because of their ability to fix CO2 as either dissolved CO2 or CO2 salts such
as Na2CO3, NaHCO3, or H2CO3, microalgae have higher photosynthetic efficiency
than terrestrial plants.2 This fact leads many researchers to believe that microalgae are
the best candidates for carbon capture. Like all plants, microalgae get their energy
from a process known as the Calvin Cycle (scheme 1). Three molecules of Ribulose
1,3 bisphosphate react with three molecules of CO2 catalyzed by the enzyme rubisco
to generate six molecules of 3-phosphoglycerate (the carbons that come from carbon
dioxide are in red). These six molecules are reduced using six molecules of ATP and
six molecules of NADPH (the ATP and NADPH are generated during the light
dependent stage of photosynthesis) to produce six molecules of glyceraldehyde 3-
phosphate. Five of these molecules are used to regenerate the three molecules of
ribulose 1,3 bisphosphate and one is used to make half of glucose (2 cycles are
necessary to produce one full glucose molecule). Under certain conditions, the
glucose is broken down and stored as triglycerides (TGs).
A few groups have proposed that these TGs, which are carbon dense, would be
1
Philip ManninoTingyu WenKaixuan Xubest stored underground so as to lower the atmospheric CO2 concentration.3 Many
other groups, however, have proposed utilizing these TGs as biofuels. The most
widely accepted method for converting these TGs into biofuel is through a process
known as transesterification1 (scheme 2). In this process, the TGs are reacted with
three equivalents of an alcohol, typically methanol, producing three long chain esters
and glycerol. The three long chain esters would then be isolated and used as biofuels.
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Philip ManninoTingyu WenKaixuan Xu
O
O-23OP
OH
OH
OPO3
-2
Ribulose 1,3-bisphosphate
3 + CO
O
3rubisco
Carbon Fixation OH
OPO3
-26
3-Phosphoglycerate
Reduction
OHC
HO
OPO3
-2
Glyceraldehyde 3-Phosphate
1 Glyceraldehyde 3-Phosphate
1/2 Glucose
Triglycerides
CH
O
OHHO
HO
6
6 ATP
6 NADPH3 ATP
Scheme 1: The Calvin Cycle
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Philip ManninoTingyu WenKaixuan Xu
H2C
HC
H2C
O
O
OO
R
O
RO
R
Triglyceride
+ 3 CH3OH
H2C
HC
H2C
OH
OH
OH
O
RO+
Alcohol Glycerol
3
Esters
Scheme 2: Transesterification
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Philip ManninoTingyu WenKaixuan XuMicroalgae lipid through transesterification reaction with microwave
irradiation:
The suitability of Nannochloropsis sp. Lipid was determined by Fourier Transform
Infra-Red Spectrometry (FT-IR). In the FT-IR spectra, every peak is assigned to a
functional group. There are two shape peaks at 2839 and 2949 cm-1, showing the
vibration of C-H bond on carbohydrates and lipid. The peak at 1012 cm -1 indicates C-
O-C bond of polysaccharides. The bending of methyl lipids was shown at 1450 cm -1.
A broad peak at 3251 cm-1 shows the O-H stretching from water and N-H stretching
from proteins of Nannochloropsis sp. Another functional group from the proteins is at
1643 cm-1, indicating amide 1 (C=O) stretching. These results show the presence of
methyl and methylene groups, indicating the methyl ester formation. FT-IR spectra
also shows that the most suitable time for microwave treatment for biodiesel
production is 30 min. (Figure 1) 1
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Philip ManninoTingyu WenKaixuan Xu
Figure 1: FT-IR spectra at three different duration of microwave treatment
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Philip ManninoTingyu WenKaixuan XuGrowth and Lipid Productivity of Nannochloropsis sp. With different
concentrations of CO2 in Gaseous and Solution Form:
Microalgae grow at higher CO2 concentration (1–10%) present higher biomass
productivity compared to those grow at atmospheric CO2 concentration. Table 1
summarizes the experiments done on microalgae for atmospheric CO2 capture. CO2
removal rates and biomass productivity when growing autotrophically were recorded.
A pre-processing of gaseous stream may be performed to concentrate CO2 to increase
the efficiency of CO2 capture by microalgae. Besides the CO2 concentration,
temperature and light intensity can also affect microalgae growth and, consequently,
the CO2 fixation rates and biomass productivity. The photosynthetic efficiency of
microalgae and CO2 solubility drop at high temperatures. Intense light penetrates
better into high-density cultures.2
Nannochloropsis sp. strain was cultivated within five different concentrations of
CO2 (1, 10, 15, 20, and 25%) (v/v). According to figure 2, CO2 concentration at 15%
gave the most rapid growth of Nannochloropsis sp., producing the highest dry
biomass weight of 0.441 g/L at day 8. The second highest dry weight is 0.392 g/L on
day 8 at 10% of CO2 concentration. Throughout the cultivation period, the
Nannochloropsis sp. strain indicated an increasing trend at the first 8 days at 10%,
15%, and 20% concentrations of CO2. At 1% CO2 as a control group, the strain grew
much slower than other concentrations. The concentration of CO2 in the atmosphere is
about 0.0387% (v/v)4, which is not as high as the best growth condition for
microalgae. However, from combustion processes, wastes gases contains more than
7
Philip ManninoTingyu WenKaixuan Xu15% CO2, showing a possibility as a source for industrial microalgae production.
From other researchers’ study, there is a specific growth rate of Nannochloropsis sp.,
increasing from 0.33-0.52/day cultivated with atmosphere air with 15% CO2.5 All the
results show that high concentration of CO2 boosts photosynthetic efficiency. Thus,
microalgae can produce a higher quantity of biomass within a shorter time. On the
other hand, the Nannochloropsis sp. strain produced maximum amount of lipid with
15% CO2 showing in Figure 3. The total lipid productivity was determined after eight
days of cultivation under five different CO2 gas concentrations. The highest yield of
lipid productivity is 18.93mg/L per day with 15% CO2.
Solute carbonates or dissolved CO2 can be absorbed directly by microalgae. In this
study, the ability of microalgae growth with carbonate salts from fuel gases was
measured by a series solutions containing HCO3- and CO32- in five concentrations
(2,5,10,15,20% (v/v)). The results show an increasing rate on the dry biomass weight
as the carbonate concentration increased. (Figure 4) The highest dry biomass of 0.55g
was found at the 20% carbonate on day 10, corresponding to the highest dry biomass
of 0.44g at the 15% CO2 gas on day 8. These observations indicate that growth rate of
microalgae in solute carbonate condition is better and faster than in CO2 gas
conditions. Figure 5 shows the effect of series fuel gas solution on lipid productivity.
Nannochloropsis sp. produce more in a more concentrated fuel gas condition. The
maximum yield of lipid productivity was found at 20% carbonate solution with 23
mg/L per day. It was also 27% higher than that produced by the 2% carbonate
solution.
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Philip ManninoTingyu WenKaixuan Xu
Table 1: Atmosphere carbon capture by microalgae.
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Philip ManninoTingyu WenKaixuan Xu
Figure 2: Comparison of dry biomass vs times of Nannochloropsis sp. strain cultivated in five different flasks with CO2 concentrations of 1, 10, 15, 20 and 25% (v/v).
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Philip ManninoTingyu WenKaixuan Xu
Figure 3: Comparison of lipid productivity vs CO2 level of Nannochloropsis sp. strain cultivated in five different flasks with CO2 concentrations of 1, 10, 15, 20 and 25% (v/v).
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Philip ManninoTingyu WenKaixuan Xu
Figure 4: Comparison of dry biomass vs times of Nannochloropsis sp. strain cultivated in sodium bicarbonate/sodium carbonate series solutions with concentration of 2, 5, 10, 15, 20% (v/v).
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Philip ManninoTingyu WenKaixuan Xu
Figure 5: Comparison of lipid productivity vs flue gas solution (%) of Nannochloropsis sp. strain cultivated in sodium bicarbonate/sodium carbonate series solutions with concentration of 2, 5, 10, 15, 20% (v/v).
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Philip ManninoTingyu WenKaixuan XuMerits and Deficiencies
Compared to terrestrial plants, microalgae have higher efficiency in converting
solar energy into biomass (ten times higher than terrestrial plants). These
microorganisms present high growth rate with some species double their cell count in
few hours. High growth rate ensures high CO2 fixation rate. 1.83 kg of CO2 can be
fixed via cultivating one kilogram of microalgae. Species such as Anabaena sp. and
Chlorella vulgaris presents high CO2 fixation rates at 1.45 g L-1 d-1 and 6.24 g L-1 d-1,
respectively.6 Compared to other plants, arable land not required with microalgae
production. They are also easier to grow. Even lower grade water (wastewater) could
be used as culture medium. Anaerobic digestion of microalgae, which includes
processes such as: hydrolysis, acidogenesis, acetogenesis and methanogenesis,
converts organic biomass into volatile fatty acids and methane under little or without
oxygen conditions.7 In addition to converting lipid to ester as biofuel (mentioned in
the beginning), some species are able to produce a considerable amount of methane.
Saccharina latissima and Ulva lactuca can produce 68.2 and 95.6 ml of methane per
gram of algae, in 36 and 42 days respectively.8 If the purity of recovered methane is
higher than 95%, the gaseous mixture can be shipped as compressed natural gas.9
Anaerobic digestion also converts the biomass into plant nutrients that can be recycled
to sustain the microalgae growth and to provide even greater yields. The production of
biofuel by microalgae is essentially a carbon neutral process with zero emission of
CO2, because the CO2 generated from biomass combustion can be reused for
microalgae growth. In addition, when compared with biofuels produced from other
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Philip ManninoTingyu WenKaixuan Xuraw materials, microalgal biofuels present high productivity and high grade oil
production (petroleum fuel substitutes). Microalgae farm has many other merits: (i)
they help protect the biodiversity of the oceans by providing shelters and serving as
feeding/ nursery grounds for many organisms8; (ii) it can be incorporated in
aquaculture systems as the oxygen produced by microalgae improves the respiration
of cultivated animals; (iii) some microalgae, like seaweed, are excellent food sources
due to its high protein and fiber content as well as low total lipid; (iv) microalgae can
synthesize a number of molecules with biological activities, such as phenolic
compounds, carotenoids, alkaloids and polysaccharides that can be used in
pharmaceutical and cosmetic industries.10
Currently, the main challenges to biofuel production by microalgae are related
to the viability of large-scale commercialization of microalgae, because it still
requires large investment and high energy consumption associated to the production
and downstream processes of biomass. Additionally, the low atmospheric CO2
concentration limits microalgae growth. Integration of a physicochemical process to
concentrate CO2 in feeding gaseous stream to microalgal cultures will enhance
microalgal productivities and CO2 capture efficiencies. Furthermore, future studies on
light distribution and nutrients are needed to enhance microalgae growth condition
and optimize productivity.11 Nevertheless, it is believed that a sustainable microalgal
biofuel production can be possible in 10 years and microalgae-based biofuels are a
promising solution for global climate change.
Large point sources, being the primary source of CO2 emission, have gained
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Philip ManninoTingyu WenKaixuan Xumuch attention over the years as an experimental field for CO2 capture methods.
However, even if CO2 emissions from large point sources were immediately reduced
to zero, climate change would still continue due to the existing greenhouse gas present
in the atmosphere. Diffuse sources constitute half of CO2 emissions. Despite being
more expensive and less efficient than CO2 capture from point sources, CO2 capture
from atmosphere is still an important complement. Microalgae play an important role
in stabilizing atmospheric CO2 concentration by capturing this pollutant directly from
air. It has the following advantages compared to other point-source carbon capture
methods: (i) it captures CO2 from any part of the economy emitted at different
location and time; (ii) an algae farm can be located anywhere; (iii) CO2 transport
infrastructure is not required; (iv)microalgae are very eco-friendly, due to the absence
of toxicity and biodegradability.12 Maity et al. (2014)13 reported a possible reduction
of half CO2 emissions with the using of microalgae based biofuel instead of
petroleum-based transportation fuels. It is expected that the operation of large-scale,
commercial biomass energy systems under stringent climate policies can help reduce
the atmospheric CO2 concentration to between 400 and 450 ppm at the end of the
century.14
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1 Saifuddin, N.; Aisswarya, K.; Juan, Y. P; Priatharsini, P. Sequestration of High Carbon Dioxide
Concentration for Induction of Lipids in Microalgae for Biodiesel Production. Journal of Applied
Sciences 2015. 15(8), 1045-1058.
2 Moreira, D.; Pires, J.C.M. Atmospheric CO2 capture by algae: Negative carbon dioxide emission
path. Bioresource Technology 2016. 215, 371-379.
3 R. Sayre. Microalgae: The Potential for Carbon Capture. BioScience 2010, 60, 722-726.
4 Kumar, A.; S. Ergas, X.; Yuan, A.; Sahu; Zhang, Q. et al., Enhanced CO2 fixation and biofuel
production via microalgae: Recent developments and future directions. Trends Biotechnical. 2010,
28, 371-380.
5 Jiang, L.; Luo, S.; Fan, X.; Yang, Z.; Guo, R. Biomass and lipid production of marine microalgae
using municipal wastewater and high concentration of CO2. Applied Energy 2011, 88, 3336-3341.
6 Ghorbani, A.; Rahimpour, H.R.; Ghasemi, Y.; Zoughi, S.; Rahimpour, M.R. A review of carbon
capture and sequestration in Iran: microalgal biofixation potential in Iran. Renew. Sustain. Energy
2014, 35, 73–100.
7 Song, M.; Pham, H.D.; Seon, J.; Woo, H.C. Overview of anaerobic digestion rocess for biofuels
production from marine macroalgae: a developmental erspective on brown algae. Korean J. Chem.
Eng. 2015, 32 (4), 567–575.
8 Nielsen, H.B.; Heiske, S. Anaerobic digestion of macroalgae: methane potentials, pre-treatment,
inhibition and co-digestion. Water Sci. Technol. 2011, 64 (8), 1723–1729.
9 N’Yeurt, A.D.; Chynoweth, D.P.; Capron, M.E.; Stewart, J.R.; Hasan, M.A. Negative carbon via
ocean afforestation. Process Saf. Environ. Prot. 2012, 90 (6), 467– 474.
10 Tabarsa, M.; Rezaei, M.; Ramezanpour, Z.; Waaland, J.R. Chemical compositions of the marine
algae Gracilaria salicornia (Rhodophyta) and Ulva lactuca (Chlorophyta) as a potential food source.
J. Sci. Food Agric. 2012, 92 (12), 2500–2506.
11 Moreira; Diana; Pires, J. C. M. Atmospheric CO2 Capture by Algae: Negative Carbon Dioxide
Emission Path. Bioresource Technology 2016, 215, 371–379.
12 Pires, J.C.M.; Alvim-Ferraz, M.C.M.; Martins, F.G.; Simoes, M. Carbon dioxide capture from flue
gases using microalgae: engineering aspects and biorefinery concept. Renew. Sustain. Energy Rev.
2012, 16 (5), 3043–3053.
13 Maity, J.P.; Bundschuh, J.; Chen, C.Y.; Bhattacharya, P. Microalgae for third generation biofuel
production, mitigation of greenhouse gas emissions and wastewater treatment: present and future
perspectives – a mini review. Energy 2014, 78, 104–113.
14 Luckow, P.; Wise, M.A.; Dooley, J.J.; Kim, S.H.; Large-scale utilization of biomass energy and
carbon dioxide capture and storage in the transport and electricity sectors under stringent CO2
concentration limit scenarios. Int. J. Greenhouse Gas Control 2010, 4 (5), 865–877.