Atmospheric CO2 Capture and Biofuel Production of Microalgae

Philip Mannino

Tingyu Wen

Kaixuan 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 best 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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Tingyu Wen

Kaixuan Xu

Scheme 1: The Calvin Cycle

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Scheme 2: Transesterification

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Tingyu Wen

Kaixuan Xu

Microalgae 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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Kaixuan Xu

Figure 1: FT-IR spectra at three different duration of microwave treatment

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Growth and Lipid Productivity of Nannochloropsis sp. With different concentrations of CO2in Gaseous and Solution Form:

Microalgae grow at higher CO2concentration (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 streammay be performed to concentrate CO2 to increase the efficiency of CO2 capture bymicroalgae. Besides the CO2 concentration, temperatureand light intensity can also affect microalgae growth and, consequently, the CO2 fixation rates andbiomass productivity.The photosynthetic efficiency of microalgae andCO2 solubility dropat high temperatures. Intense lightpenetrates 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 15% 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 observationsindicate 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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Kaixuan Xu

Table 1: Atmosphere carbon capture by microalgae.

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Kaixuan 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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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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Kaixuan 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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Tingyu Wen

Kaixuan 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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Tingyu Wen

Kaixuan Xu

Merits and Deficiencies

Compared to terrestrial plants, microalgae havehigher 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 raw 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 10years and microalgae-based biofuels are a promising solution for global climate change.

Large point sources, being the primary source of CO2 emission, have gained much 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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[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 Control2010, 4 (5), 865–877.