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Carbon dioxide fixation and biomass production from combustion flue gas using energy microalgae
Energy xxx (2015) 1e11
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Carbon dioxide fixation and biomass production from combustion flue gas using energy microalgae Bingtao Zhao a, *, Yaxin Su b, Yixin Zhang a, Guomin Cui a a b
School of Energy and Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2014 Received in revised form 19 May 2015 Accepted 21 May 2015 Available online xxx
Algae-based bioenergy has been regarded as the next generation of renewable energy. To fix CO2 from flue gas and harvest algal biomass for energy conversion, three energy microalgae, Chlorella sp., Isochrysis sp. and Amphidinium carterae, were investigated in 1-L bubble column photobioreactors with an aeration of 15% CO2 at the flue-gas level. According to the potential on CO2 fixation and biomass production, Chlorella sp. was selected as the dominant species due to its superiority to the other species, with a specific growth rate of 0.328 d1, a biomass production rate of 0.192 gL1 d1 and a CO2 fixation rate of 0.353 gL1 d1. Furthermore, Chlorella sp. was cultured under varied physicochemical parameters, including CO2 concentrations, aeration rates and toxic compounds (SO2, NO and Hg2þ) to assess its performances. The maximum specific growth rate, biomass production rate and CO2 fixation rate were found to be 0.372 d1, 0.268 gL1 d1 and 0.492 gL1 d1 at a CO2 concentration of 10%; 0.375 d1, 0.274 gL1 d1 and 0.503 gL1 d1 at an aeration rate of 0.1 vvm; and 0.328 d1, 0.192 gL1 d1 and 0.353 gL1 d1 in the absence of toxic compounds, respectively. The results provide a basis for microalgal-based CO2 emission reduction and bioenergy utilization in pilot-scale applications. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Energy microalgae CO2 fixation Biomass production Flue gas Physicochemical parameters
1. Introduction Carbon dioxide (CO2) from human activity has been demonstrated to be responsible for global warming and the greenhouse effect [1]. Anthropogenic CO2 emissions, particularly the emissions from fossil fuel combustion, have become important scientific, environmental and even international economic and political issues. Stable, safe and environmentally acceptable post-combustion CCS (CO2 capture and storage) as well as product recycling technologies, are necessary in addition to improvements in current energy efficiency and the development of other renewable energies. According to scientific principles, post-combustion CO2 capture can be roughly categorized as chemical absorption, physicochemical adsorption, membrane, cryogenics, CLC (chemical looping combustion) and terrestrial sequestration [2]. A potential and promising biologic approach, microalgae-based CO2 fixation and energy/resource utilization, has received significant attention over the last two decades due to its techno-economic feasibility and
* Corresponding author. Tel.: þ86 21 55271751; fax: þ86 21 55272376. E-mail address: [email protected] (B. Zhao).
environmental friendliness [3e13]. In essentials, the microalgae essentially biologically fix and store CO2 via photosynthesis, which can convert water and CO2 into organic compounds without secondary pollution. Microalgal-CO2 fixation features potential advantages over other carbon capture and storage approaches, such as a wide distribution, high photosynthesis rate, good environmental adaptability and easy operability. Additionally, the microalgal biomass can be harvested after CO2 fixation to produce microalgal biofuel that can be utilized as a renewable or sustainable energy source (Fig. 1). Although microalgal-based CO2 fixation and biomass production are promising, this approach suffers from many specific challenges that need to be overcome in scientific research and engineering practice. Most of microalgae appear to be able to produce biomass and fix atmospheric CO2 (0.038%, v/v) (Table 1), e.g. Isochrysis galbana [14,15], Chaetoceros calcitrans [15], Chlorella vulgaris [16] and Chlorella sp [17]. Despite differences in culture modes, operating conditions and photobioreactors, the data indicated that these species had a specific growth rate of 0.183e0.645 d1 and biomass production rate of 1 1 0.011e0.840 gL d . Furthermore, some microalgae can be used to produce biomass and fix CO2 at low concentrations (0.038e5%, v/
http://dx.doi.org/10.1016/j.energy.2015.05.123 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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Fig. 1. Flowchart of microalgae-based CO2 biofixation, biomass production and energy conversion from combustion flue gas.
v) (Table 1), including Porphyridium purpureum 1380-1A, which is effective at 2% CO2 [18], Chlorella vulgaris, which is effective at 0.1e2.8% CO2 [19], Chlorella sp., which is effective at 0.038e5% CO2 [20] and Chlorella sp. AG10002, which is effective at 0.5e5% CO2 [21]. The specific growth rates of these species ranged from 0.127 to 0.885 d1, and their biomass production rates ranged from 0.074 to 1.060 gL1 d1. However, due to the differences in the CO2 concentration level between the atmosphere and the combustion flue gas, the adaptability and performance of these microalgae in high
CO2 concentrations, including at flue-gas CO2 concentrations (usually >10%, v/v), are unknown. Usually, the ability of microalgae to produce biomass and fix CO2 varies by species and culture condition. A previous study indicated that CO2 concentrations exceeding 1% (v/v) negatively affect most of the microalgae by inhibiting the Rubisco activity in the CCM (carbon concentrating mechanism) [19]. However, the selfadaption of some improved microalgae can overcome this effect. Therefore, microalgae species have been modified using special
Table 1 Summary of biomass performance on microalgae-based CO2 fixation. Microalgae Used for atmospheric CO2 fixation Isochrysis galbana Isochrysis galbana Chaetoceros calcitrans Chlorella vulgaris Chlorella sp. Used for low concentration CO2 fixation Porphyridium purpureum 1380-1A Chlorella vulgaris Chlorella sp. Chlorella sp. AG10002 Used for high concentration CO2 fixation Nannochloris sp. Nannochloropsis sp. Phaeodactylum tricornutum Chlorococcum littoral Chlorella sp. KR-1 Chlorella vulgaris Euglena gracilis Scenedesmus obliquus Chlorella kessleri Spirulina sp. Scenedesmus obliquus €geli Aphanothece microscopica Na Chlorella sp. MTF-7
CO2 concentration (%, v/v)
Specific grow rate (d1)
Biomass production rate (gL1 d1)
Ref.
0.038 0.038 0.038 0.038 0.038
0.183e0.207 0.645 0.387 0.293e0.416 0.288e0.512
0.100e0.126 0.011 0.007 0.436e0.840 0.139e0.751
[14] [15] [15] [16] [17]
2 0.1e2.8 0.038e5 0.5e5
0.250 NA 0.127e0.605 0.792e0.885
0.304 0102e1.060 0.074e0.207 0.192e0.335
[18] [19] [20] [21]
15 15 15 20 15 0.038e30 10 0.038e18 0.038e18 0.038e12 0.038e12 15 2e25
0.700 0.657 0.501 0.078 1.547 0.528e0.745 0.599 0.216e0.261 0.199e0.257 0.330e0.440 0.150e0.220 0.067e0.611 0.312e0.411
0.256 0.229 0.180 0.400 1.400 0.092e0.387 0.114 0.064e0.090 0.061e0.090 0.040e0.200 0.040e0.140 0.008e0.800 0.183e0.358
[22] [22] [22] [23] [24] [25] [26] [27] [27] [28] [28] [29,30] [31]
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methods, including selective isolation, pre-culture and habituated culture, to enhance the ability to fix CO2 and produce biomass. They were found to be able to endure and grow in high concentrations of CO2 (usually >10%, v/v) (also shown in Table 1), including the microalgae Nannochloris sp. and Nannochloropsis sp., Chlorococcum littoral and Phaeodactylum tricornutum [22], Chlorococcum littoral [23], Chlorella sp. KR-1 [24], Chlorella vulgaris [25], Euglena gracilis [26], Scenedesmus obliquus and Chlorella kessleri [27], Spirulina sp. €geli and Scenedesmus obliquus [28], Aphanothece microscopica Na [29,30] and Chlorella sp. MTF-7 [31]. They were found to be able to grow at 10e25% CO2 with specific growth rates ranging from 0.067 to 1.547 d1 and biomass production rates ranging from 0.008 to 1.400 gL1 d1. In particular, the microalgae Chlorella H-84 [32], Chlorella sp. KR-1 [33], Chlorella ZY-1 [34] and Chlorella sp. T-1 [35] were found to grow at CO2 concentrations of up to 40%, 70%, 70% and 100%, respectively. Furthermore, certain special and improved microalgae could adapt to the toxic compounds in flue gas [24,33] and mobile exhausts [36]. Nevertheless, their performances in comprehensively varied flue gas conditions were not addressed, especially at varied gas flow rates and in the presence of trace metals in flue gas. Moreover, the parameters related to biofixation and biomass production (e.g. pH value and dissolved oxygen) have not yet been explored in detail. At present, the effective fixation of CO2 and biomass production from combustion flue gas via microalgae continues to face many challenges. Most importantly, microalgae must not only contain high amounts of lipids, which are easily converted into high quality biofuel, but they must also perform well in a complex flue gas environment for pilot-scale applications, which is not possible when using microalgae that are habituated to in vitro culture or genetically induced in bench-scale experiments. Energy microalgae appear to meet these needs, but their characteristics of CO2 fixation and biomass production need to be comprehensively examined due to the complexity of flue gas conditions (e.g., 10e20% CO2 concentration, high flow rate and the presence of toxic compounds, such as SO2, NOx, and trace metals, etc.). Additionally, large amounts of microalgae are required for the large-scale applications, which is difficult to achieve using modified microalgae cultured at the lab-scale. Currently, comprehensive reports that systemically examine the behavior and characteristics of these energy microalgae in the fixation of CO2 and production of biomass in response to varied flue gas conditions are rare. This work experimentally investigated microalgae-based CO2 fixation and biomass production from simulated combustion flue gas to provide a reference for pilot-scale applications. First, the CO2 fixation and biomass production of three candidate natural energy microalgae species, Chlorella sp., Isochrysis sp. and Amphidinium carterae, were compared at the flue-gas CO2 concentration level. On this basis, a high-performance species was selected to further examine the effects of varying physicochemical parameters, including the CO2 concentration, aeration rate and addition of toxic compounds (SO2, NO and Hg2þ), on the performance and behavior-
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related parameters, such as pH value and dissolved oxygen, of this species. 2. Materials and methods 2.1. Microalgae species and culture medium The microalgae candidates Chlorella sp., Isochrysis sp. and Amphidinium carterae were used in this work, because they are important energy microalgae representatives of the class chlorophyceae, chrysophyceae and dinophyceae [37e40]. The oil content (based dry weight) was found to be 33% for the Chlorella sp., 29% for the Isochrysis sp. and 23% for the Amphidinium carterae, respectively. Moreover, the great advantages of them are that they are easily available, widely distributed and potential application prospect. Their physical and morphological properties are shown in Table 2. All microalgae were cultured in f/2 medium in artificial seawater with 74.79 gL1 NaNO3, 5.66 gL1 NaH2PO4$2H2O, 15.35 gL1 Na2SiO3$9H2O, 23.0 mgL1 ZnSO4$7H2O, 178.1 mgL1 MnCl2$4H2O, 7.26 mgL1 Na2MoO4$2H2O2 and 11.9 mgL1 CoCl2$6H2O. The trace elemental solution contained 9.99 mgL1 CuSO4$5H2O, 4.36 gL1 Na2EDTA$2H2O, 4.15 gL1 FeC6H5O7$5H2O, 5 mgL1 Cyanocobalamin (VB12), 5 mgL1 Biotin (VH) and 1 gL1 Thiamine-HCl (VB1). 2.2. Experimental system and conditions The experimental microalgae culture system is illustrated in Fig. 2. The microalgae were cultured in bench-scale bubble column photobioreactors (diameter of 65 mm and height of 420 mm) with a working volume of 1 L. All photobioreactors were placed in a 250L digitally controlled light incubator (SPX-250GB, Hede Corperation, Shanghai, China), which was equipped with an ultraviolet sterilamp and a sterilizing PTFE membrane filters in ventilation system to ensure the pre-sterilization and aseptic condition during the culture process. The cultivations were operated at 18 ± 1 C (the local average annual temperature), a light intensity of 84 mmolm2 s1 (the local average light intensity) and a lightedark cycle of 12:12 h (approximate day/night ratio). The bubble column photobioreactors were continuously aerated with the simulated flue gas, which consisted of CO2 and N2 with or without the toxic compounds SO2 and NO. Hg2þ was dynamically and drop-wise added to the culture medium via an HgCl2 solution and was converted in the flue gas to the desired concentration. All cultures were inoculated with the microalgae species at the same initial number concentration (initial cell concentration) of 1 106 cells mL1, and the culture period was set as 7 days. The initial pH value was 8.0 ± 0.1, and the pH value was not artificially controlled during the entire culture process, with the exception of the control experiment for the SO2 effect. The detailed experimental design is shown in Table 3.
Table 2 Physical and morphological properties of energy microalgae used in this work. Microalgae
Chlorella sp.
Isochrysis sp.
Amphidinium carterae
Class Size Appearance Cell morphology (640)
Chlorophyceae 3e5 mm Green, spherical
Chrysophyceae 5e7 mm Golden, 2 flagellate
Dinophyceae 12e17 mm Yellow, 4 flagellate
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Fig. 2. Schematic diagram of experimental system. 1-Simulated flue gas cylinder; 2-Valve; 3-Flow meter; 4-Mixer; 5-Gas inlet; 6-Light incubator; 7-Bubble photobioreactor; 8-Light; 9-pH meter; 10-Dissolved oxygen meter; 11-Spectrophotometer; 12-Digital biomicroscope; 13-Computer for image and data processing; 14-Gas outlet.
Table 3 Design of the experiment. Stage
Microalgal species
CO2 concentration (%, v/v)
N2 concentration (%, v/v)
Aeration rate (vvm)
Toxic compounds (ppm)
I
Chlorella sp. Isochrysis sp. Amphidinium carterae Dominant species screened
15 15 15 10, 15, 20 15 15 15 15
85 85 85 90,85,80 85 85 85 85
0.2 0.2 0.2 0.2 0.1, 0.2, 0.3 0.2 0.2 0.2
/ / / / / 0, 100, 250, 250 (controlled pH) for SO2 0, 100, 250 for NO 10,20,30 mg m3 for Hg2þ
II
2.3. Assay methods
2.4. Characterizing parameters
The number of microalgae cells in the microalgae suspension sample was determined using an observation system that included a bright-line hemocytometer (Qiujing, Shanghai, China) and a digital bio-microscope XSP-30 (Phenix optics Group Co., Ltd, Jiangxi, China). The cell biomass concentration was determined by measuring the absorbance at 680 nm (A680) using a visible spectrophotometer SP-721E (Shanghai Spectrum Instruments Co., Ltd, Shanghai, China); correspondingly, the dry biomass was measured after the microalgae cell sample was filtered through a pre-dried and pre-weighed 0.45 mm membrane filter and then dried at 105 C for 12 h. Light intensity was measured adjacent to the photobioreactor using a Basic Quantum Meter LX-1010B (Jinda Technologies Co. Ltd, Shenzhen, China). The pH value of the culture medium samples was directly measured everyday using a pH meter P-II (Shenqi Automation Instrumentation Co. Ltd, Shanghai, China) calibrated with pH 4.01 and 9.18 standard buffer solutions. The DO (dissolved oxygen) value in the culture medium samples was also directly measured using a self-calibrating dissolved oxygen meter AZ-8403 (Hengxin Instrumentation Co. Ltd, Taiwan). The uncertainty analysis according to the preliminary investigation using controlled parallel experiments indicated that, under the designed conditions, the maximum relative errors of the experimental data were in the range of ±5%. All experimental data in this work were obtained based on this measurement accuracy.
The SGR (specific growth rate) was used to describe the microalgae biomass kinetics in this work:
k ¼ ðln Xt ln X0 Þ=ðt t0 Þ
(1)
where k is the SGR (d1), and Xt and X0 are the biomass concentration (based on dry cellular weight) (gL1) at culture time t and t0, respectively. The BPR (biomass production rate), i.e., the linear growth rate or average growth rate, was used to estimate the microalgae biomass productivity according to the following equation:
p ¼ ðXt X0 Þ=ðt t0 Þ
(2)
where p is the BPR (gL1 d1). Correspondingly, the CFR (CO2 fixation rate) could be estimated according to the biomass production rate and carbon content in microalgae biomass. some investigations statistically estimated it via an approximate molecular formula of microalgae biomass with CO0.48H1.83N0.11P0.01 (C, O, H, N, and P represent the elements carbon, oxygen, hydrogen, nitrogen and phosphorus, respectively) [11,40], which is derived from the data reported by Grobbelaar (2004) [41]. However, the elemental composition is usually varied with environmental conditions, culture processes and in particular with microalgae species. To characterize the CFR for most
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unicellular microalgae, this work uses the following equation for calculation:
f ¼ pCC MCO2 MC
(3)
where f is CFR based on the initial microalgae biomass (gL1 d1) and p is the microalgae biomass production rate. CC is the carbon contents (wt) in the three microalgae biomass and can be experimentally determined by elemental analysis. In this work CC were found to be 50.1% for Chlorella sp., 46.8% for Isochrysis sp. and 47.2% for Amphidinium carterae, respectively. MC and MCO2 are the molar mass of Carbon and CO2, respectively. 3. Results and discussion 3.1. Performance of energy microalgae candidates in response to flue gas CO2 concentration levels The performances of three energy microalgae at a typical fluegas CO2 concentration of 15% are shown in Fig. 3. Note that differences in the physical size, moisture content and cell structure resulted in their different initial biomass concentrations, even though the cell numbers did not differ (Table 4). These cells did not uniformly adapt to the simulated flue gas environment. Microalgae Chlorella sp. and Isochrysis sp. could grow, but Amphidinium carterae showed poor performance under this condition. Specifically, almost all Amphidinium carterae cells entered decline after aeration with simulated flue gas for one day. The microalgae Chlorella sp. and Isochrysis sp. still exhibited the difference even though could both grow under these conditions. Specifically, Chlorella sp. performed better than Isochrysis sp. Moreover, a significant lag phase was not observed for Chlorella sp., while a 4-day lag phase was identified for Isochrysis sp. Microalgal-CO2 fixation and biomass production are closely associated with the physicochemical conditions (e.g. CO2 concentration, pH value, light intensity, etc.) and hydrodynamic conditions (e.g. gas-liquid-solid (gas-medium-microalgae cell) flow and mass transfer, etc.). However, the microalgae species plays a vital role in CO2 fixation and biomass production, as it essentially determines the difference in the effect of these factors on the microalgae CCM (carbon concentrating mechanism). For example, Chlorella has been considered one of the energy microalgae with high potential for
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application in complex and harsh environments, such as flue gas. Some Chlorella may have good performances via special modifications (e.g., using biochemical screening, habituation and improvement), such as an SGR of 0.671 d1, BPR of 0.917 gL1 d1 and CFR of 1.723 gL1 d1 values for Chlorella sp. KR-1 [33] and an SGR of 0.677 d1, BPR of 0.950 gL1 d1 and CFR of 1.768 gL1 d1 values for Chlorella ZY-1 [34]. However, most improved microalgae cultured in at high CO2 concentrations (10e25%) yielded SGR values ranging from 0.067 to 1.547 d1 and BPR values ranging from 0.008 to 1.400 gL1 d1 [22e31]. Compared with these microalgae, the energy microalgae Chlorella sp. and Isochrysis sp., but not Amphidinium carterae, used in this work showed above-average performances. The microalga species Chlorella sp. adapted relatively quickly to flue gas-aerated culture conditions (SGR of 0.328 d1, BPR of 0.192 gL1 d1 and CFR of 0.353 gL1 d1). Moreover, it also performed better than in static culture (without aeration) at atmospheric CO2 concentration levels (SGR of 0.288 d1, BPR of 0.139 gL1 d1 and CFR of 0.262 gL1 d1), even though the pH value of the culture medium sharply decreased from 8.0 to approximately 5.3. The microalga species Isochrysis sp. performed moderately. The flue gas conditions (SGR of 0.183 d1, BPR of 0.111 gL1 d1 and CFR of 0.190 gL1 d1) and the atmospheric environment (SGR of 0.189 d1, BPR of 0.118 gL1 d1 and CFR of 0.202 gL1 d1) differed little in terms of the cell performance. The difference in the performance in a flue gas (almost declined) and atmospheric environment (SGR of 0.261 d1, BPR of 0.171 gL1 d1 and CFR of 0.296 gL1 d1) was most pronounced for Amphidinium carterae, suggesting that it may be not directly suitable for CO2 fixation from flue gas. Because the microalgae species Amphidinium carterae is very sensitive to environmental factors, especially to the CO2 concentration, a sharp decline in the pH value of the culture medium also inhibits the metabolism of this species. Additionally, the shear effect of the gaseliquid fluid flow caused by bubbling aeration is more pronounced due to the lack of cellulose walls, leading to a hard growth. However, once the loads of the above unfavorable factors were reduced, the microalgae Isochrysis sp. and Amphidinium carterae are expected to perform better on CO2 fixation and biomass production. Based on the comparison with Isochrysis sp. and Amphidinium carterae, it was demonstrated that the microalgae Chlorella sp. had better performance on CO2 fixation and biomass production. Therefore, it can be selected as the dominant species for further investigation.
3.2. Effects of varied flue gas parameters on the performance of the dominant microalgae Chlorella sp.
Fig. 3. Performance of three energy microalgae under 15% flue gas CO2 level (AR ¼ 0.2 vvm and no toxic compounds presence).
3.2.1. CO2 concentration The effect of the flue gas CO2 concentration on the carbon fixation, biomass production, pH value and DO value for Chlorella sp. is illustrated in Fig. 4 and Table 5. Overall, the presence of high concentrations of CO2 exerted a negative effect, although microalgae Chlorella sp. could grow in culture over time. The SGR, BPR and CFR at three CO2 concentrations decreased and ranged from 0.372 to 0.293 d1, from 0.268 to 0.146 gL1 d1 and from 0.492 to 0.268 gL1 d1, respectively, over the entire culture period. Specifically, the biomass of Chlorella sp. negatively correlated with the CO2 concentration. Moreover, the pH value in the medium showed similar trends at the various CO2 concentrations. When aerating the medium with simulated flue gas for 1 day, the pH value sharply decreased from 8.0 to below 5.6 for 10% CO2, to 5.5 for 15% and to 5.2 for 20% CO2. The pH value slightly increased thereafter. Similar trends were observed for the DO value.
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Table 4 Performance parameters for three energy microalgae at 15% CO2. Microalgal species
X0 (gL1)a
X7 (gL1)
k (d1)
p (gL1 d1)
f (gL1 d1)
Chlorella sp. Isochrysis sp. Amphidinium carterae
0.150 0.300 0.230
1.493 1.080 0.016
0.328 0.183 /
0.192 0.111 /
0.353 0.190 /
a
X0 is 1 106 cells mL1 in number concentration.
According to gaseliquid mass transfer theory, the increase in the CO2(g) concentration increases the CO2(l) concentration via dissolution and increases Hþ and HCO 3 concentrations (decreasing pH value) via hydrolysis. These changes usually do not favor the growth of most microalgae according to the conventional CCM, which states that CO2 concentrations exceeding 1% can inhibit the activity of CA (carbonic anhydrase) and formation of CCM [19,42]. In this work, the typical flue-gas CO2 concentration (10%e20% v/v) is much higher than the atmospheric CO2 concentration (0.038% v/ v). The growth of microalgae chlorella sp. was inhibited as the CO2 concentration increased. Despite this inhibitory effect, Chlorella sp. continued to grow well under these conditions and no decrease in growth was observed, indicating that chlorella sp. can adapt and change the CCM in response to high concentrations of CO2, giving it the potential of continuous growth. In addition, although the flue gas CO2 caused the pH value to decrease more compared to the atmospheric CO2, the resultant pH value was not extremely low. As the photosynthesis and metabolism proceed, the pH value slightly increased. Similarly, aeration of CO2 gave rise to decrease of the DO. Also, as photosynthesis proceeded, the DO slightly recovered. In essence, this result demonstrated that the microalgae chlorella sp. can endure high CO2 concentrations and low pH values. A similar potential was also found in various improved Chlorella that can tolerate high CO2 concentrations of up to 70% (e.g. Chlorella sp. KR-1 [34] and Chlorella ZY-1 [35]) and even 100% (e.g., Chlorella sp. T-1 [32]) and a pH value as low as 3.5. However, it should be noted that the adaptability to the CO2 concentration varied by Chlorella species and the optimum CO2 concentration was usually found in the range of 0.038%e10% [31]. Overall, although the CO2 concentration has an adverse impact, the microalgae Chlorella sp. readily adapts to this impact and continues to fix CO2 and produce biomass at high levels.
Fig. 4. Effect of CO2 concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (AR ¼ 0.2 vvm and no toxic compounds presence).
3.2.2. Aeration rate The effect of the flue gas aeration rate on the carbon fixation, biomass production, pH value and DO value for microalgae Chlorella sp. is shown in Fig. 5 and Table 5. The aeration rate also had negative effect although microalgae Chlorella sp. could continue to grow as the culture time elapsed. Specifically, the biomass of Chlorella sp. negatively correlated with the CO2 concentration. The SGR, BPR and CFR changed in 0.375e0.263 d1, 1 1 1 1 0.274e0.114 gL d and 0.503e0.209 gL d , respectively, at the three aeration ratios during the entire culture period. Moreover, the pH value of the medium differed little between the three aeration rates and ranged from 8.0 to 5.5 due to the diffusive equilibrium in the gaseliquid phases when the simulated flue gas was aerated for 1 day. However, the DO value recovered more rapidly at low CO2 concentrations. The microalgae culture process usually requires an appropriate turbulent flow and mixing in the gas-liquid-solid (CO2-mediummicroalgae) phases to enhance mass transfer, allow light to reach cells, equalize the temperature and nutrients, remove produced oxygen and prevent the aggregation and sedimentation of microalgae. The aeration rate is an important factor in a gas aerating culture in addition to the bubble size. As expected, an appropriate turbulence effect due to an appropriate aeration rate helps to improve microalgae performance in terms of carbon fixation and
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Table 5 Performance parameters of Chlorella sp. at varied flue-gas CO2 concentration, aeration rate and toxic compounds concentration. Items
Concentration
X7 (gL1)
k (d1)
p (gL1 d1)
f (gL1 d1)
CO2 concentration
10% 15%a 20% 0.1 0.2a 0.3
2.025 1.493 1.170 2.070 1.493 0.945
0.372 0.328 0.293 0.375 0.328 0.263
0.268 0.192 0.146 0.274 0.192 0.114
0.492 0.353 0.268 0.503 0.353 0.209
0 ppma 100 ppm 250 ppm 250 ppm (pH ¼ 5.5) 100 ppm 250 ppm 10 mg m3 20 mg m3 30 mg m3
1.493 0.593 0.128 0.548 1.223 0.855 0.788 0.705 0.615
0.328 0.196 / 0.185 0.300 0.249 0.103 0.096 0.088
0.192 0.063 / 0.057 0.153 0.101 0.091 0.079 0.066
0.353 0.116 / 0.105 0.281 0.186 0.167 0.145 0.121
Aeration rate (vvm)
Toxic compounds Free SO2
NO Hg2þ
a
Using same conditions.
biomass production, but extremely high aeration rates may result in strongly shear stress, especially in the processes of bubble generation, bubble deformation (e.g., bubble coalescence and breakup) and gaseliquid interface formation [43,44]. Therefore, the effect of the aeration rate on microalgal-CO2 fixation and biomass production is complex. It was also found to be associated with the microalgae species and culture conditions. The recommended aeration rate is 0.05e1.00 for most bubbling cultures. Moreover, the optimum aeration rate varies by microalgae species and photobioreactor configuration, e.g. 0.025e1.00 vvm was proposed to be cost-effective for 5% or 10% (v/v) CO2 aeration and 0.05 vvm for a flat-panel photobioreactor [45]. In the present work, aeration rates higher than 0.1 vvm appear to hinder the CO2 fixation and biomass production of Chlorella sp. 3.2.3. Typical toxic compounds The effects of the pollutant characteristics, including different SO2, NO and Hg2þ concentrations, on the carbon fixation, biomass production, pH value and DO value for Chlorella sp. are presented in Figs. 6e8 and Table 5. (1) SO2 According to Fig. 6(a), the presence of SO2 in the flue gas significantly inhibited the growth of Chlorella sp. Although Chlorella sp. had a 2-day lag phase, it continuously grew during the 6-day culture period at an SO2 level of 100 ppm, with an SGR of 0.196 d1, BPR of 0.063 gL1 d1 and CFR of 0.116 gL1 d1. However, once the SO2 concentration increased to 250 ppm, Chlorella sp. was almost completely inhibited and gradually declined in concentration after the 2-day lag phase. Additionally, the biomass of Chlorella sp. also decreased after 6 days in culture at 100 ppm SO2, which was caused by an accumulated effect of the pH value (Fig. 6(b)). In addition, the DO value recovered faster in the absence of SO2 (Fig. 6(c)). The effect of SO2 on the microalgae growth was primarily attributed to the indirect effect of the pH value, i.e. if the pH of the culture medium is artificially maintained constant via neutralization, the growth characteristic for some microalgae almost do not differ from those in the absence of SO2 [18]. Fig. 6(b) shows that the threshold pH value for Chlorella sp. is approximately 4.7. It means that Chlorella sp. cannot survive below this pH. However, further investigation showed that Chlorella sp. could grow when the pH value was artificially maintained at 5.5, but the biomass produced in response to 15% CO2 with 250 ppm SO2 was less than that
produced in response 15% CO2 without SO2 or 100 ppm SO2. This finding actually indicates that the inhibition of Chlorella sp. by SO2 is associated with not only the pH value (Hþ concentration in cul ture medium) but also with the SO2 4 and HSO4 concentration, which result from SO2 hydrolysis suggesting that SO2 4 and HSO4 are microalgae growth inhibitors [46,47]. Overall, compared with other microalgae species that were completely inhibited at 50 ppm SO2, Chlorellas sp. moderately tolerates SO2. (2) NO Fig. 7(a) shows that the presence of NO in the flue gas partly inhibits the growth of Chlorella sp. The SGR, BPR and CFR were reduced from o 0.328e0.249 d1, from 0.192 to 0.101 gL1 d1 and from 0.353 to 0.186 gL1 d1, respectively, at the three NO concentration levels during the 6-day culture period. No significant lag phase was observed, even at high NO concentrations. Unlike the effect of SO2, NO did not significantly change the pH (Fig. 7(b)). In addition, the DO value recovered faster in the absence of NO (Fig. 7(c)). The effect of NO on the carbon fixation and biomass production for Chlorella sp. is considered to be complex. It seems to be less significant than that of SO2 at the same concentration, NO did not made Chlorella sp. decline even at high NO concentration (250 ppm). Unlike water-soluble SO2, NO cannot easily directly impact the growth of microalgae via the pH of the culture medium. The decrease in the pH value of the culture medium is mainly attributed to CO2 instead of NO. However, the presence of NO is in associated with physiological conditions of microalgae cells. Usually, its influence on the growth of microalgae has dual characters. Extremely low concentrations of NO may even be absorbed by the culture medium and transformed into NO2, which can serve as a source of nitrogen nutrition for microalgae that can process inorganic forms [8]. However, this positive influence is limited and depends on the microalgae species. For most of microalgae, increased NO concentrations decrease the growth rate. NO concentrations higher than 300 ppm may give rise to the decline of microalgae [24,48]. (3) Hg2þ Fig. 8(a) shows that even extremely low concentrations of Hg2þ can strongly inhibit Chlorella sp. In the concentration range of 10e30 mg m3, the inhibited effect of Hg2þ did not make change remarkably. Moreover, because CO2 aeration was the only source of
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Fig. 5. Effect of aeration rate on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15% and no toxic compounds presence).
Hþ, the pH curve (Fig. 8(b)) and the DO curve (Fig. 8(c)) are similar to those for NO. As one of the most important trace metals from combustion flue, mercury usually exists in the forms of elemental mercury (Hg0), oxidized mercury (Hg2þ) and particle-bound mercury (Hgp). Hgp can usually be captured by high-efficiency particle collectors, while many mercury control technologies often oxidize Hg0 into Hg2þ in order to achieve high-efficiency capture. Hg2þ in flue gas was observed to inhibit the microalgae because it is water-soluble and destroys chlorophyll, directly resulting in the reduction of the photosynthetic efficiency [49].
Fig. 6. Effect of SO2 concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15%, AR ¼ 0.2 vvm).
3.3. Analysis on application 3.3.1. Scale-up In practice, the bubble column photobioreactor can be scaled up for microalgae-based including Chlorella sp.-based CO2 fixation and biomass production because it is one of the most simple closed culture setups. In addition to the geometrical dimensions, the scale-up process depends on the physicochemical parameters of the flue gas, including the CO2 concentration, toxic pollutants, initial density of the microalgae, temperature,
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Fig. 7. Effect of NO concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15%, AR ¼ 0.2 vvm).
Fig. 8. Effect of Hg2þ concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15%, AR ¼ 0.2 vvm).
light, nutrients and pH value, as well as on the hydrodynamic parameters, including mass transfer, flow and mixing. The present result can be used to determine the optimized operating parameters. Theoretical, semi-empirical, dimensional analysis and CFD (computational fluid dynamics) approaches could then be used to scale up this process for pilot application. The industrial culture of microalgae is relatively easy due to the convenience and availability of seawater and f/2 broth, which is the most common culture medium. Moreover, when using improved culture modes, including semi-continuous culture,
continuous culture and advanced multistage culture system in industrial applications, the load of the CO2 concentration, aeration rate and toxic compound concentration as investigated in this work can be indirectly decreased or buffered in these processes. They can actually improve microalgal-CO2 fixation and biomass production. Other important culture conditions, such as the temperature, light (including intensity and lightedark cycle), nutrients, initial density, flow and mixing exert complex and nonlinear effects on the microalgal-CO2 fixation and biomass production [50].
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3.3.2. CO2 fixation efficiency Although a consensus on the effectiveness of fixation of CO2 by microalgae was not completely reached [51], microalgae-based CO2 fixation and biomass production are considered to be promising. Due to the self-limiting nature of biochemical processes, microalgae can barely reach the same CO2 capture efficiency as mature chemical approaches, and also have the feature with a relatively long life cycle period. The CO2 fixation efficiency of Chlorella sp. cultured in a bubble column photobioreactor was approximately 16e58% [20]. It does not rival the CO2 capture efficiency of absorption (e.g. ammonia and amine), absorption and membrane approaches, which usually reach 90% or higher. Moreover, the microalgae biomass usually requires a harvest cycle of 3e7 days or longer. Therefore, new high-efficiency photobioreactors and culture modes (e.g. continuous culture) need to be developed to improve the CO2 fixation performance and shorten the biomass harvest cycle. Nevertheless, microalgae-based CO2 fixation and biomass production are usually preferable to be used when emphasizing the economic performance, environmental friendliness and product utilization for energy resources in addition to carbon emission reduction, because this biochemical process consumes low amounts of external energy (despite utilizing closed cultivation), does not emit secondary pollutants and generates energy-oriented products that can be further converted into biofuel. Additionally, the toxic compounds in flue gas have negative effect on microalgae-CO2 fixation and biomass production in most cases. From another standpoint, microalgae are at least partially able to remove them, e.g., Dunaliella tertiolecta was able to remove 50%e60% NO in continuous 15 days when there was 2% O2 [52] while NOA-113 had 40 mgd1 removal rate for NO [53]. However, the removal performances depend upon the microalgae specie, the biomass density and the toxic compound concentrations. The removal rates for the different toxic compounds need to be further investigated for the microalgae Chlorella sp. used in this work. 3.3.3. Energy conversion of microalgae-based biomass Chlorella sp. was demonstrated to be a potential bio-medium or biomaterial for CO2 control from combustion sources, although follow-up studies are required, including studies that improve the microalgae performance in harsher flue gas environments and studies that convert the microalgae biomass into bioenergy for utilization via physicochemical extractions, biochemical processes and thermochemical technologies with high efficiency [54e56]. In particular, the contents of lipid (oil) and fatty acid in the microalgae are varied with microalgae specie and culture parameters [20,37], e.g., the lipid content of Chlorella fusca was found to increase when the microalgae were grown at increasingly higher concentrations of CO2 [57] but Chlorella sp. had inconsistent performance [20]. For the purpose of energy utilization, therefore, their optimization is still an important issue. 4. Conclusions The potential of three natural energy microalgae candidates, Chlorella sp., Isochrysis sp. and Amphidinium carterae, to biologically fix flue gas CO2 and produce biomass was investigated. At 15% CO2, Amphidinium carterae was completely inhibited and Isochrysis sp. was also inhibited could grew relatively slowly. In comparison, Chlorella sp. performed better and was selected as the dominant microalgae. Chlorella sp. performed well in terms of CO2 fixation and biomass production, although the CO2 concentration (10e20%, v/v), aeration rate (0.1e0.3 vvm) and presence of toxic compounds
(0e250 ppm SO2 and 0e250 ppm NO and 10e30 mg m3) in the flue gas negatively affected its growth. The scale-up of this approach for pilot applications that improves the convenience, fixation efficiency of CO2 from flue gas and conversion of microalgae-based biomass into biofuels needs to be further explored. Acknowledgments This work was jointly sponsored by National Natural Science Foundation of China (Nos. 50806049 and 51278095), Innovation Program of Shanghai Municipal Education Commission (No. 09YZ209), China Scholarship Council (No. 201208310168) and The Hujiang Foundation of China (No. D14001). Nomenclature AR CC CCO2 CSO2 CNO C2þ Hg DO f k MC MCO2 p pH X X0 X7 Xt
aeration rete content of carbon in microalgae biomass CO2 concentration in simulated flue gas SO2 concentration in simulated flue gas NO concentration in simulated flue gas Hg2þ concentration in simulated flue gas Dissolved oxygen value CO2 fixation rate (CFR) based on initial microalgae biomass (gL1 d1) specific growth rate (SGR) for microalgae (d1) molar mass of carbon (g mol1) molar mass of CO2 (g mol1) biomass production rate for microalgae (gL1 d1) pH value microalgal biomass concentration (gL1) microalgal biomass concentration at time t0 (gL1) microalgal biomass concentration at 7th day (gL1) microalgal biomass concentration at time t (gL1)
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Carbon dioxide fixation and biomass production from combustion flue gas using energy microalgae Bingtao Zhao a, *, Yaxin Su b, Yixin Zhang a, Guomin Cui a a b
School of Energy and Power Engineering, University of Shanghai for Science and Technology, 516 Jungong Road, Shanghai 200093, China School of Environmental Science and Engineering, Donghua University, 2999 North Renmin Road, Shanghai 201620, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 December 2014 Received in revised form 19 May 2015 Accepted 21 May 2015 Available online xxx
Algae-based bioenergy has been regarded as the next generation of renewable energy. To fix CO2 from flue gas and harvest algal biomass for energy conversion, three energy microalgae, Chlorella sp., Isochrysis sp. and Amphidinium carterae, were investigated in 1-L bubble column photobioreactors with an aeration of 15% CO2 at the flue-gas level. According to the potential on CO2 fixation and biomass production, Chlorella sp. was selected as the dominant species due to its superiority to the other species, with a specific growth rate of 0.328 d1, a biomass production rate of 0.192 gL1 d1 and a CO2 fixation rate of 0.353 gL1 d1. Furthermore, Chlorella sp. was cultured under varied physicochemical parameters, including CO2 concentrations, aeration rates and toxic compounds (SO2, NO and Hg2þ) to assess its performances. The maximum specific growth rate, biomass production rate and CO2 fixation rate were found to be 0.372 d1, 0.268 gL1 d1 and 0.492 gL1 d1 at a CO2 concentration of 10%; 0.375 d1, 0.274 gL1 d1 and 0.503 gL1 d1 at an aeration rate of 0.1 vvm; and 0.328 d1, 0.192 gL1 d1 and 0.353 gL1 d1 in the absence of toxic compounds, respectively. The results provide a basis for microalgal-based CO2 emission reduction and bioenergy utilization in pilot-scale applications. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Energy microalgae CO2 fixation Biomass production Flue gas Physicochemical parameters
1. Introduction Carbon dioxide (CO2) from human activity has been demonstrated to be responsible for global warming and the greenhouse effect [1]. Anthropogenic CO2 emissions, particularly the emissions from fossil fuel combustion, have become important scientific, environmental and even international economic and political issues. Stable, safe and environmentally acceptable post-combustion CCS (CO2 capture and storage) as well as product recycling technologies, are necessary in addition to improvements in current energy efficiency and the development of other renewable energies. According to scientific principles, post-combustion CO2 capture can be roughly categorized as chemical absorption, physicochemical adsorption, membrane, cryogenics, CLC (chemical looping combustion) and terrestrial sequestration [2]. A potential and promising biologic approach, microalgae-based CO2 fixation and energy/resource utilization, has received significant attention over the last two decades due to its techno-economic feasibility and
* Corresponding author. Tel.: þ86 21 55271751; fax: þ86 21 55272376. E-mail address: [email protected] (B. Zhao).
environmental friendliness [3e13]. In essentials, the microalgae essentially biologically fix and store CO2 via photosynthesis, which can convert water and CO2 into organic compounds without secondary pollution. Microalgal-CO2 fixation features potential advantages over other carbon capture and storage approaches, such as a wide distribution, high photosynthesis rate, good environmental adaptability and easy operability. Additionally, the microalgal biomass can be harvested after CO2 fixation to produce microalgal biofuel that can be utilized as a renewable or sustainable energy source (Fig. 1). Although microalgal-based CO2 fixation and biomass production are promising, this approach suffers from many specific challenges that need to be overcome in scientific research and engineering practice. Most of microalgae appear to be able to produce biomass and fix atmospheric CO2 (0.038%, v/v) (Table 1), e.g. Isochrysis galbana [14,15], Chaetoceros calcitrans [15], Chlorella vulgaris [16] and Chlorella sp [17]. Despite differences in culture modes, operating conditions and photobioreactors, the data indicated that these species had a specific growth rate of 0.183e0.645 d1 and biomass production rate of 1 1 0.011e0.840 gL d . Furthermore, some microalgae can be used to produce biomass and fix CO2 at low concentrations (0.038e5%, v/
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Fig. 1. Flowchart of microalgae-based CO2 biofixation, biomass production and energy conversion from combustion flue gas.
v) (Table 1), including Porphyridium purpureum 1380-1A, which is effective at 2% CO2 [18], Chlorella vulgaris, which is effective at 0.1e2.8% CO2 [19], Chlorella sp., which is effective at 0.038e5% CO2 [20] and Chlorella sp. AG10002, which is effective at 0.5e5% CO2 [21]. The specific growth rates of these species ranged from 0.127 to 0.885 d1, and their biomass production rates ranged from 0.074 to 1.060 gL1 d1. However, due to the differences in the CO2 concentration level between the atmosphere and the combustion flue gas, the adaptability and performance of these microalgae in high
CO2 concentrations, including at flue-gas CO2 concentrations (usually >10%, v/v), are unknown. Usually, the ability of microalgae to produce biomass and fix CO2 varies by species and culture condition. A previous study indicated that CO2 concentrations exceeding 1% (v/v) negatively affect most of the microalgae by inhibiting the Rubisco activity in the CCM (carbon concentrating mechanism) [19]. However, the selfadaption of some improved microalgae can overcome this effect. Therefore, microalgae species have been modified using special
Table 1 Summary of biomass performance on microalgae-based CO2 fixation. Microalgae Used for atmospheric CO2 fixation Isochrysis galbana Isochrysis galbana Chaetoceros calcitrans Chlorella vulgaris Chlorella sp. Used for low concentration CO2 fixation Porphyridium purpureum 1380-1A Chlorella vulgaris Chlorella sp. Chlorella sp. AG10002 Used for high concentration CO2 fixation Nannochloris sp. Nannochloropsis sp. Phaeodactylum tricornutum Chlorococcum littoral Chlorella sp. KR-1 Chlorella vulgaris Euglena gracilis Scenedesmus obliquus Chlorella kessleri Spirulina sp. Scenedesmus obliquus €geli Aphanothece microscopica Na Chlorella sp. MTF-7
CO2 concentration (%, v/v)
Specific grow rate (d1)
Biomass production rate (gL1 d1)
Ref.
0.038 0.038 0.038 0.038 0.038
0.183e0.207 0.645 0.387 0.293e0.416 0.288e0.512
0.100e0.126 0.011 0.007 0.436e0.840 0.139e0.751
[14] [15] [15] [16] [17]
2 0.1e2.8 0.038e5 0.5e5
0.250 NA 0.127e0.605 0.792e0.885
0.304 0102e1.060 0.074e0.207 0.192e0.335
[18] [19] [20] [21]
15 15 15 20 15 0.038e30 10 0.038e18 0.038e18 0.038e12 0.038e12 15 2e25
0.700 0.657 0.501 0.078 1.547 0.528e0.745 0.599 0.216e0.261 0.199e0.257 0.330e0.440 0.150e0.220 0.067e0.611 0.312e0.411
0.256 0.229 0.180 0.400 1.400 0.092e0.387 0.114 0.064e0.090 0.061e0.090 0.040e0.200 0.040e0.140 0.008e0.800 0.183e0.358
[22] [22] [22] [23] [24] [25] [26] [27] [27] [28] [28] [29,30] [31]
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methods, including selective isolation, pre-culture and habituated culture, to enhance the ability to fix CO2 and produce biomass. They were found to be able to endure and grow in high concentrations of CO2 (usually >10%, v/v) (also shown in Table 1), including the microalgae Nannochloris sp. and Nannochloropsis sp., Chlorococcum littoral and Phaeodactylum tricornutum [22], Chlorococcum littoral [23], Chlorella sp. KR-1 [24], Chlorella vulgaris [25], Euglena gracilis [26], Scenedesmus obliquus and Chlorella kessleri [27], Spirulina sp. €geli and Scenedesmus obliquus [28], Aphanothece microscopica Na [29,30] and Chlorella sp. MTF-7 [31]. They were found to be able to grow at 10e25% CO2 with specific growth rates ranging from 0.067 to 1.547 d1 and biomass production rates ranging from 0.008 to 1.400 gL1 d1. In particular, the microalgae Chlorella H-84 [32], Chlorella sp. KR-1 [33], Chlorella ZY-1 [34] and Chlorella sp. T-1 [35] were found to grow at CO2 concentrations of up to 40%, 70%, 70% and 100%, respectively. Furthermore, certain special and improved microalgae could adapt to the toxic compounds in flue gas [24,33] and mobile exhausts [36]. Nevertheless, their performances in comprehensively varied flue gas conditions were not addressed, especially at varied gas flow rates and in the presence of trace metals in flue gas. Moreover, the parameters related to biofixation and biomass production (e.g. pH value and dissolved oxygen) have not yet been explored in detail. At present, the effective fixation of CO2 and biomass production from combustion flue gas via microalgae continues to face many challenges. Most importantly, microalgae must not only contain high amounts of lipids, which are easily converted into high quality biofuel, but they must also perform well in a complex flue gas environment for pilot-scale applications, which is not possible when using microalgae that are habituated to in vitro culture or genetically induced in bench-scale experiments. Energy microalgae appear to meet these needs, but their characteristics of CO2 fixation and biomass production need to be comprehensively examined due to the complexity of flue gas conditions (e.g., 10e20% CO2 concentration, high flow rate and the presence of toxic compounds, such as SO2, NOx, and trace metals, etc.). Additionally, large amounts of microalgae are required for the large-scale applications, which is difficult to achieve using modified microalgae cultured at the lab-scale. Currently, comprehensive reports that systemically examine the behavior and characteristics of these energy microalgae in the fixation of CO2 and production of biomass in response to varied flue gas conditions are rare. This work experimentally investigated microalgae-based CO2 fixation and biomass production from simulated combustion flue gas to provide a reference for pilot-scale applications. First, the CO2 fixation and biomass production of three candidate natural energy microalgae species, Chlorella sp., Isochrysis sp. and Amphidinium carterae, were compared at the flue-gas CO2 concentration level. On this basis, a high-performance species was selected to further examine the effects of varying physicochemical parameters, including the CO2 concentration, aeration rate and addition of toxic compounds (SO2, NO and Hg2þ), on the performance and behavior-
3
related parameters, such as pH value and dissolved oxygen, of this species. 2. Materials and methods 2.1. Microalgae species and culture medium The microalgae candidates Chlorella sp., Isochrysis sp. and Amphidinium carterae were used in this work, because they are important energy microalgae representatives of the class chlorophyceae, chrysophyceae and dinophyceae [37e40]. The oil content (based dry weight) was found to be 33% for the Chlorella sp., 29% for the Isochrysis sp. and 23% for the Amphidinium carterae, respectively. Moreover, the great advantages of them are that they are easily available, widely distributed and potential application prospect. Their physical and morphological properties are shown in Table 2. All microalgae were cultured in f/2 medium in artificial seawater with 74.79 gL1 NaNO3, 5.66 gL1 NaH2PO4$2H2O, 15.35 gL1 Na2SiO3$9H2O, 23.0 mgL1 ZnSO4$7H2O, 178.1 mgL1 MnCl2$4H2O, 7.26 mgL1 Na2MoO4$2H2O2 and 11.9 mgL1 CoCl2$6H2O. The trace elemental solution contained 9.99 mgL1 CuSO4$5H2O, 4.36 gL1 Na2EDTA$2H2O, 4.15 gL1 FeC6H5O7$5H2O, 5 mgL1 Cyanocobalamin (VB12), 5 mgL1 Biotin (VH) and 1 gL1 Thiamine-HCl (VB1). 2.2. Experimental system and conditions The experimental microalgae culture system is illustrated in Fig. 2. The microalgae were cultured in bench-scale bubble column photobioreactors (diameter of 65 mm and height of 420 mm) with a working volume of 1 L. All photobioreactors were placed in a 250L digitally controlled light incubator (SPX-250GB, Hede Corperation, Shanghai, China), which was equipped with an ultraviolet sterilamp and a sterilizing PTFE membrane filters in ventilation system to ensure the pre-sterilization and aseptic condition during the culture process. The cultivations were operated at 18 ± 1 C (the local average annual temperature), a light intensity of 84 mmolm2 s1 (the local average light intensity) and a lightedark cycle of 12:12 h (approximate day/night ratio). The bubble column photobioreactors were continuously aerated with the simulated flue gas, which consisted of CO2 and N2 with or without the toxic compounds SO2 and NO. Hg2þ was dynamically and drop-wise added to the culture medium via an HgCl2 solution and was converted in the flue gas to the desired concentration. All cultures were inoculated with the microalgae species at the same initial number concentration (initial cell concentration) of 1 106 cells mL1, and the culture period was set as 7 days. The initial pH value was 8.0 ± 0.1, and the pH value was not artificially controlled during the entire culture process, with the exception of the control experiment for the SO2 effect. The detailed experimental design is shown in Table 3.
Table 2 Physical and morphological properties of energy microalgae used in this work. Microalgae
Chlorella sp.
Isochrysis sp.
Amphidinium carterae
Class Size Appearance Cell morphology (640)
Chlorophyceae 3e5 mm Green, spherical
Chrysophyceae 5e7 mm Golden, 2 flagellate
Dinophyceae 12e17 mm Yellow, 4 flagellate
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Fig. 2. Schematic diagram of experimental system. 1-Simulated flue gas cylinder; 2-Valve; 3-Flow meter; 4-Mixer; 5-Gas inlet; 6-Light incubator; 7-Bubble photobioreactor; 8-Light; 9-pH meter; 10-Dissolved oxygen meter; 11-Spectrophotometer; 12-Digital biomicroscope; 13-Computer for image and data processing; 14-Gas outlet.
Table 3 Design of the experiment. Stage
Microalgal species
CO2 concentration (%, v/v)
N2 concentration (%, v/v)
Aeration rate (vvm)
Toxic compounds (ppm)
I
Chlorella sp. Isochrysis sp. Amphidinium carterae Dominant species screened
15 15 15 10, 15, 20 15 15 15 15
85 85 85 90,85,80 85 85 85 85
0.2 0.2 0.2 0.2 0.1, 0.2, 0.3 0.2 0.2 0.2
/ / / / / 0, 100, 250, 250 (controlled pH) for SO2 0, 100, 250 for NO 10,20,30 mg m3 for Hg2þ
II
2.3. Assay methods
2.4. Characterizing parameters
The number of microalgae cells in the microalgae suspension sample was determined using an observation system that included a bright-line hemocytometer (Qiujing, Shanghai, China) and a digital bio-microscope XSP-30 (Phenix optics Group Co., Ltd, Jiangxi, China). The cell biomass concentration was determined by measuring the absorbance at 680 nm (A680) using a visible spectrophotometer SP-721E (Shanghai Spectrum Instruments Co., Ltd, Shanghai, China); correspondingly, the dry biomass was measured after the microalgae cell sample was filtered through a pre-dried and pre-weighed 0.45 mm membrane filter and then dried at 105 C for 12 h. Light intensity was measured adjacent to the photobioreactor using a Basic Quantum Meter LX-1010B (Jinda Technologies Co. Ltd, Shenzhen, China). The pH value of the culture medium samples was directly measured everyday using a pH meter P-II (Shenqi Automation Instrumentation Co. Ltd, Shanghai, China) calibrated with pH 4.01 and 9.18 standard buffer solutions. The DO (dissolved oxygen) value in the culture medium samples was also directly measured using a self-calibrating dissolved oxygen meter AZ-8403 (Hengxin Instrumentation Co. Ltd, Taiwan). The uncertainty analysis according to the preliminary investigation using controlled parallel experiments indicated that, under the designed conditions, the maximum relative errors of the experimental data were in the range of ±5%. All experimental data in this work were obtained based on this measurement accuracy.
The SGR (specific growth rate) was used to describe the microalgae biomass kinetics in this work:
k ¼ ðln Xt ln X0 Þ=ðt t0 Þ
(1)
where k is the SGR (d1), and Xt and X0 are the biomass concentration (based on dry cellular weight) (gL1) at culture time t and t0, respectively. The BPR (biomass production rate), i.e., the linear growth rate or average growth rate, was used to estimate the microalgae biomass productivity according to the following equation:
p ¼ ðXt X0 Þ=ðt t0 Þ
(2)
where p is the BPR (gL1 d1). Correspondingly, the CFR (CO2 fixation rate) could be estimated according to the biomass production rate and carbon content in microalgae biomass. some investigations statistically estimated it via an approximate molecular formula of microalgae biomass with CO0.48H1.83N0.11P0.01 (C, O, H, N, and P represent the elements carbon, oxygen, hydrogen, nitrogen and phosphorus, respectively) [11,40], which is derived from the data reported by Grobbelaar (2004) [41]. However, the elemental composition is usually varied with environmental conditions, culture processes and in particular with microalgae species. To characterize the CFR for most
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unicellular microalgae, this work uses the following equation for calculation:
f ¼ pCC MCO2 MC
(3)
where f is CFR based on the initial microalgae biomass (gL1 d1) and p is the microalgae biomass production rate. CC is the carbon contents (wt) in the three microalgae biomass and can be experimentally determined by elemental analysis. In this work CC were found to be 50.1% for Chlorella sp., 46.8% for Isochrysis sp. and 47.2% for Amphidinium carterae, respectively. MC and MCO2 are the molar mass of Carbon and CO2, respectively. 3. Results and discussion 3.1. Performance of energy microalgae candidates in response to flue gas CO2 concentration levels The performances of three energy microalgae at a typical fluegas CO2 concentration of 15% are shown in Fig. 3. Note that differences in the physical size, moisture content and cell structure resulted in their different initial biomass concentrations, even though the cell numbers did not differ (Table 4). These cells did not uniformly adapt to the simulated flue gas environment. Microalgae Chlorella sp. and Isochrysis sp. could grow, but Amphidinium carterae showed poor performance under this condition. Specifically, almost all Amphidinium carterae cells entered decline after aeration with simulated flue gas for one day. The microalgae Chlorella sp. and Isochrysis sp. still exhibited the difference even though could both grow under these conditions. Specifically, Chlorella sp. performed better than Isochrysis sp. Moreover, a significant lag phase was not observed for Chlorella sp., while a 4-day lag phase was identified for Isochrysis sp. Microalgal-CO2 fixation and biomass production are closely associated with the physicochemical conditions (e.g. CO2 concentration, pH value, light intensity, etc.) and hydrodynamic conditions (e.g. gas-liquid-solid (gas-medium-microalgae cell) flow and mass transfer, etc.). However, the microalgae species plays a vital role in CO2 fixation and biomass production, as it essentially determines the difference in the effect of these factors on the microalgae CCM (carbon concentrating mechanism). For example, Chlorella has been considered one of the energy microalgae with high potential for
5
application in complex and harsh environments, such as flue gas. Some Chlorella may have good performances via special modifications (e.g., using biochemical screening, habituation and improvement), such as an SGR of 0.671 d1, BPR of 0.917 gL1 d1 and CFR of 1.723 gL1 d1 values for Chlorella sp. KR-1 [33] and an SGR of 0.677 d1, BPR of 0.950 gL1 d1 and CFR of 1.768 gL1 d1 values for Chlorella ZY-1 [34]. However, most improved microalgae cultured in at high CO2 concentrations (10e25%) yielded SGR values ranging from 0.067 to 1.547 d1 and BPR values ranging from 0.008 to 1.400 gL1 d1 [22e31]. Compared with these microalgae, the energy microalgae Chlorella sp. and Isochrysis sp., but not Amphidinium carterae, used in this work showed above-average performances. The microalga species Chlorella sp. adapted relatively quickly to flue gas-aerated culture conditions (SGR of 0.328 d1, BPR of 0.192 gL1 d1 and CFR of 0.353 gL1 d1). Moreover, it also performed better than in static culture (without aeration) at atmospheric CO2 concentration levels (SGR of 0.288 d1, BPR of 0.139 gL1 d1 and CFR of 0.262 gL1 d1), even though the pH value of the culture medium sharply decreased from 8.0 to approximately 5.3. The microalga species Isochrysis sp. performed moderately. The flue gas conditions (SGR of 0.183 d1, BPR of 0.111 gL1 d1 and CFR of 0.190 gL1 d1) and the atmospheric environment (SGR of 0.189 d1, BPR of 0.118 gL1 d1 and CFR of 0.202 gL1 d1) differed little in terms of the cell performance. The difference in the performance in a flue gas (almost declined) and atmospheric environment (SGR of 0.261 d1, BPR of 0.171 gL1 d1 and CFR of 0.296 gL1 d1) was most pronounced for Amphidinium carterae, suggesting that it may be not directly suitable for CO2 fixation from flue gas. Because the microalgae species Amphidinium carterae is very sensitive to environmental factors, especially to the CO2 concentration, a sharp decline in the pH value of the culture medium also inhibits the metabolism of this species. Additionally, the shear effect of the gaseliquid fluid flow caused by bubbling aeration is more pronounced due to the lack of cellulose walls, leading to a hard growth. However, once the loads of the above unfavorable factors were reduced, the microalgae Isochrysis sp. and Amphidinium carterae are expected to perform better on CO2 fixation and biomass production. Based on the comparison with Isochrysis sp. and Amphidinium carterae, it was demonstrated that the microalgae Chlorella sp. had better performance on CO2 fixation and biomass production. Therefore, it can be selected as the dominant species for further investigation.
3.2. Effects of varied flue gas parameters on the performance of the dominant microalgae Chlorella sp.
Fig. 3. Performance of three energy microalgae under 15% flue gas CO2 level (AR ¼ 0.2 vvm and no toxic compounds presence).
3.2.1. CO2 concentration The effect of the flue gas CO2 concentration on the carbon fixation, biomass production, pH value and DO value for Chlorella sp. is illustrated in Fig. 4 and Table 5. Overall, the presence of high concentrations of CO2 exerted a negative effect, although microalgae Chlorella sp. could grow in culture over time. The SGR, BPR and CFR at three CO2 concentrations decreased and ranged from 0.372 to 0.293 d1, from 0.268 to 0.146 gL1 d1 and from 0.492 to 0.268 gL1 d1, respectively, over the entire culture period. Specifically, the biomass of Chlorella sp. negatively correlated with the CO2 concentration. Moreover, the pH value in the medium showed similar trends at the various CO2 concentrations. When aerating the medium with simulated flue gas for 1 day, the pH value sharply decreased from 8.0 to below 5.6 for 10% CO2, to 5.5 for 15% and to 5.2 for 20% CO2. The pH value slightly increased thereafter. Similar trends were observed for the DO value.
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Table 4 Performance parameters for three energy microalgae at 15% CO2. Microalgal species
X0 (gL1)a
X7 (gL1)
k (d1)
p (gL1 d1)
f (gL1 d1)
Chlorella sp. Isochrysis sp. Amphidinium carterae
0.150 0.300 0.230
1.493 1.080 0.016
0.328 0.183 /
0.192 0.111 /
0.353 0.190 /
a
X0 is 1 106 cells mL1 in number concentration.
According to gaseliquid mass transfer theory, the increase in the CO2(g) concentration increases the CO2(l) concentration via dissolution and increases Hþ and HCO 3 concentrations (decreasing pH value) via hydrolysis. These changes usually do not favor the growth of most microalgae according to the conventional CCM, which states that CO2 concentrations exceeding 1% can inhibit the activity of CA (carbonic anhydrase) and formation of CCM [19,42]. In this work, the typical flue-gas CO2 concentration (10%e20% v/v) is much higher than the atmospheric CO2 concentration (0.038% v/ v). The growth of microalgae chlorella sp. was inhibited as the CO2 concentration increased. Despite this inhibitory effect, Chlorella sp. continued to grow well under these conditions and no decrease in growth was observed, indicating that chlorella sp. can adapt and change the CCM in response to high concentrations of CO2, giving it the potential of continuous growth. In addition, although the flue gas CO2 caused the pH value to decrease more compared to the atmospheric CO2, the resultant pH value was not extremely low. As the photosynthesis and metabolism proceed, the pH value slightly increased. Similarly, aeration of CO2 gave rise to decrease of the DO. Also, as photosynthesis proceeded, the DO slightly recovered. In essence, this result demonstrated that the microalgae chlorella sp. can endure high CO2 concentrations and low pH values. A similar potential was also found in various improved Chlorella that can tolerate high CO2 concentrations of up to 70% (e.g. Chlorella sp. KR-1 [34] and Chlorella ZY-1 [35]) and even 100% (e.g., Chlorella sp. T-1 [32]) and a pH value as low as 3.5. However, it should be noted that the adaptability to the CO2 concentration varied by Chlorella species and the optimum CO2 concentration was usually found in the range of 0.038%e10% [31]. Overall, although the CO2 concentration has an adverse impact, the microalgae Chlorella sp. readily adapts to this impact and continues to fix CO2 and produce biomass at high levels.
Fig. 4. Effect of CO2 concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (AR ¼ 0.2 vvm and no toxic compounds presence).
3.2.2. Aeration rate The effect of the flue gas aeration rate on the carbon fixation, biomass production, pH value and DO value for microalgae Chlorella sp. is shown in Fig. 5 and Table 5. The aeration rate also had negative effect although microalgae Chlorella sp. could continue to grow as the culture time elapsed. Specifically, the biomass of Chlorella sp. negatively correlated with the CO2 concentration. The SGR, BPR and CFR changed in 0.375e0.263 d1, 1 1 1 1 0.274e0.114 gL d and 0.503e0.209 gL d , respectively, at the three aeration ratios during the entire culture period. Moreover, the pH value of the medium differed little between the three aeration rates and ranged from 8.0 to 5.5 due to the diffusive equilibrium in the gaseliquid phases when the simulated flue gas was aerated for 1 day. However, the DO value recovered more rapidly at low CO2 concentrations. The microalgae culture process usually requires an appropriate turbulent flow and mixing in the gas-liquid-solid (CO2-mediummicroalgae) phases to enhance mass transfer, allow light to reach cells, equalize the temperature and nutrients, remove produced oxygen and prevent the aggregation and sedimentation of microalgae. The aeration rate is an important factor in a gas aerating culture in addition to the bubble size. As expected, an appropriate turbulence effect due to an appropriate aeration rate helps to improve microalgae performance in terms of carbon fixation and
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Table 5 Performance parameters of Chlorella sp. at varied flue-gas CO2 concentration, aeration rate and toxic compounds concentration. Items
Concentration
X7 (gL1)
k (d1)
p (gL1 d1)
f (gL1 d1)
CO2 concentration
10% 15%a 20% 0.1 0.2a 0.3
2.025 1.493 1.170 2.070 1.493 0.945
0.372 0.328 0.293 0.375 0.328 0.263
0.268 0.192 0.146 0.274 0.192 0.114
0.492 0.353 0.268 0.503 0.353 0.209
0 ppma 100 ppm 250 ppm 250 ppm (pH ¼ 5.5) 100 ppm 250 ppm 10 mg m3 20 mg m3 30 mg m3
1.493 0.593 0.128 0.548 1.223 0.855 0.788 0.705 0.615
0.328 0.196 / 0.185 0.300 0.249 0.103 0.096 0.088
0.192 0.063 / 0.057 0.153 0.101 0.091 0.079 0.066
0.353 0.116 / 0.105 0.281 0.186 0.167 0.145 0.121
Aeration rate (vvm)
Toxic compounds Free SO2
NO Hg2þ
a
Using same conditions.
biomass production, but extremely high aeration rates may result in strongly shear stress, especially in the processes of bubble generation, bubble deformation (e.g., bubble coalescence and breakup) and gaseliquid interface formation [43,44]. Therefore, the effect of the aeration rate on microalgal-CO2 fixation and biomass production is complex. It was also found to be associated with the microalgae species and culture conditions. The recommended aeration rate is 0.05e1.00 for most bubbling cultures. Moreover, the optimum aeration rate varies by microalgae species and photobioreactor configuration, e.g. 0.025e1.00 vvm was proposed to be cost-effective for 5% or 10% (v/v) CO2 aeration and 0.05 vvm for a flat-panel photobioreactor [45]. In the present work, aeration rates higher than 0.1 vvm appear to hinder the CO2 fixation and biomass production of Chlorella sp. 3.2.3. Typical toxic compounds The effects of the pollutant characteristics, including different SO2, NO and Hg2þ concentrations, on the carbon fixation, biomass production, pH value and DO value for Chlorella sp. are presented in Figs. 6e8 and Table 5. (1) SO2 According to Fig. 6(a), the presence of SO2 in the flue gas significantly inhibited the growth of Chlorella sp. Although Chlorella sp. had a 2-day lag phase, it continuously grew during the 6-day culture period at an SO2 level of 100 ppm, with an SGR of 0.196 d1, BPR of 0.063 gL1 d1 and CFR of 0.116 gL1 d1. However, once the SO2 concentration increased to 250 ppm, Chlorella sp. was almost completely inhibited and gradually declined in concentration after the 2-day lag phase. Additionally, the biomass of Chlorella sp. also decreased after 6 days in culture at 100 ppm SO2, which was caused by an accumulated effect of the pH value (Fig. 6(b)). In addition, the DO value recovered faster in the absence of SO2 (Fig. 6(c)). The effect of SO2 on the microalgae growth was primarily attributed to the indirect effect of the pH value, i.e. if the pH of the culture medium is artificially maintained constant via neutralization, the growth characteristic for some microalgae almost do not differ from those in the absence of SO2 [18]. Fig. 6(b) shows that the threshold pH value for Chlorella sp. is approximately 4.7. It means that Chlorella sp. cannot survive below this pH. However, further investigation showed that Chlorella sp. could grow when the pH value was artificially maintained at 5.5, but the biomass produced in response to 15% CO2 with 250 ppm SO2 was less than that
produced in response 15% CO2 without SO2 or 100 ppm SO2. This finding actually indicates that the inhibition of Chlorella sp. by SO2 is associated with not only the pH value (Hþ concentration in cul ture medium) but also with the SO2 4 and HSO4 concentration, which result from SO2 hydrolysis suggesting that SO2 4 and HSO4 are microalgae growth inhibitors [46,47]. Overall, compared with other microalgae species that were completely inhibited at 50 ppm SO2, Chlorellas sp. moderately tolerates SO2. (2) NO Fig. 7(a) shows that the presence of NO in the flue gas partly inhibits the growth of Chlorella sp. The SGR, BPR and CFR were reduced from o 0.328e0.249 d1, from 0.192 to 0.101 gL1 d1 and from 0.353 to 0.186 gL1 d1, respectively, at the three NO concentration levels during the 6-day culture period. No significant lag phase was observed, even at high NO concentrations. Unlike the effect of SO2, NO did not significantly change the pH (Fig. 7(b)). In addition, the DO value recovered faster in the absence of NO (Fig. 7(c)). The effect of NO on the carbon fixation and biomass production for Chlorella sp. is considered to be complex. It seems to be less significant than that of SO2 at the same concentration, NO did not made Chlorella sp. decline even at high NO concentration (250 ppm). Unlike water-soluble SO2, NO cannot easily directly impact the growth of microalgae via the pH of the culture medium. The decrease in the pH value of the culture medium is mainly attributed to CO2 instead of NO. However, the presence of NO is in associated with physiological conditions of microalgae cells. Usually, its influence on the growth of microalgae has dual characters. Extremely low concentrations of NO may even be absorbed by the culture medium and transformed into NO2, which can serve as a source of nitrogen nutrition for microalgae that can process inorganic forms [8]. However, this positive influence is limited and depends on the microalgae species. For most of microalgae, increased NO concentrations decrease the growth rate. NO concentrations higher than 300 ppm may give rise to the decline of microalgae [24,48]. (3) Hg2þ Fig. 8(a) shows that even extremely low concentrations of Hg2þ can strongly inhibit Chlorella sp. In the concentration range of 10e30 mg m3, the inhibited effect of Hg2þ did not make change remarkably. Moreover, because CO2 aeration was the only source of
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Fig. 5. Effect of aeration rate on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15% and no toxic compounds presence).
Hþ, the pH curve (Fig. 8(b)) and the DO curve (Fig. 8(c)) are similar to those for NO. As one of the most important trace metals from combustion flue, mercury usually exists in the forms of elemental mercury (Hg0), oxidized mercury (Hg2þ) and particle-bound mercury (Hgp). Hgp can usually be captured by high-efficiency particle collectors, while many mercury control technologies often oxidize Hg0 into Hg2þ in order to achieve high-efficiency capture. Hg2þ in flue gas was observed to inhibit the microalgae because it is water-soluble and destroys chlorophyll, directly resulting in the reduction of the photosynthetic efficiency [49].
Fig. 6. Effect of SO2 concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15%, AR ¼ 0.2 vvm).
3.3. Analysis on application 3.3.1. Scale-up In practice, the bubble column photobioreactor can be scaled up for microalgae-based including Chlorella sp.-based CO2 fixation and biomass production because it is one of the most simple closed culture setups. In addition to the geometrical dimensions, the scale-up process depends on the physicochemical parameters of the flue gas, including the CO2 concentration, toxic pollutants, initial density of the microalgae, temperature,
Please cite this article in press as: Zhao B, et al., Carbon dioxide fixation and biomass production from combustion flue gas using energy microalgae, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.123
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Fig. 7. Effect of NO concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15%, AR ¼ 0.2 vvm).
Fig. 8. Effect of Hg2þ concentration on (a) Biomass of Chlorella sp., (b) pH of culture medium and (c) DO value in culture medium (CCO2 ¼ 15%, AR ¼ 0.2 vvm).
light, nutrients and pH value, as well as on the hydrodynamic parameters, including mass transfer, flow and mixing. The present result can be used to determine the optimized operating parameters. Theoretical, semi-empirical, dimensional analysis and CFD (computational fluid dynamics) approaches could then be used to scale up this process for pilot application. The industrial culture of microalgae is relatively easy due to the convenience and availability of seawater and f/2 broth, which is the most common culture medium. Moreover, when using improved culture modes, including semi-continuous culture,
continuous culture and advanced multistage culture system in industrial applications, the load of the CO2 concentration, aeration rate and toxic compound concentration as investigated in this work can be indirectly decreased or buffered in these processes. They can actually improve microalgal-CO2 fixation and biomass production. Other important culture conditions, such as the temperature, light (including intensity and lightedark cycle), nutrients, initial density, flow and mixing exert complex and nonlinear effects on the microalgal-CO2 fixation and biomass production [50].
Please cite this article in press as: Zhao B, et al., Carbon dioxide fixation and biomass production from combustion flue gas using energy microalgae, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.05.123
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3.3.2. CO2 fixation efficiency Although a consensus on the effectiveness of fixation of CO2 by microalgae was not completely reached [51], microalgae-based CO2 fixation and biomass production are considered to be promising. Due to the self-limiting nature of biochemical processes, microalgae can barely reach the same CO2 capture efficiency as mature chemical approaches, and also have the feature with a relatively long life cycle period. The CO2 fixation efficiency of Chlorella sp. cultured in a bubble column photobioreactor was approximately 16e58% [20]. It does not rival the CO2 capture efficiency of absorption (e.g. ammonia and amine), absorption and membrane approaches, which usually reach 90% or higher. Moreover, the microalgae biomass usually requires a harvest cycle of 3e7 days or longer. Therefore, new high-efficiency photobioreactors and culture modes (e.g. continuous culture) need to be developed to improve the CO2 fixation performance and shorten the biomass harvest cycle. Nevertheless, microalgae-based CO2 fixation and biomass production are usually preferable to be used when emphasizing the economic performance, environmental friendliness and product utilization for energy resources in addition to carbon emission reduction, because this biochemical process consumes low amounts of external energy (despite utilizing closed cultivation), does not emit secondary pollutants and generates energy-oriented products that can be further converted into biofuel. Additionally, the toxic compounds in flue gas have negative effect on microalgae-CO2 fixation and biomass production in most cases. From another standpoint, microalgae are at least partially able to remove them, e.g., Dunaliella tertiolecta was able to remove 50%e60% NO in continuous 15 days when there was 2% O2 [52] while NOA-113 had 40 mgd1 removal rate for NO [53]. However, the removal performances depend upon the microalgae specie, the biomass density and the toxic compound concentrations. The removal rates for the different toxic compounds need to be further investigated for the microalgae Chlorella sp. used in this work. 3.3.3. Energy conversion of microalgae-based biomass Chlorella sp. was demonstrated to be a potential bio-medium or biomaterial for CO2 control from combustion sources, although follow-up studies are required, including studies that improve the microalgae performance in harsher flue gas environments and studies that convert the microalgae biomass into bioenergy for utilization via physicochemical extractions, biochemical processes and thermochemical technologies with high efficiency [54e56]. In particular, the contents of lipid (oil) and fatty acid in the microalgae are varied with microalgae specie and culture parameters [20,37], e.g., the lipid content of Chlorella fusca was found to increase when the microalgae were grown at increasingly higher concentrations of CO2 [57] but Chlorella sp. had inconsistent performance [20]. For the purpose of energy utilization, therefore, their optimization is still an important issue. 4. Conclusions The potential of three natural energy microalgae candidates, Chlorella sp., Isochrysis sp. and Amphidinium carterae, to biologically fix flue gas CO2 and produce biomass was investigated. At 15% CO2, Amphidinium carterae was completely inhibited and Isochrysis sp. was also inhibited could grew relatively slowly. In comparison, Chlorella sp. performed better and was selected as the dominant microalgae. Chlorella sp. performed well in terms of CO2 fixation and biomass production, although the CO2 concentration (10e20%, v/v), aeration rate (0.1e0.3 vvm) and presence of toxic compounds
(0e250 ppm SO2 and 0e250 ppm NO and 10e30 mg m3) in the flue gas negatively affected its growth. The scale-up of this approach for pilot applications that improves the convenience, fixation efficiency of CO2 from flue gas and conversion of microalgae-based biomass into biofuels needs to be further explored. Acknowledgments This work was jointly sponsored by National Natural Science Foundation of China (Nos. 50806049 and 51278095), Innovation Program of Shanghai Municipal Education Commission (No. 09YZ209), China Scholarship Council (No. 201208310168) and The Hujiang Foundation of China (No. D14001). Nomenclature AR CC CCO2 CSO2 CNO C2þ Hg DO f k MC MCO2 p pH X X0 X7 Xt
aeration rete content of carbon in microalgae biomass CO2 concentration in simulated flue gas SO2 concentration in simulated flue gas NO concentration in simulated flue gas Hg2þ concentration in simulated flue gas Dissolved oxygen value CO2 fixation rate (CFR) based on initial microalgae biomass (gL1 d1) specific growth rate (SGR) for microalgae (d1) molar mass of carbon (g mol1) molar mass of CO2 (g mol1) biomass production rate for microalgae (gL1 d1) pH value microalgal biomass concentration (gL1) microalgal biomass concentration at time t0 (gL1) microalgal biomass concentration at 7th day (gL1) microalgal biomass concentration at time t (gL1)
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