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Application of a gas-liquid mixing pump in biogas purification and the coproduction of nano calcium carbonate Xi Liu, Honghua Jia, Jun Zhou, Xiaoyu Yong, Ping Wei, and Min Jiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01759 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 28, 2016

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Figure 1. Process diagram of the gas-liquid mixing pump technology (1, gas cylinder; 2, gas buffer; 3, rotameter; 4, gas adjusting valve; 5, Ca(OH)2 slurry tank; 6, turbine flowmeter; 7, pressure and vacuum gauge; 8, gas-liquid mixing pump; 9, retaining valve; 10, safety valve; 11, pressure valve; 12, liquid adjusting valve; 13, end gas collection; 14, gas-liquid separator) figure 1 44x23mm (600 x 600 DPI)

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Figure 2. The lift of gas-liquid mixing pump when delivering different materials figure 2 59x42mm (600 x 600 DPI)


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Figure 3. The shaft power of gas-liquid mixing pump when delivering different materials figure 3 59x42mm (600 x 600 DPI)


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Figure 4. The efficiency of gas-liquid mixing pump when delivering different materials figure 4 59x42mm (600 x 600 DPI)


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Figure 5. The lift of gas-liquid mixing pump in varied gas flow rate figure 5 59x42mm (600 x 600 DPI)


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Figure 6. The shaft power of gas-liquid mixing pump in varied gas flow rate figure 6 59x42mm (600 x 600 DPI)


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Figure 7. The efficiency of gas-liquid mixing pump in varied gas flow rate figure 7 59x42mm (600 x 600 DPI)


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Figure 8. The CO2¬ content of the end gas in varied gas flow rate figure 8 59x42mm (600 x 600 DPI)


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Figure 9. The XRD pattern of the calcium carbonate sample in gas-liquid mixing pump figure 9 61x44mm (600 x 600 DPI)


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Figure 10. The TEM image of the calcium carbonate sample in gas-liquid mixing pump experiments figure 10 84x84mm (300 x 300 DPI)


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Figure 11. The SEM image of PPS-PTFE coating figure 11 84x68mm (300 x 300 DPI)


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Application of a gas-liquid mixing pump in biogas purification and the coproduction of nano calcium carbonate X. Liu1,2, H.H. Jia1,2, J. Zhou1,2*, X.Y. Yong1,2, P. Wei1, M. Jiang1 1

College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China; 2

Bioenergy Research Institute, Nanjing Tech University, Nanjing 211816, China

* Corresponding author. Tel: +86-25-58139929, Fax: +86-25-58139929, E-mail: [email protected]

ABSTRACT: The CO2 removal is one of the key technological requirements in biogas purification. A technique using a gas-liquid mixing pump to capture CO2 and coproduce nano calcium carbonate was devised and tested in this work. The characteristic curves and CO2 absorption efficiency of the pump were investigated. The CO2 content of the end gas in all experiments was less than 2%, and the calcium carbonate samples showed that calcite was produced with favorable particle size. A solution was proposed to deal with gas inlet blockage by preparing polyphenylene sulfide (PPS) - polytetrafluoro ethylene (PTFE) coating. The coating had super-hydrophobicity, self-cleaning ability, unique microstructure and good adhesion to the substrate. The gas inlet coated with PPS-PTFE could remain clean for at least 12 hours, suggesting that the method used in this work was suitable. This work provides a novel approach for CO2 absorption and biogas purification.

Keywords: Biogas purification, Nano calcium carbonate, Gas-liquid mixing pump, PPS, PTFE.

1. Introduction As a promising renewable energy 1, biogas can help alleviate environmental pollution and the energy crisis. The purification of biogas is one of the key technological stages in its 1


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application 2. In particular, the removal of CO2 can raise the biogas caloric value to meet the compressed natural gas standard 3, which is crucial in its promotion. In addition to some established CO2 capture technologies such as water scrubbing 4, chemical absorption 5, pressure swing adsorption 6, and membrane separation 7, researchers are searching for more economical approaches that can utilize CO2 effectively. Using a Ca(OH)2 suspension to react with CO2 and coproduce nano calcium carbonate is one of these practises. Nano calcium carbonate is a high performance filler and coating widely used in many industrial fields 8, and it’s production can compensate for the cost of CO2 capture to some extent. The gas-liquid mixing pump is a kind of vortex pump with both gas and liquid inlets. Using the negative pressure generated by the liquid flow, the pump can suck in gas and efficiently mix it with water 9. The pump is generally applied in aeration, ozonated water production and air floatation 10. To the best of our knowledge, there have been no concern raised in the literature about using a gas-liquid mixing pump as a reactor to capture CO2. Thus, the purification of biogas and coproduction of nano calcium carbonate is a new research field. In this work, a technique using a gas-liquid mixing pump to capture CO2 and coproduce nano calcium carbonate was devised and tested, and a polyphenylene sulfide (PPS) polytetrafluoroethylene (PTFE) coating was prepared to deal with gas inlet fouling, which is a problem that has arisen in experiments. This work provides a novel approach to CO2 absorption and biogas purification.

2. Materials and methods 2.1.

Materials

The Ca(OH)2 used in this work was industrial grade (91.7% purity). The feed gas, containing approximately 40% CO2 (volume fraction), was provided by the biogas plant in Nanjing Tech 2


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University. The grade 304 stainless steel corrosion specimen was provided by Shanghai Luosong Electromechanical Equipment Co., Ltd (Shanghai, China). The PPS powder was provided by Zhejiang NHU Materials Co., Ltd (Shaoxing, China) and was used after sift through a 500 mesh sieve. The PTFE emulsion was provided by Aladdin Industrial Corporation (Shanghai, China). Other materials, including anhydrous ethanol, n-butanol, silicon dioxide, and sodium dodecylbenzene sulfonate, were supplied as analytical grade and used as received. The gas-liquid mixing pump (25QY-2) was provided by the Nanfang Pump Industry Co., Ltd (Hangzhou, China), and had a rated flow of 33.3 L/min. 2.2. Gas-liquid mixing pump experiments The experimental apparatus was assembled according to the process diagram shown in figure 1. The Ca(OH)2 slurry was fed into the gas-liquid mixing pump and adjusted to a certain flow rate, then open the gas valve and adjust the flow rate. The feed gas was stored in the cylinder and induced into the buffer tank before being fed into the pump. The pressure of the buffer tank was close to zero. The Ca(OH)2 and CO2 was mixed and reacted in the pump and then transported into the separator. The end gas was collected from the gas outlet of the separator and analyzed using an infrared gas analyzer (HP-FX02, Nanjing Hope Instrument Co., Ltd, Nanjing, China). The slurry flowed out of the separator and back to the tank. It kept circulating throughout the process until the pH value dropped to near 7. The CO2 content from the gas analyzer, pressure in the inlet and outlet pressure gauge, and power value shown by the power meter (ELE-3d3y, Yueqing Elecall Co., Ltd, Yueqing, China) were recorded. After finishing the experiments, the slurry was filtrated, diluted three times with pure water, washed three times with anhydrous ethanol, and dried in the drier. Samples of CaCO3 were collected for X-ray diffraction (XRD: Smartlab TM 9KW, Rigaku, Tokyo, Japan) and transmission electron microscopy (TEM: JEM-2100, JEOL, Tokyo, Japan) characterization. The lift of the pump was calculated by formula (1) according to Bernoulli’s principle. The 3


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effective power of the pump was calculated by formula (2). The shaft power of the pump was calculated by the active power divided by the electrical efficiency (79.6 %). The active power was recorded by the power meter and the efficiency of the pump was calculated by formula (3). He = ( z 2 − z 1 ) +

p 2 − p1 u 2 − u12 + 2 ρg 2g

Pe = ρ gHeqv

η =

Pe Pa

(1) (2) (3)

where He is the effective lift of the pump (m); z1 and z2 are the heights of the centers of the inlet and outlet pressure gauges, respectively (m); p1 and p2 are the pressure of the inlet and outlet pressure gauges, respectively (Pa); u1 and u2 are the flow rates of the inlet and outlet, respectively (m/s); ρ is the density of the liquid (kg/m³); qv is the volumetric flow rate (m³/s); Pe is the effective power of the pump (W); Pa is the shaft power of the pump (W); and η is the efficiency of the pump (1).

Figure 1. Process diagram of the gas-liquid mixing pump technology (1, gas cylinder; 2, gas buffer; 3, rotameter; 4, gas adjusting valve; 5, Ca(OH)2 slurry tank; 6, turbine flowmeter; 7, pressure and vacuum gauge; 8, gas-liquid mixing pump; 9, retaining valve; 10, safety valve; 11, pressure valve; 12, liquid adjusting valve; 13, end gas collection; 14, gas-liquid separator)

2.3. Preparation of the PPS-PTFE coating for gas inlet anti-blockage 4


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The gas inlet of the gas-liquid mixing pump (as shown in the figure S1 of the supporting information) is where CO2 and Ca(OH)2 first contact each other and generate CaCO3 particles. The CaCO3 particles would gradually block the inlet when the pump runs for more than six hours. Preventing blockage of the gas inlet is therefore a critical issue for the gas-liquid mixing pump process. The super hydrophobic surface has a self-cleaning ability 11, and has the potential to maintain the gas inlet clean and unobstructed. A hydrophobic coating was prepared in this work to protect the gas inlet from clogging. For practical use, the hydrophobic coating of the gas inlet should have three properties. (1) Favorable hydrophobicity. The contact angle between water and the coating should be 150o or more to meet the super hydrophobicity standard. (2) Chemical inertia. The gas inlet is placed in an alkaline environment, and therefore the coating should be resistant alkali to maintain its function (3) Good adhesion to the metal substrate. The pump housing is an environment with intense turbulence, and the coating should therefore bind firmly to the 304 stainless steel substrate to maintain its function. The polytetrafluoroethylene (PTFE) film has strong a chemical resistance and low surface energy, which gives it hydrophobic properties. The polyphenylene sulfide (PPS) film adheres strongly to metal substrates 12, 13. Therefore, the PPS-PTFE complex coating had the potential to solve the blockage problem. We prepared the PPS-PTFE coating according to a procedure proposed in the literature 14. The details about the preparation procedure can be found in the supporting information.

2.4. Characterization of the PPS-PTFE coating Whereas the gas inlet does not have plain surface, it is difficult to do contact angle and scanning electron microscopy (SEM) characterization, we therefore prepared the PPS-PTFE coating on the stainless steel corrosion specimen (the same material as the gas inlet) to do the 5


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characterizations. Contact angle values were obtained using a drop shape analyzer (DSA 100 Krüss, Hamburg, Germany) at ambient temperature with 5 µL of pure water on the surface of the samples. The morphological analysis was performed using a scanning electron microscope (TM3000, Hitachi, Tokyo, Japan). The adhesion of the coating to the substrate was tested by the cross cut method according to Chinese standard GB/T 9286-1998. 2.5. Anti-blockage test of the PPS-PTFE coated gas inlet The experiments compared the anti-blockage effect of the coated and uncoated gas inlets under the same conditions. The pump was constantly fed with CO2 gas and Ca(OH)2 slurry until the gas flow dropped to near zero. The working time of the pump was recorded at this point. The mass fraction of Ca(OH)2 slurry was 2% and the flow rate was 33.3 L/min. The flow rate of the feed gas was 2 L/min. When the pH dropped to near 7, another batch of Ca(OH)2 slurry was applied.

3. Results and discussion 3.1. Gas-liquid mixing pump experiments 3.1.1. Characteristic curves of the gas-liquid mixing pump when delivering different materials The experiments compared characteristic curves of the gas-liquid mixing pump working with three different materials: pure water, Ca(OH)2 slurry and gas-slurry mixture with 2 L/min gas flow rate. The curves reveal the law of the pump when delivering various materials and could be used to determine the optimum working condition. The parameters are selected based on the conventional operation conditions of the pump. e.g. the liquid flow rates are set near the rated flow rate. The gas flow rates are set near the recommended gas flow rate (10% of the total flow rate). Figure 2 shows the lift curves of the pump with the three materials. As the total flow rate increased, the lift curves of all three materials experienced a sharp, almost linear drop. The 6


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steep descent in the characteristic curves were in line with the practical operation of vortex pumps 15. The lift of water was higher than that of the Ca(OH)2 slurry and the gas-liquid mixture under the same conditions. This was mainly due to the fact that when transporting a solid-liquid mixture, the solid phase cannot transfer the pressure energy as well as the liquid phase can. On the other hand, the solid phase cannot diffuse freely, it is reasonable to deduct the volume of the solid phase when calculating the lift 16-18. The lift of the gas-liquid mixture was slightly higher than that of the Ca(OH)2 slurry because both gas and liquid are fluid and can transfer pressure 19. Figure 3 presents the shaft power curves of the pump for the three materials. It can be seen that the law of power was similar to that of the lift. The shaft power curves of all three materials displayed a sharp, almost linear drop when the total flow rate increased. The law for water was slightly higher than that of the slurry and mixture under the same conditions. Figure 4 presents the efficiency curves of the pump for the three materials. It can be seen that all of the curves rose at first then displayed a falling trend, reaching their peaks at around the rated flow of the pump (33.3 L/min). The most efficient flow rate was usually set as the rated flow in the design and testing of pumps. The results of this study proved that the most efficient flow for transporting a gas-liquid or solid-liquid mixture was also the rated flow. The efficiency of water was slightly higher than that for the slurry and gas-liquid mixture, indicating that when a solid or gas phase is involved in transporting material, the performance of the gas-liquid mixing pump will deteriorate.

7


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Figure 2. The lift of gas-liquid mixing pump when delivering different materials

Figure 3. The shaft power of gas-liquid mixing pump when delivering different materials

Figure 4. The efficiency of gas-liquid mixing pump when delivering different materials 8


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3.1.2. Characteristic curves of the gas-liquid mixing pump and the CO2 capture efficiency under varied gas flow The experiments compared characteristic curves and the end gas CO2 content of the gas-liquid mixing pump working with Ca(OH)2 slurry and varied gas flow rate. Figure 5 presents the lift curves of the pump working under total flow rates of 26.6, 30, 33.3, 36.6, and 40 L/min. As the gas flow rate increased, all of the lift curves dropped slightly. When working under a total flow rate of 36.6 L/min, the pump could reach a maximum gas flow rate of 4.2 L/min. The pump could suck in relatively more gas in the high efficiency zone (30-36.6 L/min flow rate) than in the small flow rate zone, because the degree of vacuum in the pump liquid inlet was small. The gas flow was small too when working in the high flow rate zone because the energy of the fluid was mainly transferred in the form of kinetic energy, and therefore the pressure energy was also small. The lift of the pump working in the high flow rate zone was small, resulting in even smaller pressure energy and a weak gas sucking ability. Liu 20, 21 and Li 22claim that the performance of the gas-liquid mixing pump evidently deteriorated when the gas holdup was high, but we did not observe any significant deterioration in performance, possibly because the gas flow in the experiments was relatively small. Figure 6 presents the shaft power curves of the pump working under total flow rates of 26.6, 30, 33.3, 36.6, and 40 L/min. There was a similar trend to that of the lift curves. As the gas flow rate increased, all of the power curves dropped slightly, indicating that when the gas holdup increased, the pump consumed less power.

9


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Figure 5. The lift of gas-liquid mixing pump in varied gas flow rate

Figure 6. The shaft power of gas-liquid mixing pump in varied gas flow rate

Figure 7. The efficiency of gas-liquid mixing pump in varied gas flow rate 10


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Figure 7 presents the efficiency curves of the pump working under total flow rates of 26.6, 30, 33.3, 36.6, and 40 L/min. There is a similar trend to that of lift and shaft power curves. As the gas flow rate increased, all the efficiency curves dropped slightly, proving that the increased gas holdup would decrease the pump’s efficiency. Several studies 18, 21 have shown that the performance of a gas-liquid mixing pump will deteriorate rapidly when gas holdup is over 15 %. High gas holdup would increase the flow resistance and could even cause a flow-cut. It can also be seen that the high efficiency zone for the pump was at about 30 L/min. It is therefore reasonable to deliver gas-liquid mixture in the high efficiency zone. Figure 8 presents the end gas CO2 content curves of the pump working under total flow rates of 26.6, 30, 33.3, 36.6, and 40 L/min. Although the lifts and efficiencies of the pump varied, the CO2 content were almost the same under the different conditions (less than 2 %), indicating that the pump can capture CO2 with considerable efficiency under various conditions.

Figure 8. The CO2 content of the end gas in varied gas flow rate

3.1.3. Characterization of calcium carbonate samples Figure 9 shows the XRD pattern of the CaCO3 sample acquired in the experiments. The 11


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pattern conforms very closely to calcite. Among the three crystal forms of CaCO3 (calcite, vaterite, and aragonite), calcite is the steadiest 23. The formation of calcite in the experiments complies with the general rules of thermodynamics The TEM image of the samples (shown in figure 10) from the experiments indicated that most crystals were approximately round or cubic, with an average size of 77.2 nm and a range of 50-100 nm.

Figure 9. The XRD pattern of the calcium carbonate sample in gas-liquid mixing pump experiments

Figure 10. The TEM image of the calcium carbonate sample in gas-liquid mixing pump experiments 12


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3.2. Characterization of the PPS-PTFE coating 3.2.1. The hydrophobicity of the PPS-PTFE coating Contact angle values were obtained using the drop shape analyzer. As shown in the figure S2 of the supporting information, the contact angle between the stainless steel surface and water changed from 78.7o to 152.2o, which had met the super hydrophobicity standard. 3.2.2. The self-cleaning ability of the PPS-PTFE coating The self-cleaning ability of the as-prepared coating was tested according to the method proposed by Su 24. Calcium carbonate powder was sprinkled on the corrosion specimen coated with PPS-PTFE and then water was dropped onto it. As shown in the figure S3 of the supporting information, the spherical water droplet rolled and removed the powder from the surface, indicating that the coating could be maintained in a clean and uncontaminated state. 3.2.3. Micro morphology of the PPS-PTFE coating The morphological analysis was performed by SEM. Figure 11 presents the micro structure of the PPS-PTFE coating at 5000× magnification. Veins resembling braided fabrics and some hill-like bumps can be clearly seen in the coating. The bumps were 4 ×4 µm in size. The intrinsic contact angle of the PTFE surface was 108° 25. According to Wenzel 26, Cassie and Baxter 27, a rough surface could help make the surface more super-hydrophobic.

13


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Figure 11. The SEM image of PPS-PTFE coating

3.2.4. The adhesion of the PPS-PTFE coating to the substrate The adhesion of the coating to the substrate was tested by the cross cut method. Six parallel lines were cut on the coating with 2 mm spacing, and then six perpendicular parallel lines were cut with the same spacing. A tape was applied to the coating and was then ripped off. If no lattice peeled off and all the cut edges were smooth, it was classed as grade 0 (first grade). The results (showed in the figure S4 of the supporting information) indicate that the PPS-PTFE coating could adhere to the substrate firmly, with none of the lattice peeled off. 3.2.5. Anti-blockage test of the PPS-PTFE coated gas inlet The anti-blockage effect of the coated and uncoated gas inlets was compared when performing the CO2 capture experiments. The gas flow rate was observed before it dropped to zero, which can be regarded as a sign of blockage. The working time of the pump was recorded at this point. The experiments were performed under the following conditions: Ca(OH)2 slurry mass fraction = 2%; liquid flow rate = 33.3 L/min; CO2 volume fraction in the feed gas = 47 %; and gas flow rate = 2 L/min. The gas buffer was large enough to maintain the pressure balance with the atmosphere. When the pH dropped to near 7, another batch of Ca(OH)2 slurry was applied. 14


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The results showed that the gas flow in the uncoated gas inlet dropped to 0 after about six hours and the blockage was evident, whereas the gas flow in the PPS-PTFE coated gas inlet did not decrease significantly for 12 hours and there was no blockage (as shown in the figure S5 of the supporting information). This proves that the PPS-PTFE coating could improve the anti-blockage ability of the gas inlet. The coated gas inlet maintained its performance in the alkaline environment, indicating that the PPS-PTFE coating had a strong chemical resistance.

5. Conclusion A technique using a gas-liquid mixing pump to capture CO2 and coproduce nano calcium carbonate was devised and tested in this work. The gas-liquid mixing pump was used here as a reactor in gas absorption for the first time. The characteristic curves and CO2 absorption efficiency of the pump was investigated when it was working with different materials and varied gas flow rates. The results indicated that the lift and shaft power decreased sharply when the gas or liquid flow rate increased. The efficiency and flow rate of the gas peaked at the rated flow rate. The CO2 concentration of the end gas in all experiments was less than 2%. Characterization of the calcium carbonate samples showed that calcite was produced, with an average particle size of 77.2 nm. A solution was proposed to deal with the gas inlet blockage, which had arisen in gas–liquid mixing pump experiments. A PPS-PTFE coating was prepared by preparing bottom, middle, and top layer suspensions, spraying them on the stainless steel substrate, and then sintering. A contact angle test showed that the coating had met the super-hydrophobicity standard and had a self-cleaning ability. SEM results showed that there were veins resembling braided fabrics and some hill-like bumps in the coating. The bumps were 4 × 4 µm in size. The rough surface made the coating more hydrophobic. The cross cut test results of the coating indicated a strong adherence of the coating to the stainless steel substrate. Experiments comparing the anti-blockage ability of the coated and uncoated gas 15


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inlets were carried out, with results showing that the gas inlet coated with PPS-PTFE could maintain clean for at least 12 hours, indicating that the method used in this work was suitable. The method used in this study has a simple process and good gas-liquid mixing effect. It could capture the CO2 effectively with a relatively low primary investment, and simultaneous get the nano calcium carbonate. It provides a novel approach for CO2 absorption and biogas purification.

Supporting information Detailed preparation method of the PPS-PTFE coating, schematic graph of the gas inlet of the gas-liquid mixing pump, contact angle comparison of the coated and uncoated surface, the self-cleaning test and cross cut test results, and the comparison of the gas inlets after the anti-blockage test.

Acknowledgements This work was financially supported by the National Basic Research Program of China (2013CB733500), the National Key Technology Support Program of China (2014BAC33B00), the National Natural Science Foundation of China (21307058, 21207065), the Jiangsu Province Science Foundation for Youths (BK20130931), and the Key Science and Technology Project of Jiangsu Province (BE2016389).

Reference: 1. Chaemchuen, S.; Kabir, N. A.; Zhou, K.; Verpoort, F., Metal-organic frameworks for upgrading biogas via CO2 adsorption to biogas green energy. Chemical Society Reviews

2013, 42, (24), 9304-32. 16


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