Na Zhang

Zh Pan

Zhien Zhang

Wenxiang Zhang

Li Zhang

Francisco M Baena-Moreno

Eric Lichtfouse

William G Lowrie

CO 2 capture from coalbed methane using membranes: a review

Keywords: Coalbed methane, CO 2 capture, Membrane, Absorption, Greenhouse effect

Coalbed methane is an abundant form of natural gas extracted from coal beds. Coalbed methane is viewed as a cleaner energy source versus petroleum and coal combustion because methane extraction, transport and use are more efficient and less polluting. However, coalbed methane contains high amounts of CO 2 that induce solidification during liquefaction. Therefore, CO 2 has to be reduced below 2% to meet the pipeline transportation standards. In addition, CO 2 capture would reduce the amount of gas emissions to the atmosphere, thus mitigating global warming. Here, we review membrane absorption, which is an advanced method for CO 2 capture from coalbed methane, by controlling the gas and liquid phases separately during the operation process. We compare CO 2 removal methods for various coalbed methane sources. Parameters influencing CO 2 removal by membrane absorption are discussed to conclude that CO 2 capture efficiency is improved by increasing the flow rate, temperature, and absorbent concentration, reducing the gas flow rate, and selecting a mixed absorbent. We also explain the principles, processes and applications of CO 2 membrane absorption.

Introduction

Coalbed methane is a clean energy source, but accidental discharge into the atmosphere wastes resources, and induces serious climate warming, threatening the environment and human living conditions [START_REF] Huang | Carbon dioxide capture using an omniphobic membrane for a gas-liquid contacting process[END_REF]. With the increasing CO 2 emissions, the greenhouse effect is increasing, and carbon dioxide capture has been recognized as a viable option to reduce carbon dioxide [START_REF] Yan | Carbon capture, utilization and storage (CCUS)[END_REF]. Currently, the methods for CO 2 capture mainly include solvent absorption, adsorption, low-temperature distillation, synthetic hydrates, and membranes [START_REF] Huang | Carbon dioxide capture using an omniphobic membrane for a gas-liquid contacting process[END_REF]Lv et al. 2012b).

With the changing energy economy in recent years, the consumption of naturally occurring gas has been increasing. Because petroleum and coal are rather exhaustible resources, a production peak is expected in the future. Thus, coalbed methane, which is a high-quality energy source, has been continuously produced (Li et al. 2018a, b). China is rich in coal resources and has the third largest coalbed methane reserve in the world after Russia and Canada (Li et al. 2018c;[START_REF] Su | A combined stimulation technology for coalbed methane wells: Part 2[END_REF]. The world's geological coalbed methane resources amount to approximately 2.68 × 10 14 m 3 . Russia, the USA, China, Canada, and Australia account for 90 vol% of the world's total coalbed methane resources (China: 12 vol.%) (Liu et al. 2018c;[START_REF] Su | A combined stimulation technology for coalbed methane wells: Part 2[END_REF].

The concentration of CH 4 in coalbed methane is about 97% [START_REF] Mallick | Energy analysis on coalbed methane (CBM) coupled power systems[END_REF], and the energy released by combustion of 1 m 3 CH 4 is 35.9 million J, which is equivalent to the combustion of 1.2 kg of standard coal [START_REF] Hao | Effect of silica sol on the sealing mechanism of a coalbed methane reservoir: new insights into enhancing the methane concentration and utilization rate[END_REF]). However, when directly discharged into air by accident or negligence, CH 4 produces a huge greenhouse effect, which is approximately 21 times stronger than that of CO 2 . Also the CH 4 destructive capacity of the ozone layer is approximately seven times higher than CO 2 [START_REF] Daeho | Development of a dynamic simulation model of a hollow fiber membrane module to sequester CO 2 from coalbed methane[END_REF][START_REF] Warmuzinski | Harnessing methane emissions from coal mining[END_REF]). Moreover, other problems are caused, such as increasing surface temperature, frequent climate variations, and aggravating greenhouse effects (Kim and Kim 2018;[START_REF] Lewis | CO 2 capture in amine-based aqueous solution: role of the gas-solution interface[END_REF]Li et al. 2018a;[START_REF] Luis | Recent developments in membrane-based technologies for CO 2 capture[END_REF]. Therefore, accidental coalbed methane emissions have caused widespread concern worldwide.

Figure 1a presents the coalbed methane productions of the world's major coalbed methane producers from 2006 to 2014, showing an increase in China. Figure 1b depicts the production and utilization of coalbed methane in China from 2011 to 2017 [START_REF] Zheng | Research on development policy of coalbed methane industry in China's coal mining areas[END_REF]. Although China's annual coalbed methane production is very high, the utilization rate is relatively low.

Since 2008, China's coalbed methane production has increased significantly to more than 2.832 × 10 8 m 3 /a (a: annually, per year) [START_REF] Moore | Coalbed methane: a review[END_REF]. The development of coalbed methane is essential for improving the safety of the domestic coal mine production, relieving the market gas supply pressure, and protecting the ecological environment [START_REF] Qin | A review on development of CBM industry in China[END_REF]. The development of coalbed methane resources not only reduces the emissions of greenhouse gases and ensures an optimized energy exploitation, but also reduces natural disasters such as explosions and spontaneous combustions [START_REF] Cheng | An intelligent gel designed to control the spontaneous combustion of coal: fire prevention and extinguishing properties[END_REF][START_REF] Fan | Coal and gas outburst dynamic system[END_REF]). As a consequence, China has introduced several policies to promote the development of the coalbed methane industry within the National Thirteenth Five-Year Plan. The Chinese national energy administration has established a strategic goal to achieve a coalbed methane production of 2.40 × 10 10 m 3 in 2020 to accelerate the cultivation and development of the coalbed methane industry and to promote a clean energy production and consumption revolution [START_REF] Kim | Techno-economic evaluation of gas separation processes for long-term operation of CO 2 injected enhanced coalbed methane (ECBM)[END_REF]Li et al. 2018a;[START_REF] Sarhosis | Economic modelling for coal bed methane production and electricity generation from deep virgin coal seams[END_REF][START_REF] Zhou | Methane drainage and utilization in coal mines with strong coal and gas outburst dangers: a case study in Luling mine, China[END_REF].

Figure 2 shows the storage state of coalbed methane in the ground and its extraction and separation processes (Liu et al. 2018a). Significant progress has been made in the construction of pipelines for coalbed methane in China, with a capacity of more than 4300 km and 18 billion cubic meters per year (https ://www.in-en.com/artic le/html/energ y-22580 91.shtml ). However, the high amounts of CO 2 in the coalbed methane exploitation cause a solidification in subsequent liquefaction. This affects the natural gas treatment process and the quality of the natural gas. Therefore, a coalbed methane decarburization is necessary to reduce the CO 2 concentration to values below 2% to meet the pipeline transportation standard [START_REF] Bhadra | Separation of methane-nitrogen mixture by pressure swing adsorption for natural gas upgrading[END_REF][START_REF] Datta | Optimization of membrane unit for removing carbon dioxide from natural gas[END_REF][START_REF] Xu | Recent advances on the membrane processes for CO 2 separation[END_REF].

CO 2 capture from other gas mixtures by hollow fiber membrane absorption, and other methods for methane decarburization methods have been reported [START_REF] Chen | Experimental study on the separation of CO 2 from flue gas using hollow fiber membrane contactors with mixed absorbents[END_REF][START_REF] Fan | Numerical study of CO 2 -enhanced coalbed methane recovery[END_REF][START_REF] Fang | Numerical simulation of enhancing coalbed methane recovery by injecting CO 2 with heat injection[END_REF][START_REF] Lee | Development of novel sub-ambient membrane systems for energy-efficient post-combustion CO 2 capture[END_REF][START_REF] Li | Analytical model and experimental investigation of the adsorption thermodynamics of coalbed methane[END_REF]. However, to the best of our knowledge, coalbed methane decarburization by membrane absorption has been rarely reported. For instance, Yan et al. (2015b) studied the use of membrane absorption in flue gas decarburization. They mainly investigated the effect of the membrane structure and various influencing factors on the decarburization efficiency. [START_REF] Anand | A technical, economic, and environmental assessment of amine-based CO 2 capture technology for power plant greenhouse gas control[END_REF] investigated the decarburization process of biogas by membrane absorption. However, these studies have not focused on coalbed methane decarburization.

Here, we explore the feasibility of using hollow fiber membrane absorption to remove CO 2 from coalbed methane. We present applications and methods for membrane absorption in decarburization systems. For example, a mathematical model is used to determine how the membrane absorption method changes the decarburization efficiency. This method improves decarburization and acid gas removal versus conventional methods (Zhang et al. 2014b). Then, the "CO 2 capture technology for coalbed methane" section compares methods for the capture of CO 2 from coalbed methane. In the "CO 2 capture by membrane absorption" section, the principles of the membrane absorption and the main factors controlling the capture of CO 2 are discussed. Furthermore, the application of membrane absorption to other gases is reported in "Membrance absorption applied to other gases" section.

CO 2 capture technology for coalbed methane

Currently, the methods for removing CO 2 from coalbed methane mainly include solvent absorption, adsorption, low-temperature distillation, synthetic hydrates, and membranes (Table 1). Solvent absorption is a gas separation technology that separates the components with the highest solubilities from a mixture with a liquid absorbent [START_REF] Chen | The corrosion and degradation of amine solution in the coal bed methane decarburization process[END_REF]. [START_REF] Lei | Simulation study on CO 2 removal technology from coal-bed methane with alkamine method[END_REF] added different additives (piperazine, ethanolamine, and diethanolamine) to methyl diethanolamine to capture CO 2 from coalbed methane. The results show that the absorption performance of the methyl diethanolamine solution increases highly after the addition of the additives. However, the regeneration ability of the mixed solution decreases to a certain extent because the regeneration performance of the additives is lower than that of methyl diethanolamine. Furthermore, it was discovered that diethanolamine has a better absorption performance than methyl diethanolamine and a better regeneration performance than ethanolamine. [START_REF] Wan | Simulation analysis of CBM decarburization process[END_REF] simulated the decarburization process of coalbed methane with methyl diethanolamine in an absorption tower and analyzed the effect of the pristine and activated methyl diethanolamine concentration on the CO 2 capture efficiency of the system. The results show that the larger the mass fraction of the methyl diethanolamine solution, the stronger the decarburization efficiency. [START_REF] Zhang | Decarburization characteristics of coalbed methane by membrane separation technology[END_REF] modeled the decarburization of coalbed methane Fig. 2 Coalbed methane production. As the coal is mined, coalbed methane is separated from coal. Coalbed methane is used after pipeline transportation and decarburization. Reproduced from Liu et al. (2018a), with permission from Elsevier with four amine solutions, i.e., diethanolamine, ethanolamine, triethanolamine, and 1-(2-hydroxyethyl) pyrrolidine. 1-(2-Hydroxyethyl) pyrrolidine induces a high decarburization efficiency even at low absorbent concentrations because it exhibits a high chemical reaction constant for CO 2 . When the 1-(2-hydroxyethyl) pyrrolidine concentration increases from 0.4 to 0.9 M, its decarburization efficiency increases by 46.9%. Furthermore, [START_REF] Chen | The corrosion and degradation of amine solution in the coal bed methane decarburization process[END_REF] considered the corrosion and degradation of an amine solution in the coalbed methane decarburization process. The advantages of the solvent absorption method are the simple equipment, low energy consumption, low investment, and high purity of the products. The disadvantages are the slow absorption and regeneration of the solution.

Pressure swing adsorption is the selective adsorption of a separated gas mixture through adsorbents under different partial pressures [START_REF] Ebner | State-of-the-art adsorption and membrane separation processes for carbon dioxide production from carbon dioxide emitting industries[END_REF]. [START_REF] Tao | CO 2 capture from high concentration CO 2 natural gas by pressure swing adsorption at the CO 2 CRC Otway site, Australia[END_REF] used a pressure swing adsorption device to remove CO 2 from natural gas with a high CO 2 content. The decarburization efficiency reaches 66% for a CO 2 content of 30-50% and pressure of 7 bar. However, the purity of the final product CH 4 is insufficient to meet the general natural gas demand. Based on the experimental data of the pressure swing adsorption, they investigated the effects of the pressure with simulations. According to their results, the CH 4 purity can reach 98% when the pressure is reduced to 1 bar. Thus, the pressure is a critical factor for achieving the required separation performance. Engelhard used the Molecular Gate™ adsorption technology to decarbonize coalbed methane by compressing the feed pressure in a more traditional pressure swing adsorption mode [START_REF] Ebner | State-of-the-art adsorption and membrane separation processes for carbon dioxide production from carbon dioxide emitting industries[END_REF]. Moreover, [START_REF] Daeho | Development of a simulation model for the vacuum pressure swing adsorption process to sequester carbon dioxide from coalbed methane[END_REF] studied the coalbed methane decarburization through vacuum pressure swing adsorption. The advantages are the mature technology, simple equipment, low operating costs, and high methane recovery rate. However, it consumes much energy and strongly depends on the adsorbents.

Cryogenic distillation usually includes a mechanical method to liquefy the gas and exploits the difference in the gas boiling points to conduct the rectification. Thus, the different gas components can be separated and a high-purity gas is obtained [START_REF] Chen | Study progress of concentrating technology of lowconcentration oxygen-containing coal-bed methane[END_REF]Lv et al. 2013). [START_REF] Wang | Study on test parameters of oxygen liquefaction cold box for low-concentration coal-bed methane[END_REF] used the cryogenic distillation technology to liquefy and separate components of coalbed methane at low temperatures and pressures simultaneously. For an increase in the methane concentration in the coalbed methane from 22 to 27%, the methane recovery rate increases slightly to approximately 98.7%. For an increase in the methane concentration from 27 to 45%, the methane recovery rate decreased slightly but was still above 98%. Moreover, [START_REF] Sun | Simulation and analysis of a liquefaction and separation process of low concentration CBM[END_REF] designed a cryogenic liquefaction separation equipment for coalbed methane with a 50% air volume and a daily processing capacity of 5.67 × 10 4 m 3 . It simultaneously completes the liquefaction and separation processes and exhibits a CH 4 recovery rate of 82.19%. At this stage, cryogenic rectification is a matured technology with a high product purity and CH 4 recovery rate. The drawbacks of the method are the high energy consumption, large investment, complex equipment, and unsuitability for low-concentration and small-scale coalbed methane fields.

Hydrate separation of CO 2 is a new carbon capture approach [START_REF] Meng | Research progress of hydrate separation technology for biogas purification[END_REF], which exploits the different pressures of the gas components in the mixture to form hydrates in a solution and achieve the separation (Liu et al. 2018b;[START_REF] Pan | Effect of silica sand size and saturation on methane hydrate formation in the presence of SDS[END_REF]). The study demonstrates that the required pressures for the CH 4 and CO 2 hydrate formation are 2.65 MPa and 1.22 MPa under equal conditions, respectively [START_REF] Wang | Hydrate capture CO 2 from shifted synthesis gas, flue gas and sour natural gas or biogas[END_REF]. Therefore, the CO 2 separation from coalbed methane through this method can be achieved by controlling the temperature and pressure. Hence, gas components of CO 2 that easily form hydrates can be separated from the gas phase to purify the coalbed methane (Lv et al. 2013). In another work [START_REF] Zhong | An experimental study of using hydrate formation to enhance the methane recovery of low-concentration CBM[END_REF], the CH 4 recovery rate reaches 46.1% at 283.4 K and 2.6 MPa. After the secondary hydration separation, the CH 4 volume fraction increases from 30 to 72%. The key of the coalbed methane hydrate separation technology is to determine the thermodynamic conditions for the separation of different components to improve the separation efficiency. The technology provides a high storage density, convenient transportation, simple process, and oxygen-free products. However, it requires a high operating pressure, constitutes a high safety risk, and exhibits high costs and a low methane recovery rate.

The membrane absorption method has attracted much attention owing to its large gas-liquid contact area, fast mass transfer rate, high operational flexibility, no liquid ubiquity and mist entrainment, and simple equipment [START_REF] Anand | A technical, economic, and environmental assessment of amine-based CO 2 capture technology for power plant greenhouse gas control[END_REF]. Compared with traditional separation methods, the membrane absorption method is efficient and requires low investments and maintenance costs for the coalbed methane treatment [START_REF] Gabelman | Hollow fiber membrane contactors[END_REF][START_REF] Kaldis | Energy and capital cost analysis of CO 2 capture in coal IGCC processes via gas separation membranes[END_REF][START_REF] Medina-Gonzalez | Improving PVDF hollow fiber membranes for CO 2 gas capture[END_REF]. [START_REF] Zeng | Application research of a new composite decarburization method for coalbed methane China[END_REF] first studied the coalbed methane decarburization by combining pressure swing adsorption and the membrane separation technology. The results show that the CH 4 loss rate can be reduced by at least 61.5% with respect to the loss rates of the individual methods. [START_REF] Daeho | Development of a dynamic simulation model of a hollow fiber membrane module to sequester CO 2 from coalbed methane[END_REF] used a hollow fiber membrane with a countercurrent flow structure to absorb CO 2 from coalbed methane. The selected coalbed methane composition in the simulations was 90% CH 4 and 10% CO 2 . According to the results, increasing the effective length and number of fiber membranes and the gas pressure or decreasing the gas flow rate can improve the CH 4 recovery rate and purity. Moreover, [START_REF] Zhang | Decarburization characteristics of coalbed methane by membrane separation technology[END_REF] used a membrane contactor for coalbed methane decarburization by establishing a mathematical model. The results show that the decarburization efficiencies can be increased by 30% and 46% for an increased liquid flow rate and absorbent concentration, respectively. The decarburization efficiency is significantly better than that of a single membrane contactor (maximal increase of 47%). However, the gas mixture only contained CO 2 and CH 4 . Thus, other gas components should be investigated in future studies.

At present, the membrane absorption method is widely applied for natural gas purification because of the high CH 4 concentrations in gas streams. [START_REF] Tang | Kinetics research on mixed solvents of MDEA and enamine in natural gas decarbonization process[END_REF] compared the effects of single methyl diethanolamine solutions and its blended solutions with diethylenetriamine or triethylenetetramine on the CO 2 absorption capacity during natural gas decarburization. They discovered that the CO 2 absorption capacity is enhanced when diethylenetriamine or triethylenetetramine is added. [START_REF] Faiz | CO 2 removal from natural gas at high pressure using membrane contactors: model validation and membrane parametric studies[END_REF] used hollow fiber membrane contactors to capture CO 2 from natural gas at high pressures up to 50 bar with physical and chemical absorbents. In addition, [START_REF] Kang | Removal of high concentration CO 2 from natural gas using high pressure membrane contactors[END_REF] investigated CO 2 capture from natural gas containing 70% CO 2 . According to the results, the CO 2 capture efficiency increases from 33.3 to 91.3% when the pressure is increased from 1 to 60 bar. In this study, the capture of high CO 2 concentrations from natural gas was investigated with a membrane contactor process at elevated pressures for the first time. Furthermore, [START_REF] Hb | A comparison of porous and non-porous gas-liquid membrane contactors for gas separation[END_REF] studied the decarburization of natural gas via membrane absorption. The advantages of this technology are the low energy consumption, low operating costs, and no secondary pollution. In addition, the membrane life could hinder further pilot-scaled applications of this technology.

Based on a comparison of multiple coalbed methane decarburization methods, the membrane absorption method exhibits advantages regarding the process flow, energy consumption, operating costs, and decarburization efficiency. Owing to the coalbed methane characteristics, simple development concept, and environmental protection, the membrane absorption method is more suitable for coalbed methane decarburization treatments and will become a hot spot in this research field.

CO 2 capture by membrane absorption

Principle

The membrane absorption method is a novel separation process that combines membrane separation and absorption. It usually employs microporous membranes. During the process, the gas and liquid phases flow through both sides of the membrane contactor and contact at a fixed gas-liquidphase interface. The membrane itself is not gas selective but acts as a barrier to separate the absorbent and gas. The CO 2 diffuses through the membrane to the liquid phase owing to a concentration gradient. Theoretically, the membrane pores allow the CO 2 gas molecules to pass the membrane section without a high-pressure driving force. Thereby, one mainly relies on the selective absorption of the absorbent to separate mixed gases [START_REF] Li | Review of CO 2 absorption using chemical solvents in hollow fiber membrane contactors[END_REF][START_REF] Liang | Effect of long-term operation on the performance of polypropylene and polyvinylidene fluoride membrane contactors for CO 2 absorption[END_REF]). The basic principle (for a hydrophobic porous membrane) is shown in Fig. 3a. The main driving force of the technology is the gas concentration difference. First, CO 2 from the gas mixture reaches the surface of the membrane pore and diffuses through the membrane pores to the gas-liquid twophase interface. Next, CO 2 reacts with the absorbent in the tube side [START_REF] Boucif | CO 2 capture in HFMM contactor with typical amine solutions: a numerical analysis[END_REF][START_REF] Genceli | Effects of carboxylated multi-walled carbon nanotubes having different outer diameters on hollow fiber ultrafiltration membrane fabrication and characterization by electrochemical impedance spectroscopy[END_REF]). After the purification, the CH 4 gas in the coalbed methane separates from the gas mixture. Figure 3b demonstrates the CO 2 separation process from coalbed methane through membrane absorption.

Effect of absorbent type

The absorbent type in the membrane absorption has an important effect on the CO 2 capture from coalbed methane. The absorbent choice should consider the absorption efficiency, mass transfer rate, chemical stability, and recycling performance. In addition, physical properties such as the degradability, toxicity, corrosivity, and volatility of the absorbent and the chemical compatibility with the membrane material must be considered. The development of the CO 2 absorbent use is depicted in Table 2.

Although water was used as absorbent owing to the easy access, the method exhibits a low capture efficiency. Afterward, amine solutions were applied worldwide [START_REF] Amrei | Comprehensive modeling and CFD simulation of absorption of CO 2 and H 2 S by MEA solution in hollow fiber membrane reactors[END_REF][START_REF] Nakhjiri | The effect of membrane pores wettability on CO 2 removal from CO 2 /CH 4 gaseous mixture using NaOH, MEA and TEA liquid absorbents in hollow fiber membrane contactor[END_REF][START_REF] Pashaei | Modeling and experimental study on the solubility and mass transfer of CO 2 into aqueous DEA solution using a stirrer bubble column[END_REF][START_REF] Stowe | Molecular insights into the enhanced rate of CO 2 absorption to produce bicarbonate in aqueous 2-amino-2-methyl-1-propanol[END_REF][START_REF] Yuan | CO 2 absorption in mixed aqueous solution of MDEA and cholinium glycinate[END_REF]. [START_REF] Marzouk | Removal of carbon dioxide from pressurized CO 2 -CH 4 gas mixture using hollow fiber membrane contactors[END_REF] conducted an experimental study on CO 2 capture in a mixed gas (9.5% CO 2 /90.5% CH 4 ). They compared the effects of water, sodium hydroxide solution, and ethanolamine solution on the CO 2 flux and capture efficiency. The results show that both sodium hydroxide solution and ethanolamine solution have higher CO 2 flux and capture efficiency than water. [START_REF] Boributh | Analytical solutions for membrane wetting calculations based on log-normal and normal distribution functions for CO 2 absorption by a hollow fiber membrane contactor[END_REF] and Al-Marzouqi et al. ( 2009) selected water and ethanolamine as absorbents to remove CO 2 from mixed gases (10% CO 2 /90% CH 4 ). They concluded that ethanolamine has a better CO 2 separation ability than water. Because sodium hydroxide and ethanolamine solutions react chemically with CO 2 . Hence, their decarburization efficiencies are significantly higher.

Because amine absorbents exhibit volatilization, large losses, high corrosivity, and poor thermal degradation in decarburization processes, their application in the CO 2 capture and separation is limited. To improve the compatibility with the membrane materials, amino acid salt solutions are employed in membrane absorption systems (Li et al. 2018c). An amino acid salt solution exhibits the same absorption performance and capacity as an amine absorbent, exhibits a low volatility and good resistance to oxidative degradation and thermal degradation [START_REF] Fang | Kinetics of CO 2 absorption in aqueous potassium L-prolinate solutions at elevated total pressure energy[END_REF]. [START_REF] Rahim | Absorption of CO 2 from natural gas using different amino acid salt solutions and regeneration using hollow fiber membrane contactors[END_REF] conducted CO 2 capture experiments with a gas mixture of 10% CO 2 and 90% CH 4 . The experimental results show that the decarburization efficiency of the amino acid salt solution is better than that of the amine solution. Furthermore, Simons et al. ( 2010) compared the effects of amino acid salt solution and ethanolamine on the decarburization efficiency in a mixed gas (20% CO 2 /80% CH 4 ). The results reveal that amino acid salt solutions exhibit competitive capability in capturing CO 2 . Moreover, with the rise of nanomaterials, research on the enhancement of CO 2 absorption with nanofluids has gradually emerged.

Nanofluids exhibit high absorption efficiency, absorption capacity, thermal conductivity, and stability [START_REF] Hajilary | CFD modeling of CO 2 capture by water-based nanofluids using hollow fiber membrane contactor[END_REF]Kim et al. 2018). [START_REF] Rezakazemi | CO 2 absorption enhancement by water-based nanofluids of CNT and SiO 2 using hollow-fiber membrane contactor[END_REF] selected carbon nanotube and SiO 2 nanofluids for the separation of CO 2 . As presented in Fig. 4, the CO 2 separation efficiencies of the carbon nanotubes and SiO 2 nanofluids as absorbents are superior to that of water. [START_REF] Taheri | Simultaneous absorption of carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S) from CO 2 -H 2 S-CH 4 gas mixture using amine-based nanofluids in a wetted wall column[END_REF] added SiO 2 and Al 2 O 3 nanoparticles to a diethanolamine absorbent to form a nanofluid for the decarburization of mixed CO 2 /CH 4 gas. The experimental results show that the decarburization efficiency of the SiO 2 / diethanolamine nanofluids is 40% higher than that of a pure diethanolamine solution. Furthermore, the decarburization efficiency of Al 2 O 3 /diethanolamine nanofluids is increased by 33%. In addition, Zhang et al. (2018a) promotes the CO 2 absorption through nanofluids.

Owing to the low regeneration and absorption rate of a single absorbent, researchers have employed mixed solutions as absorbents and achieved great progresses. Zhang and Shu (2014) compared the reproducibility and CO 2 absorption properties of sodium glycinate and potassium glycinate by adding triethanolamine. The results show that the addition of low triethanolamine concentrations can increase the employed a mixed methyl diethanolamine/N-aminoethyl piperazine solution as absorbent and polypropylene hollow fiber membranes to conduct a comprehensive parametric study on the CO 2 absorption behavior. According to the results, the N-aminoethyl piperazine and methyl diethanolamine mixture has a higher CO 2 absorption capability. [START_REF] Mesbah | Theoretical study of CO 2 separation from CO 2 /CH 4 gaseous mixture using 2-methylpiperazine-promoted potassium carbonate through hollow fiber membrane contactor[END_REF] developed a mathematical 2D model and used a hollow fiber membrane contactor to separate CO 2 from a CO 2 / CH 4 gas mixture with a mixed solution of 2-methylpiperazine and potassium carbonate as absorbent. They discovered that increasing the 2-methylpiperazine concentration from 0.1 to 0.5 mol/L can significantly improve the CO 2 capture efficiency (up to 98%). Table 3 summarizes the conditions and parameters of the membrane absorption tests conducted in the literature.

According to the previously presented analysis, the mixed solution has a higher absorption efficiency and recycling efficiency when subjected to a decarburization treatment than a single absorbent. This is because the mixed solution can overcome the disadvantages of a single absorbent and concentrates the advantages of different absorbents, thereby improving the absorption efficiency and increasing the absorption capacity.

In summary, the main gas component selected by most researchers is CH 4 , which is also the main component of coalbed methane. Therefore, the previously mentioned research studies are also applicable to coalbed methane decarburization. In practical applications, different proportions of the solution should be mixed based on the actual needs to achieve a high decarburization efficiency and high reproducibility of the solution.

Effect of gas-phase parameters

The gas-phase parameters mainly include the gas composition, pressure, and flow rate of coalbed methane. These parameters have a great influence on the absorption process of a CO 2 film. Generally, the capture efficiency of CO 2 decreases with increasing gas velocity. However, the mass transfer effect works oppositely. This is because the residence time of the gas phase in the membrane decreases sharply. Hence, the boundary layer of the gas phase becomes thinner gradually, and a part of the CO 2 is separated from the membrane contactor before fully reacting with the absorbent, which weakens the capture efficiency. Simultaneously, the increase in gas velocity reduces the resistance of the mass transfer process, thus enhancing the mass transfer effect between the gas-liquid reactions. Lv et al. (2012a) studied the capture efficiency of CO 2 at different flow rates with a polypropylene hollow fiber membrane module. The results show that the capture efficiency decreases with increasing gas flow rate and increases with increasing absorbent flow rate.

Daeho (2018) used a hollow fiber membrane with a countercurrent flow structure to absorb and remove CO 2 from coalbed methane. The composition of the selected coalbed methane in the simulation process was 90% CH 4 and 10% CO 2 . According to the numerical simulation results, the purification efficiency of CH 4 is inversely proportional to the change in the gas flow rate. Furthermore, [START_REF] Zhang | Comparisons of various absorbent effects on carbon dioxide capture in membrane gas absorption (MGA) process[END_REF] simulated the effect of the absorbent flow rate on the CO 2 capture efficiency. The results show that when the gas flow rate increases from 0.28 to 1.40 m/s, the CO 2 capture efficiency decreases by approximately 50%. [START_REF] Yan | Experimental study on the separation of CO 2 from flue gas using hollow fiber membrane contactors without wetting[END_REF] obtained similar trends with a potassium glycinate solution. Moreover, [START_REF] Zhang | Decarburization characteristics of coalbed methane by membrane separation technology[END_REF] selected coalbed methane (14% CH 4 /86% CO 2 ) as feed gas and used an alcohol amine solution to decarbonize coalbed methane. The results show that the CO 2 concentration at the membrane outlet increases with increasing gas flow rate. When 0.5 M diethanolamine, ethanolamine, triethanolamine, and 1-(2-hydroxyethyl) pyrrolidine and gas flow rates of 0.3-0.8 m/s are applied, the CO 2 capture efficiency in the membrane decreases by 55.7%, 51.2%, 47.5%, and 59.9%, respectively.

Logically, an increasing system pressure increases the CO 2 concentration in the gas phase. However, with increasing gas pressure, the driving force of the mass transfer process increases. Hence, more CO 2 molecules can enter the gas-liquid interface through a membrane pore and react with the absorbent, which improves the CO 2 capture efficiency [START_REF] Cozma | Modeling and simulation of high pressure water scrubbing technology applied for biogas upgrading[END_REF][START_REF] Dindore | CO 2 absorption at elevated pressures using a hollow fiber membrane contactor[END_REF]). Mansourizadeh 2010) used a polyvinylidene fluoride hollow fiber membrane to absorb CO 2 in a mixed gas and analyzed the effect of the gas-phase pressure on the mass transfer. When the CO 2 pressure increases from 1 × 10 5 to 6 × 10 5 Pa, the CO 2 flux increases from 1.25 × 10 -3 to 6.5 × 10 -3 mol/m 2 s, and the CO 2 capture efficiency increases by 50%. [START_REF] Razavi | CFD simulation of CO 2 capture from gas mixtures in nanoporous membranes by solution of 2-amino-2-methyl-1-propanol and piperazine[END_REF], [START_REF] Mansourizadeh | Experimental study of CO 2 absorption/stripping via PVDF hollow fiber membrane contactor[END_REF], and [START_REF] Gao | Hybrid behavior and mass transfer performance for absorption of CO 2 into aqueous DEEA/ PZ solutions in a hollow fiber membrane contactor[END_REF] validated this conclusion in subsequent studies.

The CO 2 content in coalbed methane is generally 10-20%. The higher the CO 2 ratio, the thicker the boundary layer of the gas phase, and the diffusion of a large amount of CO 2 in the membrane pores is hindered, which decreases the total mass transfer coefficient. Simultaneously, parts of the CO 2 and absorbent have not completely reacted and leave the membrane contactor, which results in a decrease in the CO 2 capture efficiency. [START_REF] Zhang | Comparisons of various absorbent effects on carbon dioxide capture in membrane gas absorption (MGA) process[END_REF] studied the effect of the CO 2 content on the capture efficiency through simulations. When the CO 2 content is increased from 10 to 20%, the decarburization efficiency is reduced by approximately 50%. At constant solubility, the CO 2 amount participating in the permeation reaction remains constant, and the amount of CO 2 increases. Thus, the decarburization efficiency decreases. [START_REF] Yan | Experimental study on the separation of CO 2 from flue gas using hollow fiber membrane contactors without wetting[END_REF] and [START_REF] Chen | Study on hollow fibre membrane contactor for the separation of carbon dioxide from carbon dioxide-nitrogen mixture[END_REF] obtained the same behavior for the efficiency through experimental studies.

The CH 4 content in coalbed methane is 86-97% (Flores and Flores 2014; [START_REF] Mallick | Energy analysis on coalbed methane (CBM) coupled power systems[END_REF][START_REF] Zhang | Decarburization characteristics of coalbed methane by membrane separation technology[END_REF]. Because of the low CO 2 content in the initial stage of coalbed methane mining, coalbed methane can be directly burned. However, as the CO 2 content in coalbed methane continues to increase, hydrated solids block the pipeline when low temperatures and high pressures are encountered during the pipeline transportation. To improve the utilization efficiency of coalbed methane and reduce safety hazards in the transportation process, a decarburization treatment is required. As mentioned above, the coalbed methane decarburization efficiency can be improved by increasing the gas-phase pressure, reducing the gas flow rate and the percentage of CO 2 during the coalbed methane decarburization treatment.

Effect of liquid-phase parameters

Liquid-phase parameters play a key role in the improvement in the CO 2 capture efficiency. The effects of the CO 2 capture efficiency on the liquid concentration, flow rate, and temperature are summarized in this section. According to the two-phase theory, the CO 2 absorption mainly occurs in the liquid-phase boundary layer [START_REF] Lewis | CO 2 capture in amine-based aqueous solution: role of the gas-solution interface[END_REF]. Starting from the shell side, CO 2 enters the liquid-phase boundary layer through the membrane pores and is quickly absorbed by the absorption liquid. As the concentration of the absorbing liquid increases, the concentration of the active component that absorbs CO 2 in the boundary layer of the liquid phase increases. This can increase the chemical reaction rate of CO 2 and the solute dissolved in the liquid phase (Yan et al. 2014a). Hence, the CO 2 concentration in the liquid film is rapidly decreased, and the liquid film resistance is lowered, which improves the CO 2 removal efficiency. [START_REF] Zhang | Comparisons of various absorbent effects on carbon dioxide capture in membrane gas absorption (MGA) process[END_REF] studied the effect of the changing absorbent concentration on the CO 2 capture efficiency through experiments. The results show that when the concentration of the absorbent increases from 0.1 to 1 mol/L, the CO 2 capture efficiency is increased by 61.8%. [START_REF] Eslami | Modeling and simulation of CO 2 removal from power plant flue gas by PG solution in a hollow fiber membrane contactor[END_REF] used numerical models to study the effect of different concentrations of a potassium glycinate solution on the CO 2 capture efficiency in polytetrafluoroethylene hollow fiber membrane contactors. According to their results, when the absorbent concentration is increased from 0.5 to 3 M, the CO 2 capture efficiency is increased by approximately 14%.

With increasing absorbent flow rate, the disturbance of the liquid in the tube is strengthened, the thickness of the boundary layer is reduced, the mass transfer resistance of the liquid phase is reduced, and more fresh absorbents react with the CO 2 molecules. Thus, the decarburization efficiency is improved [START_REF] Zhang | Decarburization characteristics of coalbed methane by membrane separation technology[END_REF]. [START_REF] Zhang | Comparisons of various absorbent effects on carbon dioxide capture in membrane gas absorption (MGA) process[END_REF] simulated the effect of the absorbent flow rate on the CO 2 capture efficiency. The results show that when the absorbent flow rate increases from 0.015 to 0.085 m/s, the CO 2 capture efficiency increases by approximately 30%. Lv et al. (2012a) used 0.5 mol/L ethanolamine to capture CO 2 with polypropylene hollow fiber membranes. The effect of the liquidphase flow on the absorption performance for 8-46 mL/min was studied at a constant feed gas flow. The results show that when the liquid-phase flow rate is increased, the CO 2 capture efficiency increases by 49%. This conclusion has been verified in subsequent studies [START_REF] Ansaripour | Experimental and numerical investigation of CO 2 absorption using nanofluids in a hollow-fiber membrane contactor[END_REF].

In addition to increasing the flow rate of the absorbent, increasing the absorbent temperature helps to increase the CO 2 absorption efficiency [START_REF] Eslami | Modeling and simulation of CO 2 removal from power plant flue gas by PG solution in a hollow fiber membrane contactor[END_REF][START_REF] Yan | Experimental study on the separation of CO 2 from flue gas using hollow fiber membrane contactors without wetting[END_REF]. [START_REF] Shirazian | Numerical simulation of mass transfer in gas-liquid hollow fiber membrane contactors for laminar flow conditions[END_REF] compared the decarburization rate of an absorbent at 298 K and 288 K. The decarburization rate increases by approximately 10% with the increase in temperature. Moreover, Mansourizadeh et al. (2010) studied the effect of absorbent temperature on CO 2 capture efficiency. When the absorbent temperature was increased from 283 to 313 K, the CO 2 mass transfer rate was increased by approximately 58.6%. [START_REF] Wu | Experimental studies on CO 2 absorption in immersed hollow fiber membrane contactor[END_REF] studied the effect of changes in the absorbent temperature on the CO 2 capture efficiency and mass transfer rate. When the absorption temperature is increased from 298 to 325 K, the CO 2 capture rate and mass transfer rate are increased by 30% and 40%, respectively.

According to the previous analysis, increasing the concentration, flow rate, and absorbent temperature can improve the CO 2 capture efficiency. However, owing to the required temperature control for the membrane material in the membrane contactor while the absorbent temperature is being changed, the actual material characteristics should be controlled to maximize the capture efficiency.

Effect of membrane contactor structure

At present, there is no study focusing on the effect of the membrane contactor structure on the CO 2 capture from coalbed methane. The main components of coalbed methane and natural gas are CH 4 and CO 2 , whereas the main components of flue gas are N 2 and CO 2 . Although their compositions are slightly different, the common essence is the removal of CO 2 from the mixture. The research on the impact of membrane contactor structures on CO 2 removal from flue gas is relatively mature which provides guidelines in the CO 2 separation from coalbed methane. Figure 5 presents the structure of a hollow fiber membrane contactor [START_REF] Zhang | CO 2 absorption in a hollow fiber membrane contactor and its sorption characteristics in a PVA facilitated transport membrane[END_REF].

Changes in the wall thickness of the fiber membrane affect the direction of the gas movement. Furthermore, increasing the wall thickness prolongs the residence time of the gas in the membrane, thereby increasing the resistance along the path, which makes the diffusion movement more complicated and leads to a decrease in the decarburization efficiency [START_REF] Rezakazemi | CFD simulation of natural gas sweetening in a gas-liquid hollow-fiber membrane contactor[END_REF]. Yan et al. (2015b) studied the effect of the fiber wall thickness on the CO 2 capture efficiency. The results show that the CO 2 capture efficiency decreases with increasing wall thickness. Furthermore, the decarburization efficiency is reduced by approximately 40% owing to the weak reaction between diethanolamine and CO 2 . [START_REF] Rezakazemi | CFD simulation of natural gas sweetening in a gas-liquid hollow-fiber membrane contactor[END_REF] reported the same conclusions.

Increasing the effective length of the fiber membrane increases the gas-liquid contact area, which results in a prolonged residence time of the gas in the membrane. Consequently, the CO 2 capture process in the gas is enhanced. [START_REF] Boributh | Effect of membrane module arrangement of gas-liquid membrane contacting process on CO 2 absorption performance: a modeling study[END_REF] selected three different membrane lengths (0.25 m, 0.5 m, and 0.7 m) to compare their CO 2 capture efficiencies. The results show that the decarburization efficiencies for a membrane length of 0.7 m increase by 15.27% (0.25 m) and 3.58% (0.5 m), respectively.

Reducing the inner diameter of the fiber membrane increases the flow rate of the absorbent, which results in an increase in the CO 2 absorption per unit time and in an improvement in the CO 2 capture efficiency. [START_REF] Rajabzadeh | CO 2 absorption by using PVDF hollow fiber membrane contactors with various membrane structures[END_REF] studied the effect of a changing inner membrane diameter on the CO 2 capture efficiency. According to the results, the capture efficiency increases with decreasing inner membrane diameter because the gas-liquid two-phase contact area increases with increasing number of hollow fiber membranes. This results in an increase in the packing density and shell-side mass transfer coefficient. [START_REF] Eslami | Modeling and simulation of CO 2 removal from power plant flue gas by PG solution in a hollow fiber membrane contactor[END_REF] investigated the effects on the CO 2 capture efficiency for a liquid flow rate of 0.1 m/s, an absorbent concentration of 0.5 mol/L, and increasing number of hollow fiber membranes from 3000 to 10,000. As a result, the capture efficiency increases from 36 to 100%. Furthermore, Faiz and Almarzouqi ( 2010) used a membrane contactor to remove CO 2 from natural gas under high pressure. The results show that the capture efficiency increases with increasing porosity and decreases with increasing tortuosity factor.

According to the previous analysis, to ensure the overall performance of the membrane module, reducing the wall thickness of the membrane, the inner diameter, and tortuosity factor and increasing the number of membranes, porosity, and effective membrane length are beneficial to the CO 2 capture. The CO 2 capture from coalbed methane through membrane contactor structures will be the future research direction.

Effects of other conditions

In addition to the previously mentioned factors, the connection mode of the membrane module, the direction of the gas-liquid flow, and the fluidity have a certain influence on the CO 2 capture efficiency. [START_REF] Zhang | Decarburization characteristics of coalbed methane by membrane separation technology[END_REF] studied the effect of a single component and series of components on the CO 2 capture efficiency in coalbed methane. According to the results, the decarburization rate of the serial module is evidently better than that of the single module. This is mainly due to the increase in the number of membrane modules, which leads to an increase in the effective membrane length and an increasing gas-liquid contact area and gas-liquid residence time in the membrane. This indirectly strengthens the mass transfer process between the two phases and improves the decarburization efficiency.

Boributh et al. ( 2011) compared the effects of single, serial, and parallel connection methods on the CO 2 capture efficiency. The results show that the best absorption performance can be obtained with a serial connection. Moreover, Zhang et al. (2018b) studied biogas (40% CO 2 /60% CH 4 ) CO 2 separation and CH 4 purification in a membrane contactor with an amine solution. By comparing the effects of single membrane module and membrane modules in series on the decarburization efficiency under equal conditions, they proved that the CO 2 separation efficiency and CH 4 purification efficiency increase by 24.4% and 9.8% with serial membrane modules, respectively. Simultaneously, they concluded that the efficiency of the countercurrent state increases by 38.9% and 25.6% compared with that of the downstream state, respectively. The previously mentioned studies demonstrate that the countercurrent flow mode can improve the CO 2 capture efficiency owing to the influence of the driving force. Therefore, the countercurrent flow mode is currently mainly employed in membrane absorption systems.

The fluidity is mainly described by a laminar flow or turbulent flow. A gas-liquid contact is more sufficient owing to severe disturbances of the turbulences, which enhance the mass transfer between the gas and liquid phases, thus promoting the separation of CO 2 . This has been confirmed in Zhang et al. (2014a, c). In summary, the coalbed methane decarburization efficiency can be improved through a membrane module and countercurrent flow. The effects of different factors on CO 2 capture efficiency are summarized in Table 4. [START_REF] Qi | Microporous hollow fibers for gas absorption II: mass transfer across the membrane[END_REF] first proposed a method for absorbing CO 2 with hollow fiber polypropylene (PP) membrane contactors. The method is widely applied because of its large gas-liquid contact area, fast mass transfer rate, no mist entrainment phenomenon, large operation flexibility, and small equipment. With the development of the membrane absorption technology in recent years, many research studies on the membrane absorption technology based on hollow fiber membranes have been conducted in China. Figure 6 presents the various application of CO 2 separation from different gases.

Membrane absorption applied to other gases

As the main source of CO 2 , flue gas accounts for approximately 80% of the greenhouse gases [START_REF] Sutanto | CO 2 removal from biogas with supported amine sorbents: first technical evaluation based on experimental data[END_REF]. The main components of the flue gas are CO 2 and N 2 , and the CO 2 ratio is generally between 10 and 20 vol% [START_REF] Masoumi | Removal of carbon dioxide by aqueous amino acid salts using hollow fiber membrane contactors[END_REF][START_REF] Zhang | Comparisons of various absorbent effects on carbon dioxide capture in membrane gas absorption (MGA) process[END_REF]. Therefore, capturing CO 2 from flue gas is an effective method to control CO 2 emissions. At present, the membrane absorption method is relatively mature Fig. 6 Application of membrane separator for CO 2 separation in different gases. The membrane separation method can be applied to a variety of gas decarburization treatments. The process of gas-liquid two-phase absorption in the contactor is also shown in the flue gas decarburization research field. Many studies have been carried out using the membrane absorption method to remove CO 2 from flue gas [START_REF] Chen | Experimental study on the separation of CO 2 from flue gas using hollow fiber membrane contactors with mixed absorbents[END_REF][START_REF] Lee | Development of novel sub-ambient membrane systems for energy-efficient post-combustion CO 2 capture[END_REF]. Because the emitted flue gas has a relatively high temperature, a high-temperature-resistant membrane material is required for the membrane absorption. Therefore, to remove CO 2 from flue gas through the membrane absorption method, future research will focus on the development of new high-temperature-resistant membrane materials.

China is a coal energy-consuming country, and energy shortages and environmental pollution will seriously hinder the sustainable development of the society [START_REF] Medeiros | Simulation of an off-shore natural gas purification process for CO 2 removal with gas-liquid contactors employing aqueous solutions of ethanolamines[END_REF]. At present, natural gas is the fastest growing energy source in the world. However, natural gas contains acid gases such as CO 2 , which not only cause serious air pollution but also accelerate the corrosion of pipelines and equipment. Furthermore, CO 2 reduces the calorific value of natural gas and increases transportation costs [START_REF] Knoope | A state-of-the-art review of techno-economic models predicting the costs of CO 2 pipeline transport[END_REF]. Therefore, natural gas must be purified to ensure the safety of natural gas storage, transportation, and use. The membrane absorption method is a process based on the combination of a conventional absorption method and membrane separation. The method does not require high precision and complicated operations of packed columns and an absorption tower when the CO 2 is removed and enables highly efficient decarburization. Therefore, it is considered as a potential candidate that can be developed for natural gas purification (Fu et al. 2012). Many studies have been carried out on this technology [START_REF] Faiz | Insights on natural gas purification: simultaneous absorption of CO 2 and H 2 S using membrane contactors[END_REF][START_REF] Kang | Removal of high concentration CO 2 from natural gas using high pressure membrane contactors[END_REF][START_REF] Rahim | Absorption of CO 2 from natural gas using different amino acid salt solutions and regeneration using hollow fiber membrane contactors[END_REF][START_REF] Tang | Kinetics research on mixed solvents of MDEA and enamine in natural gas decarbonization process[END_REF]. However, owing to the high pressures required for the decarburization process of natural gas, the membrane absorption method suffers from many technical problems in the decarburization treatment. The membrane material has high requirements under high pressures, which might decrease the anti-wetting property of the membrane material. Thus, research on membrane materials with high-pressure resistances and anti-wetting properties would promote natural gas decarburization.

In response to the growing energy crisis, biogas is currently considered as a renewable energy source that can replace traditional fossil fuels [START_REF] Amrei | Comprehensive modeling and CFD simulation of absorption of CO 2 and H 2 S by MEA solution in hollow fiber membrane reactors[END_REF]Li et al. 2018c). The main components of biogas are CO 2 and CH 4 , and the CO 2 content is generally between 25 vol% and 55 vol% [START_REF] Scholz | Transforming biogas into biomethane using membrane technology[END_REF]. However, initial biogas contains much CO 2 . Thus, the direct use will reduce the calorific value of biogas. It is essential to decarbonize biogas to improve its efficiency and make it a substitute for natural gas. Because membrane absorption exhibits a high decarburization efficiency, many researchers have used this technology to study biogas decarburization [START_REF] Sutanto | CO 2 removal from biogas with supported amine sorbents: first technical evaluation based on experimental data[END_REF]Yan et al. 2014aYan et al. , b, 2015a;;Zhang et al. 2014c). However, the biogas phase has a higher CO 2 content than coalbed methane and natural gas. Hence, the continuous The main emissions of ships include CO 2 , NOx, SOx, CO, hydrocarbons, and particulate matter (Eyring 2005a). Emissions from ship exhaust increase the SO 2 concentration in the atmosphere, which directly harms humans, marine life, and the stability of the entire ecosystem. Because the international community's requirements for environmental protection have increased, ship exhaust emission standards have become more stringent (Eyring 2005b). Owing to the limitations of narrow space in ship exhaust treatment, it is difficult to achieve flexible operation by traditional capture methods. By contrast, the membrane absorption methods are more suitable owing to their unique advantages (large gas-liquid contact area, flexible installation and operation, and simple equipment). At present, membrane absorption is applied to the SO 2 capture from ship exhaust. [START_REF] Qiu | Optimization of microstructure and geometry of hydrophobic ceramic membrane for SO 2 absorption from ship exhaust[END_REF] studied the optimization of the microstructures and geometries of hydrophobic ceramic membranes for SO 2 absorption from ship exhaust gases. According to the results, the optimized efficiency can be increased by 16%, and the optimal film thickness and inner film diameter are 1.0 mm and 4.0-6.0 mm, respectively. So far, no scientific article has mentioned the use of membrane absorption to remove CO 2 from ship exhaust gas. This will be the future development direction of the membrane absorption technology.

Conclusion

The membrane absorption method for coalbed methane decarburization is a key technology related to the application prospects of coalbed methane. A variety of separation techniques have been applied for coalbed methane decarburization. The membrane absorption method has the advantages of large gas-liquid contact areas, fast mass transfer rates, no mist entrainment phenomenon, a large operation flexibility, and small equipment. It has been widely applied for CO 2 capture in flue gas, biogas, and natural gas. Although the gas composition and content of coalbed methane are different from those mentioned previously, the decarburization processes are similar. Therefore, we have summarized research studies with different CO 2 and CH 4 mixing ratios. The following conclusions can be drawn:

1. Increasing the temperature, concentration, and flow rate of the absorbent during the membrane absorption process and reducing the gas flow rate have a positive effect on the coalbed methane decarburization efficiency. 2. The newly applied solvents (e.g., amino acid salts and nanofluids) exhibit high decarburization efficiencies as absorbents. Furthermore, single absorbents can be mixed to achieve a better decarburization effect. 3. By changing the structure of the membrane contactor (e.g., increasing the effective membrane length and reducing the membrane wall thickness and membrane inner diameter), the CO 2 can entirely react with the absorbent, which improves the CO 2 capture efficiency. 4. The CO 2 capture can be improved by changing the gasliquid flow state and flow direction, such as the flow state of the countercurrent and turbulent flow in the decarburization process.

At present, the membrane absorption method has good application prospects for purification treatments of coalbed methane, flue gas, natural gas, biogas, and ship exhaust. However, unresolved technical problems make large-scale industrial applications impossible and require further research. Based on the current and possible future applications of the membrane absorption technology to achieve large-scale applications, the following recommendations are given:

1. At present, the membrane absorption method mainly uses the simulated gas component for CO 2 separation. However, because acid gases such as H 2 S and SO 2 can still interfere with the experimental results in the actual Polypropylene A mass transfer model for the CO gas composition, the influence of these interference factors on the degassing effect of the actual gas must be considered for more realistic simulation results. 2. The research on high-efficiency absorbents should focus on the absorption characteristics of novel amino acid salt solutions and nanofluids in membrane contactors owing to their higher chemical reaction rates and absorption efficiencies and because they are environmentally friendly, thermally stable, and membrane coated. 3. When developing a membrane absorption model, the model should be continuously optimized based on the differences in the absorbents, membrane materials, and other influencing factors. Furthermore, the kinetics and thermodynamics of the reaction of new absorbents with CO 2 should be strengthened. Further comprehensive analyses should be conducted to optimize the conditions and maximize the CO 2 absorption efficiency. 4. Expanding the application field of the CO 2 absorption through the membrane absorption method: Ship exhaust contains much CO 2 . However, at present, researchers do not focus on the capture of CO 2 from ship exhaust through membrane absorption.

Fig. 1 a

 1 Fig. 1 a Gas production of the world's major coalbed methane-producing countries. The coalbed methane production in the USA far exceeds that of the other four countries, and the coalbed methane production in China exhibits an upward trend (bcm: billion cubic meters). Reproduced from Ju et al. (2019), with permission from Elsevier; b China's coalbed methane extraction and utilization from 2011 to 2017. With the continuous development and utilization of energy, the utilization of coalbed methane has shown an increasing trend

Fig. 3 a

 3 Fig.3a CO 2 transport in a membrane contactor. The main driving force of the technology is the gas concentration difference. First, CO 2 from the gas mixture reaches the surface of the membrane pore and diffuses through the membrane pores to the gas-liquid two-phase interface. Next, CO 2 reacts with the absorbent in the tube side; b process flow diagram of gas separation. After the mixed gas is separated by the membrane contactor, the absorbent is subjected to gas-liquid separation treatment to recycle the absorbent. At the same time, the decarburized gas is collected and stored

Fig. 4

 4 Fig.4Effects of gas flow on CO 2 capture efficiency of different nanofluids. The CO 2 separation efficiencies of the carbon nanotubes (CNT) and SiO 2 nanofluids as absorbents are superior to that of water. Reproduced from[START_REF] Rezakazemi | CO 2 absorption enhancement by water-based nanofluids of CNT and SiO 2 using hollow-fiber membrane contactor[END_REF], with permission from Elsevier

Fig. 5

 5 Fig.5Schematic of hollow fiber membrane contactor. The components of the membrane contactor and the direction of gas-liquid flow can be clearly seen from the structural drawing. Reproduced from[START_REF] Zhang | CO 2 absorption in a hollow fiber membrane contactor and its sorption characteristics in a PVA facilitated transport membrane[END_REF]

Table 2

 2 Development of absorbent usage for membrane absorption

	Absorbent	Benefits	Drawbacks	References
	Water	Rich in resources and pollution-free	Low absorption efficiency	Marzouk et al. (2010)
	Alkali	High absorption efficiency, large capacity	Strongly corrosive and non-renewable	Lin and Chu (2015)
	Inorganic salt	Large absorption capacity and renewable	High-resolution energy consumption	Li et al. (2018c)
	Amine	Fast chemical reaction rate, mature technology	Corrosive, high-resolution energy consumption Amrei et al. (2014)
	Amino acid salt Good compatibility with membrane, non-toxic, strong	Poor regeneration	Fang et al. (2015)
		thermal stability
	Nanofluids	Large specific surface area and surface activity, high	High equipment requirements and low output	Kim et al. (2018)
		absorption efficiency and stability

Table 3

 3 Membrane absorption test conditions and parameters

	References	Faiz and Al-Marzouqi (2011)			Faiz and Al-Marzouqi (2009)				Marzouk et al. (2010)			Al-Marzouqi et al. (2009)				Al-Marzouqi et al. (2016)			Al-Marzouqi et al. (2008)			Kang et al. (2017)		Atchariyawut et al. (2007)							Khaisri et al. (2010)
	Membrane material Comment	Polypropylene A scheme of two membrane modules	in series was recommended for the	simultaneous absorption of CO 2 and H 2 S	Polyvinylidene fluoride, polypropylene The wetting effects on the decarburiza-	tion efficiency of polypropylene and	polyvinylidene fluoride membranes	were investigated	Polytetrafluoroethylene Novel design of hollow fiber mem-	brane modules suitable for high-	pressure operations	Polypropylene, silicon rubber Three different membrane modules	were used in this study; the poly-	propylene module exhibited the best	performance in terms of CO 2 flux	Tetrafluoroethylene and ethylene First report on the long-term opera-	copolymers tional stability of HFM modules	under industrial conditions	Polypropylene A comprehensive mathematical 2D	model was developed and combined	with the chemical absorption for CO 2 absorption	Polyvinylidene fluoride Capture of high CO 2 concentrations from natural gas with high-pressure	membrane contactors	Polyvinylidene fluoride The experiments were performed in	a membrane contactor constructed	with (0.2 μm pore size) micropo-	rous polyvinylidene fluoride hollow	fibers. Sodium chloride was used as	an additive in the sodium hydroxide	aqueous solution	Polytetrafluoroethylene The membrane mass transfer resist-	ance calculated with the Wilson plot	method predicted the CO 2 concentra-tion profile with a higher accuracy	than the theoretical method
	Gas component	5% CO 2 , 5% H 2 S, 90% CH 4		10% CO 2 , 10% H 2 S, 80% CH 4			9.5% CO 2 , 90.5% CH 4		10% CO 2 , 90% CH 4			5% CO 2 , 95% CH 4		10% CO 2 , 90% CH 4		45-70% CO 2 , 55-30% CH 4		20% CO 2 , 80% CH 2 , 50% 4 , 50% CO CH 4					15% CO 2 , 85% CH 4		40% CO 2 , 60% CH 4
	Absorbent	Potassium carbonate			Ethanolamine				Water, diethanolamine, ethanolamine,	sodium hydroxide, triethylenetet-	ramine	Diethanolamine, diethylenetriamine,	ethylenediamine, ethanolamine,	tetraethylenepentamine, triethylene-	tetramine, water, sodium hydroxide	Sodium hydroxide, diethanolamine,	potassium carbonate		Ethanolamine, sodium hydroxide			Methyl diethanolamine		Water, sodium hydroxide, ethanola-	mine, sodium chloride						Ethanolamine			Water, triethanolamine, diethan-	olamine, ethanolamine, potassium	argininate

Table 4

 4 Summary of the influence of different factors on CO 2 capture efficiency Influencing factor Effect on CO 2 capture efficiency References Gas phase CO 2 content A higher CO 2 concentration results in a decrease in the CO 2 capture efficiency Yan et al. (2007), Zhang (2016) Pressure Increasing gas pressure improves the CO 2 capture efficiency Cozma et al. (2015), Dindore et al. (2004), Gao et al. Inner diameter Reducing the inner diameter of the fiber membrane results in an improvement in the CO 2 capture efficiency Eslami et al. (2011, Rajabzadeh et al. (2009) Fluidity Turbulent flow can promote the CO 2 capture efficiency Zhang et al. (2014a) Module number A serial connection improves the CO 2 capture efficiency Boributh et al. (2011), Zhang et al. (2019)

	(2018)

absorption from biogas to potassium argininate solutions was developedZhang et al. (2014c)

Acknowledgements This work is supported by the Basic Science Center Program for Ordered Energy Conversion of the National Natural Science Foundation of China (No. 51888103), Liaoning Province Natural Science Fund Project Funding (No. 201602470), and Program for Liaoning Excellent Talents in University (No. LJQ2014038).


domain: sdv sde

text:
Kinetics of growth responses of a root encountering an obstacle

Kinetics of growth responses of a root encountering an obstacle

Future

  Marie-Béatrice BOGEAT-TRIBOULOT, UMR Silva, INRA -Nancy Evelyne KOLB, UMR PMMH Root responses to axial mechanical resistance 2 Soil is heterogeneous  interfaces, obstacles Macroscopic responses  growth reduction or stop  or root bending and growth axis reorientation Cellular responses:   osmotic pressure   Turgor pressure  counteracts soil pressure   mucilage exudation  reduces soil-root frictions  favours penetration   growth in diameter  resistance to bucking  favours penetration  Cell wall properties , … Sensing, signaling and molecular responses • time-lapse photography : dt=6' • high resolution : 2µm / pixel particule image velocimetry & kinematics GROWTH COMPONENTS -Root growth rate = maximal Velocity (V max ) -Maximal Elemental Elongation Rate (EER max ) -Growth Zone length (GZ) keeps the root in the focal plan but root is free (not braced) • Obstacle = 2mm diameter circle or 3 mm long flat 6 If the root is free, a short apical touch does not affect growth rate If the contact root-obstacle is a bit longer/stronger, light and delayed reduction of root growth rate ns Brace root more tightly  Consider incidence angle  Consider root diameter  Use sensors with different stiffness : disentangle time and force  Other growth parameters : EERmax, turgor  Simon Berrar (Master 2)  Antoine Cambien (Master 1)  Thibaut Gaillot (CPP) Thank you for your attention

Questions

 How sensitive is root growth to an axial contact / pressure ?  What is the minimum force that reduces root growth rate?  How fast is axial growth rate reduced?

Adventitious root of poplar cutting as a model