Post-combustion carbon capture by membrane separation, Review

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Post-combustion carbon capture by membrane separation, Review Magda Kárászováa, Boleslav Zacha, Zuzana Petrusováa, Vojtěch Červenkaa, Marek Bobákb, ⁎ Michal Šyca, Pavel Izáka,b, a b

Institute of Chemical Process Fundamentals of the CAS, v.v.i., Rozvojová 135, 165 00 Prague, Czech Republic MemBrain s.r.o., Pod Vinicí 87, 471 27 Stráž pod Ralskem, Czech Republic

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon dioxide Membrane separation Post-combustion capture (PCC) Carbon capture and storage (CCS) Flue gas

The human-induced increase in average global temperature since pre-industrial times (1850–1900 average) has already reached 1.0 °C in 2017 so there is a strong call for effective carbon capture and storage/utilization technology especially in the energy sector where the CO2 emissions are the largest. Membrane technologies are often declared to be a good option. This work tries to find how the commercially available modules are doing from the economical and practical point of view and if there are some new membrane materials which could be feasible and practically applicable in the power plat post-combustion CO2 separation. The main conclusion is that membrane technology is potentially suitable for fuel gas purification in the future but there are still some issues to be solved such as for example membrane resistance for humid feed stream, fouling and long-term stability of thin selective layer.

1. Introduction Combustion is a significant industrial source of air pollution worldwide. Therefore, a variety of flue gas treatment systems are used and they are an essential part of many technologies that include combustion [1–7]. Often, a variety of pollutants have to be removed from flue gas, which affects the extent and complexity of the flue gas treatment system. Some aspects of flue gas treatment have a long history, e.g. the removal of particulate matter. Other problematics are relatively new, such as the reduction of NOx, or removal of polychlorinated dibenzodioxins and dibenzofurans (PCDD/Fs) and/or mercury. Because of the global temperature rising of which anthropogenic CO2 is considered to be the major cause, there is a demand for CO2 capture technologies. Now, one of the key strategies to achieve lower greenhouse gas emissions is carbon pricing, either in the form of carbon allowances or a carbon tax. The carbon tax can currently (2019) range

from a few (e.g. Japan) to over one hundred (e.g. Sweden) €/ton of CO2 equivalent [8]. The carbon allowance price has reached almost 30 €/ton of CO2 equivalent (July 2019) within the European Union Emission Trading Scheme (EU ETS). The EU ETS is one of the first and so far the biggest carbon markets worldwide and regulates the greenhouse gas emissions from large scale facilities in the energy (e.g. power plants) and industry sectors [9]. This concerns 31 countries (EU + Iceland, Norway, and Lichtenstein), moreover, additional countries, such as China, South Korea, Canada, Japan, New Zealand, Switzerland, and the United States, have implemented separate national or regional systems within the EU ETS [10]. According to the policy of greenhouse gas control of the EU, the emissions of greenhouse gases should be 21% lower in 2020 and 41% lower in 2030 compared to 2005. The price of a CO2 emission allowance rocketed to the current price from 4.38 €/ton of CO2 equivalent in May 2017 [11,12] and may reach 40 €/ton [13] by 2023. Writing such an article in 2017, the main question would have

Abbreviations: [APTMS][Ac], 3-(trimethoxysilyl)propan-1-aminium acetate; Barrer, non-SI unit of gas permability; CA, cellulose acetate; CNG, compressed natural gas; EU ETS, European union emission trading scheme; [emim][BF4], 1-ethyl-3-methylimidazolium tetrafluoroborate; [emim][Tf2N], 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide; gen1, gen2, fist generation, second generation; GO, graphene oxide; GPU, gas permeation unit; hum, humid feed flow; IL, ionic liquid; MEA, monoethanolamine; MMM, mixed matrix membrane; MOF, metallic-organic framework; NOx, nitrogen oxides; LM, liquid membrane; LNG, liquefied natural gas; PA, polyamide; PAN, polyacrylonitrile; PCDD/Fs, dibenzodioxins/dibenzofurans; PDMS, polydimethyl siloxane; PE, polyethylene; PEBA, polyether block amide; PEG, polyethylene glycol; PEI, polyether imide; PEO-PBT, poly(ethylene oxide)–poly(butylene terephthalate); PES, polyether sulofone; PI, polyimide; PIM, polymer with intrinsic microporosity; PIM-1, polymer with intrinsic microporosity of the fist generation; PolyIL, polyionic liquid; ppm, parts per million; PSf, polysulfon; PTMEG, polytetramethylene Ether Glycol; PU, polyurethane; PVA, polyvinyl alcohol; PVC-g-POEM, poly(vinyl chloride)-g-poly(oxyethylene methacrylate); SILM, supported ionic liquid membrane; SOx, sulphur oxides; STP, standard conditions (i.e. 273.15 K and 101.325 kPa); TRP, thermally rearranged polymers; UF, ultrafiltration; ZIF, zeolitic imidazolate framework ⁎ Corresponding author at: Institute of Chemical Process Fundamentals of the CAS, v.v.i., Rozvojová 135, 165 00 Prague, Czech Republic. E-mail address: [email protected] (P. Izák). https://doi.org/10.1016/j.seppur.2019.116448 Received 9 August 2019; Received in revised form 16 December 2019; Accepted 16 December 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Magda Kárászová, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116448

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been if any method of CO2 capture from power plant flue gas was feasible compared to purchase of the emission allowance but, due to the gradual change in politics and prices of CO2 emission allowances, the situation is changing and, nowadays, the question is rather which technology is the most feasible. There are several possibilities of reducing the emissions of CO2 from combustion processes such as oxy-fuel, pre-combustion capture, chemical looping combustion, and post-combustion capture [1], which can be used nearly at any combustion plant. The most common form of post-combustion CO2 capture is monoethanolamine (MEA) absorption but in principle, other absorbents can be used [14]. Other options of post-combustion CO2 capture are based on adsorption and are represented by pressure swing adsorption, temperature swing adsorption, and calcium looping [1]. It is worth mentioning that, in terms of postcombustion CO2 capture, MEA absorption is at the time the only commercially used technology [15,16]. However, both the capital and operating costs of MEA absorption are high and even coal-fired power plants that use the separated CO2 for enhanced oil recovery struggle with economic sustainability of the process, some projects have even been abandoned [17]. Membrane separation might be a promising way with operating parameters beyond current technologies as they often have low energy consumption, low operational costs, small footprint, and easy scale-up and incorporation into existing technologies [18]. Membrane technology has successfully been applied in various applications such as reverse osmosis instead of distillation [19], natural gas purification instead of amine absorption systems [20], biogas purification up to CNG quality [21], removal of heavy metals from water [22], membrane crystallization applied in CO2 capture [23,24], etc. Concerning CO2 capture by membrane processes, progress can be observed in the last 10 years [25–28]. This review focuses on CO2 separation from flue gas by the commercial membrane modules and results from pilot plant experiments, progress in the development of new membrane materials and if they are able to face the conditions of the flue gas and the discussion if the membranes are really able to align to the MEA scrubbing technology.

membrane and swelling of the membrane by water. The permeance and selectivity recovered slightly after some time of operation but did not reach the initial values (Table 1). NOx and SO2 passed through the membrane with CO2. Scholes et al. concluded that the biggest challenge was humidity fluctuation in combination with difficult temperature regulation of the experimental setup. The humidity typically causes significant changes in the transport properties of the membrane [34]. In 2015, White et al. [35] implemented a pilot-scale system with membrane Polaris™, which was sized to treat a stream of flue gas containing one ton of CO2 per day. This amount of generated CO2 was approximately equivalent to a 0.05 MWe of a coal-fired power plant. The process design had proposed a two-step separation process. The first step used a combination of a small amount of feed compression and permeate vacuum to generate a pressure ratio and capture approx. 50% of the inlet CO2. The subsequent step utilized combustion air as a sweep gas to drive the CO2 recovery up to 90%. MTR membranes Polaris™ with pure-gas CO2 reached permeance values of 1000–2000 GPU (345 kPa, 23 °C) and CO2/N2 selectivity of 50–60. An experiment with flue gas generated by the combustion of pulverized coal was performed for 1800 h. The content of CO2 after the first step of membrane treatment was 63% in the permeate and under 5% in the retentate. During an uninterrupted operation that lasted nearly 400 h, the total carbon capture rate ranged between 83 and 91% of input CO2. The work was extended in 2017 and it was shown that the membrane modules were stable. The total operation time with flue gas was 9133 h [33]. PolyActive™ membranes are nowadays widely tested. The temperature dependence of transport properties of different multilayer PolyActive™ membranes was presented in Brinkmann et al. [29]. Temperature influenced both permeance and selectivity. The permeance increased with temperature while the selectivity decreased. The optimum may be found at approx. 40 °C, where the permeance reaches 2000 GPU and CO2/N2 selectivity is approximately 45. The structure and thickness of PolyActive™ membranes may be found in Schuldt et al. [36]. It was found, that PolyActive™ swells with CO2 at higher CO2 fugacity (over 8 bar) [36], which leads to lower selectivity. Brinkmann et al. [37] studied the pressure distribution in envelope membrane modules with PolyActive™ separation layers. They compared the experimental data from their previous work with a simulation based on a free-volume model and module-flow patterns. A simulation of a virtual power plant with different types of membrane modules incorporated was performed, assuming different operating conditions. The energy consumption of the whole process was evaluated. The membrane separation process could compete with MEA absorption only when 70% recovery and 95% purity of CO2 were required and when the envelope model with counter-current flow was used. The results of the simulation published by Brinkmann et al. [37] were used by Pohlmann et al. [38], who decided to verify the results in the building of a pilot-scale unit. The unit consisted of envelope modules with PolyActive™ separation layers. The effective membrane area was 12.5 m2. Flue gas from a power plant containing 14.5% of CO2, 6.5% of O2, 50–100 ppm of SO2, 76–91 ppm of NOx, and 14% of H2O was used. The feed and permeate pressures were 1.265 and 0.050 bar (abs.), respectively. The main benefit of this work is that the authors tried to identify the most sensitive situations at which the membrane

2. Commercial membrane modules and experimental membrane materials in the context of carbon capture at coal-fired power plants There is a limited number of commercial membranes that can be used for post-combustion CO2 separation [29–33]. Most of them have already been tested at coal-fired power plants. The results are summarized in Table 1. The operation conditions and the main issues observed during testing are shortly described below. PRISM™ by Air Products was tested by Scholes et al. [32] at the Victorian brown coal-fired power plant. The flue gas was compressed to 150 kPa. The flow rate of the dried flue gas entering the membrane module at 45 °C was 3.5 kg/h. The module was placed in clean flue gas after the blower discharge of a direct contact cooler. The main problem consisted in the condensation of water on the membrane. The permeance and the selectivity decreased dramatically after a few hours of membrane operation, which was explained by the plasticization of the Table 1 Commercially available membrane modules tested with flue gas. Manufacturer

Membrane

Permeance [GPU]

CO2/N2 selectivity

Polymer

Reference

Air Liquide Air Products MTR

Medal PRISM™ Polaris™ gen 1 Polaris™ gen 2 PolyActive™ PermSelect®

referred only normalized 760 1000 2000 1480 32.5

50 13 50 49 55 12

PI PSf PE-PA copol. PE-PA copol. PEO-PBT PDMS

[30] [32] [33]

Helmholtz-Zentrum PermSelect

2

[29] [31]

PEDM/ZIF-8@GO-6–6 PIM-1 PIM-1 - Schiff base network Pebax/Zeolite Y composite membrane, with three layers on top of a polyethersulfone Pebax without zeolite composite membrane, with three layers on top of a polyethersulfone Biomax PES with zeolite Y nanoparticles and Pebax/PEG 200 polymer selective layer PEI (PVC-g-POEM) Asymetric PES hollow fibers Polybenzoxazol Polyvinylamine as a fixed carries Semi-alicyclic aromatic polyimide Ce0.8Sm0.2O1.9(SDC)/Li2CO3-Na2CO3 ceramic carbonate dualphase membrane

Amino-modified mesoporous ceramic membranes

MOF PIM PIM in MMM MMM in composite Composite Composite Composite Hollow fiber TRP Fixed carriers Polymeric (modified PI) Ceramic

Ceramic

3

* For the thickness of membrane of 1 µm, calculated from permeance.

MOF MOF MOF MOF MOF

MMM MMM MMM

Membrane [ATMPS][Ac] in polyimide 84 [Emim][Tf2N] and (Al(i-C3H7O)3) + SAPO 34 in PSf Pebax®1657/[emim][BF4] gel membrane GO-IL in Pebax 1657 polyionic liquid with free ionic liquid content on epoxide amine with facilitated carrier transport CA functionalized by Tf2N anion Pebax® membrane with pseudopeptide biconjugate additives Amino-functionalized polyhedral oligomeric silsesquioxane (amine-POSS®) nanoparticles dispersed in polyvinyl alcohol (PVA) Pebax/PTMEG PU with NiO nanoparticles Contained Fe(BTC), benzene 1,3,5 tricarboxylate and iron octahedral clusters, with removable terminal ligands of H2O and OH incorporated in Matrimid NH2-MIL-53(Al) in CA PU/UiO-66 (Zr) PU/MIL-101 (Cr) ZIF/300/Pebax 1657 ZIF/300/Pebax 1657

Membrane type SILM SILM LM in composite LM in MMM LM in MMM MMM MMM MMM

Table 2 Experimental membrane materials.

30*

222.2* 8.6

22

300 28 192

45

58.2 19 22.4 35 10 25

25 34 42 73.9 59.2

15.5 75* 83 102.1 466.2 475 7500 7600 795* 1420* 745* 35* 50*

60 68 23

CO2/N2 selectivity 40 20.3 42 71 140 26.8 44

350 350 218

Permeability [Barrer]* 24* 7.24* 280 143 900 8.9* 194 22.2*

SO2 poisoned CO2 permeation, temp 550–750 °C

Selectivity from numer. model

Tamb and 4 bar Tamb and 4 bar 298 K, 12 bar dry feed CO2:N2 15:85 (v/v) 298 K, 12 bar CO2:N2 15:85 (v/v) feed relative humidity 85% Tests at 25 °C under 1 bar

Depends on PTMEG content Depending on nanoparticle content

Humidity effect: increase of CO2 permeance

Did not enhance the CA properties

Note

[64]

[53] [54] [55] [56] [56] [57] [58] [59] [60] [61] [62] [63]

[50] [51] [51] [52] [52]

[47] [48] [49]

Reference [39] [40] [41] [42] [43] [44] [45] [46]

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unit could fail. The flue gas should be pre-treated before entering the membrane module. According to the authors the reasonable treatment includes the removal of dust, condensate, and most of the water vapor. Generon membranes and PermSelect® have so far not been tested with flue gas from a power plant. We can see that although the commercial membrane modules have already been tested on some level in the real conditions, there are still many issues to be solved especially those connected with the durability of the membrane materials. That is why a great effort is still put into the development of new membrane materials. Twenty-seven membrane materials of different kinds are presented in Table 2. The data are published since 2013 and declared by their authors as convenient for flue gas separation. Four of these materials were tested at the pilot scale in power plants and the results show what should be solved in the near future for wider applicability in the flue gas treatment as can be seen from brief descriptions below. Sandru et al. [61] implemented a pilot-scale membrane unit with polyvinylamine fixed carrier separation layers. The effective area of the membrane was 1.5 m2. The unit was tested at Sins hard coal power plant in Portugal for 6.5 months. The flue gas brought to the membrane unit contained 12% of CO2, 6% of O2, approx. 200 mg/m3(STP) of SO2 and the same concentration of NOx. The operating conditions were: feed gas flow of 6–24 m3(STP)/h, the temperature of 45 °C, vacuum pressure in the permeate between 10 and 20 kPa (average of 13 kPa). The values of obtained CO2 permeances were between 0.2 and 0.6 m3(STP)/(m2 bar h), and CO2/N2 selectivity between 80 and 300. The results were described by the authors as stable, considering the harsh conditions such as plant outages and high NOx concentrations. Eiberger et al. [64] published an article on the usage of aminomodified mesoporous ceramic membranes in trial tests at a lignitefueled power plant and a hard-coal-fueled power plant. The tested membranes had an asymmetrical structure with macroporous α-Al2O3 support, coated with a mesoporous γ-Al2O3 or 8YSZ interlayer. The microporous functional top layer was made of amino-functionalized silica. Microanalysis showed the membrane surface was plugged by a deposition of fly ash and gypsum; water condensation in mesopores was observed and was accompanied by precipitation of sulfate crystals on the membrane surface; the interlayer made of γ-Al2O3 showed irreversible phase transformation, excluding this material for membrane contact with flue gas from a coal-fired power plant; the interlayer showed a significant increase of hydrothermal stability, but was still degraded by water condensation and crystal formation; which was partly reduced by deposition of SiO2 based top layer on the membrane. For long-term exposure, pre-treatment was necessary, especially drying of the flue gas and removal of residual ash particles. Choi et al. [59] ran a pilot-scale membrane separation unit connected to a liquefied natural gas (LNG) fired boiler (commonly used in power plants). The separation layers of the hollow fiber membrane modules were made of polyethersulfone (PES). The modules were first tested with a model mixture of CO2/N2. Thereafter, the pilot-scale unit was built and tested with flue gas containing 10.8% of CO2, 2% of O2 and 87.2% of N2. The membrane separation was divided into four stages. The permeance of CO2 through the used membrane reached 40 GPU. The feed flow rate was 500 l/min, the feed pressure was 588 kPa in the first stage, the permeate pressure was 19.6 kPa. Under such operating conditions, it was possible to capture 90% of CO2 of 99% purity. The experimental results were also compared with results obtained from a simulation performed using the Aspen Custom Modeler. In some cases, the simulation showed different results, which, according to the authors’ declaration, are not fully understood. Even one supported ionic liquid membrane (SILM) was prepared on a larger scale and was tested with flue gas. Klingberg et al. [65] built the pilot-scale unit using polyacrylonitrile (PAN) ultrafiltration membranes with [emim][Tf2N] ionic liquid. The method of preparation of a large scale flat sheet envelope membrane module was developed. The

ultrafiltration (UF) membrane was coated by the methanol solution of the ionic liquid. The prepared membrane modules were exposed to flue gas from a lignite-fired power plant. An experiment that lasted 335 h was performed. The CO2 permeance and CO2/N2 selectivity of the membrane were dependent on the composition of the coating solution: more methanol – higher permeance and lower selectivity. There was a significant increase of CO2 permeance and decrease of CO2/N2 selectivity before and after the flue gas exposition because of losses of the ionic liquid during the experiment as could be expected from the knowledge of liquid membranes. The basic research has focused on various membrane materials in recent years. CO2 separation is studied in two main configurations of membranes – flat sheet and hollow fibers. The representative materials are summarized in Table 2. Liquid membranes offer high diffusivity and tunability for a specific separation process. However, the leakage of the liquid has to be avoided and the membrane stability has to be solved. Another recently tested material is ceramic porous membranes. They are able to capture up to 90% of CO2 from flue gas [66,67]. Polymeric membranes also show very good permeability and selectivity. However, they suffer from the plasticization when CO2 is separated from flue gas. Contrary, MMMs and metallic-organic frameworks (MOFs) seems to be promising membrane material combining the transport and separation properties of polymeric matrix with the stabilization by nanoparticle/metallic fillers. Therefore, the prevention of defects is realized thanks to the good compatibility between polymeric matrix and fillers [51,68]. Modified MOFs can be applied for the humid feed gas while common MOFs usually suffer in the contact with humid feed. However, they have to overcome the complex purification of flue gas due to other impurities that have to be also separated [68]. Therefore, the key role plays also the membrane durability and its lifetime for any kind of membrane materials. The main method of comparison of membrane materials is the Robeson plot. During the years, the authors got used to that the main goal is to prepare the membrane material that can overcome the actual upper bound. However, it is necessary to reflect if the material may be turned into a thin, defect-free and homogeneous film of sufficient area and mechanical properties. 3. Progress in the potential of membrane materials in last years Since 2010 when Merkel et al. published the CO2 capture process based on Polaris membrane and declared that membranes can compete with the MEA scrubbing [69] under mild pressure conditions, many studies based on similar assumptions trying to find optimal membrane characteristics and describing the relationship between the properties of the membrane and energy consumption have been published [70–78]. However, the required membrane area is often very high. Therefore, it is necessary to find a very permeable membrane material that will be also selective to decrease the required membrane area. Secondly, the question if the optimal membrane material already exists has to be answered. That is why we decided to use the approach of Roussanaly et al. [79], who developed a numerical model based on a model coal-fired power plant producing 4.6 Mt of CO2 per year. Roussanaly et al. compared membrane separation and MEA absorption and subsequently published a “case study” of possible utilization of membrane separation of CO2 in different plants including the coal-fired power plant producing 2 706 ton/h of flue gas (578 ton/h of CO2) [80] to estimate optimal membrane properties for the flue gas treatment (permeability/permeance, selectivity) so they could be at least of the same cost as MEA absorption or even cheaper. We applied the Roussanalýs coal-fired power plant case assuming the optimal cost carbon capture ratio (50–90% [81]) to see if the “experimental membrane materials” presented in Table 2 and membrane modules presented in Table 1 would stand the test. The curves are borders between the areas of different savings compared to MEA absorption. Robeson upper bound [82] was 4

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Fig. 1. “Experimental membrane materials” (Table 2) for which permeability is known and their cost-effectiveness compared to MEA absorption; “dry” means dry feed flow and “hum” abbreviates humid feed flow.

decrease of permeance [86]. The level of CO2 capture is leaded by the membrane selectivity and permeability. Therefore, these two parameters strongly influences the energy needed for the membrane PCC. However, many membrane materials follow the trade-off: high permeability is accompanied by the lack of selectivity and vice versa. Fortunately, the permeability factor can be overcome by manufacturing of a thin selective layer (less than 500 nm) on a very permeable porous support. Therefore, selective membrane can offer also excellent permeability. Reaching a maximum CO2 permeability can save the energy needed for a compression of flue gas before the entering the membrane module [87–89]. The energy can be also saved when the CO2/N2 selectivity increases. Fortunately, pilot experiments are available nowadays contrary to the time before 2007 [87]. It was found that the increases of CO2/N2 selectivity from 20 to 40 leads to lower than half energy consuming for Polyactive® membrane and further increase of selectivity is accompanied by slower decrease of energy consumption [90]. On the other hand, not only the price is necessarily considered. The other factors such as a potentially toxicity of solvents used for an absorption method has to be taken into an account. Thanks to the low environmental impact, membrane separation for PCC has open the doors nowadays. It is necessary to point out that although the economical and optimization studies suggest that membrane processes can be feasible compared to MEA scrubbing, taking into account the facts from the pilot plant tests, we still have a few membrane materials that are really utilizable in flue gas treatment.

also added to see if it gives valuable information about membranes in coal fire plant CO2 separation. It is necessary to point out that the analysis assumes that the flue gas would be compressed up to 55 bars. This feed pressure results from the decision to keep the membrane area at a reasonable level. The study considers the overall cost of the process including the cost of membrane material, compression, cooling, evacuating and energy for the process. More details may be found in the original work. Roussanaly himself enumerated about 70 membrane materials of which only a few are able to compete with MEA absorption in the case of a coal fire plant. Here we present two figures based on the analysis. The first one (Fig. 1) for the materials from Tables 1 and 2 for which permeability is known and the second one (Fig. 2) for the materials from Tables 1 and 2 for which the permeance is known. Both figures suggest that 1) Permeance higher than 500 GPU and selectivity higher than 40 could make the membrane separation of CO2 from flue gas competitive to MEA absorption 2) The membrane material with permeability higher than 200 Barrer and selectivity higher than 40 would be feasible assuming that thin film of less than 500 nm could be prepared. Commercial modules are feasible compared to MEA absorption assuming their best performance. On the other hand, some commercial modules e.g. PRISM™ may be utilized if the problem of their stability under real flue gas conditions was solved. Applying the analysis to the “experimental membrane materials”, we can see that PIM and PolyIL with free IL content exceeded significantly the optimal membrane properties. So far, it has not been tested if the PolyIL with free IL content may be turned into a membrane module. The case of PIM is also interesting. However, an attempt to create PIM hollow fibers resulted in fibers that were too brittle [83]. Another problem with PIM is aging [84]. Lau et al. [85] presented a possible solution to this problem of PIM aging, but no subsequent work has been published to confirm the promising results. Besides that, the PIM-1 membranes have already been tested with flue gas and it was found that wetness and impurities in flue gas cause a significant

4. Conclusions Membrane separation have very good potential to separate CO2 from flue gas. The sectors covered by the European Union Emissions Trading System (which include the power and heat generation) should reduce their overall greenhouse gas emissions by 43% in 2030 (compared to 2005). Therefore, the separation of CO2 from flue gas might be an inevitable step for incineration processes. So far, amine absorption is the only large scale process for CO2 separation from flue gas. However, commercial applications show that it is very expensive in terms of both capital and operating costs which 5

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Fig. 2. Commercial modules (Table 1) and “experimental membrane materials” (Table 2), for which the permeance is known, and their cost-effectiveness compared to MEA absorption; “gen1” means the first generation of PolarisTM membrane and “gen 2” means the second generation of PolarisTM membrane.

References

creates space for the application of alternative technologies, such as membrane separation. Most of the commercially available membrane modules have already been tested in real power plants. PolyActive™ and Polaris™ seem to be the most promising materials that are commercially available. Some of the experimental membranes would be feasible assuming that it is possible to create a thin film and subsequently the membrane module. However, some materials only showed very high sensitivity in the contact with all components from real flue gas. The membranes should be applicable in the future but there is still a gap between studies on optimization or cost evaluation of membrane separation processes in post-combustion CO2 capture and the technological state-of-the-art. Especially, the long-term stability of the membrane materials and their resistance to water vapor present in real flue gas has to be solved. Generally, any technology for CO2 capture would be energy-consuming and expensive so, from this point of view, membrane technologies which are waste-free and their application research and development of new materials promises early applicability of them in PCC, especially at smaller applications like cement plants.

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Declaration of Competing Interest The authors declared that there is no conflict of interest.

Acknowledgements The authors gratefully acknowledge the Czech Science Foundation grant No 18-05484S for financial support. M. Karaszova, B. Zach and M. Šyc would like to acknowledge EU structural funding in Operational Programme Research, Development and Education, project No. CZ.02.1.01/0.0/0.0/16_026/0008413 “SPETEP” Strategic Partnership for Environmental Technologies and Energy Production. M. Bobák would like to acknowledge the project TE02000077 “Smart Regions Buildings and Settlements Information Modelling, Technology and Infrastructure for Sustainable Development” supported by Technology Agency of the Czech Republic. 6

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