1. Technologies for Carbon Dioxide Capture and Storage
The ever-increasing atmospheric CO2 concentration calls for immediate action to reduce possibilities for global catastrophic risks to the planet and to the very existence of civilization. This is why the development of technologies for carbon capture and storage (CCS) is crucial for preserving the future and the quality of life. During the last decades, the development of CCS technologies has been studied widely and intensively to find the balance among efficiency of use, production, and maintenance costs. Notably, the advancement of technologies for carbon capture is equally as important as the new options for carbon storage.
The reduction of CO
2 emissions generally relies on its separation from flue gas from the combustion reactions. The technologies for CO
2 separation include six major methods, which are
[1]:
The CO
2 absorption technologies generally can be distinguished between chemical and physical absorption. The chemical absorption is then subdivided into formulation and operation. The chemical formulation involves a CO
2 reaction with single amines (derivatives of ammonia), amine blends (solutions of two or more amines in varying compositions), and caustics (strong alkaline chemicals). At the same time, chemical operation is based on the use of a chemical adsorber or stripper and rotating columns to absorb CO
2, where the rotation increases the chemical reactivity and the absorption efficiency
[2]. The second absorption type, physical absorption of CO
2, involves the use of units with various solvents and sorbents for gas separation, such as selexol, rectisol, fluor (econamine), purisol, and others
[1][3][4][5][6].
Technologies for CO
2 adsorption can be distinguished between adsorbent beds and the use of regeneration cycles. The fixed-bed adsorption generally utilizes activated carbon, alumina (synthetically produced Al
2O
3), silica, zeolites, metal organic frameworks (MOFs), hydrotalcites, amine supported adsorbents, and polymers
[1][7][8][9][10][11][12][13][14]. Regeneration cycles, in turn, involve pressure swing adsorption, temperature swing adsorption, steam, or moisture adsorption
[1][15][16][17].
The calcium looping technology uses the regenerative calcium cycle, where to separate CO
2 from flue gas, a metal is reversibly reacted between its carbonate form and its oxide form. Within this approach for CO
2 separation, the technology basically involves a loop of calcination and carbonation to remove CO
2 from flue gas
[1][18].
The cryogenic technology includes a set of conventional and unconventional methods. Among these, the cryogenic distillation is amidst the most common conventional approaches. In turn, most unconventional methods involve various cryogenic fluids, heat exchangers, and cryogenic packed beds to fix CO
2 [1][19][20][21].
The membrane technology is based on gas separation using specially designed membranes, for instance, a polyphenylene oxide (PPE), poly dimethyl siloxane (PDMS) membrane, or polypropylene (PP) or ceramic bed systems to fix CO
2 [1][22][23][24].
The technology behind the biological CO
2 separation using microalgae involves carbon bio-fixation, for example, through enzyme-catalytic hydrolysis. The idea behind microbial bio-fixation involves CO
2 fixation for microalgae cell growth
[1][25][26][27].
Different technologies require divergent approaches; they are also characterized by different use and maintenance costs, as well as the range of applications and scale. Currently, among main technologies used for CO
2 capture is
[28][29][30][31][32][33]:
Unfortunately, most of these approaches are expensive and high-energy demanding, which therefore limit widespread use of the given technology.
The cryogenic distillation is amongst most used technologies for CO
2 adsorption and gas separation
[29]. It is low-temperature CO
2 capture technology, which relies on phase change. The technology can be applied to a range of CO
2 concentrations. Regrettably, it is high-energy demanding and expensive, and thus the use on large industrial scale cannot be economically justifiable. At the same time, this technology can be recommended for smaller-scale applications
[1].
The membrane purification involves CO
2 capture in polymeric membranes. The membrane purification technology requires comparatively less energy than cryogenic distillation and is also much easier to operate; however, this technique is limited to minor CO
2 concentrations and thus again cannot be justified for an industrial scale use
[30][34].
The electrochemical reduction of CO
2 is among promising future technologies; however, there are numerous challenges for large-scale production
[35]. Separation of CO
2 from flue gas using electrochemical cells is complex and energy demanding process. In this context, experiments have shown that this technology can be used to separate CO
2 from flue gas streams produced by coal combustion for electricity generation; however, for this to happen, elevated temperature molten carbonate electrochemical cells would be required and the presence of any kind of contaminants would impact the electrolyte within the cell and thus also the effectiveness of CO
2 separation. At the same time, experiments with low-temperature cells also continue and their applications are continuously advancing, which show comparatively higher work efficiency than the high-temperature cells
[36].
Yet another approach—CO
2 adsorption with liquids, in general with alkylamine solution or chilled ammonia, collides with an obstacle—is a complex, expensive, and high-energy demanding technique. Moreover, evaporation of these liquids eventually causes equipment corrosion, and thus this method may not be among long-term solutions
[37].
In many aspects, the CO
2 adsorption using solids is among the most reasonable options as this method demands the least energy, can be used in the long-term, and some of the materials can be reused and applied on a large industrial scale. In addition, organic or inorganic sorbents are available with high selectivity in respect to CO
2, high efficiency, and stability during their exploitation
[33][38][39]. Use of porous solid materials, such as clays, zeolites, activated carbon, MOFs, metal oxides, layered double hydroxides (LDHs), and porous silica have shown to be very effective for selective CO
2 capture
[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55]. Notably, the majority of CO
2 adsorption studies have been reported in high pressure and elevated temperatures as it is known that with increasing pressure, CO
2 adsorption level would also increase
[56]. However, in regard to GHG removal from the atmosphere, CO
2 adsorption at atmospheric pressure also has to be accounted for, and therefore it is crucial to expand on this topic in further studies
[57].
The most common technologies for CO
2 capture from large stationary sources are generally based on a selective CO
2 absorption using aqueous solutions of amines, such as monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), diglycol-amine (DGA), and 2-amino-2-methyl-1-propanol (AMP)
[58]. Regrettably, such an approach has several disadvantages over other equivalent solutions for carbon capture; for example, some known disadvantages are equipment corrosion or loss of amines due to evaporation. Moreover, solvent regeneration requires high energy consumption, and thus alternative approaches that are less energy demanding are necessary. In fact, this necessity has promoted the development of new strategies for CO
2 capture that also involve the development of new appropriate solid material sorbents based on cheap substances, for instance, on clay minerals.
2. Materials for Carbon Dioxide Capture Technologies
It is reasonable to consider that several groups of sorbents, both representing solid and liquid phase materials, are suggested for the development of the CCS technologies due to their high efficiency and selectivity to CO
2 capture, long-term applicability, and economic justifiability. In this regard, the materials with high potential for CO
2 capture include a variety of porous materials and liquids of both natural and artificial origin, such as
[59][60][61][62][63]:
-
Metal organic frameworks (MOFs);
-
Graphene organic frameworks (GOFs);
-
Covalent organic frameworks (COFs);
-
Metal oxides;
-
Homogenous porous silica;
-
Zeolites;
-
Activated carbon;
-
Clay minerals;
-
Molecular basket sorbents (MBSs);
-
Ionic liquids.
It has been reported that MOFs and GOFs form three-dimensional structures with narrow and homogenous pore size distribution, and this allows reversible retention of CO
2 molecules
[59][60][61][64][65]. A number of MOFs with designed pore and channel size and large surface area are viewed as materials for CO
2 sorption and storage
[32][66].While MOFs and GOFs show significant advantages in gas selectivity and separation in comparison to traditional adsorbents, the main disadvantages of these structures, however, are their relatively low thermal and hydrolytic stability, which are crucial for adsorbent regeneration and low yields. Yet another obstacle is the excessive cost, which is required to prepare these materials on a large industrial scale
[59]. At the same time, properties of MOFs can be further improved by several means
[67][68]. The main improvements that can be made to MOF composites are increased porosity and sorption capacity, as well as improved special functionality. Moreover, numerous structural modifications and improved kinetics in the synthesis of MOFs can be made, that all together can significantly improve their effectiveness towards CO
2 sorption
[69].
Other structures—COFs—are an equal alternative to MOFs and GOFs, as they can be either two- or three-dimensional highly porous organic solids, and they have already been used in the photocatalytic and electrocatalytic systems for large scale CO
2 conversion to CO. However, such photocatalytic systems have poor overall CO
2 selectivity and low effectiveness for CO
2 adsorption
[70][71][72][73][74][75]. Moreover, COF applications on an industrial scale are hampered by their low stability in the presence of water
[76][77]. Simultaneously, studies show an effective CO
2 adsorption on numerous metal oxides, such as Fe
2O
3, TiO
2, ZrO
2, or Al
2O
3 in the temperature below 0 °C, but unfortunately, the high energy support to maintain the required temperature is high, making this approach high-energy demanding
[78]. Notably, the use of such solids as CaO, MgO, or mixed oxides coming from hydrotalcites generally is of low cost; they are highly available and have high overall capacity to capture CO
2. However, use of these oxides is affected by the same complications as other metal oxides
[79].
In the last several decades, experimentation with porous silica in regard to CO
2 adsorption has been carried out. Experiments with porous silica with different textural parameters such as mesocellular foams—MCM-41, MCM-48, SBA-15, hexagonal mesoporous silica, and others—have shown that CO
2 capacity is directly proportional to the microporosity
[79][80][81]. However, in comparison, MOFs show much better CO
2 adsorption capacity than silica
[50][59][80][82][83][84][85].
Zeolites and activated carbon, among all materials, reach by far the highest CO
2 adsorption values. In fact, inorganic zeolites were the first porous materials to be investigated for the ability to sequester CO
2; however, their production is expensive, thus alternatives with similar properties are being sought
[59][86][87][88][89]. Zeolites, activated carbon, and other solid porous sorbents are capable to capture CO
2 via physical and chemical adsorption mechanisms
[58]. Zeolites can bind CO
2 either by chemical binding or by including both chemical and physical sorption pathways.
Clay minerals are a low-cost alternative with similar CO
2 adsorption capacity to zeolites and activated carbon, thus their use on an industrial scale can be economically justifiable. Clays have high pore and channel size variability, large surface area, a wide variety of structures responsible for CO
2 sorption, and large void volume for carbon storage. Furthermore, clays can be chemically modified to ensure high clay-based sorbent selectivity in respect to other gasses present in flue gas, such as H
2N
2, O
2, CH
4, or CO
[32][59]. For instance, Wang et al. (2013) have reported that the support material accounts for over 70% of the total capital cost for sorbent preparation. In this respect, due to the easy availability, excellent thermal and chemical stability, surface properties that can be adjusted for various properties, and low cost, clay minerals can be considered one of the most justifiable materials for CCS
[90]. For example, one of the top sequesters for CO
2 is the exchanged fluorohectorite clay, which belongs to the smectite group
[91]. In addition, clay minerals can be used as the base material for other technologies, such as MBSs. The concept of MBS is used to denote sorbents that may selectively capture CO
2 onto a functional basket. Sorbents of this type are typically prepared by immobilizing an amine-functional polymer onto a porous carrier. Recent studies point to the benefits of MBS, such as, superior sorption–desorption characteristics, high sorption capacity, selectivity towards CO
2, high regeneration abilities, and high stability. In addition, MBSs have much less corrosion potential and require much lower energy consumption when compared to the conventional methods, such as amine scrubbing
[92]. It is worth mentioning that, while sorption processes have been studied in high detail and general regularities are known, desorption processes are not yet fully understood. At the same time, recent studies show that CO
2 desorption in natural clays and clay modification products, including MBSs, can be controlled both by pore structures and by chemically functionalized groups
[93]. However, understanding which factor prevails requires further investigation. Nevertheless, it is believed that desorption can be triggered by changes in clay properties that may endure pressure or temperature change, yet further studies are required.
The last considerable group of materials with a potential for CO
2 fixation are ionic liquids. These are non-volatile liquids with modifiable structure and high CO
2 uptake capacity
[94]. Ionic liquids have been explored in various chemical and biological applications; however, use of them comes with a number of potential weaknesses in comparison to clay minerals; this includes weak thermodynamic properties, corrosivity, and toxicity
[95]. One of the least substantial options for industrial-scale carbon capture is the precipitation of calcium carbonate in underground reservoirs. Crystallization of calcium carbonate requires a liquid, such as groundwater, thus this method can be viewed within the ionic fluids. However, the crystallization is slow geological process that is sensitive to pressure and temperature changes, and thus this approach is not an effective long-term or industrial-scale solution
[96]. Therefore, the use of clay minerals and clay modification to support an increased level of CO
2 fixation are both justifiable and welcomed in regard to reducing GHG emissions.