Carbon Dioxide Separation Technologies: Applicable to Net Zero

Abstract

Carbon dioxide (CO₂) emissions from burning fossil fuels play a crucial role in global warming/climate change. The effective removal of CO₂ from the point sources or atmosphere (CO₂ capture), its conversion to value-added products (CO₂ utilization), and long-term geological storage, or CO₂ sequestration, has captured the attention of several researchers and policymakers. This review paper illustrates all kinds of CO₂ capture/separation processes and the challenges faced in deploying these technologies. This review described the research efforts put forth in gas separation technologies. Recent advances in the existing gas separation technologies have been highlighted, and future directives for commercial deployment have also been outlined.

1. Introduction

Energy is the primary source of greenhouse-gas (GHG) emissions, with a share of around 76% (mainly CO₂ emissions). Though COVID-19 triggered an exceptional decrease in global GHG emissions in 2020, the largest-ever annual rise in CO₂ emissions witnessed a CO₂ rise from 31.5 Gt to 36.6 Gt in 2021 [1]. To achieve the COP26 targets established for net zero, carbon-capture utilization and storage (CCUS) technologies could be the bottleneck. CCUS is predominantly employed to capture CO₂ produced from different industrial sources, such as steel plants, power plants, chemical industries, and thermal-electric power plants. The most conventional approaches for carbon capture are precombustion, postcombustion, and oxyfuel combustion methods [2]. Although there has been extensive research in direct air-capture approaches, the high capital cost has been a significant challenge to deploying this technique. In addition, the development of CO₂ separation techniques (to separate CO₂ from flue or fuel–gas mixtures) has also gained a significant attraction for the economical deployment of carbon-capture technologies. In this direction, adsorption, absorption, microbial, membrane separation, and environmentally friendly techniques such as ‘gas hydrate-based’ separation and biological processes have grown significantly around the world [2,3,4]. The adsorption, absorption, and cryogenic distillation processes are the most mature CO₂ separation methods with high separation efficiency. CO₂ capture through Gas hydrate-based and membrane separation methods is highly effective due to their low energy consumption. However, these technologies also have drawbacks, mainly temperature requirements, energy intensity, and CO₂ concentration dependency [3,5,6,7,8,9]. The industrial applications of CO₂ capture through different methods have numerous drawbacks, restricting this process from being used commercially. Therefore, the current review article discusses all the CO₂ separation technologies, listing their advantages and disadvantages. Moreover, the paper also identifies a futuristic overview for improving efficiency and the possible deployment opportunities of different methods.

2. CO₂ Separation/Capture Technologies

Research and technological advancements have created multiple novel carbon dioxide (CO₂) separation methods. The uncontrollable release of notorious anthropogenic GHGs, especially carbon dioxide, has caused major environmental issues. The harmful effect of CO₂ has motivated the development of technologies dedicated to achieving net-zero emission goals while evaluating their efficacy, economics, and environmental impacts [10]. The flue gas properties (such as composition, temperature, and pressure conditions) are also important parameters for selecting the appropriate process of CO₂ separation [11]. The current scenario requires investments to mitigate carbon emissions effectively (CO₂ capture and sequestration) [12]. Some primary CO₂ capture techniques investigated globally are adsorption, absorption, chemical looping combustion, membrane separation, microbial/algal separation, hydrates-based separation, and cryogenic distillation method [2,3]. Figure 1 compares the advantages and disadvantages of different CO₂ capture/separation techniques mentioned above. As shown in Figure 1, absorption, adsorption, and membrane-based separation methods offer high separation efficiency. Though cryogenic separation is the most mature technique, the process is highly energy intensive.

2.1. Absorption

The absorption method is widely applicable in the petroleum, coal, and natural-gas industries for separating CO₂ [13]. Removing CO₂ from a gas stream using different absorbents (physical and chemical) has been used in the industrial sector for over 50 years. The absorption method is broadly divided into two types: physical and chemical absorption [14,15]. Chemical absorption is the process by which a solvent absorbs CO₂ and produces chemical compounds. These chemical components are later reused by removing the absorbed CO₂ through different techniques. However, if the solvent is chemically inert, it does not interact with CO₂. CO₂ is chemically absorbed in two steps: the treated gas is initially introduced into counter-flowing interaction with the solvent stream. The solvent absorbs CO₂ from the flue gas stream during this phase. As the solvent warms up, CO₂ is desorbed in a stripping column, further migrating to the top of the column, where pure CO₂ is recovered, compressed, and stored [14,16,17].

On the other hand, physical solvents do not undergo any reaction with CO₂, making them more desirable for CO₂ separation processes. Henry’s law of equilibrium in vapor–liquid mixtures governs the physical absorption process. It states that the relative gas pressure in equilibrium with the solvent at any given temperature is directly proportional to the amount of a gaseous phase dissolved in a unit volume of the solvent. Since the physical absorption process is pressure dependent, it performs better than chemical absorption at higher partial pressures of CO₂, such as in an integrated gasification combined cycle (IGCC) power plants [14,18,19]. The coal, natural gas, and petroleum industries extensively use absorption techniques to segregate CO₂ [13]. Kim and Yang [20] studied the competence of hollow-fiber membranes in CCS with various aqueous absorbents. The capability of the hollow polytetrafluoroethylene (PTFE) membrane filter was measured at varying temperatures. They conclude that the absorption rate of CO₂ increases with the temperature rise. Sensitive absorbents such as 2-amino-2-methyl-l-propanol (AMP) and monoethanolamines (MEA) are vastly active agents to achieve augmented rates of CO₂ absorption. Figure 2 shows the different categories of absorption medium broadly categorized into physical and chemical absorption. The physical and chemical processes are further divided into five subcategories: rectisol process, purisol process, selexol process, amine-based process, and inorganic chemical process.

In recent years, research has been undertaken on CO₂ capture from fossil-fuel emission sources [21,22]. The CO₂ absorption in aqueous media is evaluated by the equation [23]:

CO₂ + 2H₂O ↔ HCO₃⁻ + H₃O⁺

Absorption of CO₂ into aqueous solvent was initially done to purify gases such as synthetic gas, hydrogen, and natural gas [22,24]. However, research that originated on CO₂ capture from fossil-fuel sources has been done with absorption [21,22]. CO₂ absorption through the membrane is the combination of gas absorption and membrane separation processes. This method perceived a remarkable perspective in the last decade for capturing CO₂ from flue gas streams [25].

The absorption process for the large implementation of CCS is the perhaps amine based which may cause equipment corrosion, solvent loss, and the production of volatile degradation constituents [26,27]. The release of nitramines and nitrosamines from the degradation of amine emissions can cause potential damage to human health [28]. Kozak et al. [29] presented a chilled-ammonia process (CAP) to capture CO₂ from flue gases produced by different industries. They suggested that this process required low energy for the regeneration of CO₂ at increased pressure and temperature, reducing the downstream compression, and is more environmentally friendly than the amine processes. Furthermore, the capacity of solvents in absorption was found to be better at lower temperatures, which necessitates the cooling of the solvent before the process and reducing the efficiency of the process [19]. Although absorption is the utmost developed CO₂ separation process due to its high efficiency and low cost, it has certain environmental drawbacks due to the disposal of the absorbent after use [2].

Table 1. Various solvents applied for CO₂ absorption [11].

Solvent	Advantages	Disadvantages
PHYSICAL ABSORPTION
DME of PEG (Selexol)	Lower energy requirement for regeneration. Low toxicity, low corrosiveness, and lower vapor pressure	Temperature and pressure dependent. Low CO2 absorption capacity
Glycol Carbonate
Glycol
Methanol (Rectisol)
Fluorinated Solvent
CHEMICAL ABSORPTION
Alkanolamines (MEA, DEA, MDEA)	Rapid reaction Greater selectivity Reversible absorption Low-cost solvent	Limited CO2 loading capacity Greater apparatus corrosion rate Higher energy consumption
Amino acid and aq. amino acid salt	Nonvolatile solvent Possesses high surface tension Better performance than MEA with similar concentration	Lower effectiveness in presence of O2
Ammonia	Unharmed in presence of SO2 and O2 in flue gas Corrosion free Lower energy requirement	Not suitable for postcombustion applications. Risk of dry CO2-NH3 explosion at high CO2 concentration.
Ionic Liquid	High thermal steadiness Low vapor pressure required Greater polarity Lower toxicity	Viscosity of the fluid increases with CO2 absorption
Aq. Piperazine (PZ)	Quick absorption kinetics Limited degradation rates during CO2 separation Insignificant thermal degradation in concentrated PZ solution Promising equilibrium characteristics	Low oxidative degradation of concentrated PZ

lists various physical and chemical CO₂ absorption agents used for separation in the industry.

2.2. Adsorption

Burning fossil fuels has led to the inexorable emission of greenhouse gases (GHG) and responsible for global warming. Carbon dioxide escape can be prevented by capturing it before it gets released into the environment. One such method that has been gaining popularity is the adsorption of CO₂ on adsorbent material at high temperatures. CO₂ adsorption at high temperatures is a significant CO₂ separation method. Adsorption is a physical process in which a solid sorbent is used to fix the CO₂ onto its surface. The adsorption process reduces energy consumption and cost during CO₂ separation. Adsorbents can be used to capture single or multilayer gases depending on the absorbent’s temperature, pore size, surface force, and pressure [2,30,31]. The process employs an adsorbent with a nanoporous surface to precisely adsorb CO₂ from the flue gas. Regeneration of the adsorbent is done by creating a vacuum environment around the adsorbent or by providing heat [32]. Generally used adsorbent materials are molecular sieves, zeolites, activated carbon, calcium oxides, lithium zirconate, and hydro-calcites [2].

Table 2. Chemical and physical adsorbents efficiency for postcombustion CO₂ [11,33].

Sorbent	Operating Pressure (kPa)	Operating Temperature (K)	CO2 Capture Capacity (mol CO2/kg Sorbent)
CHEMICAL ADSORBENTS
Mesoporous (MgO) [34]	101	298	1.8
CaO nanopods [11]	101	873	17.5
CaO derived from nano-CaCO3 [11,34]	101	923	16.7
CaO-MgAl2O4 (spinel nanoparticles) [34]	101	923	9.1
Nano CaO/Al2O3 [34]	101	923	6.0
Lithium–Silicate [34]	-	993	8.18
CaO [34]	100	873	17.3
PHYSICAL ADSORBENTS
Activated Carbon [35]	110	303	1.58
NiO-ACs [35]	101	298	2.227
Na-Y [36]	101.32	273	4.9
NaKA [35]	101.32	373	3.88
MWNT [37]	101	303	1.7
CNT at (Cu3(btc)2) [36]	1818	298	13.52
MOF-177 [38]	4545	298	33.5
Pd-GNP Nanocomposite [39]	1111	298	4.5

enlists different physical and chemical adsorbents used in the postcombustion capture of CO₂.

Adsorption is broadly categorized into chemical and physical adsorption processes. Chemical adsorption or ‘chemisorption’ is driven by chemical reactions at the contact surface. Metal salts and metal oxides are compounds that constitute most chemical adsorbents. ‘Physiosorption’ or physical adsorption does not affect the chemical structure of the adsorbent during adsorption. Inorganic porous materials such as zeolites, hydrotalcite, and activated carbons (AC) are widely used physical adsorbents (refer to Figure 3) [30,37,40,41]. Activated carbon is an economical material with a large surface area and flexible pore structure when treated with activating agents. However, effective CO₂ separation through AC is possible when the AC possesses weak binding energy with carbon dioxide [42]. Zeolites are hydrophilic, yet strong CO₂ adsorption agents. However, upon interaction with water, the strength of the links between the interconnected substances reduces, decreasing the adsorption capability of the zeolites. Applying metal–organic frameworks (MOFs) as adsorbents is a new approach. Metal ions or ion clusters are the essential components of MOFs, amalgamated by organic linkers and bridging agents to form stable coordination bonds. MOFs have advantages such as ease of synthesis and design with large porosity and modified pore features. Silica, a non-carbonaceous material, has a large surface area, small pore size, and high mechanical stability. Materials made of mesoporous silica use amine-based compounds to trap CO₂ [40,42,43,44]. An effective adsorbent should have the following properties: (i) good mechanical strength, (ii) high sorption kinetics, (iii) high selectivity, and (iv) stable adsorption capacity [27].

The various pathways for carrying out the adsorption process are [13]:

1.

Pressure swing adsorption (PSA);

2.

Temperature swing adsorption (TSA);

3.

Electrical swing adsorption (ESA);

4.

Vacuum Swing Adsorption (VSA).

The recovery of CO₂ captured during the adsorption process through processes such as pressure-temperature swing adsorption (PTSA), vacuum swing adsorption (VSA), pressure swing adsorption (PSA), and temperature swing adsorption (TSA) processes where PSA and TSA are the most widely used techniques. PSA adsorbs CO₂ onto a solid adsorbent surface at fluctuating pressures between maximum and minimum permissible pressure limits. TSA is the process of CO₂ recovery through variations of temperature using hot air or steam. The PSA method is favorably implemented in industrial applications due to its high recovery efficiency (85%) and lower application cost than TSA. However, TSA is observed to be 95% effective in recovering CO₂ from adsorbed surfaces, although it has a longer regeneration time than PSA. The requirement of high temperature during TSA and high energy during PSA methods are the most significant drawbacks of these two methods [6,7,45,46]. Yong et al. [47] reviewed the various adsorbents at high temperatures. They studied material from carbon-based adsorbents with high adsorption capacity for CO₂ separation at surrounding temperature and pressure to other agents such as zeolites, metal oxide sorbents, and hydrotalcite-like compounds (HTlcs). It is of the utmost importance to understand that the choice of adsorbents depends on the operating conditions of the process. MgAl₂O₄, CaO– and nano CaO/Al₂O₃ are the most effective chemical adsorbents. The regeneration of chemical adsorbents is complex, even though they have high adsorption capacity and selectivity [30]. Figure 3 shows different materials used as adsorbents for CO₂ separation.

Figure 3. Materials used for CO₂ capture as adsorbents (reproduced with permission from [48]).

Figure 3. Materials used for CO₂ capture as adsorbents (reproduced with permission from [48]).

3. Membrane Separation

Membrane-gas absorption technology is based on the combination of two processes, membrane separation, and gas absorption. This technique witnessed tremendous potential in the last decade for the recovery of CO₂ from flue gas streams [25]. The application of membranes is an effective separation mechanism for selectively segregating components from a gas stream. It comprises a selective semipermeable barrier that allows the passage of only certain elements to separate via various mechanisms. Membranes can either be inorganic or organic, and nonporous or porous. Membrane processes are further categorized as gas-separation and gas-absorption membranes based on their corresponding designs and functioning limitations [13].

Membrane technology has emerged as an operational and favorable substitute for gas separation. Zhang et al. [49] in their comprehensive review discussed the development of various membranes mainly engaged for CO₂/CH₄ separation methods and also proceeded to draw out the dissimilarities between different membranes. Membranes are generally made up of inorganic and polymeric materials or are of a mixed matrix (a combination of both inorganic and polymeric constituents). Figure 4 represents the principle of CO₂-membrane gas absorption. The crossover of CO₂ through the selective semi-permeable membrane is carried onto the absorption liquid for further processing. Compared to other resistances, the mass-transfer resistance of CO₂ diffusion from the gas phase via a permeable membrane to the liquid phase is minimal. However, the resistance to mass transfer significantly rises when the membrane pores are fully or partially occupied with a solvent (wet). Thus, membranes appropriate for membrane-gas absorption should have the following characteristics: (1) hydrophobic properties to prevent membrane impregnation and increased mass-transfer rigidity; (2) a relatively high porosity to minimize membrane resistance; and (3) high chemical stability to endure the potent solvents typically used as the sorbent materials for CO₂.

Membrane based CO₂ separation is expected to spectate considerable development in the productivity of membranes from various sources [51]. Various degrees of membrane selectivity can be attained depending on the material used in the membrane and its qualities. This approach uses far less energy than absorption or cryogenic fractional processes. For membrane diffusion processes, metallic, polymeric, and ceramic membranes have been developed. The membrane separation process is also used to separate additional gases from natural gas, such as CO₂ [2]. Various degrees of membrane selectivity can be attained depending on the material used in the membrane and its qualities. This approach uses far less energy than absorption or cryogenic fractional processes. Metallic, polymeric, and ceramic membranes are used for membrane-diffusion processes. With ever-growing concerns around global warming and militating against climate change, CO₂ separation is posed to witness a further development in mechanisms and efficiency, with researchers putting in noble efforts in developing and deploying membranes for CO₂ separation from various sources with increased efficiencies [51].

The primary premise is to use the variations in physical qualities and chemical composition of specific elements in a gas mixture and a selective membrane to diffuse the gas constituents at varying rates through the membrane material. The different components of the gas have different diffusion speeds as they move through the membrane, and the differential pressure helps determine the flow through the membrane. The driving force behind this procedure is the variation in concentration of the component on both sides of the membrane [52,53]. Brunetti et al. [54] carried out a general assessment of the existing CO₂ separation methods based on membranes and paralleled it to other separation technologies such as cryogenic and adsorption. They observed that the efficacy of the membrane method is heavily influenced by flue gas properties such as low CO₂ pressure and strength of the membrane, the critical barriers to implementing this technology.

Specific limitations that pose a barrier in the application of membrane separation are brittleness, the difficulty of commercial-scale manufacture, and cost. CO₂-induced plasticization remains a challenging issue in the use of polymer membranes for the separation of CO₂ from natural gas [54,55,56,57].

Table 3. CO₂ removal efficiency of different polymeric membranes under varying temperature conditions with different absorbents [58,59,60,61,62,63].

Membrane Material	Analysis Condition/Absorbent	Gas Mixture	Temperature (in K)	CO2 Removal (%)
PVDF [58]	DI Water	CO2	299.15	1.84
PVDF [58]	0.5 M NaOH	9:91 CO2:CH4	298.15	94.00
Poly Propane (PP) [59]	3 M Potassium glycinate	14% CO2/air	298.15	86.29
PEI [60]	-	CO2	298.15	60.32
PEI-fSiO2 [60,61]	2 M Sodium taurinate	CO2	298.15	83.40
PEEK [62]	1.64 M activated K2CO3	13:87 CO2:N2	37–57	87.40
PEEK [63]	2.93 M DEA	13:87 CO2:N2	37–57	93.10

lists the performance of in-use membranes at different temperatures and varying absorbents.

4. Hydrate-Based Separation

Hydrate-based CO₂ technology is gaining traction as a promising technique. The primary chemical used in this approach is water, which is injected into the target gas system. The hydrate-based system is highly favorable in contrast to the described options due to the absence of chemical absorbents, which considerably benefits economic considerations [64]. Before learning the fundamentals of hydrate-based CO₂ capture (HBCC) technology, it is critical to understand that a gas hydrate is a clathrate water structure with gas molecules such as CH₄, CO₂, N₂, and H₂ captured in a solid cage structure of the water molecules. Weak van der Waal forces are involved to intact the molecules of gas and water. The lower hydrate-production pressure of CO₂, in contrast with other components mixed with CO₂, makes hydrate formation easier [65]. Even though a substantial progress has been made in improving the rate of hydrate formation, commercial scale process has not developed yet [66]. Xia et al. [67] studied the structural characteristics and occupational behavior of the hydrate formed to capture CO₂ from a biogas mixture using Raman spectroscopy. The group used THF + DMSO and TBAT + DMSO systems to facilitate CO₂ hydrate formation. It was observed that the presence of DMSO promoted the hydrate formation, increasing the selectivity of CO₂. Raman spectroscopy further revealed that the simulated biogas formed structure II hydrates and semiclathrate frameworks with THF + DMSO and TBAB + DMSO systems. Figure 5 gives information about the hydrate-based gas separation process. The optimum temperature and pressure conditions are attained to form hydrate crystals capturing the selective gas molecules in the system. For example, out of feed gas mixture (A + B), B is preferentially occupying the hydrate cages and can be separated as solid hydrate crystals while B component can be recovered from the gas phase.

Moreover, Figure 6 lists different gas systems, which can be separated by hydrates-based technology. It further signifies that CO₂ separation is performed for precombustion (CO₂/H₂), postcombustion (CO₂/N₂), and upgraded natural gas streams consisting of methane and a CO₂ mixture [68]. Postcombustion carbon capture performs CO₂ separation from flue gas consisting of N₂ and O₂. The flue gases undergo subsequent compression and cooling to facilitate the formation of hydrates and further separation of the gas mixture.

Nevertheless, studies on thermodynamic additives (e.g., tetrahydrofuran) have shown a practical impact on CO₂ hydrates formation in CO₂ + N₂ systems, making gas separation more efficient [69]. Precombustion CO₂ capture separates CO₂ from the fuel–gas mixture consisting of H₂. This process removes CO₂ from the fuel–gas mixture comprising CO₂ (40%) and H₂ (60%). Typically, the fuel–gas is generated at elevated pressures (on the order of 2–7 MPa), which can be directly relevant for hydrate formation after introducing a cooling process. Babu et al. thoroughly discussed hydrate-based separation processes for capturing CO₂ from fuel–gases. They studied different reactor combinations and different kinetic and thermodynamic promoters to assess the overall economic feasibility of the method [70]. They concluded that hydrate based gas separation (HBGS) technology was the most environmentally friendly gas separation amongst all other techniques since it uses water as a solvent. Further, the ease of performing at any high-pressure container makes its industrial application feasible. They also concluded that the HBGS method resulted in higher gas-storage capacity than any existing gas separation method.

Hydrate separation of CO₂ begins with creating hydrates by exposing the CO₂-containing exhaust-gas stream to water at high pressure. The production of the hydrate allows for the collection of CO₂. The resulting hydrate is further separated, producing pure CO₂. Hydrates offer enormous potential for capturing and storing significant amounts of gas. More studies and developments on this unique technology are expected shortly, providing an improved option for CO₂ sequestration [4,13]. Yang et al. [71] studied the impacts of reaction time, additives, and pressure on hydrate formation. They also investigated the effect of introducing various gases and additives while using several cycles on the continuous commercial hydrate-based gas separation systems. Li et al. [72] studied the continual hydrate-based gas separation process while introducing a novel and more accessible multiphase isothermal flash-computation approach.

Moreover, gas hydrates have also been considered a promising solution for the geological storage of CO₂ due to their high stability and the sizeable gas-holding capacity in hydrate structures [4]. The impact of additives on hydrate formation is significant in applying gas hydrate technology in gas separation and storage processes. Several researchers have studied thermodynamic and kinetic promoters to enhance the hydrate formation rate. Arora and colleagues studied the impact of SDS and zeolite on CO₂ hydrate formation kinetics in two fixed-bed silica. It was observed that SDS had promoted effects on hydrate formation rate, whereas zeolites had more delayed hydrate-formation kinetics than SDS. The gas intake per mole of water for the SDS system was very high compared to zeolites [73]. Kumar et al. [74] studied the impact of fly-ash contaminants on hydrates-based gas separation for CO₂ capture from a flue gas mixture in the presence of THF, SDS, and SDBS promoters. It was observed that the presence of fly-ash did not impact the phase equilibrium conditions for hydrate formation while enhanced the efficacy of the HBGS process by reducing the induction time and elevating the formation kinetics. Kumar et al. [75] studied the impact of SDS and THF in a CO₂/N₂/SO₂ gas mixture. The study observed that THF at a concentration of 5.56 mol% showed the maximum gas uptake. Veluswamy et al. [76] studied CO₂ hydrate formation using tetrahydrofuran (THF) and sodium dodecyl sulfate (SDS). They discovered that the gas uptake in the CO₂ system with 0.6 wt. % SDS at 1 °C showed the best results as compared to the THF system. They also concluded that the guest gas particles played an important role in affecting the activity of the thermodynamic promoter (THF) in similar operating conditions and driving forces. However, the use of THF and SDS has significant environmental issues, which enhances the need to explore biobased additives by manifolds.

Chemical additives show significant results in CO₂ separation using gas hydrates technology however, most of them are harmful to the environment. Developing environmentally friendly bioadditives for efficient hydrate formation requires extensive scientific research. Employing bioadditives in the selective separation of CO₂ has been studied by several researchers over time. In this direction, Amino acids have demonstrated the significant improvement in the hydrate formation kinetics. Liu et al. [77] compared the CO₂ hydrate formation kinetics of four amino acid solutions, L-norleucine, L-leucine, L-tertleucine, and L-isoleucine. Their study concluded that L-isoleucine showed significant results in CO₂ hydrate-formation kinetics with an increased gravimetric capacity. A recent study conducted by Singh et al. [78] investigated the kinetics of methane and carbon dioxide hydrates formation in the presence of biobased additives such as methionine and chitosan at low concentrations in stirred and unstirred conditions. They concluded that both methionine and chitosan behaved as kinetic promoters of CO₂ hydrate formation. They also observed that the additives had a good effect on the hydrate-dissociation kinetics with an increase in their concentration. Investigation of the hydrate-formation rate in the presence of amino acids found in different plant extracts requires more scientific interference for finding environmentally benign solutions for CO₂ hydrate formation. Bioadditives are generally investigated to be effective at low concentrations, which make their application industrially viable and environmentally benign.

5. Cryogenic Distillation

The cryogenic distillation method is a nontraditional distillation process, where a column is employed to separate the liquid components. The cryogenic distillation method is used to efficiently remove gaseous components within the mixture based on the basic concept of separation based on boiling point difference. It operates at low temperatures and high pressure. However, it depends on the liquid-mole ratio in a cryogenic distillation column [2]. The liquid-mole fraction determines the conditions for generating solid CO₂ in a cryogenic distillation column [79]. The distillation occurs at exceedingly low temperatures and high pressures and is an energy-intensive method that costs 600–660 kWh/ton of CO₂ retrieved in the liquid state. However, the system’s efficiency ranges from 90–95% [80]. While the fractional distillation column in the normal process separates the components of a liquid, the cryogenic distillation method is utilized to effectively separate gaseous components in the mixture based on the basic principle of separation, which is based on the phase equilibrium conditions. It finds vast commercial applications for CO₂ separation from gas streams having comparatively higher CO₂ concentrations (more than 50%) [2,13]. Figure 7 shows the impact of CO₂ concentration on the binary CH₄-CO₂ system’s P-T phase envelope. Altering the CO₂ levels in a gaseous mixture changes the bubble and dew points due to varying temperature and pressure conditions. As seen from the P-T phase envelope, every mixture with a certain CO₂ level has a unique critical point with various pressures and temperature circumstances. The temperature of the dew and bubble points at constant pressure reduces as the CH₄ content in the binary mixture increases, and vice versa.

Moreover, the CO₂ freezing lines are also impacted by the component shift [81]. Figure 8 demonstrates a process-flow diagram of the cryogenic CO₂ separation method. The most promising benefit of cryogenic CO₂ capture is that the process can operate without any chemical absorbent. Cryogenic distillation also finds wide application in the removal of CO₂ from high-pressure gases [13].

Figure 9 represents the block-flow diagram of the cryogenic CO₂ capture process, dividing the procedure into two parallel processes. The method converts liquefied natural gas into gaseous-phase natural gas, simultaneously segregating air into different components. The technique separates the components at different pressure and temperature conditions. The mechanism for the process of CO₂ separation can be summarized as follows:

First, the flue gas containing CO₂ is cooled to a desublimation temperature (−100 to −350 °C). After solidifying the CO₂, it is separated from the other light-gas components. Further, solid CO₂ is subjected to high-pressure conditions (100–200 atm). Through this process, CO₂ recovery can reach up to 99.9% of the flue gas [2].

High product purity is the key benefit of cryogenic CO₂ collection processes and it does not need any organic solvents, which makes the cryogenic CO₂ capture technique a green technology. However, the gaseous mixture must undergo an appropriate thermodynamic phase analysis before designing a cryogenic CO₂ collection technique.

6. Biological CO₂ Capture/Separation

The enormous upsurges in the world population and urbanization have increased the need for energy consumption, eventually leading to a significant boost in CO₂ emissions. The CO₂ emission has risen by 0.85 °C since 1880, and it has been predicted that an increase of 1.4–5.8 °C would be experienced by the end of the decade [84]. The primary requirement is to lower CO₂ emissions, which can be achieved by using fewer fossil fuels and increasing carbon capture and sequestration [4,65]. Bio-CCU, i.e., biologically mediated CO₂ capture and utilization, is found to be one of the promising biotechnologies to decrease CO₂ emissions considerably [85]. Photosynthetic and non-photosynthetic methods of CO₂ fixing are the two primary biological methods. Microalgae cultivation, afforestation, and ocean fertilization are photosynthesis-based techniques that fix CO₂ in the sunshine and transform it into valuable goods. Contrarily, non-photosynthetic processes repair CO₂ in the absence of sunshine by using biological systems. The agronomy of algal biomass, whether cyanobacteria (prokaryotic) or eukaryotic (microalgae) microorganisms are superior to other photosynthetic methods. A technique with considerable potential, CO₂ fixation by microalgae converts water and CO₂ into organic molecules without additional energy, causing no pollution due to biological processes for CO₂ capture. Microalgal CO₂ fixation depends on nutrient availability, temperature, pH, and light conditions during cultivation, and the CO₂ concentration, pollutant gases, and the type of algae. Microalgae biomass is made up of lipids, proteins, and carbs. Algal biomass protein has already been used to improve animal diets. Compared to typical crops such as canola, soybean, or a corner acre of land, the microalgae yields are nearly 30 to 50 times higher. Algae are grown in ponds and bioreactors with large surfaces that maximize sun exposure, resulting in a land- and water-intensive process. Whereas non-photosynthetic processes yield large amounts of biomass, their life-cycle analyses are subpar. The main advantage is these biological systems’ adaptability to different settings and feedstock. Additionally, they can withstand low CO₂ concentrations and contaminants (such as NOx and SOx) in the carbon products frequently present in industrial releases. These factors significantly influence the capital and operating costs of biological-conversion technologies. Due to the high feedstock cost, such as sugars, and low output yields, large-scale commercialization of microbial synthesis has always been a challenge. Contrarily, CO₂ is used by photosynthetic algae and cyanobacteria, reducing the feedstock cost, although they grow slowly compared to industrial-level production. Along with the aforementioned biological pathways, additional pathways have been created for significant CO₂ fixation from the environment, including farming crops and cattle, valorizing biomass, afforestation, and marine fertilization. These strategies all use photosynthetic cells in their operations. The following sections contain a thorough examination of the benefits and drawbacks of each biological technique [86,87,88,89,90]. Samipour et al. [91] studied the drawbacks of biological-process implementation in the industrial sector. They segregated the problems into two distinct categories: biological and process-related challenges. They concluded that these issues comprised ineffective natural processes and energy sources. The sensitivity of the process and tight control conditions are also some drawbacks of the system. They suggested artificial biology-guided metabolic engineering from a holistic perspective to produce successful strains and an efficient method of related streamlined operations with optimal operating regulations and procedures. Optimized conditions that mimic natural circumstances are essential for developing well-organized techniques to industrialize CO₂ biotransformation and biofixation. Figure 10 depicts the microalgae agronomy with CO₂ consumption and their uses as biofuel. The flowchart shows the treatment of byproducts generated from postcombustion processes with water and nutrients and later used for microalgae cultivation. The microalgal biomass can be used further for different processes and as a feedstock for the postcombustion CO₂ capture method, continuing the cycle.

Wan et al. [93] experimented with very-high selectivity and permeability GO/MILM, which was catalyzed biogenetically. The membrane possesses moderate viscosity and hence is effective in CO₂ separation. Biological CO₂ separation may also inculcate the use of biopolymers produced by living organisms or from natural sources. They have properties such as biodegradability, composting, and environmental sustainability, which provide them with tremendous usability in the gas separation processes [94].

7. Comparison of Various CO₂ Capture/Separation Process

Table 4. Comparison of different parameters of CO₂-separation processes [2,3,5,6,7,8,9,16,83,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119].

Parameter	Chemical Absorption [2,5,100,102,105]	Physical Absorption [2,5,16,100,102,105]	Adsorption [3,5,6,7,46,101,102,112]	Chemical Looping [8,9,83,102,103,108,114,118,119]	Hydrate-Based Separation [107,109,110,111,117]	Membrane [98,99,102,115,116]	Cryogenic [5,102,104,113]	Biological CO2 Separation [95,96,97,102,103,104]
Technique	Amine, Chilled Ammonia, and Amino Acid Salt Solvent	Rectisol, Selexol, Etc. Integrated Gasification Combined Cycle	Pressure- Temperature Swing adsorption	FeO, CuO, MnO and NiO	Natural gas hydrate cage structures	Polymeric inorganic and mixed membrane	Cryogenic distillation	Microbial and algal CO2 separation agents
CO2 Concentration	<30.4	>59.3	28–34	3–8	35	11.8	<90	3–5
CO2 Capture (%)	95	>90	<85	52–60	99	90	99.9	30
Cost ($/Ton)	26.2	25.1	6.94	16–24	40–100	3–10	32.7	1–40
Purity	99	<99	99.98	>96	99	95	99.95	-

enlists different CO₂-separation methods based on their CO₂-capture efficiency, cost-per-unit CO₂ removal, feed-stream concentration, and purity of the CO₂ stream collected. Techniques such as chemical looping and biological CO₂ separation require a shallow CO₂-concentration feed stream as the input. Adsorption provides the purest CO₂ stream, as compared to the other methods. Membrane technology shows the best results in terms of cost and efficiency. Further research with membranes in focus could undoubtedly produce a long-lasting, economical, and efficient solution to CO₂ separation issues existing in the world’s energy sector.

Moreover, Figure 11 demonstrates the technology readiness level (TRL) of different CO₂ separation processes. Researchers have studied the techno-economic aspects of different CO₂ segregation methods and classified them based on the lab scale or/and industrial application [120].

Several studies have been conducted on CO₂ separation over the years, addressing the challenges in the existing technologies. Kenarsari et al. [121] reviewed the existing CO₂ separation technologies such as absorption, adsorption, membrane separation, chemical looping, and cryogenic separation. Addressing the benefits and drawbacks of these processes, they concluded that implementing any particular technology could not effectively resolve the CO₂ separation and capture challenges. Further, they suggested that combining the available technologies would provide an effective CO₂ separation method for industrial application. Zoccali et al. [122], in their review, discussed the recent advances in column technology and the hardware required for online coupling of supercritical fluid extraction (SFE) and chromatograph (SFC). Their study explained the advantages of employing supercritical fluids for the extraction and separation techniques and their benefits over conventional solvent-based methods. Moreover, they highlight that SFE possesses a high selectivity and easy abstraction of residual CO₂ after fluid-extraction processes. Further, the elimination of expanded CO₂ gas could be achieved after the depressurization of the system. Yang et al. [123] discussed several essential CO₂-separation techniques, including absorption, adsorption, membrane technique, and hydrate-based and chemical-looping processes. They also discussed the different CO₂ storage methods such as forestation, mineral carbonation, and ocean fertilization techniques. They suggested that membrane application for CO₂ separation is an environmentally benign, cost-effective, and low space-consuming process which could achieve high selectivity and stability by employing mixed-matrix membranes instead of polymer membranes. Furthermore, they also suggested that the sequestration of CO₂ through forestation and ocean fertilization could help to delay or prevent unwanted atmospheric CO₂ release.

Overall, this paper conducts a detailed review of the existing CO₂-separation technologies, emphasizing hydrate-based technology. Further, this paper provides a table addressing parameters such as technique, cost, and efficiency for different separation processes. The paper also addresses the technology-readiness level of different methods, broadly segregating them into different categories.

8. Future Scopes

The ever-growing technological advancements in CCUS techniques are being widely studied by researchers all over the globe. Developments in CO₂ separation techniques are obligatory to increase their efficacy. Nevertheless, certain challenges in the separation techniques limit their application in industrial sectors. Economic constraints play a significant role in employing CCUS techniques, providing scope for examining cost-effective methods for CO₂ separation. Herein, we suggested the following research opportunities in the sector of CO₂ separation techniques:

• Environmentally benign substitutes need to be developed for adsorption and absorption process;
• The use of biopolymers in gas-separation membranes requires extensive research;
• Process-related challenges need to be overcome to enhance the use of biological CO₂ separation techniques at industrial levels.

In addition to the suggestions made here, several other challenges in the CCS require scientific attention for further deployment, such as:

• Identify suitable methods to optimize the energy consumption of different CO₂ capture/separation processes;
• Longevity of the CO₂ source (for example, cement plants, power plants, and steel);
• Distance between the CO₂ generator and the storage facility;
• Onshore versus offshore storage and transportation;
• Transportation infrastructure type (pipeline trunks, road and rail networks, and cargo dock facilities).

9. Conclusions

The emission of greenhouse gases (GHGs) has become an extensive issue in the fast-growing world. The release of GHGs from municipal solid waste, industrial activities, deforestation, and fossil-fuel burning has resulted in the growing global-warming potential. Capturing the produced CO₂ would not only help to reduce GHGs in the atmosphere but also provide a significant carbon dioxide source for industrial applications. Selecting the best carbon-capture technology is a complex topic as a variety of technological and economic aspects should be considered, including feed-gas compositions and parameters, the desired purity of the processed gas, facility costs (investment and maintenance), possible economic experience, process dependability, and operational flexibility. On the other hand, the vast number of active CO₂ capture installations and the relatively simple operation mechanisms of individual units allow for the formulation of broad criteria for selecting the right way to capture CO₂. The technical issues associated with CO₂ separation are well recognized and, in most cases, adequately solved. Identifying processes for optimized energy consumption plays an important role in deciding the most appropriate CO₂ separation methods from different sources. Zach et al. [124] conducted a study to suggest a user-friendly optimization tool for enhancing the performance of membrane-based CO₂ separation at the lowest possible energy expense. They derived a mathematical co-relation which was verified using a hypothetical waste-to-energy system to produce desirable results. Similar models must be identified and/or derived for emerging technologies such as hydrate-based separation to enhance their efficiency and optimize the technology. This paper reviews different CO₂ separation techniques and their advantages and disadvantages to determine the best technique for CO₂ separation. It has been observed that adsorption remains the most efficient technology for CO₂ separation. However, developing membrane applications and gas hydrates technology for gas separation using natural polymers and environmentally friendly additives, respectively could reduce the cost of these two processes.

It is essential to identify the most feasible method for CO₂ capture to implement CCUS effectively in different industrial sectors. This research suggests that gas hydrate based techniques would be one of the most feasible approaches for efficient CO₂ capture/ separation and storage. Scientific investigation of biological CO₂ capture, such as artificial photosynthesis, is also suggested a promising approach. The increasing global temperature must be controlled to save the earth and the human race.