The presence of water vapor in gas streams is a significant technical issue for restricting the large-scale development of carbon capture. The polarity of H2O often leads to the decrease or even failure of CO2 capture rate of adsorbents. In addition, it also causes parasitic losses such as temperature and pressure drop to the system, and even causes equipment corrosion and adsorbent poisoning, thus greatly increasing the extra energy consumption and cost. In order to solve the above bottleneck, understanding the mechanism of H2O/CO2 co-adsorption and developing the highly efficient CO2 adsorbent with reasonable cost, low regeneration energy consumption and insensitivity to H2O are the important basis for the realization of effective CO2 adsorption capture under wet gas streams. At present, due to the dispersion in multiple fields and different emphasis points, there is a lack of summary on the mechanism analysis of the influence of H2O on CO2 adsorption, and it is difficult to form a relatively unified view. In this paper, the co-adsorption process of CO2 and H2O are reviewed in detail from the macro and micro levels. Firstly, according to the fundamental research of co-adsorption mechanism, the progress in the fields of competitive adsorption, moisture swing adsorption and “breathing effect” are reviewed and briefly evaluated. Secondly, based on the application research of co-adsorption, the status and progress of adsorbent development and technology improvement of wet gas CO2 adsorption are described. Furthermore, the CO2 adsorption capture level under different wet gas sources is also briefly evaluated. Finally, the shortcomings of the current research are summarized and the future directions are prospected. This paper attempts to summarize, analyze and compare the CO2/H2O co-adsorption processes in various fields, which may provide effective guidance for CO2 adsorption capture in wet gas source.
Contents
1 Introduction
2 Fundamental research on CO2/H2O co-adsorption mechanism
2.1 Competitive adsorption
2.2 Moisture swing adsorption
2.3 Breathing effect
2.4 Evaluation of CO2/H2O co-adsorption mechanism
3 Application research on CO2/H2O co-adsorption mechanism
3.1 Adsorbents for CO2adsorption capture in wet gas
3.2 Technology for CO2adsorption capture in wet gas
3.3 Performance evaluation of CO2adsorption capture in wet gas
Jie Zhao, Shuai Deng, Li Zhao, Ruikai Zhao.
CO2 Adsorption Capture in Wet Gas Source: CO2/H2O Co-Adsorption Mechanism and Application.
Progress in Chemistry, 2022, 34(3): 643-664 DOI:10.7536/PC210215
In order to achieve the goal of controlling the global average temperature rise within 1.5 degrees Celsius in 2050, active intervention in carbon emissions is one of the necessary measures in the future[1,2]. From the current research, Carbon capture and storage (CCS) technology is still a hot issue of Carbon reduction[3]. CCS can be used as an intermediate medium in the transition from fossil fuels to renewable energy to solve the problem of CO2 emissions in the fossil fuel era, while providing breathing space for the development of renewable technologies. Traditional CO2 capture technologies, such as solvent absorption, membrane separation, adsorption cycle and so on, are combined with fixed carbon emission sources such as power plants and cement plants to separate and capture CO2[4][5][6]. Different from this, Direct air capture (DAC) technology is flexible and convenient, and is not limited by space and time, which can complement the advantages of fixed carbon source carbon capture technology and accelerate the decarbonization process through "negative carbon"[7].
One of the keys to the high efficiency of adsorption carbon capture technology is the adsorbent. In previous studies, some porous materials, such as zeolites, Metal-organic frameworks (MOFs), amino or alkali metal solids, have shown high CO2/N2 selectivity, CO2 absorption rate and good stability[8][9][10]. However, the screening or characterization environment of existing adsorbents is often limited to dry environment, and the applicability of wet environment is not fully investigated, while the stability of adsorbents usually changes dramatically in wet environment, resulting in the change of adsorption performance of CO2. Humidity changes caused by water vapor are widespread in many industrial gas streams, such as industrial flue gas, natural gas, biofuel gas, etc. See Figure 1 for a summary of the research literature on carbon capture with water vapor[11][12][13].
Fig. 2 shows the results of adsorption of CO2 and H2O in 14 different materials, including metal-organic frameworks, zeolites, mesoporous silica, and activated carbon, under coal-fired power plant flue gas conditions[14]. Blue represents the adsorption amount of the H2O in the multi-component, and red represents the adsorption amount of the CO2 in the multi-component CO2, the N2, and the H2O.Green represents the amount of CO2 adsorbed in the absence of H2O and N2 under the same pressure conditions. It can be seen from the figure that most of the adsorption materials with metal cations or anions are easy to adsorb a large number of H2O, and will affect the adsorption of CO2. In view of the unavoidable problem that industrial gas stream contains water vapor, the common solution is to use drying equipment to pre-remove water before the waste gas passes through the adsorption bed[15,16]. Because of the large amount of waste moisture to be treated, the investment in carbon capture equipment is increased, and the dehydration energy consumption of the dehydration unit often causes additional operating costs.
Air direct carbon capture technology has initially been shown to be effective in separating CO2 from humid atmospheric environments, however, due to the lower concentration of CO2 in the atmosphere (400 ppm), this requires a higher concentration than H2O.The affinity and binding energy of adsorbents for CO2 need to be stronger in order to ensure the preferential enrichment of CO2 in the presence of water, which results in challenges such as high cost and high regeneration energy consumption of direct air capture technology[17][18].
It has also been shown that the presence of water vapor promotes the separation of gas mixtures. For example, due to the clustering of H2O around the unsaturated metal sites of CuBTC adsorption materials, the interaction between metal ion adsorption sites and CO2 is enhanced, and the phenomenon of water promoting CO2 adsorption occurs under low pressure conditions[19]. Based on the existing literature, it is found that the adsorption of water on CO2 has both "positive" and "negative" effects. Therefore, how to reduce the inhibition of water and enhance the effective driving of water in different carbon capture scenarios needs to be fully understood and grasped from the macro and micro mechanisms. In this paper, the research progress of co-adsorption of CO2 and H2O in different carbon capture scenarios in recent years will be summarized and summarized from three aspects: adsorption mechanism, material research and development, and improvement of carbon capture process.The advantages and common problems in the current research were put forward, and the prospects for deepening the understanding of the mechanism of CO2/H2O co-adsorption process from the perspective of thermodynamics were put forward.
2 Basic study on co-adsorption mechanism of CO2/H2O;
Due to the different physical and chemical characteristics of adsorbents, the mechanism of co-adsorption of CO2 and H2O is also different. For the adsorption materials with physical adsorption as the main way, the H2O molecules usually compete with the CO2 molecules. For the adsorption materials based on chemical adsorption, enhancing the reactivity of CO2 and basic groups is an important way to improve the adsorption performance of CO2, which is widely used in the field of variable humidity adsorption. In the existing literature, it is found that there is a special class of flexible porous materials, which can selectively adsorb mixed components by using its unique "breathing (opening) effect". Based on this, this section will review the co-adsorption mechanism from three areas: competitive adsorption, wetting change adsorption and "respiration effect" of flexible materials, and finally summarize the thermodynamic process of CO2/H2O co-adsorption based on thermodynamic principles.
2.1 Competitive adsorption
Competitive adsorption refers to the competitive phenomenon of multi-component adsorption on the surface of adsorbent[20]. When the two components compete for the same active center, the interaction between the component and the adsorbent determines the adsorption capacity. Generally, the larger the adsorption capacity, the stronger the interaction between the component and the adsorbent. When the two active sites are different, the competitive adsorption usually shows that after one component is adsorbed, the other component can not be adsorbed again. For the adsorption materials with physical adsorption, the inhibition of CO2 adsorption by H2O is a more serious problem than that of competing gas molecules with higher concentrations.
From the point of view of physical and chemical characteristics, the distribution of ions or functional groups on the surface of porous solid materials is complex, and adsorbate molecules usually show energy heterogeneity in the process of surface migration, which is more obvious in polar adsorbate molecules[21]. Therefore, when the polar H2O molecule exists in the adsorption layer, the strong interaction between the adsorbent and it, i.e., the hydrophilicity of the adsorbent surface, needs special consideration. It can be seen from Table 1 that the dipole moment and quadrupole moment of CO2 and H2O are obviously different, and the dipole moment of CO2,H2O molecules is the main factor for strong interaction with the adsorption material.After hydration, H2O molecules tend to form a hydrogen bond network, which can separate the cations present in the adsorption material from the framework and stabilize them in the pore center, preventing CO2 from adsorbing on the active sites.
Qiao et al. from the National University of Singapore used Monte Carlo simulation to study the adsorption behavior of CO2 on zeolite 13X in wet flue gas[21]. The structural analysis of the radial distribution function of Figure 3A shows that the H2O molecule is able to force the CO2 molecule to transfer from the Na+ active site of the framework structure to the α-pore center, while enhancing its interaction with the Na+ site. This result is consistent with the simulation result of Smit et al. Of the University of California, Berkeley: it can be seen in Figure 3B that with the increase of H2O molecules, the distance between the cation Na+ in the zeolite and the O atom in the H2O decreases from 3.55 Å to 2.7676 Å, and the distance between the hydrogen atoms in the H2O-H2O decreases continuously, which strengthens the interaction between them, thus forming a hydrogen bond network structure[22]. However, when there are independent CO2 binding sites and H2O binding sites in the pores of the adsorption material, and there is no overlap, the adsorption performance of the CO2 may be enhanced due to the presence of H2O molecules.
The pore structure of the adsorbent has an important effect on the adsorption. Both BET specific surface area and pore volume can reflect the pore structure of the adsorbent. Generally speaking, the adsorption capacity of micropores is stronger than that of mesopores. Li et al. screened 15 MOFs materials with low pore size (< 10 10 Å) by high-throughput for GCMC simulation under wet flue gas conditions after combustion, and found that MOFs materials with high CO2/H2O adsorption selectivity usually have small pore structure (4.5 ~ 5.77 Å) and low porosity, because the van der Waals interaction between narrow pores and CO2 is stronger than that of H2O[70]. In addition, the H2O molecules can move freely in the weak interaction environment, and have little effect on the adsorption of CO2. When the interaction between H2O and adsorbent is strong enough to make it adsorbed on the surface, the adsorption of CO2 depends on the adsorption strength of H2O on MOFs.
Jeong et al. Divided the zeolite into three types of structures: the binding sites of :(1)CO2 and H2O are clearly separated (site distance greater than 3 3 Å, SAS structure), so the adsorption of CO2 and H2O molecules occurs in different channels[23]; The binding sites of (2)CO2 and H2O overlap each other (site distance less than 1 1 Å, AWW structure), while the binding sites of (3)CO2 and H2O are close but not completely overlapped, and the adsorption occurs in the same pore (OWE structure). The screening results are shown in fig. 3C. In the OWE zeolite structure, H2O molecules tend to adsorb near the channel wall, while CO2 molecules tend to adsorb from the channel wall to the center of the channel or cage. Although the binding sites of CO2 and H2O are physically close in space, they are separated in energy. In order to distinguish the effects of Coulomb and Lennard-Jones (LJ) interactions between CO2 and H2O, they studied three different cases: (1) only LJ interaction; (2) Coulomb interaction only; (3) with both LJ and Coulomb interactions, the long-range Coulomb interaction between CO2 and H2O was found to be the main reason for the improved adsorption performance of CO2 (Fig. 3D, e).
According to the Brunauer-Emmett-Teller (BET) model of solid surface adsorption, the adsorption enthalpy of molecules is proportional to the polarity of adsorbents and the polarizability of adsorbed molecules, respectively, without considering the specific chemical bonds and only considering the van der Waals interaction between atoms, which reflects the interaction between adsorbents and adsorbates to some extent[24][25][26]. From the point of view of thermodynamics, the competitive adsorption behavior of CO2 and H2O can also be understood by the desorption enthalpy of the adsorbate. Purdue et al. Found that (40 kJ/mol),H2O molecules have higher desorption enthalpy (up to 78 kJ/mol) than CO2 molecules in the adsorption simulation of mixed components in the range of 273 ~ 373 K, so the thermodynamic driving force is greater[21].
Li et al. Proposed an extended Langmuir-BET (LBET) adsorption model, in which the competitive effect between CO2 and H2O on the adsorbent interface was analyzed, and the synergistic relationship between CO2 and H2O molecules was also considered[27]. The model assumes that the easily adsorbed H2O molecules form a water layer as a secondary adsorption site, and further adsorb CO2 through hydrogen bonding interactions and hydration reactions. The model is in good agreement with the experimental data of coadsorption of CO2/H2O binary components on activated alumina, and the adsorption of CO2 decreases slightly at lower relative humidity. When the relative humidity is high, because the synergistic effect of H2O and CO2 is enhanced, it is considered that the CO2 and the H2O molecular layer of the secondary adsorption site form a complex. In addition, this phenomenon can also be seen from the change trend of adsorption heat, the adsorption heat of pure component H2O is 69. 58 kJ/mol, and the adsorption heat of H2O molecular layer forming secondary adsorption sites is reduced to 43. 3 kJ/mol. This value is basically consistent with the liquefaction heat of H2O (44. 2 kJ/mol), in which the H2O molecules exist in the form of polymers, while the CO2 molecules are adsorbed on the H2O molecular layer with a lower adsorption heat (16. 6 kJ/mol, 26 kJ/mol for pure component CO2)[28]. The competitive adsorption between CO2 and H2O increased the adsorption heat of CO2, while the synergistic effect decreased the adsorption heat of CO2.
In fact, the essence of the adsorption principle is that the surface atoms of condensed matter have the tendency to adsorb foreign ions or molecules to reduce their surface free energy due to the existence of dangling bonds on the surface, and at room temperature and pressure, the adsorption Gibbs free energy should be a thermodynamic quantity that directly reflects the strength of the adsorption tendency[29]. According to Helmholtz equation, Gibbs free energy is the result of enthalpy and entropy[30]. The enthalpy of adsorption reflects the thermal effect of the adsorbate in the adsorption process, while the entropy of adsorption indicates the spontaneous transition of the molecule from a disordered state in the bulk phase to an ordered state in the adsorption phase[31]. This means that attention should be paid not only to the adsorption enthalpy change, but also to the adsorption entropy change when analyzing the competitive adsorption process.
Joos et al. Used GCMC simulation and DFT calculation to analyze the competitive adsorption behavior of CO2 and H2O on silicalite, and found that the adsorption entropy difference between the adsorbed molecules and desorbed molecules was the main driving force to realize the adsorption of CO2 at high temperature and desorption at low temperature[32]. This method broadens the research scope of multicomponent competitive adsorption thermodynamics and is of great significance. However, the deficiency of this study is that the calculation process assumes that the adsorption enthalpy of CO2 and H2O remains unchanged under the simulated temperature and pressure conditions, so there are some ideals and limitations. The change of the thermodynamic state point of the adsorption process should be paid attention to in the subsequent calculation.
Table 2 summarizes the research status of the competitive adsorption process of CO2 and H2O. In view of the influence of H2O on the adsorption of CO2, the existing research is mainly based on experiments, focusing on the capture rate of CO2 and energy consumption. The co-adsorption mechanism of CO2 and H2O was mainly based on molecular simulation, involving adsorption selectivity, adsorption enthalpy change and other evaluation indicators. There is a lack of basic thermodynamic data, such as adsorption Gibbs free energy, on the coadsorption mechanism of CO2/H2O, and it is difficult to form a generalized view.
2.2 Hygroscopic adsorption
Different from the traditional adsorption separation technology, the humidity change adsorption technology is to balance the adsorbent -CO2 system by adjusting the environmental humidity, and finally complete the adsorption separation of CO2 by adsorption-desorption-regeneration cycle. The concept of humidifying adsorption separation of CO2 was first proposed by Lackner of Columbia University. They used a quaternary ammonium (CH3)4N+) ion resin to study the mechanism of humidifying adsorption process[42]. The quaternary ammonium group has higher binding energy to the CO2, and the influence of the interfacial H2O on the adsorption capability of the CO2 is controllable. Therefore, the binding capacity between the adsorbent surface and the CO2 can be directly regulated and controlled by adjusting the change of environmental humidity (water vapor partial pressure), so as to realize the adsorption-desorption process of the CO2 gas. The humidity-change adsorption process mainly includes: (1) When the environment is dry or the humidity is very low, the basic groups on the surface of the adsorbent preferentially adsorb the CO2;(2); when the environment is at a higher humidity or the interface hydration degree is high, the CO2 will be gradually released; And (3) that adsorbent can be recover and regenerated aft being dried, so that the adsorption performance of the CO2 can be obtained circularly, and the desorb CO2 can be directly utilized or compressed and transported to a burial site.
The thermodynamic cycle process of humidification adsorption is analyzed from the thermodynamic point of view, as shown in Figure 4. The whole adsorption-desorption-regeneration cycle is divided into four States: (1) state 1 (Empty-Dry), dry ionic resin, CO2 unadsorbed; (2) state 2 (Full-Dry), Dry ionic resin, CO2 has been adsorbed; (3) state 3 (Full-Wet), Wet ionic resin, the CO2 forms a with the group; (4) state 4 (Empty-Wet), Wet ionic resin, the CO2 is released. After the state 4, the wet ionic resin which does not adsorb the CO2 is exposed to the air and can reach the initial state again, thereby completing the closure of the whole cycle. Lackner et al. pointed out that States 1-2, 2-3, and 3-4 are all spontaneous processes, so the free energy change is negative. Under the assumption of ideal gas, the free energy required for the adsorption and separation of 1 mol CO2 from air is:
The Pin is the concentration of the inlet CO2, and the (400 ppm),Pout is to filter out part of the CO2.The concentration of CO2 at the rear outlet (less than 400 ppm),P0 is the ambient pressure. The free energy provided by evaporation of 1 mol of water is:
Where R is the relative humidity in a dry environment. It can be known that the number of H2O molecules required to separate one CO2 molecule from the air is:
The method proves that the free energy provided by the evaporation of the H2O in the humidity change adsorption process can meet the free energy required for adsorbing and separating the CO2. Lackner et al. Considered the irreversibility of the actual adsorption separation process, and the lowest energy consumption of the more realistic air capture is 30 kJ/mol CO2, and its thermodynamic efficiency can exceed 90%. Compared with the traditional post-combustion flue gas capture technology (thermodynamic efficiency of 45%), this technology can theoretically offset the higher part of the energy consumption.
Based on anion exchange resin, Wang et al. analyzed the adsorption Gibbs free energy change in the process of variable humidity adsorption by establishing a thermodynamic model coupling the phase equilibrium and chemical equilibrium of the “H2O-CO2- resin "system (Figure The adsorption reaction free energy of 5a,b),CO2 is linearly related to the relative humidity of the interface, and the adsorption of CO2 leads to the H2O desorption of the interface."[43]. The results show that the humidifying regeneration behavior is directly related to the activity of the bound H2O on the adsorbent surface, and the activity of the bound H2O is determined by the ambient humidity. The moisture-change adsorption behavior can be understood as the desorption of one CO2 molecule requires the adsorption of multiple H2O molecules from the environment, and the H2O molecules are the driving force leading to the desorption of CO2, which is essentially different from the traditional competitive adsorption mechanism.
From the point of view of adsorption kinetics, the CO2 adsorbents based on basic groups have the following special properties: (1) chemisorption; (2) Interface-bound H2O participate in chemical reactions between anions; (3) The binding H2O has a great influence on the reaction equilibrium and kinetics. The humidity-change adsorption process can be described by the following reversible reaction equation:
In the above formula, under the dry condition, the reaction equilibrium shifts to the left side, and the CO2 exists in the form of ; When the system is wet, the equilibrium shifts to the right, where the products are more abundant than the reactants, and the anions prefer to exist in the form of . Therefore, the equilibrium constant of the adsorption process is greatly affected by humidity. When the humidity changes, in order to keep the whole system in balance, the four components OH-, , and H2O molecules in the adsorption material will react and diffuse. Based on the ion transport characteristics, Ge et al. Proposed a dynamic model of moisture-changing adsorption from the basic diffusion reaction and gave a numerical solution[44]. Then a humidity fluctuation frequency response technique (MSFR) was developed to conduct humidity fluctuation frequency response experiments, and the accuracy of the model was verified. The results showed that the interfacial binding H2O affected both the thermodynamic equilibrium and the kinetic reaction rate of the humidity-change adsorption process. The higher the humidity was, the greater the adsorption rate of the quaternary ammonium PILs adsorption CO2 was. The lower the humidity is, the larger the apparent adsorption rate constant is. This is due to the inorganic diffusion of CO2 molecules, and the high concentration of CO2 will have a significant impact on the dynamic characteristics of the adsorption of CO2 by quaternary ammonium PILs. They analyzed the microscopic mechanism of the wetting adsorption process by DFT, and the molecular structure of PILs is shown in Figure 6a[45]. For PILs, the location of the anion OH- is the preferential adsorption site of CO2. At the same time, with the decrease of the number of H2O around , the dissociation of H2O is stronger. The interaction between the CO2 and these ions may involve the following basic steps.
The DFT calculation results show the possible reaction pathways involved in the adsorption of CO2 on PILs with hydrated , as shown in fig. 6B. The product of anion can be formed through reaction path A (equations 4 and 7) or reaction path B (equations 5 ~ 8), where the activation energy of reaction 1 is as high as 179.83 kJ/mol, because the process involves the bending of bonds, and the linear CO2 is transformed into a Y-type O=C(OH)2. Reaction path B can be thought of as a process of proton transfer from the H2O to the ion, in which the H atom in the bound H2O is transferred to the O atom in the anion,The formed OH- can easily react with the CO2 (Formula 7), and the reaction activation energy is 34.66 kJ/mol CO2. Since the reaction in Equation 6 does not have any energy barrier, the total activation energy of reaction path B is much smaller than that of reaction path A, which indicates that reaction path B is the most favorable reaction path.
MIT Langer et al. Developed humidity gradient power generation technology based on the regulation of interfacial energy by polymer material interface combined with H2O[46]. In 2015, Columbia University developed a high energy density "artificial muscle" material based on bacterial spore materials that can sense humidity changes, which can use the evaporation energy of water to drive light sources to generate electricity[47]. Thus it can be seen that the interfacial hydration H2O has unique physical and chemical characteristics, which makes the humidifying adsorption technology play a huge role in the fields of energy, environment and life science.
Through the above research, it is found that the variable humidity adsorption technology plays an important role in carbon capture from wet gas sources, and in future research, it is necessary to continue to further explore the thermodynamic and kinetic models based on the adsorbent system.Used to elucidate the underlying mechanism, which will serve as a theoretical basis for other adsorbents to be used in similar studies and be used to guide the rational design of more adsorbent materials.
2.3 Breathing effect
The development and utilization of the response properties represented by the "respiration effect" is an important way to realize the functional application of adsorption materials[48]. "Respiration effect" refers to the phenomenon that the structure of adsorption materials will change significantly under external stimulation, which is mainly manifested by reversible changes in the diversity and size of pore structure. This special "breathing" phenomenon, also known as the opening effect, mostly occurs in flexible MOF materials, but also in special flexible zeolites[49]. This breathing behavior is related to the organic functional group of the material, and is also affected by the dipole moment interaction between the host and guest molecules[50]. Llewellyn et al. Designed an elastic MOF (fig. 7A) with a certain amount of crystalline H2O in the lattice gap and a relaxed unit cell structure[54]. Under the condition of heating or vacuum, the MOF will breathe dynamically, and the crystalline H2O in the lattice gap will be breathed out step by step, and the unit cell structure will shrink. When returned to room temperature and air conditions, the unit cell structure returned to the initial relaxed state containing H2O.
The flexible zeolite MER reported by Bassem et al. Of the University of St. Andrews has a flexible structure, which can coordinate the cations outside the framework during the removal of H2O, so that the structure is strongly distorted and distorted, thus realizing the change of pores from narrow to wide[55]. The structural change of the flexible zeolite mainly depends on the number and size of exchangeable cations, as well as the interaction between available lattice sites, adsorbed molecules and cations, and the framework. Flexible zeolite MER exhibits a rich gas adsorption behavior, which can be optimized by adjusting the structure. Fig. 7B shows the competitive adsorption behavior of CO2/CH4 in the K+ modified MER. When the structure is narrow pore form, the MER is non-porous to the pure gas CH4, and when a small amount of CO2 is adsorbed, the structure of the MER is changed into a wide pore form through "breathing", and the CH4 and the CO2 can be simultaneously adsorbed. Because of the stronger interaction between K+ and CO2, MER showed the adsorption selectivity of CO2.
Therefore, the "respiration effect" of flexible MOFs and zeolites can be used to screen the CO2 in high humidity flue gas. Snurr et al. studied the adsorption behavior of CO2/CH4/H2O mixed components in HKUST-1 by GCMC and DFT, as shown in Fig. 8[53]. Pre-adsorption of a small amount of H2O molecules in HKUST-1 can increase the CO2/CH4 adsorption selectivity. HKUST-1 has a Cu (II) metal-centered framework structure. Under appropriate activation conditions, H2O can change the affinity of HKUST-1 for adsorbates by proton transfer and exchange with unsaturated coordination sites . When the content of H2O is 4 wt%, the site of coordinative unsaturation of HKUST-1 is hydrated by the molecules of H2O, which leads to the widening of pore diameter, so the CO2 adsorbed by HKUST-1 is more than that in dry state. When the content of H2O increased to 8 wt%, the adsorption capacity of CO2 began to decrease due to the large number of H2O molecules, which blocked the free space within the pores of HKUST-1. This also explains the special phenomenon of some MOF materials reported in Section 2.1.1: under low pressure, a small amount of H2O will promote the adsorption of CO2, while under high pressure, H2O will inhibit the adsorption of CO2.
Respiratory behavior of MIL-53 was reported by Llewellyn et al[54]. The narrow pore of this zeolite is a special structural form caused by the hydrogen bonding interaction between the H atom in the H2O molecule and the O atom in the carboxyl group. After dehydration at high temperature, MIL-53 exhibits a macroporous structure due to the absence of interaction. It is observed that the hydrated MIL-53 is in the form of narrow pores when the CO2 pressure is lower than 5 bar, and the narrow pores and large pores coexist in the MOF when the pressure is increased. When the CO2 pressure reaches 15 bar, MIL-53 is all in large pore form. This behavior is expected to have wide applications in gas adsorption and separation.
At present, there are still few studies on the adsorption performance of flexible porous solid adsorption materials for carbon capture from moisture sources.However, this unique breathing behavior undoubtedly makes flexible MOFs or zeolites a promising host material for renewable energy gas storage, and also provides a new idea for the development of adsorption materials suitable for carbon capture technology from wet gas sources.
2.4 Evaluation of CO2/H2O co-adsorption mechanism
Evaluating the energy change of CO2 and H2O in the process of adsorption is helpful to understand the complex mechanism of competitive and cooperative adsorption involved in the process of CO2/H2O co-adsorption from the thermodynamic point of view. Table 3 lists the literatures on thermodynamic analysis of coadsorption of CO2 and H2O in recent years. It can be seen that the adsorption enthalpy is still an important parameter to analyze the thermodynamic process of co-adsorption, while the adsorption entropy and Gibbs free energy are lack of relevant calculation and research. At present, due to the different effects of H2O vapor on CO2 under different adsorption materials, the great difference of system output and the various combinations of existing research methods, it is impossible to quantitatively assess the carbon capture process of wet gas source under a unified framework. Therefore, it is urgent to understand the energy change of co-adsorption of CO2 and H2O from the perspective of thermodynamics in order to reduce the energy consumption and cost of carbon capture technology from wet gas sources.
2.4.1 Evaluation method
Based on gas separation theory and Helmholtz thermodynamic equation, Smit et al analyzed the competitive adsorption process of CO2 and H2O on zeolite MFI with appropriate assumptions, and pointed out the potential of adsorption/desorption process driven by adsorption entropy[32]. Based on this method, a thermodynamic evaluation of the wet gas source carbon capture technology is carried out and a CO2/H2O co-adsorption thermodynamic carbon pump model is proposed, and a simplified diagram is shown in fig. 9. The ΔG1 and the ΔG2 represent the Gibbs free energy change of the H2O and the CO2 from the gas phase to the adsorption phase, respectively. ΔG3 represents the difference between CO2 and H2O Gibbs free energy change. If ΔG3<0, it means that the Gibbs free energy difference drives the thermodynamic carbon pump to preferentially adsorb CO2; If ΔG3>0, it means that the Gibbs free energy difference drives the thermodynamic carbon pump to adsorb H2O. Adsorptive selectivity α indicates the ability of adsorbents to separate CO2 from multi-components, which is related to the adsorption capacity and the initial molar mass of the mixture.With the help of thermodynamic carbon pump model, the adsorptive selectivity of CO2/H2O is related to the difference of Gibbs free energy between CO2 and H2O, so as to evaluate its thermodynamic performance.
2.4.2 Evaluation result
Table 4 lists the thermodynamic simulation calculation results of four representative adsorption materials of zeolite MFI, AFI and metal organic framework CuBTC, MgMOF-74 according to the above model. The initial partial pressure of H2O was 3%, 1%, 0.5% and 1% in MFI, MgMOF-74, CuBTC and AFI, respectively. The partial pressure of the CO2 was 15%, and the remainder was filled with N2. The relative adsorption amount of CO2 and H2O was significantly correlated with their respective adsorption Gibbs free energy. (1) MFI: Before 328 K, there are Δ < Δ and Δ < Δ, where H2O is preferentially adsorbed; After 338 K, there are Δ < Δ, Δ < Δ, and CO2 is preferentially adsorbed. (2) MgMOF-74 and AFI: before the low temperature section, there is a Δ < Δ ,Δ < Δ and Δ < Δ after 308 K,Δ < Δ (AFI is 318 K), the value of enthalpy change of adsorption can not directly determine who has the priority to obtain adsorption advantage, because the adsorption entropy change is also an important driving force to drive the adsorbate molecules in the adsorption process. (3) CuBTC: Δ < Δ in all temperature ranges, and Δ < Δ at 368 K, where CO2 gains a competitive advantage. The above results show that for CuBTC adsorption materials, the H2O is preferentially adsorbed at low temperature, and the adsorption enthalpy is the main driving force. At high temperature, the CO2 is preferentially adsorbed, and the adsorption entropy becomes the main driving force. For MgMOF-74 and zeolite MFI, AFI, the preferential adsorption of H2O molecules at low temperature and CO2 molecules at high temperature are the main driving forces for the adsorption enthalpy change.
Fig. 10 show that CO2/H2O adsorption selectivity as a function of temperature for four adsorption materials. The adsorption isotherm and enthalpy were calculated by GCMC, and the adsorption entropy and Gibbs free energy were calculated by DFT. The adsorption selectivity of CO2/H2O calculated by the molecular pump simulation of CO2/H2O coadsorption thermodynamics is in good agreement with the adsorption selectivity of CO2/H2O calculated by the ratio of CO2/H2O adsorption amount in the temperature range of 288 ~ 388 K, which indicates the accuracy of the model. In the low temperature range of 288 ~ 328 K, the adsorption selectivity of CO2/H2O of the four adsorption materials is less than 1, and the adsorption selectivity ability of the four adsorption materials for CO2 is in the order of AFI > MgMOF-74 > CuBTC > MFI. In the high temperature range of 328 ~ 338 K, the CO2/H2O adsorption selectivity of AFI, MFI and MgMOF-74 adsorption materials began to increase gradually and was greater than 1, and their adsorption selectivity ability was in the order of AFI > MgMOF-74 > MFI > CuBTC. From the analysis in Table 3, it can be seen that the adsorption selectivity of CO2/H2O is obviously dependent on their respective adsorption Gibbs free energy, so this conclusion can be used for preliminary screening of various adsorption materials. Zeolites such as AEL and FAU were selected to verify the universality of the model. It can be seen from fig. 11 that when the ΔG3<0, the CO2/H2O adsorption selectivity of most of the adsorption materials is greater than 1, and the more negative the ΔG3 value is, the greater the CO2/H2O selectivity value is. Conversely, when the ΔG3>0, the adsorption selectivity of the CO2/H2O is less than 1, indicating that the adsorption process has lost its competitive advantage CO2.
In this section, the detailed mechanisms of the co-adsorption of CO2 and H2O in the competitive adsorption field, the variable humidity adsorption field, the respiration effect of flexible adsorption materials and the thermodynamic carbon pump field are described, respectively. Through the review, it can be found that there are similarities and differences in the research of the co-adsorption mechanism of CO2/H2O in the four fields. From the point of view of adsorption mode, in the process of competitive adsorption, CO2 and H2O molecules snatch the same adsorption site or H2O molecules are adsorbed as the second adsorption site of CO2, which is mostly physical adsorption. The wetting-change adsorption process depends on the affinity of the basic functional groups on the surface of the adsorbent to the CO2, and belongs to chemical adsorption. The "respiration effect" of flexible adsorbents is a reversible change in pore structure size through external stimulation, which belongs to the category of physical change. However, from the point of view of thermodynamics, the adsorption-desorption process of CO2 is highly related to the number of H2O molecules and adsorption energy, whether it is competitive adsorption or humidification adsorption. In the competitive adsorption process, the difference between the Gibbs free energy of the CO2 and that of the H2O serves as a driving force; The driving force for the adsorption of CO2 depends on the humidity difference during the humidification adsorption process, and its thermodynamic nature is the Gibbs free energy change of H2O molecules, see Figure 12.
3 Study on the Application of CO2/H2O Co-adsorption
The "landing" of the above mechanism and principle research is mainly marked by the development of efficient adsorbents and processes, and most of them serve carbon capture applications. The representative studies in recent years are summarized as follows.
3.1 Wet gas source CO2 trapping adsorption material
Adsorbents are the core of the development of carbon capture technology from wet gas sources, which can directly affect its feasibility and efficiency. In view of the development and application of adsorbents, a variety of efficient adsorption materials suitable for carbon capture technology have been reported at home and abroad. Zeolites and MOFs are the most widely used. In order to eliminate the adverse effect of H2O vapor on the adsorption of CO2 in the adsorption cycle process, it is necessary to continue to find porous materials with hydrothermal stability and high CO2/H2O adsorption selectivity, or to modify the existing adsorption materials. For variable-humidity adsorption technology, the specific surface area and adsorption capacity of the porous structure of the adsorption material need to be further improved in addition to ensuring that the adsorption material has the characteristics of functional group hydrophilicity and strong alkalinity. This section focuses on the adsorption of CO2 under humid conditions, focusing on the widely used zeolites, MOFs, silica-based molecular sieves and modified adsorption materials.
3.1.1 Zeolites
Natural zeolite was first discovered in 1756 and belongs to natural aluminosilicate minerals[63]. Because there are negative charges in the structure unit, there must be cations around to balance the charges of the framework, generally alkali metals or ions, so zeolites have a strong affinity for polar molecules, especially H2O. In addition to natural zeolites, many scholars have developed and synthesized artificial zeolites with similar structures and properties to natural zeolites, also known as molecular sieves. Zeolite molecular sieves have been divided into various structural types according to the polyhedron composed of oxides, including A-type, X-type and other structures[64]. Zeolite 13X is considered to be one of the ideal adsorbents for gas separation due to its low cost and high CO2 selectivity, as shown in Figure 13A.
However, zeolite 13X is extremely sensitive to H2O, and even a small amount of H2O can rapidly decrease the adsorption performance of the CO2. Wang et al. determine that binary adsorption isotherms of H2O/CO2 in zeolite 13X and 5A sample, noting that when H2O is adsorbed near its saturation load, the capacity of CO2 is one order of magnitude lower than that of the dry component[65]; Xu et al. Studied the coadsorption behavior of CO2 and H2O on zeolite 13X, and the results showed that the adsorption capacity of CO2 was reduced by 10% in the presence of a small amount of H2O compared with the adsorption capacity of pure components[15]. The H2O has a stronger dipole moment and has a stronger interaction with the cations in the zeolite, so it can be easily adsorbed. However, a few studies showed completely opposite results. Rege and Yang studied the coadsorption behavior of trace CO2 and H2O on γ-Al2O3 and zeolite 13X, and found that H2O could promote the adsorption of CO2 at low pressure.It should be noted that the adsorption amount of both CO2 and H2O increases, which may be due to the fact that the adsorbed H2O molecules under low pressure provide additional adsorption sites for CO2, rather than robbing "shared" sites[66]. However, this phenomenon disappears rapidly with the increase of pressure. Fig. 14 summarizes the changes in the adsorption capacity of CO2 of various zeolite materials under dry and wet conditions. It can be seen that due to the existence of H2O, the CO2 adsorption performance of most zeolite materials will be reduced to varying degrees, and only a small number of zeolite materials will have enhanced CO2 adsorption performance.
It can be seen that although zeolite has the advantages of thermal stability, high CO2/N2 adsorption selectivity, easy synthesis and low cost, its strong hydrophilicity limits the development and application of CO2 capture technology in wet gas source. If zeolite can be Hydrophobically modified according to its physical and chemical characteristics, it can not only maintain high adsorption capacity and adsorption selectivity of CO2, but also reduce the affinity for H2O to a certain extent.The method has the advantages of simplifying the production and maintenance processes of equipment for trapping the CO2 under the wet gas source in the circulating adsorption process, obviously reducing the cost for trapping the CO2, and having important guiding significance and application value for slowing down the discharge of the point-source type CO2.
3.1.2 Metal-organic frameworks (MOFs)
Metal-organic framework (MOF) materials are new highly crystalline porous nanomaterials with periodicity, porous state and spatial topological network structure, which are mainly formed by small molecular organic ligands and metal ions or clusters through intermolecular forces or coordination bonds, as shown in Fig. 13 B ~ d. Compared with traditional adsorbents such as zeolites, activated alumina and activated carbon, metal-organic frameworks (MOFs) have been widely used in gas adsorption and separation technologies in recent years due to their high pore volume, high specific surface area, designability and high selectivity of CO2[67].
Yaghi et al. reported 9 kinds of MOFs materials, and found that the CO2 adsorption capacity of MOF-177 material is 33. 5 mol/kg at 298 K and 3. 5 MPa; the CO2 adsorption capacity of MOF-210 material can reach 74. 2 mol/kg at 298 K and 50 bar, which is a very ideal CO2 adsorption material[68]. Banerijee et al reported a series of ZIFs and found that the CO2 adsorption capacity of ZIF-68, 69 and 70 materials has made a breakthrough, especially ZIF-69, under the conditions of 273 K and 1 bar, 1 volume of adsorbent can adsorb 82. 6 volumes of CO2, that is to say, its adsorption capacity is as high as 82. 6 times, which greatly improves the application prospect of MOFs materials in carbon capture technology[69].
MOFs have poor water stability, and their CO2 adsorption capacity will be reduced to varying degrees when exposed to humid air or H2O vapor. Because H2O can destroy the coordination bonds between organic ligands and metal ions in MOFs materials, thus destroying the structure. Davis et al. Studied the adsorption behavior of CO2 on MIL-100 (Fe) material under humid conditions[75]. When the relative humidity is 3%, the adsorption capacity of CO2 is 26 mg/G, and the adsorption heat is-43 ~ -34 kJ/mol. When the relative humidity reaches 10%, the adsorption capacity of CO2 decreases slightly, and the adsorption heat is-56 kJ/mol. However, when the relative humidity reached 20%, the adsorption capacity of CO2 suddenly increased to 66 mg/G. This result indicates that the pre-adsorbed H2O molecules form a "micropore pocket" in the mesopores of MIL-100 (Fe), so that the CO2 molecules can fill the micropore pocket at a lower pressure, as shown in fig. 15.
However, for some MOFs, a small amount of water can promote the adsorption of CO2. Bahamon et al. measured the adsorption isotherm of CO2/N2/H2O ternary mixture on CuBTC, and found that H2O would promote the adsorption of CO2 at low pressure[112]. In other mixed-gas adsorption studies, the same phenomenon was found: the CO2/H2 selectivity of the soc-MOF increased at low pressure when a trace of water was added to the CO2/H2 mixture[19]. The conclusion that the CO2 adsorption properties of MOF materials at low pressure are little affected by water molecules is also verified by Anderson et al.: All predicted materials maintain good selectivity at low pressure, and among these materials, about 75% of the selectivity is not affected by water under gas-gas conditions[74]. Due to the existence of metal ions and clusters, the interaction mechanism between MOF materials and H2O is complex, which needs to be analyzed from the physical and chemical characteristics of the structure and energy. For MOF materials which are seriously affected by H2O, it is necessary to improve their hydrothermal stability under higher pressure and reduce their affinity for H2O in subsequent studies. Boyd et al. Published a study on MOFs in the field of wet flue gas carbon capture in Nature, which also clarified the research strategy, that is, synthesizing hydrophobic MOFs or enhancing the hydrophobic properties of existing MOFs by surface modification[33].
3.1.3 Silicon-based molecular sieves
Kresge et al. Synthesized M41S series (MCM-41, MCM-48, MCM-50) molecular sieves based on silicon, which opened a new era of molecular sieve science[72]. Silicon-based molecular sieves have large pore size, specific surface area and wall thickness, and also have high chemical and thermodynamic stability[73]. Anderson reported titanium silicalite on Nature in 1994. They used high-resolution electron microscopy, electron diffraction, solid-state nuclear magnetic resonance and other methods to carry out physical and chemical analysis on the structure of the material, and unveiled the mystery of titanium silicalite ETS-4 and ETS-10 for the first time, both of which showed strong adsorption characteristics on the microporous structure[74]. Subsequently, silica-based molecular sieves have been gradually applied to heterogeneous catalysis, advanced inorganic materials, adsorption and separation, and many other fields[75][76].
Silicon-based molecular sieves are considered to be the ideal adsorption materials in the field of CO2 capture of wet flue gas, which are not easily affected by humidity. Yoon, Xijiang University, Korea, published in Science in 2015 on the development and performance of microporous silicon-based molecular sieves in the capture of high humidity flue gas CO2[77]. They reported a highly stable microporous copper silicate SGU-29 with specific adsorption sites of H2O and CO2, but no shared adsorption sites of CO2 and H2O, see Figure 13e.
The results show that the adsorption capacity of CO2 is 115 cm3/cm3 at 298 K, 100 mbar and 90% absolute humidity, which is much higher than that of traditional zeolite molecular sieves. SGU-29 is therefore able to easily adsorb both CO2 and H2O from moist flue gas without interfering with each other. As shown in fig. 16, where / represents the ratio of the amount of adsorption of the CO2 under wet to dry conditions, a value closer to 1 indicates that the adsorbent material is less affected by the H2O. The ratios of SGU-29 and AM-6 to ETS-10 were all close to 1, showing good adsorption performance for CO2. These silica-based molecular sieves have similar heterogeneous structures, so they can effectively adsorb CO2 in the process of carbon capture from wet flue gas. Although the research on microporous silica-based molecular sieves in high humidity flue gas carbon capture is still very limited, the SGU-29 silica-based molecular sieves proposed by Yoon et al. Provide new ideas and contributions for the design and development of new materials.
3.1.4 Surface modification
In order to overcome the problem that adsorption materials such as zeolites and metal organic frameworks (MOFs) can not efficiently capture the CO2 of wet gas sources, many scientists have paid attention to the modification of materials, which are mainly divided into physical modification methods and chemical modification methods[78]. Physical modification is to increase the pore utilization rate of the material without changing the chemical structure, so as to improve the adsorption performance of the material. For example, by properly increasing the activation temperature, the pore volume of MOF-5 can be increased and some solvent molecules can be removed, thus improving its hydrogen storage capacity[79]. Chemical modification refers to changing the chemical structural characteristics of the adsorbent material, such as replacing atoms on the adsorbent with different groups (OH-, Cl-), thereby improving the affinity for the target adsorbate[80]. The chemical modification method can significantly improve the adsorption selectivity of the adsorption material, which plays an important role in the multi-component gas adsorption separation.
Coelho et al. Proposed three modification methods to enhance the interaction between MOFs and CO2: (1) the introduction of highly polar functional groups (— OH, — CN, —NO2, etc.) Can induce the polarization of CO2 molecules and thus enhance the affinity with adsorbents[57]; (2) producing uncoordinated metal sites; (3) MOFs channels are functionalized with basic nitrogen-containing groups, see Figure 13 (f ~ I). The results show that due to the presence of Fe3+ in the porous material MIL-100 (Fe), the adsorption capacity of CO2 increases by more than 5 times in humid environment. The amino-modified MIL-53 (Al) was studied by Z Zárate et Al. The NH2-MIL-53(Al) has — OH group and has a strong affinity for CO2[81]. When the relative humidity was 5%, the CO2 adsorption capacity reached the maximum (4. 6 wt%). Due to the CO2 limitation effect induced by H2O, the amount of CO2 trapped in the wet condition was 1.7 times higher than that in the dry condition.
Due to the strong affinity of zeolite for H2O molecules, it can not effectively trap CO2 in high humidity flue gas. Hydrophobicity of zeolite can be effectively improved by increasing the ratio of silica to alumina in the structure of zeolite and eliminating the hydroxyl groups on the surface of zeolite[82]. It was found that the stability of zeolite increased with the increase of the ratio of silica to alumina. Hydrophobic functional groups can be used to eliminate the hydroxyl groups on the surface of zeolite, which can not only improve the surface hydrophobicity, but also effectively control the pore size of zeolite. Zapata et al. Treated the surface of zeolite HY with organic silanization, which can significantly improve the hydrophobic properties of zeolite, and the density of acid sites has not been significantly reduced[83]. Liu et al. Wrapped the surface of zeolite 13X with a layer of hydroxyl-rich mesoporous silica shell material, and the results showed that the hydrophobic performance of zeolite 13X@mSiO2 was improved, and the adsorption performance of CO2 was also significantly improved[84]. The method can provide effective guidance for the bottleneck problem that the commercial zeolite 13X cannot adsorb the CO2 in the high-humidity flue gas.
For the wetting-change adsorption method, due to the lower concentration of CO2 in the atmospheric environment, the binding energy between the adsorbent and the CO2 must be larger to ensure the absorption. At present, most of the researches focus on the porous solid materials modified by amino groups. He et al. used carbon black as a carrier to synthesize a colloidal crystal with a quaternary ammonium group through a reversible template method, and the synthesis path is shown in Figure 17. The adsorbent has a good porous structure, specific surface area and dynamic performance, and can selectively adsorb CO2 in a humid environment, reaching 0.37 mol/kg[85,86]. Hou et al. obtained a new humidity-changing adsorption material by connecting quaternary ammonium groups on the surface of natural bamboo fibers. The results showed that the material could withstand higher humidity changes. When the humidity level did not exceed 70%, the adsorption capacity of CO2 increased with the increase of humidity[87]. Above 70%, the adsorption capacity of CO2 decreased with the increase of humidity, showing the regeneration effect of humidification. Adsorbents for humidifying CO2 have in common the presence of a polymeric backbone, a quaternary ammonium cation covalently attached to the backbone, and a strongly basic partner anion, such as or OH-. In the current research, the functionalized ionic liquid polymers (PILs) are the most widely used humidity-changing adsorption materials.
To sum up, the adsorption performance of CO2 in humid environment can be significantly improved by surface modification or modification of adsorption materials. Table 5 lists the changes of CO2 adsorption capacity of various materials in humid environment. It can be found that the CO2 adsorption performance of adsorption materials can be significantly improved by the modification method. However, the stability and regeneration of surface groups still need to be evaluated in detail in terms of kinetics and thermodynamics. In addition, it also needs to pay attention to the cost and large-scale application, which still faces enormous challenges.
3.2 CO2 capture technology for wet gas sources
Adsorption-cycle method is an industrial gas separation technology developed rapidly in recent years, which is widely used in CCS industry. The typical processes such as Temperature swing adsorption (TSA), pressure swing adsorption (PSA) and Vacuum Pressure swing adsorption (VPSA) can make the purity and recovery rate of Temperature swing adsorption in the dry flue gas capture reach more than 90% and 80%, respectively[96]. Although the technology of separating and drying flue gas has been mature, the actual flue gas in power plants often contains water vapor, NOx, SOx and other impurities, especially water vapor will cause many adverse effects on the carbon capture system, such as temperature drop, pressure drop and equipment corrosion, resulting in a significant increase in energy consumption and cost of the adsorption cycle process[97]. Therefore, how to improve the carbon capture process and further reduce the energy consumption and cost of CO2 in the capture wet gas source has become a hot research topic at home and abroad in recent years.
3.2.1 Swing adsorption technique
The core principle of wet gas carbon capture technology based on swing adsorption mechanism is to use the free energy generated by various potential differences to drive continuous adsorption. At present, its high efficiency mainly focuses on how to effectively remove water vapor to increase the adsorption capture rate of CO2. For carbon capture from wet flue gas after combustion, the adsorption process widely studied at present is to use the adsorption characteristics of different adsorbents to increase the adsorption bed for dehydration, that is, Multilayer adsorption process[98,99]. The principle of multi-layer adsorption process is to add multiple adsorption bed layers in the adsorption bed, and use more than two adsorbents to disassemble CO2 and H2O for simultaneous adsorption. The adsorbents used to adsorb the H2O in advance are weak adsorbents such as activated carbon and activated alumina, which conform to the linear adsorption isotherm model, while the main adsorption layer is made of porous nano-adsorption materials such as zeolite and MOFs, which have high CO2 adsorption performance. This method can effectively remove water vapor while reducing the high energy consumption and cost caused by large-scale pre-drying of wet flue gas, and has been widely used in gas separation, hydrogen purification and other fields.
Webley et al. proposed an improved double-layer PSA process cycle, and the whole cycle mainly consists of three steps: adsorption, countercurrent vacuum pumping and repressing, as shown in Figure 18[100]. This study not only improved the percentage limit of water vapor in the feed (higher than 4%), but also accurately controlled the water concentration peak surface in the adsorbent bed and the CO2 separation effect according to the bed ratio and the volume ratio of flushing gas/filling gas, so as to avoid the poisoning of the main layer adsorbent by water vapor, control the vacuum level in the regeneration process, and reduce the energy consumption in the CO2 capture process.
Hefti et al., ETH Zurich, proposed a new temperature swing adsorption cycle process with a condenser[101]. Compared with the traditional four-step circulation method, the six-step circulation of the process is characterized by the addition of two condensers, so that water vapor can be condensed at the feed inlet. In addition, a protective layer consisting of activated alumina is used to remove part of the water vapor present in the tail of the adsorption bed. The process can effectively capture the CO2 in the high wet flue gas, so that the recovery rate and purity of the CO2 can reach 90% and 95% (dry gas level) respectively. Wang et al. further emphasized the stability of the process to achieve reasonable process performance with higher water vapor content in the feed gas[107].
Multilayer pressure swing adsorption integrated the CO2 adsorption and water removal process in the same adsorption device, which avoided the pre-drying stage. However, this technology needs to pay attention to the change value of water carrying area at all times, otherwise the water will often penetrate into the tower, and the risk of process failure is very high. The ultimate strategy for the deleterious effects of water vapor in the adsorption cycle CO2 capture technology is to use a water-tolerant/hydrophobic adsorbent. However, the development of such materials is a great challenge, which requires a comprehensive understanding of the competition mechanism from the perspective of kinetics and thermodynamics, respectively, according to the different characteristics of CO2 and H2O molecules.
Joos team of Ghent University proposed an inverted carbon capture technology with high temperature adsorption and low temperature desorption of CO2 according to the difference of temperature dependence between CO2 and H2O, as shown in Figure 19[32]. Step HALD1 is an adsorption process that absorbs wet flue gas at 400 K without cooling; Step HALD2 is a regeneration cycle process in which the adsorption bed is filled with saturated water, and when in contact with the hot waste again, the waste heat is used to dehydrate, making the CO2 competitive again. Wate no longer affect that adsorption of the CO2 in this process, but assists the adsorption/desorption cycle process. Compared with traditional CCS technology, this technology uses the heat released by water to drive the recovery of CO2 without additional energy input for regeneration.
3.2.2 Air direct capture CO2 technology
The principle of DAC technology is to use the atmosphere as the transport medium of CO2, and to enrich the CO2 directly from the low concentration of gas partial pressure (40 Pa) by chemical adsorbent[102]. At present, the research directions of DAC technology based on adsorbents are divided into three categories: (1) the development of efficient adsorbents[103]; (2) Design of the adsorption chamber of the contactor; (3) Design and control of adsorption process.
The DAC process based on amine-based solid adsorbents has been widely studied to reduce the energy consumption during adsorbent regeneration, but there are still some challenges to overcome, such as the co-adsorption process of CO2 and H2O. Although the presence of a small amount of water in the DAC technology based on amine-based solid adsorbents can promote the adsorption of CO2 to some extent, from the energy point of view, the high energy loss associated with water may offset this advantage. Veneman et al.'s Study on the co-adsorption behavior of CO2 and H2O on amine-based solid adsorbents showed that the presence of H2O would increase the cost of carbon capture by 2-3 US dollars per ton of CO2[104]. In order to cope with the adverse effects of water vapor, carbon capture technology, which combines direct air capture technology with cyclic adsorption technology such as temperature swing and pressure swing, has been gradually developed.
Different from the traditional adsorption and separation methods, Moisture swing adsorption swing adsorption (MSA) is a carbon capture technology which is gradually developed and matured on the background of direct air capture technology.The method provides a brand-new perspective and method for effectively adsorbing the CO2 under the humidity condition, and is a trapping method for controlling the chemical adsorption capability of an interface to the CO2 by utilizing the strong alkalinity and hydrophilicity of the surface functional group of the chemical adsorbent and adjusting the partial pressure of water vapor in the environment, and the schematic diagram is shown in Figure 20[105].
The key to the development of moisture-change adsorption technology is to develop adsorption materials with moisture-change adsorption characteristics, which usually have high specific surface area, complex porous structure and fast adsorption rate of regenerated adsorbents. Table 6 summarizes the relationship between the adsorption capacity and the half-adsorption time of various moisture-changing adsorption materials. It can be seen from the table that the adsorption capacity of the adsorption materials developed at the present stage and the half-adsorption time roughly show a reverse trend, that is, the adsorption capacity of the adsorbent with fast adsorption speed is generally small; Adsorbents with larger adsorption capacity have larger adsorption rate. Therefore, the development of adsorbent materials with high hygroscopic adsorption characteristics is still the focus of hygroscopic adsorption technology research.
3.3 Performance Evaluation of CO2 Capture Technology for Wet Gas Source
The energy consumption of carbon capture technology is one of the important obstacles to its large-scale application. At present, most of the studies are focused on the CO2 capture technology under dry conditions, ignoring the influence of H2O molecules in the actual separation process of CO2. Based on this, there is an urgent need to evaluate and analyze the energy efficiency of wet source carbon capture technology.
3.3.1 Evaluation method
(1) Mixed gas separation model. The carbon migration process can be considered as driven by the carbon potential difference, where the carbon potential is defined as the degree of enrichment of carbon or carbon-containing components in the space[110]. Driven by the carbon potential difference, the CO2 can spontaneously diffuse from the high-volume fractional source to the low-volume fractional sink. Carbon capture, on the contrary, is a reverse carbon flow that uses energy such as input work or heat to achieve the enrichment of CO2 from low concentration to high concentration. Therefore, although the principles of various carbon capture technologies are different, from the perspective of energy and mass conversion, they all use energy input to make up for the carbon potential difference, and then realize the reverse flow of carbon. House et al., based on the first and second laws of thermodynamics, carried out the thermodynamic analysis of ideal energy consumption for the general CO2 capture process by using the basic theory of gas separation[111]. The model summarizes and evaluates traditional carbon capture technologies, and the thermodynamic model is shown in Figure 21. CO2 ideal separation minimum work Wmin refers to the minimum useful work that must be consumed in the separation process in theory, which is used to characterize the difficulty of the separation process in theory.
(2) Mixed gas cycle model. The adsorption material suitable for wet gas source needs to meet the requirements of good CO2 adsorption performance and low regeneration energy consumption. In the swing adsorption loop technology, the most common regeneration technologies are PSA, TSA, VSA and so on. In order to evaluate the effectiveness of the co-adsorption of CO2 and H2O on the actual carbon capture technology, a typical swing adsorption cycle TSA was taken as an example to analyze its energy efficiency performance. The thermal regeneration energy consumption Qthermal is the energy required for the heating stage and the desorption stage in the TSA process, which mainly includes the sensible heat required for heating the adsorbent and the energy required for overcoming the endothermic desorption process, as shown in Equation 9[112].
Where is the heat capacity, is the temperature difference between adsorption and desorption processes, is the adsorbent density, V is the adsorption layer volume, ε is the porosity, and is often expressed as an average of the whole surface coverage, which is related to the adsorption capacity under adsorption and desorption conditions and can be expressed as .
3.3.2 Evaluation result
Fig. 22 shows the variation of the minimum work of separation in zeolite MFI, AFI and MOF materials MgMOF-74 and CuBTC with the initial concentration of H2O when the initial concentration of CO2 is 10% to 15% and the component is set as CO2/(H2O+N2). In this model, the H2O and the N2 are treated as a kind of gas, and the separation work is affected by both the initial concentration of the CO2 and the collection rate. It can be seen from the figure that with the increase of the initial H2O concentration, the minimum separation work of CO2 under the four adsorption materials shows an upward trend in different degrees. This is due to the fact that the initial mole fraction of CO2 decreases with the addition of H2O molecules during the separation process, resulting in an increase in the minimum work. It should be noted that the increase of H2O molecules will also bring different degrees of impact on the adsorption performance of CO2, so the capture rate of CO2 will also change. It can be seen from Section 2.4 that for CuBTC adsorbent, under low pressure, H2O molecules will promote the adsorption of CO2 and thus increase the capture rate of CO2, so the separation work will eventually show an upward trend; For MFI, MgMOF-74 and AFI adsorption materials, the H2O molecules always inhibited the CO2, and the reason why the separation work kept an upward trend may be that the effect of H2O on the initial concentration of CO2 was greater than that on the adsorption performance.
Fig. 23 shows the effect of different H2O contents and adsorption temperatures on the energy consumption of the TSA process. It can be seen from the figure that the energy consumption of TSA process is greatly affected by the adsorption temperature and H2O. When the moisture content exceeds a certain value, for example, when the adsorption temperature is 300 K and the water content is 3%, the energy consumption will suddenly increase exponentially. This is due to the existence of a large number of H2O molecules, resulting in a higher adsorption enthalpy. Fig. 24 shows the energy consumption of the TSA process as a function of the difference in adsorption Gibbs free energy of CO2 and H2O, Δ. It can be seen that if Δ < Δ < 0, the TSA energy consumption increases rapidly with the increase of Δ. However, it should be emphasized that when the Gibbs free adsorption energy difference between CO2 and H2O is more than 2 kJ/mol, the energy consumption reaches 203. 99 kJ/kg. This is obviously inconsistent with the actual process, indicating that the adsorption behavior of CO2 is almost impossible at this time. When Δ < Δ < 0, it is found that the energy consumption is 3. 16 kJ/mol with the increase of Δ, such as Δ = -2 kJ/mol, while the energy consumption value increases to 9. 2 kJ/mol when Δ = 5 kJ/mol.The TSA energy consumption level increases in the opposite direction during this process. It can be seen that the TSA energy consumption is significantly related to the Gibbs free energy difference between CO2 and H2O, and in this case, the energy consumption is the smallest in the range of -3 to 0 kJ/mol for Δ.
4 Conclusion and prospect
The rapid development of carbon capture industry under the pressure of "negative carbon" emission has put forward new requirements for CO2 adsorption process. In order to reduce the adverse effects of impurities in gas source on CO2 capture, especially the H2O between industrial flue gas and atmospheric environment, it is necessary to break through the bottleneck restriction and derive new technical concepts suitable for CO2 capture in wet gas source. In this paper, the co-adsorption mechanism of CO2 in the presence of H2O was reviewed in detail from both basic and applied aspects.
The mechanisms of coadsorption between (1)CO2 and H2O are different in the fields of competitive adsorption, humidification adsorption and respiration effect of flexible materials. From the adsorption mode, the competitive adsorption was mainly caused by van der Waals force, the CO2 and the H2O could snatch the same adsorption site at the same time, or the H2O molecules were adsorbed as the second adsorption site of the CO2. The moisture-changing adsorption process is mainly caused by the force between the chemical bond of the surface functional group and the adsorbate, which has a strong selectivity to the target adsorbate and is mostly chemical adsorption. The "respiration effect" of flexible adsorbents is a special property of materials, which can use external stimuli to make the pore structure change reversibly for selective adsorption.
The coadsorption mechanism of (2)CO2 and H2O can be used in many fields, but it can be preliminarily unified from the point of view of thermodynamics, that is, the difference of Gibbs free energy between CO2 and H2O is an important thermodynamic driving force in the coadsorption process, which is affected by both adsorption enthalpy and adsorption entropy. The deficiency of the current study is the acquisition of thermodynamic parameters.In the future, the theoretical calculation method of the co-adsorption process of CO2 and H2O should be clarified and standardized.
(3) Adsorbents are the core of the development of CO2 capture technology from wet gas sources.In this paper, various types of adsorbents suitable for the co-adsorption of CO2 and H2O and the breakthrough progress in their fields are described, mainly including zeolites, organometallic frameworks, silica-based molecular sieves and surface-modified adsorption materials. Meanwhile, the CO2 capture process for separating high humidity flue gas and air is discussed, and it is pointed out that the development of high-quality adsorbents with low cost, low energy consumption and stable H2O is the key issue for the CO2 capture process at present. Finally, the energy consumption level of the wet gas source carbon capture process is evaluated. For the separation model, the minimum separation work of the CO2 increases with the increase of the initial H2O concentration; It is found that the energy consumption of TSA is significantly related to the difference of Gibbs free energy between CO2 and H2O, and there is a certain range of Δ to minimize the energy consumption.
In this paper, the co-adsorption process of CO2 and H2O was preliminarily summarized from the aspects of mechanism explanation, material development and process improvement, aiming to further deepen the understanding of carbon capture technology by wet gas adsorption. Due to the lack of thermodynamic data of adsorption phase or different thermodynamic evaluation systems, there are still some limitations in the evaluation of existing studies, and further accumulation and in-depth comparison of data mining and processing are needed in the future.
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