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Adsorption refers to the uptake of CO₂ molecules onto the surface of another material. Based on the nature of interactions, adsorption can be classified into two types: (i) physical adsorption and (ii) chemical adsorption. In physical adsorption, the molecules are physisorbed because of physical forces (dipole–dipole, electrostatic, apolar, hydrophobic associations, or van der Waals) and the bond energy is 8–41 kcal mol⁻¹, while in chemical adsorption, the molecules are chemisorbed (chemical bond; covalent, ionic, or metallic) and the bond energy is about 60–418 kcal mol⁻¹ [11].

A theoretical advantage of adsorption against other processes is that the regeneration energy should be lower compared to absorption because the heat capacity of a solid sorbent is lower than that of aqueous solvents. However, other parameters, such as working capacity and heat of adsorption, should also be considered [12]. The higher the heat of adsorption, the stronger the interaction between the CO₂ molecules and adsorbent‐active sites and thus the higher the energy demand for the regeneration. The potential disadvantages for adsorbents include particle attrition, handling of large volumes of sorbents, and thermal management of large‐scale adsorber vessels.

Solid sorbents can be classified according to the temperature range where adsorption is performed. Low‐temperature solid adsorbents (<200 °C) include carbon‐based, zeolite‐based, metal–organic framework (MOFs)‐based, several alkali metal carbonate‐based, and amine‐based solid adsorbents. Intermediate‐temperature (200–400 °C) solid adsorbents include hydrotalcite‐like compounds or anionic clays, while high‐temperature (>400 °C) sorbents refer to calcium‐based adsorbents and several alkali ceramic‐based adsorbents.

Usually, adsorption takes place in packed or fluidized beds, as can be seen in Figure 1.3. For the packed bed case, the adsorbent is loaded into a column, the flue gas flows through the void spaces between the adsorbent particles, and CO₂ gets adsorbed onto the surface of the particles. In fluidized beds, the flue gas flows upward through a column above the minimum fluidization velocity and the adsorbent particles are as such suspended in the gas flow. Regardless of the process configuration, the adsorbent selectively adsorbs CO₂ from the flue gas and is subsequently regenerated to complete the cyclic adsorption process.

Figure 1.3 The adsorption process: (a) difference of physisorption and chemisorption, (b) a packed bed configuration, and (c) a fluidized bed configuration. Source: Adapted from Global CCS Institute (https://www.globalccsinstitute.com/archive/hub/publications/29721/co2-capture-technologies-pcc.pdf).

Cyclic adsorption processes alternate between the adsorption and desorption modes of operation. Based on the intensive variable that is cycled, the adsorption processes are broadly classified as pressure swing adsorption (PSA) or temperature swing adsorption (TSA), as can be seen in Figure 1.4. If the cycle switches between adsorption at atmospheric pressure and desorption under vacuum, then it is called vacuum swing adsorption (VSA). Pressure vacuum swing adsorption (PVSA) cycles have an adsorption step above atmospheric pressures and desorption under vacuum [13].

Figure 1.4 Comparison of TSA and PSA for the regeneration of solid adsorbents. H = high; L = low.

Source: Adapted from Rackley [73].

In a packed bed configuration, regeneration is accomplished by heating the CO₂‐loaded adsorbent to liberate CO₂. During this time, the flue gas is diverted to a second packed bed, which continues to adsorb CO₂ from the gas. By alternating the flue gas between two packed beds that alternatively undergo absorption and regeneration in a cycle, CO₂ can be continually removed from the flue gas. In a fluidized bed, the sorbent is circulated between an absorber vessel where it contacts the flue gas and a regenerator vessel where it is heated to liberate gaseous CO₂.

Usually, the PSA process is preferred to other cyclic operations when the process is carried out at elevated pressures. Otherwise, when the concentration of the adsorbate is low (0–15 vol%), or when the process is at low pressure, other options such as TSA may need to be considered. For a low‐concentration adsorbate, the PSA technology may result in a much longer desorption step, whereas for low‐pressure processes, the installation should also include additional vacuum pumps and compressors, both resulting in a more complicated process, increased capital cost, and reduced efficiency [8]. A potential option that could overcome these issues is vacuum pressure swing adsorption (VPSA).

TSA can work both for low and elevated pressures; however, it is usually preferred when the adsorption step is carried out at a low temperature. Consequently, the main advantage of TSA over PSA is its ability to separate efficiently strong‐bonded adsorbates onto adsorbents, as for the case of chemisorption. However, a major drawback of TSA is the high energy intensity of the desorption process compared to PSA. Other alternatives to TSA include microwave swing adsorption (MSA) [14] and electric swing adsorption (ESA) [15] that could offer potential energy savings and faster heating rates; however, these technologies are still at low technology readiness level (TRL).

Generally, TSA is usually preferred for post‐combustion CO₂ capture at low temperature and atmospheric pressure, while PSA usually is the right choice for pre‐combustion CO₂ capture at elevated temperatures, as in the case for an IGCC plant configuration. As a post‐combustion arrangement, PSA and TSA are assessed as TRL 6.

Adsorption equilibria, kinetics, and regeneration ability are key factors to evaluate the performance of an adsorbent. Fast adsorption/desorption kinetics, influenced by functional groups present, as well as the pore size and distribution in the support, are essential for an efficient CO₂ adsorption process and control of the cycle time and the required amount of adsorbent. Other selection criteria include high CO₂ selectivity, mechanical strength after multi‐cycling, chemical stability/tolerance to impurities, high availability, and, lastly, low costs.

Figure 1.5 Calcium looping system as post‐combustion configuration.

Source: Adapted from Abanades [16].

Figure 1.6 Chemical looping combustion. Me_xO_y/Me_xO_y−1 denotes the recirculation oxygen carrier material.

Source: Adapted from Abanades et al. [17]. © Elsevier.