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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. MexOy/MexOy−1 denotes the recirculation oxygen carrier material.
Source: Adapted from Abanades et al. [17]. © Elsevier.
1.2.4.2 High‐Temperature Solids Looping Technologies
The most common types of high‐temperature solids looping technologies are calcium and chemical looping combustion. Calcium looping uses CaO as a sorbent, which produces CaCO3 at approximately 650 °C (Figure 1.5). Chemical looping is a two‐step conversion process where the fuel reacts with almost pure O2 as in the oxyfuel process, while a metal oxide acts as an oxygen carrier and reacts with the fuel, obtaining CO2 and water (Figure 1.6). In both cases, the metal oxide or CaO is regenerated.
Note that calcium looping can be considered as post‐combustion or pre‐combustion, while chemical looping can be considered as oxy‐combustion or pre‐combustion depending on the configuration [16].
Because of the high operation temperature, the advantage of this process is the potential recovery of energy for steam production, which can be used for additional power production and reduce the efficiency penalty in the power plant.
Calcium looping has shown a significant evolution over the past 15 years from lab scale to pilot testing, reaching a TRL of 6. The main research focus to cut down the costs over the next years is on the sorbent, reactors (configurations and interconnections), and process designs [17]. If used in the industrial sector, calcium looping can be beneficially integrated in the cement production facility because of the use of solids from the capture system in the production. In this regard, the CLEANKER project aims to scale up a calcium looping process in a cement production environment, which will increase the TRL of this technology up to 7.4
Chemical looping has reached a TRL of 6 as oxyfuel arrangement while a TRL of 3 as pre‐combustion system. The main research areas on chemical looping are focused on the reactor design, oxygen carrier development, and prototype testing. Moreover, more than a thousand materials have been tested at the laboratory scale. At a larger scale (0.3–1 MW), the accumulated operational experience is more than 7000 hours [17]. A detailed review of the main process routes under development within the chemical looping systems is included in Ref. [17].
1.2.4.3 Membranes
Membranes are porous structures able to separate different gases at different rates because of their different permeation [8]. These can be used not only in post‐ and pre‐combustion processes but also in oxyfuel for oxygen separation. In post‐combustion, the main interest in these systems is their low energy requirements compared to the traditional chemical absorption process.
The energy needs are reduced to those from the compressor and vacuum pump. Moreover, membrane systems are easy to start and operate, have no emissions associated, and are modular, offering installation advantages [8]. However, the separation mechanism of membranes is based on the difference of CO2 partial pressure. In post‐combustion, because of the relative low CO2 concentration in the flue gas to be treated (approximately 4–12% for power plants), this driving force would not be enough to achieve high CO2 capture ratios through simple configurations. However, membranes could offer advantages for partial capture arrangements and generally more complex arrangements are used to reach a full capture rate (90%). In pre‐combustion, because of the higher partial pressure of CO2 in the gas to be treated, membranes can be more effective. In any case, the gas containing CO2 must be cooled down to meet the temperature limitations of the membrane [18] and that could be a drawback (Figure 1.7).