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).
Figure 1.7 Scheme of a single‐stage membrane system.
Source: Adapted from Mores et al. [18].
Table 1.1 Advantages of each type of membrane [21].
Source: Adapted from Wang et al. [21].
| Type of membrane | Advantages |
|---|---|
| Ceramic | Good selectivity–permeability Easier to manufacture larger areas |
| Polymeric | Good thermal stability and mechanical strength |
| Hybrids | Aiming to show the advantages of both ceramic and polymeric membranes |
There are two main characteristics to define a membrane material for CO2 capture: permeability, which will impact on the CO2 separation ratio and selectivity, which will define the CO2 concentration in the output gas. From a techno‐economic perspective, the optimum values for selectivity and permeability would be a function of the gas to be treated, as studied in Ref. [19]. The ratio of the permeability to the thickness of the membrane will be of high importance as that will characterize the permeance (commonly measured as gas permeation units [GPU]). To maximize the permeance without impacting the mechanical stability, the membranes are typically a dense layer supported by a porous layer [20].
The membrane materials can be divided into ceramic, polymeric, and hybrid (Table 1.1). Moreover, the design of the membrane‐based system will be a key factor on the separation process. Firstly, the membrane module will be the key factor. The main modules for polymeric membranes are described as a spiral wound, a hollow fiber, and an envelope [21].
The majority of the membranes used currently for post‐combustion are based on polymeric materials [20], and a large list of polymers have been studied in the literature, including polyimides, polysulfones, and polyethylene oxide. The most advanced processes have reached currently a TRL of 6. Because of the modularity membranes offer, although sometimes predicted, it is not clear if there will be a fast development toward higher TRLs [21].
1.2.4.4 Chemical Absorption
The basic configuration of chemical absorption (Figure 1.8) includes the reaction of a liquid solvent with CO2 in a column called absorber at a relatively low temperature, 40–60 °C, and its desorption in another column called desorber or stripper, generally at a high temperature, 100–140 °C. It must be noted that process modifications and solvent enhancements might modify those process conditions.
Figure 1.8 General chemical absorption configuration
The absorption of CO2 into liquid solvents takes place by three phenomena: chemical reaction, physical absorption, and diffusivity. Depending on the compound and the conditions, one phenomenon will be predominant over the others.
Chemical solvents are more attractive candidates for typical post‐combustion processes, with relatively low partial pressures of CO2 (10–15% in coal power plants and 4–8% for gas‐fired power plants). Chemical absorption follows a standard configuration such as in Figure 1.8. However, new configurations have appeared to enhance the process, increase the efficiency, and/or decrease the capture costs.
Chemical absorption with amines is by far the most advanced carbon capture process and the only one that reached a TRL of 9 [2]. The most tested solvent is aqueous monoethanolamine (MEA) solution, although it does not represent any more the benchmark solution as consolidated alternatives show enhanced properties. Two large‐scale facilities have used enhanced systems, the Boundary Dam Capture plant [2] and Petra Nova. One of the main pathways to get more efficient chemical absorption processes and cut down costs is the development of new solvents. However, many solvents are
