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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].