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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 emerging and only few have been tested at large scale, maintaining the TRL of other new systems still low. A review of commercial solutions and relevant projects can be found, for example, in Ref. [22]. The main criteria for the selection of a solvent are included in Table 1.2.
Primary amines are of high interest because of their fast reaction with CO2. However, the main drawback is their high energy consumption for the solution regeneration. Several alternatives are emerging to decrease such penalty, the most common one being the use of tertiary amines. However, the CO2 absorption in tertiary amines is much slower. Consequently, other alternatives are emerging, such as the use of blends combining primary and tertiary amines (commonly called “promoted tertiary amines” or “activated tertiary amines”). Numerous alternatives have emerged during the past years; perhaps it is difficult to establish the best alternative.
Table 1.2 Desired solvent properties and its impact on the absorption process [75].
Source: Adapted from Mathias et al. [75].
| Solvent property | Impact on the absorption process |
|---|---|
| High capacity and low heat of absorption | It is linked to the energy requirements per ton of CO2, but the absorption capacity is connected to heat (thermodynamics) and independent variation is limited |
| High mass transfer and chemical kinetics | It reduces equipment size or the capacity by operating near the equilibrium limit |
| Low viscosity | It reduces the pumping costs and potentially increases the mass transfer and the heat transfer rate |
| Low degradation tendency | It reduces the solvent make‐up and the regenerator can operate at higher pressure/temperature, increasing the thermal efficiency |
| Low toxicity/environmentally friendly | It becomes more important if toxic by‐products are released during volatility losses |
| Cost and availability | It will impact on reaching commercial scale |
| Low fouling tendency | It will impact on the operation |
A potential substitute of traditional solvents is the use of compounds that, at unloaded or loaded conditions, separate into two phases, called biphasic solvents. There are two types of biphasic solvents, namely, liquid–liquid or solid–liquid, depending on the phases in solution. The main advantage is that only one phase needs to be regenerated, and consequently, the stripper size is reduced, and the energy consumption is potentially lower. Consequently, numerous biphasic solvents have been studied in the literature (e.g. in Ref. [23]).
Another strategy is to add enzymes, such as carbonic anhydrase (CA) [24]. CA increases the kinetic constant of the absorption of CO2 in aqueous amine and dilute carbonate solutions by catalyzing the CO2 hydration. The impact will depend on the compounds in solution, as the regeneration of the enzyme regeneration rate will vary. The challenges enzymes offer are their pH and thermal stability, lifetime, and sensitivity