Engineering Solutions for CO2 Conversion. Группа авторов

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Engineering Solutions for CO2 Conversion - Группа авторов


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1.7 Scheme of a single‐stage membrane system.

      Source: Adapted from Mores et al. [18].

      Source: Adapted from Wang et al. [21].

Type of membrane Advantages
Ceramic
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 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

Schematic illustration of the general chemical absorption configuration

      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.

      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


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