## 1 Introduction
Pre-combustion CO\({}_{2}\) capture refers to a pre-treatment of fuel (such as gasification or partial oxidation) to produce a gaseous stream (synthesis gas) relatively rich in CO\({}_{2}\) (typically 40% by volume) and hydrogen which can then easily be separated. Oxy-fuel combustion involves oxygen separation from air and combustion of a hydrocarbon fuel (or coal) with oxygen in a closed cycle producing an exhaust stream of almost pure CO\({}_{2}\) without need for further separation. Post-combustion CO\({}_{2}\) capture is the most versatile and well-established end-of-pipe option that involves the successful separation of CO\({}_{2}\) after combustion takes place. Although the process requires no significant engine or plant modifications, there are challenges related to the relatively low CO\({}_{2}\) levels (typically 5-15% by volume) and the variability of the exhaust stream [14]. The gaseous mixture can be separated using a number of diverse but well-established and technologically mature processes that include physical or chemical absorption, adsorption, membrane separation, or combinations of the above. Chemical absorption is by far the most developed CO\({}_{2}\) capture method and is based on the selective reaction between a CO\({}_{2}\)-containing gaseous mixture and a liquid solvent. In this way, the mass transfer following the dissolution of CO\({}_{2}\) in the liquid phase is further enhanced by the chemical reaction promoted by the concentration difference [14]. Subsequently, the CO\({}_{2}\)-loaded solvent is usually driven to a regeneration process in which the reverse reaction occurs, releasing pure CO\({}_{2}\) and lean solvent. This process can either be temperature- or pressure-based, with the most prevalent one being the thermal regeneration in a fashion similar to distillation. Solvent regeneration ensures its recirculation in the system, thus minimizing liquid flowrates and solvent cost. On the other hand, chemical or physical adsorption systems are based on the binding of CO\({}_{2}\) on a solid adsorbent in a reactive or non-reactive step, respectively, and its subsequent release in a regeneration step similar to that of absorption-based systems. Adsorption systems primarily offer reduced energetic demands for sorbent regeneration; however, they involve solids handling which can prove challenging [15]. Finally, membrane-based systems (gas permeators or membrane contactors) can be employed to separate CO\({}_{2}\) from gaseous mixtures. Dense polymeric membranes can be used for selective CO\({}_{2}\) removal from the gaseous mixture (gas permeators), while porous membranes (either polymeric or ceramic) are able to provide very high specific mass transfer area (membrane contactors), resulting in increased CO\({}_{2}\) capture efficiencies in a membrane gas absorption mode of operation. However, it must be noted that the technology readiness level (TRL) of the membrane-based technologies is lower than the previously mentioned ones and their high application potential has to be demonstrated in an operating environment [16]. In general, solvent-based separation is often linked to somewhat increased energy demands for regeneration for the case of chemical absorption or generally admitted low efficiencies for physical absorption, while on the other hand, membrane-based separation alone results in inadequate efficiencies, mainly due to the small CO\({}_{2}\) concentrations (<15%) found in conventional flue gas streams [17]. Adsorption systems can instead easily benefit from the temperature or pressure control of the process. However, sorbent deactivation due to chemical degradation as well as sintering or attrition render them susceptible to high operating and maintenance costs [18]. Post-combustion carbon capture offers significant advantages as a decarbonization measure for the shipping industry.