# Analysis of Technologies for Carbon Dioxide Capture from the Air
###### Abstract  
The increase in CO\({}_{2}\) concentration in the atmosphere has prompted the research community to find solutions for this environmental problem, which causes climate change and global warming. CO\({}_{2}\) removal through the use of negative emissions technologies could lead to global emission levels becoming net negative towards the end of this century. Among these negative emissions technologies, direct air capture (DAC), in which CO\({}_{2}\) is captured directly from the atmosphere, could play an important role. The captured CO\({}_{2}\) can be removed in the long term and through its storage can be used for chemical processes, allowing closed carbon cycles in the short term. For DAC, different technologies have been suggested in the literature, and an overview of these is proposed in this work. Absorption and adsorption are the most studied and mature technologies, but others are also under investigation. An analysis of the main key performance indicators is also presented here and it is suggested that more efforts should be made to develop DAC at a large scale by reducing costs and improving efficiency. An additional discussion, addressing the social concern, is indicated as well. atmospheric CO\({}_{2}\); direct air capture; CO\({}_{2}\) reduction; CO\({}_{2}\) removal; key performance indicators; carbon capture technologies +
Footnote †: journal: Applied Sciences  
(power-to-gas (PtG), power-to-liquid (PL) and power-to-chemical (PtC)), closing the carbon cycle in the short term (direct air carbon capture and utilization--DACCU) [7, 13, 14, 15]. There are some advantages of DAC. DAC is independent of CO\({}_{2}\) source location, because CO\({}_{2}\) is distributed homogeneously throughout the atmosphere, minimizing or eliminating CO\({}_{2}\) transportation costs when the capture plant is situated near the utilization or storage site [16]; the capture plant can be located in a particular place where the the renewable energy can be exploited at the minimum cost [17]; DAC can tackle emissions from the transport sector and other small sources that constitute about 50% of total emissions; air is ubiquitous, with few contaminants that could cause the degradation of solvents/sorbents (e.g., NO\({}_{\omega}\), SO\({}_{\omega}\) and particulates) [18]; arable land is not required, avoiding loss of agricultural land [9]; the technology contributes to the achievement of Sustainable Development Goals (SDGs) 13, 9, 6 (by-product water could be used as fresh water in general), 8 (creation of new green jobs), 7 (exploitation of renewable energies), 11 and 12 (production of fuels and materials from atmospheric CO\({}_{2}\)) and 3 (positive effect on health) [17]. However, there are also some disadvantages: the long term storage and sequestration of CO\({}_{2}\) are critical (costs, long-term monitoring, potentially induced seismicity and leakage) [19]; low CO\({}_{2}\) partial pressure in the air, about 400 ppm, 350 times lower than coal flue gas so that large volumes of air should be processed to capture a significant amount of CO\({}_{2}\)[7, 13, 20]; high energy and cost requirements due to the dilute nature of CO\({}_{2}\) in the air, requiring a large volume of air to be treated [21]. DAC technologies include absorption, adsorption, ion exchange resin, mineral carbonation, membrane, photocatalysis, cryogenic separation, electrochemical approach and electrodialysis approaches [6, 18, 22, 23, 24, 25, 26]. Absorption and adsorption technologies are the most analyzed and studied in the literature [18]. DAC technologies are characterized by thermodynamic issues related to the separation of CO\({}_{2}\) from the air, which are more significant than in the separation of CO\({}_{2}\) from the flue gas [27]. Thermodynamic calculation provides the minimum amount of energy for a defined separation depending on the temperature, pressure and initial and final concentrations of the streams.