Decarbonisation Technology - February 2023

utilise a phase change to separate CO 2 in the form of a liquid or solid from lighter gases (Berstad, Anantharaman and Neksa, 2013). The method offers various benefits such as high purity product, avoiding the need for toxic chemicals, and can be applied to a range of CO 2 concentrations. The process also recovers all gas moisture and most gas impurities, which are less volatile than CO 2 (NOx, SOx, Hg, PM) in separable streams. CCC involves a physical separation process based on the differences between the boiling points and the desublimation properties of the components in the gas mixture. Therefore, phase equilibria data are indispensable to define the pressure-temperature conditions in which the CO 2 in a mixture remains as a liquid, gas or solid. The two phases provide two types of cryogenic methods: liquid-vapour separation or conventional cryogenic methods (cryogenic distillation above 193ºK) and solid- vapour separation or non-conventional methods (cryogenic liquid heat exchangers) and packed bed. Since cryogenic separation offers high CO 2 recovery rates and purity levels of 99+%, this technology is gaining considerable attention and exists in the form of hybrid technologies as well (Font-Palma, Cann and Udemu, 2021), (Baxter, et al ., 2021), (Maqsood, et al., 2014). Song et al. reviewed major strategies and technologies for CO 2 capture from fossil fuel combustion and summarised the characteristics of cryogenic technologies for CO 2 capture (Song, et al., 2019). Development challenges for cryogenic technology include cold energy sources, capture costs, and impurities. The results of this investigation indicated that cryogenic CO 2 capture processes could be easily retrofitted to existing industrial emission facilities and avoid the challenges associated with chemical solvents or physical sorbents, with the remark that opportunities exist for the future development of cryogenic-based technologies. Sustainable Energy Solutions (SES) conducted field tests to capture nominally 1 ton of CO2 / day in utility-scale power plants, cement plants, heating plants, and other industrial sites that burn natural gas, biomass, coal, shredded tyres, municipal waste, and combinations of these fuels. These tests captured 95-99% of the CO2

from gases with initial CO 2 contents between 4% and 28% and achieved a CO 2 purity of at least 99%. SES is upscaling the system to a commercial scale (10-80 tons of CO2 per day). This has demonstrated the potential for CCC to contribute to energy storage and DAC innovatively and cost-effectively (Sustainable Energy Solutions, 2021). experiencing a renaissance as their applications expand due to positive features, including higher energy efficiency, flexibility, and sustainability compared to conventional approaches. There are four methods for electrochemical CO 2 capture and recovery: • A pH-swing method • Methods that rely on the binding affinity of CO2 Electrochemical CO2 capture Electrochemical CO 2 capture methods are molecules to redox-active species • Molten carbonate cell method • Hybrid electrochemical processes that combine CO 2 capture with direct conversion. Sharifian and coworkers recently reviewed scientific progress made by electrochemical CO 2 capture processes (Sharifian, Wagterveld, et al., 2021). Approaches using pH-swing to leverage the carbonate equilibrium are most widely studied due to their straightforward operation and the absence of toxic or expensive chemicals. In theory, a mild pH-swing over ca. 2-3 pH units would capture 98+% of the CO 2 . However, in practice, the reaction kinetics of such a mild swing are slow. A wider pH range (ca. 5-6 pH units) or the use of enzymes (carbonic anhydrase) may be needed to improve the kinetics. Electrochemical processes have the potential to be energy efficient as they target molecules directly instead of the medium surrounding them (Stern, Simeon and Herzog, 2013), (Wang, Herzog and Hatton, 2020). Such CO2 capture methods can be applied to all CO 2-containing streams at any concentration including direct capture from air (Eisaman, et al., 2009) or ocean (de Lannoy, et al., 2018), (DiMascio, et al., 2010), can be retrofitted, and allow small footprints with a flexible geometry (Voskian and Hatton, 2019), (Shaw and Hatton, 2020). An electrochemical pH-swing is induced via electrolysis, bipolar membrane electrodialysis