These kinds of technologies enable the removal of CO₂ from flue gases, when applied downstream from combustion devices (boiler or gas turbines) or can produce carbon-free fuels in the context of pre-combustion processes [1]. Carbon separation may be a consequence of CO₂ retention in sorbents or solvents or due to a chemical reaction between CO₂ and the separation agent [1]. The latter involves solvents such as amines, being aqueous solutions of monoethanol amine the most commercially widespread process – especially in the context of gas sweetening [2]. CO₂ is chemically absorbed and, after that, the solvent is regenerated in a stripper column. CO₂ is obtained at high purity in a partial condenser, while steam must be provided for the operation of the reboiler of the column. This is a highly intense separation, with energy usage of 3.5 MJ/kg of captured CO₂ [2]. 

Different configurations have been studied with the aim of reducing the energy penalty of the amine process configurations [3]. Furthermore, adsorption and membrane-based separation technologies have been considered, applied as post- and pre-combustion units [4-6]. It must be noted that the driving force for the separation, in this kind of processes, relies on the difference between the CO₂ partial pressure in the flue gas and in the separation agent. Consequently, the energy penalty to increase CO₂ purity from the flue gas or fuel gas to storage compatible compositions (95% CO₂  mole fraction) is lower in the case of high molar fraction CO₂ streams [1]. Physical separation technologies are then the most suitable options in the case of gasification plants. 

Captured CO₂ is pressurised, reaching critical conditions (close to 7.5 MPa), for enabling long distance transport and geological storage [1]. This involves power usage due to the gas compression. Thus, staged and intercooled compression stages have been the subject of optimisation [1]. 

It must be noted that CO₂ capture technologies allow the decrease of direct CO₂ emissions whilst enabling co-capture of certain air pollutants [7]. However, plants with equipped CO₂ removal technologies exhibit larger fuel consumption to keep constant output delivery due to the energy penalty of the capture technologies. This may cause larger upstream emissions, and, in some cases, thermal and chemical degradation of separation agents could also produce emissions from ammonia and amine compounds [8]. Environmental comparison of the different technologies should then pay attention to life cycle impacts across the full system. 

Captured CO₂ can also be employed as feedstock for the production of certain chemicals [9], fuels [10], or utilised to enrich agriculture greenhouses [11]. CO₂ utilisation is a field of ongoing research, as a way to mitigate capture related costs, and as a deposition method in areas which are not close to geological storage hubs.

[1] IPCC. Special Report on Carbon Dioxide Capture and Storage. 2005 

[2] CAESAR. European best practice guidelines for assessment of CO2 capture technologies.

[3] Ahn H, Luberti M, Liu Z, Brandani S. Process configuration studies of the amine capture process for coal-fired power plants. Int. J. Greenh. Gas. Con. 2013; 16: 29-40.  

[4] Oreggioni G D, Brandani S, Luberti M, Baykan Y, Friedrich D, Ahn H. CO2 capture from syngas by an adsorption process at a biomass gasification CHP plant: Its comparison with amine-based CO2 capture. International Journal of Greenhouse Gas Control .2015; 35: 71-

[5] Ferrari M C, Bocciardo D, Brandani S. Integration of multi-stage membrane carbon capture processes to coal-fired power plants using highly permeable polymers. Green Energy & Environment. 2016.

[6] Luberti M, Oreggioni G D, Ahn H. Design of a Rapid Vacuum Pressure Swing Adsorption (RVPSA) Process for Post-combustion CO2 Capture from a Biomass-fuelled CHP Plant. Journal of Environmental Chemical Engineering. 2017; 5: 3973-3982. 

[7] Rao AB, Rubin E. A technical, economic, and environmental assessment of amine-based CO2 capture technology for power plant greenhouse gas control. Environmental Science & Technology. 2002; 20: 4467-4475.

[8] Kornneef, J., Van Keulen, T., Faaij, A., Turkenburg, W., 2008. Life cycle assessment of a pulverized coal power plant with post-combustion capture, transport and storage of CO2. International Journal of Greenhouse Gas Control 2, 448–467.

[9] Fernandez Dacosta C, Van de Spek M, Hun C R, Oreggioni G D, Skagestad R, Parihar P, Gokak D T, Stromman A H, Ramirez A. Prospective techno-economic and environmental assessment of carbon capture at a refinery and CO2 utilisation in polyol synthesis. Journal of CO2 Utilization. 2017; 21:405-4222. 

[10] Schakel W, Oreggioni G D, Singh B, Stømann A, Ramirez A. Assessing the techno-environmental performance of CO2 utilisation via dry reforming of methane for the production of dimethyl ether. Journal of CO2 utilization. 2016; 16: 138-149 

[11] Oreggioni G D, Luberti M, Tassou S A. Agriculture greenhouse CO2 utilization in anaerobic-digestion-based biomethane production plants: A techno-economic and environmental assessment and comparison with CO2 geological storage. Applied Energy. 2019;242:1753-176.