Lawrence Chen
With the acceleration of climate change due to CO2 emissions being such a prevalent crisis, individuals and companies alike have been desperately pursuing ways to mitigate our anthropogenic carbon footprint within recent decades. One strategy that has been gaining significant traction and news coverage, especially during this year’s COP28, has been carbon capture. Carbon capture technologies seem promising when envisioning a green future due to their potential to abate the emissions of existing fossil fuel power plants, support sustainable hydrogen fuel production, and even allow for a negative carbon footprint. But how does carbon capture work, and is it really humanity’s miraculous solution that people make it out to be?
The first major approach to carbon capture is point source carbon capture, in which CO2 is selectively removed from an output source with a high CO2 concentration, such as the flue stream of a fossil fuel plant. This particular process is known specifically as post-combustion carbon capture, and the primary methods of post-combustion capture are solvent-based capture, physisorption, chemisorption, and membrane-based capture.
In solvent-based capture, CO2 is reactively absorbed at the interface between gaseous CO2 and a liquid solvent, such as monoethanolamine. These solvents aim to diffuse CO2 into the bulk liquid, and they typically have a viscosity tradeoff. Aqueous solvents that are less viscous are more efficient in diffusion, but require more heat when regenerating the solvent for reuse since the water has to be evaporated away. Viscous solvents face the opposite problem, as they have lower thermal requirements but have slower rates of CO2 mass transfer from gas to liquid.
In physisorption, porous physical adsorbents use weak dipole forces to attract CO2 to pore walls. In general, the larger the pore size, the faster the diffusion of CO2 takes place. However, small pores are ideal for maximizing the surface area to volume ratio, and therefore the maximum adsorption capacity as well. Physical adsorbents excel in having low energy requirements due to containing zero moisture and forming weaker dipole bonds. Conversely, carbon capture via chemisorption is the process of CO2 adsorption with covalent bonds. The increased strength of these bonds as opposed to physisorption’s dipole forces increases CO2 affinity, but comes with the cost of higher energy requirements and the potential formation of attractions with molecules other than CO2.
Membrane-based capture simply involves passing a gas stream through a permeable membrane, selectively capturing CO2 due to the various gases in the stream having unique relative permeabilities. This option is promising due to its simplicity, energy efficiency, and lower water requirements, but can require large initial costs to establish the infrastructure. Membranes are also sensitive to trace impurities in the flue stream.
The second primary category of carbon capture is known as direct air capture (DAC), in which CO2 is removed directly from the ambient atmospheric air. Instead of being retrofitted at point sources, DAC operations require the construction of entire plants. This method sounds especially appealing because plants can be deployed anywhere and the process theoretically makes a negative carbon footprint possible. However, the efficiency of the process is very limited due to how low the concentration of CO2 is in a sample of regular air–only 400 parts per million. DAC is therefore a very expensive operation due to the sheer amount of air that must be circulated in order to capture a meaningful volume of CO2–and that’s not even mentioning the significant investment of constructing DAC plants in the first place. Solvents and solid sorbents are typically the capture materials of choice in DAC facilities, and solid sorbent systems are generally preferable as they consume less water during operation–1.6 tonnes of water per tonne of captured CO2 as opposed to up to 7 for sorbent-based DAC.
Clearly, carbon capture is a promising concept but every current approach has significant limitations. The consensus that most climate scientists have come to is that carbon capture cannot be used to justify the continual operation of fossil fuel plants as opposed to transitioning to renewable energy, but that carbon capture will play an essential part in decarbonizing a select few sectors (namely the chemical, steel, and cement industries) on humanity’s path to net zero emissions.
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