How CO2 is Captured

From factory exhaust to purified gas

Exploring how CO2 is captured before it reaches the atmosphere

CO2 Capture Technologies

CO2 produced during industrial processes can be captured at various stages, including:
  • Pre-combustion: Carbon is separated from fossil fuels before the combustion step.
  • Oxy-fuel combustion: A highly concentrated CO2 stream is captured after combusting fossil fuels in the presence of pure oxygen.
  • Post-combustion: CO2 is captured directly from flue gas at the point of emission.

Absorption: The Industry Standard

Several technologies have been developed for capturing CO2. Among them, chemical absorption is one of the most mature and widely applied methods. In this process, CO2-containing gas streams are absorbed into a solvent, forming unstable compounds that can later be treated, typically through heating, to release the captured CO2. Common solvents used include mono-ethanolamine (MEA), ammonia, and hydroxide solutions [1]. These systems are very selective but also need large amounts of heat or electrical energy to release the CO2 afterwards.
In addition to chemical absorption, CO2 can also be captured through physical absorption using membranes or liquids as water or ionic liquids [2] [3]. Both are based on the principles of Henry’s Law. These methods offer lower energy requirements and therefore greater scalability potential. Membrane-based carbon capture presents are a promising approach, offering compact system design and the potential for high efficiency once the technology reaches maturity. This method operates based on the differential permeation rates and selectivity of gases through various membranes. Despite its advantages, the current limitations such as low gas selectivity and high material costs have hindered its commercial viability. Among the available options, polymer membranes in the form of hollow fibers are the most commonly used in this strategy. However, challenges such as solvent stability, membrane fouling and corrosiveness remain significant obstacles to their broader implementation.

Cryogenic Separation

Another CO2 capture method currently being demonstrated at pilot scale is cryogenic separation, which relies on very low temperatures to exploit the different condensation or sublimation points of CO2 compared to other gases in the flue gas stream. One common cryogenic technique is condensation, which begins with compressing the gas to 20 bar and -20˚C drying and cooling the gas stream, allowing CO₂ to condense into a liquid while other gases remain in their gaseous state. However, this method is only effective to gas streams with high CO2 concentrations (typically >50%) [4]. Alternatively, another cryogenic technology utilises external cooling loops, where flue gas is cooled just above the CO2 frosting temperature which then sublimised at -80˚C. Under these conditions, CO2 precipitates as dry ice, and can be converted into liquid form for storage and transportation [5] by containing it and increase the temperature. The disadvantage is that all the gas needs to be cooled down leading to a high energy demand. Therefore, the most recent innovations aim to reduce the energy demands of cryogenic processes by heat integration. For example, some systems now employ cold nitrogen gas as a refrigerant, combined with regenerative cooling and high-efficiency pre-coolers to create more cost-effective capture methods [6]. While cryogenic CO2 capture offers the key advantage of producing high-purity CO2, its high energy consumption and operational costs remain major challenges for its widespread adoption.

Adsorption: Trapping CO2 on Solids

While the three techniques mentioned above are already in commercial use to varying degrees, several other carbon capture technologies are under active development, though their economic feasibility for industrial-scale deployment remains limited. One such approach is adsorption, in which CO₂ is captured by binding to the surface of solid materials such as zeolites, activated carbon, or metal oxides [7]. Despite its potential, adsorption faces several challenges that hinder its commercial adoption. These include low CO₂ selectivity, competitive adsorption of water vapor, and the difficulty of regenerating the adsorbents [8]. This technology is applied in configurations such as temperature swing adsorption (TSA) and pressure swing adsorption (PSA).

Which Technology Fits Your Process?

CO₂ capture technologyAbsorptionAdsorptionCryogenicMembrane
PrincipleChemical/physical binding of CO₂ to sorbentsSurface binding of CO₂ to a solid sorbentCondensation and desublimation temperatures of CO₂ compared to other gassesSeparation based on permeation and selectivity of membranes
AdvantageEasy to scale up; low cost; high CO₂ selectivitySimple, low energy requirementHigh purity of captured CO₂; no chemical reagentsCompact capture module, simple
DisadvantageSolvent stability and corrosivenessLow CO₂ selectivity due to higher affinity toward water vapourHigh energy consumption and high capital costHigh cost, low selectivity and low stability of the membrane
Maturity levelCommercialNon-commercialPilot scaleNon-commercial
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