It is inevitable that the world will still rely on fossil fuels in the coming years. To reduce greenhouse emissions produced during their combustion, carbon capture and storage (CCS) need to be adopted to mitigate climate change effects. For this matter, different technologies have been investigated for an efficient carbon capture process. Among these technologies, liquid amine scrubbing is the most mature method. However, new technologies involving porous materials are arising in the quest to increase carbon capture capacity and economic advantages.
Liquid amine scrubbing
The amine scrubbing method was created in the 1930s to separate CO2 and hydrogen from natural gas to improve natural gas quality. Recently, amine scrubbers have been adapted for CCS due to their technology maturity, which allows their use at the industrial scale for post-combustion carbon capture. Due to this technology's longevity, several commercial plants have been constructed to capture CO2 from flue gas.
Amines are organic compounds that show high CO2 solubility when used as solvents. The low solvent cost and good trapping effect favor the absorption with aqueous amine solutions. The most used amine solutions include Diethanolamine (DEA), Methyldiethanolamine (MDEA), Diisopropanolamine (DIPA), and Aminoethoxy Ethanol (Diglycolamine) (DGA).The chemical absorption with liquid amines is carried out in two steps: 1) the flue gas reacts with the solvent to capture CO2; 2) the CO2-rich solution is sent to a stripper to regenerate the solvent at temperatures between 120 and 150 °C. This method can capture 85-95% carbon dioxide in the flue gas with high purity above 99.9%.However, amine scrubbing also presents disadvantages such as:
- high-energy penalty for solvent regeneration,
- thermal or oxidative degradation of amines to toxic chemical such as ammonia or harmful nitro-compounds
- foaming, equipment corrosion and evaporation loss
- environmental issues due to waste disposal with volatile degradation compounds, which have been identified as potentially harmful to human health and the environment.
Adsorption using MOFs
Among porous materials for CO2 capture, Metal-Organic Frameworks (MOFs) have shown remarkable adsorption capacity and selectivity in post-combustion carbon capture. MOFs possess a unique chemical structure due to their hybrid nature (metal centers and organic ligands). They have an extraordinary surface area and pore volume and allow control over the pore structure (size and shape).
MOFs offer a facilitated and rational framework design for custom fabrication. Besides, they can be modified in post-synthetic processes to enhance a specific property for a determined application. Taking advantage of post-synthetic modification, MOFs have been tailored with amine residues to improve their CO2 adsorption properties. For instance, an amine-modified MOF showed enhanced CO2 capture capacity compared to the unmodified material (CO2 capacity increased up to four times). In this way, by using post-synthetic modifications, the high affinity of amines towards carbon dioxide is exploited in porous materials such as MOF, a unique feature offered by these materials. Moreover, unlike amines, MOFs are more thermally stable in temperatures up to 200 °C.
The major drawback of amine scrubbing technology is the energy-intensive regeneration step, performed in temperatures between 120 and 150 °C. In this sense, MOFs can considerably decrease energy consumption since MOF regeneration can be performed at temperatures as low as 90 °C at 1 bar, so that available low-temperature waste heat could operate the regeneration step. Besides the lower energy requirement, Metal-Organic Frameworks show good recyclability. Tests performed in the amine-modified MOF show a minimal capacity loss after 15 cycles, making them a promising option for industrial applications.In comparison to aqueous amine technology, MOFs offer:
- enhanced stability
- superior selectivity over nitrogen
- lower energy requirements
- comparable working capacity
Owing to recent advances in MOF technology for carbon capture, they are one of the most promising options as amine technology replacement. Furthermore, other gas separation applications can benefit by implementing MOFs, such as biogas upgrading and hydrocarbon separations.
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