Breaking Barriers: Exploring Membrane Technology in Carbon Capture
Carbon dioxide (CO₂) stands as the foremost greenhouse gas, wielding significant influence over the planet’s global warming trajectory. Its prevalence in the atmosphere, primarily attributed to combustion processes involving carbon-rich fuels and oxygen, contributes significantly to the escalation of global temperatures. In prior discussions exploring the basics of carbon capture and the utilization and predominant method of absorption for CO₂ capture, we now turn our attention to another innovative technology in the realm of carbon dioxide capture: membrane technology.
Membrane Technology
A membrane is a barrier between two fluid phases, facilitating the active transport of specific solute molecules based on existing gradients. In membrane separation, the desired solute is selectively transported from one side of the membrane to the other within a module, while the bulk fluid is discharged on the initial side. The effectiveness of membrane separation hinges on the solute’s affinity for the membrane and the membrane’s selectivity (Alami et al., 2020).
Various membrane modules feature diverse arrangements of membranes, either in series or parallel, forming intricate systems or networks (Abanades et al., 2015). Initially, naturally sourced membranes were standard in module construction. However, concerns about their durability and purity led to the widespread adoption of synthetic membranes, now prevalent in the industry. Synthetic membranes include polymeric and inorganic types, with polymeric membranes comprising organic and organic-inorganic hybrids. These membranes can exhibit porous or non-porous characteristics determined by the materials and fabrication methods. Porous membranes enable separation through Knudsen diffusion, while non-porous ones rely on atomic, molecular, or ionic diffusion mechanisms, often in tandem with facilitated transport. In general, inorganic membranes are renowned for their superior performance and stability.
Fundamental of Membrane in Carbon Capture
The advent of membrane technology in CO₂ capture is a recent development spurred by advancements in membrane materials and processing techniques (Dey et al., 2023). Gas separation via membranes gained traction in the 1980s, using cellulose acetate membranes to extract CO₂ from natural gas streams. However, large-scale deployment of membrane systems for gas separation proved challenging in the past, with only limited industrial applications emerging recently. The capture mechanism operates based on a pressure gradient across the membrane. Efficiency increases when this pressure gradient is substantial. Consequently, capturing CO₂ from mixtures with low CO₂ concentrations, indicated by low partial pressures of CO₂, poses challenges to the capture process (Linjala, 2021).
In this article, we will explore the application of membrane technology in three key sectors of carbon capture: pre-combustion, oxy-fuel combustion, and post-combustion capture.
Pre-Combustion Membrane Technology
Pre-combustion CO₂ capture using membranes remains relatively underexplored compared to other methods (Merkel et al., 2010). This approach focuses on separating CO₂ and H₂ from syngas production, boasting notably lower costs than post-combustion processes. Additional equipment, like compressors or vacuum pumps, is required to pressurize the feed stream, contributing to this cost advantage (Alami et al., 2020). Membranes for pre-combustion CO₂ capture typically fall into H₂-selective and CO₂-selective. H₂-selective membranes allow H₂ gas to permeate while retaining CO₂ in the retentate. Conversely, CO₂-selective membranes exhibit high selectivity for CO₂, facilitating its preferential permeation.
Typically, metallic or inorganic membranes are employed to capture CO₂ from the air before it enters the combustion chamber. Metallic membranes, known for their exceptional thermal stability, are commonly favored for pre-combustion CO₂ capture (Ji et al., 2017). Examples include zeolite-based membranes and those incorporating metal-organic frameworks, which are also explored for H₂/CO₂ separation. Recently, mixed matrix membranes (MMM) and ceramic-composite membranes have been trialed for pre-combustion CO₂ capture (Shah et al., 2021). These membranes reportedly demonstrate improved CO₂ permeability and H₂/CO₂ selectivity.
Oxy-Fuel Combustion Membrane Technology
The primary challenge with post-combustion CO₂ capture arises from the presence of N₂ in the flue gas exiting the combustion chamber. Unlike conventional combustion processes, oxy-fuel combustion employs pure oxygen, eliminating N₂ from the exhaust gas — consequently, the combustion of fuel yields CO₂ and water vapor. While water vapor can be condensed from the exhaust stream, the remaining CO₂-rich gas is readily compressible, storable, and utilizable.
Compared to cryogenic air separation, a prevalent CO₂ capture method during oxy-fuel combustion, membrane-based CO₂ capture faces obstacles due to its high operational temperature requirement (1173 K) and expensive membranes (Ji et al., 2017). Moreover, challenges such as high-temperature sealing and ensuring membrane setups’ chemical and mechanical stability persist as technical hurdles to be overcome before commercializing membrane-based CO₂ capture in oxy-fuel combustion systems. An example of such a membrane is the ion-transport ceramic non-porous membrane (Duan et al., 2015).
Post-Combustion Membrane Technology
Most membrane processes for carbon capture focus on post-combustion CO₂ separation, where CO₂ is extracted from industrial flue gases, distinguishing it from other components like H₂ and N₂. During combustion processes, the concentration of CO₂ in flue gases typically ranges from 3% to 15%, posing challenges for standalone membrane capture systems. In post-combustion capture scenarios, strategies such as feed-side compression, permeate-side vacuum pumping, and the utilization of larger membrane areas may be necessary to enhance capture efficiency in low-pressure flue gas environments, albeit at a significant increase in capture costs. To mitigate expenses in conditions characterized by low partial pressure of CO₂, prioritizing high membrane permeability over high selectivity is recommended (Nakao et al., 2019; IEGHG, 2019).
Polymeric membranes, including cellulose acetate, polysulfone, polyethersulfone, polyvinyl alcohol, polyvinylidene difluoride, and polyimide membranes, are commonly used. Polyimides, known for their robust thermal, mechanical, and chemical properties and variable CO₂ permeability, excel in CO₂ separation (Chao et al., 2021). Inorganic membranes, on the other hand, offer higher selectivity, lower CO₂ permeability, and superior chemical and thermal stability.
Recent advancements have led to the development of various technologies improving the CO₂ separation capacity of membrane operations, including mixed matrix membranes (MMMs), ceramic porous membranes, metal-organic frameworks, and composite membranes (Chen et al., 2015). Adding inorganic fillers at micro- or nano-scales into polymeric matrices has been found to enhance membrane thermo-mechanical stability. Alongside traditional membranes, there is a high demand for commercial and advanced polymeric separation membranes such as Pebax®, Matrimid®, PolyActive™, Polaris™, PermSelect™, PRISM™, sulfonated polyether-ether-ketone (SPEEK), and sulfonated-polystyrene-b-poly(ethylene-r-butylene)-b-polystyrene (S-SEBS) for preparing MMMs and composite membranes for post-combustion CO₂ capture. Additionally, materials like polymer intrinsic microporosity (PIM) films and PolyILs (polymeric membranes infused with ionic liquids) are currently under active research for membrane-based post-combustion CO₂ separation.
Future Advancements
Advancements in membrane manufacturing for CO₂ capture should prioritize achieving high CO₂ permeability and selectivity. However, a balance between these two factors is crucial for optimizing membrane performance in the future. Membranes utilized for CO₂ enrichment and for separating N₂, H₂, and O₂ must exhibit long-term thermo-mechanical and chemical stability while being efficient and cost-effective (Kárászová et al., 2020). Polymeric membranes must endure membrane aging and plasticization, while inorganic membranes require improved operational flexibility. All membranes should be manufactured with minimal complexity, scalability, and affordability. The emerging trend of utilizing bio-based “green composite” materials shows promise for producing advanced composite membranes tailored for specific gas separation needs. Green composites, a focal point in current research, offer environmentally friendly alternatives with lower carbon footprints and enhanced sustainability compared to conventional materials (Dey & Ray, 2018).
Industrial Utilization of Membrane Capture
MTR stands as a prominent innovator in membrane capture technology, offering the commercially available Polaris membrane renowned for its high permeability and CO₂-selective properties, applicable to both pre- and post-combustion processes. Constructed from thin-film composite polymer structures, the Polaris membrane has garnered attention for its efficacy (Figueroa et al., 2008).
To address the challenge of treating vast volumes of low-pressure flue gas, MTR has proposed a two-step process, as outlined by IEGHG (2019). In the initial membrane stage, a minor amount of feed compression (2.4–3 bar) and slight vacuum (0.07–0.2 bar) applied to the permeate stream generate the necessary pressure differential for CO₂ removal, capturing approximately half of the CO₂. Subsequently, an air sweep utilizing combustion air in the second stage achieves final CO₂ removal, resulting in an impressive overall CO₂ capture efficiency of 90% (White et al., 2019). However, a drawback of this approach is the requisite modification of the power station boiler to accommodate the elevated CO₂ content in the air.
In Conclusion…
Membrane-based technologies are promising for carbon capture applications, offering efficient and cost-effective solutions across various separation processes. Advancements in membrane manufacturing aim to optimize CO₂ permeability and selectivity, requiring a delicate balance to enhance overall membrane performance. Ensuring long-term stability, efficiency, and affordability of membranes remains paramount, necessitating resilience against aging, plasticization, and operational demands. Bio-based “green composite” materials represent a promising avenue for producing advanced membranes with reduced environmental impact. As research progresses and technological innovations continue, membrane-based carbon capture stands poised to play a pivotal role in mitigating greenhouse gas emissions and advancing toward a sustainable future.
Membrane technology, a relatively recent addition to carbon capture methods, continues to evolve and expand. The content covered in this article represents merely a fraction of the progress achieved in this field. Stay tuned for future discussions as we delve deeper into the advancements and innovations within membrane-based carbon capture technologies.
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