The Absorption Solution: A Key Player in Carbon Emission Reduction
Carbon dioxide capture stands at the forefront of efforts to mitigate carbon emissions stemming from the utilization of fossil fuels (Wang et al., 2014). Having previously examined the fundamentals of carbon capture, our focus now shifts towards a pivotal technology within this realm: absorption. Classified primarily under post-combustion carbon capture, absorption represents a cornerstone approach to capturing carbon dioxide emissions.
In recent years, the post-combustion carbon capture landscape has witnessed a surge in research attention, with absorption emerging as a dominant technique (Kumar et al., 2021). Remarkably, within this domain, two primary systems have garnered significant traction: low-temperature-based solid CO₂ capturing systems, known as low-temperature adsorption, and liquid CO₂ capture material systems, denoted as absorption (Hira et al., 2023). Our exploration will center on the latter, delving into the intricacies of liquid CO₂ capture material systems.
Understanding Absorption Technology
Liquid amine-, ionic liquid-based and several other CO₂ capture systems represent the predominant types of liquid-based CO₂ capture methodologies (Hira et al., 2023). These systems leverage the reactivity of CO₂, facilitating high collection efficiency and rapid absorption rates. Notably, using solvents or liquid form in the absorption process allows for seamless integration into existing fuel-fired power stations, positioning it as an up-and-coming short-to-medium-term CO₂ capture solution.
Absorption-based techniques hold a significant share in the landscape of post-combustion carbon capture technologies, constituting over 60 percent of the methodologies employed (Bhown & Freeman, 2011). This prevalence underscores the effectiveness and widespread adoption of absorption as a carbon capture approach. In contrast to adsorption, where CO₂ molecules adhere to the surface of solid particles, absorption involves the uptake of CO₂ into the bulk phase of another material, such as dissolving CO₂ molecules into a liquid solution.
The absorption cycle operates on a temperature-dependent acid-base reaction, where CO₂, being acidic, reacts with a basic solution at flue gas temperatures ranging from 40 to 60 degrees Celsius. Following this reaction, the resulting “loaded” solution undergoes regeneration in a separate vessel, where heating reverses the reaction, liberating gaseous CO₂. Subsequently, the collected CO₂ is dried, compressed, and transported to a storage reservoir. This cyclic process enables efficient capture and storage of CO₂ emissions, contributing to efforts aimed at reducing carbon footprints (Gao et al., 2020; IEGHG, 2019).
The depicted absorption scheme offers the advantage of minimal energy consumption. Three distinct sources facilitate energy regeneration within the system: firstly, the energy absorbed or released is harnessed to provide heat to the solvent; secondly, the energy generated during the reaction process aids in CO₂ capture; and thirdly, latent heat is utilized for the solvent evaporation step. For instance, in a specific scenario, water serves as the medium for aqueous amine solvents, exemplifying this energy-efficient process.
Now, let us explore the several common absorption technologies: amine-based liquid solvents, ionic liquids, multiphase absorbents, and water-lean solvents. We will delve into the strengths and weaknesses associated with each approach.
Amine-Based Liquid Absorption
Liquid amines are the most commonly utilized absorbents for carbon dioxide capturing and utilization (CCU), renowned for their impressive absorption rates, low viscosity, substantial CO₂ potential, and thermal/chemical stability (Hira et al., 2023). However, despite these advantages, amine-based CO₂ absorption systems encounter significant challenges, including high energy requirements for restoration, amine decomposition, and equipment corrosion during operation. Typically, the absorber releases filtered gas with a CO₂ removal effectiveness of approximately 90 percent, followed by a thermal fluctuation process to liberate the trapped CO₂ from the enriched solution within an elevated heat range of 100 to 120°C. While amine-based absorbents demonstrate notable advantages in absorbing CO₂ at reduced pressure and in dilute flue gas environments, the high energy consumption for desorption presents a notable drawback, reflecting the inherent resistance behavior of chemical absorption.
Conventional Amine
Monoethanolamine (MEA) has enjoyed commercial use as a robust amine absorbent for over five decades, with other liquid amines such as N-MDEA and DEA also commonly utilized and researched (Hira et al., 2023). The CO₂ absorption performance is primarily dictated by the chemical structure of the chosen amines, categorized based on the quantity of hydrogen atoms bound to the nitrogen atom into primary amine (PA; -NH₂ ligand), secondary amine (SA; -NH ligand), and tertiary amine (TA; -N ligand) amines. The order of binding energy between CO₂ and amines typically follows the sequence: PA < SA < TA. The reaction for CO₂ bindings for PA, SA, and TA are as follows.
1. For PAs and SAs: CO₂ + 2R₁R₂NH ⇋ R₁R₂NH₂⁺ + R₁R₂NCOO⁻
2. For TAs: CO₂ + H₂O + R₁R₂R₃N ⇋ 2R₁R₂R₃NH⁺ + HCO₃⁻
However, the principal drawbacks of amine-based absorption methods, including high energy requirements for regeneration, amine degradation, and equipment erosion, stem from intrinsic properties such as poor thermal stability, corrosive nature, and high CO₂ desorption enthalpy (Hira et al., 2023). MEA experiences degradation pathways involving carbamate polymerization and oxidative and thermal degradation, necessitating the use of additional solvent additives. Exposure to NOₓ leads to MEA degradation, resulting in the formation of diethanolamine (DEA), a secondary amine, which further reacts to produce harmful nitrosamines and nitramines (Fostås et al., 2011). Furthermore, while sulfur dioxide (SO₂) at certain levels may inhibit the oxidative degradation of MEA, it significantly enhances thermal degradation as sulfites promote the generation of ammonia through degradation processes (Zhou et al., 2013). Additionally, the presence of oxygen exacerbates the oxidative degradation of MEA (Chi and Rochelle, 2002).
Consequently, significant research efforts have been directed towards finding alternative solvents to conventional amines, with various absorbents based on liquid amines developed to enhance carbon capture capabilities and mitigate associated downsides. These include blended amine solvents, promising commercial applications in the near future, ongoing investigations into CO₂ absorption/desorption mechanisms, solvent screening, and catalyst-assisted regeneration processes to optimize efficiency and reduce energy demands.
EFG+ by Fluor
Fluor, a key player in the development of MEA-based carbon capture technologies, has introduced the Econamine FG Plus (EFG+) technology tailored for post-combustion applications. The advanced EFG+ process incorporates an upgraded solvent formulation, still anchored by MEA as the primary absorbent. EFG+ boasts the ability to employ higher concentrations of amines alongside inhibitors designed to resist corrosion and degradation, thereby enhancing reaction rates and increasing loading capacity (Reddy et al., 2003). This versatile process finds applicability across various sectors including power generation, refineries, and chemical industries. With a track record spanning nearly two decades, EFG+ has been successfully implemented in numerous commercial ventures, with 27 licensed plants currently operational (Fluor, 2020).
Piperazine-AMP Blend
A notable contender challenging MEA is an aqueous amine blend comprising piperazine (PZ) and 2-amino-2-methyl-1-propanol (AMP). PZ, recognized for its high loading capacity and rapid absorption rate, is paired with AMP, a sterically hindered amine known for forming unstable carbamates upon CO₂ reaction, necessitating minimal regeneration energy (Bougie and Iliuta, 2012). In comparison to MEA, PZ/AMP exhibits lower energy demands, reduced costs, and slower degradation rates. Adjusting the PZ/AMP ratio allows for tailoring the process to favor either enhanced mass transfer (higher ratio) or diminished reboiler duty (lower ratio) (Khan et al., 2015). However, PZ/AMP confronts challenges related to solvent degradation leading to the emission of harmful amines, and as of now, there are no commercial developers for PZ/AMP-based carbon capture technologies.
KS-1 and KS-21 by Mitsubishi Heavy Industries
Mitsubishi Heavy Industries (MHI) has developed advanced amine capture solvents known as KS-1 and KS-21, both based on sterically hindered amines, in collaboration with The Kansai Electric Power Company (KEPCO) for their KM CDR capture process applicable across various flue gas scenarios. While KS-1, an earlier iteration, has demonstrated commercial viability (TRL 9), KS-21 represents an enhanced version aimed at achieving superior techno-economic performance. KS-21 has undergone successful demonstrations at a small industrial scale (TRL 7). When compared to MEA, KS-1 exhibits advantages such as lower regeneration energy requirements, higher loading capacity, and reduced degradation rate and corrosivity. With a loading capacity up to 40% higher than MEA, KS-1 enables lower solvent circulation rates, thereby reducing equipment size and capital costs (Iijima, 2005).
CANSOLV by Shell
Shell’s CANSOLV process presents another commercially established post-combustion capture method based on amines, attaining a Technology Readiness Level (TRL) of 9. Beyond CO₂ recovery, this process offers the additional benefit of recovering SO₂ emissions as a valuable by-product, convertible into useful forms such as sulphuric acid or liquid SO₂. The captured CO₂ finds utility primarily in enhanced oil recovery operations (Shell, 2020).
Ionic Liquid Absorption
Ionic liquids (ILs) represent a class of organic salts characterized by a designable organic cation, anion, and neutral molecules, showcasing notable ionic interactions alongside the van der Waals forces typical of many liquid states. These distinctive ionic interactions contribute to their characteristics such as low vapor pressures, thermal stability, miscibility, and elevated viscosity (Butt et al., 2023). This class of substance offers intrinsic structural adaptability, lower corrosion, good thermal stability, low volatility, and higher solubility of CO₂ (Aghaie et al., 2018). These properties position them as promising candidates for CO₂ capture, requiring lower regeneration heat compared to solvents based on different amines. Notably, their primary advantage lies in CO₂ capturing, highlighting their potential to absorb CO₂ while minimizing regeneration energy.
Physical absorption of functional ionic liquids is commonly influenced by structural properties and the interaction between ionic liquid and atmospheric CO₂ (Palomar et al., 2019). Imidazolium-based ionic liquids, for instance, were initially proposed for capturing CO₂ emissions, with modifications such as fluorination of the anionic part and increasing alkyl group chain lengths enhancing CO₂ solubility in physical ILs. However, physical ILs exhibit limitations, including restricted absorption speed, selectivity, and potential, attributed to minimal interaction forces between ILs and CO₂ and high viscosity hindering mass transfer.
Functional ionic liquids hold promise in advancing CO₂ capture efficiency by incorporating different active groups for synthetic absorption (Hira et al., 2023). Despite their advantages, such as low volatility, higher stability, and lower corrosivity, challenges such as high viscosity and cost impede widespread adoption. Task-specific ILs and molecular modeling techniques offer avenues for optimizing viscosity, reaction enthalpy, and cost while blending ILs with amine solutions or co-solvents can mitigate viscosity issues and offset cost concerns associated with regeneration heat. However, a comprehensive feasibility and economic analysis are imperative before widespread implementation.
Multi-Phase Absorbents
Multi-phase absorbents, also known as phase-separating absorbents, undergo a phase change upon reaching a certain level of absorption. This transformation enables the CO₂-lean solvent, initially in a liquid phase upon entering the absorber, to form a separate immiscible liquid phase or precipitate into a solid phase during absorption. The distinctive phase differentiation between the CO₂-lean and CO₂-rich solvents facilitates their easy separation, consequently reducing the large regeneration volumes characteristic of liquid absorbents. These multi-phase absorbents can be divided into two categories: liquid-solid and liquid-liquid phase-separating absorbents (Bui et al., 2018).
Aqueous Ammonia (NH₃)
AAmmonia-based solvents represent one of the most established forms of multi-phase absorbents. Ammonia reacts with CO₂ to produce ammonium and bicarbonate ions, further forming compounds like ammonium carbamates, carbonates, and bicarbonates. Depending on process conditions, the solvent may precipitate and form a slurry during absorption. Precipitation can be mitigated by maintaining low ammonia concentrations or employing high-temperature conditions, albeit at the expense of absorption capacity. Aqueous ammonia capture offers several benefits, including its common and cost-effective nature, non-corrosive properties, good absorption capacity, and low reboiler duty. Additionally, ammonia can capture NOₓ and SOₓ emissions by forming compounds like ammonium sulphate and ammonium nitrate, which can be recovered and utilized. Challenges associated with ammonia-based capture include the risk of unwanted precipitation and high volatility, leading to solvent losses and harmful ammonia emissions. (Yang et al., 2014).
Chilled Ammonia Process by GE
GE, formerly Alstom, has endeavored to enhance aqueous ammonia capture with its Chilled Ammonia Process (CAP). By cooling the aqueous ammonia solvent to temperatures ranging from 0 to 10°C, ammonia’s solubility in water is enhanced, thereby improving CO₂ absorption capacity. Cooling also reduces ammonia vaporization, consequently lowering harmful ammonia slip. Due to the low temperatures involved, the solvent precipitates at lower loading compared to non-chilled processes. GE has explored both solid-mode and non-solid mode CAP processes, successfully demonstrating the technology in various pilot-scale applications, including synthetic gases and flue gases from coal, gas, and oil combustion (Augustsson et al., 2017).
UNO-MK3 by CO₂CRC
CO₂CRC has developed the precipitating UNO-MK3 process based on an aqueous potassium carbonate (K₂CO₃) absorbent, aiming to achieve a 90% capture rate in pre- and post-combustion applications. Aqueous potassium carbonate boasts numerous advantages as a CO₂ capture absorbent, including low regeneration energy, low volatility and corrosivity, non-toxicity, good oxidative degradation resistance, applicability to high absorption temperatures, and the ability to capture SOₓ and NOₓ emissions. SOₓ and NOₓ emissions react with K₂CO₃ to form potassium sulphate (K₂SO₄) and potassium nitrate (KNO₃), which can be recovered and utilized as fertilizers. However, the process faces challenges such as slow reaction rates, which can be improved through the use of catalysts or promoters like boric acid, amines, or amino acids. With unpromoted solvents, the capture rate remains low at around 20–25% (Mumford et al., 2012; Mumford et al., 2015; Smith et al., 2014; Hu et al., 2016).
Hot-Chilled Ammonia Process
Hot carbonate processes utilize concentrated carbonate salt solutions (e.g., K₂CO₃) at elevated temperatures to chemically absorb CO₂ and potentially SO₂. While hot carbonate processes have been employed in various industries to remove CO₂ from industrial gases, they remain relatively novel in carbon capture applications (Chapel et al., 1999).
Hot Potassium Carbonate Process by Stockholm Exergi
The Hot Potassium Carbonate (HPC) process, based on the Sargas process, is a simplified version of the Benfield process designed to eliminate acidic compounds like CO₂ and H₂S from industrial gases. The HPC process configuration is straightforward: compressed flue gas undergoes absorption with hot potassium carbonate solvent, forming bicarbonate. Subsequently, the bicarbonate is directed to an unpressurized desorber column, regenerating back to carbonate and releasing CO₂ and H₂O. The carbonate is then recirculated for absorption after energy recovery. While the technology has been commercially operated in gas refineries and trialed in pilot-scale carbon capture operations with coal-fired flue gases, it remains at the pilot-scale level (Levihn et al., 2019).
DMX Process by IFPEN
DMX, based on 1,3-dipropyl-7-methylxanthine, is a liquid-liquid phase-separating process developed by IFPEN for post-combustion capture applications. Upon absorption, the solvent divides into two immiscible liquid phases, with the CO₂-rich phase separated for regeneration. The process offers advantages including high absorption capacity, low regeneration energy, high degradation resistance, non-corrosiveness, and lower operational costs compared to conventional methods (Broutin et al., 2017).
Water-Lean Solvents
Water-lean (or non-aqueous) solvents have been developed to reduce the high energy-intensity that is characteristic to conventional aqueous solvents. This high energy-intensity is a result of regeneration of large volume flows that mainly consist of water. By using solvents that have lower specific heat values than water the amount of regeneration energy can be reduced (Heldebrandt et al. 2017). The principle of operation with water-lean absorbents is otherwise same as with other liquid absorbent capture processes. The most advanced water-lean solvents are mainly based on organic liquids and amine blends.
eCO₂Sol by Research Triangle Institute
The eCO₂Sol (researched under the name NAS) is a novel non-aqueous solvent developed by the Research Triangle Institute (RTI) for post-combustion applications. Advantages of the process are low regeneration energy requirement (potentially 2.0 GJ/tCO₂), high stability, high loading capacity, and low corrosivity. (Zhou et al. 2018; RTI 2020.)
In Conclusion…
Advancements in carbon capture technologies continue to evolve, with a diverse array of approaches being explored to address the challenges of mitigating CO₂ emissions. From multi-phase absorbents offering improved separation efficiency to novel solvent formulations like water-lean solvents and non-aqueous solvents, the field is witnessing innovative solutions aimed at reducing energy intensity and enhancing capture efficiency. Each technology, whether aqueous ammonia-based solvents like the Chilled Ammonia Process by GE or precipitating absorbents like UNO MK 3 by CO₂CRC, presents unique advantages and challenges, underscoring the complexity of carbon capture endeavors. Despite the progress made, several challenges remain, including slow reaction rates, solvent degradation, and high energy requirements for regeneration. Continued research and development efforts are essential to address these challenges and advance the deployment of carbon capture technologies to combat climate change effectively.
Absorption stands as one of the select carbon capture methodologies available, and this article merely scratches the surface of the various absorption technologies in existence. Perhaps in the future, there will be an opportunity to delve deeper into these technologies, exploring them in greater detail.
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