Systems Design and Economic Analysis of Direct Air Capture of CO2 through Temperature Vacuum Swing Adsorption Using MIL-101(Cr)-PEI-800 and mmen-Mg2(dobpdc) MOF Adsorbents

Amine-Functionalized Defective MOFs for Direct Air Capture by Postsynthetic Modification

Amine-functionalized defective metal-organic frameworks (DM) showed promise for direct air capture (DAC) of CO2 under ambient conditions. In this work, chromium-based DM was functionalized via a two-step postsynthetic modification with ethylenediamine (EDA), tris(2-aminoethyl)amine (TAEA), and polyethylene-polyamines (PEPA). Characterization by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and scanning electron microscopy (SEM) confirmed successful synthesis and structural integrity. Among the samples, 1:1-PEPA-DM exhibited the best performance, with a CO2 adsorption capacity of 1.26 mmol/g, a regeneration energy of 75.1 kJ/mol, and only 26.62% capacity loss after 12 cycles in ambient air. In contrast, 1:1-TAEA-DM showed a high regeneration energy (158.61 kJ/mol) and a 95.17% capacity loss. Physically impregnating PEPA resulted in a lower capacity (0.94 mmol/g) and a loss of 76.32% after 12 cycles. These results highlight covalent PEPA grafting as a promising strategy for developing durable DAC sorbents.

Advancements and Challenges in Adsorption-Based Carbon Capture Technology: From Fundamentals to Deployment

Carbon dioxide (CO2) adsorption on solid sorbents represents a promising technology for separating carbon from different sources and mitigating anthropogenic emissions. The complete integration of carbon capture technologies in various industrial sectors will be crucial for a sustainable, low-carbon future. Despite developing new sorbents, a comprehensive strategy is essential to realize the full potential and widespread adoption of CO2 capture technologies, including different engineering aspects. This study discusses the pathway for deploying adsorption-based carbon capture technology in fundamental material science aspects, thermo-physical properties behavior at the molecular level, and industrial pilot scale demonstrations. When integrated with process simulation and economic evaluations, these techniques are instrumental in enhancing the efficiency and cost-effectiveness of the capturing processes. While advancements in adsorption-based carbon capture technologies have been notable, their deployment still encounters significant hurdles, including technical, economic, and environmental challenges. Leveraging hybrid systems, renewable energy integration, and the strategic application of emerging machine learning techniques appear promising to address global warming effectively and will consequently be discussed in this investigation.

Polymer Sorbent Design for the Direct Air Capture of CO2

Anthropogenic activities have resulted in enormous increases in atmospheric CO2 concentrations particularly since the onset of the Industrial Revolution, which have potential links with increased global temperatures, rising sea levels, increased prevalence, and severity of natural disasters, among other consequences. To enable a carbon-neutral and sustainable society, various technologies have been developed for CO2 capture from industrial process streams as well as directly from air. Here, direct air capture (DAC) represents an essential need for reducing CO2 concentration in the atmosphere to mitigate the negative consequences of greenhouse effects, involving systems that can reversibly adsorb and release CO2, in which polymers have played an integral role. This work provides insights into the development of polymer sorbents for DAC of CO2, specifically from the perspective of material design principles. We discuss how physical properties and chemical identities of amine-containing polymers can impact their ability to uptake CO2, as well as be efficiently regenerated. Additionally, polymers which use ionic interactions to react with CO2 molecules, such as poly(ionic liquids), are also common DAC sorbent materials. Finally, a perspective is provided on the future research and technology opportunities of developing polymer-derived sorbents for DAC.

Accelerated Testing of PEI-Silica Sorbent Pellets for Direct Air Capture

Amine-based sorbents have shown exceptional CO2 uptake for direct air capture (DAC). However, amine degradation is a major issue for this class of materials, hindering their deployment for large-scale DAC. In this study, a comprehensive evaluation of polyethylenimine (PEI) sorbents was conducted to understand their degradation under process-relevant environments for the DAC of CO2. A solvent-minimized silica-supported PEI-sorbent powder synthesis method using centrifugal mixing was developed. Unlike traditional solvent-assisted impregnated sorbent synthesis methods, the centrifugal mixing method enabled a 94% reduction in volatile and toxic organic solvent use in pelletized sorbent synthesis. The pelletized sorbents exhibited CO2 adsorption capacities consistent with traditional fabrication methods for PEI-based solid sorbents (about 1 mmol/g). The pelletized sorbent degradation behavior was evaluated at three different regeneration temperatures (80, 100, and 120 °C) under nitrogen (N2), ambient air (21% O2), and saturated dry and wet (75% relative humidity (RH)) CO2 environments using fixed-bed breakthrough (BT) experiments. Additionally, accelerated testing (AT) protocols that mimic industrial DAC conditions were developed to assess the long-term stability of the PEI-silica pellets. Our results indicate that the sorbent degrades rapidly (ca. 94% within 24 h) at 120 °C in ambient air (21% O2), demonstrating the detrimental impact of oxygen when compared to an O2-free environment. AT performed for 100 h (equivalent to 33, 100, and 100 cycles) continuously at 80, 100, and 120 °C reveals that dry CO2-induced degradation of the PEI-silica sorbent pellets is 30-40% and 40-50% more than the degradation measured in wet CO2 and inert (pure N2) environments.

Exploring Geometric Properties and Cycle Design in Packed Bed and Monolith Contactors Using Temperature-Vacuum Swing Adsorption Modeling for Direct Air Capture

This study presents a comprehensive comparison between the packed bed and monolith contactor configurations for direct air capture (DAC) via process modeling of a temperature-vacuum swing adsorption (TVSA) process. We investigate various design parameters to optimize performance across different contactor geometries, including pellet size, monolith wall thickness, active sorbent content in monoliths, and packed bed structure configurations, considering both a traditional long column (PB40) and multiple shorter columns configured in parallel (PB5). Our parametric analysis assesses specific exergy consumption, sorbent, and volume requirements across different operating conditions of a five-step TVSA cycle. For minimizing sorbent requirements, PB5 and monoliths with over 80% sorbent loading were the best-performing contactor designs with overlapping performance in the low-exergy region. Beyond this region, PB5 faced limitations in reducing sorbent requirements further and was constrained by a maximum velocity at which it is sensible to operate without substantially increasing the exergy demand. In contrast, monoliths decreased sorbent requirements with minimal exergy increase due to reduced mass transfer resistances and lower pressure drop associated with their thin walls. The analysis of volume requirement-specific exergy Pareto fronts revealed that PB5 was less competitive with this metric due to the requirements for additional void space in the contactor configuration. The study also revealed that optimal sorbent loading for reducing volume requirements in monoliths differed from those minimizing sorbent usage, with the most effective loading being below 100%. Thus, the optimal contactor design varies depending on the goals of minimizing sorbent and volume requirements, and the choice and design of the contactor will depend on the relative costs of these factors. Lastly, our findings challenge the assumption that higher velocities are always preferable for direct air capture, suggesting instead that the operating velocity depends on the contactor configuration.

Incorporating Diurnal and Meter-Scale Variations of Ambient CO2 Concentrations in Development of Direct Air Capture Technologies

To be implemented on climate-relevant scales, direct air capture of CO2 (DAC) will require large capital-intensive facilities and careful attention to cost minimization. In making decisions among potential sites for DAC facilities, all of the factors that will impact process cost and efficiency should be considered. In this paper we focus on a factor that has previously received little attention in the DAC community, namely variations in atmospheric conditions on hourly time scales and length scales of meters. We present data curated from extensive previous studies of biosphere-atmosphere fluxes with observations of CO2 concentration, temperature, and relative humidity (RH) with hourly resolution from many sites in North America. These include locations where typical diurnal variations in CO2 concentration during summer months exceeds 150 ppm. These variations are larger than the seasonal variations that exist between averaged CO2 concentrations in winter and summer, and they are highly correlated with diurnal variations in temperature and RH. Diurnal variations are dependent on the height above ground at which CO2 concentrations are measured, with smaller variations existing at heights of 10 m or more than at ground level. We illustrate the potential implications of these short-term variations for the operation and optimization of a DAC process with process-level calculations for a specific adsorption-based process using amine-rich adsorbents.

Assessment of the Potential of Electrochemical Steps in Direct Air Capture through Techno-Economic Analysis

Direct air capture (DAC) technologies are proposed to reduce the atmospheric CO2 concentration to mitigate climate change and simultaneously provide carbon as a feedstock independent of fossil resources. The currently high energy demand and cost of DAC technologies are challenging and could limit the significance of DAC processes. The present work estimates the potential energy demand and the levelized cost of capture (LCOC) of liquid solvent absorption and solid adsorption DAC processes in the long term. A consistent framework is applied to compare nonelectrochemical to electrochemical DAC processes and estimate the LCOC depending on the electricity price. We determine the equivalent cell voltage needed for the electrochemical steps to achieve comparable or lower energy demand than nonelectrochemical processes. The capital expenses (CapEx) of the electrochemical steps are estimated using analogies to processes that are similar in function. The results are calculated for a range of initial data of CapEx and energy demand to include uncertainties in the data.

On Comparing Packed Beds and Monoliths for CO2 Capture from Air Through Experiments, Theory, and Modeling

This study compares the performance of amine-functionalized γ-alumina sorbents in the form of 3 mm γ-alumina pellets and of a γ-alumina wash-coated monolith for CO2 capture for direct air capture (DAC). Breakthrough experiments were conducted on the two contactors to analyze the adsorption kinetics and performance for different gas feeds. A constant pattern analysis revealed dominant mass transfer resistances in the gas film and in the pores, with axial dispersion also observed, particularly at higher concentrations. A 1D, physical model was used to fit the experiments and thus to estimate mass transfer and axial dispersion coefficients, which appear to be consistent with the hypotheses derived from constant pattern analysis. A dual kinetic model to describe mass transfer was found to better describe the tail behavior in the monolith, whereas a pseudo-first-order model was sufficient to describe breakthroughs on packed beds. A substantial two-order magnitude decrease in mass transfer coefficients was noted when reducing the feed concentration from 5.6% to 400 ppm CO2, thus underscoring the significant mass transfer limitations observed in DAC. Comparison between the contactors revealed notably higher mass transfer coefficients in the monolith compared to the packed beds, which are attributed to shorter diffusion lengths and lower equilibrium capacity. While the faster mass transfer coefficients observed in the monolith experiments led to reduced specific energy consumption and increased adsorption productivity compared to the packed bed at 400 ppm, no significant improvement was observed for the same process at the higher concentration of 5.6% CO2 in the feed. This finding highlights the need to tailor the contactor design to the specific gas separation requirements. This research contributes to the understanding and quantification of mass transfer kinetics at DAC concentrations in both packed bed and monolith contactors. It demonstrates the crucial role of the contactor in DAC systems and the importance of optimizing the adsorption step to enhance productivity and DAC performance.

Assessing Impacts of Atmospheric Conditions on Efficiency and Siting of Large-Scale Direct Air Capture Facilities

The cost and efficiency of direct air capture (DAC) of carbon dioxide (CO2) will be decisive in determining whether this technology can play a large role in decarbonization. To probe the role of meteorological conditions on DAC we examine, at 1 × 1° resolution for the continental United States (U.S.), the impacts of temperature, humidity, atmospheric pressure, and CO2 concentration for a representative amine-based adsorption process. Spatial and temporal variations in atmospheric pressure and CO2 concentration lead to strong variations in the CO2 available in ambient air across the U.S. The specific DAC process that we examine is described by a process model that accounts for both temperature and humidity. A process that assumes the same operating choices at all locations in the continental U.S. shows strong variations in performance, with the most influential variables being the H2O gas phase volume fraction and temperature, both of which are negatively correlated with DAC productivity for the specific process that we consider. The process also shows a moderate positive correlation of ambient CO2 with productivity and recovery. We show that optimizing the DAC process at seven representative locations to reflect temporal and spatial variations in ambient conditions significantly improves the process performance and, more importantly, would lead to different choices in the sites for the best performance than models based on a single set of process conditions. Our work provides a framework for assessing spatial variations in DAC performance that could be applied to any DAC process and indicates that these variations will have important implications in optimizing and siting DAC facilities.

Techno-Economic Assessment of Electromicrobial Production of n-Butanol from Air-Captured CO2

Electromicrobial production (EMP), where electrochemically generated substrates (e.g., H2) are used as energy sources for microbial processes, has garnered significant interest as a method of producing fuels and other value-added chemicals from CO2. Combining these processes with direct air capture (DAC) has the potential to enable a truly circular carbon economy. Here, we analyze the economics of a hypothetical system that combines adsorbent-based DAC with EMP to produce n-butanol, a potential replacement for fossil fuels. First-principles-based modeling is used to predict the performance of the DAC and bioprocess components. A process model is then developed to map material and energy flows, and a techno-economic assessment is performed to determine the minimum fuel selling price. Beyond assessing a specific set of conditions, this analytical framework provides a tool to reveal potential pathways toward the economic viability of this process. We show that an EMP system utilizing an engineered knallgas bacterium can achieve butanol production costs of <$6/gal ($1.58/L) if a set of optimistic assumptions can be realized.

Solar-Powered Direct Air Capture: Techno-Economic and Environmental Assessment

Direct air capture (DAC) of CO2 has gained attention as a sustainable carbon source. One of the most promising technologies currently available is liquid solvent DAC (L-DAC), but the significant fraction of fossil CO2 in the output stream hinders its utilization in carbon-neutral fuels and chemicals. Fossil CO2 is generated and captured during the combustion of fuels to calcine carbonates, which is difficult to decarbonize due to the high temperatures required. Solar thermal energy can provide green high-temperature heat, but it flourishes in arid regions where environmental conditions are typically unfavorable for L-DAC. This study proposes a solar-powered L-DAC approach and develops a model to assess the influence of the location and plant capacity on capture costs. The performed life cycle assessment enables the comparison of technologies based on net CO2 removal, demonstrating that solar-powered L-DAC is not only more environmentally friendly but also more cost-effective than conventional L-DAC.

Comprehensive review and assessment of carbon capturing methods and technologies: An environmental research

A majority of the primary contributors of carbon dioxide (CO2) emissions into the environment have really been out of human-made activities. The levels of CO2 in the atmosphere have increased substantially since the time of the industrial revolution. This has been linked to the use of fossil fuels for energy production, as well as the widespread production of some industrial components like cement and the encroaching destruction of forests. An extreme approach is now necessary to develop the right policies and address the local and global environmental issues in the right way. In this regard, CO2 capturing, utilization, and storage are reliable options that industrial facilities can initiate to overcome this problem. Therefore, we have evaluated the two leading technologies that are used for carbon capture: direct (pre-combustion, post-combustion, and oxy-combustion) and indirect carbon (reforestation, enhanced weathering, bioenergy with carbon capture, and agricultural practices) capturing to provide their current status and progresses. Among the considered processes, the post-combustion techniques are widely utilized on a commercial scale, especially in industrial applications. Technology readiness level (TRL) results have showed that amine solvents, pressure-vacuum swing adsorption, and gas separation membranes have the highest TRL value of 9. In addition, the environmental impact assessment methods have been ranked to evaluate their sustainability levels. The highest global warming potential of 219.53 kgCO2 eq./MWh has been obtained for the post-combustion process. Overall, through this comprehensive review, we have identified some critical research gaps in the open literature in the field of CO2-capturing methods where there are strong needs for future research and technology development studies, for instance, developing stable and cost-effective liquid solvents and improving the adsorption capacity of commercialized sorbents. Furthermore, some research areas, like novel process design, environmental and economic impact assessment of capturing methods with different chemicals and modeling and simulation studies, will require further effort to demonstrate the developed technologies for pilot and commercial-scale applications.

Materials for Direct Air Capture and Integrated CO2 Conversion: Advancement, Challenges, and Prospects

Direct air capture and integrated CO2 conversion (DACC) technologies have emerged as promising approaches to mitigate the increasing concentration of carbon dioxide (CO2) in the Earth's atmosphere. This Perspective provides a comprehensive overview of recent advancements in materials for capturing and converting atmospheric CO2. It highlights the crucial role of materials in achieving efficient and selective CO2 capture as well as catalysts for CO2 conversion. The paper discusses the performance, limitations, and prospects of various materials in the context of sustainable CO2 mitigation strategies. Furthermore, it explores the multiple roles DACC can play in stabilizing atmospheric CO2.

Developing Versatile Contactors for Direct Air Capture of CO2 through Amine Grafting onto Alumina Pellets and Alumina Wash-Coated Monoliths

The optimization of the air-solid contactor is critical to improve the efficiency of the direct air capture (DAC) process. To enable comparison of contactors and therefore a step toward optimization, two contactors are prepared in the form of pellets and wash-coated honeycomb monoliths. The desired amine functionalities are successfully incorporated onto these industrially relevant pellets by means of a procedure developed for powders, providing materials with a CO2 uptake not influenced by the morphology and the structure of the materials according to the sorption measurements. Furthermore, the amine functionalities are incorporated onto alumina wash-coated monoliths that provide a similar CO2 uptake compared to the pellets. Using breakthrough measurements, dry CO2 uptakes of 0.44 and 0.4 mmol gsorbent-1 are measured for pellets and for a monolith, respectively. NMR and IR studies of CO2 uptake show that the CO2 adsorbs mainly in the form of ammonium carbamate. Both contactors are characterized by estimated Toth isotherm parameters and linear driving force (LDF) coefficients to enable an initial comparison and provide information for further studies of the two contactors. LDF coefficients of 1.5 × 10-4 and of 1.2 × 10-3 s-1 are estimated for the pellets and for a monolith, respectively. In comparison to the pellets, the monolith therefore exhibits particularly promising results in terms of adsorption kinetics due to its hierarchical pore structure. This is reflected in the productivity of the adsorption step of 6.48 mol m-3 h-1 for the pellets compared to 7.56 mol m-3 h-1 for the monolith at a pressure drop approximately 1 order of magnitude lower, making the monoliths prime candidates to enhance the efficiency of DAC processes.

Low-concentration CO2 capture using metal-organic frameworks - current status and future perspectives

The ever-increasing atmospheric CO₂ level is considered to be the major cause of climate change. Although the move away from fossil fuel-based energy generation to sustainable energy sources would significantly reduce the release of CO₂ into the atmosphere, it will most probably take time to be fully implemented on a global scale. On the other hand, capturing CO₂ from emission sources or directly from the atmosphere are robust approaches that can reduce the atmospheric CO₂ concentration in a relatively short time. Here, we provide a perspective on the recent development of metal-organic framework (MOF)-based solid sorbents that have been investigated for application in CO₂ capture from low-concentration (<10 000 ppm) CO₂ sources. We summarized the different sorbent engineering approaches adopted by researchers, both from the sorbent development and processing viewpoints. We also discuss the immediate challenges of using MOF-based CO₂ sorbents for low-concentration CO₂ capture. MOF-based materials, with tuneable pore properties and tailorable surface chemistry, and ease of handling, certainly deserve continued development into low-cost, efficient CO₂ sorbents for low-concentration CO₂ capture.

Single-Walled Zeolitic Nanotubes: Advantaged Supports for Poly(ethylenimine) in CO2 Separation from Simulated Air and Flue Gas

Previous research has demonstrated that amine polymers rich in primary and secondary amines supported on mesoporous substrates are effective, selective sorbent materials for removal of CO₂ from simulated flue gas and air. Common substrates used include mesoporous alumina and silica (such as SBA-15 and MCM-41). Conventional microporous materials are generally less effective, since the pores are too small to support low volatility amines. Here, we deploy our newly discovered zeolite nanotubes, a first-of-their-kind quasi-1D hierarchical zeolite, as a substrate for poly(ethylenimine) (PEI) for CO₂ capture from dilute feeds. PEI is impregnated into the zeolite at specific organic loadings. Thermogravimetric analysis and porosity measurements are obtained to determine organic loading, pore filling, and surface area of the supported PEI prior to CO₂ capture studies. MCM-41 with comparable pore size and surface area is also impregnated with PEI to provide a benchmark material that allows for insight into the role of the zeolite nanotube intrawall micropores on CO₂ uptake rates and capacities. Over a range of PEI loadings, from 20 to 70 w/w%, the zeolite allows for increased CO₂ capture capacity over the mesoporous silica by ∼25%. Additionally, uptake kinetics for nanotube-supported PEI are roughly 4 times faster than that of a comparable PEI impregnated in SBA-15. It is anticipated that this new zeolite will offer numerous opportunities for engineering additional advantaged reaction and separation processes.

Direct Air Capture of CO2 through Carbonate Alkalinity Generated by Phytoplankton Nitrate Assimilation

Despite the consensus that keeping global temperature rise within 1.5 °C above pre-industrial level by 2100 reduces the chance for climate change to reach the point of no return, the newest Intergovernmental Panel on Climate Change (IPCC) report warns that the existing commitment of greenhouse gas emission reduction is only enough to contain the warming to 3-4 °C by 2100. The harsh reality not only calls for speedier deployment of existing CO₂ reduction technologies but demands development of more cost-efficient carbon removal strategies. Here we report an ocean alkalinity-based CO₂ sequestration scheme, taking advantage of proton consumption during nitrate assimilation by marine photosynthetic microbes, and the ensuing enhancement of seawater CO₂ absorption. Benchtop experiments using a native marine phytoplankton community confirmed pH elevation from ~8.2 to ~10.2 in seawater, within 3-5 days of microbial culture in nitrate-containing media. The alkaline condition was able to sustain at continued nutrient supply but reverted to normalcy (pH ~8.2-8.4) once the biomass was removed. Measurements of δ¹³C in the dissolved inorganic carbon revealed a significant atmospheric CO₂ contribution to the carbonate alkalinity in the experimental seawater, confirming the occurrence of direct carbon dioxide capture from the air. Thermodynamic calculation shows a theoretical carbon removal rate of ~0.13 mol CO₂/L seawater, if the seawater pH is allowed to decrease from 10.2 to 8.2. A cost analysis (using a standard bioreactor wastewater treatment plant as a template for CO₂ trapping, and a modified moving-bed biofilm reactor for nitrate recycling) indicated that a 1 Mt CO₂/year operation is able to perform at a cost of ~$40/tCO₂, 2.5-5.5 times cheaper than that offered by any of the currently available direct air capture technologies, and more in line with the price of $25-30/tCO₂ suggested for rapid deployment of large-scale CCS systems.

The Capture and Transformation of Carbon Dioxide in Concrete: A Review

Concrete is one of the most commonly used engineering materials in the world. Carbonation of cement-based materials balances the CO2 emissions from the cement industry, which means that carbon neutrality in the cement industry can be achieved by the carbon sequestration ability of cement-based materials. Carbon dioxide is a symmetrical molecule and is difficult to separate. This work introduces the important significance of CO2 absorption by using cement-based materials, and summarizes the basic characteristics of carbonation of concrete, including the affected factors, mathematical modeling carbonization, and the method for detecting carbonation. From the perspective of carbon sequestration, it mainly goes through carbon capture and carbon storage. As the first stage of carbon sequestration, carbon capture is the premise of carbon sequestration and determines the maximum amount of carbon sequestration. Carbon sequestration with carbonization reaction as the main way has been studied a lot, but there is little attention to carbon capture performance. As an effective way to enhance the carbon sequestration capacity of cement-based materials, increasing the total amount of carbon sequestration can become a considerably important research direction.

Recent advances in direct air capture by adsorption

Significant progress has been made in direct air capture (DAC) in recent years. Evidence suggests that the large-scale deployment of DAC by adsorption would be technically feasible for gigatons of CO₂ capture annually. However, great efforts in adsorption-based DAC technologies are still required. This review provides an exhaustive description of materials development, adsorbent shaping, in situ characterization, adsorption mechanism simulation, process design, system integration, and techno-economic analysis of adsorption-based DAC over the past five years; and in terms of adsorbent development, affordable DAC adsorbents such as amine-containing porous materials with large CO₂ adsorption capacities, fast kinetics, high selectivity, and long-term stability under ultra-low CO₂ concentration and humid conditions. It is also critically important to develop efficient DAC adsorptive processes. Research and development in structured adsorbents that operate at low-temperature with excellent CO₂ adsorption capacities and kinetics, novel gas-solid contactors with low heat and mass transfer resistances, and energy-efficient regeneration methods using heat, vacuum, and steam purge is needed to commercialize adsorption-based DAC. The synergy between DAC and carbon capture technologies for point sources can help in mitigating climate change effects in the long-term. Further investigations into DAC applications in the aviation, agriculture, energy, and chemical industries are required as well. This work benefits researchers concerned about global energy and environmental issues, and delivers perspective views for further deployment of negative-emission technologies.

Plastic Waste Product Captures Carbon Dioxide in Nanometer Pores

Plastic waste (PW) and increasing atmospheric carbon dioxide (CO₂) levels are among the top environmental concerns presently facing humankind. With an ambitious 2050 zero-CO₂ emissions goal, there is a demand for economical CO₂ capture routes. Here we show that the thermal treatment of PW in the presence of potassium acetate yields an effective carbon sorbent with pores width of 0.7-1.4 nm for CO₂ capture. The PW to carbon sorbent process works with single or mixed streams of polyolefin plastics. The CO₂ capacity of the sorbent at 25 °C is 17.0 ± 1.1 wt % (3.80 ± 0.25 mmol g^-1) at 1 bar and 5.0 ± 0.6 wt % (1.13 ± 0.13 mmol g^-1) at 0.15 bar, and it regenerates upon reaching 75 ± 5 °C. The CO₂ capture cost from flue gas via this technology is estimated to be <$21 ton^-1 CO₂, much lower than competing CO₂ capture technologies. Hence, this PW-derived carbon material should find utility in the capture of CO₂ from point sources of high CO₂ emissions while providing a use for otherwise deleterious PW.

Electrochemical CO2 reduction - The macroscopic world of electrode design, reactor concepts & economic aspects

For the efficient electrochemical conversion of CO2 into valuable chemical feedstocks, a well-coordinated interaction of all electrolyzer compartments is required. In addition to the catalyst, whose role is described in detail in the part "Electrochemical CO2 Reduction toward Multicarbon Alcohols - The Microscopic World of Catalysts & Process Conditions" of this divided review, the general cell setups, design and manufacture of the electrodes, membranes used, and process parameters must be optimally matched. The authors' goal is to provide a comprehensive review of the current literature on how these aspects affect the overall performance of CO2 electrolysis. To be economically competitive as an overall process, the framework conditions, i.e., CO2 supply and reaction product treatment must also be considered. If the key indicators for current density, selectivity, cell voltage, and lifetime of a CO2 electrolyzer mentioned in the techno-economic consideration of this review are met, electrochemical CO2 reduction can make a valuable contribution to the creation of closed carbon cycles and to a sustainable energy economy.

Modeling of Vacuum Temperature Swing Adsorption for Direct Air Capture Using Aspen Adsorption

The paper evaluates the performance of an adsorption-based technology for CO2 capture directly from the air at the industrial scale. The approach is based on detailed mass and energy balance dynamic modeling of the vacuum temperature swing adsorption (VTSA) process in Aspen Adsorption software. The first step of the approach aims to validate the modeling thanks to published experimental data for a lab-scale bed module in terms of mass transfer and energy performance on a packed bed using amine-functionalized material. A parametric study on the main operating conditions, i.e., air velocity, air relative moisture, air temperature, and CO2 capture rate, is undertaken to assess the global performance and energy consumption. A method of up-scaling the lab-scale bed module to industrial module is exposed and mass transfer and energy performances of the industrial module are provided. The scale up from lab scale to the industrial size is conservative in terms of thermal energy consumption while the electrical consumption is very sensitive to the bed design. Further study related to the engineering solutions available to reach high global gas velocity are required. This could be offered by monolith-shape adsorbents.

Moving beyond 90% Carbon Capture by Highly Selective Membrane Processes

A membrane-based system with a retentate recycle process in tandem with an enriching cascade was studied for >90% carbon capture from coal flue gas. A highly CO2-selective facilitated transport membrane (FTM) was utilized particularly to enhance the CO2 separation efficiency from the CO2-lean gases for a high capture degree. A techno-economic analysis showed that the retentate recycle process was advantageous for ≤90% capture owing to the reduced parasitic energy consumption and membrane area. At >90% capture, the enriching cascade outperformed the retentate recycle process since a higher feed-to-permeate pressure ratio could be applied. An overall 99% capture degree could be achieved by combining the two processes, which yielded a low capture cost of USD47.2/tonne, whereas that would be USD 42.0/tonne for 90% capture. This FTM-based approach for deep carbon capture and storage can direct air capture for the mitigation of carbon emissions in the energy sector.

Technological Options for Direct Air Capture: A Comparative Process Engineering Review

The direct capture of CO₂ from ambient air presents a means of decelerating the growth of global atmospheric CO₂ concentrations. Considerations relating to process engineering are the focus of this review and have received significantly less attention than those relating to the design of materials for direct air capture (DAC). We summarize minimum thermodynamic energy requirements, second law efficiencies, major unit operations and associated energy requirements, capital and operating expenses, and potential alternative process designs. We also highlight process designs applied toward more concentrated sources of CO₂ that, if extended to lower concentrations, could help move DAC units closer to more economical continuous operation. Addressing shortcomings highlighted here could aid in the design of improved DAC processes that overcome trade-offs between capture performance and DAC cost. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A Comparative Study of Different Sorbents in the Context of Direct Air Capture (DAC): Evaluation of Key Performance Indicators and Comparisons

Direct air capture can be based on an adsorption system, and the used sorbent (chemisorbents or physisorbents) influences process. In this work, two amine-functionalized sorbents, as chemisorbents, and three different metal organic frameworks, as physisorbents, are considered and compared in terms of some key performance indicators. This was carried out by developing a mathematical model describing the adsorption and desorption stages. An independent analysis was carried out in order to verify data reported in the literature. Results show that the equilibrium loading is a critical parameter for adsorption capacity, energy consumption, and cost. The considered metal organic frameworks are characterized by a lower equilibrium loading (10−4 mol/kg) compared to chemisorbents (10−1 mol/kg). For this reason, physisorbents have higher overall energy consumptions and costs, while capturing a lower amount of carbon dioxide. A reasonable agreement is found on the basis of the operating conditions of the Climeworks company, modelling the use of the same amine cellulose-based sorbent. The same order of magnitude is found for total costs (751 USD/tonneCO2 for our analysis, compared to the value of 600 USD/tonneCO2 proposed by this company).

Sub-Ambient Temperature Direct Air Capture of CO2 using Amine-Impregnated MIL-101(Cr) Enables Ambient Temperature CO2 Recovery

Due to the dramatically increased atmospheric CO₂ concentration and consequential climate change, significant effort has been made to develop sorbents to directly capture CO₂ from ambient air (direct air capture, DAC) to achieve negative CO₂ emissions in the immediate future. However, most developed sorbents have been studied under a limited array of temperature (>20 °C) and humidity conditions. In particular, the dearth of experimental data on DAC at sub-ambient conditions (e.g., -30 to 20 °C) and under humid conditions will severely hinder the large-scale implementation of DAC because the world has annual average temperatures ranging from -30 to 30 °C depending on the location and essentially no place has a zero absolute humidity. To this end, we suggest that understanding CO₂ adsorption from ambient air at sub-ambient temperatures, below 20 °C, is crucial because colder temperatures represent important practical operating conditions and because such temperatures may provide conditions where new sorbent materials or enhanced process performance might be achieved. Here we demonstrate that MIL-101(Cr) materials impregnated with amines (TEPA, tetraethylenepentamine, or PEI, poly(ethylenimine)) offer promising adsorption and desorption behavior under DAC conditions in both the presence and absence of humidity under a wide range of temperatures (-20 to 25 °C). Depending on the amine loading and adsorption temperature, the sorbents show different CO₂ capture behavior. With 30 and 50 wt % amine loadings, the sorbents show weak and strong chemisorption-dominant CO₂ capture behavior, respectively. Interestingly, at -20 °C, the CO₂ adsorption capacity of 30 wt % TEPA-impregnated MIL-101(Cr) significantly increased up to 1.12 mmol/g from 0.39 mmol/g at ambient conditions (25 °C) due to the enhanced weak chemisorption. More importantly, the sorbents also show promising working capacities (0.72 mmol/g) over 15 small temperature swing cycles with an ultralow regeneration temperature (-20 °C sorption to 25 °C desorption). The sub-ambient DAC performance of the sorbents is further enhanced under humid conditions, showing promising and stable CO₂ working capacities over multiple humid small temperature swing cycles. These results demonstrate that appropriately designed DAC sorbents can operate in a weak chemisorption modality at low temperatures even in the presence of humidity. Significant energy savings may be realized via the utilization of small temperature swings enabled by this weak chemisorption behavior. This work suggests that significant work on DAC materials that operate at low, sub-ambient temperatures is warranted for possible deployment in temperate and polar climates.

Carbon dioxide capture with zeotype materials

This review describes the application of zeotype materials for the capture of CO2 in different scenarios, the critical parameters defining the adsorption performances, and the challenges of zeolitic adsorbents for CO2 capture.

Perspective - the need and prospects for negative emission technologies - direct air capture through the lens of current sorption process development

We provide a perspective on the development of direct air capture (DAC) as a leading candidate for implementing negative emissions technology (NET). We introduce DAC based on sorption, both liquid and solid, and draw attention to challenges that these technologies will face. We provide an analysis of the limiting mass transfer in the liquid and solid systems and highlight the differences.

Controlled Synthesis of Metal-Organic Frameworks in Scalable Open-Porous Contactor for Maximizing Carbon Capture Efficiency

Metal-organic frameworks (MOFs) are a class of microporous materials that have been highlighted with fast and selective sorption of gas molecules; however, they are at least partially unstable in the scale-up process. Here, we report a rational shaping of MOFs in a scalable architecture of fiber sorbent. The long-standing stability challenge of MOFs was resolved by using stable metal oxide precursors that are subject to controlled surface oxide dissolution-growth chemistry during the Mg-based MOF synthesis. Highly uniform MOF crystals are synthesized along with the open-porous fiber sorbents networks, showing unprecedented cyclic CO₂ capacities in both flue gas and direct air capture (DAC) conditions. The same chemistry enables an in situ flow synthesis of Mg-MOF fiber sorbents, providing a scalable pathway for MOF synthesis in an inert condition with minimal handling steps. This modular approach can serve both as a reaction stage for enhanced MOF fiber sorbent synthesis and as a "process-ready" separation device.

Photovoltaic-driven microbial protein production can use land and sunlight more efficiently than conventional crops

Population growth and changes in dietary patterns place an ever-growing pressure on the environment. Feeding the world within sustainable boundaries therefore requires revolutionizing the way we harness natural resources. Microbial biomass can be cultivated to yield protein-rich feed and food supplements, collectively termed single-cell protein (SCP). Yet, we still lack a quantitative comparison between traditional agriculture and photovoltaic-driven SCP systems in terms of land use and energetic efficiency. Here, we analyze the energetic efficiency of harnessing solar energy to produce SCP from air and water. Our model includes photovoltaic electricity generation, direct air capture of carbon dioxide, electrosynthesis of an electron donor and/or carbon source for microbial growth (hydrogen, formate, or methanol), microbial cultivation, and the processing of biomass and proteins. We show that, per unit of land, SCP production can reach an over 10-fold higher protein yield and at least twice the caloric yield compared with any staple crop. Altogether, this quantitative analysis offers an assessment of the future potential of photovoltaic-driven microbial foods to supplement conventional agricultural production and support resource-efficient protein supply on a global scale.

Understanding the Effect of Water on CO2 Adsorption

Carbon capture from large sources and ambient air is one of the most promising strategies to curb the deleterious effect of greenhouse gases. Among different technologies, CO₂ adsorption has drawn widespread attention mostly because of its low energy requirements. Considering that water vapor is a ubiquitous component in air and almost all CO₂-rich industrial gas streams, understanding its impact on CO₂ adsorption is of critical importance. Owing to the large diversity of adsorbents, water plays many different roles from a severe inhibitor of CO₂ adsorption to an excellent promoter. Water may also increase the rate of CO₂ capture or have the opposite effect. In the presence of amine-containing adsorbents, water is even necessary for their long-term stability. The current contribution is a comprehensive review of the effects of water whether in the gas feed or as adsorbent moisture on CO₂ adsorption. For convenience, we discuss the effect of water vapor on CO₂ adsorption over four broadly defined groups of materials separately, namely (i) physical adsorbents, including carbons, zeolites and MOFs, (ii) amine-functionalized adsorbents, and (iii) reactive adsorbents, including metal carbonates and oxides. For each category, the effects of humidity level on CO₂ uptake, selectivity, and adsorption kinetics under different operational conditions are discussed. Whenever possible, findings from different sources are compared, paying particular attention to both similarities and inconsistencies. For completeness, the effect of water on membrane CO₂ separation is also discussed, albeit briefly.

Guidelines for Techno-Economic Analysis of Adsorption Processes

Techno-economic analyses (TEAs) of CO2 capture technologies have risen in popularity, due to growing interest in meeting CO2 emissions reduction targets. Adsorption processes are one of the technologies proposed for CO2 capture, and although difficult, standardization of TEAs for adsorption should be attempted. The reason is that TEAs are often referred to as input data to other forms of modeling, to guide policy, and to act as summaries for those unfamiliar with adsorption processes. Herein, we discuss the aspects that should be considered when conducting TEAs for CO2 adsorption processes, and we present the implications of choices made at the TEA stage and offer guidance on best practice. our aim is to make TEAs of adsorption processes widely accessible to the adsorption community, and more generally to communities engaged in the evaluation of CCS technologies.

Effect of Extended Aging and Oxidation on Linear Poly(propylenimine)-Mesoporous Silica Composites for CO2 Capture from Simulated Air and Flue Gas Streams

Physical aging or degradation of amine-containing polymers and supported amine adsorbents is a critical issue that could limit the practical application of such materials for CO₂ capture. However, to date, there is a scarcity of studies that evaluate the long-term stability of amine-based sorbents without the exclusive use of accelerated aging tests. Here, we demonstrate that extended aging (∼2 years) of linear poly(propylenimine) (LPPI) confined in mesoporous silica (SBA-15) supports does not drastically impact the CO₂ adsorption performance under simulated flue gas (10% CO₂) and direct air capture (DAC, 400 ppm CO₂) conditions, although the behavior of the aged sorbents and polymers in the two CO₂ concentration regimes differs. The sorbents made with aged LPPI have modestly decreased CO₂ uptake performance (≲20% lower) compared to the fresh polymers, with overall good CO₂ cycling performance. The data indicate that only slow degradation occurs under the deployed ambient storage conditions. Even after extended aging, the LPPI-based sorbents preserved their ability to display stable temperature-swing cycling performance. In parallel, the impact of blending LPPI polymers of different number-average molecular weights, M_n, is evaluated, seeking to understand its impact on adsorbent performance. The results demonstrate that the blends of two M_n aged LPPI give similar CO₂ adsorption performance to adsorbents made from a single-M_n LPPI, suggesting that molecular weight will not negatively impact adsorbent performance in the studied M_n range. After an accelerated oxidation experiment, the aged LPPI sorbents retained a larger portion of the samples' original performance when cycling under simulated flue gas conditions than under DAC conditions. However, in each case, the oxidized sorbents could be cycled repeatedly with consistent uptake performance. Overall, these first of their kind extended aging tests suggest that LPPI-based amine adsorbents offer promise for long-term, stable use in carbon capture applications.