Prediction of CO2 capture performance of a direct air capture unit under representative atmospheric flow conditions using large eddy simulation

Global warming threatens the entire planet, and solutions such as direct air capture (DAC) can be used to meet net-zero goals and go beyond. This study investigates using DAC in a 5-step temperature vacuum swing adsorption (TVSA) cycle with adsorbents' Li-X and Na-X, readily available industrial zeolites, to capture and concentrate CO2 from air in cold climates. From this study, we report that Na-X in cold conditions has the highest known CO2 adsorption capacity in air of 2.54mmol/g. This combined with Na-X's low CO2 heat of adsorption, and fast uptake-rate in comparison to other benchmark materials, allowed for Na-X operating in cold conditions to have the lowest reported DAC operating energy of 1.1 MWh/tonCO2. These findings from this study show the promise of this process in cold climates of Canada, Alaska, Greenland, and Antarctica to be part of the solution to global warming.

Limits to Paris compatibility of CO2 capture and utilization

The economic, environmental, and energetic performance of direct air capture (DAC) processes based on solid sorbents depends significantly on ambient air conditions and the availability of renewable resources. High ambient temperature or low humidity leads to higher energy consumption and lower CO2 productivity; lack of renewable resources may make the direct air capture process not viable. With this work, we investigated how the performance of sorbent-based direct air capture plants varies when changing ambient conditions and how the system should be optimally designed and operated to match the time-dependent variations. To this end, we formulated a new modeling framework, where thermodynamic modeling of adsorption processes is bridged to mixed integer linear optimization via a portable linear model of DAC. The process is based on a vacuum-temperature swing cycle, whose performance was obtained with a rate-based thermodynamic model at varying ambient conditions for an exemplary sorbent representative of different amine-functionalized materials. The optimal design and operation were investigated for (i) a stand-alone DAC system installed at three different geographical locations and (ii) a DAC system embedded in a multi-energy hub aimed at supplying the DAC energy demand from renewable resources. We found that DAC performance is optimal when the process can adjust the operating variables according to the weather profile and when CO2 can be produced flexibly over time, for example, by adopting a buffer storage tank. Other operation strategies are suboptimal but might require less sophisticated control systems. Moreover, the results suggest that capturing costs are significantly smaller in cold and humid conditions. This conclusion holds for both the stand-alone and the integrated DAC systems. However, for the latter, cold and humid conditions are favorable only when abundant renewable energy is available and can be supplied at low costs, for example, via wind farms. These conclusions remain true over a wide range of technical and cost assumptions.

Large-eddy and RANS simulations were performed to examine the details of the heat-transfer mechanisms in a U-duct with a high-aspect ratio trapezoidal cross section at a Reynolds number of 20,000. ANSYS-Fluent was used to perform the simulations. For the large-eddy simulations (LES), the WALE subgrid-scale model was employed, and its inflow boundary condition was provided by a concurrent LES of incompressible fully-developed flow in a straight duct with the same cross section and flow conditions as the U-duct. The grid resolution required to obtain meaningful LES solutions were obtained via a grid sensitivity study of incompressible fully-developed turbulent flow in a straight duct of square cross section, where data from direct numerical simulation (DNS) and experiments are available to validate and guide the simulation. In addition, the grid used satisfies Celik’s criterion, and resolves the Kolmogorov’s −5/3 law. Results were also obtained for the U-duct by using RANS, and three widely used turbulence models were examined — realizable k-ε with the two-layer model in the near-wall region, shear-stress transport (SST) model, and stress-omega full Reynolds stress model (RSM). Results obtained from LES showed unsteady flow separation to occur immediately after the turn region, which none of the RANS models could predict. By being able to capture this unsteady flow mechanism, LES was able to predict the measured heat-transfer downstream of the U-duct. The maximum relative error in the predicted local heat-transfer coefficient was less than 10% in the LES results, but up to 80% in the RANS results.

Direct air capture technologies have gained prominence as vital tools for atmospheric carbon dioxide removal, with four major categories, namely liquid solvent-based, solid sorbent-based, electrochemical, and emerging hybrid systems, demonstrating varying degrees of maturity and feasibility. Liquid solvent-based direct air capture, including systems using potassium hydroxide, amines, and advanced ionic liquids or deep eutectic solvents, benefits from high CO2 reactivity and established chemical regeneration processes, but faces limitations from high thermal energy demands, solvent degradation, and environmental handling concerns. Solid sorbent-based systems, such as those utilizing amine-functionalized materials or metal-organic frameworks, offer low-temperature regeneration and modular designs, yet often suffer from variable adsorption capacity under different humidity levels and degradation over multiple cycles. Electrochemical direct air capture is a rapidly advancing field that uses redox-active materials or ion-exchange membranes to reversibly bind and release CO2 using electrical energy. These systems enable operation under ambient conditions with high selectivity and reduced thermal input, though challenges persist in terms of redox material stability and scalability. Other emerging methods, such as cryogenic, photocatalytic, mineralization-based, and biological direct air capture, offer innovative pathways to reduce energy use or permanently sequester CO2, but remain at early developmental stages. While significant advances have improved energy efficiency, cost-effectiveness, and operational stability across direct air capture technologies, further research is needed to enhance long-term material performance, develop low-cost, scalable reactor designs, and improve integration with renewable energy systems. Future studies should prioritize techno-economic assessments, lifecycle analysis, and hybrid approaches that combine the strengths of multiple direct air capture pathways to achieve cost-effective and durable carbon removal at gigaton scales.

Alkali and alkaline earth metal oxides dispersed on high surface area γ-alumina were studied for the selective adsorption of CO2 under simulated ambient direct air capture (DAC) and subsequent temperature programmed desorption. Thermogravimetric analysis showed Na2O/γ-Al2O3 had superior adsorption and desorption via temperature programed desorption (TPD) relative to other supported metal oxides. Parametric and laboratory aging studies were conducted under simulated direct air capture of 400 ppm CO2, reflecting seasonal and locational ambient temperature and humidity conditions. Initial studies were performed on granular carriers and subsequently on washcoated ceramic monoliths. Aging studies were conducted on sample compositions of 10% Na2O/γ-Al2O3 granules and 10% Na2O + PX-80 γ-Al2O3 washcoat, deposited on a cordierite automotive-catalytic converter type monolith designed for low pressure drop in high air flow rates. Both selective adsorption and desorption of CO2 showed high stability with no signs of deactivation after 400+ hours of cyclic testing under a wide variety of simulated ambient conditions.

Large eddy simulation (LES) with various subgrid-scale (SGS) models was introduced to numerically calculate the transient flow of the hydraulic coupling. By using LES, the study aimed to advance description ability of internal flow and performance prediction. The CFD results were verified by experimental data. For the purpose of the description of the flow field, six subgrid-scale models for LES were employed to depict the flow field; the distribution structure of flow field was legible. Moreover, the flow mechanism was analyzed using 3D vortex structures, and those showed that DSL and KET captured abundant vortex structures and provided a relatively moderate eddy viscosity in the chamber. The predicted values of the braking torque for hydraulic coupling were compared with experimental data. The comparison results were compared with several simulation models, such as SAS and RKE, and SSTKW models. Those comparison results showed that the SGS models, especially DSL and KET, were applicable to obtain the more accurate predicted results than SAS and RKE, and SSTKW models. Clearly, the predicted results of LES with DSL and KET were far more accurate than the previous studies. The performance prediction was significantly improved.

Negative emissions technologies are gaining widespread acceptance as crucial tools in achieving climate goals, such as keeping global temperatures below 2 °C of pre-industrial levels by 2100. Two technologies central to carbon dioxide removal efforts are direct air capture and Bioenergy with carbon capture and storage. While both technologies have undergone extensive study, only a few studies have explored the potential of using biomass as an energy source for direct air capture technology. This is despite bioenergy with carbon capture having the ability to provide carbon-negative heat and power, as well as its potential impact on the climate mitigation goals of the century. This study aims to investigate the feasibility of meeting the energy requirements of a direct air capture unit using bioenergy. Combining these units will result in compounded negative emissions for the integrated system. The objective is to examine the thermal and electrical requirements of the two primary approaches used in direct air capture design: the liquid solvent and solid sorbent direct air capture units, and to calculate the compounded negative emissions achieved by integrating them with bioenergy. The results of this study demonstrate that for a direct air capture plant capturing 1 mega ton of carbon dioxide per year, approximately 1200 and 2400 tons of biomass per day would be sufficient to meet the energy needs of the solid sorbent and liquid solvent direct air capture systems, respectively. The combined capture efficiency of both types of direct air capture systems integrated with bioenergy stands at 91.19% to 93.9% with overall carbon captured up to 1.51 mega tons of carbon dioxide per year. Over the century, integrating bioenergy into direct air capture units can remove gigaton levels of carbon from the atmosphere without disrupting the demand–supply dynamics of existing and future energy systems.

Quantifying the interaction of the atmosphere and water surfaces is of great importance for water resources management, climate studies of ocean-atmosphere exchange and regional climate in coastal areas. Atmospheric dynamics over water surfaces have generally received less attention than land-atmosphere interactions due to difficulties in operating field studies. In this research we are trying to improve the physical parameterizations of lake-atmosphere processes by integrating measurements and modeling studies. The Lake-Atmosphere Turbulent EXchanges (LATEX) field measurement campaign is analyzed to understand air-water interactions over Lac Leman (Geneva). Large eddy simulations are used to study sensible and latent heat fluxes over heterogeneous wet surfaces. We present parameters of interest for land-surface modeling, i.e. the surface energy budget and the roughness lengths for momentum, heat and water vapor that are embedded in the Monin-Obukhov similarity theory used in atmospheric models. The storage of energy in the lake is a very important term, yet methods used to quantify it, relying on temperature profile measurements from a fiber optic and the theory of conduction of heat, are not successful. We revisit classical wet surface evaporation estimation methods that include this challenging term and derive an evaporation formulation based on sensible heat flux measurements. We then focus on numerical (large eddy) simulations of the flow above a water surface. Small-scale turbulence (the so-called subgrid scales, SGS) over the lake that cannot be captured in large eddy simulations is investigated. The measurements of LATEX allowed, for the first time, the study of subgrid-scale turbulent transport of water vapor over a lake, which reveals itself well correlated with the transport of heat. Results from an a priori analysis of subgrid-scale fluxes and dissipations indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces. We use the EPFL-LES code, with its scale-dependent Lagrangian dynamic SGS model, to simulate flows over transition from dry land to wet surfaces. We observe the fetch requirement for evaporation formulations and compare to results from simplified Lagrangian footprint models. We explore the limits of applicability of Monin-Obukhov similarity theory that are due to the finite fetch.

Reducing CO2 emissions alone will not suppress global warming, and it is necessary to capture the CO2 that has been cumulatively emitted into the atmosphere as well. For this reason, negative CO2 emission technology, a technology to capture CO2 from the atmosphere, is considered essential. Especially, direct capture of CO2 from the air, so-called direct air capture (DAC) has attracted much attention as one of promising technologies, because of the high potential capacity of CO2 capture. In general, absorption, adsorption, and membrane separation are known as representative CO2 capture technologies, and DAC is basically based on these technologies. In particular, DAC using absorption and adsorption methods has already reached the level of plant scale, but the desorption process of captured CO2 from the absorbent or adsorbent consumes a large amount of heating energy and water. On the other hand, membrane separation is generally considered as a most cost- and energy-efficient process among these capture technologies, but DAC by membrane separation has not been considered at all due to the immaturity of the membrane performance for CO2 capture, especially CO2 permeance. However, recent developments in membrane technology have brought the possibility that membrane processes can be considered as a new approach to DAC. In this article, the potential of membrane technologies as DAC is discussed and future technology target is proposed.Graphical abstract

Direct air capture is a negative emission technology that captures CO2 directly from the air. It is shown to be a promising tool for fighting climate change, yet still a work in progress. Direct Air Capture of CO2 provides an overview of this technology, starting with an overview in Chapter 1 of major climate change events, moving into a comprehensive review of negative emission technologies in Chapter 2, including direct air capture. Chapter 2 covers some of the challenges associated with direct air capture and the feasibility of utilizing such a process for large-scale applications. Chapter 3 presents a literature review of sorbents under investigation for direct air capture. The advantages and disadvantages of each approach for direct air capture are extracted from results published in the literature and are summarized along with areas of ongoing work. Parallel to ongoing research on developing high-performing sorbents for direct air capture, companies and startups have begun testing pilot to commercial scale direct air capture plants. Chapter 4 summarizes the efforts of such institutions. Global CO2 markets under development to construct commercialization pathways for direct air capture, such as enhanced oil recovery, synthetic fuels, cement, greenhouses, and food and beverages, are also reviewed in Chapter 4. The digital primer concludes with the authors’ view on the prospects of direct air capture technology for fighting climate change. Information provided in all chapters is carefully referenced to relevant literature so the reader may dive deeper into the details if interested. The authors hope this digital primer will bring inspiration and ideas to young scientists.

Current climate targets require negative carbon dioxide (CO2) emissions. Direct air capture is a promising negative emission technology, but energy and material demands lead to trade-offs with indirect emissions and other environmental impacts. Here, we show by life-cycle assessment that the commercial direct air capture plants in Hinwil and Hellisheiði operated by Climeworks can already achieve negative emissions today, with carbon capture efficiencies of 85.4% and 93.1%. The climate benefits of direct air capture, however, depend strongly on the energy source. When using low-carbon energy, as in Hellisheiði, adsorbent choice and plant construction become more important, inducing up to 45 and 15 gCO2e per kilogram CO2 captured, respectively. Large-scale deployment of direct air capture for 1% of the global annual CO2 emissions would not be limited by material and energy availability. However, the current small-scale production of amines for the adsorbent would need to be scaled up by more than an order of magnitude. Other environmental impacts would increase by less than 0.057% when using wind power and by up to 0.30% for the global electricity mix forecasted for 2050. Energy source and efficiency are essential for direct air capture to enable both negative emissions and low-carbon fuels. Direct air capture (DAC) of CO2 has garnered interest as a negative emissions technology to help achieve climate targets, but indirect emissions and other environmental impacts must be better understood. Here, Deutz and Bardow perform a life-cycle assessment of DAC plants operated by Climeworks, based on industrial data.

Emission trajectories produced by integrated assessment models increasingly suggest that gigatonnes of carbon removal will be required to stabilize atmospheric greenhouse gas concentrations at safe levels. This can be accomplished using the direct air capture of carbon dioxide, among other technologies. Process models of these systems assume that they would operate at standard ambient temperature and pressure, when capture rates vary with ambient conditions, including temperature, relative humidity, and other factors. Here, we build an open-source model of a liquid solvent direct air capture technology and analyze its capture performance as a function of hourly varying ambient environmental conditions across Canada. We find that, in the cool climate considered, capture performance is degraded due to both varying environmental conditions and the intermittent operation that could result. Our findings can be used to calibrate policy and investment decisions, and to support engineers in making operational design choices.

Direct CO2 capture from air with aqueous and nonaqueous diamine solutions: a comparative investigation based on 13C NMR analysis

A large eddy simulation (LES) model, with ice phase included, has been used to study the marine convective boundary layer filled with snow. Extensions to Moeng's LES model include the diagnosis of cloud ice mixing ratio, snow precipitation, and the parameterization of detailed microphysical processes. Model simulations are compared with cold air outbreak field observations over Lake Michigan, as well as with the liquid phase LES results for the same atmospheric conditions. The buoyancy flux and vertical velocity variance profiles generated by the ice phase LES are found to be more consistent with the observations than those generated by the liquid phase LES results. The incorporation of the ice phase into the LES model has also improved the agreement of vertical velocity skewness (Sw) between observations and LES model results. It has also been found that the presence of precipitation, and the associated microphysical processes, has a significant effect on the structure of the convective boundary...