Direct air capture with solid sorbents: Adsorbent design, process engineering, and CO2 conversion

To effectively mitigate the global warming problem caused by excessive CO2 emissions, the implementation of direct air capture (DAC) technology has emerged as one of the most promising strategies for capturing CO2 from the atmosphere. The key to DAC technology hinges on the development of high-performance solid sorbent materials that demonstrate high CO2 adsorption capacity and gas separation selectivity, particularly under low CO2 partial pressure conditions. Herein, we have successfully developed a class of MOF@carboxylated wood sponge (MOF@CWS) hybrid sorbents, capable of efficient CO2 capture from low-concentration (less than 10,000 ppm) CO2 sources, achieved by embedding the porous MOF into carboxylated wood sponges (CWS) substrate via an in situ growth route. Within the MOF@CWS series, the CO2 uptake capacity of Mg-MOF-74@CWS is 3.61 and 2.65 mmol/g at 1 bar, 273 and 298 K, respectively, significantly higher than those of CWS and HKUST-1@CWS. Moreover, this material exhibited outstanding DAC performance, with the CO2 sorption capacity at 273 K up to 0.56 mmol/g from ambient air (ca. 400 ppm of CO2), surpassing most other solid sorbents. The obtained Mg-MOF-74@CWS also demonstrated exceptional CO2/CH4 separation performance, primarily due to the unique pore structure and augmented interaction between the CO2 molecules and the hybrid sorbents. The results of this study indicate that Mg-MOF-74@CWS has potential as an efficient solid sorbent for the DAC of CO2.

The extraction of CO2 directly from the atmosphere (Direct Air Capture) is commonly employed using supported amine sorbents. This adsorption technology is under rapid development with novel sorbents materials emerging and with processes being demonstrated on increasingly larger scale. Optimization of such processes require accurate knowledge on sorbent characteristics and knowledge on how operational variables change process performance. This study is primarily focused on the latter, where we aim to quantify the influence of operational parameters on the energy duty and CO2 productivity. In addition, we examined the influence of weather conditions on the adsorption rate. For this, we developed a dynamic model of the complete temperature-pressure swing adsorption cycle (TPSA). The model is validated by experimental results on a kg-scale direct air capture system. The impact of selected operational variables is assessed by two-dimensional sensitivity analyses. We show that desorption temperature is preferably high, limited by the chemical stability of the sorbent material in this particular case. In addition, the sorbent working capacity should be large when opting for an optimization towards energy duty, whereas it reaches a clear optimum in terms of CO2 productivity. Finally, we conclude that weather conditions and diurnal variations can significantly affect the performance of a direct air capture process and should certainly be considered during design and operation. With these insights and the developed model, this study provides a sound basis for further process development and optimization of direct air capture using fixed bed technology combined with solid amine sorbents.

The impact of Direct Air Capture during the last two decades: A bibliometric analysis of the scientific research, part I

Climate change triggered by the increasing amount of CO2 released into the atmosphere has become a global priority. Direct Air Capture (DAC) of CO2 has been regarded as one of the disruptive technologies to realize negative emission. Among the multiple strategies employed, adsorption on solid amines is a promising choice owing to its low energy requirements. Since the water vapor ubiquitously exists in ambient air, understanding the role it plays in DAC by the amine-functionalized solid sorbents is of vital importance. Due to the diversity in amine and support features, both promotion and deterioration of water impact on CO2 adsorption were revealed. Herein we present a multifaceted review of the trends and innovations in DAC by solid amines under humid conditions. The effect of water was discussed over four groups of materials, based on the support categories, i.e., (a) mesoporous silica, (b) mesoporous alumina, (c) mesoporous carbon and other inorganic supports, and (d) cellulose and other organic supports.

A novel contactor for reducing the cost of direct air capture of CO2

Investigation of water co-adsorption on the energy balance of solid sorbent based direct air capture processes

Direct air capture (DAC) is an emerging negative CO2 emission technology that aims to introduce a feasible method for CO2 capture from the atmosphere. Unlike carbon capture from point sources, which deals with flue gas at high CO2 concentrations, carbon capture directly from the atmosphere has proved difficult due to the low CO2 concentration in ambient air. Current DAC technologies mainly consider sorbent-based systems; however, membrane technology can be considered a promising DAC approach since it provides several advantages, e.g., lower energy and operational costs, less environmental footprint, and more potential for small-scale ubiquitous installations. Several recent advancements in validating the feasibility of highly permeable gas separation membrane fabrication and system design show that membrane-based direct air capture (m-DAC) could be a complementary approach to sorbent-based DAC, e.g., as part of a hybrid system design that incorporates other DAC technologies (e.g., solvent or sorbent-based DAC). In this article, the ongoing research and DAC application attempts via membrane separation have been reviewed. The reported membrane materials that could potentially be used for m-DAC are summarized. In addition, the future direction of m-DAC development is discussed, which could provide perspective and encourage new researchers' further work in the field of m-DAC.

The world is increasingly facing the direct effects of climate change triggering warnings of a crisis for the healthy existence of humankind. The dominant driver of the climate emergency is the historical and continued accumulation of atmospheric CO2 altering net radiative forcing on the planet. To address this global issue, understanding the core chemistry of CO2 manipulation in the atmosphere and proximally in the oceans is crucial, to offer a direct partial solution for emissions handling through negative emissions technologies. Many technologies have been proposed to develop a strategic and economic solution for carbon capture, storage, and utilization. In this paper, we review recent advances in technologies proposed for carbon capture and release via electrochemical process for point source/flue gas, direct air capture (DAC), and ocean/seawater capture. Electrochemical approaches to carbon capture are favorable in terms of reaction conditions, their ability to be incorporated into transformation processes, modularity, low relative carbon footprint, and compatibility with the availability of renewable electricity sources. We offer a critical comparative analysis of land- and ocean-based capture technologies to help guide future research and innovation.

This study develops CaZr-based dual-functional materials (DFMs) for integrated direct air capture (DAC) and an in situ reverse water–gas shift (RWGS) reaction, termed IDACU-RWGS. The baseline CaZr material, composed of Ca(OH)2 and ZrO2, exhibited limited CO2 conversion efficiency. To address this, six non-noble metal catalysts (K, Ce, Co, Cu, Ni, and Fe) were screened, with Fe identified as the optimal single catalyst due to its ability to enhance CO2 capture and conversion while maintaining 100% CO selectivity via a redox mechanism. In contrast, Ni improved capture and conversion but favored methanation, resulting in low CO selectivity. A bimetallic Fe–Ni catalyst system was subsequently designed to combine the advantages of both metals. The optimized CaZrFe2Ni1 DFM demonstrated good performance at 650 °C, achieving a CO2 capture capacity of 8.33 mmol/g, CO2 conversion of 83.4%, CO selectivity >99%, and stable performance over five cycles with minimal degradation. Characterization results (XRD, SEM, BET, CO2-TPD, H2-TPR, and XPS) revealed that the incorporation of Fe and Ni promotes the formation of oxygen vacancies and enhances CO2 activation. This work presents a promising DFM strategy for efficient and selective integrated DAC and RWGS conversion.

Direct air capture (DAC) technology is still facing some challenges that need to be addressed before the technology can be widely deployed. One major challenge is the high overall system energy requirement, and it is the main bottleneck in DAC commercialization. This study aims to unlock the potential of integrating DAC with Heat Ventilation and Air Conditioning (HVAC) systems. A thermodynamic model is used to test the effect of varying the adsorption and desorption conditions on DAC unit performance, and to assess the characteristics of DAC unit positioned in six different locations within air the handling unit (AHU). The results show that by only locating the DAC unit in Position 6 (i.e., positioning the DAC unit after the exhaust fan in AHU) in comparison to a stand-alone unit, the efficiency of the DAC system is improved at varying times through the summer season in Qatar. Specifically locating DAC unit at Position 6 increases DAC efficiency from 5.3% to 27.19% for 8th of June 2022 at 11:00 am in Qatar. Additionally, using the DAC unit with other filters in AHU allows for full air recirculation for 10 h. Based on this scenario and the weather data for 20th of July at 12:00 pm, the hottest day in Qatar in 2022, a reduction of 44% in cooling load is achieved. Locating the DAC unit at a point before the air is supplied from AHU is the best, however, in case of full recirculation of air, positioning the DAC unit after the exhaust fan will achieve better performance.

The “Direct Air Capture Recovery of Energy for CCUS Partnership (DAC RECO2UP)” project employs a team approach and supports the U.S. Department of Energy Office of Fossil Energy and Carbon Management’s (DOE-FECM) goal to decrease the cost of capture through the testing of existing direct air capture (DAC) materials in integrated field units that produce a concentrated carbon dioxide (CO2) stream of at least 95% purity. Solid-amine CO2 adsorption-desorption contactor technology, proven in the laboratory, is undergoing high-fidelity design/validation. Recoverable energy is readily available from a large number of commercial locations where DAC can be deployed; therefore, advancing the fidelity of energy recovery to directly reduce the cost of DAC is a key project objective. In addition, many commercial facilities have low-concentration CO2 vents that are uneconomical to treat alone but could provide more efficient mass and thermal transport to DAC systems with integrated energy recovery and flexible CO2 treatment capabilities. Technology scale-up leverages past research and occurs in a commercially relevant environment at the National Carbon Capture Center. Prescreening techno-economic analysis, risk assessments, and life cycle analysis are being performed by experienced team members. Results of the project will address critical technical barriers that, when solved, will improve the capital and operating costs of DAC while validating commercial relevance of cost and product quality/need.

Direct air capture (DAC) is increasingly recognized as a necessary puzzle piece to achieve the Paris climate targets. However, the current high cost and energy intensity of DAC act as a barrier. Short-term strategies for initial deployment, technology improvement, and cost reduction are needed to enable large-scale deployment. We assess and compare two near-term pathways leading to the same installed DAC capacity and thus yielding the same cost reductions: its combination with CO2 storage as direct air carbon capture and storage, or its deployment for CO2 utilization as direct air carbon capture and utilization e.g. for synthetic fuels, chemicals, and materials; we characterize these as Direct and Spillover pathways. Drawing on the Multi-level Perspective on Technological Transition as a heuristic, we examine both technical and immaterial factors needed to scale up DAC under the two pathways, in order to assess the pathways’ relative advantages and to identify possible short-term bottlenecks. We find neither pathway to be clearly better: the Direct pathway offers technical advantages but faces regulatory barriers that need to be resolved before deployment, while the Spillover pathway offers market and governance advantages but faces challenges related to hydrogen production and increasing resource needs as it scales up. There may be reasons for policymakers to therefore pursue both approaches in a dynamic manner. This could involve prioritizing the Spillover pathway in the short term due to possibly fewer short-term regulatory barriers and its ability to produce net-zero emission products for existing and accessible markets. Once short-term governance obstacles have been addressed, however, the Direct pathway may allow for more efficient scaling of DAC capacity and cost reductions, especially if by then the needed infrastructure and institutions are in place.

Batteries or silos: Optimizing storage capacity in direct air capture plants to maximize renewable energy use

Liquefied synthetic methane from ambient CO2 and renewable H2 - A technoeconomic study

The utility of MOF-based materials in direct air capture (DAC) application to ppm-level CO2