Direct Air Capture Systems Research Articles

Optimal Design and Operation of Solid Sorbent Direct Air Capture Processes at Varying Ambient Conditions

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.

Rail-based direct air carbon capture

The direct capture of carbon dioxide from the environment is increasingly becoming an urgent necessity to mitigate the worst effects of climate change. However, the high energy demands require creative implementation strategies to minimize the diversion of already-stretched conventional resources toward this cause. To alleviate these issues, creative implementation strategies must be devised to lower the barrier to economic applicability of DAC systems so that they can be widely deployed. To this end, the work described herein presents innovative technology for deploying specially designed, self-contained DAC railcars on both diesel and electrified rail lines outfitted with battery arrays, CO₂ direct air capture systems, compression equipment, and ancillary gear that uniquely exploits the substantial sustainable energy generated on-board the train through regenerative braking as well as from solar panels mounted on compatible railcars. The units are equipped with large intakes that extend up into the slipstream of the moving train and collect CO₂ feedstock air by fluidic, ramjet-type processes thus obviating the need for the fans required by land-based systems and places no demand on energy or land resources. Unloaded daily at crew change or fueling stops into regular CO₂ tank cars, the network will curate delivery of the harvested CO₂ to on-route sites for permanent underground sequestration, or delivery to end-users as feedstock for the circular carbon economy. The technology will harvest meaningful quantities of CO₂ at far lower costs and has the conservative potential to reach annual productivity of 0.45 gigatons by 2030, 2.9 gigatons by 2050, and 7.8 gigatons by 2075 with each car having an annual capacity of 3,000 tonnes of CO₂ in the near term and more as the technology progresses.

Environmental performance of different sorbents used for direct air capture

Currently, conventional carbon dioxide (CO 2 ) mitigation solutions may be insufficient to achieve the stringent environmental targets set for the coming decades. CO 2 removal (CDR) technologies, such as direct air capture (DAC), capturing CO 2 from the ambient air, are required. In this research, an independent life cycle assessment (LCA) of DAC adsorption systems based on three physisorbents (metal organic frameworks) and two chemisorbents (amine functionalized sorbents) is presented. These capture processes have been optimised by us in previous work. Results show that for the overall capture process, negative CO 2 emissions are ensured by using a cellulose-based amine sorbent (cradle-to-gate) ensuring even the net removal of CO 2 from the atmosphere (cradle-to-grave). Processes using physisorbents have poorer performances. Chemisorbents yield operating conditions allowing lower impacts on the environment. In 2050, these processes could reduce climate change but can generate other environmental impacts. With the aim to have better environmental performances of DAC systems, future research should be focused on improving the physical properties of sorbents such as the silica gel based amine sorbent to increase their capture capacities. If metal organic frameworks are to be used, it is necessary to drop their energy consumption (by increasing the loading) and the required mass of sorbent.

Minimizing the effect of oxygen on supported polyamine for direct air capture

Direct air capture (DAC), as a typical technology of negative emission technologies, is indispensable to neutralizing mobile carbon emissions. The polyamine-loaded adsorbent with high CO2 capacities at extremely low CO2 concentration while maintaining mild regeneration conditions is a promising choice for actual DAC systems. This work comprehensively investigated the effect of oxygen in the air on the selectivity of polyamine-functionalized adsorbents towards CO2 in the adsorption process and the oxidative stability in the desorption process. Results indicate high selectivity toward CO2 and oxidative resistance of adsorbents can be realized by accurately controlling the operating temperature. The oxidation of supported polyamine follows a three-phase mechanism. The oxidative stability of adsorbents varied at oxidation phases distinguished by different operating temperatures. Instructed by that three-phase oxidation mechanism, the optimal regeneration temperature of 80 °C was suggested for supported PEI and TEPA adsorbents. This optimal regeneration temperature can achieve favorable CO2 capture performance and maintain superior oxidative stability. Based on these findings, an air-assisted temperature swing adsorption DAC system was proposed to collect CO2 from the air for agricultural greenhouse.

Fluorescent Probe of Aminopolymer Mobility in Bulk and in Nanoconfined Direct Air CO2 Capture Supports

Poly(ethylenimine) (PEI) is widely recognized as an efficient carbon capture medium. When loaded onto mesoporous oxide supports, the polymer becomes particularly attractive for direct air capture (DAC) applications given the high surface area of the composites, the low volatility of the polymer, and the excellent cyclability of the system. As polymer segmental mobility is coupled with CO2 uptake and diffusion, understanding how that mobility is influenced by nanoconfinement will ultimately be critical to the development of more efficient DAC systems. Here, we discuss our development of a fluorescent probe molecule based on tetrakis(4-hydroxyphenyl)ethylene. As the fluorescence intensity of this molecule and the shape of the emission spectra are strongly dependent on the viscosity of the supporting medium, doping PEI-composites with this fluorescent probe can provide sensitive indication of polymer glass transition and/or melting temperatures across a wide range of temperatures (−100 to +100 °C). Herein, we demonstrate how this molecule can be used as a ratiometric probe to study bulk PEI dynamics and confinement effects in mesoporous silica as influenced by pore functionality, polymer fill fraction, and polymer architecture.

Direct air capture system: a feasibility study for Izmir

ABSTRACT This study aims to reduce 1% of the annual emissions in the city of Izmir. In line with this target, this study worked on the integration of a liquid solvent-based DACS system within Izmir. Primarily, the emissions within the city boundaries were calculated based on the calculation method of each sector. For the mentioned target, each component’s energy requirement of the KOH solvent-based DACS system was calculated in detail. According to these calculations, the system’s total energy requirement, economic, and area calculations were obtained. As a result, the total emission amount was calculated as 11,809,362 tCO2. Total needed energy, area, and cost for DACS system resulted to be 1,169 kJ kg−1, 12,527 m2, and $136.6 million, respectively. In addition to numerical results, DACS is the most logical solution apart from increasing environmental awareness for creating a better, stronger, and environmentally friendly city for the near future.

Modeling and Process Operability Analysis o f a Di rec t A i r Ca p t ure System

The climate crisis has become an important subject of study since it has impacted every single life on the planet. Aggravated by the large amounts of carbon dioxide released to the atmosphere, such crisis has influenced environmental regulations around the world. To lower the accumulation of this greenhouse gas in the atmosphere, several new technologies are currently being explored in the field of carbon capture and sequestration. This research focuses primarily on Direct Air Capture (DAC) technologies since they have advantages such as ease in installing and placing their facilities. Thus, the capture could be carried out near energy sources and storage sites, hence reducing costs and emissions especially of geographically remote CO2 producing processes. The DAC system design can be customized to meet the desired CO2 specifications in the captured product. In this work, a 3-stage membrane module arrangement for DAC is chosen due to its CO2 separation capabilities. A novel framework based on operability analysis is also employed to find a feasible design region for the DAC system that can operate at different pollution levels. The system is simulated employing AVEVA Process Simulation (an equation-oriented software) coupled with a Python script interface as well as MATLAB to perform process operability calculations. The results show that a feasible design for DAC can be successfully established with the proposed framework accounting for product and byproduct impurities.

Process optimization of a fixed bed reactor system for direct air capture

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 sorbent materials emerging and with processes being demonstrated on increasingly larger scale. Optimization of such processes requires accurate knowledge on sorbent characteristics and knowledge on how operational variables affect process performance. This study primarily focuses on the latter, where we aim to quantify the influence of operational parameters on the energy duty and CO2 productivity. In addition, we examine the influence of weather conditions on the adsorption rate. For this, we develop a dynamic model of the complete temperature-vacuum swing adsorption cycle (TVSA). This model was validated by experimental results on a kg-scale direct air capture system. The impact of selected operational variables was 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 high 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.

Exploratory Performance Modeling of the Sorbent-based Direct Air Capture System

The continued growth in carbon dioxide (CO2) emissions will necessitate deployment of negative emissions technologies (NETs) to meet Paris stabilization climate targets [1]. Direct air capture (DAC) is a NET that can capture dispersed source CO2 already in the atmosphere [2] [3]. DAC systems would complement the application of CO2 capture and storage (CCS) applied for significant emissions reductions. Working DAC systems have been demonstrated at relatively small scales by several companies, including Climeworks, Global Thermostat and Carbon Engineering, proving the technical feasibility of such systems. However, the performance, cost, life cycle environmental impacts and efficacy of current DAC systems is still a subject of debate. There exists a growing need for understanding how DAC systems can be designed and employed to deliver cost-effective and environmentally sustainable negative emissions and be scaled up for rapid deployment. In this study, we explore the design space for a sorbent-based DAC system by developing a DAC performance model for three systems alongside existing models from literature [4] [5]. We identify process parameters such as cycle operation parameters, sorbent thermophysical characteristics and thermal energy supply for further sensitivity analysis that could improve the performance of the DAC system given suitable energy supply options. Our investigation into these process sensitivities yields an understanding of how DAC processes might be optimized for net negative CO2 emissions. Our key findings highlight low carbon intensive thermal energy options such as industrial heat pumps and natural gas fired boilers or natural gas combined heat and power plants as suitable options for integration with a DAC system to deliver net negative emissions and isolate key variables in heat pump systems for optimal negative emissions.

Role of negative emissions technologies (NETs) and innovative technologies in transition of Japan’s energy systems toward net-zero CO2 emissions

This paper explores the role of negative emissions technologies (NETs) in energy systems, bioenergy with carbon capture and storage (BECCS) and direct air capture (DAC) with geological carbon storage (DACCS) in particular, using a bottom-up energy system model TIMES-Japan that participated in the 35th study of the Stanford Energy Modeling Forum (EMF 35 JMIP) focusing on the energy transitions for the long-run climate goals. Modeling results show that large-scale deployment of NETs is essential to achieve the net-zero vision of Japan’s long-term strategy, however, these NETs might not be enough in the case of the highest energy service demands. Within the feasible solution space, earlier deployment of BECCS with domestic biomass can contribute effectively to achieve the target with the support of the DACCS at the later period if both technologies are available. It shows feasible results without DACCS only in the lowest energy service demands, implying the importance of urgent research, development, and deployment of DACCS. Furthermore, this study shows that earlier deployment of DAC system with CO2 utilization in fuel production is a cost-effective way to lead the large-scale deployment of the DAC as NETs.

Assessment of an Integrated Direct CO2 Air Capture System Using Wind Power with Compressed Air Energy Storage in Northern Regions

This work examines the techno-economic viability for replacing, in remote Northern Canadian villages not connected to the grid, local diesel generation systems with a wind turbine potentially combined with compressed air energy storage and/or CO2 direct-air capture systems. The compressed air energy storage configurations analyzed include a geological reservoir at constant volume, a multi-stage compressor with intercooler, a multi-stage turbine and a heat exchanger. The direct-air capture technology analyzed uses a solid sorbent with temperature swing adsorption. The analysis is based on a model village of 1000 inhabitants with an average electrical demand of 1000 kW with daily morning and evening peaks, and a daily average effective capacity factor varying according to a sinusoidal probability density distribution. It is found that the direct-air carbon capture potential of a compressed air energy storage system appropriately sized for the present application is negligible owing to the small flow volume and low atmospheric CO2 concentration. Furthermore, the integration of compressed air energy storage does not lead to appreciable reductions in diesel generation and related carbon emissions. Overall, the most interesting cases considered in the present study use either the smallest wind turbine, sized for the average power demand, requiring the smallest initial investment, offering the fastest payback and reducing carbon emissions by 95% over the current case, or a wind turbine twice as powerful, covering most demand peaks, such that payback time doubles but overall carbon emission reductions are also doubled.

Process Optimization of a Fixed Bed Reactor System for Direct Air Capture

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.

Design of steam-assisted temperature vacuum-swing adsorption processes for efficient CO2 capture from ambient air

Direct air capture (DAC) is an efficient, negative-carbon-emission technology that enables the capture of distributed emissions and removes location restrictions on capture facilities. However, current DAC demonstration plants are still too costly to be commercialized. In this work, a three-step steam-assisted temperature vacuum-swing adsorption (S-TVSA) cycle based on a packed column was designed for use in DAC systems, and the CO2 and H2O capacities and kinetics of the adsorbents were considered in detail. By operating the steam purge step at reduced pressures, steam at temperatures lower than 100 °C can be supplied by cheap thermal sources. In addition, the adsorption of H2O during the steam purge step can release heat for CO2 regeneration. Parameter sensitivity analysis reveals the trade-off relationship between the performance and energy consumption of DAC system with the S-TVSA cycle. The optimal case with a variational steam purge step operating at 90 °C and 0.3 bar achieves a CO2 productivity of 4.45 mol kg−1 day−1 and an energy requirement of 0.295 MJ mol−1. If the heat energy for the purge steam comes from solar energy or low-grade industrial waste heat, which represents 80.6% of the total energy consumption, the DAC system with S-TVSA cycle will be competitive with post-combustion CO2 capture technologies. Note that the productivity can be increased by up to 280% with only 32.8% of the initial energy consumption by using novel adsorbents with higher capacities and kinetics, potentially making S-TVSA cycles highly efficient for DAC systems.

Moisture-Driven CO2 Sorbents

Summary An energy-saving system containing ion-exchange or nanoporous materials and carbonate ions is proposed, which is capable of capturing CO2 from ambient air simply by controlling the amount of water (moisture) in contact with the sorbent. The system binds CO2 from the air when the surrounding is dry, whereas it desorbs CO2 when it is wet. A design of such CO2 sorption and desorption systems is investigated using quantum mechanics simulations and is verified by experiments. Its working mechanism is revealed as the free energy change of the chemical reaction of the carbonate ions and water molecules; the free energy change decreases when the number of water molecules in the materials decreases. The influence of pore size, spacing of cations, and surface hydrophobicity of the sorbents on CO2 capture efficiency are elucidated. The study sheds light on ways to optimize an efficient direct air capture system and therefore contributes to the development of “negative emission technologies.”

A hyperbox classifier model for identifying secure carbon dioxide reservoirs

Carbon management technologies such as carbon dioxide capture and storage and direct air capture systems will be needed to mitigate climate change in the coming decades. Both of these technologies will depend on the availability of secure geological storage sites that can permanently hold carbon dioxide with minimal risk of leakage. Machine learning tools that can characterize candidate storage sites based on geological data can aid decision-makers in planning carbon management networks. In this work, a mixed integer linear programming model is developed to generate a binary hyperbox classifier for determining the integrity of a candidate storage site. The model is calibrated and validated using literature data on natural carbon dioxide reservoirs, resulting in a set of IF-THEN rules that are readily interpreted by decision-makers. The approach developed here also includes rule simplification features and the capability to account for statistical Type I (false positive) and Type II (false negative) errors. Different sets of rules can be generated using the model based on user-defined number of hyperboxes. The best set of rules can be selected based on a combination of its performance with the validation data and consistency with expert knowledge. Using the case study for identifying secure CO2 reservoirs, the set of rules which resulted in zero false positives using the validation data was generated using three hyperboxes. However, an alternative set of rules which falsely predicted two out of three insecure sites as positive provides simpler rules indicating CO2 density and reservoir depth as the most important criteria.

CaCO3 growth in conditions used for direct air capture

A fluidized environment is used in a direct air capture system for the purpose of crystallizing carbonate ions from an alkaline liquid. The crystallization of calcium carbonate, in the conditions necessary for a direct air capture system, requires a pH range that has not been studied in the current literature. Beaker and lab-scale fluidized bed experiments were used to assess the effect of temperature, surface area availability, and pH on primary and secondary nucleation. The highest retention was found in the conditions where mass transport is favored for the beaker experiments, i.e., high loading, small pellet sizes, and low temperatures. Lab-scale reactor experiments confirmed these findings with the highest retention of 58.8 ± 1.8% observed for mid-size pellets. The pellets and fines produced had the calcite morphology. An inversion of morphology from calcite to vaterite was found as pH was lowered to 11.8.

Pricing CO2 Direct Air Capture

Negative emissions technologies such as direct air capture systems are an important tool to impede climate change. Recently in Applied Energy, Azarabadi and Lackner reported a generalized cost model for direct air capture of CO2. Their findings emphasize the importance of sorbent cycle duration and stability in minimizing total system cost.

Techno-economic assessment of CO2 direct air capture plants

CO2 direct air capture (DAC) has been increasingly discussed as a climate change mitigation option. Despite technical advances in the past decade, there are still misconceptions about DAC's current and long-term costs as well as energy, water and area demands. This could undermine DAC's anticipated role in a neutral or negative greenhouse gas emission energy system, and influence policy makers. In this study, a literature review and techno-economic analyses of state-of-the-art DAC technologies are performed, wherein, DAC technologies are categorised as high temperature aqueous solutions (HT DAC) and low temperature solid sorbent (LT DAC) systems, from an energy system perspective. DAC capital expenditures, energy demands and costs have been estimated under two scenarios for DAC capacities and financial learning rates in the period 2020 to 2050. DAC system costs could be lowered significantly with commercialisation in the 2020s followed by massive implementation in the 2040s and 2050s, making them cost competitive with point source carbon capture and an affordable climate change mitigation solution. It is concluded that LT DAC systems are favourable due to lower heat supply costs and the possibility of using waste heat from other systems. CO2 capture costs of LT DAC systems powered by hybrid PV-Wind-battery systems for Moroccan conditions and based on a conservative scenario, without/with utilisation of free waste heat are calculated at 222/133, 105/60, 69/40 and 54/32 €/tCO2 in 2020, 2030, 2040 and 2050, respectively. These new findings could enhance DAC's role in a successful climate change mitigation strategy.

Capturing CO2 from air: Technical performance and process control improvement

Direct air capture (DAC) is a technology for collecting and concentrating carbon dioxide from ambient air. If driven with renewable power, DAC is potentially a negative CO2 emissions technology that can compensate emissions from non-point sources such as aviation, shipping and land-use change. This study presents the results of 10 days capture campaign done between May and July 2018 plus a process control improvement test.The bench scale DAC device is composed of eight beds containing amine-functionalized adsorbent and follows a temperature and vacuum swing adsorption (TVSA) operation cycle. The outlet CO2 concentration reached less than 100 ppm at the start of the adsorption. Dynamic atmospheric conditions (varying T, relative humidity) affect the capture profile of the beds. Desorption is accomplished by heating the bed up to around 80 °C coupled with vacuum. The product gas has purity range of 95-vol% to 100-vol% CO2. Major impurities are O2, N2, and H2O. The DAC system also produced water at molar ratio of 3.9 moles H2O per mole of CO2. Water production is affected by air humidity.Measurement-based process control increased the production to 3.4 kg CO2 per cycle with specific energy requirement of 10 kW h kg−1. The thermal energy requirement accounted for 76% of the total energy input during the improvement test.

Carbon dioxide direct air capture for effective climate change mitigation based on renewable electricity: a new type of energy system sector coupling

Pathways for achieving the 1.5–2 °C global temperature moderation target imply a massive scaling of carbon dioxide (CO2) removal technologies, in particular in the 2040s and onwards. CO2 direct air capture (DAC) is among the most promising negative emission technologies (NETs). The energy demands for low-temperature solid-sorbent DAC are mainly heat at around 100 °C and electricity, which lead to sustainably operated DAC systems based on low-cost renewable electricity and heat pumps for the heat supply. This analysis is carried out for the case of the Maghreb region, which enjoys abundantly available low-cost renewable energy resources. The energy transition results for the Maghreb region lead to a solar photovoltaic (PV)-dominated energy supply with some wind energy contribution. DAC systems will need the same energy supply structure. The research investigates the levelised cost of CO2 DAC (LCOD) in high spatial resolution and is based on full hourly modelling for the Maghreb region. The key results are LCOD of about 55 €/tCO2 in 2050 with a further cost reduction potential of up to 50%. The area demand is considered and concluded to be negligible. Major conclusions for CO2 removal as a new energy sector are drawn. Key options for a global climate change mitigation strategy are first an energy transition towards renewable energy and second NETs for achieving the targets of the Paris Agreement.