Microwave-based CO2 desorption for enhanced direct air capture: experimental validation and techno-economic perspectives

This study explores the feasibility and potential techno-economic advantages of employing microwaves for direct air capture (DAC) applications. The experimental setup resembles an industrial-scale microwave system, utilizing a single-mode applicator and zeolite 13x beads arranged in a panel configuration. This configuration essentially represents a miniaturized version of a potential DAC plant based on microwaves. The results demonstrate that microwave irradiation rapidly and efficiently desorbs the CO2 from sorbents with approximately 90% desorption achieved in 10 min—substantially shorter than conventional conduction-based methods. The desorption process occurred at a low temperature of about 50 °C, in contrast to nearly 120 °C in conventional bulk heating, due to selective heating near CO2 binding sites. Our results support that desorption duration and temperature could be further reduced by applying more uniform heating while intensifying the selective process. Based on our research and recent literature, we propose three key techno-economic advantages of designing a DAC system with microwaves that are unattainable by the conventional approach. A reduced regeneration time could allow for a more compact system design while maintaining throughput. The selectivity of microwave absorption could drastically reduce energy demand, bringing it close to the sorbent’s thermodynamic energy limits. Furthermore, the low-temperature process could inhibit the thermal degradation of amines on the sorbents, which is unavoidable in conventional processes. Potential resonant CO2 desorption by forming nonthermal plasma (NTP) is discussed. Our research highlights the feasibility and significance of employing advanced regeneration methods in the development of next-generation DAC systems.


Introduction
Industrial-scale excess CO 2 removal from the atmosphere is essential to keep global average temperature increases within 1.5 • C-2 • C above pre-industrial levels (Gasser et al 2015, Masson-Delmotte et al 2018, Rogelj et al 2018).According to various studies, nearly 90% of the strategies that aim to limit warming to 2 • C, and all approaches that strive to keep it to 1.5 • C necessitate the large-scale deployment of negative emissions technologies (NET) by the end of this century (Intergovernmental Panel on Climate Change 2014, EASAC 2018, Masson-Delmotte et al 2018).This is particularly urgent given the prediction that the 1.5 • C limit will likely be breached by the early 2030s (Friedlingstein et al 2022).
Direct air capture (DAC), a form of NET, is one industrialized method of reducing CO 2 levels in the atmosphere.One mature embodiment of DAC utilizes specially engineered porous sorbents that have a preferential affinity for CO 2 over other gases.These sorbents can passively and selectively remove CO 2 upon exposure to the atmosphere.The DAC process can yield a purified stream of CO 2 through a temperature-vacuum swing (TVS) process, where the CO 2 -saturated sorbents are heated to about 100 • C under low vacuum.As of 2023, there are 27 active DAC facilities which collectively remove about 0.01 million tonnes of CO 2 (MtCO 2 ) per year (IEA 2023).The global DAC capacity is projected to approach 75 MtCO 2 yr −1 by 2030 and needs to surpass 1000 MtCO 2 yr −1 by 2050 (Calvin et al 2023).The cost of CO 2 removal via DAC is currently estimated to be $775 per tonne based on a kilotonnesscale pilot plant (Orbuch 2020).The cost is significantly higher than the cost of by-product CO 2 from diverse industrial processes, which is traded globally at approximately $19/tCO 2 (Parsons Brinckerhoff andGlobal CCS Institute 2011, Greenwood 2018).The high cost of DAC can be linked to the inefficiencies of the TVS process.Due to non-uniform and slow heating profile, DAC based on TVS requires nearly 7-14 times more energy than what is required to desorb CO 2 from sorbents-determined by the enthalpy of desorption (Deutz and Bardow 2021).This slow conduction-based regeneration usually takes several hours to complete, which necessitates an oversized system and thus significantly increases the cost.The cost of TVS-based DAC is projected to decrease to about $200/tCO 2 with further system optimization and learning (Berger et al 2022).Despite this, the current TVS-based DAC is not expected to meet the cost target set by the U.S. department of energy, which aims to reduce the cost of DAC to $100/tCO 2 by 2032 (Department of Energy n.d.).Current attempts to improve the regeneration process include using steam purging to either substitute or supplement conductive heat transfer (Sinha et al 2017, Gebald et al 2019).
One promising approach involves replacing TVS with microwaves to induce quick, lower-temperature CO 2 desorption.Microwaves can decrease the overall energy demand by enabling CO 2 desorption at lower temperatures through selective heating near CO 2 binding sites (Vallee andConner 2006, Ji et al 2023) and by reducing the processing time due to an accelerated heating rate (Jones et al 2002, Falciglia et al 2018).The application of microwaves has been found to make CO 2 desorption 4-17 times faster than the conventional bulk heating methods from diverse sorbents including zeolites, activated carbon, amine-functionalized silica, and crystalline sorbent (Chronopoulos et al 2014, Webley and Zhang 2014, Nigar et al 2016, Jang et al 2023).When used in DAC, microwave-based regeneration could also reduce the system costs by minimizing the total quantity of sorbents needed and reducing electricity use for heating and vacuum pump operation (Webley andZhang 2014, Jang et al 2023).
In this study, we closely examine the application of microwaves for the DAC process using a setup that represents a potential design.Zeolite 13x sorbents were prepared in a thin, panel-like configuration, similar to typical sorbent arrangements found in commercial DAC plants (Wurzbacher et al 2019).
These configured sorbents were loaded into a singlemode microwave applicator, resembling a setup of a mature industrial-scale microwave system commonly used in food processing.This setup allows us to verify near-complete desorption of CO 2 from sorbents, reduced processing time, and lower energy demand when employing microwaves for DAC.We quantify CO 2 desorption by measuring changes in sorbent mass and studying mass transport of major gaseous species using a model we have developed.We also analyze the bulk heating profile and uniformity of the sorbents using infrared thermography and a purpose-built finite element model (FEM).The overall energy consumption is estimated by accounting for the primary energy dissipation and loss processes during CO 2 desorption.

Adsorbent characterization and preparation
Commercial zeolite 13X beads (4-8 mesh, Sigma-Aldrich) were used in this study for its relatively high CO 2 selectivity and microwave absorption.The CO 2 uptake capacity of the zeolites were measured with dry industrial-grade CO 2 (>99% purity) and a pressure sorption analyser (ASAP 2050, Micrometrics).Up to 4 mmol g −1 of CO 2 was adsorbed at 25 • C, which decreased to around 0.03 mmol g −1 when zeolites were heated to 90 • C under vacuum conditions near 1 Torr (figure S1).Dielectric loss of zeolites was empirically determined to monotonically increase with increasing temperature through heating tests and simulations (section S2 in the SI).Thus, thermal desorption of CO 2 from zeolites is possible using microwaves.For CO 2 desorption experiments, approximately 5 grams of zeolite beads were arranged in a panel shape within a prefabricated perforated holder made of mica sheets measuring approximately 1 × 3 × 3 cm.

Microwave applicator setup
The CO 2 desorption experiments using microwaves were performed at the Plasma Science and Technology Laboratory at the University of Michigan.The system used in this study comprised of a single-mode (TE 10 dominant) rectangular waveguide microwave applicator, equipped with a vacuumcompatible reaction chamber that holds sorbents, and auxiliary equipment to inject gases and create vacuum (figure 1(a)).The reaction chamber can house zeolite beads packed into a panel shape and can be rotated 180 • under vacuum to evenly apply microwave on both sides.This setup is essentially a miniaturized version of a potential commercialscale design where a larger, thin sorbent panel can be treated with microwave in a uniform fashion-either with a horn antenna, slotted waveguide, or phased arrays.Similar configurations are used in industrial applications where shaped foodstuffs are conveyed into microwave cavities for processing (Resurreccion et al 2013).Further details are provided in section S3 in the SI.

Procedures of CO 2 desorption experiment
The CO 2 desorption experiment was made by replacing the infrared thermometer with the auxiliary gas preparation system as shown in figure 1 and applying microwave as determined by the heating tests.The reaction chamber was clamped with the auxiliary system to form a vacuum seal.Each experiment employed approximately 5 grams of new zeolite samples and followed a three-step process of (1) sorbent outgassing, (2) CO 2 adsorption, and (3) CO 2 desorption with microwaves (figure 1(b)).The mass of the reaction chamber which contains zeolites was measured in between each step using an analytical balance (AT201, Mettler Toledo).To avoid potential contamination, particularly from water, the chamber was filled with argon gas before being removed from the microwave applicator for the measurements.
Step 1-Sorbent outgassing: Zeolites were outgassed by heating them above 150 • C with 84 W of microwave under vacuum (<10 Torr) for 80-120 min.The outgassing was considered complete when the chamber pressure stabilized below 0.1 Torr, close to the ultimate pressure measured with the absence of sorbent.The mass of the chamber was measured once the chamber cooled down to ambient level.
Step 2-CO 2 adsorption: The chamber pressure was lowered to 0.1 Torr and pure CO 2 gas was introduced using a mass flow controller.CO 2 adsorption was complete after 65-100 min when the pressure stabilized.The mass of the chamber containing CO 2rich sorbents was recorded.
Step 3-CO 2 desorption: CO 2 desorption was initiated by evacuating the chamber with the vacuum pump at full capacity.The microwave source was powered on after 30 s once chamber pressure was reduced to nearly 8 Torr, nearly 1% of the ambient level.Microwaves were initially applied at 216 W for one minute, followed by 55 W to rapidly heat zeolites to above 100 • C. Zeolites were rotated 180 • every minute to uniformly heat both sides of the packed structure.The total chamber pressure was recorded throughout the experiment.Desorption was deemed complete (typically after 30 min of microwave input) when the chamber pressure stabilized near 0.1 Torr.Upon completion, the mass of the chamber that contains regenerated sorbent was measured.

Dielectric heating and heat transfer study
Dielectric heating of zeolite 13x and the associated energy losses through conductive and radiative heat loss, waveguide heating, and electromagnetic leakage was studied using an infrared thermometer (A325sc, FLIR) and a 3D finite element model (FEM) built with COMSOL.The infrared thermometer was used to measure surface temperature of the packed zeolites and waveguide during microwave application.The FEM included all primary system componentsincluding packed zeolites, mica holder, reaction chamber, Surfaguide, and Faraday cages-using experiment-based dimensions with simplified representations to reduce computational time.The accuracy of the FEM results was confirmed with the infrared thermography during the microwave application at the packed zeolites.See section S5 in the SI for further details on temperature measurement and simulation setup.

CO 2 desorption rate estimation
During regeneration, the CO 2 desorption rate can be calculated by disaggregating the total chamber pressure (P tot ) into contributions from initial constituent gases (P x , x = CO 2 , argon, and air), desorbed CO 2 ( PCO2 ), and leaked in air ( Pair ) using equations ( 1)-( 8) (White 2006, Roth 2012).This calculation relies on a simplified system model wherein gasescomprising initial constituents, desorbed CO 2 , and leaked in air-are evacuated using a vacuum pump operating at a constant pumping speed (S 0 ) through a single pipe (figure 2(a)).This high-fidelity calculation accounts for gas-specific viscosity (µ x ), temperature (T x ), and conductance (C x ) and also considers dynamically changing gas conductance from viscous (C x,v ) to intermediate (C x,i ) flow regimes during regeneration (equations ( 4)-( 8)).Notably, this gas transport model addresses the accumulation of desorbed CO 2 within the chamber resulting from restricted pipe conductance.The temperature of the desorbed CO 2 is estimated by finding a value that aligns the total amount of desorbed CO 2 from equations ( 1)-( 8) with the measured mass change in the chamber before and after desorption.Initially, the temperature of air and argon within the chamber is presumed to be at ambient level, then it is expected to equilibrate with the CO 2 temperature upon the commencement of microwave-induced regeneration, due to rapid gas diffusion in the vacuum (Wurzbacher et al 2016).A surrogate analysis with a residual gas analyzer (RGA100, Stanford Research Systems) confirmed the primary gas constituents and the absence of contaminants during measurements.Comprehensive details on the gas transport model, including the assumptions, full mathematical formulation, and calculation procedures as well as gas composition study are provided in section S4 in the SI.
2.6.Microwave power consumption and applicator efficiency Microwave power dissipated during CO 2 desorption ( Qd ) was determined by summing the power expended to overcome the enthalpy barrier, heat various components such as zeolites, mica holder, adsorbed and desorbed CO 2 , as well as the power lost through radiative heat transfer as shown in figure 2(b) (equation ( 9)).Parameters like desorption or heating rates, were established through studies on gas transport and dielectric heating, or were based on typical values found in literature as summarized in table S2.The efficiency (η) of the microwave applicator depends on the power consumed for desorption relative to the input power ( Qi ) (equation ( 10)).The power leaked into the environment ( Ql ) due to an imperfect setup is evaluated by considering power reflected from zeolites ( Qr ), dissipated during regeneration, transmitted through zeolites ( Qt ), reflected from the dummy load ( Qrr ), and used to heat waveguides ( Qw ) (equation ( 11)).Full mathematical formulations are provided in section S5.1 in the SI.

Microwaves induce rapid, low-temperature CO 2 desorption
The results of this study demonstrate significant advantages associated with using microwaves over conventional bulk heating methods, while showcasing important design considerations that need to be addressed before advancing the technology to an industrial-scale.The CO 2 desorption process is enabled primarily by dielectric heating that rapidly increase the temperature of the packed zeolites.The simulated temperature profile of the packed zeolites after a minute of microwave irradiation is shown in figure 3(a).The simulated temperature profile is confirmed with the infrared temperature measurements.The volumetric heating induced by microwaves create a pronounced temperature gradient that decreases from core (>200 • C) to the outer surface (<100 • C).
The CO 2 desorption initiates from hot core and its overall rate of desorption decreases as the CO 2 adsorbed at the colder surface desorbs at slower rates.Overall, desorbing 42%, 80%, and 99% of the adsorbed CO 2 required 82, 450 and 1380 s of microwave input, respectively, under the application conditions described in the methodology.The overall desorption rate using microwaves is faster than that of the conventional process involving bulk heating which typically requires a few hours to desorb most of the CO 2 (Swiss Federal Office of Energy 2017).
Our results affirm that a waveguide-based applicator can achieve near-total desorption of CO 2 from packed sorbents for DAC applications.The mass change of the packed zeolites, measured before and after microwave application, indicates a cyclic capacity of 4.1 mmol g −1 .This capacity aligns with the CO 2 adsorption capacity independently measured using the sorption analyzer.The overall quantity of the desorbed CO 2 , as calculated with the mass transport model (figure 2(a)) matches this value.This calculation factors in an estimated gas exit temperature of 45 • C-55 • C due to the thermal regeneration process.The estimated temperature of the effluent gas, according to the mass transport model, aligns well with the measurement made by Webley (Webley and Zhang 2014).In Webley's study, the temperature of the exiting CO 2 upon microwave-induced regeneration within a multi-mode cavity was found to be elevated by 40 • C.

Selective process induced by microwaves
The lower temperature of the desorbed CO 2 gas (45

Net microwave energy demand during CO 2 desorption
The net amount of microwave energy consumed during CO 2 desorption can be estimated by summing the energy dissipated through various processes, such as heating materials, overcoming enthalpy barrier of desorption, heat loss to the environment, and energy  leakage during the measurement as described in the methodology.The information to compute the net energy consumed is based on data measured during the experiments as well as values estimated using the gas transport model and FEM developed in this study.We estimated that about 11 J and 1.5 J of microwave energy are consumed every second to desorb CO 2 during and after the rapid heating, respectively, as shown in figure 4.During phase of rapid heating, the majority of the dissipated microwave energy goes towards heating the zeolite (35%) and adsorbed CO 2 (18%), and facilitating CO 2 desorption (29%).As the zeolites are heated with microwaves, heat loss to the surrounding environment through conductive and radiative transfer increases over time.After the phase of rapid heating, when microwave power is reduced, the temperature of the zeolites stabilizes at a relatively constant level.At this stage, the dissipated microwave power is primarily utilized to offset energy loss through conduction and radiation while continuing to desorb the remaining CO 2 .Following a few minutes of microwave application, radiative heat loss becomes the primary process of power dissipation.
The elongated processing time caused by nonuniform heating can significantly impact the overall energy demand and purity of the recovered CO 2 .Considering 4.1 mmol g −1 of the cyclic working capacity of zeolite, close to 2.4 GJ/tCO 2 of energy is used to desorb 42% of the CO 2 in the initial rapid heating phase.However, desorbing 99% of the CO 2 can require nearly 4 GJ/tCO 2 due to a longer microwave application requirement.At the same time, the lengthened processing time negatively impacts the purity of the recovered CO 2 due to air leaked into the system during regeneration.Air was measured to leak into the reaction chamber throughout the experiment at a rate of 0.002 Torr s −1 .The leaked-in air limited the purity of the recovered CO 2 to below 90% (figure S5).The CO 2 purity continuously decreased with prolonged treatment.Since some level of leakage is unavoidable in any vacuum system, expedited regeneration through more uniform heating can help minimize dilution and the overall energy demand of the regeneration process.

Experimental limitations
The microwave power measurement revealed that only about 5% of the incident power was utilized to induce CO 2 desorption and associated heat transfer processes.The remaining 95% did not contribute to sorbent regeneration due to the (1) impedance mismatching that caused nearly 37% and 27% of the microwaves to be reflected from and transmitted through zeolites respectively, and (2) inefficient shielding that leaked 31% of the power to the environment.The FEM simulation also confirmed significant reflection and transmission.This inefficiency in microwave application can be minimized by matching impedance and proper shielding.Efficiencies of incumbent industrial microwave equipment typically exceeds 80%, which provides a benchmark for DAC based on microwaves (Radoiu and Mello 2022).

Perspectives on techno-economic merits of using microwaves for DAC
The findings in this study suggest that replacing bulk heating-based sorbent regeneration in DAC with microwaves can offer numerous techno-economic advantages.The reduction in regeneration time can lead to substantial cost savings by downsizing the system, given that sorbents are projected to account for 80% of the overall system cost of DAC (National Academies of Sciences Engineering and Medicine 2019).For example, a standard sorbent-based DAC operation can be estimated to involve two hours of CO 2 adsorption followed by one hour of regeneration.If the regeneration period is cut down to 20 min-a value similar to what is observed in this study-it would enhance the throughput of the DAC process, thus requiring 22% fewer sorbents.This could lead to system-wide cost savings of 18%, provided microwaves do not negatively impact the degradation of sorbents.
Microwaves could substantially reduce the overall energy demand of DAC by facilitating selective processes.In this study, microwaves have reduced the CO 2 desorption temperature from conventional levels (>100 • C) to around 50 • C. Microwave energy was focused on the CO 2 binding sites due to the heterogenous dielectric properties within the zeolite framework (Polisi et al 2019).Further temperature reduction is possible by intensifying the selective process.Recent research suggests that applying a strong pulse of microwave (up to 100 W) to amine-grafted silica could limit the temperature rise during regeneration to about 10 • C (Ji et al 2023).By further intensifying microwave absorption exclusively on CO 2 binding sites, nonthermal CO 2 desorption becomes a potential outcome.This could involve using a stronger, shorter microwave pulse on microwave-transparent substrate appended with amine groups.Both free CO 2 gas and amine groups exhibit low dielectric loss near ambient conditions and are effectively transparent to microwaves.Upon adsorption, ionic products formed by CO 2amine pairs and co-adsorbed water could selectively focus microwave absorption at or near CO 2 binding sites.This suggests that microwaves have the potential to reduce the energy demand of DAC to the enthalpy of desorption-the thermodynamic energy limit of a sorbent.This advantage is crucial, considering that conventional DAC would still require around 9.4 GJ/tCO 2 even with future improvements and optimization, seven times more than the enthalpy of desorption (Deutz and Bardow 2021).
More importantly, low-temperature or even nonthermal CO 2 desorption enabled by microwaves could minimize sorbent degradation.This could considerably improve the techno-economic performance of DAC given that sorbents are projected to make up approximately 80% of the system cost in future DAC implementations (National Academies of Sciences Engineering and Medicine 2019).Conventional thermal regeneration deactivates amines on sorbents through reactions with oxygen and with CO 2 at elevated temperatures (Didas et al 2014, Min et al 2018).Lowering sorbent regeneration temperature can drastically limit reductions in CO 2 uptake capacity (Heydari-Gorji and Sayari 2012).Commercial DAC plants based on TVS cannot lower regeneration temperature and instead implement diverse measures such as pre-purging and steam injection to minimize sorbent degradation (Gebald et al 2019).But sorbents still need to be renewed frequently due to the capacity loss during regeneration (Deutz and Bardow 2021).While limited tests have demonstrated sorbent stability under microwave irradiation (Nigar et al 2016, Ji et al 2023), further studies on microwaveinduced degradation is needed.Realizing these proposed techno-economic benefits require careful process optimization and system design.

Resonant CO 2 desorption through microwave-induced NTP
Lastly, microwaves may enable resonant CO 2 desorption by forming NTP.Microwave-induced NTP has been demonstrated to sequentially populate higher vibrational modes from the ground state (Snoeckx and Bogaerts 2017).The excited vibrational energy can then decay into molecule-surface bonds, leading to resonant desorption either through surface heating (Chuang et al 1985) or bond breakage (Gortel et al 1983).We formed plasma during regeneration in a separate experiment using higher microwave power (>300 W).The white visual signature, CO 2 environment, and relatively mild temperature of the chamber suggested the formation of CO 2 NTP, but no further investigation was made.While such resonant phenomena with NTP could facilitate low-energy CO 2 desorption, further research is needed to explore its potential use for DAC.

Summary and Conclusions
This study experimentally demonstrates that microwaves can be used to design advanced DAC systems and yield various techno-economic advantages.Our experiments were based on employing 5 g of commercial zeolite 13× beads, assembled in a panel formation suitable for DAC application, and desorbing CO 2 by applying microwave from both sides of the panel to uniformly heat sorbents.Dielectric heating of the 3D zeolite structure was studied using a FEM developed with COMSOL.Gas desorption and transport during regeneration were studied using an analytic model which disaggregates the measured total pressure into partial pressure of constituent gases.This mass transport model accommodates the accumulation of desorbed CO 2 inside the reactor due to limited pipe conductance, and accurately estimates the desorption rates of CO 2 that align well with the measured weight changes of the sorbents.
Zeolites were rapidly heated to 135 • C, on average, in roughly a minute of microwave application under a vacuum below 10 Torr.Overall, desorbing 90% of the adsorbed CO 2 took approximately 10 min, a substantial improvement from over an hour required by the conventional conduction-based regeneration.The duration of regeneration could be further minimized closer to a minute if the system setup could be refined to enhance heating uniformity.The analysis also confirmed that the microwavebased process desorbs CO 2 at lower temperature, close to 50 • C, compared to conventional processes based on bulk heating (>100 • C).Microwaves induce low-temperature desorption by preferentially reacting with CO 2 binding sites, rather than heating bulk zeolite structure.Overall, we estimated that about 11 J and 1.5 J of microwave energy are consumed every second to desorb CO 2 during and after the initial rapid heating phase, respectively.
These findings suggest that an advanced DAC system designed with microwaves could offer substantial techno-economic benefits which are not attainable with conventional TVS process.Advantages include potential capital cost reduction through system downsizing enabled by a shortened regeneration time.The energy demand can be reduced close to the enthalpy of desorption of sorbents, the thermodynamic energy limits for CO 2 removal, by facilitating low-temperature or nonthermal regeneration with microwaves.Importantly, microwaves could also greatly extend the lifespan of sorbentscurrently limited to about 3000 cycles-by minimizing thermal oxidation.It is important to emphasize that these substantial techno-economic advantages can be accomplished through a novel sorbent regeneration technique involving microwaves, all without making any modifications to the existing sorbents.Additional research avenues can include exploring sorbent-microwave interactions, designing microwave applicator optimized for DAC, conducting a techno-economic assessment of microwavebased DAC, or investigating resonant CO 2 desorption with NTP.These efforts can enhance our understanding of alternative DAC methods and their potential advantages.

Figure 1 .
Figure 1.(a) schematics of the experimental setup that resembles industrial-scale microwave system, (b) experiments and simulations conducted in this study.

Figure 2 .
Figure2.(a) molar transport of each gas species is calculated using gas-specific parameters and simplified system geometry, (b) dissipated microwave power is estimated by balancing the power across the control volume.

Figure 3 .
Figure 3. (a) Simulated isothermal contour of packed zeolites after a minute of microwave input, (b) calculated molar percentage of CO2, air, and argon gas during microwave application.

Figure 4 .
Figure 4. Net microwave power dissipated through different energy processes, including heating materials, desorbing CO2, heat lost to the environment, and leaked power.Effective processing power indicates microwave power used to induce CO2 desorption, including heat loss terms.
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