Emerging trends in direct air capture of CO₂: a review of technology options targeting net-zero emissions

Yasser Abdullatif ^ab, Ahmed Sodiq ^b, Namra Mir ^a, Yusuf Bicer ^a, Tareq Al-Ansari ^a, Muftah H. El-Naas ^c and Abdulkarem I. Amhamed *^b
aCollege of Science and Engineering, Hamad Bin Khalifa University, Qatar Foundation, Education City, Doha, Qatar
^bQatar Environment and Energy Institute (QEERI), Doha, Qatar. E-mail: aamhamed@hbku.edu.qa
^cGas Processing Center (GPC), Qatar University, Doha, Qatar

First published on 15th February 2023

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Yasser Abdullatif

Mr Yasser M. Abdullatif has a Bachelor of Science in Mechanical Engineering from Qatar University and a Master of Science in Sustainable Energy graduate from Hamad Bin Khalifa University (HBKU) in 2020. He is currently a PhD student and Research Assistant at QEERI, Hamad Bin Khalifa University. His research focuses on Direct Air Capture, thermodynamic analysis, CFD modeling and experimental evaluation of sorbents characteristics. He also works on Nanofluids, Engine Emissions Reduction, Multi Generation Systems Analysis, and Water Desalination technologies. His research is interdisciplinary between Mechanical Engineering, Energy, and Environmental Engineering.

Ahmed Sodiq

Dr Ahmed Sodic holds a BSc, MSc in Chemical Engineering and a PhD in Sustainable Energy from Obafemi Awolowo University, Nigeria, Masdar Institute of Science and Technology, UAE and Hamad Bin Khalifa University, Qatar, respectively. He is currently a Postdoctoral Researcher at Qatar Environment and Energy Research Institute (QEERI) with research focus on Direct Air Capture of CO2 using the HVAC systems with. He has published several articles, book chapters and patent applications in various fields such as Carbon Capture, Sustainable Energy, Electrochemistry and HVAC Systems.

Namra Mir is a Research Assistant at Hamad bin Khalifa University in the Division of Sustainable Development with the College of Science and Engineering in Doha, Qatar. She received her MSc in Sustainable Energy from Hamad bin Khalifa University in 2021. Her thesis focused on sustainable photo-electrodialysis desalination and its integration with renewable energy. She completed her BSc in Chemical Engineering in 2019 from Qatar University, Doha, Qatar.

Yusuf Bicer

Dr Yusuf Bicer received his PhD in mechanical engineering from the University of Ontario Institute of Technology in Oshawa, Canada. He completed his BS in Control Engineering (2012) and a master's degree in Energy Science and Technology (2014) at Istanbul Technical University, Turkey. His PhD thesis focused on photoelectrochemical-based hydrogen and ammonia production options. He also worked for more than two years at Istanbul Practical Gas and Energy Technologies Research Engineering Industrial Trade Inc. on natural gas and solar energy applications.

Tareq Al-Ansari

Dr Tareq Al-Ansari completed his undergraduate Beng. In Mechanical Engineering from the University College of London, an MPhil in Engineering for Sustainable Development from the University of Cambridge, and a PhD from Imperial College London. He is currently an Associate Professor and division head at the Division of Sustainable Development in the College of Science and Engineering at Hamad Bin Khalifa University (HBKU).

Muftah H. El-Naas

Dr Muftah El-Naas is a Research Professor at Qatar University's Gas Processing Center, where he's served as Director and held the QAFCO industrial Chair professorship. He has a B.A.Sc in Chemical Engineering from UBC, Canada, M.Eng & PhD in Chemical Engineering from McGill University, Canada. His expertise includes: water treatment, CO2 capture and conversion, biotechnology, membrane separation, and plasma technology. His recent research focuses on developing environmentally friendly technologies for the Oil and Gas industry. Dr El-Naas has authored over 200 papers, 2 books, several book chapters, and patents.

Abdulkarem I. Amhamed

Dr Abdulkarem Amhamed is a Senior Scientist, CO2 Cluster Program Director, with 22 years of experience in the oil and gas industry including Industry 4.0 and Sustainable Processes Design. He earned his doctorate and MSc degrees in Chemical Engineering from Bradford University, UK, and the University of Salford UK, respectively. He is currently the CO2 Cluster Program Director at QEERI, technical lead for a sustainable urban project, and developing a new technology for Direct CO2 capture from the atmosphere (DAC-AC). He has held many senior research and management positions in industry, attracted over $15 million in funding.

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Introduction

The increase in the Earth's temperature took a new turn in 1950 and has steadily continued through the 21st century. By the year 2020, however, the global mean temperature was 1.02 °C above the preindustrial era.^1,2 The Paris agreement was signed to maintain the global mean temperature below 2 °C to avoid catastrophic global warming consequences.³ By achieving the Paris agreement objective, 9 to 10 billion people will not be exposed to heatwaves, and 85 million people will not be affected by flooded rivers. Moreover, five hundred million people will not suffer from water stress, and 3 million square kilometers will remain viable for agriculture (half of the country like India).⁴ The review papers published by 97% of climate experts and the Intergovernmental Panel on Climate Change (IPCC) reported that the origin of climate change is the high emission rate of Green House Gases (GHGs), mainly CO₂.⁵ The GHGs cause global warming because these gases can absorb the infrared radiation reflected from the earth and reemit it, thereby raising the earth temperature. GHGs such as methane (CH₄), halocarbons, nitrous oxide (N₂O) and ozone (O₃) exist in the atmosphere with low concentration while the high concentration GHGs are water vapor (H₂O) and carbon dioxide (CO₂) as shown in Fig. 1.⁶

The most concentrated GHGs in the atmosphere are water vapor and CO₂ and are the leading cause of global warming. In fact, water vapor is more concentrated and can absorb more infrared waves than carbon dioxide.⁸ The global temperature increases due to anthropogenic activities, especially when humans use fossil fuels that emit CO₂ and with higher CO₂ concentrations, the atmosphere gets hotter and more humid.⁹ Thus, reducing the CO₂ emissions in the atmosphere will result in low water vapor concentration and reduced global mean temperature.^10,11 Preventive and remediation methods can reduce CO₂ emissions. The preventive approach includes using renewable energy systems and improving systems efficiency,¹² while the remediation includes capturing, storing, and utilizing CO₂.¹³ Although the whole world exerts efforts to reduce CO₂ emission by preventive methods, billions of tons of CO₂ is still being emitted into the atmosphere. Considering the continuous emissions, IPCC proposed CO₂ capture as a necessary.

There are two types of CO₂ capture technologies: the conventional stationary sources¹⁴ and the direct air capture (which removes CO₂ directly from the atmosphere). The conventional CO₂ capture technology prevents the emitted GHGs from spreading to the atmosphere.^15,16 As shown in Fig. 2, the conventional CO₂ capture technique is subdivided technology to limit CO₂ emissions¹⁷ into pre-combustion, oxy-fuel combustion, and post-combustion capture.^18,19 In pre-combustion carbon capture, a production of syngas (hydrogen and carbon monoxide mixture) from fuel reforming is followed by CO₂ separation process.²⁰ The oxy-fuel combustion technology includes fossil fuel burning in the presence of pure oxygen.²¹ The post-combustion approach involves capturing CO₂ from flue gas (end of pipe treatment approach).^16,22 The three mentioned conventional carbon capture techniques prevent emissions of CO₂ to the atmosphere from burning of fossil-based fuels. On the other hand, Negative Emissions Technologies (NETs) including direct air capture technology (DAC) creates an outlet to directly capture the existing CO₂ from the atmosphere to achieve negative emission goals.²³ Although the IPCC proclaimed that fossil fuel usage was needed to be immediately reduced in 1990,²⁴ fossil fuel global primary energy consumption represented 84.3% in 2019.²⁵ As a result, the latest report from IPCC stated that the contribution of NETs are required to stabilize the CO₂ emission at double the preindustrial levels by the second half of 21st century.²⁶ Moreover, the need for rapid deployment of negative carbon technologies were proposed by Paris talks and Nation Research Council (NRC) to meet the 2 °C limit and net-zero carbon in the later part of 21st century goals.^27,28

Negative emissions technologies (NETs) are carbon dioxide removal (CDR) techniques from the atmosphere, such as direct air capture (when carbon removal is achieved through physicochemical processes such as adsorption, absorption, ocean alkalinity enhancement and soil mineralization) or indirect air capture (when carbon removal is achieved through biological processes such as bioenergy with carbon capture and storage (BECCS), ocean fertilization, afforestation, biochar and algae culture).^29,30 Under direct air capture, there is adsorption, which is the capture of CO₂ in the pores of solid sorbents (physisorption)^31,32 or the reaction of acidic natured CO₂ with basic sites on solid materials (chemisorption).^32,33 Alternatively, CO₂ can be captured in the liquid volume in the process known as absorption. In absorption, reactive compounds (such as NaOH, Ca(OH)₂, KOH and amines) are dissolved in liquid phase, which allows CO₂ from air to be retained within the liquid volume.³⁴ Ocean alkalinity enhancement is an absorption process in which CO₂ is absorbed in ocean water and reacts with ocean minerals to neutralize ocean acidification.^35,36 This process alone captures about one-third of the global CO₂ emissions from the atmosphere.³⁴ Soil mineralization is another direct air capture approach, it is the largest land-based carbon sink on the planet and it is capable of capturing about 3 GtC year⁻¹.³⁷ Soil carbon mineralization follows three approaches,³⁸ (i) buildup of organic carbon as a result of plant growth; (ii) rock weathering (helping the breakdown of inorganic carbon in soil solution); and (iii) precipitation of carbonate materials. However, ocean alkalinity enhancement and soil mineralization were rarely investigated in the literature and more studies are required to show if the technology can be used at scale.³⁹ Detailed pictorial representations of NETs are shown in Fig. 3 below. BECCS is the most researched NETs under indirect air capture.⁴⁰

CO₂ from the atmosphere is captured naturally by microorganisms and plants through photosynthesis, thereby producing biomass. Since biomass is considered a clean source of energy, the biomass obtained can be used to generate electricity via thermoelectric power plant, thereby translating to negative emissions,^34,42 however, its use is dependent on the availability of biomass, land and storage.⁴³ Afforestation is another indirect air capture that has the capacity to store large amount of CO₂. Just like soil mineralization, afforestation is a technique that can be deployed anytime because it is readily available.⁴⁴ Planting new trees or better management of the existing forests can increase the natural rates of carbon capture from the atmosphere,⁴⁵ although it has been reported in the literature that this may lead to loss of biodiversity,⁴⁶ loss of valuable land for crops,⁴⁷ and increase in loss of ice and local warming in high altitude forest.⁴⁸ Ocean fertilization increases biomass productivity on the ocean floor when micronutrients (such as iron) and macronutrients (such as phosphorus and nitrogen) are dissolved into the ocean, whereby the expected microorganism sinks to the ocean floor.⁴⁹ Algae (microalgae and seaweeds) culture has been reported to have photosynthetic efficiency that is ten times higher than plants.⁵⁰ Microalgae culture has the capacity to capture CO₂ from different sources, such as from flue gas point sources and distributed sources like the atmosphere.⁵⁰ The most remarkable part of CO₂ biosequestration procedure is the recycling of part of biomass in the form of biochar. Biochar is produced through pyrolysis or gasification of biomass.⁵¹ Currently, biomass remains are burned or decomposed in soil; and as a result, large amount of the CO₂ captured through photosynthesis is emitted to the atmosphere. When biomass is converted to biochar for soil nourishment, CO₂ emission may be reduced by as much as 1.8 Gt year⁻¹ without CCS.⁵²

About half of CO₂ yearly emissions are from distributed sources. The fact that DAC systems can remove CO₂ from both distributed and point sources, are not location specific, do not have contamination issues such as (NO_x and SO_x), and has small footprint, shows the necessity of deploying DAC over point sources and other NETs.⁵³ However, capturing CO₂ from dilute air is an energy intensive process. The minimum CO₂ separation energy required in case of dilute air (400 ppmCO₂) was calculated to be about 20 kJ per molCO₂,^16,29–31 while the minimum energy required to capture from flue gas using benchmark aqueous MEA is 8.4 kJ per molCO₂.⁵⁴ DAC is a relatively new technology that is still in its early commercial stages. The early startups that have contributed immensely to DAC commercialization are Carbon Engineering in Canada, Climeworks in Switzerland and Global Thermostat in the United States.⁵⁵ Different DAC studies have shown that chemisorption is more relevant to DAC than physisorption (activated carbon, zeolites, and metal–organic frameworks) because physisorption has poor performance in capturing CO₂ at low concentration streams in the presence of water vapor.^56,57

The high energy requirements of DAC lead to high capital and operating costs, which is a major challenge of the existing technologies.⁵⁸ The costs are reported in the literature for three main types of DAC systems, which are high temperature aqueous solutions,^59–64 low temperature solid sorbents,^65-67 and moisture swing solid sorbents.⁶⁸ However, these reported costs were not comparable due to different assumptions and outputs. Fasihi et al.⁶⁹ recalculated these costs based on fixed assumption and reported cost range of 115-388, 120-244 and 99 EUR per ton CO₂ for high temperature aqueous solutions, low temperature solid sorbents and moisture swing solid sorbents, respectively. A DAC cost of Gigaton scale at less than $100 per ton CO₂ by 2050 is needed for the technology to achieve great climate impact, which highlights the need for cost reduction in DAC technologies.⁷⁰ Many studies have been carried out to estimate the potential cost of DAC to show if it would be a climate change valuable solution in the future. Simon et al.⁷¹ examined a generic DAC technology, and claimed that capturing CO₂ at 220 EUR per ton CO₂ is currently possible but indicated that the calculated cost could be provided by more research into kinetics and capturing thermodynamics. Moreover, the authors estimated a range of cost from 75 to 800 EUR per ton CO₂ based on pessimistic and optimistic scenarios. House et al.⁷² pointed out that the literature underestimated the cost of DAC, and the current cost of DAC is as high as 750 EUR per ton CO₂, however, the cost has the potential to reach 225 EUR per ton CO₂ with a technological breakthrough. In fact, most of the studies in the literature show that DAC technology cost will be reduced with time. The cost of capturing CO₂ was estimated to reach 30, 71 and 105 EUR per ton CO₂ based on optimistic, realistic, and pessimistic scenarios by Broehm et al.⁷³ Nemet and Brandt⁷⁴ also estimated a low DAC cost of 45, 23 and 14 EUR per ton CO₂ by 2029, 2050 and 2100, respectively based on the assumed 10% learning rate and a lifetime of 50 years, which is higher than other assumptions in the literature by 20 years. Mahdi et al.⁶⁹ proposed that the line of research should be more intensive on low temperature solid sorbent compared to high temperature aqueous solutions because it has higher potentials for cost reduction to reach 54 EUR per ton CO₂ for low temperature systems compared to 71 EUR per ton CO₂ for high temperature system by 2050 based on 15% learning curve. The reason for this potential is the fact that low temperature solid sorbents can use low thermal grade heat, does not required external water and has high modularity.

In the recent years, considerable efforts have been made by carbon capture researchers to collate work on direct air capture of CO₂ in the form of review articles. Several review articles have been published on DAC sorbent materials and technology options.^75–80 For example, the work of Deng et al.,⁷⁵ Zhu et al.,⁷⁶ Cherevotan et al.,⁷⁷ and Shi et al.⁷⁸ focused on various forms of sorbent materials for DAC applications, while McQueen et al.⁷⁹ provided analysis on current as well as future DAC technologies, and Erans et al.⁸⁰ assessed DAC technologies with a focus on techno-economic and socio-political challenges. A search through the literature showed few review articles that combine all the highlighted elements in a single publication. This work, nonetheless, combines the review of DAC sorbent systems with technology options. Moreover, to the best of our knowledge, this work is the first article to review DAC integration with HVAC systems – an emerging technological option.

As a comprehensive review, the article starts by highlighting the importance of using porous material to capture CO₂ from the atmosphere over any other process. All the absorption and adsorption-based technologies are assessed in detail with tables to allow for the comparison. A general overview on the commercialization of DAC is presented, and for the first time, a detailed review of the articles that touch on DAC-HVAC systems' integration is carried out. The work highlights the main challenges and benefits of the integration, presents mathematical models for the integration evaluation, proposes certain suitable technologies based on the conducted reviews and shows a brief technoeconomic study of HVAC-DAC unit integration to demonstrate the cost benefits therein.

Description of DAC

The direct air capture (DAC) system setup consists of sorbents, contact area, and regeneration module. The sorbent is a liquid or solid material that attracts CO₂ either chemically or physically. For CO₂ capture to occur, ambient air is exposed to the sorbent material through the contact area, and upon saturation with CO₂ or as desired, the sorbent material undergoes regeneration in which it is separated from the captured CO₂ to get concentrated CO₂ stream. The sorbent material should be reversible so that it can be used many times. The removal of CO₂ from ultra-diluted air by heating, cooling, air compression or using membranes requires an extremely high-energy consumption.⁷⁸ For example, 2.2 MJ mol⁻¹ of CO₂ of air-cooling is required to form dry ice and 7 MJ per mole of CO₂ is required for 1 bar pressure drop through membrane separation.⁸¹ On the other hand, binding CO₂ with sorbent materials consumes little or no energy, but the most energy required in the process is associated with removing high concentrated CO₂ from the sorbents.⁷⁸ In the literature, sorbents associated with DAC technology are classified into two, liquid⁶⁸ and solid⁸² sorbents. Solid sorbents do not lose heat to evaporation as liquids; it has better kinetics and is more effective in preventing the loss of volatiles to the atmosphere.⁸³ The liquid (absorption-based systems) and solid (adsorption-based systems) sorbents used in DAC systems are described below.

Absorption based systems

Absorption is a technique used to remove CO₂ from a gas stream into the bulk of liquid sorbent based on chemical or physical interactions. The absorption of CO₂ can be classified into the absorbent types such as alkanolamines absorption,⁸⁴ dual alkali absorption,⁸⁵ aqueous ammonia absorption,⁸⁶ sodium carbonate slurry absorption and chilled ammonia absorption. The absorption processes used in DAC systems are limited to the chemical sorbents with strong CO₂ binding affinities.⁷⁸ The following will include only the absorbents that are used in DAC systems.

Adsorption based systems

Adsorption is a reversible process where the solid adsorbent captures the molecules, atoms or ions of gases and liquids on its surface by physical means like van der Waals forces or by forming chemical bonding. The reversibility of adsorption process is a function of temperature and pressure, which means the adsorption and desorption capability can vary by varying both temperature and pressure. CO₂ can be adsorbed at high pressure and then released when the pressure is lowered in a process known as pressure swing adsorption (PSA). The same process could be implemented by alternating the temperature in a process called temperature swing adsorption (TSA).¹⁶ The solid adsorbents will be divided in the present work among the three main categories, which are physisorption, chemisorption, and moisture swing adsorption.

In chemisorption, CO₂ binds strongly to the adsorbent by chemical bonding, which has the capacity to capture CO₂ even in ambient air. However, it needs high energy to release the CO₂ from the sorbents through heating like what occurs in the calcination process.¹⁰² On the other hand, the physisorption requires less regeneration energy but it has low CO₂ selectivity and capacity for atmospheric concentrations.⁵⁶ Another process that does not consume much energy in releasing CO₂ is the electrochemical CO₂ capture, but it has been reported that it is only effective if CO₂ concentration falls between 15% and 30%.¹⁰² Finally, moisture swing adsorption combines the advantages from both chemisorption and physisorption, as it offers a high selectivity and capacity with low regeneration energy. In the moisture swing adsorption, CO₂ can bind to the sorbent materials if it is dry, while the separation occurs if the material is wet. The energy consumption in the process is associated with water evaporation for drying the sorbent material.¹⁰³ Different physisorption, chemisorption and moisture swing sorption materials will be discussed in the later sections.

Other CO₂ capture classifications

Other CO₂ sorbents

Commercialization of DAC

Many companies work on direct air capture as shown in Fig. 12. Keith established Carbon Engineering company in Canada in 2009, in which Bill Gates partly funded it.⁶⁹ Carbon Engineering initially used sodium hydroxide solution as a sorbent with a desorption temperature of about 900 °C. The company has recently optimized the solvent used from NaOH to KOH to enhance CO₂ capture efficiency of their system.²³⁰ The company introduced a carbon capture unit that can capture 1 ton of CO₂ per day in 2015 with a target to integrate a fuel production system based on the captured CO₂. The company reported that it could produce 1 barrel of fuel per day using their AIR-TO-FUEL pilot in 2017. The cost of capture, purification, and compression of CO₂ to 150 bar was estimated to be in the range of 75-113 € per tCO₂.⁶³ Another top DAC startup in the world today is Climeworks. Gebald and Wurzbacher established Climeworks company in Switzerland in 2009. Climeworks uses amine-modified sorbent to capture CO₂ at a desorption temperature of 100 °C. Climeworks collaborated with Audi and Sunfire to launch a DAC unit that converted the captured CO₂ into synthetic diesel in 2014.²³¹ Two other commercial-scale DAC units were installed in Switzerland and Iceland to feed a greenhouse with CO₂ and for the mineralization process, respectively in 2017. The company has a goal to achieve a cost of 75 € per tCO₂ for the large-scale plants. In 2021, the Orca unit was launched in Iceland to capture CO₂ from ambient air and store it through mineralization. The plant runs purely on renewable energy and can capture 4000 tons of CO₂ per year.²³² Eisenberger established Global Thermostat company in the USA in 2010. Global Thermostat uses a low temperature-based system to capture CO₂ from both point sources and ambient air at the desorption temperature between 85 and 95 °C.²³³ The company launched a running unit that uses amine-modified monolith as a sorbent in California. It was reported that the waste heat with mentioned desorption temperatures could be used in a plant with a capacity of 40000 tons of CO₂ per year, and they target a capture cost of 11–38 € per tCO₂.⁶⁵ O'Connor established Antecy company in Finland in 2010. It was also working on a low-temperature sorbent that can be regenerated at a temperature between 80–100 °C²³⁴ later in 2019. Antecy and Climeworks companies were fully merged to achieve a better and stronger technology.²³³ Another DAC system with a CO₂ capacity of 1.387 tCO₂ per year uses the VSA regeneration method with regeneration temperature between 70 and 80 °C, the company was established by Oy Hydrocell.²²⁹ Moreover, the company provides laboratory-scale regenerative and non-regenerative CO₂ scrubbers units.²³⁵ There is also Skytree company located in the Netherlands. The information about the technology used by Skytree is limited, but they use electrostatic absorption and moisture for regeneration and benzylamines functionalized sorbent.²³⁶ Besides Skytree, there is also Infinitree which was established in New York in 2014, and it launched a laboratory scale unit that uses moisture swing technology in capturing CO₂.^237,238 All other mentioned companies provide CO₂ purity of more than 99% except companies like Skytree and Infinitree, which provide low purity CO₂, for example, Infinitree offers CO₂ purity between 3% and 5% for growing algae.²³⁸ Other newly established companies are creating innovating ideas to make direct air capture more visible such as Aircapture, Carbon Capture, Carbyon, Heirloom, Mission Zero Technologies, and Solitaire Power. Bill Gross established Carbon Capture company in California in the USA in 2019.²³⁹ Carbon Capture company uses molecular sieve technology to remove CO₂ from air with a pre-dehumidifier stage powered by low-cost renewable energy. They reported that their system could produce a ton of clean distilled water with every ton of CO₂ capture.^240,241 Carbyon is another DAC company that was established in the Netherlands in 2019. They target a cost-effective system based on amine-modified thin membrane and fast swing technology where there is a rotating drum with a modified material to capture CO₂ effectively. They claimed that the CO₂ adsorption takes 30 seconds and additional 30 seconds is needed for regeneration. They currently target CO₂ capture cost of 50 EUR per ton.²⁴² Direct air capture using carbon mineralization is developed by Heirloom, a company established in South India. The carbonate minerals are decomposed into pure CO₂ stream and oxide minerals in their system. The oxide mineral is exposed to ambient air to naturally absorb CO₂ without any fans.^241,243 Mission Zero technology was also launched in England in 2020 and uses electrochemical method for sorbent regeneration.²⁴¹ Soletair Power was established in Finland in 2016, it uses buildings as carbon sinks by using resin-based DAC technology. The company proclaimed that their products boost human activity and fight climate change. They have two main products: indoor CO₂ capture unit and building HVAC integration unit. Their products can reduce CO₂ concentration in the airflow between 200–300 ppm and capable of adsorbing 20 tons of CO₂ per year. However, the building HVAC integration unit is permanently fixed to the building HVAC system, whereas the indoor unit is mobile and can be moved depending on the desired location.^241,244

DAC and HVAC integration potential

Buildings consume almost 40% of the world's energy demands, and about 50 to 82% of buildings' energy is consumed in the HVAC systems.²⁴⁵ The necessity to provide fresh indoor air, especially during and after COVID-19,²⁴⁶ and the rising energy demands, challenge researchers to reduce energy losses in the HVAC systems.²⁴⁷ Minimizing energy demands in buildings can be achieved in two ways: increasing the efficiency of the HVAC system (inside uses) or capturing the lost energy to be used for valuable purposes (outside uses).^247,248 The building energy management systems (BEMS) were used to decrease energy consumption in HVAC systems based on the classifications shown in Fig. 13. The inside use means the recovered energy from the HVAC system is used within the HVAC system equipment to raise its efficiency in achieving thermal comfort, while the outside use means the energy is used in another system to provide a useful output such as electricity.²⁴⁹ Besides the mentioned two classifications, DAC with HVAC system integration can be proposed to combine both inside and outside uses. The inside uses include reducing indoor CO₂ levels to improve indoor air quality (IAQ) and HVAC energy reduction using higher air recirculation ratios, while the outside uses include a more efficient DAC system by capturing from more elevated CO₂ concentrated streams (indoor air), applying cooler adsorption and using moisture swing adsorption (MSA) within the HVAC system.

Benefits of DAC and HVAC integration

Reducing indoor CO₂ levels is mandatory because indoor CO₂ accumulates due to human metabolism or emissions from indoor sources.²⁵⁰ The high levels of CO₂ exposure lead to severe cognitive effects. Studies have shown that the CO₂ concentrations in the range of 4000 to 10000 cause malaise, headache and lethargy, while higher levels of 10000 to 30000 may cause metabolic changes, non-narcotic central nervous system and electrolyte imbalances. The indoor CO₂ concentration can be controlled by simply replacing the indoor air with outdoor air through ventilation, however, ventilation is an energy-intensive process that adds more energy requirements for fans, heater, cooler, humidifier and dehumidifiers. Moreover, in some cases ventilation is not a solution due to the use of air exchange rate reduction strategies or in case of the need of protecting people from outdoor hazards (shelter in place). Gall et al.²⁵¹ targeted reducing the CO₂ level in indoor environment such as sleeping microenvironments, school classrooms and shelter in place (SIB) facilities as it can exceed the permissible exposure limit (PEL) of 5000 ppm in 8 hours set by Occupational Safety and Health Administration (OSHA). The study investigated four alkaline metal oxides and hydroxides Ca(OH)₂, Mg(OH)₂ soda lime and MgO in indoor air cleaning applications. The results showed that mg-containing sorbent has slow kinetics, then soda lime was used, and 1.7 kg was enough to reduce 80% of indoor CO₂ levels and not exceed the OSHA PEL in SIB while it reduced the CO₂ levels from 2599 ppm to 550–750 ppm in low ventilated bedrooms. The simulated pressure drop was 300 pa which can be achieved by typical fans from HEPA filter-containing portable air cleaner. However, CO₂ saturated the sorbent after 40 hours based on assumed regular occupancy, implying the need for sorbent replacement, leading to high cost.

Secondly, the HVAC energy reduction can be achieved by increasing the air recirculation rate. The buildings' energy consumption is high and contributes to one-third of the total global CO₂ emissions. However, the use of CO₂ capture device can reduce the CO₂ levels in building, it does not allow the full recirculation of air because of other pollutants like volatile organic compounds and dust, which are harmful to humans. Most air pollutants other than CO₂ can be captured using filters such as HEPA H14, carbon filter, G3/4 pre-filter and F7/8 fine-filter. Although combining these filters with a CO₂ capture unit removes all types of air pollutants,²⁵² the build-up of oxygen is still an issue that requires ventilation.²⁵³ Based on Kim et al.²⁵⁴ assumption and calculations, the recirculation of air for 10 hours leads to approximately 20.12–20.174% oxygen ratio in the air, while the acceptable oxygen levels are between 19.5% and 23.5% (by volume), which allows the full recirculation of air within that period (10 hours).

A more efficient DAC system that captures from higher CO₂ concentrated streams (indoor air) was simulated by Zhao et al.²⁵⁵ and the results showed that the optimal second law efficiencies for indoor CO₂ concentration of 3000 ppm, 2000 ppm and 1000 ppm are 44.57%, 37.55% and 31.6%, respectively. The effect of Indoor CO₂ concentration on indoor CO₂ filters was further investigated experimentally by Hu et al.,²⁵⁷ the results showed that the higher the inlet concentration (1000, 1500, 2000), the steeper the breakthrough curve, which signifies a higher CO₂ capture rate. The above studies show the potential benefits of the higher indoor CO₂ levels on DAC unit performance. Applying cooler CO₂ adsorption and its effects on DAC unit performance was further studied by Zhao et el.,²⁵⁵ the results showed an increase in second law efficiency from 15.38% to 39.41% when adsorption temperature changed from 323 to 298 K under 3000 ppm CO₂ level conditions. It was explained that decreasing adsorption temperature increases the adsorbent CO₂ capacity and reduces the minimum separation work. The previous results showed the potential for energy savings by simply locating the DAC unit in the cold region of the HVAC stream.

Finally, the moisture swing approach in capturing and producing CO₂ is considered the least energy consuming technique among others. However, it needs more water compared to other regeneration techniques.²⁵⁸ Shi et al. conducted an experiment on five different samples, showing that the higher relative humidity of air decreases the equilibrium CO₂ concentration.²⁵⁹ The previous study highlights the potential of integrating a moisture swing-based DAC system with the HVAC system as different air streams with different humidity already exist within the HVAC system. Bryan and Salamah proposed the integration of CO₂ collectors inside the HVAC system. The system is located to capture CO₂ from the exhaust stream before leaving the building as shown in Fig. 14. It uses ion-exchange resin sorbent and moisture swing to adsorb and regenerate CO₂. The advantage of their proposed system is that it uses the HVAC fan-free energy and the higher concentration of CO₂ in the exhaust stream. Based on their calculation, the system can offset 0.1% of USA carbon emissions if the system is installed in 50% of the existing commercial buildings.²⁵⁶

Modeling of DAC and HVAC integration benefits

Different methods were used to evaluate the performance of using CO₂ capture system within the built environment. Gall et al.²⁵¹ conducted an experiment to evaluate the performance of indoor air cleaner and track the indoor CO₂ concentration after using a CO₂ capture unit, based on the setup shown in Fig. 15, by measuring the variation of indoor CO₂ concentration with respect to time with and without the air cleaner. The indoor CO₂ concentration level (C_room) in mole CO₂ m⁻³ for a room that uses a CO₂ capture unit can be modeled by the below equation.

Vroom dCroom/dt = QCa − QCroom − QfCroom + QfCf,Last node	(20)

where, V_room is the room volume (m³), Q is the outdoor air ventilation rate through the room (m³ s⁻¹), C_a is the concentration of CO₂ in outdoor air (molesCO₂ m⁻³), C_room is the concentration of CO₂ in the room as it enters the reactor, Q_f is the volumetric flow rate through the reactor (m³ s⁻¹), and E is the total CO₂ emission rate from occupants in the room (moles s⁻¹). Q and E can be obtained based on the procedure described by Persily et al.,²⁶⁰ the boundary condition for the above equation is such that at t = 0, C_room = 0.0163 molesCO₂ m⁻³ (or 400 ppm CO₂) or whatever the initial concentration of room is and C_f,20 (mol CO₂ m⁻³) is the concentration of CO₂ in the flow exiting the reactor at the last node which can be calculated based on below equations

Vf,1·dCf,1/dt = QfCroom − QfCf,1 − yk[M1]Cf,1Vf,1	(21)

d[M1]/dt = −k[M1][Cf,1]	(22)

Vf,i+1·dCf,i+1/dt = QfCf,i − QfCf,i+1 − yk[Mi+1]Cf,i+1Vf,i+1	(23)

d[Mi+1]/dt = −k[Mi+1][Cf,i+1]	(24)

Here, the index i ranges from 1 to 19, V_f,i is the volume of reactor element i, C_f,i is the concentration of CO₂ in the well-mixed reactor element i, M_i is the mass of unreacted sorbent in element i.

To solve the above equation, parameters such as y(carbonation yield) and k(reaction constant) and boundary conditions need to be identified based on experimental measurements. The carbonation yield or total removed CO₂ was calculated by measuring the CO₂ concentration at the inlet and outlet of the indoor air cleaner and determining the time-integrated difference between both. The calculated mass of absorbed CO₂ was divided by the initial mass of sorbent and converted to moles to determine the molar yield (y) in moleCO₂/mole_sorbent, for reaction constant (k) estimation, the sum of squared errors between measured and modeled values of C_f was calculated based on the following formula

	(25)

Different meaningful values of K were used to model C_f, and then the value that achieved the minimized SSE function was used for the calculation, and finally, the boundary conditions were applied.

Another study that presents a model to track CO₂ concentration but added the energy-saving calculations by raising the air recirculation ratio was conducted by Kim et al.²⁵⁴ They carried out experimental and numerical modeling to analyze the effect of using a CO₂ capture device on indoor CO₂ concentration, allowable air recirculation ratios, and the energy-saving potentials. The new system consists of both AHU and CO₂ capture units, which was compared to the conventional ventilation system. The modelling was implemented based on two configurations, as shown in Fig. 16.

The required outdoor airflow rate (V_s) is correlated with the increase in indoor CO₂ concentration (C_g) and CO₂ generation rate per person (N) with the following equation;

Cg = N/Vs	(26)

While for the mechanically ventilated rooms, the equation will be;

V·dC/dt = QC0 − QC(t) + G(t)	(27)

V represents the room volume, Q is the volume flow rate, C₀ is the outdoor CO₂ concentration C(t) is the indoor CO₂ concentration and G(t) is the CO₂ generation rate per person, both are functions of time. The integration of the previous equation with the assumption of Q, C₀ and G(t) to be constant lead to the following equation;

C(t) = C0 + G(t)/Q + (C(0) − C0 − G(t)/Q)e−It	(28)

I = Q/V	(29)

By adding CO₂ capture unit to the case of air recirculation, the equation will be as follow;

C(t) = C(t − 1) − A(t − 1) + G(t)/Q + (C(0) − ((t − 1) A(t − 1))−G(t)/Q)e−It	(30)

where A(t) is the rate of CO₂ adsorption. Based on the experimental work, an equation was derived with an assumption of 800 m³ h⁻¹ air flow rate and 1 kg adsorption capacity. The equation can calculate the amount of CO₂ adsorbed in ppm for certain time.

	(31)

	(32)

The cooling and heating loads also were calculated using the following formula;

QC = mair(hin − hout)	(33)

Qh = mairCp(Tin − Tout)	(34)

One more model was also presented by Baus and Nehr,²⁶¹ who investigated the possibility of attaching DAC unit to the HVAC system as shown in Fig. 17. Experimental measurements were conducted for four different buildings in Germany and the data was used in a numerical model to assess the building CO₂ budget and potential energy saving by increasing air recirculation.

Based on the four evaluated buildings, the following equations were used to estimate building CO₂ load and potential energy saving to be 114.3–530.2 kgCO₂ per day and 0.34–2.57 MW h per ton.

	(35)

AC = pair,hum·feed·Cp,amb·ΔT + 1.2Qv·ρH2O·feed	(36)

Where m_i is the mass of either CO₂ or H₂O in (g), t_i is the time in (h), Kⁱ₁ is the emission rate due to occupancy (g h⁻¹), Kⁱ₂ is the rate increasing due to ambient air feed (g h⁻¹), Kⁱ₃ is the rate change from recycled air, Kⁱ₄ is air exchange rate in (1/h), Kⁱ₅ is the performed rate of CO₂ capture or dehumidification, _AC is the energy consumption in HVAC, p_air,hum is the specific density of humid air, _feed is the ambient air rate (m³ h⁻¹), C_p,amb is the specific heat of ambient air, ΔT is the temperature difference between the design point and the ambient temperature, Q_v is water enthalpy of vaporization and ρ_H2O is the absolute humidity of air.

The effect of both CO₂ concentration level and cooler adsorption was modeled by a thermodynamic model presented by Zhao et al.²⁵⁵ The model describes the performance of TVSA based indoor CO₂ capture unit and can be used as a method of comparison between different capturing technologies. It combines the thermodynamic second law of efficiency calculation, adsorption isotherms, and regeneration energy. The model can assess the effect of range of parameters such as adsorption temperature (T_ads), desorption temperature (T_des), desorption pressure (P_des), indoor CO₂ level (feed gas CO₂ concentration x_A,1), CO₂ recovery and produced CO₂ purity on DAC thermodynamic efficiency (η_TVSA). The calculation can be performed using the following equations;

	(37)

where T_H is the temperature of heat source, T_L is the temperature of cooling source, T₀ is the ambient temperature and W_min is the minimum separation work, which can be calculated as follow;

	(38)

where W_ac is the actual work done which, is divided into the work done for pressurization W_pump, input heat per cycle Q_H and adsorption cooling energy per cycle Q_L. Q_H and Q_L can be calculated for TVSA cycle using the below formulas based on assumed single gas adsorption (CO₂) by treating other components as inert gases, equilibrium capacity is reached, uniform temperature and CO₂ concentration in the sorbent are attained. All other above parameters have been fully defined by zhao et al.²⁵⁵

	(39)

	(40)

WCTVSA = qmax − qmin	(41)

where q_max is the maximum equilibrium CO₂ loading in the TVSA cycle; q_min is the minimum equilibrium CO₂ adsorption amount in the TVSA cycle, while (ΔH(q)) is the desorption enthalpy and (q) is the adsorption capacity, which is calculated from the isotherm curves that change from material to another. The above model used amine-functionalized cellulose and its isotherm curves parameters and calculations were obtained from ref. 163

Other models available in the literature, which are associated with filter design and adsorption kinetics were conducted by Hu et al.²⁵⁷ In the study, they ran an experiment to maintain acceptable IAQ levels by capturing excess CO₂. Activated carbon filter impregnated with magnesium and calcium oxide was used as the capturing material. The filter performance was assessed based on initial removal efficiency, η₀

	(42)

and pressure drop, which can be calculated based on Ergun equation with the interpretation of each parameter in ref. 257 et al.

	(43)

The results showed that impregnated activated carbon with MgO promotes better adsorption performance than CaO. Moreover, the experiment and model showed that the initial efficiency increases as the air velocity decreases; however, the lower air velocity leads to a higher pressure drop, indicating that both parameters should be optimized for the best performance. Lee et al.²⁶² investigated the use of amine-modified Y-type zeolite in CO₂ adsorption and desorption cycles under indoor environmental conditions. Three types of amines were impregnated in the zeolite (TEPA, MEA and IPA), and TEPA achieved the highest capacity of 158 mg g⁻¹-adsorbent per hour under indoor CO₂ concentration of 1000 ppm.²⁶² The experimentally obtained concentrations of CO₂ with respect to time were successfully fitted to the first-order kinetic model and it was used to calculate the rate constants k. The highest k value between the three amines was achieved by TEPA.

Economic analysis of DAC and HVAC integration

Across different studies in the literature, it has been found that the key parameters affecting the capital and operating costs of DAC solid sorbent are the sorbent working capacity, cycle time, sorbent lifetime, desorption temperature and vacuum pressure.⁷⁹ Many of the economic assessments performed on DAC in the literature show varied results due to differences in assumptions and methodologies; these can make it difficult for direct comparisons.⁸⁰ The cost for current large-scale DAC systems ranges roughly between $80 per tCO₂ to $1133 per tCO₂, and in the future it is expected to drop to around $34 to $260 per tCO₂.²⁶³ The improvements in contactor designs, sorbent properties, and heat integration are some of the parameters predicted to lower economic and environmental impacts.²⁶³ Bioenergy with carbon capture and sequestration (BECCS) is a competitor for DAC and its cost ranges between $20–100$ per tCO₂ which is lower than that of DAC. Coastal blue carbon and terrestrial carbon removal are other alternatives with lower costs around $0–20 per tCO₂. However, these technologies have drawbacks such as available land area, demand for wood and forestry management.⁷⁰

Capital expenditures (CAPEX), operating expenditures (OPEX) and the sorbent are the main factors that affect the overall cost of DAC. The type of sorbent selected can also affect the overall cost with liquid sorbents costing less. Climeworks that uses a solid sorbent has an overall cost of $600 per tCO₂.¹⁶⁰ CAPEX contributes to most of the overall cost for this technology. Individual equipment components for the process have costs ranging from $0.13–420 million dollars.⁷⁰ OPEX such as maintenance, labour, waste removal and makeup are necessary to keep the process running. For both liquid and solid sorbents, the OPEX is below $100 per tCO₂ with solid systems having lower OPEX around $5 to $50 per tCO₂.²³⁸ Therefore, it is necessary to conduct research to reduce costs for a more economically favourable process. Since energy is required for heating, the source of energy also influences the cost as well as the environmental impacts. Fossil fuel-based energy sources are cheaper but have a higher impact on the environment. Solar energy results in the cost being around $430 per tCO₂, which is much higher than a fossil fuel based energy source such as coal.²⁶⁴ Since solid adsorbent systems require regeneration temperatures of around 80–120 °C, low-grade waste heat can be used.²⁶⁵

DAC coupled with HVAC can result in reduced energy requirements for ventilation systems. Additionally, when integrating with existing HVAC systems, there are available blower (fans) and existing system (duct, utilities) to use for potentially better CO₂ capture at low temperatures. This should reduce the CAPEX for DAC as they are already available. Ji et al.²⁶⁶ coupled DAC with a building air conditioning system in a study, which significantly reduced the heat demand resulting in lower OPEX. They also found that not much additional cost was needed for retrofit. In a theoretical study, Baus and Nehr²⁶¹ found that coupling DAC with HVAC in recirculation can potentially lower the energy requirements in buildings; resulting in lower operation costs. It was also highlighted that CO₂ absorbers can be expensive and can have stability issues in the long run needing replacement. Moreover, sorbents are exposed to thermal and mechanical stress in addition to reactive chemicals, which can lower the lifetime of the sorbents. Therefore, the technical feasibility of HVAC/DAC depends on sorbents with long lifetimes and affordable costs. The economic viability is quantified by the energy saving potential of HVAC/DAC integration in recirculation mode, which lowers OPEX. Therefore, the success of the HVAC/DAC coupling depends on the DAC technology itself and the effectiveness of integration of DAC into the energy infrastructure of the building. Even though the DAC technology is in its infancy, coupling with HVAC in buildings provides an energy saving potential, which can encourage the technology to enter the mass market.²⁶¹

Challenges and future recommendation in DAC-HVAC integration

The HVAC and DAC integration brings complications as well as potential savings. These complications apply to some challenges that can be explained in the following points.

• For energy efficiency purposes, full air recirculation is recommended, but the build-up of VOC requires additional filters, which increases the pressure drop in the air supply stream.

• The integration of DAC unit with the HVAC systems implies additional pressure drop in the supply air stream, which requires CO₂ filter/sorbent material geometry's optimization.

• For the CO₂ filter to achieve building negative emission concept, the effluent air CO₂ level should be lower than the ambient CO₂ level, as a result, the minimum acceptable indoor CO₂ level should be investigated.

• The filter location should be positioned in the coldest stream; however, the coldest stream would have the highest relative humidity. The humid air will cause water adsorption and brings more energy consumption for water regeneration. This discussion raises a research challenge to find material that co-adsorb the least water molecules.

• It is important for future research in DAC-HVAC integration to utilize the cold stream for adsorbent cooling after temperature-dependent regeneration.

• For recommendations, HVAC indoor environment requires the use of environmentally friendly materials (sorbents) in order to safeguard the health of the occupants.

• Finally, investigating a way to combine humidity swings in HVAC and moisture swing adsorption of CO₂ without mixing the produced CO₂ with air again is recommended.

Conclusions

This work has reviewed the literature on emerging trends in direct air capture of CO₂. The discussion is directed at the main drivers (sorbent systems and regeneration options) of DAC for commercialization. It is well established that a good sorbent should have high CO₂ selectivity, capacity, free binding energy, low regeneration energy, thermal stability, and cyclic stability. The two main capture options in DAC, the liquid and solid sorbents, have been tested for these characteristics. In general, the liquid sorbents are volatile, which implies heat loss due to evaporation and have lower kinetics compared to solid sorbents. For the aqueous solution of metal hydroxides, they need high temperature (up to 900 °C) for regeneration, although amine solutions have been reported with lower regeneration temperature. For solid sorbents, especially the physisorption materials, the major issue that affects their performance is their strong affinity for moisture in the atmosphere. If the strong water affinity issue is resolved, MOFs nanocomposite and porous frameworks are good sorbent candidates as they have high selectivity towards CO₂ over N₂ and other gases in the atmosphere. For chemisorption materials, depending on the category, the major issue affecting their performance is poor cyclic stability, which leads to leaching of amines during operations, and the poor cyclic stability is compounded when water is co-adsorbed. Although covalently bound polymeric amines on solid supports that are known as hyper-branched amino silica (HAS) have been able to solve this issue as reported. HAS offer high capacity, stability, easy preparation, low cost, and excellent regeneration compared to other categories of chemisorption materials.

As for the energy requirements for DAC system if high purity CO₂ is desired, it should be noted that there is no significant difference in the required energy between absorption and adsorption systems. More precisely, the thermal and electrical energy requirements for solid sorbent are 3–6 GJ tCO₂⁻¹ and 1.5 GJ tCO₂⁻¹, respectively. While for liquid sorbent, the thermal and electrical energy requirements are 5.25–8.1 GJ tCO₂⁻¹ and 1.3–1.8 GJ tCO₂⁻¹, respectively, depending on the contactor configuration and packing materials. However, moisture swing adsorption (MSA) offers opportunities if high purity CO₂ is not desired, because it has low energy consumption compared to other methods. The mechanism of MSA opens up potentials for integrating DAC with other systems such as HVAC for creating DAC system with minimum energy requirements. As illustrated in the economic analysis of DAC-HVAC integration, research efforts are already looking promising in DAC-HVAC integration; however, health impact of the sorbents to be used in HVAC systems needs to be investigated because of the sensitivity of the HVAC systems to human wellbeing. Investigation of the suitable sorbents, regeneration method, filter position, and filter design is required to gain the most of the proposed integration.

Conflicts of interest

On behalf of all authors, the corresponding author states that there exists no conflict of interest either financially or through other personal considerations that may compromise or have the appearance of compromising the researchers' professional judgment in conducting or reporting this study.

Nomenclature

(a-CNCs)	Acetylated cellulose nanocrystals
(MCM-41)	Mobil composition of matter
°C	Degree celsius
AAILs	Amino acid ionic liquids
AC	Activated carbon
APC-ILs	Aminopolycarboxylate-based ILs
APS	Aminopropyltriethoxysilane
BIF-20	A zeolite with high density of exposed B–H bonding
BN	Boron nitride
CN	Carbon nitride
CNF	Cellulose nanofibers
Con.	Concentration
Da	Dalton
DAC	Direct air capture
EMS	Energy management system
FS	Fumed silica
g	Gram
GCPS	Global Congress on Process Safety
GHG	Green house gases
GJ	Gigajoule
HAS	Hyperbranched amino silica
HIPE	High internal phase emulsion
HMDS	Hexamethyldisilazane
HVAC	Heating, ventilation, and air conditioning
IER	Ion exchange resin
ILs	Ionic liquids
IPCC	Intergovernmental panel on climate change
K+	Potassium cation
kJ	Kilo joule
kW h	Kilowatt hours
L	Litre
m	Meter
m3	Meter cube
MCF	Mesoporous cellular foam
MCN	Mesoporous carbon nitride
MEA	Monoethanolamine
Mg	Magnesium
mg	Milligram
MJ	Megajoule
MOF	Metal–organic frameworks
Na+	Sodium cation
NDDCTs	Natural draft from natural draft dry cooling towers
NFC	Nano fibrillated cellulose
NG	Nanostructured graphite
NPs	Nanoparticles
NRC	Nation Research Council
Pa	Pascal
PEG	Poly(ethylene glycol)
PEHA	Pentaethylenehexamine
PEI	Poly(ethylenimine)
PMMA	Poly(methyl methacrylate)
PPA	Poly(allylamine)
PPI	Poly(propylenimine)
ppm	Parts per million
PPN	Porous polymer networks
PSA	Pressure swing adsorption
PVA	Polyvinyl alcohol
PVDF/Si-R	Polyvinylidene fluoride and superhydrophobic silica nanoparticles
R1NH2	Primary amines
R1R2NH	Secondary amines
R1R2R3N	Tertiary amines
Ref.	Reference
Reg.	Regeneration
RFAS4	Four letter from (resorcinol, formaldehyde, 3-aminopropyltriethoxysilane)
RHA	Rice husk ash
SBA-15	Santa barbara amorphous-15
SER	specific energy requirement
SG	Silica aerogels
SynA	Mesoporous y-alumina-supported PEI composite
t	Ton
TCSA	Temperature concentration swing adsorption
TEPA	Tetraethylenepentamine
TPD	Temperature programmed desorption
TSA	Temperature swing adsorption
TVSA	Temperature-vacuum swing adsorption
VSA	Vacuum swing adsorption
WC	Working capacity
ZIF-8	Zeolitic imidazolate framework
Zr-SBA-15	Zirconium-containing mesostructured SBA-15

Greek letters

€	Euro
ΔH	Enthalpy of the reaction
IPA	Isopropanol amine
IPA	Isopropanol amine
M	Molecular weigh

Subscripts

th

Thermal

Chemical formulas

(CH4)	Methane
(CO2)	Carbon dioxide
(H2O)	Water
(N2O)	Nitrous oxide
(O3)	Ozone
Ca(OH)2	Calcium hydroxide
CaCO3	Calcium carbonate
CaO	Calcium oxide
H2SO4	Sulfuric acid
He	Helium
K2SO4	Potassium sulfate
KOH	Potassium hydroxide
MgO	Magnesium oxide
N2	Nitrogen
Na2CO3	Sodium carbonate
NaBO2	Sodium metaborate
NaOH	Sodium hydroxide
SiO2	Silicon dioxide
TiO2	Titanium dioxide

Acknowledgements

Authors acknowledge the financial support from Qatar Environment and Energy Research institute (QEERI), Hamad bin Khalifa University (HBKU) (member of Qatar Foundation), Qatar National Research Fund, NPRP12C-0821-190017, GSRA8-L-1-0506-21032, and Open Access funding provided by the Qatar National Library, Doha, Qatar.

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