In the future, CO2 could be extracted directly from the air on a larger scale. The first industrial prototypes for direct air capture are already in operation. However, the processes and plants are energy-intensive and expensive.
The concentration of CO2 in the air now stands at 417 ppm - it was 280 ppm before the start of the Industrial Revolution, over a period of several thousand years. The Intergovernmental Panel on Climate Change (IPCC) calls for "deep, rapid and sustained reductions in greenhouse gas emissions." In addition, it may also be necessary in the future to actively remove CO2 from the atmosphere and thus limit the greenhouse effect to a tolerable level.
Possible options include reforestation, the rewetting of moors and the sequestration of CO2 by accelerating the weathering of rock. In addition, so-called direct air capture (DAC) technologies are also being discussed, i.e. large plants that filter CO2 directly from the ambient air.
DAC processes are based on various principles: absorption or adsorption, but membrane technologies or air separation are also possible. However, absorption and adsorption have the highest technical maturity. In the book Carbon Capture, Jennifer Wilcox explains the processes in detail.
CO2 absorption in solvents
In absorption, CO2 is either physically dissolved or chemically bound in a liquid - for example an aqueous monoethanolamine or alkali/hydroxide solutions. Subsequently, the solvent saturated with CO2 is regenerated in a coupled process. This produces a reasonably pure CO2 stream that can be concentrated and stored or used elsewhere, while the recycled solvent is recirculated and can absorb CO2 from the air.
The process takes place in multi-stage columns. As the number of stages in a column increases, so does the CO2 yield. The air is blown into the system by fans, while the liquid is moved by pumps. Fans and pumps, as well as the heat input required to release the CO2 from the solvent, account for a large proportion of the energy used and thus also the operating and maintenance costs. For example, when a sodium hydroxide solution is used, the CO2 is desorbed from the solution at temperatures of around 900 °C. The heat required is 1420 to 100 °C. The heat requirement is between 1420 and 2250 kWh per ton of CO2.
Porous solids adsorb CO2
A different principle comes into play in adsorption. Here, the CO2 is bound to the surface of a porous solid with high CO2 binding affinity, either via weak intermolecular forces or via strong covalent bonds. Suitable filter materials include amino-modified mesoporous silica, zeolites, or metal-organic frameworks. The pore size and distribution determine how quickly the material is saturated and how much CO2 it can bind.
By heating it up to 80 to 100 °C and under vacuum conditions, the CO2 can then be desorbed from the filter material. Alternatively, the CO2 can also be removed by moistening the filter, which is already possible at temperatures of around 45 °C. Operating and maintenance costs are closely related to the heat required for regeneration of the filter material and to the fan power with which the air is blown through the filter.
Circulating huge volumes of air
The underlying processes are already being used in industry. There, however, they are applied to exhaust gases with CO2 concentrations of 8 to 14 %, whereas the concentration of CO2 in the atmosphere is only 0.03 %. Fans must therefore feed vast amounts of air to the DAC systems so that the process can effectively filter the climate gas out of the air. This is expensive and energy-intensive. According to Gautam Sen, capturing a fixed amount of CO2 from the air consumes 1.8 to 3.6 times more energy than applying the same processes to industrial exhaust, as he points out in the article Research & Development Pathways/Challenges in Direct Air Capture of CO2.
Jennifer Wilcox also points out the low efficiency of DAC processes when applied to ambient air rather than exhaust gases. According to Wilcox, an average 500-MW coal-fired power plant emits about 11,000 tons of CO2 per day. To capture the same amount of CO2 from the air, she calculates, "For example, assuming an airflow velocity of 2 m/s, capturing 11,000 t of CO2 per day directly from the air requires an area of about 133,000 m2 to process 2.31 × 1010 m3 of air per day."
Too energy-intensive as the sole solution
In the chapter Technical Support for Long-Term Deep Decarbonization, researchers at Tsinghua University cite a study that says taking 30 Gt out of the atmosphere - today, global emissions are about 40 Gt per year - would mean building about 30,000 large DAC factories. By comparison, fewer than 10,000 coal-fired power plants operate worldwide today.
Scientists believe it is realistic that to offset continued chronically high CO2 emissions, DAC technologies would consume up to a quarter of the world's energy production by 2100 - about 300 exojoules. This amount is equivalent to the annual energy demand of China, the U.S., the EU and Japan, or the global supply of coal and natural gas in 2018.
Several DAC plants in operation
Nevertheless, several industrial prototypes of DAC plants are already in operation, and at the very least there is hope that further research and development projects will produce viable options to at least flank measures to reduce CO2 emissions.
An overview of current projects, technologies used, and approximate costs is provided by Gautam Sen:
50 to 80 US dollars/t-CO2
Carbon Engineering (Canada)
Combination of KOH and Ca(OH)2 as solvent
136 US dollars/t-CO2, with heat and electricity costs of 4 US dollars/GJ and 7 US cents/kWh, respectively
Climeworks (Switzerland) and Union Engineering (Denmark)
600 US dollars/t-CO2
Global Thermostat (USA)
Amine-based chemical solution on porous ceramic honeycomb structure
10 to 35 US dollars/t-CO2 for capture of 1 million tons/year
Center for Negative Emissions of Arizona State University
Ion exchange resin
30 to 200 US dollars/t-CO2
Carbon sink (USA)
Amine-based ion exchange
Less than 20 US dollars/t-CO2, provided that power plant waste heat can be used
No substitute for emissions reduction
Despite the high energy consumption, experts are confident, according to Gautam Sen, that DAC processes could compete with other CO2 capture processes in terms of cost if they were deployed on a massive scale. Nevertheless, it remains questionable whether the technology can even be scaled up at the necessary rate, he said.
In the chapter Climate Action: The Feasibility of Climate Intervention on a Global Scale of the book Climate Geoengineering: Science, Law and Governance, Kimberly A. Gray concludes, "However, at the current state of the art and under current economic conditions, none of the techniques for removing CO2 from the air could be used alone or in combination to achieve the 2°C warming target without unacceptable biophysical or economic impacts." Thus, aggressive mitigation of greenhouse gas emissions remains the centerpiece of a climate action plan, he said.