What Is Direct Air Capture? Does It Work?

Direct air capture is the process of pulling in air from the atmosphere and then using chemical reactions to separate out the carbon dioxide (CO2) gas. The captured CO2 can then be stored underground or used to make long-lasting materials such as cement and plastics. The goal of direct air capture is to use a technological fix to decrease the overall concentration of CO2 in the atmosphere. By doing this, direct air capture could work alongside other initiatives to help mitigate the devastating effects of the climate crisis.

According to the International Energy Agency, an energy modeling organization, there are 15 direct air capture plants operating in the United States, Europe, and Canada. These plants capture over 9,000 tons of CO2 every year. The United States is also developing a direct air capture plant that will have the ability to remove 1 million tons of CO2 from the air per year.

The U.N.'s Intergovernmental Panel on Climate Change (IPCC) has warned that global CO2 emissions need to be reduced by 30% to 85% before the year 2050 in order to keep CO2 levels in the atmosphere below 440 parts per million by volume, and global temperatures from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit). Can direct air capture contribute to those reductions?

In order to slow the progression of climate change, scientists and economists from the IPCC agree that long-term measures are needed to reduce the amount of human-made greenhouse gas emissions. Direct air capture has been widely criticized as not doing enough on its own to bring down the amount of harmful CO2 in the atmosphere. It also costs more per ton of CO2 captured than other climate crisis mitigation strategies.

How Does Direct Air Capture Work?

Direct air capture uses two different ways to remove CO2 directly from the atmosphere. The first process uses what’s called a solid sorbent to soak up the CO2. An example of a solid sorbent would be a basic chemical that lays on the surface of a solid material. When air flows over the solid sorbent, a chemical reaction occurs and binds acidic CO2 gas to the basic solid. When the solid sorbent is full of CO2 it is either heated to between 80 C and 120 C (176 F and 248 F) or a vacuum is used to absorb the gas from the solid sorbent. The solid sorbent can then be cooled and used again.

The other type of direct air capture system uses a liquid solvent, and it is a more complicated process. It starts with a large container where a basic liquid solution of potassium hydroxide (KOH) flows over a plastic surface. Air is pulled into the container by large fans, and when the air that contains the CO2 comes in contact with the liquid, the two chemicals react and form a type of carbon-rich salt. 

The salt flows into a different chamber where another reaction happens that creates a mix of solid calcium carbonate (CaCO3) pellets and water (H2O). The calcium carbonate and water mix is then filtered to separate the two. The final step of the process is to use natural gas to heat the solid calcium carbonate pellets to 900 C (1,652 F). This releases the high-purity CO2 gas, which is then collected and compressed. 

The leftover materials are recycled back into the system to be used again. Once the CO2 has been captured, it can be permanently injected underground into rock formations to help bring aging oil wells back to life or used for long-lasting products like plastics and construction materials.

Direct Air Capture vs. Carbon Capture and Storage

Many experts believe that both direct air capture and carbon capture and storage systems (CCS) are essential pieces of the climate crisis mitigation puzzle. On a fundamental level, both technologies reduce the amount of CO2 that could mix into the atmosphere. However, unlike direct air capture, CCS uses a chemical to capture CO2 directly at the source of the emissions. This prevents CO2 from ever entering the atmosphere. For example, CCS might be used to capture and compress all of the CO2 in the emissions from a coal-fired power plant stack. Direct air capture, on the other hand, would collect the CO2 that has already been released into the air by the coal-fired power plant or other fossil fuel-burning operations. 

Direct air capture and CCS both use basic chemical compounds such as potassium hydroxide and amine solvents to separate CO2 from other gases. Once the CO2 is captured, both processes must then compress, move, and store the gas. While CCS is a slightly older process than direct air capture, they are both relatively new technologies that could benefit from further development. 

Because CCS removes CO2 at its source, it can only be used where there is fossil fuel combustion, like industrial facilities and power plants. In theory, direct air capture can be used anywhere, although placing it near sources of electricity or where CO2 can be stored would increase its efficiency.

Current DAC Initiatives and Results

According to the World Resources Institute, there are three leading direct air capture companies in the world: Climeworks, Global Thermostat, and Carbon Engineering. Two of the companies utilize solid sorbent technology to remove CO2, while the third uses liquid solvent carbon engineering. The number of operational and pilot plants varies from year to year, but the world's first commercial-grade DAC facility currently removes 900 tons of CO2 per year, and there are several commercial facilities under construction.

For the past 15 years, a direct air capture pilot plant in Squamish, British Columbia, Canada, has used renewable electricity and natural gas to fuel a liquid solvent process which can remove one ton of CO2 per day. This same company is currently building another direct air capture facility that will be able to capture 1 million tons of CO2 per year. 

Another direct air capture plant being built in Iceland will be able to capture 4,000 tons of CO2 per year and will then permanently store the compressed gas underground. The company building this plant currently has 15 smaller direct air capture plants around the world.

Pros and Cons

The most obvious advantage to direct air capture is its ability to reduce atmospheric CO2 concentrations. It can not only be used more widely than CCS, it also uses up less space to capture the same amount of carbon as other carbon sequestration techniques. In addition, direct air capture can also be used to create synthetic hydrocarbon fuels. But in order to be effective, the technology must be sustainable, inexpensive, and scalable. So far, direct air capture technology has not advanced enough to meet these requirements. 

Pros

Companies that specialize in direct air capture technology are currently developing new, larger direct air capture plants with the capability of capturing up to 1 million tons of CO2 per year. If enough smaller direct air capture units are produced, they could capture as much as 10% of human-generated CO2. By injecting and storing the CO2 underground, the carbon is permanently removed from the cycle.

Because it relies on capturing CO2 from the atmosphere and not directly from fossil fuel emissions, direct air capture can function independently of power plants and other fossil fuel-burning factories. This allows for more flexible and widespread placement of direct air capture plants.

Compared to other carbon capture techniques, direct air capture does not require as much land per ton of CO2 removed.

In addition, direct air capture could reduce the need to extract fossil fuels, and it could further decrease the amount of CO2 we release into the atmosphere by combining captured CO2 with hydrogen to produce synthetic fuels, such as methanol. 

Cons

Direct air capture is more expensive than other carbon capture techniques such as reforestation and afforestation. Some direct air capture plants currently cost between $250 and $600 per ton of CO2 removed, with estimates ranging from $100 to $1,000 per ton. According to researchers from the RFF-CMCC European Institute on Economics and the Environment, future costs of direct air capture are uncertain because they will depend on how quickly the technology advances. Conversely, reforestation can cost as little as $50 per ton.

Direct air capture’s high price tag comes from the amount of energy it requires to remove CO2. The heating process for both liquid solvent and solid sorbent direct air capture is incredibly energy intensive because it requires chemical heating to 900 C (1,652 F) and 80 C to 120 C (176 F to 248 F), respectively. Unless a direct air capture plant relies solely on renewable energy to produce heat, it still uses some amount of fossil fuel, even if the process is carbon negative in the end.