Among the many pathways being pursued for global decarbonization, one technology seeks to mimic the very thing forests and other vegetated areas do naturally — remove carbon dioxide (CO2) directly from the air.
Direct air capture (DAC) technology may not yet be getting the same attention as other decarbonization pathways, but it’s being widely explored for its potential as a solution that actually negates CO2 already present in the atmosphere. This negative-emissions approach utilizes chemical processes to strip CO2 from ambient air. The collected CO2 can then be compressed for permanent sequestration in underground geological formations. Alternatively, CO2 can be utilized for enhanced oil recovery or for beneficial reuse in applications such as e-fuels or other industrial applications.
“DAC can truly be a novel approach as the technology can avoid many of the limitations associated with carbon capture from point sources such as flue stacks,” says Justin Schnegelberger, manager of development engineering for the Burns & McDonnell Energy Group. “Hard-to-abate industries that face challenges reducing carbon emissions from their own products, processes or activities are increasingly looking at DAC.”
There are a number of processes under development for direct air capture. While the specific technologies differ, the general concept is similar. Ambient air is moved over and through chemically active media that strip out CO2.
Many of the technologies currently in the most advanced stages of development and implementation utilize one of two general processes. The chemical compositions of either will vary, depending upon the given technology solution supplier:
Under both methods, the CO2 binds with the active chemicals. Once the media are saturated with CO2, heat is applied to advance the process.
In a liquid solvent-based system, the CO2-rich solution is moved to a regeneration vessel where the solvent is heated to approximately 900 degrees C (1,652 degrees F), breaking chemical bonds and releasing the CO2.
In a solid adsorbent system, the saturated media typically remains in the contactor vessel, where it is sealed off and heated to between approximately 80 and 120 degrees C (176 and 248 degrees F), breaking chemical bonds and releasing the CO2.
The CO2 gas is then collected and compressed prior to transport from the site for sequestration or for another end use.
DAC technology is an energy intensive process. In active DAC technology, fans driven by electric motors move air through the process. Thermal energy is required for process regeneration (stripping CO2 from the capture media subsequent to removal from ambient air). Additional energy is also required for process cooling requirements, CO2 compression and other balance-of-plant equipment, systems and functions.
Because DAC technology is generally intended to be a negative-emissions resource, it is often the goal for DAC facilities to utilize non-fossil or other non-CO2-emitting resources for both thermal and electrical supply. If fossil-based energy sources are used to supply either thermal or electrical energy needs, the CO2 emissions from these sources will need to be factored into calculations that determine the net CO2 reduction from the DAC facility. For this reason, DAC facilities may be prone to be located in regions with an abundance of power from renewable sources.
With DAC technology not being tied to an emissions point source, there is a great deal of flexibility for siting of sequestration areas and energy sources. This can allow for integration with various renewable energy resources. For example, a 4-kiloton-per-year DAC facility is currently operating in Iceland that takes advantage of abundant geothermal energy for its power and process heat needs. A similar 40-kiloton-per-year geothermal facility being developed by Climeworks is currently in construction in Iceland.
DAC technology is gaining traction as a potentially viable solution for hard-to-abate industries like transportation and aviation that face decarbonization challenges. In these industries, DAC would be utilized to remove CO2 from the atmosphere to offset CO2 emissions produced from internal combustion engines, jet engines or other sources. The ultimate goal is to achieve net zero or net-negative CO2 emissions.
A number of companies such as Airbus, United Airlines, Audi, Swiss Re, Microsoft, JP Morgan Chase and others are moving forward with programs in which DAC facilities provide carbon dioxide removal (CDR) services. Companies purchase CDR credits from DAC facilities to offset produced CO2 or result in net negative emissions. Airbus, for example, previously announced an agreement with Oxy Low Carbon Ventures (1PointFive) for such a program. Under the agreement, Airbus purchases the capture and permanent sequestration of 100,000 metric tons of CO2 per year for four years by a DAC facility currently under construction by Oxy in the Permian Basin of West Texas. That plant is projected to have a total CO2 removal capacity of 500,000 metric tons.
According to the International Energy Agency, there are 18 mostly small DAC plants currently operating worldwide, but an enormous scale-up is expected by 2050, resulting in capacity to annually capture more than 60 million metric tons of CO2. This growth will require advancements in DAC technology along with modularization of components and advancements in the availability of cost-effective renewable or low-carbon electricity to improve the overall economics of construction and operation.
Financial and tax incentives recently made available by Congress will help by providing billions of dollars for grants, loans or tax credits. The Inflation Reduction Act that was passed in mid-2022 sweetens tax credits already available under Section 45Q of the U.S. tax code for both point source and DAC facilities. The Infrastructure Investment and Jobs Act, enacted in 2021, provides $3.5 billion for DAC hubs and an additional $115 million for DAC technology development.
Siting for DAC facilities must weigh in the availability and abundance of non-CO2-emitting energy resources and proximity to pipelines, sequestration fields and offtake possibilities. In order to achieve net zero CO2 emissions, negative emissions solutions and carbon removal technologies will absolutely be required. DAC could prove to be a key option in the global movement toward decarbonization.