Carbon Dioxide Removal Program

Direct Air Capture

The CDR Program is advancing intentional technological interventions for the purpose of creating negative emissions through durable storage. While these approaches differ significantly in their energy use, land use, and social implications, they will all require substantial R&D efforts to enable a responsible and efficient CDR industry that is responsive to the climate crisis. Challenges include the dilute concentration of CO₂ in the atmosphere, large water and land use requirements, carbon life cycle effectiveness, low-carbon energy integration, low-cost durable storage, and impacts of process pressure drop on overall energy use and system cost. The development of robust life cycle analysis and monitoring, reporting and verification (MRV) methods are critical to ensuring effective and permanent CO₂ removal for all CDR approaches. Note that CDR does NOT refer to existing systems that remove CO₂ from the atmosphere; for example, CDR does NOT include CO₂ removal through existing trees or by planting new trees to offset wildfire losses.

Biomass Carbon Removal and Storage

Biomass carbon removal and storage (BiCRS) is a CDR approach involving the use of biomass (e.g., algae, municipal waste, agricultural and wood residues) to remove CO₂ from the atmosphere and store it underground or in long-lived products, without affecting food security, rural livelihoods, biodiversity conservation, and other important valuesii. Carbon dioxide is produced from the combustion, gasification, pyrolysis, or other conversion of biomass to generate electricity or produce hydrogen, and the resulting CO₂ emissions are captured and then stored in a manner that prevents entry into the atmosphere. Challenges associated with BiCRS include water and land requirements; cultivating, transporting, and processing microalgae at large scale; and difficulties in measuring, monitoring, and crediting carbon removal.

One common type of BiCRS is bioenergy with carbon capture and storage (BECCS), which combines energy production using plant biomass to produce electricity, liquid fuels, and/or heat with the capture and storage of the CO₂ produced when using the bioenergy. Because the biomass draws CO₂ from the atmosphere as it grows, BECCS can be considered a negative-emissions technology. The co-firing of biomass (e.g., switchgrass, wood pellets, corn stover) with fossil fuels reduces the CO₂ life cycle footprint of existing power plants. However, large-scale implementation of BECCS is constrained by land availability for competing uses (i.e., food production) and low power plant efficiency.

Enhanced Mineralization

Enhanced mineralization (also known as enhanced weathering or accelerated weathering) is a CDR approach that uses technologies or modified land-use approaches to accelerate the decomposition of calcium- and magnesium-rich silicate rocks, which chemically react with CO₂ to form solid carbonate minerals, resulting in the removal of CO₂ from the atmosphere. There are several types of mineralization processes: in situ (e.g., CO₂ reactions in geologic formations underground), ex situ (e.g., CO₂ reactions that involve extraction, transport, and grinding of minerals), and surficial (e.g., CO₂ reactions with minerals distributed across land or coastal areas). Enhanced mineralization can help stabilize and permanently store waste from certain industries (e.g., mine tailings). Research is needed to assess this potential across different waste streams and to ensure that real benefits in environmental remediation are seen by local communities.

Direct Ocean Capture

Ocean-based carbon removal (also referred to as marine CDR (mCDR) or direct ocean capture (DOC)) is a CDR approach that removes CO₂ from the atmosphere and upper ocean and securely stores the excess carbon either in marine or geological reservoirs. The ocean acts as a large natural reservoir for CO₂, removing a substantial fraction of the excess atmospheric CO₂; however, due to several physical, geochemical, and biological processes that influence the exchange between air and oceans, an excess of CO₂ in the oceans can occur, leading to changes in seawater chemistry- (i.e., ocean acidification) and negative impacts on ecosystems. Ocean-based CDR approaches have the potential to promote atmospheric CO₂ uptake by the ocean while mitigating ocean acidification, and thus contribute to the restoration of ecosystems.

A report released in 2021 by the National Academies of Sciences, Engineering, and Medicine titled “A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration” discusses the benefits, risks, and potential of ocean-based CDR as well as six ecosystem- and technological-based approaches to be considered.iii The approaches span a wide range of biotic (i.e., nutrient fertilization, artificial up- and down-welling, seaweed cultivation, and ecosystem recovery) and abiotic (e.g., electrochemical engineering processes and alkalinity enhancement) methodologies. The CDR Program is focused on the advancement of abiotic approaches. Electrochemical methods consume electricity to drive the separation of acid and base solutions to extract CO₂ from seawater in the form of a gas or as mineral carbonates. Ocean alkalinity enhancement is the chemical alteration of seawater chemistry via addition of alkalinity through various mechanisms (e.g., adding large amounts of pulverized silicate or carbonate rock) thereby “locking” CO₂ into other forms of dissolved inorganic carbon (DIC), which is expected to promote atmospheric CO₂ influx into the ocean.

Sorbents

Sorbent-based CO₂ capture involves the chemical or physical adsorption of CO₂ using a solid sorbent. Like solvents, solid sorbents are usually regenerated by increasing temperature or reducing pressure to release the captured CO₂. In addition, sorbent-based capture technologies exhibit greater modularity than solvent-based systems, providing an advantage for large-scale operation. Optimizing the long-term stability and heat transfer properties of sorbents are some of the major challenges, which can be addressed through sorbent washcoating techniques. R&D objectives include novel sorbent support structures, such as fibers, monoliths, and laminates, and low-cost durable sorbents that have high selectivity for CO₂ from dilute sources, high CO₂ adsorption capacity, and resistance to oxidation. System advancements include sorbent process intensification techniques, novel reactor designs, and enhanced process configurations, such as novel stationary or rotating beds for CO₂ adsorption and desorption.

Electrochemical Novel Concepts

Electrochemical-based CO₂ capture uses pH swing cycle that changes conditions between basic and acidic to capture and release CO₂. Electrochemical processes offer potential advantages by having a lower regeneration energy requirement than existing amine-based systems. Additionally, hydrogen is produced as a byproduct of the electrochemical process, which can be used to produce hydrocarbons and valuable chemical products. Electrochemical processes can be easily integrated with renewable power sources to reduce the cost and improve the efficiency of CO₂ capture systems. The challenge with electrochemical processes involves the design of electrochemical cell and gas-liquid contactors for large-scale applications. R&D objectives include the development of improved electrode materials, membrane contactors, and scaling up systems for single-stack and/or multi-stack modular configurations.