Direct Air Capture: Assessing Impacts to Enable Responsible Scaling

Glossary of Terms

Capture media: Capture media are chemicals that can selectively bind to carbon dioxide gas in ambient air. Capture media can be liquid solvents or solid sorbents.

Carbon capture and use/storage (CCUS): CCUS captures emissions at the source to prevent them from entering the atmosphere (e.g., by capturing CO2from a cement plant’s smokestack). Carbon capture is a form of mitigation.

Carbon removal: Approaches that pull carbon dioxide that has already been emitted out of the atmosphere. Natural carbon removal includes tree planting and agricultural approaches that add more carbon to the soil, and technological carbon removal includes engineered approaches like direct air capture that captures CO2 via chemical reactions.

Community Benefit Agreement (CBA): A contractual agreement between a community and project developer that legally ensures community benefits in exchange for community support of a project.

Contactor: The contactor is the unit in a DAC plant that brings ambient air in contact with a solvent or sorbent.

Criteria air pollutants: The EPA sets National Ambient Air Quality Standards (NAAQS) for six common air pollutants in the United States, required by the Clean Air Act: ground-level ozone, particulate matter, carbon monoxide, lead, sulfur dioxide, and nitrogen dioxide.

Direct air capture (DAC): DAC pulls ambient air over a capture medium (e.g., solvent or sorbent) that chemically reacts with CO2, removing it from the air. The captured carbon dioxide can then be stored underground or used in products.

Distributive impacts considerations: Considerations related to the distribution of negative and positive impacts that different DAC stakeholders experience, particularly in communities where DAC plants are sited.

Environmental justice: Environmental justice is the fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income, with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies. This goal will be achieved when everyone enjoys,“[t]he same degree of protection from environmental and health hazards, and equal access to the decision-making process to have a healthy environment in which to live, learn, and work” (U.S. EPA 2021a).

Geologic storage: End-use option for captured CO2 that involves injection into deep rock formations that trap and sequester it permanently. Carbon dioxide may also be injected into certain types of rock, where it reacts to form carbonates and is permanently stored in solid form.

Gt: Gigaton, or one billion tonnes. One Gt of CO2 equivalent is roughly equivalent to emissions from the annual energy used in 120 million homes (U.S. EPA 2015).

High-/low-quality heat: The relative ease with which energy converts into mechanical energy; low-quality heat does not as easily convert to work because it is less concentrated and organized. Many sources of industrial waste heat are low quality and can serve the energy requirement for sorbent plants, whereas solvent plants require high-quality heat. Low- and high-quality heat are typically associated with low and high temperatures, respectively, but differ from temperature.

Mitigation: Mitigation activities avoid or reduce emissions going into the atmosphere (e.g., by replacing a coal power plant with wind turbines). Mitigation is distinct from carbon removal, which removes previously emitted CO2.

Mt: Megaton, or one million tonnes; 1 Mt of CO2 equivalent is roughly equivalent to emissions from the annual energy used in 120,000 homes (U.S. EPA 2015).

Oxygen-fired kiln (oxyfuel combustion): Natural gas can be combusted in a high-oxygen environment rather than air, which makes postcombustion capture of CO2 easier since flue gas is predominantly water and CO2.

Procedural considerations: These considerations enable stakeholders to participate and be involved in decision-making in a fair way around environmental matters (Clayton 2000), such as DAC plants. Such considerations will be particularly necessary in communities where DAC plants are sited.

Tonne: Metric ton.

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