Glossary

45Q tax credits: US government incentive for carbon storage. The CO₂ must be from an anthropogenic source. In February 2018, the value of the tax credit was raised to $35/ton CO₂ injected for EOR, $50/ton CO₂ injected for geologic storage.
Absorption: Physical or chemical separation process in which molecules enter into the bulk phase of a solvent.
Acid gases: A gas that forms an acidic solution when mixed with water. Carbon dioxide is an acid gas.
Adsorption: Physical or chemical separation process in which molecules adhere to a surface.
Anthropogenic: Resulting from human activity.
Bioenergy with CCS (BECCS): Applying carbon capture to bioenergy processes, such as a biomass-fired power plant or a biomass to liquid fuels process. Interest in BECCS has risen due to the potential for negative emissions.
Caprock: In carbon storage, the caprock is an impermeable rock layer that overlies the permeable rock layer where the CO₂ is stored.
Carbon cycle: Refers to the cycling of carbon dioxide between the atmosphere, terrestrial biosphere (soils and vegetation), and the ocean.
Carbon dioxide capture and storage: From the IPCC Special Report on CCS: “A process consisting of the separation of CO₂ from industrial and energy-related sources, transport to a storage location and long-term isolation from the atmosphere.”
Carbon footprint: The amount of CO₂ emissions associated with an activity or product.
Climate change: The change in the climate due to anthropogenic greenhouse gas emissions. One major change is the increase in global mean temperature, so sometimes referred to as global warming.
Coal gasification: The process that converts coal to a gas, termed a syngas. For power production, the syngas is fed to a turbine. Alternatively, the syngas can be used as a feedstock for chemicals or fuels.
Combined cycle: Power production using both a Brayton cycle (gas turbine) and a Rankine cycle (steam turbine).
Criteria pollutants: In the United States, air pollutants with national air quality standards. The six criteria pollutants are carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide.
Cryogenic processes: Processes occurring at very low temperatures.
Direct air capture: The removal of CO₂ from the air by engineered systems.
Energy penalty: The percentage reduction in output from a power plant due to the addition of carbon capture.
Enhanced oil recovery (EOR): Sometimes called tertiary recovery, these are techniques for extracting more crude oil from an oil reservoir. CO₂ EOR is one such technique, where CO₂ is injected in an oil reservoir in order to mobilize the crude oil and allow it to flow to a production well.
Flue gas: Exhaust gases from combustion, usually vented through a flue (i.e., chimney or smoke stake).
Geo-engineering: The deliberate large-scale manipulation of Earth’s systems to counteract the impacts of climate change.
Greenhouse effect: Greenhouse gases in the atmosphere trap infrared radiation coming from a planet’s surface, warming the planet to a temperature above what it would be without the greenhouse effect. CO₂ is one such greenhouse gas.
Heat Rate: The amount of heat energy in a fuel required to produce a kWh of electricity, given in MJ/kWh_e. The conversion efficiency is defined as 3.62 divided by the heat rate.
Induced seismicity: Human-made triggering of seismic energy (i.e., earthquakes or tremors).
Negative emissions: Removing CO₂ from the atmosphere.
Metal-organic frameworks: A class of adsorbents that combine metals and organic compounds.
Oxy-combustion capture: Carbon capture technique that eliminates nitrogen in flue gases by combusting in high purity oxygen instead of air, greatly simplifying the capture process.
Permeability: Measure of the ability of fluids to flow through a geological formation.
Porosity: Measure of the void space within a geological formation.
Post-combustion capture: Carbon capture from flue gases.
Pre-combustion capture: Carbon capture before a gas mixture is combusted, usually from a syngas.
Seismic: Geological surveying methods involving artificially produced vibrations.
Specific gravity: Ratio of the density of a substance to the density of water, which is 1000 kg/m³.
Sublimation: Phase transition from a solid directly to a vapor. Since CO₂ does not exist as a liquid at atmospheric pressure, it goes directly from vapor to solid as it cools at the sublimation point of −78.5°C.
Supercritical pulverized coal: A type of power plant to convert coal to electricity. Coal is pulverized to small particles to be fed to a boiler that produces steam to drive a steam turbine to produce electricity. Supercritical refers to the relatively high steam temperature (~565°C) and pressure (~243 bar) that yields high conversion efficiencies.
Syngas (synthesis gas): A gas consisting primarily of carbon monoxide and hydrogen, made from gasification of coal or biomass.
Tonne: A metric ton or 1000 kilograms. It is ~10 percent larger than a short ton, which weighs 2000 pounds.
Underground Injection Control Program: In the United States, program established by the Safe Drinking Water Act to regulate all US injection wells to protect drinking water. There are six classes of injection wells, with CO₂ EOR wells falling under Class II and CO₂ storage wells falling under Class VI.