Capturing CO₂

Capturing CO₂ is most cost-effective at point sources, such as large carbon-based energy facilities, industries with major CO₂ emissions, natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Extracting CO₂ from air is possible although the lower concentration of CO₂ in air compared to combustion sources complicates the engineering and increases the cost of capture.¹Carbon capture and storage. (2021, June 23). In Wikipedia. https://en.wikipedia.org/wiki/Carbon_capture_and_storage

Carbon capture, utilization, and storage projects have as a starting point to capture CO₂ from these point sources, or directly from the atmosphere, with minimum cost and energy penalty. What is an energy penalty? The energy penalty can be looked at in two slightly different ways: the reduction in the net electrical energy from a power plant caused by the addition of capture technology, or the increase in the required fuel for a power plant to maintain its power output after addition of capture technology.

Capturing Technology Overview

Post-Combustion Capture

Post-combustion capture refers to capturing CO₂ from a flue gas generated after combusting a carbon-based fuel, such as coal or natural gas. In conventional fossil fuel power plants, coal or natural gas is burned with air to generate heat energy which is converted to electricity. Of the 4 trillion kilowatt hours of electricity generated in the U.S. in 2019, about 23% was from coal and 38% was from natural gas. With over 60% of the electricity in the U.S. produced from fossil fuel power plants, deployment of post-combustion capture technologies is vital to reduce CO₂ emissions.²Department of Energy. (n.d.). Post-combustion CO₂ capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture/post-combustion

Systems for Post-combustion Capture

Solvent-based CO₂ capture involves chemical or physical absorption of CO₂ from flue gas into a liquid carrier. The absorption liquid is regenerated by increasing its temperature or reducing its pressure to break the absorbent-CO₂ bond.³Department of Energy. (n.d.). Post-combustion CO₂ capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture/post-combustion

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 CO₂.⁴Department of Energy. (n.d.). Post-combustion CO₂ capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture/post-combustion

Membrane-based CO₂ capture uses permeable or semi-permeable materials that allow for the selective transport and separation of CO₂ from flue gas.⁵Department of Energy. (n.d.). Post-combustion CO₂ capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture/post-combustion

Pre-Combustion Capture

Pre-combustion capture separates CO₂ from gasification and reforming processes in which a gaseous fuel, or “synthesis gas (syngas)”, is formed, consisting mainly of hydrogen (H₂), carbon monoxide, and CO₂. In an integrated gasification combined cycle power plant, a carbon-based fuel (i.e., coal) is reacted with steam and oxygen under pressure to form syngas, which is used to fuel a gas turbine generator to produce electricity. The recovered heat is used to produce steam that also drives a turbine generator designed to generate electricity. The carbon is captured from the syngas before it is combusted in the gas turbine.⁶Department of Energy. (n.d.). Pre-combustion CO₂ capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture/pre-combustion

Systems for Pre-combustion Capture

Systems for pre-combustion also involve solvents, sorbents, and membranes. In pre-combustion capture, membranes are used to separate CO₂ and H₂ in syngas and produce a concentrated CO₂ stream. ⁷Department of Energy. (n.d.). Pre-combustion CO₂ capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture/pre-combustion

Capture from Industrial Sources

CO₂ capture from industrial facilities, such as petroleum refineries, iron and steel processing plants, and ethanol plants—in which CO₂ emissions may be present at a higher concentration than coal-fired power plants—is a vital element in reducing CO₂ emissions.⁸Department of Energy. (n.d.). Carbon capture. U.S. Department of Energy. Retrieved June 24, 2021, from https://netl.doe.gov/coal/carbon-capture

Negative Emissions Technologies

Direct Air Capture

Negative emissions technologies aim to remove CO₂ from the atmosphere, with the resultant carbon stored or utilized. One concept now commercially available is to apply direct air capture (DAC), which pulls in atmospheric air, then through a series of chemical reactions, extracts the CO₂ from it while returning the rest of the air to the environment.

Plants and trees do this naturally every day as they photosynthesize, but DAC technology does it much faster, with a smaller land footprint, and delivers the carbon dioxide in a pure, compressed form that can then be stored underground or reused.⁹Carbon Engineering. (n.d.). Our Technology. Retrieved June 24, 2021, from https://carbonengineering.com/our-technology/

Through DAC, a facility directly captures CO₂ from the ambient air (.04 volume% CO₂) and generates a concentrated stream of CO₂. During the process, ambient air flows over a sorbent that selectively removes the CO₂. Then the CO₂ is released as a concentrated stream for storage or reuse, while the sorbent is regenerated and the CO₂-depleted air is returned to the atmosphere.¹⁰American Physical Society. (2011, June 1). Direct air capture of CO₂ with chemicals. Retrieved November 3, 2020, from https://www.aps.org/policy/reports/assessments/upload/dac2011.pdf

DAC technology is scalable and geographically flexible, offering broad potential as a negative-emissions technology. Currently, costs per ton of CO₂ captured are a major barrier facing DAC implementation (~$100/t CO₂ captured)¹¹https://carbonengineering.com/our-technology/. Nevertheless, research and commercial projects are moving forward. In 2020 the U.S. Department of Energy awarded a total of $21 million to projects for DAC technologies. And a number of companies are partnering on DAC projects.¹²USDOE. (2020, September 1). Department of Energy invests $72 million in carbon capture technologies. Retrieved 11/3/2020 from https://www.energy.gov/articles/department-energy-invests-72-million-carbon-capture-technologies