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 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 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
What is one benefit of direct air capture?
- Solvent-icon-USDOE: U.S. Department of Energy
- Sorbents-icon-USDOE: U.S. Department of Energy
- Membrane-icon-USDOE: U.S. Department of Energy