As far as scientific, technical and engineering terms go, “carbon capture, utilization and storage” is relatively self-explanatory.
CCUS technologies seek to remove CO2 or carbon dioxide emitted from power stations, factories and other industrial facilities or directly from the atmosphere, then either store the greenhouse gas permanently underground, both onshore and off, or use it to create products and services. Strategies that safely and efficiently transport compressed CO2 are also a significant aspect of the field.
There are numerous CO2 capture technologies in use and under development, according to the International Energy Agency:
Chemical absorption is a process in which CO2 reacts with a chemical solvent to form a weakly bonded compound, which can be later separated using heat. This technology has been widely used for decades in both small and large-scale projects. It’s highly selective and results in a relatively pure CO2 stream, which makes it well suited for industrial flue gases.
Physical separation uses either a solid surface (adsorption) or a liquid solvent (absorption) to capture CO2 molecules, which can later be released via an increase in temperature or pressure. This technique is used mainly in natural gas processing and the production of ethanol, methanol and hydrogen.
In oxy-fuel separation, fuel is combusted in the presence of nearly pure oxygen, which means the resulting flue gas is composed almost entirely of CO2 and easily removed water vapor. This technology is in the large prototype phase and has been implemented in coal power plants and cement factories.
Membrane separation uses polymeric or inorganic filters that allow CO2 to pass through while retaining other gases. Use of this technology varies—according to the IEA, it’s in the demonstration stage in natural gas processing, with only one large-scale capture plant. Membranes for treating syngas and biogas are available commercially, while ones for flue gas treatment are under development.
Calcium looping is another technology currently in the development stage. Lime is used as a sorbent to capture CO2 from a gas stream. The resulting calcium carbonate is transported to a second reactor, where it is separated, and the lime reused.
Chemical looping also requires two reactors. In the first, small metal particles bind with oxygen in the air to form a metal oxide. In the second, the oxide reacts with fuel, resulting in the production of energy and a pure stream of CO2. The metal is then reused in the first reactor. This technique has been used in 35 pilot projects that burn coal, gas, oil and biomass.
Direct separation, currently being tested in pilot projects, strips CO2 from cement production by indirectly heating limestone.
Two projects are currently testing the feasibility of supercritical CO2 power cycles, in which CO2 above its critical temperature and pressure is used to drive turbines. Nearly pure oxygen is used to burn fuel, meaning flue gas is made up only of CO2 and water vapor.
In April 2021, the U.S. Department of Energy awarded $99 million on projects to demonstrate new large-scale carbon capture technologies. This illustrates the potential for innovation and the support available to scientists and engineers in this field.
Once captured, CO2 can be transported via pipeline, ship, truck and train, with the latter two more commonly used for shorter distances and smaller quantities. Pipelines are the most cost-effective method, having been used for many years—North America is home to a network of CO2 pipelines totaling over 5,000 miles.
Using Captured CO2
Carbon dioxide has been used in enhanced oil recovery for decades. In one process, CO2 (or another gas) is injected into a reservoir to force oil toward where it can be pumped out. The gas also improves flow by dissolving into the oil and lowering its viscosity. This technique is used in the majority of EOR production in the U.S. As part of the CO2 remains in the reservoir, it is considered long-term storage as well.
CO2 can also be converted for other uses.
When converted into solid mineral carbonates (mineralization), it can be used to make aggregates for use in concrete, asphalt and construction fill. CO2 can also be used instead of water to “cure” concrete during mixing.
Other processes can convert CO2 for use in the creation of synthetic fuels, polymers and new materials, to accelerate the growth of algae and to produce beverages, fire extinguishers and other products.
This facet of CCUS is also ripe for innovation and funding. For fiscal year 2022, the Department of Energy requested $38 million for research into early-stage utilization technologies with the potential to develop markets for CO2-based products.
At Colorado School of Mines, which recently launched an Integrated Carbon Capture, Utilization and Storage Initiative, areas of technological research into CCUS include geologic storage; capturing CO2 via capture membranes and chemical looping; converting CO2 into commodity chemicals and fuels; and using the gas for enhanced oil recovery. Faculty are also investigating carbon pricing, industry policy, regulatory approaches and the use of data analytics to determine the success of projects.
This research informs the CCUS educational programs at Mines, which includes a fully online certificate in carbon capture, utilization and storage.
John Bradford, geophysics professor and vice president for global initiatives at Colorado School of Mines, gives an overview of carbon capture in an Explainer episode of The Conveyor podcast below: