Carbon capture and sequestration (CCS) is the process of trapping carbon dioxide at its source, transporting it, and storing it underground so it doesn’t enter the atmosphere. The idea is straightforward: instead of letting CO2 from power plants and factories drift into the sky, you catch it and lock it away in geological formations deep beneath the earth’s surface. About 230 million metric tons of CO2 are already used or captured globally each year, though that figure needs to grow dramatically to make a meaningful dent in climate change.
How CO2 Gets Captured
There are three main approaches to capturing carbon dioxide from industrial sources, and they differ based on when in the process the CO2 is separated.
Post-combustion capture pulls CO2 out of exhaust gases after fuel has already been burned. This is the most common approach because it can be retrofitted onto existing power plants and factories. The flue gas passes through a chemical solvent, typically an amine-based liquid, which absorbs the CO2. The solvent is then heated to release the concentrated CO2 for storage.
Pre-combustion capture converts fuel into a mix of hydrogen and CO2 before combustion happens. The CO2 is separated using a physical solvent, and the remaining hydrogen is burned for energy. This method works well in facilities designed for it from the start but is harder to add to plants already in operation.
Oxy-combustion takes a different route entirely. Instead of burning fuel in regular air (which is mostly nitrogen), it burns fuel in nearly pure oxygen. The result is an exhaust stream that’s mostly CO2 and water vapor. Condensing the water out leaves concentrated CO2 ready for transport. No chemical solvents needed.
The Energy Cost of Capture
Capturing CO2 isn’t free in energy terms. Running the capture equipment requires a significant chunk of the power a plant generates, a penalty that varies by method and fuel type. For coal-fired plants in the near term, the energy penalty ranges from 23 to 30 percent, meaning a plant with capture equipment needs roughly 31 percent more coal per kilowatt-hour than a plant without it. Natural gas plants fare better, with penalties between 10 and 28 percent. Pre-combustion capture on natural gas is the least energy-intensive option, with a penalty around 10 percent compared to about 17 percent for coal.
These numbers are expected to improve. Projections suggest the energy penalty could drop to 4 to 9 percent beyond 2030 as the technology matures. The theoretical minimum, if you could run a perfectly efficient system, is just 5.1 percent for a typical coal plant’s exhaust stream. That gap between the theoretical floor and real-world performance is where most of the engineering effort is focused.
Direct Air Capture: A Different Approach
Point-source capture grabs CO2 where concentrations are high: smokestacks, cement kilns, steel furnaces. Direct air capture (DAC) does something much harder. It pulls CO2 directly from the ambient atmosphere, where concentrations are roughly 0.04 percent, hundreds of times more dilute than industrial exhaust. That dilution makes DAC far more energy-intensive and expensive.
The tradeoff is flexibility. Point-source capture can only address emissions from stationary industrial facilities. DAC can theoretically offset emissions from sources that are difficult or impossible to capture at the point of release, like transportation and wildfires. Both technologies are progressing, but DAC remains limited by prohibitive costs, while point-source capture is closer to commercial viability.
Getting CO2 Where It Needs to Go
Once captured, CO2 must be compressed and transported to a storage site. Pipelines are the primary method for large-scale transport, carrying CO2 in a dense, high-pressure state. Ships are another option, particularly for offshore storage sites, though they currently operate at a much smaller scale and carry CO2 at lower pressures and temperatures than pipelines.
The CO2 arriving at a storage site may need additional compression if its pressure doesn’t match what’s required for injection underground. Continuous monitoring at the wellhead tracks pressure, temperature, and flow rate throughout the injection process. The pipeline infrastructure needed for widespread CCS is substantial, comparable in ambition to existing natural gas pipeline networks.
Where CO2 Gets Stored Underground
Geological sequestration means injecting CO2 deep underground into rock formations that can trap it permanently. Four types of formations are suitable: deep saltwater-filled rock layers (saline aquifers), depleted oil and gas reservoirs, coal seams too deep to mine, and basalt formations. The most promising sites are at depths below 800 meters, where pressure and temperature keep CO2 in a dense, fluid-like state that takes up far less space than it would as a gas.
A good storage site needs a solid cap layer, essentially impermeable rock sitting above the storage formation that prevents CO2 from migrating upward. Sites with slow groundwater movement are preferred because they reduce the chance of CO2 dissolving into and being carried by water. Closed or semi-closed geological structures, where rock layers form natural traps, offer the best long-term containment.
Monitoring for Leaks
Storing CO2 underground only works if it stays there. Monitoring technologies verify that injected CO2 remains contained, and detection systems differ depending on whether storage is onshore or beneath the seafloor.
For sub-seabed storage, monitoring follows a layered approach. Acoustic systems mounted on the seafloor can detect CO2 bubbles or liquid droplets escaping from the seabed. If a leak is suspected, autonomous underwater vehicles equipped with sensors that measure acidity and dissolved CO2 levels map the distribution of leakage points. Ongoing surveillance then uses remotely operated vehicles or automated systems that rise and descend through the water column, collecting data at multiple depths to track how far any leaked CO2 has spread. Onshore sites use similar principles: ground-level sensors, soil gas sampling, and seismic imaging to track the CO2 plume as it moves within the storage formation over time.
Putting Captured CO2 to Use
Not all captured CO2 goes underground. Carbon capture, utilization, and storage (CCUS) includes pathways where CO2 becomes a raw material. The largest single use is fertilizer production, where 130 million metric tons of CO2 go into manufacturing urea each year. The oil and gas industry consumes another 70 to 80 million metric tons for enhanced oil recovery, injecting CO2 into aging wells to push out additional oil.
Beyond these established uses, CO2 shows up in food and beverage production, metalwork, cooling systems, fire suppression, and greenhouses where it accelerates plant growth. More experimental applications are gaining ground. CO2 can serve as a feedstock for synthetic fuels, including methane, methanol, gasoline, and aviation fuel. These CO2-derived fuels are particularly appealing for sectors like aviation, where electrification is extremely difficult. CO2 can also replace fossil fuels as a raw material in producing chemicals and polymers.
One of the less energy-intensive pathways involves reacting CO2 with industrial waste products like iron slag or coal fly ash to create building materials. CO2 can replace water in concrete curing or be used to produce construction aggregates, small particulates used in building materials. These reactions form stable carbonate minerals, locking the carbon into solid form. That said, utilization alone delivers a fraction of the climate benefit that permanent geological storage does. Scenario analyses suggest CO2 use within the energy system provides less than 13 percent of the emissions reductions that storage would deliver.
Financial Incentives in the US
The US federal government offers tax credits under Section 45Q to encourage carbon capture projects. The base credit is $17 per metric ton of CO2 captured and stored in geological formations, and $12 per metric ton for CO2 used in enhanced oil recovery or other utilization. Direct air capture facilities receive $36 per metric ton at the base rate.
These amounts increase fivefold for facilities that meet prevailing wage and registered apprenticeship requirements, bringing the effective credit to $85 per metric ton for geological storage and $180 per metric ton for direct air capture. These credits have been a major driver behind new project announcements, particularly for DAC facilities where the higher credit helps offset the technology’s steep costs.