CCS stands for carbon capture and storage, a set of technologies that separate CO2 from industrial emissions before they reach the atmosphere, then lock that CO2 away deep underground. The basic idea is straightforward: instead of letting power plants, cement factories, and refineries vent carbon dioxide into the air, you intercept it, compress it, move it to a suitable location, and inject it into rock formations where it can stay for thousands of years. Modern CCS-equipped facilities are designed to capture around 90% of the CO2 from their exhaust streams.
The Three Stages: Capture, Transport, Storage
Every CCS project follows the same three-step sequence. First, CO2 is separated from the other gases produced during combustion or industrial processing. Second, that concentrated CO2 is compressed and moved, usually by pipeline, to a storage site. Third, it’s injected deep into underground rock formations where a layer of impermeable caprock prevents it from migrating back to the surface.
Each stage involves its own engineering challenges, and the overall cost and effectiveness of a project depend on how well all three work together.
How CO2 Gets Captured
There are three main approaches to the capture step, and which one a facility uses depends largely on what kind of industrial process it’s attached to.
Post-combustion capture is the most intuitive. A power plant or factory burns fuel normally, and the CO2 is pulled out of the exhaust afterward. This can be done through chemical or physical absorption (where a solvent grabs the CO2 molecules), membrane separation (where gases pass through a filter that selectively blocks CO2), or cryogenic distillation (where the exhaust is cooled until CO2 condenses out). Post-combustion systems can be retrofitted onto existing facilities, which makes them especially relevant for plants that are already operating.
Pre-combustion capture takes a different approach. Instead of cleaning up exhaust after the fact, the fuel is partially converted into a mixture of hydrogen and CO2 before it’s burned. Because this process produces a gas stream with a higher concentration of CO2, less energy is needed to separate it out compared to post-combustion methods. Pre-combustion capture is commonly paired with gasification plants that convert coal or natural gas into hydrogen-rich fuel.
Oxy-fuel combustion simplifies the separation problem entirely. Rather than burning fuel in regular air (which is about 78% nitrogen), the fuel is burned in nearly pure oxygen. The result is an exhaust stream made up almost entirely of CO2 and water vapor. Removing the water is easy, so you’re left with a concentrated CO2 stream without needing complex chemical separation.
A newer variation called chemical looping combustion avoids mixing fuel and air altogether. Instead, metal oxide particles carry oxygen from one reactor to another, keeping the fuel and air streams completely separate. This inherently produces a concentrated CO2 stream without the energy penalty of traditional separation.
How CO2 Gets Moved
Once captured, CO2 needs to travel from the industrial site to a storage location, which can be tens or hundreds of miles away. Pipelines are the primary method, though CO2 can also be moved by ship, rail, or truck for smaller volumes or longer distances.
For pipeline transport, CO2 is compressed into a supercritical state, a condition where it behaves like something between a liquid and a gas. In this form, it flows efficiently through pipelines, though the pressures involved can exceed what traditionally designed pipelines handle. U.S. federal regulations currently only cover pipelines carrying CO2 in a supercritical state at concentrations above 90%. The infrastructure requirements are significant: CO2 pipelines need to be specifically engineered for higher pressures and must account for the fact that CO2 behaves differently from natural gas or oil if a leak occurs.
Where It Gets Stored
The final destination for captured CO2 is a geological formation deep underground. The most promising storage sites are deep saline aquifers, which are porous rock layers saturated with saltwater that sits far below freshwater sources. These formations have the largest estimated storage capacity worldwide. CO2 can also be injected into depleted oil and gas reservoirs, which have the advantage of being well-studied from decades of extraction.
A suitable storage site needs three things: enough porous space to hold the CO2 (capacity), an impermeable caprock layer above it to prevent upward migration (containment), and rock properties that allow CO2 to be pumped in at a practical rate (injectivity). For offshore storage, practical considerations narrow the options further. Researchers have identified criteria including water depths of no more than 300 meters, locations within 200 miles of a coastline, and sites outside Arctic and Antarctic regions.
Once injected, CO2 is held in place by multiple trapping mechanisms. Initially it’s trapped physically beneath the caprock, similar to how natural gas accumulates underground. Over time, it dissolves into the surrounding saltwater and eventually reacts with minerals in the rock to form solid carbonates, becoming permanently locked in place.
Using Captured CO2 Instead of Burying It
Not all captured CO2 goes straight into permanent storage. CCUS, with the added “U” for utilization, refers to projects that put captured carbon to commercial use. The most established application is enhanced oil recovery (EOR), where CO2 is injected into aging oil fields to push out crude that conventional pumping can’t reach. CO2-EOR is the most mature utilization pathway and could potentially unlock an additional 500 billion barrels of oil globally by 2050.
Beyond oil recovery, captured CO2 can be converted into methanol, synthetic natural gas, aviation fuel, and basic chemicals like ethylene and propylene. Mineralization processes turn CO2 into building materials. Biological approaches use engineered microorganisms or algae to convert CO2 into biofuels, bioplastics, animal feed ingredients, or high-value chemicals like pigments and proteins. Most of these pathways are still scaling up, but they represent a potential revenue stream that could offset the high cost of capture.
What Could Go Wrong
The biggest concern with geological storage is leakage. Modeling studies have assessed storage security over timescales of 10,000 years, and the results point to abandoned wells as the most significant risk factor. Old oil, gas, or water wells that penetrate a storage formation can act as pathways for CO2 to escape if their cement seals have degraded over time. The density of abandoned wells in a given area, the proportion of those wells with compromised seals, and the difficulty of locating undocumented wells are among the most influential variables in leakage risk models.
Natural pathways through fractures or faults in the caprock represent another potential leak route, though these tend to be less significant than well-related risks at well-characterized sites. Effective monitoring programs focus on identifying and tracking abandoned wells during injection, with plans for remediation if problems are detected. Regulators can most effectively improve storage security by ensuring thorough well surveys and ongoing monitoring rather than relying solely on site selection.
CCS Projects Operating Today
CCS is no longer theoretical. The United States leads globally with 22 operating projects and a combined capture capacity of 19.1 million tonnes of CO2 per year. Canada adds another eight projects capturing 9 million tonnes annually. Brazil’s Santos Basin pre-salt project, operated by Petrobras and running since 2013, has a capacity of 10.6 million tonnes, making it one of the largest single installations in the world. Australia contributes two projects capturing 3.3 million tonnes per year.
New projects continue to move forward. The Liverpool Bay CCS project in the UK, part of the HyNet industrial hub, plans to inject around 4.5 million tonnes of CO2 per year into depleted gas fields beneath the Irish Sea. Hub-style projects like this one, where multiple industrial emitters share transport and storage infrastructure, are increasingly seen as a way to bring costs down and make CCS viable for smaller facilities that couldn’t justify dedicated infrastructure on their own.