The basic idea
Carbon capture and storage, often shortened to CCS, is a group of technologies that aim to stop carbon dioxide from reaching the atmosphere. Instead of letting the gas leave a smokestack, it is separated, compressed and then either stored permanently or put to some use. Related approaches try to remove carbon dioxide that is already in the air.
The idea is not new, and some projects have operated for decades, often connected to oil and gas production or chemical manufacturing. Even so, the technology covers a small share of global emissions, and opinions differ about how large its role should be.
The main steps
Most CCS systems follow three stages.
Capture. Carbon dioxide has to be separated from other gases. There are several broad approaches. Post-combustion capture removes the gas from exhaust after fuel has been burned, usually with a chemical solvent. Pre-combustion capture converts fuel into a gas mixture first and separates the carbon dioxide before the fuel is burned. Oxy-fuel combustion burns fuel in nearly pure oxygen so the exhaust is mostly carbon dioxide and water vapor, which are easier to separate. Capture works best where the exhaust is rich in carbon dioxide, such as at some fertilizer, cement, steel or hydrogen plants.
Transport. Captured gas is typically compressed and sent by pipeline, and in some cases by ship or truck. Pipelines require planning and permission, and routes can raise local concerns.
Storage or use. The most discussed storage option is deep underground in porous rock formations capped by layers that are meant to keep the gas in place, such as depleted oil and gas fields or saline formations. Monitoring is used to check that the gas stays where it was put. Some captured carbon dioxide is used instead, for example in making certain chemicals, building materials or fuels, or injected into oil fields to recover more oil. How long the carbon stays out of the atmosphere differs a lot between these uses.
Where it is proposed to be used
Supporters usually point to a few cases.
- Heavy industry. Cement, for example, releases carbon dioxide from the chemical reaction itself, not just from fuel. Few alternatives remove that portion of emissions today.
- Power generation. Fitting capture to fossil-fuel or biomass power plants could cut their emissions, though it adds equipment and uses energy.
- Hydrogen production. Making hydrogen from natural gas produces carbon dioxide, and capturing it is one route to lower-emission hydrogen fuel.
- Removal from the air. Methods such as direct air capture aim to take carbon dioxide out of the atmosphere, which some scenarios for net zero targets assume will be needed to offset hard-to-avoid emissions.
Uncertainties and criticisms
CCS is debated, and a fair summary includes the concerns as well as the potential.
Cost and energy use. Capture equipment is expensive to build, and separating and compressing the gas takes energy. That energy has to come from somewhere, which can reduce the net benefit. Cost estimates vary widely by industry and site.
Capture rates. A plant may not capture all of its carbon dioxide. Reported rates differ between projects, and what is counted, including emissions from producing and moving the fuel, affects how large the real reduction is. A life cycle assessment is one way to examine this, although its results depend on assumptions.
Storage confidence. Geologists generally consider suitable formations capable of holding carbon dioxide for very long periods, but suitable sites are not available everywhere, and long-term monitoring and responsibility for any leaks are open policy questions.
Fossil-fuel lock-in. Critics worry that expecting future capture could slow the shift away from fossil fuels, or prolong the life of high-emitting plants. Supporters respond that some sectors have few other options. Both arguments are part of the wider energy transition debate.
Use versus storage. If captured carbon dioxide is used to produce more oil, the overall climate effect depends on how that is accounted for.
Delivery record. Many announced projects have been delayed, scaled back or cancelled, while a smaller number operate at scale. This is one reason forecasts differ so much.
Policy and economics
Because capture adds cost without producing a product people will pay extra for, projects often depend on policy support. This may be tax credits, grants, regulation, or carbon pricing that makes emitting more expensive than capturing. Rules on liability, permitting and monitoring also shape what is feasible. These details differ by country and change over time, so it is worth checking current information for any specific location.
A way to think about it
CCS is better seen as one possible tool among many, not a single solution. How useful it turns out to be will depend on cost, on how reliably it works at scale, on where storage is available and on how it is accounted for in emissions reporting. Anyone reading a claim about a project can usefully ask four questions. How much carbon dioxide is captured? Where does it go? What energy does the process use? And who checks the result over the long term?