How Carbon Capture Works

There are three main steps to carbon capture and storage (CCS):

Let's take a more detailed look at the trapping and separation process:

Carbon is taken from a power plant source in three basic ways: post-combustion, precombustion and oxy-fuel combustion [source: National Energy Technology Laboratory].

A fossil fuel power plant generates power by burning fossil fuel (coal, oil or natural gas), which generates heat that turns into steam. That steam turns a turbine connected to an electricity generator. Another word for the burning process is combustion.

With post-combustion carbon capture, the CO2 is grabbed after the fossil fuel is burned. The burning of fossil fuels produces something called flue gases, which include CO2, water vapor, nitrogen and sulfur dioxide.

In a post-combustion process, CO2 is separated and captured from the flue gases that result from the combustion of fossil fuel. This process is the most commonly used technique in carbon-capture technology. It's a convenient strategy because it can be deployed at both new and preexisting coal-fired power plants. However, there are some drawbacks. In order to work, post-combustion carbon capture requires some physically large equipment — and it can make turbines less efficient [source: Elhenawy].

With precombustion carbon capture, carbon is trapped and removed from fossil fuels before the combustion process ends.

Coal, oil or natural gas is heated in steam and oxygen, resulting in a synthesis gas, or syngas. The gas mostly contains CO2, hydrogen (H2), and carbon monoxide (CO). Later, a separate reaction converts water (H2O) into hydrogen. While that's going on, some of the carbon monoxide is transformed into carbon dioxide. The end result is a gas mixture loaded with H2 and CO2 [source: U.S. Department of Energy].

It's easy to isolate, capture and sequester the CO2 from that mix. Meanwhile, engineers can use the hydrogen for other energy production processes.

Precombustion carbon capture is usually more efficient than the postcombustion strategy. However, the equipment comes with a higher price tag. Besides, older power plants tend to be less suited for this technique than some new ones [source: Elhenawy].

With oxy-fuel combustion carbon capture, the power plant burns fossil fuels — but not in ordinary air. Instead, the fuels are burned in a gas mixture containing lots and lots of pure oxygen. This results in a flue gas whose two main components are CO2 and water. Afterward, it's possible to separate out the CO2 by compressing and cooling the water [sources: National Energy Technology Laboratory and National Resources of Canada].

Certain aspects of oxy-fuel combustion carbon capture are inexpensive, but the process has a high cost overall. (Pure oxygen isn't cheap.) Also, there are some concerns about its applicability. A 2020 review published in the journal Catalysts argued that the relevant technology "needs to be proved for large scale operations" [source: Elhenawy].

On the positive side, oxy-fuel combustion capture can be used at both old and new coal-burning power plants [source: Elhenawy].

Now, here's an important question: Once the carbon is captured, how is it transported to a storage location? Keep reading to find out.

After carbon dioxide (CO2) is captured, the next step is transporting it to a storage site. The usual method of transporting CO2 is through a pipeline.

Pipelines have been in use for decades, and large volumes of gases, oil and water flow through pipelines every day. Carbon dioxide pipelines are an existing part of the infrastructure in the U.S. and many other countries. In fact, there are now more than 4,039 miles (6,500 kilometers) of CO2 pipelines distributed across Africa, Australia, the Middle East and North America. Most were created for a process called Enhanced Oil Recovery (EOR), but some are connected to CCS projects [source: Noothout].

You can put a pipeline just about anywhere, including underground or underwater. They can be found running through such diverse environments as deserts, farmlands, mountain ranges and oceans. [source: Intergovernmental Panel on Climate Change].

Pipelines may be connected to processing plants or power plants that rely on fossil fuels, as well as natural sources of CO2. The purity of a line's CO2 supply may be affected by the kinds of technology used at its source [source: Noothout].

In some cases, the CO2 might travel as far as it can in the pipe, then transition to a tanker truck, tanker ship or pressurized cylinders to finish its journey. Note that there's an asphyxiation risk if a massive amount of CO2 escapes into the atmosphere. As with tanks that transport natural gas and other hazardous materials, good construction is key. That, and good driving.

Getting back to pipelines, they can transport CO2 in three states: gaseous, liquid and solid. Solid CO2 is commonly known as dry ice, and it's not cost-effective to transport CO2 as a solid.

Pipelines commonly transport carbon dioxide in its gaseous state. Said gas needs to be compressed before it's moved from Point A to Point B. According to the National Energy Technology Laboratory, the ideal pressure range is between 1500 and 2200 PSI (or 10,342 and 15,168 KPA).

Engineers must be on guard against impurities in the CO2 stream, like hydrogen sulfide and water. The latter has been known to corrode pipelines, but that's just the tip of the iceberg. Under high pressure and low temperatures, the water in these pipes may form natural-gas hydrates, solid crystals that can clog up your lines. Scientists are still devising ways to handle such impurities [sources: Onyebuchi and Bai].

In the world of construction, safety is a top priority. If a pipe ruptures near a populated area, the sudden release of CO2 gas in large quantities could have serious repercussions for both the public health and the environment. To keep industrial digging equipment from accidentally striking the pipes, planners can bury them deep underground. Also, when possible, laying pipelines down far away from cities, towns and the like might be advisable [source: Onyebuchi].

DNV, a prominent risk management and quality assurance company based in Norway, released new safety procedures for CO2 transport pipelines in 2021. Meanwhile, the United Kingdom's Health and Safety Executive now has an extensive list of guidelines covering everything from corrosion to land usage.

Pipeline costs fluctuate depending on the route of the pipeline (through heavily congested areas, mountains, offshore); the quality of the materials; the equipment involved; how much labor is required; and other expenses.

After we collect and transport all that carbon dioxide (CO2), we're going to need somewhere to put it. But where? In some sort of giant storage unit? A huge tank out in the desert? Will we need more landfills to hold our CO2 waste?

Don't worry, the answer to all those questions is "no." There are a few places we've found to store CO2, including several underground. In fact, there's research that suggests the United States alone has enough subsurface space to potentially hold 1.8 trillion tons (1.71 trillion metric tons) of carbon dioxide in deep aquifers, permeable rocks and other such places [source: Cunliff and Nguyen].

Let's talk about the logistics of underground storage. Deep underground, CO2 can be kept at pressures of over 1,057 PSI (72.9 atm) and at temperatures above 88 degrees Fahrenheit (31.1 degrees Celsius).

When those specific conditions are met, CO2 becomes supercritical. In that state, carbon dioxide takes on properties normally associated with both gases and liquids. Supercritical CO2 has a low viscosity, just like a gas. But at the same time, it's also got the high density of a liquid [sources: National Energy Technology Laboratory and Imaging Technology Group].

Because it can seep into the spaces in porous rocks, a great amount of CO2 can be stored in a relatively small area. Oil and gas reservoirs are well suited to store CO2 as they consist of layers of porous rock formations that have trapped oil and gas for years [source: Center for Science Education].

CO2 is artificially injected into underground rock formations below the Earth's surface. These natural reservoirs have overlying rocks that form a seal, keeping the gas contained. There can be risks to underground storage, though, and we'll discuss those a bit later.

Basaltic rock formations also make attractive CO2 storage spots. Volcanic in origin, basalt is one of the most common types of rock in the Earth's crust. Researchers have found that when CO2 reacts with the magnesium and calcium basalt naturally contains, it can be transformed into solid minerals, specifically dolomite, calcite and magnesite [source: Cartier].

Then we have coal deposits. Sometimes, the ones that have been written off as "unmineable" can hold very large quantities of captured CO2. Inside, it's possible to store the gas at lower pressures — and thereby save money [source: Talapatra].

In addition to underground storage, we're also looking at the ocean for permanent CO2 storage. Historically, there's been a lot of discussion about potentially dumping CO2 straight into the ocean — at depths greater than 9,842 feet (3,000 meters). That far below the surface, carbon dioxide is actually denser than water. So hopefully, the dumped CO2 would be trapped in place for some time [source: Center for Science Education].

Ocean carbon storage is largely untested, and there are many concerns about the safety of marine life and the possibility that the carbon dioxide would eventually make its way back into the environment.

Next, we'll look at some of these concerns in more detail and find out if carbon ca­pture and storage is a viable solution for our future.

Although carbon capture and storage may seem like a miracle solution, it's not without concern or controversy.

To begin, it's important to remember that carbon capture and storage (CCS) is not a license to continue emitting CO2 into the atmosphere. Whatever the future holds for CCS, other emission-reduction efforts will still be necessary. However, CCS provides a way to clean up some of our existing power plants.

According to a 2020 report from the Global CCS Institute, there are now "65 commercial CCS facilities in various stages of development globally."

Yet some critics worry about the economics of CCS. Electric cars and solar panels are commodities that can be marketed and sold to individuals and private organizations. But in contrast, finding ways to monetize captured CO2 has proven difficult.

Another drawback? Current CCS technologies actually require a lot of energy to implement and run. Besides, they depend on water — and lots of it — for cooling and processing purposes [sources: Magneschi and Rosa].

Given this need for H2O, there've been debates over how CCS might (or might not) contribute to water scarcity. In 2020, a team led by Lorenzo Rosa at the University of California, Berkeley simulated the effects of retrofitting every large coal-fired power plant in the world with four different kinds of CCS technology.

To quote their paper, which the journal Nature Sustainability published May 4, 2020, "certain geographies lack sufficient water resources to meet the additional water demands of CCS technologies."

And this is just one of the environmental concerns folks have raised about carbon capture and storage.

What happens if the carbon dioxide leaks out underground? It's hard to predict what the distant future holds for CO2 we've already trapped below the Earth's surface. Implementing good regulations — and choosing quality storage sites — might make an enormous difference down the road.

There are a few potential ways for recaptured CO2 to leak to the surface. Ironically, the wells built to inject it underground in the first place could become a possible escape route later on. So could abandoned oil and gas wells — or natural faults [source: Dunne].

One 2018 projection claims leaks are unlikely if "realistically well-regulated storage" is put into effect. This contradicts some earlier research on the matter [sources: Dunne and Alcalde].

Some opponents of CCS believe that, viable or not, the focus is all wrong. They say we should be focusing on ways to wean ourselves off fossil fuels, but CCS prolongs the lives of power plants that rely on them.

On the other side of the divide, CCS supporters believe renewables are only part of the solution. In their view, we'll probably need to combine these with carbon capture tech in order to have any serious hope of thwarting catastrophic climate change.

There are still many questions about the role carbon capture and storage will ultimately play in helping us alleviate the greenhouse effect and fight climate change. But one thing's for certain: Carbon dioxide emissions are a worldwide problem.