We’re pumping carbon dioxide (CO₂) and other greenhouse gases into the atmosphere at an alarming rate. What if we could prevent carbon emissions or suck carbon dioxide out of the air? Carbon capture and storage accomplishes the first and its newer cousin direct air capture accomplishes the second.

The idea of carbon capture was first proposed in 1938. The first large-scale injection of carbon dioxide into the ground occurred in 1972 at the Sharon Ridge oilfield in Texas. A Norwegian project called Sleipner, started in 1996, represents the first commercial carbon capture and storage operation.

At least 18 projects are in operation today with five more under construction and 20 in some stage of development. So far, however, they have combined to sequester only 1% of the carbon dioxide necessary to meet the 2040 goals of the Paris Agreement.

Most of these projects aim to boost production of oil and gas fields. Many environmentalists, therefore, consider carbon capture only a way to greenwash a dirty industry. Others, including me, consider that we won’t stop burning fossil fuels as quickly as we would like. Carbon capture and storage should at least mitigate the damage.

But by now, more industries besides fossil fuels have turned to it. This article will examine carbon capture and storage first and then direct air capture.

Carbon capture and storage

At first, oil and gas companies used carbon capture to force additional fuels out of the ground after they no longer produced using conventional drilling. Only recently has anyone stored the carbon to help the environment.

Carbon capture traps carbon at its emission source, such as a fossil fuel power plant. It happens by three basic methods:

Post-combustion carbon capture takes place in the smokestack after the fuel is burned. Older plants can retrofit with this technology, and it keeps up to 90% of the emissions out of the atmosphere. It takes a lot of energy to compress it enough to transport it, however.

Pre-combustion carbon capture heats fossil fuels in pure oxygen before they’re burned. The process runs the resulting gases through a catalytic converter and feeds the output (a mixture of carbon dioxide and hydrogen) into the bottom of a flask.

As the gas rises, it meets an amine that has been poured into the top. The amine binds the carbon dioxide and falls to the  bottom of the flask. The hydrogen continues to rise and exits the flask. Then the carbon/amine mixture is heated, causing the carbon dioxide to rise and exit while the amine sinks to the bottom. The process basically separates and captures two pure gases and recycles the amine.

Pre-combustion capture costs less and captures a higher concentration of carbon dioxide. Again, it captures up to 90% of the plant’s emissions. It is impossible to retrofit old plants with this technology, however.

Oxy-fuel combustion carbon capture burns the fuel in pure oxygen. The resulting emissions comprise steam and carbon dioxide. Cooling and compressing this stream separates the two components. It can keep 90% of emissions out of the atmosphere.

Transportation, storage, and a warning

The captured carbon dioxide moves mostly through pipelines. It travels mostly as either a compressed gas or in liquid form. If the pipeline ends before it reaches the storage area, the carbon dioxide can finish the trip in a tanker truck or ship.

Generally speaking, the pipelines are safer than trucks or ships. And carbon dioxide pipelines appear to present less environmental risk than oil or natural gas pipelines. But we’ve never yet built a pipeline than never leaks and never causes harm when it does.

Spent oil and gas wells make a great place to store the captured carbon dioxide. The fuels themselves occupied porous layers of rock far beneath the surface. The captured gas fills the same spaces.

Carbon dioxide can also be forced into volcanic rock, specifically basalt. There, the gas eventually solidifies and becomes limestone.

Scientists  have estimated that the planet can store about 10 trillion tons of carbon dioxide. Lest that seem like nearly infinite storage, it amounts to only 100 years of human-created emissions. It makes little sense to burn fossil fuels and then develop expensive and energy intensive technology to deal with the environmental consequences.

Carbon capture and storage works. It’s just not a magic bullet for solving our greenhouse gas problems.

Storing the captured carbon dioxide treats it as a waste disposal problem We need to outgrow the concept of waste and consider everything a potential resource.

What can we do with all that carbon dioxide?

In principle, carbon dioxide can be made into any fuel or chemical that now comes from petroleum. The technology to do so cost effectively does not yet exist, but plenty of research teams are working to develop it.

Potentially, captured carbon dioxide could become a trillion-dollar business. The soft drink industry can use it for carbonating beverages. The gas can also serve as a raw material for fertilizer, concrete, nanomaterials, protein meal for fish farming or even aviation fuel.

Using it as fuel, of course, returns the carbon to the atmosphere, but it uses no petroleum. Instead, it essentially recycles carbon dioxide and makes a circular economy. It might help the industry scale up to where it makes a real impact.

A project called SkyMine can even use carbon dioxide to make such useful chemicals hydrochloric acid, sodium hypochlorite, and sodium bicarbonate. That’s right: baking soda.

Direct air capture

New carbon capture technologies don’t take carbon from combustion. A particularly promising one, direct air capture, sucks carbon dioxide out of the air.

It may reduce the airline industry’s carbon footprint. Major airlines, including United and Delta, have invested in direct air capture companies as part of their sustainability strategies.

Frequent flyers can also donate to carbon capture products in exchange for perks. A collective called Tomorrow’s Airexists to facilitate the effort. Everyone has noticed the reduction in carbon emissions caused by the pandemic. Frequent flyers who care about the environment have begun to join organizations such as Tomorrow’s Air. A travel company called Explorer’s Passage offsets its emissions 1.25 times by investing in direct air capture.

A Swiss company called Climeworks has developed modular machinery with a fan that draws air into a collector. A filter catches the carbon. When the collector is full, it is heated to the boiling point of water, which releases pure carbon dioxide. The carbon dioxide, when combined with water, is piped underground. There it reacts with natural basalt formations and, in a couple of years turns to stone.

This process requires a lot of energy. Only with renewable energy can it be economically viable. Climeworks’ facility in Iceland operates from a geothermal power plant. Its facility in Switzerland relies on a waste-to-energy incinerator. Other direct air capture facilities, by Climeworks or other companies, use other renewable energy.

Growing demand from corporations and consumers, as well as government support, should drive the cost down.

Some advantages of direct air capture

Direct air capture can take carbon out of the air more efficiently than even forests. Each collector captures the same amount of carbon dioxide as 2,000 trees. It can take 50 years for a new tree to grow enough to make much difference. Wildfires, of course, release tremendous quantities of carbon back into the atmosphere.

Reforestation has many advantages, of course, but direct air capture can store more carbon, and in a place where it can’t easily get back into the air. It can also prevent the oceans from absorbing excess carbon dioxide, which makes it more acid.

Direct air capture collectors don’t need arable land to operate. And since the concentration of carbon dioxide is the same everywhere in the world, any location is suitable for this technology.

Of course carbon dioxide from direct air capture has all the same uses as from conventional carbon capture and storage. For example, the baking soda we all know and love ultimately comes from the ground. Why not pull it out of thin air instead?

Sources:

Can carbon capture make flying more sustainable? / Chloe Berge, National Geographic. February 5, 2021

How carbon capture works / Debra Ronca, How Stuff Works. July 9, 2008

You asked: does carbon capture technology actually work? / Anuradha Varanasi, Earth Institute of Columbia University.