What is carbon capture?

Picture a kitchen with a charcoal stove and a hood above it. While the smoke is thick and rising in one place, the hood catches it easily.

Now let that smoke spread through the whole house. To clear it you would have to pull every room of air through a filter. Same smoke, same kitchen, and a job hundreds of times harder.

That gap is the most useful thing to understand about carbon capture. Catching carbon dioxide at a chimney is one problem. Catching it out of open air is a different problem wearing the same name.

Two very different jobs

Carbon capture is a family of methods with one aim. Keep carbon dioxide out of the air, or take back what is already there.

The first kind is point source capture. A cement works, a steel mill or a fertiliser plant sends its exhaust through special equipment. That equipment strips out the carbon dioxide before the gas leaves the chimney. In that exhaust the gas can be hundreds of times more concentrated than in open air.

The second kind is direct air capture. Machines pull in ordinary outdoor air and take the carbon dioxide out of it. Ordinary air is only about 0.04 percent carbon dioxide. That is roughly four molecules in every ten thousand.

Both go by the name carbon capture. They differ in cost, energy and purpose. Point source capture stops new emissions. Direct air capture takes back old ones. Only the second is a true removal.

How chimney capture works, step by step

1. Exhaust gas leaves the furnace or kiln, then is cooled and cleaned of dust.
2. The gas rises up a tower while a liquid trickles down through it. The liquid grabs carbon dioxide and lets the rest pass.
3. The loaded liquid moves to a second tower and is heated. The heat makes it let go of the carbon dioxide, now almost pure.
4. The liquid, back to its old state, returns to the first tower to work again.
5. The carbon dioxide is dried and squeezed until it behaves almost like a liquid, then sent to a pipeline or a tanker.
6. It is pumped underground, or used in a product.

Step three is the costly one. Heating the liquid takes a great deal of steam, and that steam has to come from somewhere. Therefore a plant with capture burns more fuel to make the same cement or the same electricity. Engineers call that the energy penalty. It is the reason capture is never free.

Designs usually aim to catch around nine tenths of the carbon dioxide in the gas stream. Real plants have often caught less, especially in their early years. Ask for measured figures over a full year rather than the design number.

Direct air capture, and why it costs so much more

The machinery works on the same idea. Air is drawn through a solid or a liquid that holds carbon dioxide. Heat or a vacuum then makes it let go.

The physics is unkind. To gather one tonne of carbon dioxide from open air, you must move a huge volume of air. The material that catches it also has to be very fussy about what it grabs. Both of those cost energy. Per tonne, direct air capture needs far more energy than chimney capture.

That leads to a hard rule. If the energy comes from fossil fuel, a machine can release more carbon dioxide than it collects. Direct air capture only makes sense on clean power, and clean power is usually more useful somewhere else first. Treat it as a tool for the last stubborn share of emissions.

Where the carbon dioxide goes

Carbon dioxide is pumped into porous rock, usually deeper than about 800 metres. At that depth the pressure squeezes it until it is dense, so it takes far less room. It sits in the tiny spaces between grains, like water in a sponge. Above it must sit a layer of tight rock that fluids cannot pass. That cap holds everything in place.

Two other homes exist. Some projects inject the gas into basalt, where it reacts with the rock over a few years and becomes a solid mineral. Others use emptied oil and gas fields. They held oil and gas for millions of years, which is good evidence that the seal works.

Sites are watched with wells and surveys. The awkward question is who monitors a site in a hundred years, and who pays if it leaks. That is a legal question more than a technical one.

What it is genuinely good at

Some emissions come from the chemistry itself, and no change of fuel will remove them.

Cement is the clearest case. Making cement means heating limestone until it breaks down, and that reaction gives off carbon dioxide on its own. You could run the kiln on clean electricity and still have that stream. Steel, lime and several chemical plants share the problem. Natural gas processing is easier again. The gas often arrives with carbon dioxide mixed in, and it must be separated anyway.

Those are the honest cases. A concentrated stream, a process that cannot simply be switched to electricity, and a plant that will run for decades.

What it is bad at

Three criticisms are fair.

First, it is expensive for each tonne avoided. Cleaner power, less waste and better efficiency usually cost less. Capture should not be first in the queue.

Second, it deals with carbon dioxide and nothing else. The other pollution from burning, and the mining or drilling upstream, are untouched.

Third, and most argued over, is delay. A promise of capture in ten years can be used to justify building a plant today. Plenty of projects have been announced with capture and then run for years without it.

Scale is worth holding in mind as well. The world captures far less than one percent of what it emits each year. Capture is a specialised tool for hard cases. Cleaner electricity, less waste and a better run grid still cut more carbon dioxide per unit of money spent.

Capture, use, and the labels that confuse people

Three sets of letters come up constantly, and they are easy to mix up.

• CCS means carbon capture and storage. The gas is caught and put underground to stay.
• CCU means carbon capture and use. The gas is caught and turned into a product, such as fizzy drinks, greenhouse feed, fuels or building materials.
• EOR means enhanced oil recovery. Carbon dioxide is pumped into an old oil field to push out more oil. A large share of all captured carbon dioxide has gone this way.

Use is where the labels mislead most. A fizzy drink gives up its carbon dioxide in days. A fuel made from it releases the carbon when it burns. Only a few uses, such as curing concrete, hold the carbon for good. So ask how long the carbon stays put, and count anything short of a century as a use rather than a store.

The same care applies to hydrogen. Hydrogen made from natural gas is sold as low carbon only because of the capture behind it. That is why it is fair to compare it against green hydrogen, which needs no capture at all.

What to check

When a capture project is announced, read for three numbers and one word.

How many tonnes a year, measured rather than designed? What share of that plant’s total emissions does the figure cover? Where does the carbon dioxide end up, and for how long? The word to look for is stored. If the project says used, ask what it is used for, and how long the carbon stays out of the air.

Just Out Tech explains new research in plain language. This article was drafted with AI assistance and checked by a human against the original source.