HORIZONS

Carbon capture technology could turn trains into giant greenhouse-gas vacuums

All aboard the CO2-choo train!

If you lined up all the world’s railroad tracks, they would be long enough to wrap around the equator 32 times.

Our dependence on (and love for) trains started in the Industrial Revolution, right when we began pouring carbon dioxide into the atmosphere. Today, trains are not terrible for the environment. Rail travel is one of the least carbon dioxide-emitting forms of transportation. And engineers have a creative plan to fight climate change with the locomotives themselves. Essentially, they want to turn trains into vacuums that suck carbon dioxide from the air.

What’s new — In a new study, a team of engineers lay out plans for train cars that can filter nearly 25 pounds of carbon dioxide out of the air for every mile they travel. The idea is manifested in CO2­Rail, a U.S.-based start-up founded by one of the study’s authors.

These adapted rail cars may one day take take carbon dioxide-heavy air in one end and spit clean air out the back. Bachman et al. Joule (2022)

The concept rests on a braking method more typically found in EVs like Teslas called regenerative braking and controversial carbon-capture technology — the novel idea that carbon dioxide can be extracted from the environment and stored or reused.

“[This energy] is already being produced. It’s already being wasted. Why not just capture it and use it?”

It would use existing rail infrastructure— passenger trains and freight alike — and retrofit the cars with vacuums and storage units to turn them into mobile carbon sinks.

The use of existing rail infrastructure, as well as regenerative braking energy, are innovative ideas for direct air carbon capture, says Ajay Gambhir. He researches low-carbon technologies at Imperial College London and was not involved in the recent study.

“There are some really nice touches to this approach,” Gambhir says.

The study was published Wednesday, July 20, in the journal Joule.

Taking cues from Tesla

The researchers’ carbon-vacuuming train cars take advantage of the energy generated by the trains every time they brake or slow down. When these ten-thousand-ton vehicles grind to a halt, the energy from their momentum typically converts to heat (that’s why it feels so hot if you stand next to a freshly stopped train). But trains can recapture this energy and store it in a battery — a process known as regenerative braking that is already used in EVs, including Teslas, which are famously fitted with one-pedal systems.

Teslas are among the EVs that use regenerative braking to help conserve energy.MediaNews Group/Boston Herald via Getty Images/MediaNews Group/Getty Images

“[This energy] is already being produced. It’s already being wasted. Why not just capture it and use it?” asks Eric Bachman, co-founder of CO2Rail and an author of the new study.

If the energy were used to power the train, it would help to prevent some CO2 emissions from the train itself. But the authors found the same amount of energy can also be used to filter up to five times as much CO2 from the air as it might save if it powers an onboard carbon capture system instead. Fitting solar panels to the trains will also help to ensure they can always power on, the study suggests.

“This energy is better used to capture CO2 using [direct air capture] rather than using it to save fuel or electricity to power the train,” Matthew Realff and Ryan Lively tell Inverse. Both study carbon capture and are professors at the Georgia Institute of Technology. Neither was involved in the new study.

According to Bachman, CO2Rail will retrofit existing tank cars with this carbon-capture technology.

Carbon capture: The numbers game

To limit the worst outcomes of climate change, most scientists agree that we need to keep global temperatures at or under 1.5 degrees Celsius above pre-industrial levels. Halting all current CO2 emissions is not dramatic enough to accomplish this goal — we also have to remove 10 billion metric tons of the molecule from the atmosphere every year, according to a 2021 report by the Intergovernmental Panel on Climate Change.

This Climeworks plant in Switzerland filters carbon dioxide from the air.Credit: Climeworks

Re-capturing carbon from the air requires a lot of money and energy. In 2021, there were 19 plants worldwide that capture carbon dioxide from the air. Together they suck in about 10,000 metric tons of CO2 annually for $150-325 per metric ton, according to the International Energy Agency. Altogether, that is up to $3,250,000 cost to remove the same amount of emissions as created by 1,260 U.S. homes, according to the EPA. There are about 70 million single-family homes in the U.S., according to some sources — one doesn’t need to do the math to realize there is a problem here.

Compare that to the railway carbon capture plan, which the authors estimate could remove 45,000 times that amount using 13,350 retro-fitted rail cars by 2030, at an estimated cost of $45 per metric ton. That equates to 450,000,000 tons of carbon dioxide — perhaps enough to account for almost all the emissions coming from homes in the U.S. If it works, that’s a massive jump in scale — but for significantly less money than the current method (it is worth noting it would still equate to billions of dollars).

“The size of the challenge — 10 billion tons of CO2 per year — is so large that we’ll need a broad set of methods to reach that goal.”

The railroad cars would operate off-the-grid using the electricity produced by the train, so that would afford more grid-distributed energy to power stationary carbon-capture plants, says Mihri Ozkan, a professor of electrical and computer engineering at the University of California, Riverside. She was not involved in the recent research.

How it works — Stationary carbon capture plants rely on large fans to suck in air. But these mobile train cars would catch air as they speed over the tracks, hitched to locomotives designed to transport people and products. The extra load requires the train to consume more energy — about the same amount that would be required to power an industrial-scale fan in a stationary carbon-capture system, study co-author Bachman says.

When the air flows into the car’s main chamber, it passes through a material designed to filter out the CO2 through adsorption. Adsorption is a process whereby particles stick to a surface — in this case, CO2. Carbon dioxide-free air flows out the back of the car. Once the maximum amount of CO2 has been adsorbed, the chamber closes off. The CO2 is the desorbed — released from the surface trap — and then liquefied under pressure.

For every 2,500 air molecules that flow through the train car, one molecule of carbon-dioxide will be trapped inside. Bachman et al. Joule (2022)

The liquid CO2 can be dropped off at the next depot. That CO2 can then either be stored underground or used to create carbon-neutral fuels, eventually returning it to the atmosphere. It is worth noting that these solutions are somewhat controversial, as the long-term consequences of humans storing trapped carbon underground are not clear.

On the horizon: Battling the hydra

We’ll have to wait for the first carbon-capture train cars to be built to get a better picture of their full potential. If all goes well on the funding side, Bachman hopes to have a prototype by the summer of 2023.

Regardless of whether this plan comes to fruition, Bachman and his team’s innovative design raise the fact that keeping global warming under 1.5 degrees Celsius above pre-industrial levels is a challenge that requires fresh ideas and new ways of thinking about how we live now — including how we move. Railway carbon capture isn’t intended to replace stationary plants, Bachman emphasizes — the world needs many of these dramatic solutions to tackle the problem.

“We’re going to need what [the plants are] doing,” he says.

Other techniques to facilitate carbon removal are also in the works, from more technology-based innovations to more organic strategies like replanting forests, says Colin McCormick. He researches CO2 removal technologies at Georgetown University. He is also the Chief Innovation Officer at Carbon Direct and was not involved in the recent study.

“It’s important to realize that the size of the challenge — 10 billion tons of CO2 per year — is so large that we’ll need a broad set of methods to reach that goal,” he says. Here here.

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Editor’s note: This story has been updated to make clear Teslas are fitted with one-pedal systems, not without a brake. We regret the error.