Rail-based carbon capture plants could help mitigate the worst effects of climate change, says a team of international researchers including University of Toronto chemists Geoffrey Ozin and Alán Aspuru-Guzik.

The researchers are working with U.S.-based start-up, CO2Rail, to design direct air capture (DAC) equipment housed inside special rail cars interspersed among standard freight and passenger cars. They would be outfitted with large, roof-top intakes that allow ambient air in the slipstream to move into a CO2 collection chamber, eliminating the need for energy-intensive fans that stationary DAC operations require.

This is important because huge volumes of air need to pass through the carbon capture system in order for it to be useful. Even though humans have poured far too much CO2 into the atmosphere, the molecules still make up only 0.04 percent of the air — that’s one in 2,500 molecules.

Once captured in sufficient volumes, the CO2 would then be unloaded at crew change or fueling stops into regular CO2 tank cars for permanent underground sequestration or for use as feedstock in fuel or fertilizers.

The team outlined its plan in July in the journal Joule, estimating that the technology could harvest meaningful quantities of CO2 at far lower costs than stationary DAC operations, and has the potential to reach annual productivity of 0.45 gigatons by 2030, 2.9 gigatons by 2050, and 7.8 gigatons by 2075.

The authors estimate the system could scale down costs to less than $50 USD per tonne. “It makes the technology not only commercially feasible but commercially attractive,” says CO2Rail’s chief technology officer Eric Bachman.

The team, including researchers in the U.S., the UK and Switzerland, is experimenting with various CO2 capture systems. “Almost every week, Geoff and I look at a new sorbent system with a new chemistry, which has just come out,” says Bachman, who hopes to have a prototype next year.

All of the sorbent systems work by a so-called swing technique, says Ozin. “Pressure, heat, electrochemistry or capacity changes can be used to control the energy demand, CO2 capture capacity, and cycle times of the adsorption-desorption process that underpins the action of a direct air capture system,” he says. “The big question is which one will function best in the freight cars of a direct air capture train.”

One of the more promising systems uses a class of organic compounds called quinones. When quinone molecules are forced to take on an extra electron, they develop a high chemical affinity for CO2 molecules and snag any that pass by. Take the extra electron away, that affinity disappears, and the CO2 is released.

As the team writes in Joule, “in other words, by applying a voltage bias, CO2 is adsorbed and by reversing the applied voltage bias, the CO2 molecules are desorbed and swept out of the system for harvest. Correspondingly, the quinone is now regenerated and ready to capture more CO2 during the next cycle.”

The DAC system is powered by on-board generated, sustainable energy such as solar panels and regenerative braking. When a train pumps the brakes, it converts the entire train’s forward momentum into electrical energy. Currently, this energy is dissipated in the form of heat and discharged out of the top of the locomotive.

Ozin, who studies how to harness sunlight to turn CO2 into a chemical feedstock for a wide range of products and fuels, says the rail system circumvents the need to use large swathes of land to build renewable sources of energy to power stationary DAC systems.

“It’s a huge problem because most everybody wants to fix the climate crisis, but few are happy to have it done in their proverbial ‘backyard’,” he says. “Rail DAC does not require special zoning, surveys or building permits and would be transient and generally unseen by the public.”

As University College London chemist Richard Catlow points out, effective carbon capture technologies will be crucial in achieving the target of net-zero, as will CO2 utilisation together with green hydrogen to produce synthetic fuels.

“A particular challenge is posed by direct air capture of CO2, which is receiving worldwide attention,” he says. “The paper of Bachman et al. on rail-based DAC is particularly intriguing and exciting and offers the opportunity of real progress in meeting this challenge.”