Modified trains could harvest CO2 directly from the air

The Sheffield team has been working with US-based start-up CO2Rail to design direct air capture (DAC) equipment that can be placed within special rail cars on already running trains to absorb carbon from the atmosphere, helping to mitigate the impact of climate change.

The DAC rail cars work by using large intakes of air that extend up into the slipstream of the moving train to move ambient air into the large cylindrical CO2 collection chamber and eliminate the need for energy-intensive fan systems that are necessary with stationary DAC operations.

The air then moves through a chemical process that separates the CO2, allowing the rest of it to travel out of the back or underside of the car and return to the atmosphere.

After a sufficient amount has been captured, the chamber is closed and the harvested CO2 is collected, concentrated, and stored in a liquid reservoir until it can be emptied from the train at a crew change or fuelling stop into normal CO2 rail tank cars.

It is then transported into the circular carbon economy as value-added feedstock for CO2 utilisation, or to nearby geological landfill sites.

Each of these processes is powered exclusively by onboard-generated sustainable energy sources that require no external energy input or off-duty charging cycles.

In addition to the DAC system, the researchers plan to install regenerative braking on trains to maximise efficiency. Currently, this energy is dissipated on trains in the form of heat and discharged out of the top of the locomotive during every braking manoeuvre.

Eric Bachman of CO2Rail Company, said: “On average, each complete braking manoeuvre generates enough energy to power 20 average homes for an entire day so it is not a trivial amount of energy.

“Multiply this by every stop or deceleration for nearly every train in the world and you have about 10⁵ times more energy than the Hoover Dam produces within that same period."

Professor Peter Styring, from the University of Sheffield and co-author of the research, said: “The direct capture of carbon dioxide from the environment is increasingly becoming an urgent necessity to mitigate the worst effects of climate change.

“Currently the enormous amount of sustainable energy created when a train brakes or decelerates is simply lost. This innovative technology will not only use the sustainable energy created by the braking manoeuvre to harvest significant quantities of CO2, but it will also take advantage of many synergies that integration within the global rail network would provide.

“The technology will harvest meaningful quantities of CO2 at far lower costs 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 with each car having an annual capacity of 3,000 tonnes of CO2 in the near term.”

Unlike stationary DAC operations, which require large areas of land to build equipment and to construct renewable sources of energy to power them, the system would be transient and generally unseen by the public.