Researchers from the University of Sheffield are working with US-based CO2Rail Company to design direct air capture equipment that can be used within special rail cars to scrub carbon dioxide from the air. The special rail cars are designed to be placed with already running trains in regular service.
The rail cars work by using large air intakes that extend up into the slipstream of the moving train to move ambient air into a large, cylindrical CO2-collection chamber, thus eliminating the need for the energy-intensive fan systems required by stationary direct air capture operations. Within the collection chamber, a chemical process separates the CO2 from the air, which then travels out of the back or underside of the car and returns to the atmosphere.
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After a sufficient amount of CO2has been captured, the chamber is closed and the harvested CO2 is 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’s then transported into the circular carbon economy as value-added feedstock for CO2 utilisation or to nearby geological landfill sites.
Each process is powered exclusively by on-board-generated sustainable energy sources that require no external energy input or off-duty charging cycles. When a train pumps the brakes, its energy braking system converts the train’s forward momentum into electrical energy in much the same way as a regenerative electric vehicle. 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.
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‘The direct capture of carbon dioxide from the environment is increasingly becoming an urgent necessity to mitigate the worst effects of climate change,’ said Professor Peter Styring, Director of the UK Centre for Carbon Dioxide Utilization at the University of Sheffield. ‘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 direct air capture operations, which require large areas of land to build equipment and to construct renewable sources of energy to power them, CO2Rail would be transient and would generally be unseen by the public.
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‘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,’ said Eric Bachman of CO2Rail Company. ‘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, and that was a hydro-electric construction project that took six years and cost US$760million in today’s dollars.
The team found that each direct air capture car can harvest about 6,000 tonnes of carbon dioxide from the air per year and more as the technology develops. Moreover, since trains are capable of hosting multiple CO2Rail cars, each train will harvest a corresponding multiple of CO2 tonnage.
With its sustainable power requirements exclusively supplied by train-generated sources that are without incremental cost, savings of 30–40 per cent per tonne of harvested CO2 can be realised from energy inputs alone.
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This, along with other significant savings, such as land, brings projected cost at scale down to less than US$50 per tonne and makes the technology not only commercially viable but commercially attractive.
‘At these price points and with its tremendous capabilities, CO2Rail is likely to soon become the first megaton-scale, first gigaton-scale, and overall largest provider of direct air capture deployments in the world,’ said Professor Geoffrey Ozin from the University of Toronto, who also worked on the research.
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The team is also working on a similar system that can remove the CO2 emissions from the exhaust of diesel-powered locomotives. With the growth of sustainably-sourced rail electrification systems, this point-source capability on diesel lines would make rail the world’s first carbon-neutral mode of large-scale transportation.
The research has been published in Joule.