Swiss start-up company Climeworks is using similarly captured CO₂ to aid photosynthesis and improve crop yield in nearby greenhouses, but as yet the price is nowhere near competitive. The plant does capture this extra CO₂, and of course could be powered by renewable energy for a healthier carbon balance – but the problem of what to do with all the captured gas remains. In Carbon Engineering's natural gas-fired plant, the whole cycle generates half a tonne of CO₂ for every tonne captured from air. This last step requires a vast amount of energy. Thus, the solid calcium carbonate is heated to 900 ☌ to recover pure CO₂. To be feasible, direct air capture needs to produce concentrated CO₂ as its product, which can either be safely stored or put to use. It also isn't a practical option for government-funded carbon storage due to the massive quantities of calcium hydroxide that would be required. Though calcium carbonate has uses in agriculture and construction, this process would be far too expensive as a commercial source. This part of the process costs relatively little energy and its product is essentially limestone – but making mountains of calcium carbonate doesn't solve our problem. Unfortunately, this isn't a small-scale problem anymore – we now need to capture billions of tonnes of CO₂, and fast. The 19th-century apparatus used in this latter procedure still features on the American Chemical Society's logo. Lithium hydroxide was the basis of the CO₂ absorbers that kept the astronauts on Apollo 13 alive, and potassium hydroxide captures CO₂ so efficiently that it can be used to measure the carbon content of a combusted substance. Other hydroxides capture CO₂ in the same way. But chemists have been doing it on small scales since the 18th century, and it can even be done – albeit inefficiently – with supplies from the local hardware store.Īs secondary school chemistry students will know, CO₂ reacts with limewater (calcium hydroxide solution) to give milky-white insoluble calcium carbonate. Given that CO₂ only accounts for 0.04% of the molecules in our air, capturing it might seem like a technological marvel. Truly tipping the balance from carbon source to carbon sink is a delicate business, and our view is that the energy costs involved and likely downstream uses of captured CO₂ mean that Carbon Engineering's "bullet" is anything but magic. Unfortunately, the big picture isn't as simple. So when recent news emerged that Canadian company Carbon Engineering has harnessed some well-known chemistry to capture CO₂ from the atmosphere at a cost of less than $100 a tonne, many media sources hailed the milestone as a magic bullet.
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