It sounds at first like science fiction: giant machines that suck carbon dioxide directly out of the air.
It may seem like the perfect solution to global warming, and there are machines that can do it. Dozens are already operating on several continents.
A number of new projects and investments have helped to bolster the nascent sector, which is comprised of a handful of small start-ups vying to perfect the technology.
But the process is complex, because on average, carbon dioxide represents only 0.04 per cent of the air around us. That level is creeping up as humans continue to burn fossil fuels, but even at such low concentrations extracting carbon dioxide from the air is still a challenge.
Nevertheless, investors and energy companies are ploughing growing sums into direct air capture ventures. In June, Switzerland-based Climeworks closed a $75m funding round, which it will use to build more carbon dioxide collectors.
Meanwhile, Canada-based Carbon Engineering has been working with Occidental Petroleum, the Houston-based energy group, to design and build the world’s biggest direct air capture plant. It will be capable of pulling a million tonnes of carbon dioxide out of the atmosphere every year — the equivalent of 40m trees. Construction is expected to start next year.
Newer start-ups are also entering the fray. Ireland-based Silicon Kingdom says it will build and install its first direct air capture machine this year.
The most common method is to use large fans to pull air across a chemical substance which binds with the CO2 molecules.
Then the CO2 is separated from the chemical agent using heat, purified, and stored.
Climeworks’ direct air capture plant has 18 CO2 collectors, and the plant is able to capture 900 tonnes of CO2 every year. But it takes heat and electricity to work.
Another method is best described as a “mechanical tree” — the wind simply blows across the structure and the CO2 in the breeze is absorbed by discs soaked in chemicals.
The collected CO2 has a variety of uses. Climeworks is selling some of the extracted CO2 gas to nearby greenhouses, where it acts as a fertiliser to help the plants grow faster, but it can also be used in fizzy drinks, and for energy, fuels and material.
Climeworks was the first to land paying customers. But they're a long way from achieving their ultimate goal, removing one per cent of global annual CO2 emissions by 2025. To do that, they'll need customers who want to sequester CO2 rather than just reuse it.
In Texas, the plant being designed by Carbon Engineering and Occidental will produce CO2 that is injected into oil wells to help improve the extraction rates of the fuel reserves from geological reservoirs — a process known as “enhanced recovery”.
Carbon Engineering says the company will also produce synthetic fuels — which could be used in aviation — at its new research facility in Canada. This can be manufactured by recycling the carbon molecules pulled from the air with hydrogen to synthesis combustible fuel.
Companies are also searching for the best way of storing CO2 — often underground in disused oil and gas fields to prevent its leakage back into the atmosphere.
Right now, it costs as much as $600 per tonne to extract CO2 from the air, which is expensive. Over time, start-ups are targeting a cost of around $100 per tonne. But other methods of sequestering carbon, such as planting a tree, cost as little as $10 per tonne.
Interest in direct air capture has been boosted by the recent stream of companies, and countries, that have pledged to cut their emissions to “net zero”. Such commitments involve cutting emissions to as low as possible, then compensating for any remaining emissions with programmes like tree planting or direct air capture.
But the Intergovernmental Panel on Climate Change estimates that we might need to extract more than 100 billion tonnes of CO2 from the atmosphere by the end of this century, to limit warming to less than 1.5C.
Direct air capture isn't the only way to do this, but it could be one tool among many.
With today's technology, direct air capture is still uneconomic. And critics say it can never be built on a large enough scale to make a difference to the climate. But at a time of rising emissions and rising global temperatures, more people are coming around to the view that this technology does have a role to play.
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