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AThe idea of pulling carbon dioxide straight out of the open air sounds like a new invention, but much of the chemistry behind one of the leading designs is more than a century old. Engineers working on large 'liquid-solvent' capture plants have deliberately borrowed equipment and reactions from industries that already handle alkaline chemicals on a vast scale, most notably the pulp and paper industry, which has long recycled its processing chemicals in a continuous loop. The appeal of this approach is reliability: almost every major unit in such a plant has decades of operating history behind it. The difficulty is that these units were never designed for a gas that makes up only about 0.04 per cent of the air, and adapting them has required a series of careful compromises.

BThe process begins at what designers call the shared front end, a stage that looks much the same whichever capture chemistry is chosen. Large fans pull outside air through an air intake and into a contactor, a structure that resembles an industrial cooling tower. Inside, the air passes over thin sheets of plastic packing that are kept wet with the capture medium. The packing is the key to the design: by spreading the liquid into a film with an enormous surface area, it allows carbon dioxide to dissolve without forcing the fans to push air through a deep pool of liquid, which would waste a great deal of energy. Even so, the volumes involved are huge. Because the gas is so dilute, a plant must move well over a thousand tonnes of air for every tonne of carbon dioxide it collects.

CIn a liquid-solvent plant the capture medium is an alkaline solution, typically potassium hydroxide dissolved in water. As air flows across the wetted packing, carbon dioxide reacts with the hydroxide and is held in the liquid as potassium carbonate. The solution is then pumped to a second unit, the causticiser, where it is mixed with calcium hydroxide. Here the carbonate is transferred from the potassium to the calcium, forming solid calcium carbonate, essentially the same material as limestone and chalk. Rather than letting this settle out as a fine sludge, modern designs grow it into small pellets around seed grains, which makes the solid much easier to separate from the liquid and to dry. The potassium hydroxide, now restored, flows back to the contactor to collect more carbon dioxide.

DThe pellets carry the captured carbon to the most demanding step in the plant: the calciner. In this kiln the calcium carbonate is heated to roughly 900°C, at which point it breaks down into calcium oxide and a concentrated stream of carbon dioxide. Because the gas leaves the calciner almost pure, it needs only dehydration and compression before it can enter a pipeline. The calcium oxide is passed to a slaker, where water is added to turn it back into calcium hydroxide, ready to be used again in the causticiser. The whole plant therefore consists of two interlocking loops, one for potassium and one for calcium, in which the chemicals circulate continuously while only carbon dioxide, water and energy enter and leave in significant quantities.

EThat reliance on heat is both the strength and the weakness of the design. Temperatures of 900°C are difficult to reach with electricity alone at a reasonable cost, so early plans have relied on burning natural gas, with the carbon dioxide from that combustion captured alongside the gas taken from the air. Critics point out that this arrangement uses a fossil fuel to remove carbon, and that leaks of methane further up the supply chain would reduce the benefit. Supporters reply that the combustion emissions are captured too, and that electrically heated calciners are being developed. Solid-sorbent plants, by comparison, release their carbon dioxide at around 100°C, which allows them to use waste heat from other industries. However, their coated filters are costly and degrade over time, whereas the liquid route relies on cheap, abundant chemicals. On present evidence, neither design is clearly superior.

FWhatever the capture route, the climate benefit depends on keeping the carbon dioxide out of the atmosphere for thousands of years. The most developed option is injection into porous rock more than a kilometre underground, usually a deep saline aquifer or a depleted oil and gas field. Several processes then hold the gas in place. At first it rises until it is stopped by an impermeable caprock, a form of containment known as structural trapping. Over time, some of it becomes stranded as tiny bubbles between rock grains, some dissolves into the salty water, and in certain rock types it reacts with minerals to form solid carbonates. In basalt formations in Iceland, for example, operators have reported that most of the injected carbon dioxide was turned into stone within about two years, far faster than geologists had once assumed.

GStorage is only credible if it can be checked. Operators typically monitor injection sites using pressure gauges in the wells, repeated seismic surveys that reveal the shape of the underground plume, and chemical sampling of groundwater and soil gas near the surface. These measurements matter because buyers of carbon removal increasingly demand proof that each tonne is permanently stored before they will pay for it. For liquid-solvent plants, this adds a further consideration when choosing a location: a site must offer not only affordable energy and room for the large contactor but also suitable geology close enough to avoid long pipelines. Few places offer all three, which helps to explain why the first large plants have been planned in only a handful of regions.