DAC: needless extravagance or the sine qua non of net zero?

Image: Climeworks’ Orca DAC plant

Although the IPCC report only considered removal through biomass, interest in so-called ‘direct air capture’ (DAC) has also soared. This concept sees huge fans draw air over materials which react with CO2; these are then heated to release the greenhouse gas in a pure form, ready either for conversion to carbon-based products or storage in deep geological formations. In the last five years, over a dozen DAC test facilities have sprung up in North America and Europe.

Recent ambitions to scale up DAC are exemplified by Occidental Petroleum’s plans to construct the world’s largest facility, in Texas, aiming to capture one million tonnes of CO2 from the air each year using technology developed by Canadian company Carbon Engineering.

The CO2 will be used to boost oil production from the Permian Basin but, with enough CO2 permanently stored to offset end-use emissions, Oxy is marketing the concept as ‘net-zero oil’. Early this year, the company awarded a contract for the design of a first phase of half a million tonnes, scheduled to start operating in 2025.

Another prominent player is Zurich-based Climeworks, which has built a handful of small facilities around Europe. Among these, a plant in Iceland gained top climate credentials by tying in with the pioneering CarbFix project – a research initiative injecting captured CO2 into basalt formations where it is relatively rapidly converted to solid carbonates. A scaled-up version, the Orca facility, storing 4000 t/year is under construction and should be completed this year. This capacity will match the largest currently operating DAC plant, a distinction held by an Alabama facility commissioned in 2019 by Global Thermostat. Another veteran of the DAC scene, this US-based company has also attracted high-profile oil sector backing in the form of a partnership with ExxonMobil.

DAC technologies can be divided into systems that use solid sorbent materials to bind the CO2, such as Climeworks and Global Thermostat, and those using liquid solutions, typified by Carbon Engineering. Both processes employ large amounts of heat energy to regenerate the active chemical and release CO2, as well as a smaller amount of electricity to operate fans and other equipment. While solid-based processes tend to require more energy, they have the advantage of being able to use low-temperature (~100°C) waste heat; Climeworks sources this heat from waste-to-energy plants or – in Iceland – geothermal energy. On the other hand, regeneration of Carbon Engineering’s hydroxide solution involves a high-temperature (900°C) calcination process fuelled by natural gas, from which the emissions are also captured.

Regardless of the technology, the highly dilute nature of CO2 in the atmosphere makes for a fundamentally energetically demanding process using large equipment, meaning DAC carries a hefty price tag. The International Energy Agency (IEA) provides a broad estimate of around 130 to 340 $/t of CO2 captured and stored, compared with costs of $20-60/t typical of capturing emissions from the source. Although several of its developers assert that DAC can go below $100/t, this disparity has led critics to label the technology a needless extravagance while more concentrated sources of CO2 still abound.

However, aside from the inevitable need to ‘correct’ atmospheric CO2 levels, DAC proponents see an economically justified place for the technology in a net-zero world. A 2019 study by Goldman Sachs highlighted that the cost of abating the final 10 Gt of greenhouse gas emissions rises sharply from around $200/t to over $1000/t. These ‘hard-to-abate’ sectors, including aviation and shipping, for which electrification is not considered feasible, are where DAC can play a key role – either by offsetting the continued use of fossil fuels, or providing atmospheric CO2 for conversion to synthetic hydrocarbons.

DAC also offers some flexibility relative to capture from point sources, in that it can be located right on top of suitable geology for CO2 storage and in regions with abundant renewable energy. Oxy frame this as the atmosphere acting as a ‘virtual pipeline’ for CO2 – from emitter to DAC facility – and speculate that plants could be optimally located in the Middle East, where solar energy and opportunities for enhanced oil recovery are plentiful.

A competing technology in the carbon removal space is bio-energy carbon capture and storage, or ‘BECCS’, which converts sustainably harvested biomass to either electricity or biofuels and stores the resulting CO2. The much more concentrated CO2 streams processed in BECCS enable lower costs to be achieved – estimated at around $60/t for power generation. However, BECCS requires around 100-1000 times the land area of DAC per tonne of CO2, and future competition for this low-carbon resource will be fierce.

The IEA’s ‘Sustainable Development Scenario’ sees DAC capture 800 Mt of CO2 per year by the time net zero is reached in 2070, with 500 Mt of this reused to create carbon-based products (over three times as much atmospheric carbon is captured by BECCS). While daunting enough, this trajectory is less ambitious than the net zero targets now set by many nations, as well as the IPCC 1.5-degree scenarios, so an even greater role for DAC may well be necessary.

Fundamentally, the cost of this ‘last resort’ climate solution can be regarded as a key benchmark for a net-zero world, as it represents the ultimate cost of offsetting any remaining ‘business as usual’ – an expense which will surely be incurred to some extent. With this in mind, current efforts to develop the technology and drive down its cost at scale seem unlikely to be wasted.

Author: Toby Lockwood, IEA Clean Coal Centre