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Removing carbon dioxide from the air: Direct air capture

September 04, 2023 | Charles Fryer

A flow diagram of a Direct Air Capture process that uses caustic soda as the absorbent and includes solvent regeneration.

Direct Air Capture of CO2, or DAC, has been hitting the headlines recently.

The idea is to slow down or stop climate change by extracting carbon dioxide from the air and burying it in the ground, or converting it into useful products. It is a beguilingly simple concept. In practice it is fiendishly difficult to achieve except at high cost, because the concentration of CO2 in the air is only about 400 parts per million (ppm).

But the potential rewards of establishing a low cost and technically successful venture are attracting many companies and organisations to invest in projects that it is hoped will become commercially viable. The major endeavours are taking place in the US, stimulated by a generous support programme.

The major font of support comes from the oddly named Inflation Reduction Act, passed in August 2022. The Act’s main component is tax credits and funding to incentivise green-energy projects.

Many of its subsidies are uncapped, meaning that, depending on how many projects are put forward by the private sector, its climate spending could cost ultimately anything from $369bn (the government’s projection). There are tax credits labelled 45Q for industrial facilities and power plants that capture and store CO2, which provide up to $85/ton per ton credit for CO2 permanently stored and up to $60/ton for CO2 consumed industrially.

Also, there are tax credits under 45Z for the sale of low carbon transportation fuels, such as SAF. These credits will be available for the next 12 years.

Another source of funding comes from the Bipartisan Infrastructure Law, under which the Department of Energy has allocated $3.5 billion over the next 5 years for investments in direct air capture (DAC) technology. This technology is considered underdeveloped at present and needs support to scale up plant size so as to reach commercial viability. The DoE announced on 11 August that two projects will be assigned a total of $1.2 billion to develop DAC technology to industrial scale:

  • Project Cypress in Louisiana, run by Battelle, Climeworks Corporation and Heirloom Carbon Technologies;
  • The South Texas DAC Hub in Kleberg County, Texas, proposed by Occidental Petroleum's subsidiary 1PointFive and partners Carbon Engineering Ltd and Worley

The DoE also launched several new initiatives aimed at bringing the cost of the DAC technology down to less than $100 per metric ton of CO2-equivalent within this decade.

Battelle plans to use DAC technology developed by Climeworks and Heirloom at a location in Louisiana. Climeworks currently owns the biggest DAC facility in the world, located in Iceland, which removes around 4,000 metric tons of CO2 each year. Heirloom is a start-up company based in California which uses limestone to remove CO2 from the air. Battelle is also partnering with Gulf Coast Sequestration to manage the storage of the CO2 underground. The target is to sequester a million tons per year by 2030.

Occidental’s operations will be developed on 106,000 acres of land, south of Corpus Christi. The target is to remove up to 30 million metric tons of CO2 every year using DAC technology once fully operational. A huge array of fans will force air through a CO2 capture device. The CO2 will then be recovered and pumped thousands of feet underground into geological formations, where it can remain for hundreds of years.

Technologies to capture CO2 directly from the atmosphere

Two approaches are currently being used to capture CO2 from the air: solid and liquid DAC.

Solid DAC (S-DAC) is based on solid adsorbents operating at ambient pressure or under a vacuum and medium temperature (80-120 °C). One version uses Zeolites as the absorption medium. A DAC plant relying on zeolites was commissioned in 2022 in Norway, with plans to scale the technology up to 2,000 metric tons of CO2/year by 2025 through the project Removr

Liquid DAC (L-DAC) uses an aqueous basic solution (such as potassium hydroxide) to absorb the CO2 as a chemical compound, from which it is then released by subjecting the compound to high temperature (between 300 °C and 900 °C).

A process which is intermediate between S-DAC and L-DAC is Passive DAC, which uses calcium hydroxide to absorb atmospheric CO2 by combining the two to form CaCO3. The CO2 is recovered by reversing this reaction using renewably powered kilns.

A more radical approach is Electro Swing Adsorption DAC, which employs an electrochemical cell in which a solid electrode adsorbs CO2 when negatively charged and releases it when a positive charge is applied.

Capturing CO2 from the atmosphere through DAC is currently very energy intensive, since the concentration of the CO2 in the atmosphere is very low, around 400 parts per million (ppm), far lower than for example in the emissions of a power station or a cement plant.

Both S-DAC and L-DAC in their present state operate using both heat and electricity.  S-DAC can be powered by a variety of renewable or low-carbon energy sources, such as heat pumps, geothermal, nuclear, solar thermal or biomass-based fuels. In the case of L-DAC, however, the current high temperature needs of today’s technology requires the use of fuels, which should be low-carbon such as biomethane or renewables-based electrolytic hydrogen. It is hoped that in the future L-DAC could operate with solely electric energy.

How necessary is DAC?

In spite of the imprimatur of respectability conferred on DAC by the funding now lavished by the US Department of Energy, some environmental activists are concerned by the moral hazard involved. That is, fossil fuel companies could use the prospect of commercial viability of DAC to backslide on their announcements to move to low carbon operation. In the limit, it becomes an excuse to do nothing to reduce carbon emissions, as DAC will surely solve the problem.

Other environmental activists take a more nuanced approach, being sceptical of the contribution that DAC can make, but acknowledging that a multiplicity of technologies must be developed in order for global climate targets to be met, since the eventually successful approaches cannot be identified at the present stage of development.

Other experts are highly sceptical of DAC, regarding it as not only a futile pipe dream, but a danger that is diverting funding and effort from other technologies for carbon abatement that are far more likely to be successful. Some climate change experts have labelled DAC as “Greenwashing at its worst” and even as “The most dangerous idea in the world”.

The flow diagram below shows a process for DAC using caustic soda as the absorbent. The diagram also includes a solvent regeneration step.

A flow diagram of a Direct Air Capture process that uses caustic soda (NaOH) as the absorbent and includes solvent regeneration.

Source: JoseZZ Creative Commons Attribution-Share Alike 4.0 International

Direct Air Capture: The world's most dangerous idea: ABLC Connect Webinar (August 2023)

A recent webinar organised by ABLC Connect, with jim lanebioeconomy daily The Digest editor Jim Lane as moderator, brought together the views of the experts in ABLC’s Due Diligence Wolfpack.

The topic was 'Direct Air Capture: Green Miracle or Greenwash'.

The experts came down heavily on the side of 'greenwash'.

Paul Bryan did a rapid calculation that removing 1 ppm of CO2 per year from the atmosphere would need the processing of 900,000 billion standard cubic feet (SCF) of air per day.

That is more than 1000 times the capacity of the entire world’s gas processing plants. But CO2 levels are rising at the rate of 2.6 ppm per year, so 2.6 times that effort would be needed just to hold atmospheric CO2 concentration at its present level (ignoring other efforts to reduce carbon levels).

That is, the volume of air that would need to be processed is 2.3 quadrillion SCF/d or 23 x 1015 litres per year.   Looked at another way, the present cost of capture and supply of CO2 is about $1200/ton.

Since 1 barrel of oil on combustion releases about half a ton of CO2 , the price of crude oil should be at least $600/bbl just to cover the cost of DAC. True, the cost will come down in time, but this illustrates the challenge.

Steven Sloane emphasised that the capex costs and energy costs are crippling for DAC. It is much cheaper to use trees or plants to absorb CO2 with the input of solar energy, with the bio-material then used as fuel or to generate CO2 via fermentation. The process can be optimised with Regenerative Agriculture, a lofty name that really means soil carbon capture and sequestration.

Steve Weiss compared DAC to magic beans -- an attractive idea but an illusion. The dilute presence of CO2 in air means that DAC is as challenging as trying to recover the salt from a bowl of guacamole.

David Dodds brought up the moral hazard of DAC -- “it means we don’t have to do anything” about climate change. He quoted an economic analysis by the model En-ROADS Climate Solutions Simulator that for DAC to be successful the price of CO2 has to be at least $500/ton.

'...DAC gives fossil fuel companies an excuse to do nothing

about reducing their carbon footprint...'

Jim Lane quoted Occidental as calculating that the capex for construction of a DAC plant is $1100 per ton of CO2. A back-of-the-envelope calculation then gives a total construction cost of $42 trillion for enough such plants to bring the world to net carbon zero -- equal to one year of the world's GDP!

Michele Rubino pointed out that if DAC requires a CO2 price of $500/ton to be viable, there are many other technologies that could well be viable at a much lower price, maybe not today, but with more modest outlay in development than the sums being talked about for DAC.

Joel Stone reverted to the technique of Regenerative Agriculture as highly promising, in which CO2 is captured and then sequestered in soil. There are three times more tons of CO2 contained in the world’s soil than in the atmosphere. For every 1% increase in the CO2 content in soil, there is an increase in water absorption of 20,000 gallons per acre. In this way not only can CO2 be sequestered, at the same time water management of soil is improved, leading to increased food productivity. Alternatively, crops that lead to production of sugars and thereafter useful bio-products can be targeted.

Steve Weiss reminded us that a 1 million ton per year DAC plant involves capex of $1 billion. So at a discounted cash flow over some 15 years there is a cost of $100/ton of CO2 just for depreciation, before taking into account fixed and energy costs. Carbon Engineering has estimated the cost of carbon to make DAC viable is $180/ton. That seems optimistic.

Steve Slome pointed out that capture and sequestration of CO2 leaves the producer with nothing in hand, other than the credits obtained from the IRA plus other support systems. The danger is that supports may not be continued by the government after their allotted time, depending on the whims of politicians. DAC can survive only with credits, and only in the US for the moment. It is better to produce something of tangible value, like wood, which can be stored, sold or usefully employed. DAC will not survive if new technologies emerge with lower costs, except by going cap in hand to the government. Using algae to sequester CO2 at least leads to a tangible product, though indications are that costs are even higher than DAC’s.

Paul Bryan rounded off the webinar stating that once a project has been allocated government funds, that project tends to consume all the money willy nilly, whatever the success or failure of the project. Which project gets chosen depends on which way political winds are blowing. No lobbyist gets left behind!

No lobbyists left behind

If DAC is not the answer, what is? Jim Lane, in a later webinar, proposed that CO2 capture from the sea could be the answer. The concentration of CO2 in the sea is 100+ times higher than it is in the atmosphere. The work of capture of CO2 has already been done. It is only the cost of extraction that remains. Then the sea will absorb CO2 from the atmosphere to restore the equilibrium, and the cycle continues.

fryer_charlesThe author of this blog post Charles Fryer founded Tecnon OrbiChem in the 1970s and remains a special advisor to Tecnon OrbiChem, its parent company ResourceWise and sister companies.

The ResourceWise group comprises the renewable fuels and feedstock intelligence provider Prima Markets, as well as forestry, pulp and paper-focused Forest2Market, Fisher International, and Wood Resources International.


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