Cashing in on carbon

If CO2 were seen as a valuable asset, direct air capture might have a future as a way to deal with the greenhouse gas

Could direct air capture be a viable technological solution to tackle climate change? It works by sucking large quantities of CO₂ out of the air, which can then be stored or reused for industrial applications. It could deal with emissions that can’t easily be captured at source, such as from cars, farms, ships and planes. The captured CO₂ could be used for industrial processes or to create carbon-neutral synthetic fuels. 

While many may remain sceptical as to whether this solution can be scaled up and commercialised, start-ups around the globe are beginning to push forward with industrial-scale demonstrator plants to prove the technology’s worth. 

In Squamish, British Columbia, a start-up led by Harvard University physicist David Keith – and funded in part by Bill Gates – has been running a pilot plant since early 2015 that captures one to two tonnes of CO₂ out of the air every day. Carbon Engineering (CE) aims to build a “first of a kind” commercial plant by 2017, and plans to use the captured CO₂ to produce low-carbon fuels.  

Geoff Holmes, business development manager at CE, explains that the firm’s direct air capture (DAC) solution is process-based, a lot like oil refining or pulp and paper, where a series of chemical unit operations take the input, air, and give the desired output, purified CO₂, which can then be used or stored.

CE’s technology integrates two processes: an air contactor and a regeneration cycle that allow for the continuous capture of atmospheric carbon dioxide and production of pure CO₂. The air contactor, much like a cooling tower, pulls atmospheric air through it. Inside the device is a tightly corrugated structure, or packing, designed to create a high surface area. The packing is ‘washed’ with a highly alkaline potassium hydroxide solution. This solution reacts with the CO₂ to form potassium carbonate (K₂CO₃), which is dissolved in water. 

In traditional industries they would continuously pump liquid over the packing. However, CE found that by pumping one big flush over the packing at the start and end of the process it could cut pump energy by around 90%, while only losing 15% of the maximum CO₂ capture rate – what Holmes describes as a “big overall gain”.

The first stage of the regeneration cycle takes the potassium carbonate solution and feeds it into a pellet reactor, which simultaneously reacts it with calcium hydroxide (Ca(OH)₂). This serves to regenerate the potassium hydroxide (KOH) for reuse in the air contactor and precipitates the CO₂ out of solution as solid calcium carbonate (CaCO₃). 

Holmes says: “The reason we wanted to make pellets rather than form a fine powder, or mud, is because you can sluice out those pellets and drain the liquid off of them and put it back in the reactor. The pellets dry and de-water much more easily than fine powder or mud, so you have very little liquid carryover and that’s very important as they must then be heated.”

Once the solid calcium carbonate has been separated from the solution, it is sent to a device called a fluid-bed calciner. The calciner operates at about 900°C, which causes the calcium carbonate to decompose into calcium oxide (CaO), during which pure CO₂ is released as a gas. The calciner burns fuel, such as natural gas, in an oxygen environment to supply the heat needed to perform this reaction. The calciner also generates heat that is used to supply electricity for the rest of the air capture plant. The CO₂ produced by burning the fuel mixes with the captured atmospheric CO₂ and all the CO₂ is sent to a final cooling, compression and clean-up stage to produce pure, pipeline-quality CO₂. This closes the overall cycle, so that only energy and water are needed to power continuous CO₂ capture.

After the solids have released their CO₂, they are sent to a mixing tank where they react with water to reform fresh calcium hydroxide. This calcium hydroxide is recycled to the pellet reactor for reuse.

Peter Styring, professor of chemical engineering and chemistry at Sheffield University and director of the EPSRC CO₂Chem Grand Challenge Network, says that the main issue with DAC isn’t the technology, which is proven to work, but what you do with the CO₂ once you’ve captured it. Considered to be a world expert in carbon capture and utilisation, Styring recently co-authored an influential paper, “Carbon Capture and Utilisation in the Green Economy,” and works with government through consultation on strategies at national and European level.

Styring says: “If you have something that takes CO₂ out of the atmosphere, utilisation is one of the options. Storage is another. But if it is going to be air captured and then used for enhanced oil recovery then it’s really a non-starter because you’re just adding to the problem as you produce more CO₂ than you actually capture.”

In Styring’s paper he states that by treating CO₂ as a commodity chemical rather than a waste it becomes “a valuable asset rather than an economic drain”.

Utilisation appears to be the most popular, and commercially viable, option for DAC start-ups, with the CO₂ generally to be sold for use in carbonated drinks or the food packing industry, as well as for greenhouse fertilisation and low-carbon liquid-fuel production. It is the latter that is gaining a great deal of attention.

CE hopes its technology will eventually be used to capture ‘residual’ emissions from cars, planes and farming processes to “help manage the 60% of emissions that do not come from point sources”. However, Holmes says: “Residual capture use is probably further out there in time. Really the more near term and more exciting opportunity that we’re trying to pursue is in using the CO₂ to manufacture fuels.” 

This can be achieved with sunlight, water and air as inputs, explains Holmes, and would utilise energy generated from solar photovoltaics to heat and split water to make hydrogen and oxygen. Captured CO₂ could then be combined with hydrogen under high heat and pressure, using a traditional Fischer-Tropsch process, to form high-purity fuel composed of long-chain hydrocarbon compounds – similar to fossil fuels – but free from sulphur and aromatics. 

CE is securing funding to bring fuel synthesis equipment to its pilot plant in British Columbia, which it aims to install next year and predicts will produce up to 400 litres of fuel per day. Holmes says: “If people like the approach and are willing to moderately incentivise it that is something that could be done at scale the year after.” 

Holmes believes that synthetic fuels could become a viable part of the energy mix if countries replicate a fuel standard similar to that in California, which awards credits for low-carbon fuels and deficits for high-carbon fuels. 

While the motivation for developing these synthetic fuels will undoubtedly be commercially driven, Styring regards it as a useful kind of shorter-term sequestration of CO₂ and recommends stockpiling these kinds of liquid and gaseous fuels. In his paper, “Carbon Capture and Utilisation in the Green Economy,” he explains: “When manufacturing chemicals from CO₂, previously emitted CO₂ will be reused before it is re-emitted, resulting in a net reduction in emitted CO₂. This does not sequester as much CO₂ as if it was stored geologically or is used to produce long-lifetime products such as a polymer or mineral; but it does provide a sustainable low-carbon pathway for the chemicals industry and a net reduction in emissions.”

He adds that this net reduction and the amount of CO₂ that can be utilised to create it should not be disregarded. “The chemicals industry needs to become more sustainable and embrace a circular economy and the use of CO₂ as a feedstock enables this,” he writes. 

Swiss start-up company Climeworks has been developing its DAC technology since 2009 and supplies Audi with CO₂ to produce synthetic methane (Audi e-gas), which serves as fuel for cars like the A3 Sportback g-tron. Dominique Kronenberg, chief operating officer at Climeworks, explains that, while fuel synthesis will undoubtedly be a big market for DAC, it is not an area that the company will explore itself, preferring to remain a supplier of CO₂ to industry. 

Climeworks is a spin-out company from the science and engineering university ETH Zurich and has developed two standalone demonstrators and a modular plant, which it sells to industry and scientific facilities for their own research purposes. Its smallest demonstrator, which resembles a typical air-conditioning unit, can capture 8kg of CO₂ from ambient air per day, while its larger CO₂-Kollektor model can extract 135kg CO₂ per day. Its capture plant features a modular design and the capacity is scaleable in multiples of 35kg of CO₂ per hour (300 tonnes of CO₂ per year). Individual modules consist of six Climeworks CO₂-Kollektors which are fitted into a standard 12m container.

The Climeworks CO₂ capture technology is based on a cyclic adsorption/desorption process on a novel filter material or sorbent. Kronenberg says: “Our main filter material is based on a biodegradable cellulose, a paperish kind of material that is chemically modified with amines. Amines are the docking stations for the CO₂.” 

Once the sorbent is saturated, the CO₂ is driven off by heating it to 100°C, delivering high-purity gaseous CO₂. The CO₂-free sorbent can be reused for up to four years. Around 90% of the energy demand for the process can be supplied by low-temperature heat, such as from solar power, waste heat from incineration plants or process heat from industry. The remaining energy is required in the form of electricity for pumping and control purposes. 

Kronenberg says that the firm is continuing to test and improve the design, particularly in increasing the capacity of how much CO₂ can be captured per mass of filter material. 

Climeworks has also been working to speed up the kinetics of the adsorption/desorption process – an area that Styring says is important for the success of the technology. He says: “The problem with DAC is that you’re talking about much lower concentration of CO₂ than you get from flue gas. If you think of the thermodynamics, there is an energy penalty for doing the capture, there is an energy penalty for doing the release of the CO₂ once you capture, but it is really the kinetics that is the driving force – how quickly you can separate CO₂ from the air.” 

Kronenberg says that typically a total adsorption/desorption cycle is in the range of 5-7 hours. “We can speed it up but then the CO₂ concentration released is less, so it is a trade-off,” he says. “We could perform the process in a day and get more CO₂ per cycle but then again the cycle time is very long – it is an optimisation to maximise CO₂ captured per day.” Currently the concentration of CO₂ is in the range 99.7%-99.9% depending on the size of the system, with the bigger the system the higher the concentration. 

Reducing energy costs is also a big focus for DAC technology. Climeworks is looking at how the filter material is arranged to reduce pressure and increase air flow to reduce the energy needed to pump air through the system. It will also continue to explore how the filter can be heated up as efficiently as possible to reduce costs associated with heat loss. 

The next step for the Zurich-based company is to build and commission an industrial-scale DAC pilot plant by next April, which will produce 1,000 tonnes of CO₂ per year to be sold to a greenhouse operator. The plant will provide Climeworks with knowledge about the operation and maintenance required and the costs of production. 

The demonstrator plants at Carbon Engineering and Climeworks will be watched closely by interested parties around the world looking to make decisions about their own investments in the technology. Styring says: “In Europe there is a huge effort to bring this technology to market. In Britain there is a bit of a lag because we’re slightly behind Europe in terms of policy. It is still set on carbon capture and storage and the sequestration of CO₂ rather than the utilisation. 

“I’m not saying we don’t need sequestration but we need a balance between the two so there is a good range of competition going so the best technology wins out.” 

DAC is an enabling technology that feeds in to both sequestration and utilisation. 

In terms of progressing the technology in the UK, it comes down to the usual questions of funding and raising political awareness. Styring is conducting research into DAC at the University of Sheffield and points to countries such as Germany and Switzerland where there is substantial government backing to push forward research. This has supported the development of small-scale German clean-technology firm Sunfire, which creates synthetic fuels using air, water and CO₂, and supplies Audi and Boeing.  

Styring is involved in two main initiatives that aim to push DAC and carbon dioxide utilisation technologies towards commercialisation. One is through  Horizon 2020 European funding, helping to design a Horizon Inducement prize for carbon dioxide reuse, with details to be announced at the SET Conference in Luxembourg this month. The other is the development of an Important Project of Common European Interest for CO₂ Reuse.

Styring says: “Direct air capture is going to be one of the hottest technologies. Progress has been good, but it is going to get greater. The more people that start to look at it the better.”

The Dresden energy technology company Sunfire is Audi’s project partner and plant operator for Audi e-diesel. The process follows the power-to-liquid principle and uses green power to produce liquid fuel using just water and carbon dioxide. Reiner Mangold, head of sustainable product development at Audi, says: “Using CO₂ as a raw material represents an opportunity not just for the automotive industry in Germany, but also to transfer the principle to other sectors and countries.”