Capturing carbon

Effective technologies for capturing and storing carbon dioxide are already at our fingertips, but what will it take for them to make a difference? Vanessa Zainzinger reports

In August 2021, the Intergovernmental Panel on Climate Change’s (IPCC) sixth assessment report offered a stark reminder we have no choice but to keep massive amounts of carbon dioxide from entering the atmosphere.¹

Within 20 years, temperatures are likely to rise by more than 1.5°C above pre-industrial levels, the report found. Once we reach that point, only removing CO₂ from the atmosphere will bring global temperatures back down. The IPCC also underscored that the necessary technologies to capture and store CO₂ have not progressed rapidly enough to play a major role in tackling the climate crisis yet. In other words, we need to get moving.

The good news is: We already have carbon-capture chemistry that works. We can remove CO₂ from point sources such as the flue gas of power plants and lock it away underground. Technologies known as direct air capture (DAC) can remove CO₂ in ambient air. Globally, carbon capture, utilisation and storage (CCUS) facilities have the capacity to capture some 40 megatonnes of CO₂ each year, according to the International Energy Agency.² However, that figure must increase at least 100-fold by 2050 to meet the scenarios laid out by the IPCC, the Global CCS Institute found.³

‘There are some big projects around the world that demonstrate companies can integrate [CCUS] technology into their systems,’ says Mai Bui of the Centre for Environmental Policy at Imperial College London. ‘But the big hurdle for CCUS has always been that there was never a motivation or incentive for companies to do it. Unless there is a regulation or policy change, or some sort of financial incentive, there’s not going to be a big uptake of the technology.’

Recently, policy, regulation and public perception, as well as the scientific consensus expressed by the IPCC and the IEA, have driven a new sense of urgency for carbon capture deployment. In the UK, Prime Minister Boris Johnson’s Ten Point Plan for a Green Industrial Revolution⁴ includes a commitment to deploy two carbon capture clusters by the mid-2020s, and a further two clusters by 2030. And companies, Bui says, are beginning to ask seriously about integrating carbon capture plans into their systems. Backed by this momentum, researchers are working hard to make carbon capture technology cheaper and more efficient, with the aim of bringing it up to scale and into the market.

There was a lot of excitement and funding about carbon capture materials over the last few years, but it can be challenging to obtain follow-on funding for scaling up and testing these materials in pilot facilities.
Joe Wood professor in chemical reaction engineering, University of Birmingham

Maturing solvents

Currently, the most mature carbon-capture technologies use solvents. These systems pump flue gas up a contact tower and through a solution that absorbs CO₂ but lets through other gases, such as nitrogen. The CO₂-rich solvent then flows to the top of a stripping tower, where heat forces the CO₂ out of the solution and up to a collection system.

Carbon-capture solvents have been used successfully for some time. Companies including Linde and Air Products sell systems based on methanol to remove CO₂ from natural gas. Most solvent systems are based on amines, such as monoethanolamine solutions. But while they work well, capturing CO₂ with amines comes at a steep cost. Fitted onto a power plant’s systems, they currently consume 30–50% of the plant’s energy output.

The way to make amine-based technology more efficient, and cheaper, is via next generation solvents, says Bui, whose work at Imperial focuses on modelling how plants can capture as much CO₂ as possible.

Most of the work on advanced solvents is being done by companies. Mitsubishi Heavy Industries is currently testing a promising solvent called KS-21 at an amine plant in Mongstad, Norway.⁵ The coal-fired Boundary Dam Power Station in the Canadian province Saskatchewan – currently the only commercial power plant that has carbon capture - uses Cansolv, a solvent system developed by Shell.⁶

These modern solvents have much lower energy requirements than conventional ones, says Bui, but data for exact modelling are limited because solvents like KS-21 and Cansolv are proprietary. Still, amines are not a one-size-fits-all technology, she adds. ‘Amine technologies are good for certain flue gas composition ranges but when you get to higher concentrations, where you just need to clean up gases, you might use adsorption-based technologies or membranes.’

Promising alternatives

Adsorbents are one of the next-generation carbon-capture technologies that can pick up where amines can’t reach. They generally have faster absorption kinetics than amines and require less energy to release the CO₂ again. Adsorbents can also hold more CO₂ per unit volume than most solvents and that means that adsorbent-based systems can be smaller, reducing capital costs, and can be stripped of their CO₂ using vacuum or modest heat.⁷

‘Adsorbents present a good alternative, but the greatest challenge is developing materials that can be effectively used under processing conditions,’ says chemical engineering professor Joe Wood at the University of Birmingham. Options range from organic-derived carbons to inorganic materials such as zeolites, silica, hydrotalcites, inorganic-organic hybrid materials such as metal-organic frameworks (MOFs) or even synthetic materials such as microporous polymers. Whilst materials such as microporous polymers have very high surface areas, others such as carbons, can be readily and cheaply prepared from naturally abundant substances and modified to increase their affinity for capturing CO₂, for example, by immobilising amines on the material surface, Wood explains.

Wood, who has developed hydrotalcite adsorbents and amine modified carbons for CCUS, has found in simulation studies that adsorbent systems can separate CO2 from hydrogen to produce a stream that is over 90% pure CO₂ and another stream that is 99.4% hydrogen. But we know little about the use of adsorbents at larger scale or how quickly research will progress, he says.

‘There was a lot of excitement and funding about carbon capture materials over the last few years, but it can be challenging to obtain follow-on funding for scaling up and testing these materials in pilot facilities. However, with renewed interest in decarbonising, maybe we could expect larger scale trials of novel technologies to follow in the 2020s.’

Wood is betting on materials based on carbons as being closer to scaling up commercially than other novel materials, such as MOFs and membranes. The latter are already used in natural gas processing and other areas where CO₂ pressure and concentrations are high. Membrane systems use selectively permeable materials that exploit small chemical and physical differences between CO₂ and the rest of a gas mixture to achieve separation.

In principle, a membrane could produce a pure stream of CO₂ from any mixture of gases. But the energy requirement and cost of doing so depends on numerous factors, such as the cost of the raw materials, the lifetime of the membrane system, maintenance costs, and the energy required to drive CO₂ through the membrane. ‘Membranes are a promising technology, but are still in their infancy, and a large number of modules may be required to treat the effluent of an industrial plant or power station,’ Wood says.

MOFs, on the other hand, tend to generate a lot of excitement among industry and researchers. The sponge-like solids can, in theory, squeeze the equivalent CO₂ surface area of a football pitch into the size of a sugar cube, all the while requiring relatively little energy. But they are not nearly developed enough to be ready for gigatonne-scale capture any time soon, says MOFs researcher Chris Hawes at Keele University, UK.

‘When we first started with MOFs the big problems were about stability,’ he says. ‘Stability is still not completely solved in terms of making new materials but we’re in pretty good shape. Now, there’s an issue with scale up, cost and resource scarcity.’ Most MOFs comprise zinc, copper or nickel but high performing materials rely on rarer metals or ligands that require multiple palladium coupling steps, chromatography and separation, Hawes says.

And while MOFs perform well at capturing CO₂ selectively over nitrogen, their water selectivity is more difficult to work out, he explains. MOFs are good at absorbing CO₂ because it’s more polar than nitrogen, but water is much more polar again. If the materials do not become more resistant to absorbing water, their efficiency is compromised because they are filled up with H2O and have no space left for CO₂.

Direct air capture

Hawes can see a more promising future for MOFs in direct air capture. Researchers led by Christopher Jones at Georgia Tech, US, are studying MOFs that contain amine material, which protects the MOF from water damage because the amine occupies the places the water attacks.⁸

Removing carbon from the air works on chemical principles similar to those for point-source capture. The big difference is the CO2 in ambient air is 100-400 times more diluted than in industrial point sources.

That changes the devices and their economics, says David Danaci, a chemical engineer at Imperial College London, UK. ‘Because it’s so much more dilute, the equipment needs to be larger because the dilution makes it more difficult to do the separation, and you need to process more gas to capture a given amount of CO₂.’

There are limits to how much gas a direct air capture (DAC) container can process without causing a pressure drop, which would crush the particles. This risk can be reduced by forcing the gas through less material, but this would again reduce the amount of CO₂ absorbed.

However you look at it, DAC is expensive. Carbon capture at an ethanol plant can cost as little as $10/t. At a coal-fired power plant, the cost is around $60/t. The costs for air capture are around $500/t, according to Swiss direct-air-capture company Climeworks. Experts generally peg $100/t as the point at which DAC will become cost competitive with other decarbonisation methods.

‘We’re a lot farther away from direct air capture than most people imagine,’ says Danaci. ‘The things that we do know are it will consume more energy than post combustion, because it’s more dilute. And it is always going to be significantly more expensive. But how much more? I don’t know.’

Danaci contends we have to continue to study and develop DAC, not least to address transport emissions that cannot be dealt with at point-source. ‘We can capture CO2 from the air today, no problem. But it is much easier to do a lot of emissions reduction with point source capture today. If you want to do the equivalent amount with DAC, it is an enormous endeavour,’ he says.

Time to start

Currently, the most obvious role for CCUS is to help reduce CO2 emissions from energy intensive industrial sectors, such as cement, chemicals and iron steel, says Peter Taylor, co-director of the UK Energy Research Centre.

‘CCUS could also be used to substantially reduce the carbon footprint of steam methane reforming, which is currently the cheapest way of producing hydrogen. This hydrogen can then be used as a fuel in heavy industry or to heat homes.’

Some CCUS could be used in the UK to reduce the carbon emissions from natural gas and so provide a low-carbon source of baseload electricity generation. However, that role may be relatively modest depending on the extent to which further new nuclear power stations are built and whether we find cost-effective ways of integrating large quantities of variable renewables into the electricity system, Taylor says.

In the longer term, CCUS offers a way to decarbonise combustion and other CO₂-emitting processes so they can remain part of a net-zero world. That’s both a selling point and a point of criticism. Critics such as the NGO Friends of the Earth say carbon capture diverts resources away from renewables and keeps polluters in business.

‘Even with carbon capture, gas is not low-carbon because not all of the CO₂ is captured, and drilling for gas in the first place still leaks emissions,’ said Paul de Zylva, senior sustainability analyst at Friends of the Earth. ‘If we are to prevent the worst of climate change, we need to switch from dirty gas to powering our homes with electricity from clean, renewable technologies.’

‘But the industry wants to keep the UK hooked on gas by converting into hydrogen, and then claiming it can capture and store carbon, which is far from proven. This is a dangerous distraction from what needs to be done.’

Researchers, however, agree we need to start deploying carbon capture now, simply because we have no time to waste.

‘We need to be throwing everything we have at this,’ says Hawes. ‘I personally would like to see that the CO₂ capture technologies that we already have are just deployed.’

And while researchers will continue to work on improving technologies and making them cheaper and more efficient, it’s going to be an ongoing process, says Bui. ‘Often people theorise, if we wait for this amazing new unicorn technology in the future, are we doing to be better off? No, we should start today. Because the longer we wait, the more behind we get in meeting our targets. And it ends up being more expensive because we pay for the effects of climate change.’

References
1 ipcc.ch/assessment-report/ar6/
2 iea.org/reports/ccus-around-the-world
3 globalccsinstitute.com/resources/publications-reports-research/scaling-up-the-ccs-market-to-deliver-net-zero-emissions/
4 gov.uk/government/publications/the-ten-point-plan-for-a-green-industrial-revolution
5 mhi.com/news/210304.html
6 shell.com/business-customers/catalysts-technologies/licensed-technologies/emissions-standards/tail-gas-treatment-unit/cansolv-co2.html
7 A. Qader et al, Energy Procedia, 2017, 114, 5855.
8 jones.chbe.gatech.edu/carbon-capture/