Capturing CO2

Controlling global warming requires us to reduce the amount of CO₂ we put into the atmosphere – and to remove some of the CO₂ we’ve added. Richard Corfield reports

‘Based on the International Energy Agency and other models, Direct Air Capture (DAC) is essential to meeting the IPCC goal of 1.5°C,’ according to Yong Ding, founder of Massachusetts, US-based Decarbontek. According to the 2015 Paris Agreement, the 1.5°C target requires not only that emissions from human activities fall to net zero by 2050, but also that we must remove past ‘fossil’ emissions from the atmosphere – carbon capture.

Growing more trees is the most obvious way to achieve carbon capture and storage, but faster ways are to capture and store CO₂ from fuel sources – Point Source Capture – and the atmosphere: Direct Air Capture (DAC). Prevention of CO₂ release from fossil fuel sources involves three technologies: pre-combustion, post-combustion and oxyfuel combustion. Normally these are applied to point-source emissions such as factories or refineries. If the resulting cleaned gas is used for making CO₂ based products, for example, fizzy drinks, the process is called carbon capture and utilisation. If the gas is stored, for example underground in rock formations of worked-out oil reservoirs, the process is called carbon sequestration. The CO₂ is usually quickly returned to the atmosphere when a product is consumed, unless it is sequestered.

The difference between pre-combustion carbon capture and post-combustion carbon capture is that in pre-combustion the fuel is gasified, and the CO₂ is then separated. In post-combustion the CO₂ is separated from the exhaust of the combustion process by amine solvent systems. Such systems are in use now and are likely to be fully deployed by 2030. Non-amine solvents, solid sorbents, fuel cells and membranes should be deployed by 2035. For oxy-fuel combustion, the combustion process occurs in a near pure-oxygen environment, rather than the normal atmosphere, which results in a more concentrated CO₂ stream, making it easier to capture.¹

Energy transitions

Since 2015, many companies have jumped on the DAC bandwagon with 22 carbon projects now under way worldwide.² Many companies focus on post-combustion CO₂ reduction from point sources; a straightforward approach using amines to capture the CO₂. DAC also uses amines to separate the CO₂ from air although the approach uses giant fans to capture CO₂ from the atmosphere. This forms the basis of the current infrastructure in CCS. Despite some apparent advantages of both pre-combustion and oxyfuel capture, these methods are very unlikely to replace post-combustion capture on a global scale without major technological advances. However, all players agree that a combination of these technologies is necessary to effectively remove CO₂ from point sources and the atmosphere.

‘Carbon dioxide is being emitted at catastrophic levels for the environment. There has never been a more critical time to take decisive action against climate change than right now,’ says Tom White, CEO of C-Capture. ‘This is a complex problem created by over a century of unsustainable practices. We need a raft of measures to limit and then reverse the impacts of climate change. There is no one silver bullet.’

Carbon capture, utilisation and storage (CCUS) is a prime part of the UK Government’s plan for a ‘green’ industrial revolution. The Department for Business, Energy and Industrial Strategy (BEIS) is required to deliver the £1bn Net Zero Innovation Portfolio between April 2021 and March 2025. This requires CCUS technologies to be developed very rapidly indeed.

In a report in March 2022, the global coalition Energy Transitions Commission emphasised the role of CO₂ removal in meeting global climate objectives. The report describes how CO₂ removal, alongside rapid and deep global decarbonisation, can give the world a 50% chance of limiting global warming to 1.5°C.³

Carbon dioxide is being emitted at catastrophic levels for the environment. There has never been a more critical time to take decisive action against climate change than right now. This is a complex problem created by over a century of unsustainable practices. We need a raft of measures to limit and then reverse the impacts of climate change. There is no one silver bullet.
Tom White CEO of C-Capture

All the major energy companies, including Shell, Exxon, Chevron, Air Liquide, BP etc, are involved in CCS projects. There are also many smaller companies such as Climeworks, C-Capture, Decarbontek who are joining an already crowded field.

Climeworks Orca plant, Iceland

Shell Catalysts & Technologies focuses on two amine-based, carbon-capture technologies: CANSOLV – which is post-combustion – and ADIP ULTRA, which is pre-combustion. According to Shell, both are ‘robust and proven’. In 2021, Exxon Mobil initiated a new company Exxon Mobil Low Carbon Solutions to fulfil the Paris Agreement targets.4 In Houston, Texas, US, Exxon is one of a group of companies working together to decarbonise the city by 2050. Ironically, Houston is ideally suited to carbon storage because of extensive geological formations along the coast and under the seabed. These could hold 500bn t of CO₂.⁴

According to ExxonMobil: ‘The International Energy Agency’s Sustainable Development Scenario estimates the need for 70 to 100 new CCS facilities to be built annually by 2050, requiring between $655bn and $1.28tn in capital spending. That also means up to 100,000 construction jobs and up to 40,000 positions to operate that equipment.’⁵ In other words, CCS is becoming big business. In Houston alone 11 new CCS companies were announced in September 2021. This will require $100bn in new investment and will generate thousands of new jobs.

Air Liquide’s in-house process is called Cryocap FG to capture CO₂ using low temperatures to produce hydrogen. The process is a hybrid of pre-combustion and oxyfuel combustion. C-Capture was founded in 2009 by Chris Rayner, Professor of chemistry at the University of Leeds, UK, following initial investigations into the reaction of CO₂ with amines as it related to potential drug molecules. ‘The chemistry of CO₂ and amines is actually surprisingly complex, but methods were being used for CCS that originated nearly 100 years ago,’ says Rayner. ‘We thought there must be a better way to do it and had learnt a lot in our earlier studies. We also had a unique approach where we initially really focused on the chemistry, but then brought in our key engineering expertise to see what was really viable on scale.’

Although amines have a long history in energy production, Rayner and his team decided to look elsewhere for solvents that work at larger scales and are environmentally benign. As Rayner says, ‘it has to be very simple chemistry’. In collaboration with colleagues Douglas Barnes and Caspar Schoolderman they eventually came up with an amine free solvent that can strip CO₂ from the post-combustion stream. The properties of the new, but proprietary solvent mean that less heat is required to strip the CO₂. It is also less corrosive and is biodegradable.

‘Many recent CCS developments use tweaks of existing chemistry and then the engineering is built on top of that,’ says Rayner. In C-Capture’s system, the chemistry is modified first and then the engineering is developed to dovetail with the new chemistry. C-Capture is now scaling up its technology including through its XLR8 CCS project which uses a new class of capture solvents that are amine and nitrogen free. These can be manufactured cheaply on a large scale and are therefore suitable for C-Capture’s plans to upsize its technologies.

‘As part of our XLR8CCS project, we will deploy our unique solvent across a wider variety of applications where our technology can significantly reduce costs of capture, including glass, energy from waste and cement, demonstrating how the UK economy can be enabled to be decarbonised more quickly at lower cost,’ says White.

‘The XLR8 CCS project will see C-Capture’s unique, next-generation carbon capture technology deployed on sites across the country – within industries that are particularly difficult to decarbonise,’ adds Rayner. He lists Hanson Cement, Energy Works Hull, Glass Futures and Pilkington Glass as hosts for feasibility studies.

Swiss-based start-up Climeworks is already a long way down the road to the 2035 deadline mandated by both the UK and the EU. Climeworks, in conjunction with Icelandic carbon fixation company Carbfix, has opened a direct air capture pilot machine named Orca in Iceland. Climeworks is not divulging the name of its solvent but it is clear that carbon capture is the aim of the operation. Orca can remove 4000t/year of CO₂ and is the first industrial plant dedicated to direct air capture of carbon.⁶ The treated CO₂ is then passed to Carbfix, which converts the CO₂ into limestone by pumping it into the ground at depth and dissolving it in water then reacting it with minerals such as basalt. This mimics the process nature uses to store CO₂ but on a faster timescale.

Yet another start-up company, Ying’s Decarbontek is working on solid polymeric sorbent fibres for CO₂ capture and removal. The porous solid fibres are prepared by an innovative one step process using a low-cost commodity polymer and high molecular weight polyamines. The sorbent fibres are then packed into devices with uniform porosity. Gases containing CO₂ can flow through the devices with minimum pressure drop and CO₂ can be efficiently captured. Carbon capture using the sorbent fibres eliminates the energy cost for solvent pumping and the treatment cost for amine emission, and uses much lower energy for sorbent regeneration, compared with a standard amine process.

A start-up with a different emphasis is Perennial – formerly Cloud Agronomics – based in Colorado, US. It uses multi-spectral imagery and computer algorithms to identify areas where soil is undersaturated with respect to carbon. Analysing the spectrum of reflected light allows an estimate of the amount of carbon in the soil. When such areas have been identified, farmers or foresters working these areas can then plant vegetation with a higher potential to remove CO₂. Perennial describes this as carbon measurement, reporting and verification.

Planting vegetation reduces atmospheric CO₂. Around 3.6bn t of CO₂ – about 10% of current CO₂ emissions – could be saved every year, during the growth phase of forests. The trouble is this would require a land mass equivalent to the entire US. Another way of doing the same thing is to increase the soil’s store of humus by using cash crops and plants with deep roots, as well as working harvest remnants back into the ground and avoiding deep ploughing. According to a study by the German Institute for International and Security affairs, targeted extraction of CO₂ into humus could save between 2 and 5bn t of CO₂ globally.⁷

Burning plant tissue returns CO₂ to the atmosphere but this CO₂ can be captured using post-combustion capture in a process referred to as BECCS⁸, which represents an opportunity to remove the gas from the atmosphere, similar to DAC. The proportion of CO₂ in the air has increased from 288ppm to 414ppm since 1850. Since that time global temperatures have increased by 1.1°C. Hence the combustion of the newly grown vegetation needs to be carefully monitored.

CO₂ capture

In post-combustion CO₂ capture, flue gas is passed to an amine-based scrubber to remove the CO₂. Monoethanolamine is the most commonly used amine, however, other amine solvents such as 2-aminomethylpropanol/piperazine will result in lower costs when brought into widespread use. ⁹

‘As a result of [its weak acidity], CO₂ can form a compound with the weak bases; amines. Pure CO₂ can be released [from the amines] upon mild heat, regenerating amines, which can be reused for CO₂ capture. It is important that the CO₂ needs to be in contact with the amine functional groups to be able to form a compound,’ says Decarbontek’s Yong Ding. ‘Therefore, the contacting device for CO₂ and amine is essential. For the liquid amine process (primarily for point of source CO₂ capture), a gas is flowing from the bottom of the contacting tower, and liquid amine is showering from the top to create efficient contact between the amine and CO₂.’

Using liquid amine has drawbacks, Ding says: ‘[These are] high viscosity and regeneration energy and low solvent loss, water loss, and corrosion to the devices.’

Amines can be solids, however, with high surface areas for increased CO₂ contact. Once again, the CO₂ can be released by a mild heating and the sorbent is regenerated. There are several advantages for solid amine-based processes. There is no emission issue, no water loss and lower temperatures can be used. The regeneration energy is lower due to the lower heating capacity of the solid.

CO₂ can also form much more stable compounds such as calcium carbonate and sodium carbonate. These compounds need extremely high temperatures to decompose, making them suitable for long term storage.

References
1 ukccsrc.ac.uk/wp-content/uploads/2022/07/BEIS-Next-Gen-Industry-Engagement-Workshop-3-2022-06-23.pdf
2 carbonbrief.org/around-the-world-in-22-carbon-capture-projects/
3 energy-transitions.org/publications/mind-the-gap-cdr/
4 corporate.exxonmobil.com/Climate-solutions/CCS-in-action
5 energyfactor.exxonmobil.com/reducing-emissions/carbon-capture-and-storage/ccs-solution-economic-benefits/
6 azocleantech.com/article.aspx?ArticleID=1322
7 swp-berlin.org/publikation/eu-climate-policy-unconventional-mitigation#hd-d17590e978
8 earth.org/data_visualization/geoengineering-what-is-beccs/
9 D. Danaci et al, Environ. Sci. Technol., 2021, 55, 10619.