Capture, use and store carbon: CCUS and CDR technologies

Climate change is mainly due to carbon dioxide emissions. To solve the problem, wouldn’t it be enough to capture this excess carbon and store it? This is the idea behind one of the great themes of ecological innovation: carbon capture and use or storage. In this dossier, we present the different technologies and their benefits:

• Part 1: Carbon capture techniques
• Part 2: Carbon storage
• Part 3: Carbon uses
• Part 4: Global approaches
• Part 5: CCUS deployment

Note a point of vocabulary:“CCUS” refers to the process at the end of a factory or power plant stack (= preventing CO2 emissions), whereas CDR (Carbon dioxide removal) involves removing carbon already present in the atmosphere. Nevertheless, for the sake of demonstration, I’m going to lump both together under the term “CCUS”, as the term “capture” is broad enough to cover both approaches.

Part 1: Carbon capture techniques

Carbon capture techniques can be divided into 3 main groups: organic capture (photosynthesis), capture at plant or power station chimney outlets, and direct air capture (DAC).

Organic carbon capture

This is the most common form of carbon capture: photosynthesis. Green plants capture sunlight, water and CO2 from the air and convert them into glucose for the plant. This is the domain of agriculture, particularly forestry and soil conservation agriculture, and of innovative forms of cultivation (microalgae in particular) and green chemistry.

Classic” carbon capture: from the chimney stack

When we speak of carbon capture or CCUS, we are often referring to “stack” technology: carbon capture at the end of a factory or power plant. This is what would enable the combustion of fossil fuels to be made low-carbon, as with “blue hydrogen” (= produced by gasification or steam reforming CCUS) for example. Traditional technology is called“post-combustion” carbon capture. It is common practice in certain industries, to limit the amount of CO2 released when necessary.

Other technologies, also post-combustion, are being developed, such asoxycombustion, in which combustion takes place in the presence of pure oxygen, producing smoke that is much more concentrated in CO2 (there is no nitrogen), which facilitates filtering.

Some people refer to “pre-combustion” carbon capture, in which the fuel is transformed into synthesis gas by gasification, then the carbon is recovered, leaving only the hydrogen. However, this is neither more nor less than hydrogen production, so I don’t classify it as one of the present technologies, especially as it doesn’t seem viable.

Note that this technology can also enable a carbon removal technology (Carbon Dioxide Removal): BECCS (Bioenergy with Carbon Dioxide Capture and Storage). This involves capturing and storing CO2 from biomass combustion. The carbon is first stored by the biomass (usually the tree), then captured during combustion. This is a “carbon-negative” operation.

Direct Air Capture (DAC)

A popular solution is to extract carbon from the ambient air. This is known as DAC (Direct Air Capture). The process is much less efficient (the price is several times higher per ton captured) than end-of-pipe capture, but it is also more flexible, as it can be located close to an installation using the captured CO2 or capable of storing it. It also makes it possible to offset carbon emissions that are too difficult to avoid.

Part 2: Carbon storage

Once the carbon has been captured, it needs to be stored. There are several ways of doing this. The best known is storage in gaseous form, notably in underground cavities. However, there are other methods: storage in solids, for example in cement or trees; storage in the ocean by precipitation; and storage in soils.

Gaseous storage

The main problem with gas is that it takes up a lot of space and escapes easily. One interesting avenue is geological storage. There are three types:

• Salt caverns
• Hydrocarbon reserves
• Aquifers

Of course, in the case of temporary storage (e.g. while awaiting transport to be used or stored elsewhere), conventional reservoirs will have to be used.

Storage in solids

Carbon is one of the fundamental elements of life, and is present in many materials. The idea is to store carbon in these solids. This is obviously the logic behind forestry (carbon is locked up in wood), but also for crops in general (even if it’s generally a more ephemeral logic). Above all, there are more and more methods for locking CO2 into materials during industrial processes, such as cement design.

Storage in water

The oceans store ten times more carbon than the atmosphere, and absorb a quarter of human emissions every year. This is achieved through two natural processes: the solubilized carbon cycle and the biological cycle. The idea of ocean CO2 storage is to take advantage of these mechanisms to capture more CO2 through the ocean. Innovative methods are emerging, such as ocean fertilization, underwater geological storage,alkalinization, deep CO2 injection or the use of carboglasses. However, these strategies are not without their complexities. They involve potential environmental risks, variable storage efficiencies and significant management challenges. What’s more, their implementation is fraught with logistical and financial challenges.

Carbon storage in soils

Agriculture can store carbon in soils. This is one of the pillars of soil conservation agriculture and the 4 for 1000 initiative.

Part 3: Carbon uses

in order to reach net zero CO2 emissions for the carbon needed in society (e.g., plastics, wood, aviation fuels, solvents, etc.), it is important to close the use loops for carbon and carbon dioxide through increased circularity with mechanical and chemical recycling, more efficient use of biomass feedstock with addition of low-GHG hydrogen to increase product yields (e.g., for biomethane and methanol), and potentially direct air capture of CO2 as a new carbon source”

IPCC, vol.III, Technical Summary, p.106 : https://www.ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_TechnicalSummary.pdf

Storing carbon is a burden: couldn’t we make a profit by using it as a resource? This is the avenue being explored by new uses for wood, in construction, green chemistry and other new industrial processes. Already today, around 230 Mt of CO2 are used every year, essentially for urea production (125 Mt/year) and for the EOR of oil&gas (70 – 80Mt). There are also other uses, but of little significance, notably in food, cooling, water treatment and greenhouses (IEA 2020)

Green chemistry

CO2 can be used in the production of several chemical products:

• Urea. This technology is already mature. (TRL 11)
• Cement. CO2 can be used to replace water in concrete design (known as “CO2 curing”) and as a base material in cement. These technologies are currently being adopted. (TRL 9-10)
• Methanol. This technology is currently being demonstrated. (TRL 7-8)
• Synthetic methane (e-methane) This technology is currently being demonstrated. (TRL 7-8)
• Synthetic liquid fuels (e-fuels): This technology is currently being prototyped (TRL 5-6).

Already today, the George Olah plant in Iceland converts 5,600 tCO2 and green hydrogen into methanol every year.

Developing wood construction

Construction is currently one of the main emitters of greenhouse gases, not least because of the use of cement, a fantastic tool whose design emits enormous amounts of CO2.

There are other uses for wood, such as for toys, but they seem to me infinitesimal and not very viable (they don’t effectively replace their alternatives).

Biomass energy

When we talk about CCUS, we’re forced to talk about the use of biomass for energy, such as wood or e-methane. According to the IAE, over 90% of the carbon destined for use (10% of the total, the rest being stored) will be used to make synthetic fuels, notably for aircraft. Nevertheless, this moves us away from the logic of carbon storage towards that of renewable energy. I refer you to the dedicated pages:[biofuels][wood-energy].

New industrial processes incorporating carbon

We can distinguish the use of carbon from its storage in materials by looking at whether the use of carbon brings real added value comparable to its cost. The best example of carbon use is wood construction … in general. Striving to make a building out of wood, whatever the cost, may be more in keeping with the logic of storage.

Among the examples of industrial processes using CO2, Covestro in Germany produces 5000t of polymer per year in Dormagen, in which CO2 is used.

Part 4: Global approaches

I’ve outlined the various aspects of CCUS. In practice, however, we’re going to distinguish between different types of approach:

Part 5: CCUS deployment

In addition to forestry, post-combustion CCUS has been practised for several decades, notably in the oil&gas industry for EOR (Enhanced Oil Recovery), which consists of sending CO2 into oil wells to extract as much as possible. A large proportion of the CO2 then remains inside.

Note that in 2009, the IEA had already drawn up a roadmap proposing to reach a storage capacity of 300 MtCO2 per year by 2020. However, it was only 40Mt at the time. This can be explained by the lack of consistency in public policies and, above all, the absence of a sufficient price on CO2 emissions. (IEA 2020)

The “classic” CCUS, post-combustion: part of oil&gas

Conventional” CCUS, i.e. post-combustion, using fumes from industrial processes as they leave the plant, has been used for several decades in combination with oil&gas. CO2 is injected into the hydrocarbon reservoir to push as much oil as possible to the surface: this is known as EOR, Enhances Oil Recovery. Most of the CO2 injected in this way remains trapped in the reservoir (to be verified).

This technology could make fossil-fuel power plants less carbon-intensive. Many of these plants were built recently, and decommissioning them represents a considerable additional cost. Adding CCUS to them would limit their impact at a more reasonable cost. It would also make it possible to reduce emissions from certain industries, such as cement, which are very difficult to decarbonize.

The birth of CCUS: DAC

Direct carbon capture is increasingly popular, gaining significant traction from 2019 onwards.

• IEA (2020), CCUS in Clean Energy Transitions, IEA, Paris https://www.iea.org/reports/ccus-in-clean-energy-transitions, License: CC BY 4.0