Carbon capture and storage (CCS, Carbon Capture and Storage) is rapidly gaining enormous importance in Germany, the European Union and around the world as a significant part of climate solutions, seeming to offer a convenient answer to the climate crisis – CO₂ is captured from the use of fossil fuels in power stations and factories and stored underground. Even so, many applications of this technology remain contentious, its costs are high, the technology is not sufficiently developed, and there is a risk that attention will be diverted away from immediate emissions reductions. The questions and answers section addresses the most frequently asked questions about CCS on the basis of scientific studies and findings and the scientific consensus.

1. What is carbon capture and storage (CCS)?

In carbon dioxide capture and storage (CCS) CO₂ is captured from exhaust gases and then stored “for ever”. Two basic principles must be distinguished: whether the carbon dioxide is captured at large industrial plants, or whether the CO₂ is filtered out of the air regardless of where it originated (Erlach et al. 2022).

The classic CCS method involves capturing emissions before they reach the atmosphere. In this case, using chemical processes, the CO₂ is captured before, during or after the relevant combustion or production processes.

The captured CO₂ is transported by pipeline, road or ship and then placed in long-term storage sites, for example underground or beneath the sea. Depleted oil or gas reservoirs, or saline aquifers (layers of porous rock containing salt water), are suitable for this purpose. In the USA and Canada, compressed CO₂ is also injected into almost exhausted oil and gas fields in order to extract the remaining hydrocarbons, a method known as “enhanced oil recovery”.

In the geological deposits of the Earth's crust, carbon dioxide is retained in various ways: sometimes it is dissolved in underground salt water, “trapped” in larger or smaller cavities, or forms tight bonds with certain minerals in the rock. The aim is always to keep the CO₂ out of the atmosphere for as long as possible.

In expert discussions of climate research and (international) climate policy, CCS technology falls under the general term “carbon management”. These fields must be clearly distinguished, because they have different applications and may have different significance for climate policy and climate protection.

Infographic from: SWP-Aktuell 2023/A 30

Conventional CCS technology, which captures carbon dioxide from burnt fossil resources, is one way of reducing emissions and can slow the rise in the CO₂ concentration in the atmosphere. The second element of carbon management is “carbon capture and utilisation (CCU; Carbon Capture and Utilization). Here too the CO₂ is captured (mainly) from fossil sources and then used as a material. However, depending on the product, the carbon dioxide is usually sequestered only temporarily.

The third area of carbon management is “carbon dioxide removal” (CDR, Carbon dioxide removal). This sector covers various options for “removing” carbon dioxide from the atmosphere – in short, for reducing the CO₂ concentration in the atmosphere. On the one hand there are nature-based solutions such as afforestation or the restoration of peatlands; on the other hand there are two specifically technological CCS solutions – the carbon dioxide is captured from the combustion of biomass (BECCS, Bioenergy with Carbon Capture and Storage) or taken directly from the air (DACCS, Direct Air Carbon Capture and Sequestration) and then injected underground.

2. What is BECCS?

There is a form of CCS known as bioenergy with capture and storage (BECCS). In this process, power stations do not capture carbon dioxide from the combustion of fossil fuels such as coal or natural gas, but from the combustion of biomass, for example wood chips. During combustion, the released CO₂ is absorbed from the surrounding air. By storing this carbon dioxide permanently, BECCS would produce a net removal of the gas from the atmosphere, which would not only slow global warming (as conventional CCS does) but could halt it in the long term.

Researchers agree: to achieve the limits on global warming of 2 °C, and 1.5 °C in particular, set out in the Paris Climate Agreement, CO₂ emissions in the atmosphere must be reduced to a certain level. Even with stringent climate protection, a certain share of emissions will be very difficult or impossible to reduce at the source itself (in agriculture, for example). Using BECCS or DACCS would be a promising way of offsetting these emissions and ultimately achieving a “net zero” balance, in which the volume of greenhouse gas emissions equals the volume “taken out” of the atmosphere.

3. What is DACCS?

In addition to conventional CCS, direct air capture and storage (DACCS) is currently at the research and development stage; here the carbon dioxide is not captured at power stations or industrial sites but is separated from the air using direct air capture (DAC, Direct Air Capture). This happens regardless of where, how and when it (CO₂) entered the atmosphere. As with CCS, the captured carbon dioxide is compressed, transported and stored in geological reservoirs (Latvia also has such a possibility, for example in Dobele district, where there is a suitable geological formation appropriate for this purpose).

DACCS could be used to capture from the atmosphere emissions from a large number of relatively small, individual sources, such as transport or agriculture. Like BECCS, DACCS technology is considered “carbon neutral”, because its use produces overall net negative emissions.

4. What is carbon capture and utilisation (CCU)?

As an alternative to storage, the captured CO₂ can also be used in industrial processes, for example in the chemical industry, to produce fertilisers, plastics or synthetic fuels (also known as “e-fuels”). This technology is called carbon capture and utilisation (CCU, Carbon Capture and Utilization). At present the potential for using this material is still small. In its Sixth Assessment Report (IPCC 2022, AR6, Volume 3, Chapter 6.4.2.5), the IPCC (Intergovernmental Panel on Climate Change) estimates it at 1-2 billion tonnes a year; however, according to the IPCC, its potential could rise to 20 billion tonnes a year by the middle of the century – in other words, very considerably. To achieve that, though, extremely rapid technological development would still be required.

Some CCU options require a great deal of energy, which is likely to limit their possible application. In many cases the products made in this way do not store the CO₂ permanently. With synthetic fuels, for example, the CO₂ is released quickly and usually enters the atmosphere after only a few days or months. When CO₂ is processed into plastics, the binding process lasts longer. If this plastic is burnt in waste incineration plants, the carbon dioxide re-enters the atmosphere. It should be noted that material use sometimes requires the CO₂ to be of very high purity or at very high pressure. Fertiliser production, for example, requires a pressure of 122 bar and 99.9% purity, which overall is a very expensive process.

5. How much CO₂ can CCS capture?

So far, very little carbon dioxide has been stored worldwide. According to the Massachusetts Institute of Technology (MIT) climate portal, in 2023 it was roughly 0.045 billion tonnes a year, which corresponds to the annual emissions of about ten million cars. For comparison: according to the Global Carbon Budget report, human activity produced 40.6 billion tonnes of CO₂ in 2022. It should be stressed that 36.6 billion tonnes came from the burning of fossil fuels, which means that only about one thousandth of the carbon dioxide released each year is currently stored using conventional CCS technology. Worse still, a large share of that quantity is currently tied to enhanced oil recovery processes, which on the one hand generate additional emissions and on the other will no longer be needed once oil and gas production is phased out.

CCS technology has so far been used at only a few sites in a handful of countries. The IPCC's Sixth Assessment Report notes that 28 plants would be operating commercially worldwide in 2022. In Norway, CCS has been operating on a smaller scale for more than 25 years and the CO₂ is stored beneath the North Sea. Denmark has just commissioned its first plant and plans to store larger quantities of CO₂. In the USA, the only plants operated to date are ones that use CO₂ to force the remaining oil and gas out of their fields. The world's only working coal-fired power station with CCS technology is in Saskatchewan, Canada.

Lists of CCS facilities compiled by various sources are available, some of them industry-related, others from critically minded non-governmental organisations or the media. The number of plants listed and the assessments given differ considerably:

• Global CCS Institute facilities database
• The Zero Emissions Platform list of European CCS and CCU projects Zero Emissions Platform
• A list of the twelve largest CCS projects in the world in the online magazine DeSmog

The potential of CCS is considerable. However, according to the IPCC's Sixth Assessment Report of 2021/22, the introduction and development of this technology has in the past taken far more time than was previously assumed.

Germany's academies of sciences reached a very similar conclusion in a joint document: “The individual stages of the process – CO₂ capture, CO₂ transport and underground CO₂ storage (CCS) – are in principle ready for use on an industrial scale. Yet the development of CCS technology and its introduction to the market has proceeded much more slowly in recent years than was forecast five to ten years ago.” (Erlach et al. 2022)

In principle this technology is expensive and energy-intensive, and storage sites have to be inspected and maintained regularly, possibly for centuries. It should be borne in mind that plants with CCS are not entirely free of CO₂ either. Because the technology increases total energy consumption, power stations that capture carbon dioxide initially produce more CO₂. The IPCC's 2005 special report on the subject states that CO₂ emissions will rise by 10-40% (SRCCS, summary for policymakers). With the current state of the technology, about 90% of these emissions can be captured (Dods et al. 2021); in practice, according to media reports, often less. Even if only ten per cent of the CO₂ emissions remain, from coal-fired power stations for instance, that is still an enormous quantity, and climate neutrality cannot be achieved this way. Research is currently under way to increase efficiency. But that would in all likelihood make the technology more expensive still. For the remaining 2% of CO₂ the costs rise sharply (Brandl et al. 2021).

6. How much CO₂ can be cleaned out of the air with BECCS or DACCS?

Only BECCS and DACCS technologies offer a way of permanently removing from the atmosphere carbon dioxide that has already been released in the past or elsewhere, thereby reducing the CO₂ content of the atmosphere (and hence the Earth's temperature). The report “The State of Carbon Dioxide Removal” gives an overview of the state of carbon removal from the atmosphere; four of its lead authors have also contributed to IPCC reports. The report notes that in the case of BECCS, storage could increase from the present 0.02 billion tonnes of CO₂ a year to 0.03-0.2 billion tonnes by 2030. In the case of direct air capture and carbon storage (DACCS), the total could rise from the present less than 0.00001 billion tonnes to as much as 0.3 billion tonnes a year over the same period. Yet even these increased volumes are relatively small compared with the roughly 40 billion tonnes of CO₂ currently emitted every year.

In its Sixth Assessment Report for 2021/2022, the IPCC cites considerably higher figures. In the long term the potential of DACCS alone is 5 to 40 billion tonnes of CO₂ a year, which means that in the long run a considerable share of emissions could be removed from the atmosphere through CO₂ “removal”. However, according to the IPCC this potential is “limited mainly by requirements for low-carbon energy and by costs”.

7. How much underground CO₂ storage capacity can realistically be used?

According to IPCC figures, roughly 10,000 billion tonnes of CO₂ could in theory be stored in geological structures worldwide (IPCC 2022, AR6, WG3, Chapter 6.4.2.5), which corresponds to roughly 250 years of global emissions.

Most (80%) of the theoretical storage capacity is in so-called “saline aquifers” – porous layers of rock containing salt. In principle, depleted oil and gas deposits are also suitable for storage. In that case the existing oil and gas production infrastructure could be used to store the CO₂.

However, not all reservoirs in which CO₂ could be stored are usable in practice. For instance, the pressure in an otherwise suitable formation may be too high for carbon dioxide to be injected. Potential storage sites may be too far from production sites, or reachable only at very high additional cost. Even so, the storage potential is very large. The IPCC writes about this in the 2022 report of Working Group 3: “

“Not all storage capacity is usable, because geological and engineering factors limit the actual storage capacity to a figure below the theoretical potential, which nevertheless still exceeds the CO₂ storage requirements through to 2100 for limiting temperature change to 1.5 °C” (IPCC 2022, AR6, WG3, Chapter 6.4.2.5).

Realistically, according to the IPCC, about 1,000 billion tonnes of CO₂ could be stored worldwide. However, storage capacity varies from region to region. In Germany, for example, depleted natural gas deposits and deep saline aquifers are particularly suitable for storage. A few years ago two researchers at the Federal Institute for Geosciences and Natural Resources concluded that in theory roughly 20 to 115 billion tonnes could be stored in Germany's saline aquifers alone; that means up to 109 billion tonnes of CO₂ could be stored in northern Germany alone. For comparison, according to the German Federal Environment Agency, roughly 0.66 billion tonnes of CO₂ were emitted in Germany in 2022. In the CDRmare project, several German marine research institutes are currently investigating the possibility of storing carbon dioxide in geological formations beneath the North Sea. According to the initial publications, the potential runs to several billion tonnes. By the end of the century, about 30 million tonnes of CO₂ a year could be stored there.

A study carried out in Austria found that up to 0.12 billion tonnes of the gas could be stored (Welkenhuysen et al. 2016), compared with the Austrian Federal Environment Agency's figures for https://www.google.com/url of 0.08 billion tonnes in 2022. In 2010 a study by the Swiss Federal Office of Energy put Switzerland's capacity at about 2.6 billion tonnes of CO₂ (Diamond et al. 2010), while figures from the Federal Environment Agency show annual emissions of about 0.045 billion tonnes.

The methods used to calculate capacity differ considerably, so the values given can be taken only as indicative.

8. How expensive is this technology?

To date, all CCS technologies have been very expensive. According to IPCC figures, capturing carbon dioxide at the place where it is released currently costs more than 50 US dollars per tonne of CO₂. According to the IPCC, the investment costs for a coal or gas power station with CCS are almost twice as high as they would be without CCS; operating costs also rise and the power station's efficiency falls, because capturing carbon dioxide requires a great deal of energy.

If storage sites are a long way from the sources of emissions, considerable costs arise in transporting the gas to the storage site, to say nothing of the costs of the long-term maintenance and safety of the storage site. The German research consortium CDRmare speaks of total costs of 150-250 euros per tonne of CO₂ injected into the North Sea.

DACCS, which not only reduces CO₂ emissions but also removes greenhouse gases from the atmosphere, is more expensive still. The IPCC report estimates the costs (up to the middle of the century) at between 100 and 300 dollars per tonne of CO₂. The report on the state of carbon dioxide removal cites prices of 780 dollars in 2020 and 1,200 dollars in 2021 for the existing pilot plants that are actually in operation.

The German Academy of Sciences sets out the reasons for the high costs plainly. CO₂ makes up only a very small proportion of the air (just 0.04% by volume). To produce one cubic metre of CO₂, at least 2,500 cubic metres of air must be “filtered”. For one tonne of CO₂ that corresponds to roughly 1.27 million cubic metres of air, even if a hundred per cent filter efficiency is achieved.” (Erlach et al. 2022)

Prices are likely to keep falling as DAC technologies develop and are rolled out; it is unclear, however, whether the cost of generating electricity at coal or gas power stations with CCS will be able to compete in the medium or long term with the cost of generating electricity from solar or wind power. These forms of energy generation are already considerably cheaper, and their costs are likely to keep falling. In other words, CCS will probably widen the existing price gap between renewable energy sources, which are generally cheaper, and fossil-fuel power stations, which are on average more expensive.

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