Carbon capture and storage (CCS) is a climate technology with great potential to manage the increasing concentration of greenhouse gases in the atmosphere. CCS works by capturing CO₂ from flue gases and storing it in the subsoil. In Denmark, CCS will be used to reduce atmospheric CO₂ emissions and to remove some of the CO₂ already emitted to the atmosphere. 

The method for capturing and separating CO₂ from flue gas is based on several well-known technologies. However, there is also a need to improve existing technologies in order to reduce energy requirements.

The most mature and widespread technology for CO₂ capture is washing CO₂ out of flue gas. The washing process can be with an amine that absorbs CO₂ on contact. The technology makes it possible to capture 90 percent (or more) of the CO₂ emitted. A number of other CO₂ capture technologies are also being developed, and you can read more about these in the Danish Energy Agency's technology catalogue on CCS. 

CCS technology has been in use since the 1970s. For example, since 1996 CO₂ capture and storage has been used in Norway to reduce CO₂ emissions from gas extraction in the North Sea. According to the Global CCS Institute's 2022 status report, there are currently 30 CCS plants in operation worldwide, 11 plants under construction and 153 plants under development. 

What is the business case for storing CO₂? 

Companies covered by the EU ETS (“Emission Trading System”) and/or covered by the forthcoming, Danish CO₂ tax can avoid these taxes by capturing and storing CO₂ instead of emitting it into the atmosphere. 

Companies covered by the EU ETS must report their greenhouse gas emissions to the Danish Energy Agency by 31 March each year. 

Companies emitting biogenic CO₂ are not covered by the EU allowances scheme. Emitters of biogenic CO₂ can instead sell climate credits to cover their carbon capture and storage costs, whereby other companies can offset their climate footprint by purchasing such credits. 

In total, allowances and taxes saved from CO₂ capture and storage, together with the possible sale of climate credits, make up the revenue streams from CCS.

Industry

With the help of CCS, CO₂ emissions can be brought down for industrial processes that rely on the combustion of fossil fuel, and so-called “process-related emissions” can also be reduced. Process-related emissions occur when limestone is burnt to make cement, for example. 

Waste incineration plants

Another good application for CCS is waste incineration plants. Despite ambitions to increase waste recycling in the future, waste disposal by incineration is expected to continue and this will emit CO₂ into the atmosphere if it is not captured. 

Combined heat and power plants (CHP) and biogas plants

Biomass-fired CHP and biogas plants are other potential sources of CO₂ where it can make sense to use CCS. CCS can be used to capture the biogenic CO₂ emitted when biomass – such as wood, cardboard and paper – is burned for energy production or when raw biogas is upgraded to natural gas quality. 

BECCS

BECCS stands for “Bioenergy CCS”. Sustainable biomass is considered to be CO₂-neutral in its long-term use because plants and trees used as biomass absorb CO₂ from the atmosphere during growth. Normally, CO₂ is released back into the atmosphere when the biomass is burned or the plants rot. When BECCS is used, however, this CO₂ is captured and stored so that the CO₂ absorbed by the biomass during its growth is not emitted into the atmosphere. This leads to negative emissions.

Direct air capture

It is possible to capture CO₂ directly from the atmosphere. This is called direct air capture (DAC), and combined with permanent storage in the subsoil, it could eventually become an important technology for achieving substantial negative emissions. At present, the technology is energy-intensive and expensive, but it holds great potential for reducing the CO₂ content in the atmosphere in the future.

Utilization of CO₂ (CCU)

Capturing CO₂ for utilization is called carbon capture and utilization (CCU). Captured CO₂ can be used in the production of plastic materials, carbonaceous PtX fuels such as e-methanol and jet fuel, and e-methanisation. CCU is a common term for the capture of CO₂ for either geological storage or utlization.

The potential for CCS in Denmark

The Climate Agreement for Energy, Industry, etc. of 20 June 2020 established that CCS is to play an important part in meeting Denmark's climate targets. This policy is in line with the UN Intergovernmental Panel on Climate Change (IPCC) and the International Energy Agency (IEA), both of which consider CCS an indispensable technology to achieve the Paris Agreement objectives. 

Capture potential in Denmark 

The potential for CO₂ capture is difficult to predict and among other things it depends on developments in the industrial sector and in the overall energy sector. The Danish Energy Agency estimates, although with some uncertainty, that the capture potential in 2040 from Danish point sources (a point source is a defined location/specific plant from which CO₂ is emitted) amounts to approximately 5.4-10.8 million tonnes per year, of which between 3.5-6 million tonnes stems from point sources with biogenic CO₂ emissions. Most of the potential (up to about 6.5 million tonnes of CO₂) in 2040 comes from point sources concentrated in five clusters around Copenhagen, Aarhus and Aalborg, and in southern Jutland. 

Danish Energy Agency report “Point sources of CO₂ – potentials for CCS and CCU (2022 update)”

Map: Estimated annual point source emissions in 2040

Map of estimated annual point-source emissions in 2040 showing the geographic location of the CO₂ point sources in Denmark as well as their relative size. They are also categorized based on the following sectors: waste management, biogas, district heating and industry.  

The point sources included emit CO₂ from fossil sources, biogenic sources and industrial processes.  

To store CO₂ safely and permanently, it must be injected into suitable geological subsoil structures, typically located 800-3,000 metres below the earth's surface. CO₂ storage be both onshore and offshore. The National Geological Survey of Denmark and Greenland (GEUS) estimates that the total storage potential in the Danish subsoil is between 12 and 22 billion tonnes of CO₂. This is between 400-700 times greater than Denmark's total annual CO₂ emissions at current levels. 

In Denmark, the CO₂ will be primarily stored in porous and permeable sandstone layers, covered by one or more clay layers, acting as a seal. Once injected, the CO₂ will penetrate the small cavities of the sandstone, and in combination with the overlying clay layer this will ensure that the CO₂ does not escape from the storage. The establishment of storage facilities in Denmark will entail strong security requirements, including comprehensive monitoring programmes, so that any spills of CO₂ can be detected and remedial action taken swiftly and effectively.

Full-scale CO₂ storage does not yet exist in Denmark, but in 2022 the Danish Energy Agency issued the first permit for a pilot and demonstration project on CO₂ storage in Denmark. 

Read more about the geological aspects of CO₂ storage on the GEUS website.

Overview map of point sources and potential storage structures in Denmark 

Web-accessible alternative to overview map of point sources and potential storage structures in Denmark 

The map shows the largest Danish point sources emitting CO₂, as well as geological structures with potential for CO₂ storage. The relative biogenic/fossil share of the point source's total CO₂ emissions is shown for each point source. The largest point sources in Jutland are Reno Nord, Aalborg Portland and Studstrupværket. The largest point sources in Zealand are Ørstedværket and Equinor Refining. The geological structures with a potential for CO₂ storage are divided into existing oil and gas fields and surveyed / non-surveyed areas. Most areas with potential for CO₂ storage are located in the northern North Sea, in central and north Jutland, and in the sea south of Lolland-Falster, Funen and Als. The map reflects data from 2018.

Graphic illustration: the interplay of the CCS value chain

Illustration: GEUS

Web-accessible alternative for graphical illustration of CCS value chain interactions
1. The first stage of the value chain is typically XXX. Here, CO₂ is captured by filtering the flue gas from CO₂ sources such as industry or energy production.

2. In the second stage of the value chain, the gas is compressed and transported via a pipeline or ship to a suitable reservoir (storage).

3. In the third stage of the value chain, the CO₂ is pumped into the reservoir (storage) until it is full.  

CO₂ is primarily captured in two ways: via chemical cleaning of flue gases stemming from combustion or incineration and by biogas upgrading.

CO2 capture from flue gas 

Flue gases have a very high CO₂ concentration compared to the atmosphere, which makes it easier to capture CO₂ here rather than directly from the air. The CO₂ is captured by treating flue gas with a chemical liquid called an amine. This takes place in a kind of filter called a scrubber, which is why the process is called an amine scrubber. 

Figure: Process for capturing CO2 from flue gas

Web-accessible alternative to the figure: Process for capturing CO₂ from flue gas

The process of capturing CO₂ from flue gas is in two steps.

First, the flue gas is captured from an exhaust stack and transferred to the absorption tower where the CO₂ content of the flue gas is washed with a chemical amine liquid that can bind the CO₂ to itself. The amine liquid is then heated, releasing the CO₂ again and allowing it to be captured. The amine liquid can be cooled and reused, and the flue gas can be returned to the exhaust stack and discharged in the normal way.

1. Absorption: The flue gas is about 40 degrees Celsius when entering the absorption tower. It is led into the tower from the bottom, so that it rises upwards. At the same time, liquid is poured over a large grid through which the gas passes. The CO₂ dissolves in the liquid, purifying the gas, while the CO₂ is captured in the amine liquid.

2. Release: To release the CO₂ from the amine liquid, it must be heated to about 120 degrees Celsius. When this happens, CO₂ is released from the liquid and can be collected as a gas. The liquid can be used again, but this requires cooling. The constant heating and cooling is energy-intensive and produces excess heat. The process is continuously being developed and improved to reduce consumption.
The technology makes it possible to capture 90 percent or more of the CO₂ content in the flue gas. 

CO₂ capture from biogas 

When using biogas from biomass, the raw biogas, typically consisting of 60-65% methane (CH₄) and 35-40% CO₂, is first upgraded to natural gas quality. The upgrading is done by removing excess CO₂ from the biogas, so that the remaining methane (CH₄) can be used in the natural gas grid. Today, there are five upgrading technologies available. Some are less mature than others. These are:

• Amine scrubbing
• Water scrubbing
• Pressure swing adsorption (PSA)
• Membrane separation
• Organic physical scrubbing

In water scrubbing, the cleaning process is physical rather than chemical. The biogas is brought into contact with water, which is either sprayed or bubbled through the gas. Since CO₂ (like sulphur gases) is more soluble in water than methane, water will capture the CO₂. This physical process requires a relatively high pressure. 

Figure: Capture of CO₂ from biogas

Web-accessible alternative to the figure: Capture of CO₂ from biogas

The figure shows the production of biogas and the upgrading of biogas to natural gas quality. Biogas is produced during the oxygen-free decay of biomass such as manure, wastewater and organic residues and waste products. This process takes place in a large tank. The biogas is then transferred to upgrading plants. Upgrading is by removing the CO₂ from the biogas, leaving pure methane (CH4) that can be used in the natural gas grid. The excess CO₂ from this process can potentially be captured and stored underground. 

After capture, the CO₂ needs to be transported to a storage site. If the CO₂ is to be transported to a storage site by truck and/or ship, it is cooled and pressurized after capture. This means that the CO₂ becomes liquid and takes up significantly less space than in its gaseous form, allowing it to be transported easily and efficiently from the capture source to the storage or use site. CO₂ can also be transported in gaseous form through pipelines, in the same way as other gases are already transported.

Several websites and reports are available with more information about CCS technology, the technical, safety and environmental aspects of CCS, and the role it can play in meeting the Paris Agreement targets.

International Energy Agency (IEA) website on carbon capture, use and storage

UN Panel on Climate Change report on CCS (2005)

Many Danish and foreign research institutions are also showing a considerable interest in CCS, and the University of Edinburgh has put together an online course with a series of virtual teaching sessions on CCS. Please note that the Danish Energy Agency is not responsible for the course content or affiliated with the university in any way.  

Online course: Climate Change: Carbon Capture and Storage

The Danish Energy Agency also regularly publishes analyses that examine various aspects of CCS technology and detail the adoption of CCS in Denmark. The Danish Energy Agency's technology catalogue on CCS describes a number of different solutions for capturing, transporting and storing CO₂. It includes various carbon capture technologies for emissions in the power and district heating sectors as well as industry, carbon sequestration from the air, and various infrastructure solutions for CO₂ transport and underground storage.

Technology catalogue for carbon capture, transport and storage (CCS)

If you want to know more about the geological aspects of CO₂ storage, you can also visit the GEUS page on CCS. 

GEUS page on CO₂ Capture and Storage (CCS)