Carbon storage: 5 most suitable places to hide CO2 away

Illustration: Carolina Diaz

Through a review of scientific and political reports, as well as interviews with Danish researchers and experts, Ingeniøren has identified the carbon storage tools and methods available to Denmark and assessed their technological maturity.

We bring you five most relevant tools:

Illustration: Carolina Diaz

1. Mineral storage

Once CO2 is captured from, for example, flue gases, it can be stored underground in various ways. One of them is binding the CO2 through a reaction with minerals in the form of mineral storage.

In Denmark, researchers at DTU and the University of Copenhagen are currently investigating carbon storage through reaction with basalt and olivine. In both cases, various forms of solid carbonate compounds are formed through a process called carbonation.

While the reaction with olivine has so far primarily been tested in laboratories with few examples of commercial use, the reaction with basalt is an approach that has already been used at industrial scale in some places. But as the International Energy Agency (IEA) mentions in its CCUS report from 2021, basalt carbonation is also at an early stage of development.

The technology is already in use in Iceland, where the company Carbfix, according to its own data, has stored 70,000 tonnes of CO2 captured from steam at the Hellisheidi geothermal plant since 2014.

The captured CO2 is dissolved in water, which is pumped into the underground, where the reaction between the reactive minerals and the CO2 takes place. At the same time, they exploit the fact that the underground is volcanically active, which brings the reaction rate to 18 months.

When it comes to Denmark, researchers at DTU are looking for catalysts that would enable carbon storage in old and cold basalt under Denmark.

Basalt is plentiful all over the globe, which is why a scientific review article in Nature points out that the potential for carbon storage through mineralisation in basalt exceeds the man-made emissions.


Illustration: Carolina Diaz

2. Storage in biochar

Biochar produced from pyrolysis of biomass is a way of utilizing the ability of plants and trees to capture CO2 combined with storage in the form of stable carbon compounds rather than degradable biomass.

If the pyrolysis plant receives all residual fibre, manure fibre, and biogas fibre in Denmark, we could potentially store five to six million tonnes of CO2. The Climate Agreement only includes storing 2 million tonnes of CO2 within agriculture.Ulrik Birk Henriksen, senior researcher at DTU Chemical Engineering

In pyrolysis, biomass is processed as residual waste from agriculture, degassed biomass from biogas plants, and sewage sludge in oxygen-poor conditions and at temperatures of 500–600 degrees Celsius. This produces biochar and pyrolysis oil and gas, and up to half of the carbon of the biomass is captured in the biochar.

If this biochar is subsequently ploughed into, for example, fields, it is expected that the carbon will be stored in the ground for centuries. Researchers from the University of Copenhagen, among others, are currently investigating how permanent a storage technology it actually is as well as its side effects, such as soil remediation or pollution.

There are several examples of pyrolysis demonstration plans in Denmark, while the first full-scale plants are currently being developed by AquaGreen and SkyClean. Despite that, the Danish Council on Climate Change states in its latest status report that pyrolysis and storage in biochar are associated with a high technological risk.

Immature to moderately mature

Illustration: Carolina Diaz

3. Storage in buildings

Today, construction is a significant source of global CO2 emissions, but Danish researchers are working on also turning construction into a carbon storage method. This can be done by building new buildings using biogenic materials such as wood, straw, and eelgrass, or by building with “carbonised” concrete.

The former is a kind of extension of biological storage, as building with, for example, wood is a way of extending the carbon storage of forests. In order to maintain a high level of carbon storage in forests, it is necessary to continuously manage forests, older trees must be felled and new ones planted. The construction industry could be a potential buyer of this wood.

A report from Aalborg University estimates that the storage potential of biogenic materials corresponds to the amount of CO2 emitted from Danish concrete consumption. At the same time, in the long term, it will be possible to keep the stored carbon from being released into the atmosphere if the construction material is burned during demolition, for example, while CO2 from the flue gas is captured and subsequently stored geologically or minerally.

However, there are still quite a few barriers to the use of biogenic building materials—even the very familiar wood. There is a lack of documentation about the fire resistant, acoustic, and moisture resistant properties of the materials as well as knowledge about the use of the materials throughout the industry.

More knowledge is also needed in relation to storage in concrete, which basically follows the same principles as mineral storage. Researchers at both DTU and Aarhus University are working on finding alternatives to cement based on mineral capture of CO2. It results primarily in calcium carbonates, which also occur over time through natural carbonation of concrete.

Immature to moderately mature

Illustration: Carolina Diaz

4. Geological storage

Storing CO2 by pumping it into porous soil layers underground under high pressure is a technology with decades of experience behind it. The oil and gas industry has been using it for a long time—although primarily without the aim of storage, but to squeeze extra hydrocarbons out of nearly exhausted fields.

The Sleipner gas field in Norway is a good example of the fact that the technology is already well studied with permanent storage in mind as well. Since 1996, more than 20 million tonnes of CO2 captured during upgrading of natural gas have been stored in the field.

The potential Danish storage areas are reminiscent of the soil under Sleipner. These are geological formations such as sandstone and limestone with many small pores in which the CO2 can be stored.

Gas Storage Denmark wants to store CO2 at its gas storage facility in Stenlille. The goal is to store 0.4 million tonnes of CO2 per year starting from 2025. We are very confident about that.Carsten Møller Nielsen, reservoir engineer at Geus

Above the porous formations at a depth of 800 meters or deeper, there has to be a “lid” in the form of a less porous soil layer, for example claystone.

The dense gas is retained in the soil by four mechanisms: capillarity that causes the gas to be distributed in the pores, mineralization in contact with existing stone, the claystone layer, and dissolution of the CO2 in water in the pores.

Geus estimates that the Danish subsoil contains enough suitable formations to store up to 22 billion tonnes of CO2, which has to be considered in relation to the annual Danish emission of 42 million tonnes of CO2e in 2020. These formations are present under land, coast, and the North Sea, where depleted oil and gas fields are also in play.

However, as the International Energy Agency (IEA) highlights, there is still considerable work to be done on a global level in turning theoretical CO2 stores into real depots.

When it comes to geological storage, maturing and thorough—and expensive—seismic surveys and drilling in the subsoil are required if new wells are to be established. If we are to use depleted oil and gas fields, we need to have a better understanding of the interaction between carbonated water and limestone, which the majority of Danish fields consist of. There are also many uncertainties about the entire CO2 value chain from carbon capture and transport to storage, not least which point sources for capture to use.

Moderately mature

Illustration: Carolina Diaz

5. Biological storage

The trees have been doing it for millions of years: they capture CO2 from the atmosphere and use photosynthesis to store carbon in biomass and soil. So do all other photosynthesizing plants that, until they decay, act as carbon stores.

In this way, CO2 can also be stored biologically with afforestation as the most effective storage measure.

In its latest status report, the Danish Council on Climate Change assessed afforestation as a mature “technology” with the potential to store an average of 10 tonnes of CO2e per hectare annually during the first 100 years of a forest’s lifespan. Less at the start—as biological storage in trees takes time—and again decreasing in the long term.

However, it depends on the types of trees and felling, as well as effective management of the forest and a sufficient supply of new forests. If, for example, a forest is planted on agricultural land, there is a risk of CO2 leakage if the demand for agricultural land leads to felling of forest elsewhere. Usually elsewhere in the world.

This underlines the issue of Earth’s limited arable land, which means that any new forests have to compete for space with food and energy crops. Therefore, storage in marine plants may be a good alternative. The Hav think tank thus estimates that eelgrass alone can store up to 47 million tonnes of CO2 over 30 years.


Sources: Erling Stenby, Ulrik Birk Henriksen and Susan Stipp, DTU; Maja Bar Rasmussen, Kresten Anderskouv, Emil Thybring, Niclas Scott Bentsen and Sander Bruun, University of Copenhagen; Carsten Møller Nielsen, Geus; Jørgen Skibsted, Aarhus University; “Biogene materialers anvendelse i byggeriet” report, Aalborg University; Danish Council on Climate Change’s “Status Outlook 2022”; Carbfix and the International Energy Agency.