FROM THE AIR TO DEEP UNDERGROUND
Direct Air Carbon Capture, (Utilisation) and Storage underground (DACCS) or in products (DACCUS)
The diagram presents a DACCS system in a landscape powered by geothermal energy. Step 1: Fans pull ambient air through modules; operation uses renewable, low‑carbon power (e.g., geothermal). Step 2: CO₂ adheres to a filter. When saturated, the filter is heated with geothermal heat and releases pure CO₂. Step 3: The CO₂ is mixed with water, forming carbonic acid/bicarbonate. Step 4: The carbonated water is injected underground through wells and reacts with calcium/magnesium in basalt. Step 5: Stable carbonate minerals (e.g., calcite) form, locking carbon permanently. Step 6: Monitoring sensors track injection, water quality, and seismic activity. An inset shows the chemical pathway from CO₂ + H₂O to carbonate.
Fans in the DACCS plant in Iceland draw in ambient air to start the process of removing CO₂ from the atmosphere. Photo: Climeworks
The CO₂ is filtered directly from the air and separated through chemical processes. It is then liquefied or mixed with water and pumped underground. There, it (more or less quickly) forms a solid mineral and is thus permanently stored in the rock (DACCS). Alternatively, the CO₂ filtered from the air can also be stored in products such as concrete (DACCUS).
The first DACCS plants in Iceland and the USA have been removing CO₂ from the atmosphere for several years. Further plants are in the planning stage, including in the USA, Kenya and Oman.
DACCS plants can be built almost anywhere where there is a suitable subsurface for storage. This can be basalt rock, as in Iceland, or a former oil and natural gas reservoir. More often, however, it is a deep-lying layer of rock containing salt water.
One challenge is the high energy requirement: since CO₂ only makes up approx. 0.04% of the air, the filtering process is very inefficient and therefore energy-intensive and expensive. For the technology to contribute to combating climate change, the energy used must also come from renewable sources and any CO₂ produced must be offset (CO₂ neutrality).
Exhibits
Basalt with naturally mineralised calcite veins
Ibergeregg
Back left: loose sand; Eawag-Beachvolley-Feld, Dübendorf
Front left: sandstone without biological CO₂ mineralisation; Boise, USA
Back right: sandstone with biological CO₂ mineralisation; Eawag-Labor, Dübendorf
Front right: sandstone with biological CO₂ mineralisation; Boise, USA
NATURAL & ACCELERATED CO₂ STORAGE IN ROCK
CO₂ can be permanently bound in rocks through mineralisation: when it enters pores or cracks, it reacts with the water and chemical elements present in the rock to form minerals such as calcite (CaCO₃). In nature, this process usually takes thousands of years. Once mineralised, however, CO₂
is stored safely and stably as part of the rock. In basalt, you can see naturally mineralised calcite veins.
Technological underground CO₂ storage uses natural mineralisation, but it has to happen faster to quickly store large amounts of CO₂ in solid form. In Iceland, this happens naturally in fresh basalt: CO₂ mineralises within two years, which is incredibly fast in geological terms. In other reservoir rocks that are much more common, such as deep, saltwater-bearing rock layers or depleted oil and natural gas fields, the CO₂ injected initially remains dissolved in the water in the rock pores. It takes a very long time for it to mineralise under the cap rock, which prevents it from escaping.
Researchers are looking for ways to accelerate mineralisation. One possibility is bacteria: they can, for example, change the chemistry in pore water so that minerals such as calcite crystallise more readily (CO₂ biomineralisation). Laboratory experiments show that this causes the pores of sandstone to fill with calcite within a short period of time (sandstone drill core in the display case). Even loose sand can become sandstone in this way, with biomineralised calcite holding the grains together like glue (loose sand and sandstone in the display case; film on the screen). While biomineralisation is successful in the laboratory on Earth’s surface, the question arises: which bacteria can contribute to biomineralisation under the extreme pressures and temperatures deep underground?
HOW IS CO₂ STORED IN MUSCHELKALK LIMESTONE?
CO₂ can be stored in porous, permeable rock such as Muschelkalk (shell-bearing limestone) if the rock is covered by impermeable layers. These prevent the CO₂ from rising again. During injection, the CO₂ mixes with the water in the rock pores. The CO₂ remains bound in the water over the long term – similarly to carbon dioxide in sparkling water. Over time, some of the dissolved CO₂ also binds with the surrounding rock, for example in the form of calcium deposits.
BEFORE INJECTION
DURING AND AFTER INJECTION
- The injected CO₂ rises in the reservoir rock and accumulates under the cap rock.
- The CO₂ slowly dissolves in the pore water.
- The CO₂-containing water is denser and therefore heavier: it sinks to the bottom of the reservoir rock.
- At the bottom of the reservoir rock, the dissolved CO₂ slowly migrates with the water along the natural flow direction.