CO2: better in the ground than in the atmosphere

05. October 2023

If we are going to counteract global warming, we will need to slash our CO₂ emissions. One promising approach is to actively remove carbon dioxide from the atmosphere and bind it in new products or store it under the ground. Norway is widely viewed as one of the pioneers of CCS technology and is using gas fields under the North Sea for that purpose. In Germany, the export and final storage of CO₂ are currently prohibited. But this could change. Young enterprises are driving the development of this technology and working on efficient methods. Here are six examples.

Capturing carbon dioxide in road surfaces

Swiss company Neustark has been working on the separation and storage of CO₂ since 2019. In July, this spin-off from ETH Zurich started up its biggest installation to date: 1,000 tonnes of CO2 are being captured for storage in Biberist, close to Solothurm. First of all, carbon dioxide is separated out in biogas plants. The liquified CO₂ is then injected into the grains of crushed concrete from demolitions. This converts it into limestone which is permanently bound to the concrete. This can then be used in road building or to produce recycled concrete. The Neustark technology can store ten kilos of CO₂ in one tonne of demolition concrete. In this way, a single plant can do in an hour what it takes 50 trees one year to do, the start-up claims.

The Swiss firm already has eleven plants in operation, and others are set to follow in Austria, France and the German city of Dresden, where Neustark is planning to capture CO₂ from a biogas plant. But even the colossal amounts of demolition concrete generated around the world each year are not unlimited. This is why Neustark also intends to store CO₂ permanently underground. The idea is for the carbon dioxide generated by Swiss biogas plants to be exported in liquified form to Norway or Iceland by rail, sea and lorry.

Sucking up CO₂ on the island of volcanoes

Neustark has not selected Iceland at random: this volcanic island’s geological features make it especially suitable for the permanent underground storage of large volumes of CO₂. Which is why another young Swiss company has also discovered Iceland. Climeworks was also set up by ETH Zurich but is relying instead on a different method. In this procedure, the carbon dioxide is filtered directly out of the atmosphere.

Huge fans suck air into the plant, where the CO₂ is captured in a special filters in a collector. Once the filter material has become saturated, the collector is closed and heated to 100 degrees Celsius. The CO₂ is released once again, dissolved in water and pumped into porous rock strata up to 1,000 metres below the surface of the ocean. Once there, it reacts chemically with the minerals in the rock – and begins to turn into stone. This process takes about two years. Once it is complete, the CO₂ should theoretically be stored in the rock forever.

High energy needs

This method is known as Direct Air Carbon Capture and Storage” (DACCS) and can also be used to remove diffuse CO₂ emissions from the atmosphere. These emissions are not those released in massive amounts from tall chimneys but those produced by cars or agriculture. As the concentration of this greenhouse gas in the atmosphere is of course much lower than it is in a biogas plant, steelworks or cement factory, DACCS requires significantly more energy than the Neustark method. Researchers from the University of Freiburg have calculated that 0.3 of a tonne of CO₂ is currently emitted per tonne of separated CO₂. The higher the share of renewables used to power the plant, the lower the CO₂ emissions.

Climeworks is accordingly attempting to cover the huge amount of energy required by its CO₂ suction device with sustainable energy from deep down in the earth. The company’s plant is supplied with electricity and heat by a neighbouring geothermal power station. Thanks to its volcanic origin, hot water from Icelandic thermal springs can be extracted from below ground in high volumes and at a correspondingly low cost.

Major project in the US

The plant is designed to store 4,000 tonnes of CO₂ per annum, 1,000 tonnes of which will be reserved for the carmaker Audi, which part-owns the company. The climate start-up has now laid the foundations for a second CO₂ capture plant on Iceland, which is scheduled to go into operation in mid-2024 and will be nine times more powerful. Climeworks aims to extract carbon dioxide from the air on an even greater scale in the US, where the plants are intended to capture a million tonnes of CO₂. This would be equivalent to the annual emissions of some 445,000 petrol cars, says the US energy ministry, which is sponsoring both projects.

That sounds like a lot. And yet, some 67 million cars are on the roads in Germany alone, most of which – for now, at least – still run on petrol and diesel. The volume of unavoidable CO₂ emissions, for example from industrial processes, which it will also not be possible to replace with fossil-free alternatives in the future, is estimated at 34 to 73 million tonnes per annum in Germany. It would therefore take 73 huge CO₂ suction units to extract these massive volumes of CO₂ from the atmosphere.

But if this is to work, these CO₂ capture devices will need to make economic sense for their operators. The DACCS method, in particular, is currently a long way away from profitability. For this to change, it will be necessary, on the one hand, for the price of CO₂ certificates to rise quite a lot, but also for technology’s energy needs to fall.

Alternative filters

Austrian company Krajete aims to achieve its goals using an alternative filter material. Krajete has teamed up with Audi in Linz to operate a plant which works with an inorganic filter material. This can absorb a large number of CO₂ molecules and is also particularly insensitive to wet conditions. It is therefore not generally necessary to heat up the air that gets drawn in in the separation process – and this saves both energy and time. The expectation is that it will be possible to filter CO₂ out of the air both more quickly and more cheaply. According to the company, the costs for one tonne of CO₂ are already in the low three-figure zone. The plant is powered by a solar installation on the company’s premises.

Biological CO₂ capture equipment

Alongside technological methods there are of course also biological ways to bind carbon dioxide: planting forests or restoring dried-out wetlands, for instance. The MacroCarbon start-up founded by marine biologist Mar Fernández-Méndez from the Alfred-Wegener-Institut is placing its faith in algae: sargassum is a brown alga that grows especially fast and, according to the researchers, can store much more CO₂ than trees. Moreover, these marine plants do not take up valuable land, and nor is fresh water needed to cultivate them.

This start-up, which is funded by the German Federal Agency for Disruptive Innovation, also has algae in its sights to produce basic materials for the chemical industry – for instance organic naphtha, a kind of crude petroleum which might be able to replace products currently derived from fossil fuels. The start-up is working with chemical giant BASF on this project.

By 2040, MacroCarbon aims to be using enormous algae farms on the high seas to bind 100 million tonnes of CO₂ emissions per annum, climbing to one gigatonne by 2050. First of all, however, the start-up will have to prove that its cultivation method actually works and that its organic crude petroleum will find enough takers. In this way, cultivating the brown alga could make an increasingly important contribution to protecting the climate.