THROUGH PLANTS INTO CONCRETE
Production of BioEnergy with Carbon Capture, [Utilisation] and Storage in products (BECCUS) or underground (BECCS)
The diagram traces the cycle from forest to construction. Step 1: Plants absorb CO₂. Step 2: Biomass is used in industry and products; residues remain. Step 3: Energy from biomass residues (combustion/biogas) produces CO₂. Step 4: A capture unit uses a solvent to separate CO₂ from the flue gas. Step 5: Heating releases pure CO₂ and the solvent is recycled. Step 6: CO₂ is liquefied and transported to a concrete processing site. Step 7: CO₂ is injected into containers of recycled concrete aggregate; it reacts with cement phases, mainly Ca(OH)₂, forming CaCO₃ (calcite). A microscope inset shows the mineral. Step 8: The CO₂‑enriched aggregate is used in road building or as recycled concrete, thereby storing CO₂ permanently as mineral carbonate.
Plants absorb CO₂ from the air through photosynthesis and incorporate the carbon into their biomass, such as leaves and roots. To generate energy, biomass waste, e.g., from the wood industry, is burned or fermented. The CO₂ produced in this process is captured before it can enter the atmosphere. It is then either stored permanently underground (BECCS) or chemically bound in long-lasting products such as concrete (BECCUS).
BECCS plants that store CO₂ underground already exist in the USA and Europe. Storage in products is less common at present. In Switzerland, CO₂ from biomass is captured in some wastewater treatment plants and then stored in concrete. This concrete is already being used in construction today.
BECC[U]S can be used to generate renewable, CO₂-neutral energy and remove CO₂ from the atmosphere at the same time. However, this dual effect is offset by the fact that the cultivation of biomass purely for BECC[U]S competes with food production if agricultural land is converted for this purpose.
Recycled concrete with stored CO₂ being poured into the track bed of the tram lines at Zurich main station.
Photo: Léon Frey (focusTerra)
Exhibit
WHICH CONCRETE CUBE CONTAINS CO₂?
These two cubes were produced using recycled demolition concrete in the form of granulate. In one of the cubes, the granulate was injected with CO₂: CO₂ reacts with the cement present in the granulate, forming calcite (CaCO₃), which is the main component of limestone. The calcite builds up in the granulate in the form of rhombohedral crystals. These crystals permanently bind the CO₂ – not only in the granulate, but also after further processing into recycled concrete. This process allows an average of 10 kg of CO₂ to be stored per tonne of demolition concrete.At first glance, the two cubes in front of you appear identical and have almost the same properties. But only one contains stored CO₂ – which one? (The answer is on the back of the cubes.)
Under the microscope, the calcite crystals are clearly visible:
Recycling concrete without added CO₂: no rhombohedral calcite crystals are present.
Microscope image: Neustark
Recycling concrete with added CO₂: calcite is present as rhombohedral crystals.
Microscope image: Neustark
(The answer is on the back of the cubes.)