The cement industry is one of the most CO₂-intensive industrial sectors – not only because of the fuels required, but also because of the material-bound CO₂ quantities. 60 per cent of the total amount of CO₂ is generated just during the calcination of limestone as a result of of the process used. The EU "CLEANKER" (CLEAN clinKER) project, a funding recipient from HORIZON2020, is testing the integration of calcium looping (CaL) in the production process as a method for separating carbon dioxide (CO₂). This CO₂ capture process will be industrially tested in a demonstration plant in a cement factory in Vernasca, Italy, with the aim to achieve a consistent clinker quality.

Innovation: Integrated Calcium Looping

Carbon capture, utilisation and storage, or CCUS, is regarded as an important approach to prevent the climate impact of unavoidable amounts of CO₂ produced by industrial processes. The aim of the project is to create a basis for this by capturing a highly concentrated CO₂ gas stream. The calcium looping process will no longer be applied as a post-combustion technology, i.e. after the production process, but will instead be integrated into the process. The concept also envisages the use of the oxyfuel combustion process with pure oxygen instead of air in the calciner. This integration saves energy and offers special potential for the cement industry, since the sorbent quicklime (CaO) used for this purpose also serves as an essential raw material component for cement production.

Efficiency: Purest possible CO₂ exhaust gas

The concept for the integration of CaL in the demonstration plant provides for two process steps: First, CO₂ is separated from the exhaust gas stream in a carbonator by binding to CaO. This is followed by calcination, a combustion process using oxyfuel technology, in which the reaction product calcium carbonate (CaCO₃) is converted back into gaseous CO₂, solid CaO and water vapour. This generates the purest possible CO₂ exhaust gas stream, which contains both the raw material and fuel-related CO₂ quantities. Most of the CaO is recycled back into the carbonator and a small partial stream is fed into the rotary kiln for clinker production. The separated CO₂ is brought to the desired purity in a treatment plant. Some of the excess heat released during CO₂ integration is recovered. This allows approximately 4,000 cubic metres per hour to be treated in the demonstration plant and the CO₂ to be separated.

Effect: Emission reduction                                                                                                                                  

Over the long term, the process integrated into the clinker burning process is expected to capture 90 per cent of the resulting CO₂ volumes – both raw material and fuel-based process emissions. Other industries can use the CO₂ obtained this way as a raw material. Geological storage is also possible. This would prevent the climate-damaging greenhouse gas from escaping into the atmosphere. In a previous EU research project CEMCAP, in which VDZ participated, costs for capture were estimated at around 60 euros per tonne of CO₂. The aim of the demonstration project is now to continue to significantly reduce these costs. The additional electrical energy required for production should therefore be limited to less than 20 per cent and the additional costs for CO₂ capture to less than 25 euros per tonne of cement. Once the technology has been demonstrated, further technical and economic analyses will investigate how the technology can be transferred to other existing plants.