Approximately two-thirds of the total CO₂ emissions generated by the cement industry arise from the calcination of calcium carbonate (CaCO₃ --> CaO + CO₂) and are therefore classified as process-related CO₂ emissions. The remaining one-third of CO₂ emissions is released through the provision of the thermal energy required for calcination and for firing cement clinker as part of the combustion process. While fuel-related CO₂ emissions have been reduced in recent years through the use of alternative fuels with high biogenic content, process-related CO₂ emissions are largely unavoidable. This is due to the mineralogical transformation of CaCO₃ into CaO (and CO₂), which is essential to the cement manufacturing process.

In the conventional cement manufacturing process, the oxygen required for fuel combustion is supplied through ambient air. As ambient air contains approximately 21% oxygen, a correspondingly large volume of gas must be passed through the plant.

As a result, the exhaust gas from a conventional kiln contains a maximum of around 25% CO₂.

The Pure Oxyfuel process used in Mergelstetten (from “oxy” for oxygen and “fuel”) was developed by thyssenkrupp Polysius. It is a clinker-burning process in which pure oxygen is introduced into the kiln instead of air, ensuring heat generation through the combustion of primary and alternative fuels without the presence of atmospheric nitrogen.
This approach increases the CO₂ concentration in the kiln exhaust gas to approximately 90%, thereby significantly enhancing the potential for CO₂ capture. The aim is to capture 100% of a cement plant’s CO₂ emissions in a cost-efficient manner. The project is also intended to establish the conditions necessary for the large-scale deployment of CO₂ capture technologies within the cement industry. The captured CO₂ can subsequently be utilised or stored (CCU, CCS).

What is CCU?
CCU (Carbon Capture and Utilisation) refers to the capture of CO₂ followed by its use as a feedstock or raw material in the production of new products. Examples include the use of CO₂ in the food and beverage industry (e.g. for carbonation in drinks), in fertilisers, in the production of synthetic fuels, or in the decarbonisation of recycled concrete.

What is CCS?
CCS (Carbon Capture and Storage) refers to the permanent and secure storage of captured CO₂ in suitable geological formations. Examples include depleted oil and gas reservoirs beneath the North Sea.