Green Lime – Membrane Reactor for Process Optimization in the CO₂-Intensive Building Materials Industry

Energy-intensive companies face increasing pressure – both from rising CO₂ taxation and from customers – to make their processes more resource-efficient and sustainable. This particularly concerns the building materials industry: the CO₂ content in the exhaust gases of cement plants typically ranges between 14 % and 33 %, while in lime plants it can exceed 40 %. In order to address this challenge, Fraunhofer IKTS and its partners ran the BMFTR project Grüner Kalk (Green Lime) to develop technologies for climate-neutral lime production by means of CO₂ capture and utilization.

CO₂-intensive industries currently use various approaches to reduce their climate impact. These include substituting fossil fuels and treating industrial exhaust gases (carbon capture and storage – CCS, or carbon capture and utilization – CCU). Lime production poses a particular challenge, however: most of the CO₂ is released during the decarbonization of limestone in the kiln. This means that CO₂ neutrality cannot be achieved solely by replacing the fuel gas or switching to an electric furnace. In the new plant concept, limestone is fed into a sealed electric kiln via lock chambers. This produces atmospheres with more than 80 vol.% CO₂. By adding green hydrogen, the CO₂ is converted to methane. Two reactor concepts were compared: biological methanation (using archaea) and chemical-catalytic methanation in a membrane reactor. In the membrane reactor, hydrogen is introduced in a controlled manner via pressure-regulated dosing. The resulting product stream is then dried and fed into pyrolysis. When the methane decomposes into elemental carbon and hydrogen, the hydrogen is returned for further conversion with CO₂. Elemental carbon, or carbon black, is a valuable resource that can be used in the chemical industry or to improve agricultural soils.

The system design around the membrane reactor was developed and implemented by Fraunhofer IKTS and subsequently integrated in the overall plant at project partner HySON. Depending on the process configuration, CO₂ conversions of around 85 % to over 99 % and complete selectivity (> 99 %) to methane were achieved in the membrane reactor under design operating conditions. Compared with biological methanation, the membrane reactor offers a much more compact design, a wider operating range (throughput and stoichiometry), good scalability and the potential to recover usable heat at temperatures above 200 °C. The next step is now to scale up the plant concept further in cooperation with industrial partners.

Services offered
 

• Design and engineering of process systems
• Application-oriented catalytic and process engineering studies (including membrane-coupled systems)
• Plant construction for catalysis and PtX processes from laboratory to container scale
• Powder and catalyst preparation from a few grams to several hundred kilograms