Chemicals from CO2 Plasma Technology Advances Energy Transition

Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB and the University of Stuttgart aim to use the climate-damaging greenhouse gas carbon dioxide as a raw material for the production of chemicals. To do this, they are developing a combined plasma and membrane process that splits CO₂ into oxygen and the chemical base material carbon monoxide.

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Stuttgart /Germany — Scientists at the Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB and the Institute for Interfacial Process Engineering and Plasma Technology (IGVP) of the University of Stuttgart now want to tackle climate change by using the climate-damaging gas as a raw material for the production of chemicals and chemical energy storage systems. In the project “Pick – Plasma-Induced CO₂ Conversion for the Storage of Renewable Energies”, which is funded by the German Federal Ministry of Education and Research as part of the “Kopernikus projects for the Energiewende”, they have been researching for two years a new approach that uses excess electricity from regenerative sources and combines plasma with membrane technology.

The idea: Carbon dioxide is split in a plasma, i.e. an ionized gas with highly reactive species. This produces oxygen and carbon monoxide (CO). Carbon monoxide is a chemical raw material that can be converted into base chemicals and chemical energy stores such as methanol or methane in existing infrastructures using conventional chemical processes. To prevent carbon monoxide and oxygen from reacting back to carbon dioxide, a new heat- and CO₂-stable membrane separates oxygen.

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Splitting CO₂ in Plasma

Scientists at the Institute of Interfacial Process Engineering and Plasma Technology (IGVP) at the University of Stuttgart have developed an electrodeless reactor to carry out the carbon dioxide splitting. Here, an atmospheric pressure plasma – excited by a microwave – is generated by means of strong electric fields. If electrons excited in this plasma collide with CO₂ molecules, this causes CO₂ to decompose into the desired products carbon monoxide and oxygen.

Perovskite Membrane for the Removal of Xxygen

To prevent CO from reacting back with the oxygen produced in the plasma to form CO₂, oxygen must be constantly removed from the reaction equilibrium. For this purpose, Fraunhofer IGB has developed a new ceramic membrane that can selectively separate oxygen. Since the membrane must be temperature- and CO₂-stable at the same time, the scientists spin special ceramic materials, so-called perovskites, together with polymers into a thin-walled capillary. The resulting green fiber is sintered at high temperatures to form a dense ceramic capillary membrane. The perovskite membrane is stable in a CO₂ atmosphere and permeable for oxygen at temperatures of 800 to 1000 °C, but not to carbon monoxide and carbon dioxide. Such a membrane has not previously been described.

Successful carbon dioxide splitting and oxygen separation

The researchers have already demonstrated that the idea of the combined plasma-membrane process works in the plasma reactor developed at IGVP. Using a special apparatus in the plasma reactor, the capillary membrane is adjusted at a precisely defined point in the plasma flame. By controlling the CO₂ gas flow and microwave power, the temperature of the plasma at the membrane level is set to 800 – 1000 °C, so that oxygen can pass through optimally.

Under these conditions, the new capillary membrane showed both very good thermal stability and very good oxygen permeability. At a kilowatt microwave output that corresponds to a temperature of around 1000 °C, 2.3 milliliters of oxygen per minute and square centimeter of membrane surface were separated. Up till now, around one third of the microwave energy has been converted into chemical energy and 22 % of the carbon dioxide has been split.

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Flexible Power Control and Easy Scalability

Both the process and the plasma membrane reactor can be easily scaled and flexibly controlled, which is a great advantage for the technical implementation. The operation of the plant can be adapted to the available regenerative energy resource and react quickly to the current energy supply. The next step on the way to demonstrating the technical feasibility is to increase the separation efficiency of the plasma membrane reactor. In combination with improved gas management, it should also be possible to further increase the energy efficiency of the process.

The process can be used wherever CO₂ is produced in enriched form: in combustion processes in power plants, in the cement and glass industries, and in breweries where CO₂ is a by-product of alcoholic fermentation. The use of CO₂ as a raw material can thus help to conserve natural resources and protect the climate. The researchers are already looking for interested companies for the further development in a second funding phase and the subsequent technical implementation.

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