ICRC 2024 - Session 5

Dr. Marc Widenmeyer, Technische Universität Darmstadt

The chemical industry faces the challenge of defossilization to counteract climate change.  One way to produce basic chemicals from renewable resources is to activate inert molecules such as CO2, H2O or N2.  A microwave plasma process is presented to activate CO2 and provide CO as a base chemical.

Microwave plasma sources are designed to generate a contact-free plasma while ensuring stable operation for a wide parameter range of gas-types, gas flow and microwave power at 2.45 GHz and 915 MHz. These plasma sources are well suited for both, the synthesis of special gases and for supporting chemical reactions with highly reactive gas species. This is the key for a wide range of industrial applications. Power-to-X applications such as Power-to-Gas and Power-to-Chemicals are prominent examples of industrial applications based on these microwave plasma sources. The contactless plasma generation is the key property of these microwave energy driven high temperature applications. Moreover, microwave plasma systems offer the best alternative to build decentralized supply networks with CO2-neutral hydrogen-based fuel gases. Such gases can in turn be employed for energy storage or for direct and location-independent use. Power-to-X technologies based on microwave plasma processes are then innovative solutions to convert surplus electrical energy from renewable sources into material resources such as hydrogen, carbon monoxide, and synthetic gases for storage and recycling, e.g., conversion of electrical energy into gaseous or liquid fuels or chemicals for long-haul trucking, shipping and aviation. This contribution reviews some of these applications.

Human activities have already surpassed six of the nine quantified planetary boundaries. Our current performance-orientated material development patterns and technological advancements within a linear value chain continuously contribute to it by a steadily increasing resource demand, greenhouse gas emissions, and accruing waste, despite growing awareness. For a sustainable future, we need to change production and use of materials by implementing a circular economy. In that context, we are investigating ceramic oxygen transport membrane materials (OTM) as an enabler of microwave plasma-based CO2 and green H2 utilization. The OTM are needed to control the oxygen partial pressure in the reaction zone helping to suppress side reactions; thereby, enhancing product efficiency. The application in a plasma environment puts a high demand on the selected material. Additionally, the material needs to be recyclable and bear low environmental impacts (EIs) to align with the European Commission's new circular economy action plan. We are implementing several pathways towards well-performing sustainable OTM: i) using classical and very recent materials science tools such as Ellingham diagrams and Fermi Level Engineering, respectively, to improve the separation performance, ii) replacing as much as possible critical elements without agitating functionality, and iii) implementing (chemical) recycling at the early-stage of materials development. All these measures are simultaneously accompanied by screening life cycle assessments to identify the major contributors to the EIs, hence establishing a roadmap for sustainability improvements solely based on robust scientific data. In summary, the presented case studies demonstrate the importance of performing a sustainability assessment during the early stages of materials or process development to genuinely progress towards a sustainable future. This recommendation is broadly applicable, while the specific results of the case studies should be only generalized or extrapolated with utmost care.

In this presentation it will be shown, how perovskite membranes can be used to separate Oxygen from a CO₂ plasma in order to avoid the back reaction of CO to CO_2.

Introduction. One of the biggest challenges for the next decades is to stop the process of global warming. A possible approach to decrease the CO₂-emission could be the use of CO₂ in power-to-X applications. Here, we present the use of MIEC hollow fiber membranes in a so-called plasma-membrane reactor to separate oxygen from a CO₂ plasma. In the plasma, the CO₂ is splitted into oxygen and CO, which can be used as a platform chemical for further syntheses (e.g. Fischer-Tropsch). 

Experimental. In the project, hollow fiber membranes from various MIEC materials (e.g. (La0.6Ca0.4)(Co0.8Fe0.2)O3-δ (LCCF)) were manufactured using a wet spinning process. Subsequently, the hollow fibers were sintered to receive gastight membranes with an outer diameter of 1.2 mm. The sintered membranes were further treated by an etching process to increase the active surface of the membranes. The oxygen permeation of the LCCF hollow fibers were tested both in a normal furnace and in a microwave induced atmospheric plasma torch under a CO₂ atmosphere.

Results and discussion. Due to the operation in a CO₂ plasma, we are especially interested in the behaviour of the hollow fiber membranes under CO₂ containing atmospheres. Measurements with the hollow fibers in the furnace reveal both a good stability in a gas composition with 50 % CO₂ and a high oxygen permeation. Thereby the permeation depends strongly on sintering condition [1] and surface treatment and oxygen permeation of up to 5.1 ml min-1 cm-2 at 1000°C can be achieved [2].

In the CO₂ plasma the capillaries are repeatedly heated up to temperatures above 1000°C within seconds without any damage and no degradation could be observed even after long time treatment. Measurements with the CO2-stable LCCF hollow fibers in the plasma verify the splitting of CO₂ into CO and O₂. In the plasma-membrane reactor, an oxygen permeation up to 1,5 ml min-1 cm-2 was achieved. It was possible to perform O₂ separation from the CO₂ plasma with modules with up to 27 fibers [3].

Accordingly, a plasma coupled membrane reactor is a reasonable option to separate oxygen and prevent the back reaction of CO₂ to generate CO.

Acknowledgments

We gratefully acknowledge the support of the German Federal Ministry of Education and Research (grant number 03SF0618C).

References

[1] Buck, F., Feldhoff, A., Caro, J., Schiestel, T., Separation and Purification Technology, 2021, 259, 118023.

[2] Buck, F., Bunjaku, O., Caro, J., Schiestel, T., Journal of the European Ceramic Society°2022, 42(4), 1537-1547.

[3] Antunes, R., Wiegers, K., Hecimovic, A., Kiefer, C. K., Buchberger, S., Meindl, A., Schiestel, T., Schulz, A., Walker, M., Fantz, U. ACS Sustainable Chemistry and Engineering, 2023, 11(44), 15984-15993.

The conversion of inert molecules (e.g., CO2, CH4, and N2) with strong chemical bonds for the synthesis of value-added synthetic fuels and platform chemicals has attracted significant interest. However, the activation of these molecules remains a great challenge due to their thermodynamical stable, requiring a substantial amount of energy for activation. Non-thermal plasma (NTP) has emerged as a promising technology for gas conversions under ambient conditions. The combination of NTP with heterogeneous catalysis has great potential for achieving a synergistic effect through the interactions between the plasma and catalysts, which can activate catalysts at low temperatures, improve their activity and stability, and lead to a notable increase in conversion, selectivity, and yield of end-products, as well as enhance the energy efficiency of the process. Furthermore, plasma processes can be switched on and off instantly, offering great flexibility in decentralised fuel and chemical production using renewable energy sources, particularly intermittent renewable energy. This presentation will discuss the opportunities and challenges in plasma-catalytic gas conversion to fuels and chemicals, including various chemical processes such as CH4 activation, CO2 conversion, and ammonia synthesis.