(737f) Concentrated Solar–Plasma Chemical Processing: Reactors Design and Implementations for CO2 Decomposition

The production of chemicals from low-value feedstock using renewable energy by environmentally benign methods is prone to have an increasingly important role towards achieving sustainable development. Particularly, the conversion of carbon dioxide (CO₂) using renewable energy can fulfill the growing need for fuels and chemicals, leading to economic growth while mitigating environmental emissions. Solar thermochemical processes, given their direct use of solar irradiation, have among the greatest sustainability potential; nonetheless, the inherent intermittency of the solar resource prevents them from continuous operation, which limits their viability. The combination of solar thermochemical with non-solar approaches powered by renewable electricity can lead to continuous-operation sustainable processes with improved viability. As an example of a solar – non-solar chemical synthesis process, the integrated use of solar thermochemical and plasmachemical approaches applied to the decomposition of CO₂ is presented.

Plasmachemical processes utilize electricity to power an electrical discharge to drive feedstock gas to a plasma (i.e. partially ionized gas) state. Solar-plasma chemical processing is motivated by the potential for synergistic interactions among solar photons and plasma species (free electrons, ions, excited molecules and atoms) to lead to enhanced process performance. Computational studies [1] have shown that CO₂ in non-local thermodynamic equilibrium state (i.e. significantly greater equivalent temperature of free electrons, typically > 1 eV ~ 11600 K, compared to the gas species temperature, typically of a few hundred ^oC) absorbs drastically more solar radiation than CO₂ in thermodynamic equilibrium, which can potentially lead to greater CO₂ decomposition.

The design of reactors for solar-plasma chemical conversion present distinct challenges, such as ensuring appropriate interaction between incident solar radiation and plasma, maintaining the integrity of the optical aperture in the proximity of plasma, and integrating catalytic monoliths that exploit solar photon- and plasma-driven reactivity. Sola-plasma reactor design rationale and representative design concepts are reviewed and discussed.

Solar-plasma chemical conversion processes can be categorized based on the ratio of solar input power (P_s) to electrical input power (P_e). If P_s > P_e, then the process is denoted plasma-enhanced solar thermochemical (PES); whereas if P_s < P_e, then it is denoted solar-enhanced plasmachemical (SEP). A solar-gliding arc plasma reactor is implemented for a representative PES process [2, 3] and a solar-microwave plasma reactor for a SEP process [4, 5]. Both reactors are evaluated for the decomposition of CO₂ at (near) atmospheric pressure conditions using up to 525 W of radiative energy flux from a high-flux solar simulator. The solar-gliding arc reactor is powered by up to 240 W from two high-voltage AC power supplies and the solar-microwave plasma reactor by up to 900 W of net power from a 2.45 GHz magnetron. The studies did not include the incorporation of catalytic monoliths in order to discern the level of solar- or plasma-enhancement. CO₂ conversion and energy efficiency as function of solar and electrical input power, flow rate, and gas composition (100% CO₂ and CO₂-N₂/Ar/H₂O/CH₄ mixtures) are presented. Process characterization included optical and high-speed imaging, electrical diagnostics, optical emission spectroscopy, and gas chromatography of products. The results show that greater CO₂ conversion and energy efficiency are obtained by the combined use of concentrated solar and plasma as compared to plasma or solar energy alone. Current efforts encompass the incorporation of different catalytic monoliths into the reactors and detailed computational multiphysics analyses of the solar-plasma CO₂ conversion processes.

Acknowledgements: The author gratefully acknowledges the work of Dr. Sina Mohsenian, Dr. Dassou Nagassou, Rasool Elahi, Benard Tabu, and Visal Veng, from which this work is based. This work has been supported by the U.S. National Science Foundation through award CBET-1552037.

References:

[1] R. Elahi, D. Nagassou, S. Mohsenian, J. P. Trelles (2020). Enhanced Solar Radiation Absorption by Carbon Dioxide in Thermodynamic Nonequilibrium: A Computational Study, Solar Energy, 195, 369-381.

[2] Nagassou, D., Mohsenian, S., Bhatta, S., Elahi, R., & Trelles, J. P. (2019). Solar–gliding arc plasma reactor for carbon dioxide decomposition: Design and characterization. Solar Energy, 180, 678-689.

[3] Nagassou, D., Mohsenian, S., Nallar, M., Yu, P., Wong, H. W., & Trelles, J. P. (2020). Decomposition of CO2 in a solar-gliding arc plasma reactor: Effects of water, nitrogen, methane, and process optimization. Journal of CO2 Utilization, 38, 39-48.

[4] Mohsenian, S., Nagassou, D., Bhatta, S., Elahi, R., & Trelles J. P. (2019). Design and Characterization of a Solar-Enhanced Microwave Plasma Reactor for Atmospheric Pressure Carbon Dioxide Decomposition. Plasma Sources Science and Technology, 28, 065001.

[5] Mohsenian, S., Nagassou, D., Elahi, R., Yu, P., Nallar, M., Wong, H.-W., & Trelles, J. P. (2019). Carbon Dioxide Conversion by Solar-Enhanced Microwave Plasma: Effect of Specific Power and Argon/Nitrogen Carrier Gases, Journal of CO2 Utilization, 34, 725-732.