Redefining the Microwave Plasma-Mediated CO₂ Reduction Efficiency Limit: The Role of O–CO₂ Association

Abstract

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The Chemical Origins of Plasma Contraction and Thermalization in CO2 Microwave Discharges

“…In this work, we combine Thomson scattering experiments and 1D numerical modeling to assess the contraction mechanisms and to thereby provide a basis for predicting the power concentration. Combined Thomson–Raman scattering yields spatially resolved values of T e and n e as well as rotational (assumed equal to translational/gas) temperatures and species concentrations of CO 2 , CO, O 2 , and O, as shown in our previous work. , Numerically, a native 1D radial fluid model, elsewhere validated against measurements of neutral species mole fractions, gas temperature, electron number density, and electron temperature, is used to interpret measured trends in electron properties. More details as well as the validation of the model can be found in the work of Vialetto et al…”

“…At low pressure, C2 emission is absent, but the Raman signal of CO2 is much stronger than the Thomson signal, and both electron temperature and density have large error margins also up to ~50%. Precise values of uncertainty were determined with the method described in our previous work 8 .…”

“…The contraction of the discharge is accompanied by an increase in peak gas temperature (Tg): from 3000-4000 K at low to more than 6000 K at high pressure [8][9][10] . Concurrently, the electron number density (ne) and ionization fraction have been shown to increase by an order of magnitude 9 .…”

“…The splitting of CO 2 to CO in (microwave) plasma can be a key technology for CO 2 utilization by providing a carbon-based feedstock for chemicals and fuels. − This, in turn, can be used for storage of intermittent renewable energy in chemical bonds or integration of, for example, aviation with renewable energy sources . Understanding and predicting the local power input in such plasma systems define which chemical routes are predominant − and are thus a crucial aspect for reactor engineering. As such, knowledge on the electron properties in the discharge is crucial; the free plasma electrons absorb microwave power and transfer this energy to the heavy particles.…”

“…The contraction of the discharge is accompanied by an increase in peak gas temperature ( T g ): from 3000 to 4000 K at low pressure to more than 6000 K at high pressure. − Concurrently, the electron number density ( n e ) and ionization fraction have been shown to increase by an order of magnitude . On the basis of these measurements, Wolf et al have argued that the plasma contraction is governed by a thermo-chemical instability .…”