Power‐to‐gas is a storage technology aiming to convert surplus electricity from renewable energy sources like wind and solar power into gaseous fuels compatible with the current network infrastructure. Results of CO2 dissociation in a vortex‐stabilized microwave plasma reactor are presented. The microwave field, residence time, quenching, and vortex configuration were varied to investigate their influence on energy‐ and conversion efficiency of CO2 dissociation. Significant deterioration of the energy efficiency is observed at forward vortex plasmas upon increasing pressure in the range of 100 mbar towards atmospheric pressure, which is mitigated by using a reverse vortex flow configuration of the plasma reactor. Data from optical emission shows that under all conditions covered by the experiments the gas temperature is in excess of 4000 K, suggesting a predominant thermal dissociation. Different strategies are proposed to enhance energy and conversion efficiencies of plasma‐driven dissociation of CO2.

A suite of diagnostics is proposed to characterize microwave plasma dissociation of CO2: laser scattering, Fourier transform infrared spectroscopy, and passive emission imaging. It provides a comprehensive performance characterization as is illustrated on the basis of experiments in a 2.45 GHz, 1 kW microwave reactor with tangential gas injection. For example, two operating regimes are identified as function of pressure: the diffuse and constricted plasma mode. Their occurrence is explained by evaluation of microwave propagation, which changes with the electron‐heavy particle collision frequency ve−h. In the diffuse mode, gas temperatures of 1500–3500 K are determined. The measured conversion degree, specific energy input, and temperature are summarized in a two‐temperature thermal model, which is solved to obtain the gas temperature at the periphery of the reactor and the size of the hot zone. Solutions are found with edge temperatures of hundreds of K, and hot zone fractions which agree with the measured behavior. The agreement shows that non‐thermal processes play only a marginal role in the measured parameter space of the diffuse discharge. In the constricted mode, the radial plasma size is independent of power. A skin depth equal to the plasma size corresponds to electron densities of 1018–1019 m−3. Temperatures in the central filament are in the range 3000–5000 K. Both discharge modes are up to 50% energy efficient in CO production. Rayleigh signals increase in the afterglow, hinting at rapid gas cooling assuming that the gas composition remains unchanged.

The strong non-equilibrium conditions provided by the plasma phase offer the opportunity to beat traditional thermal process energy efficiencies via preferential excitation of molecular vibrations. Simple molecular physics considerations are presented to explain potential dissociation pathways in plasma and their effect on energy efficiency. A common microwave reactor approach is evaluated experimentally with Rayleigh scattering and Fourier transform infrared spectroscopy to assess gas temperatures (exceeding 10(4) K) and conversion degrees (up to 30%), respectively. The results are interpreted on a basis of estimates of the plasma dynamics obtained with electron energy distribution functions calculated with a Boltzmann solver. It indicates that the intrinsic electron energies are higher than is favorable for preferential vibrational excitation due to dissociative excitation, which causes thermodynamic equilibrium chemistry to dominate. The highest observed energy efficiencies of 45% indicate that non-equilibrium dynamics had been at play. A novel approach involving additives of low ionization potential to tailor the electron energies to the vibrational excitation regime is proposed.

An electron cyclotron emission ͑ECE͒ receiver inside the electron cyclotron resonance heating ͑ECRH͒ transmission line has been brought into operation. The ECE is extracted by placing a quartz plate acting as a Fabry-Perot interferometer under an angle inside the electron cyclotron wave ͑ECW͒ beam. ECE measurements are obtained during high power ECRH operation. This demonstrates the successful operation of the diagnostic and, in particular, a sufficient suppression of the gyrotron component preventing it from interfering with ECE measurements. When integrated into a feedback system for the control of plasma instabilities this line-of-sight ECE diagnostic removes the need to localize the instabilities in absolute coordinates.

Non‐oxidative methane activation is carried out in a microwave plasma reactor for coupling to higher hydrocarbons. Fourier transform infrared spectroscopy (FTIR) was used to measure absolute concentrations of the major hydrocarbon species. Hydrogen concentration was also independently inferred from pressure‐based change in molar flow measurements. By closing both the carbon and hydrogen balance, from stoichiometry of the reactions, the amount of deposits was obtained as well. Additionally, core gas temperatures up to 2500 K were measured with Raman scattering when nitrogen acted as probing molecule in sample mixture discharges. At low gas temperatures, ethane and ethylene were significant products based on plasma chemistry, with ethane selectivities reaching up to 60%. At higher gas temperatures, thermal effects become stronger shifting the selectivity toward acetylene and deposits, resembling more with equilibrium calculations. The energy efficiency of the methane conversion reached up to 15% from which 10% represented coupling efficiency to higher hydrocarbons. It is concluded that there is an interplay between plasma and thermal chemistry where plasma generates radicals and final distribution is set by thermodynamics.