Homogeneous CO₂ conversion by microwave plasma: Wave propagation and diagnostics

Abstract

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.

Plasma‐driven dissociation of CO2 for fuel synthesis

“…[14] The value of T rot is assumed to be equal to the translational gas temperature T g. The latter is supported by the good agreement between the simulated data of T rot and those of T g measured previously in our laboratory using Rayleigh scattering on MW plasmas created under conditions similar to those used in the present study. [4] The value of T vib serves as an indicator for the VT non-equilibrium at given T g . For the bands Δν = 0 and +1, the observed spectra match well with the simulated spectra for T g = 5700 K and T vib = 9000 K. For the bands with Δν = −1 and −2, the simulated spectra depart significantly from the measured ones.…”

“…Due to a high VT relaxation rate at T g = 5700 K thermal dissociation of CO 2 is predominant at this temperature. [2,4,9] 4.2 | CO 2 dissociation experiments using the 2.45 GHz MW Reactor at DIFFER Several experiments were performed using different reactor configurations (flow type, field enhancement, and quenching) of the 2.45 GHz plasma source (InitSF) at DIFFER. Experiments and configurations are listed in Table 1.…”

“…Similar observations have been reported previously. [4,9,19] Quenching of the plasma has a beneficial effect on η. In this study, quenching could enhance η to a maximum value of ∼47% at a plasma pressure of 200 mbar.…”

“…At a temperature of 3000 K, an optimal energy efficiency of 50% can be achieved under ideal quenching conditions. [4] This, however, does not take into account the possibility of heat recirculation, which can potentially lead to a higher overall energy efficiency. The quenching performance is determined by the instantaneous gas cooling rate, which is optimal between 10 7 and 10 9 K s…”

Tar Removal by Nanosecond Pulsed Dielectric Barrier Discharge

“…The CO 2 concentration decreases almost linearly with the discharge energy causing an adequate increase in CO concentration. This phenomenon has been observed by many researchers in diverse non-thermal plasma sources, such as DBD [44,45], gliding arc [46,47] and microwave discharge [48,49]. It was found that high energy electrons in these electrical discharges induce vibrational excitation of CO 2 molecules, which is a highly efficient channel for CO 2 decomposition [50].…”