Paper PS+SE-MoM4
CO₂ Splitting by Dielectric Barrier Discharge at Atmospheric Pressure: Understanding the Influence of Electrical Regimes and Electrical Configurations

Monday, November 7, 2016, 9:20 am, Room 104D

Dielectric barrier discharges (DBDs) are commonly used to generate cold plasmas at atmospheric pressure. In this experimental work, a flowing tubular DBD is used for the CO₂ splitting into O₂ and CO. The influence of the frequency (from 16 to 28 kHz), the power (from 30 to 100 W), the role of the barrier thickness (2.0, 2.4 and 2.8 mm), the kind of dielectric material (alumina, mullite, pyrex, quartz), and the effect of a pulsed AC discharge (so-called burst mode) are investigated on the filamentary behavior of the plasma and on the CO₂ conversion, by means of mass spectrometry measurements correlated with electrical diagnostics. Their influence on the gas and electrode temperature is also evidenced through optical emission spectroscopy and infrared imaging. A new methodology is developed to investigate the microdischarge properties. For this purpose, electrical measurements, based on a numerical method, are carried out to explain the conversion trends and to characterize the microdischarges through their number (N_md), their lifetime (L_md), their intensity (i_pl) and the induced electrical charge (Q_pl) for a given analysis time. These extracted data are usually underestimated or poorly described in literature.

It is shown that, when the applied power is modified, the conversion depends mostly on the Q_pl and not on the effective plasma voltage (V_pl,eff). Similarly, a better conversion is observed at low frequencies, where a more diffuse discharge with a higher V_pl,eff than at higher frequency is obtained. Moreover, increasing the barrier thickness decreases the capacitance while preserving the electrical charge. As a result, the voltage over the dielectric (V_diel) increases and a larger N_md is generated, which enhances the CO₂ conversion. Furthermore, changing the dielectric material of the barrier, while keeping the same dimensions, also affects the conversion. The highest CO₂ conversion and energy efficiency are obtained for quartz and alumina. From the electrical characterization, we clearly demonstrate that the most important parameters are the somewhat higher V_pl,eff (yielding a higher electric field and electron energy involved in CO₂ dissociation) for quartz, as well as the higher plasma current (thus larger electron density) and the larger N_md (mainly for alumina due its higher roughness, but also for quartz due to its higher V_diel). Finally, a comparison between DBD ignited in burst mode and pure AC mode is achieved. Decreasing the duty cycle from 100% (pure AC mode) to 40% leads to a rise in the conversion due to a larger N_md and a higher voltage.