High-pressure CO2 dissociation with nanosecond pulsed discharges

We investigate the conversion of CO2 into CO and O-2 with nanosecond repetitively pulsed (NRP) discharges in a high-pressure batch reactor. Stable discharges are obtained at up to 12 bar. By-products are measured with gas chromatography. The energy efficiency is determined for a range of processing times, pulse energy, and fill pressures. It is only weakly sensitive to the plasma operating parameters, i.e the extent of CO2 conversion is almost linearly-dependent on the specific energy invested. A conversion rate as high as 14% is achieved with an energy efficiency of 23%. For long processing times, saturation in the yield and a drop in efficiency are observed, due to the increasing role of three-body recombination reactions, as described by zero-dimensional detailed kinetic modeling. The modeling reveals the presence of three-stage kinetics between NRP pulses, controlled by electron-impact CO2 dissociation, vibrational relaxation, and neutral elementary kinetics. Transport effects are shown to be important for CO2 conversion at high pressures. For fill pressures beyond 10 bar, CO2 may locally transit into supercritical states. The supercritical plasma kinetics may bypass atomic oxygen pathways and directly convert CO2 into O-2. This work provides a detailed analysis of plasma-based high-pressure CO2 conversion, which is of great relevance to future large-scale sustainable carbon capture, utilization, and storage.