High Energy Efficiency Plasma Conversion of CO₂ at Atmospheric Pressure Using a Direct-Coupled Microwave Plasma System

Energy and conversion efficiencies of the carbon dioxide (CO₂) dissociation process at atmospheric-pressure conditions are investigated using a direct-coupled continuous microwave plasma system (MPS). Gas chromatography and mass spectrometer measurements were performed on the gas mixture postplasma treatment, over a range of specific energy inputs (SEIs) (0.06-0.6 eV mol^-1, in order to determine species mass fractions of CO₂, carbon monoxide, and oxygen. In this region of SEI, energy efficiency is maximized at the cost of conversion efficiency. The corresponding maximum observed quantities for conversion efficiency and energy conversion efficiency (ECE) occurred at conditions close to the end of the test range and are, respectively, 9% and 0.0048 L min^-1W^-1. The maximum throughput of CO₂ converted was 0.59 L min^-1, at an SEI of 0.1 eV mol^-1 and an ECE of 0.004 L min ^-1W^-1. Optical emission spectroscopy was used to approximate the rotational and vibrational temperatures of the plasma system to be in the range of 7000-8000 K. This paper suggests that an MPS is the most energy efficient method in dissociating CO₂ under atmospheric pressure and, therefore, most suitable to be used in a CO₂ recycling setup, which can further increase conversion rates.