Fast gas quenching of microwave plasma effluent for enhanced CO2 conversion

Improvement of the CO2 conversion using a 2.45 GHz microwave plasma torch in reverse vortex configuration with cooled effluent channels at atmospheric pressure is reported. This configuration allows for fast gas quenching by efficient gas cooling of the effluent, which leads to conversion and energy efficiency results at 900 mbar that are very similar to the highest conversion and energy efficiency values typically obtained at lower pressures (100-200 mbar). The convective cooling in long cooled channels proved to be effective, particularly in the configuration with multiple channels, where the effective cooling surface was increased by a factor of 2.4. In comparison with the forward vortex the reverse configuration did not improve conversion, it did improve plasma stability, demonstrating that the observed effect can be attributed to the convective gas cooling in the effluent by gas-surface interaction. High specific energy inputs (> 5.5 eV/ molecule) were achieved in experiments where narrow gas inlets were used, resulting in 8 times higher inlet CO2 velocities that stabilised the plasma, allowing conversion values up to 57% at 900 mbar. The plasma stabilisation was most effective at low flow rates, where high gas velocity confined the plasma to the quartz tube centre and allowed coupling higher power to the plasma (high SEI). Additionally, experiments without a vacuum pump at atmospheric pressure were performed, yielding identical results as at 900 mbar, demonstrating that the vacuum pump is not needed to achieve high conversion rates with the use of the reverse vortex configuration with cooled effluent channels.