Carbon pathways, CO2 utilization, and in situ product removal in low temperature plasma methane conversion to methanol

This study examines a non-equilibrium "cold" plasma reaction system for the partial oxidation of methane as well as the water-gas shift reaction and CO2 reforming of methane. Cylindrical and planar plasma reactors are used. Both systems have a glass dielectric interposed between two metal electrodes. The electrodes are supplied with kV ac power with frequencies ranging from 50 Hz to 100 Hz. The results show that lowering the reaction temperature from 75 degrees C to 28 degrees C in the partial oxidation experiments increases overall organic oxygenate liquid selectivities from 24% to 52%. The loa er temperatures reduce these products' vapor pressures to their equilibrium partial pressures allowing condensation within the reactor. Experiments involving CO and H2O, and CO2 and H-2 confirm that the water-gas shift reaction pathway occurs along with the partial oxidation of methane. Results of carbon dioxide reforming of methane indicates that the CO2:CH4 ratio strongly affects the results. The ethane and hydrogen selectivity decrease when the CO2:CH4 ratio is reduced from 1:2 to 1:1. This decrease in partial pressure of methane decreases direct coupling to ethane. Increasing the partial pressure of CO2 from a 1:2 to 1:1 CO2:CH4 ratio causes increased CO2 conversion to CO. The selectivity for methanol under reforming conditions is less than 1%. A further increase in the CO2:CH4 ratio from a 1:1 to 2:1 causes an increase in ethane and hydrogen selectivity indicating that an oxidative coupling pathway may play a role under these conditions. Finally, the results show that the presence of helium enhances methane conversion and alters the reaction selectivities despite lowering methane partial pressures, thus indicating a third body effect on methane activation.