Plasma-Assisted Chemical-Looping Combustion: Low-Temperature Methane and Ethylene Oxidation with Nickel Oxide

The chemical reaction network of low-temperature plasma-assisted oxidation of methane (CH4) and ethylene (C2H4) with nickel oxide (NiO) was investigated in a heated plasma reactor through time-dependent species measurements by electron- ionization molecular beam mass spectrometry (EI-MBMS). Methane (ethylene) oxidation by NiO was explored in temperature ranges from 300-700 degrees C (300-500 degrees C) and 300-800 degrees C (300- 600 degrees C) for the plasma and nonplasma conditions. Significant enhancement of methane oxidation was observed with plasma between 400 and 500 degrees C, where no oxidation was observed under nonplasma conditions. For the oxidation of methane at higher temperatures, three different oxidation stages were observed: (I) a period of complete oxidation, (II) a period of incomplete CO oxidation, and (III) a period of carbon buildup. For the C2H4 experiments, and unlike the CH4 experiments, the plasma resulted in a significant amount of new intermediate oxygenated species, such as CH2O, CH3OH, C2H4O, and C2H6O. Carbon deposits were observed under both methane and ethylene conditions and verified by X-ray photoelectron spectroscopy (XPS). ReaxFF (reactive force field) simulations were performed for the oxidation of CH4 and C2H4 in a nonplasma environment. The simulated intermediates and products largely agree with the species measured in the experiments, though the predicted intermediate oxygenated species such as CH2O and C2H6O were not observed in experiments under nonplasma conditions. A reaction pathway analysis for CH4 and C2H4 reacting with NiO was created based on the observed species from the MBMS spectra along with ReaxFF simulations.

Automated summary

The chemical reaction network of low-temperature plasma-assisted oxidation of methane and ethylene with nickel oxide was studied using electron-ionization molecular beam mass spectrometry. Plasma significantly enhanced methane oxidation between 400 and 500 degrees C, while no oxidation was observed without plasma. Different oxidation stages were observed for methane at higher temperatures. Ethylene experiments showed the formation of new intermediate oxygenated species under plasma conditions. Carbon deposits were observed in both methane and ethylene conditions. A reaction pathway analysis was created based on experimental observations and simulations.