Microwave plasma-based dry reforming of methane: Reaction performance and carbon formation

We investigate atmospheric pressure microwave (MW) plasma (2.45 GHz) conversion in CO2 and CH4 mixtures (i.e., dry reforming of methane, DRM) focusing on reaction performance and carbon formation. Promising energy costs of similar to 2.8-3.0 eV/molecule or similar to 11.1-11.9 kJ/L are amongst the best performance to date considering the current state-of-the-art for plasma-based DRM for all types of plasma. The conversion is in the range of similar to 46-49% and similar to 55-67% for CO2 and CH4, respectively, producing primarily syngas (i.e., H-2 and CO) with H-2/CO ratios of similar to 0.6-1 at CH4 fractions ranging from 30% to 45%. Water is the largest byproduct with levels ranging similar to 7-14% in the exhaust. Carbon particles visibly impact the plasma at higher CH4 fractions (> 30%), where they become heated and incandescent. Particle luminosity increases with increasing CH4 fractions, with the plasma becoming unstable near a 1:1 mixture (i.e., > 45% CH4). Electron microscopy of the carbon material reveals an agglomerated morphology of pure carbon nanoparticles. The mean particle size is determined as similar to 20 nm, free of any metal contamination, consistent with the electrode-less MW design.