(220g) A Novel Pulsed-Plasma Catalytic Reactor for Dry Reforming of Methane

Concerns over greenhouse gases have increased interest in the Dry Reforming of Methane (DMR) which produces hydrogen and carbon monoxide from the reaction of two greenhouse gases (CO₂ and CH₄) over a catalyst. Currently, DMR is primarily a catalytic process which operates at temperatures between 700°C - 900°C, and 10 to 20 bar using a 1–1.5 ratio of CH₄/CO₂. Unfortunately, these conditions also promote the water-gas shift reaction, which produces additional CO₂. We have developed a non-thermal, pulsed-plasma catalytic DRM reactor which operates at ambient temperatures and pressures. When combined with an integral monolithic catalyst bed this reactor demonstrated high conversions (60 to 95%) of both methane and carbon dioxide with high yields of hydrogen and carbon monoxide (50 to 80%). To achieve this, we developed a solid-state, MOSFET-based HV pulse generator with controllable rise times (<5 ns), pulse duration (0.1 to 10 ms), pulse shape, and frequency (10-10,000 Hz), providing improved operational flexibility and better energy efficiency than older pulse generating circuits. The reactor incorporates a point-to-plane electrode arrangement with an integral monolithic reactor which effectively couples the excited state plasma to the catalyst. The catalysts employed are stable binary metal oxides tailored for low-temperature plasma DRM reactions. Bench scale reactor tests were conducted using a feed of methane and carbon dioxide diluted in argon. To evaluate the reaction kinetics, the partial pressure of the reactants and products were measured in real time via an on-line mass spectrometer, while the excited state species were simultaneously monitored using emission spectrometry. Tests were made with the plasma alone, and the plasma plus 4 different catalyst formulations. No significant reactions were observed for the plasma without a catalyst, or for the catalyst without a plasma. The catalytic reaction kinetics were measured for a range of input power, voltages, pulse length & frequency, and electrode geometries. The feed ratio of CO₂ to CH₄ was found to be of great significance in the overall conversion and the yield of hydrogen and CO, with near stoichiometric reactant ratios proving best. The stoichiometric ratio of carbon monoxide to hydrogen in the products (from ~1:1 up to 6:1) was dependent on the ratio of the metal oxides employed in the catalyst, and can be correlated to the strength of carbon dioxide adsorption on the catalyst surface. Based on the kinetic studies and emission spectroscopy results, we propose a surface moderated reaction model which explains the high reactant conversions and product yields observed. Estimates of the energy efficiency of the bench-scale process, and a design for a larger scale reactor, indicate the potential of this novel reactor for practical applications.