(560ey) Methane Pyrolysis in Molten Metal/Salt Two-Phase Reactors
Methane is the lowest cost source of hydrogen today. Although steam methane reforming is the most cost effective technology for hydrogen production today, it generates more than 5 kg of CO2 byproduct for every kg of hydrogen. Catalytic methane pyrolysis, catalyzed by conventional heterogeneous catalysts, produces hydrogen without CO2 but the carbon coproduct poisons the catalyst. This can be avoided by using a molten metal alloy catalyst. Performing methane pyrolysis on molten metal catalysts can eliminate catalyst coking. In molten metal bubble columns, the separation of the produced carbon in a continuous process is possible as the low-density solid carbon floats on the surface of the liquid. However, using molten metals, it is difficult to produce high purity carbon without metal contamination. In this study, the production and separation of solid carbon from pyrolysis of methane with reduced metal contamination is achieved using a two-phase bubble column with a catalytic molten NiBi alloy covered with molten salt. The immiscibility of the molten metals and molten salts provided the conditions to produce a multiphase metal/salt/carbon system to separate carbon from the molten liquid efficiently. In the rising gas bubbles containing reactant and product gases with metal vapors, the solid carbon gas can entrain NiBi vapor from the liquid metal reactor region into the molten salt phase where it can condense as a metal film. It is proposed that when the gas-filled bubbles pass through the metal-salt interface, the surface metal film ruptures and metal particles are produced which return to the dense liquid metal phase and the low-density carbon is lifted to the top of the column. HRTEM analysis of the carbons revealed structural differences between carbon generated in the NiBi single-phase and NiBi/salt two-phase reactors. The carbons produced in the metal/salt two-phase reactors contained carbon black aggregates and carbon nanotubes structures while in the NiBi reactor graphitic carbon is observed. The metal contamination in the carbon produced from NiBi/salt two-phase reactor is less than 5 wt. % compared to 83 wt. % obtained from reactors with only NiBi.