Halogen-Mediated Methane Pyrolysis for CO₂-Free Hydrogen Production

Abstract

While low-cost natural gas resources remain abundant, a cost-effective option for the production of hydrogen without direct carbon dioxide emissions is pyrolysis, whereby a hydrocarbon is thermochemically decomposed into hydrogen and solid carbon. A major engineering challenge is providing the large quantities of heat required for the endothermic reaction. We report results for a pyrolysis process in which methane is reacted with sufficient chlorine or bromine such that the partial oxidation reaction to form solid carbon is autothermal, with higher reaction rates and higher equilibrium conversion than with methane alone. The autothermal reaction of 1:1 CH4/Br2 has an equilibrium methane conversion of over 95%, and the results demonstrate over 80% methane conversion in equimolar chlorine or bromine at 950 °C. No catalyst is required; however, in the process, hydrogen and hydrogen halides produced must be separated and the halogen must be regenerated by electrolysis or chemical oxidation. The CO2-free hydrogen cost of production for the halogen-mediated methane pyrolysis process is estimated to be $2.2–$2.4/kg H2, which is lower than that for water electrolysis ($3.5/kg H2) and comparable to conventional methane reforming combined with carbon capture and sequestration ($2.4/kg H2).