A low carbon methanol process using natural gas pyrolysis in a catalytic molten metal bubble reactor

Abstract

Due to its abundance and relatively low cost, natural gas will continue to be a common feedstock for methanol production, at least in the near to medium term. The present work focuses on natural gas pyrolysis in a catalytic liquid metal bubble reactor, combined with CO2utilization, as an alternative to traditional steam reforming processes. The pyrolysis reactor consists of multiple vertical tubes filled with molten Cu0.45Bi0.55alloy and heated in the radiant zone of a fired heater. The reactor geometry and operating conditions are optimized by rigorously accounting for the coupling of bubble flow hydrodynamics with the catalytic and non-catalytic kinetics of methane pyrolysis in molten metals. Pyrolysis generates hydrogen and solid carbon, which rise first through the liquid metal and then through a layer of molten salt (e.g., NaBr) whose purpose is to minimize the loss of liquid metal entrained with the carbon. The carbon forms a recoverable layer above the salt, and the hydrogen reacts with captured CO2in aseparate methanol synthesis reactor. The cradle-to-gate and cradle-to-grave CO2emissions are 0.074 and 1.45 t CO2-eq/t methanol, respectively, for median supply chain emissions of natural gas and captured CO2. This is a lower carbon footprint than other practical pyrolysis and steam methane reforming processes. Negative cradle-to-gate CO2emissions are achievable if natural gas and captured CO2are sourced from supply chains with low emissions. The estimated carbon production cost of $0.27/kg iswell below the recent market price of carbon black in the United States. The production cost of carbon is most sensitive to the fixed capital investment of the plant, the captured CO2cost, the natural gas and methanol costs, and the discount rate.