Carbo- and Methanothermal Reduction of Tungsten Trioxide into Metallic Tungsten for Thermochemical Production of Solar Fuels

The WO3/W redox pair is proposed as an alternative option for the production of solar fuels due to high reactivity and selectivity in two-step thermochemical redox cycles. This study addresses the high-temperature solar step involving the use of carbonaceous materials (carbon or methane) as reducing agents to lower the temperature of the reduction step in the WO3/W cycle, which makes the process compatible with the use of concentrated solar energy as the source of process heat. The carbothermal reduction of tungsten trioxide to tungsten by using carbon in the form of graphite, carbon black, and activated carbon was investigated with a thermobalance and a packed-bed tubular reactor, whereas methanothermal reduction was studied by using a solar-driven thermogravimetric reactor. The WO3/C powder reactivity was analyzed as a function of temperature, carbon type, and stoichiometry of the reactant mixture. The reaction was complete upon heating to 1280 degrees C when using excess carbon in the mixture, with metallic tungsten and carbon monoxide as the main products. A high specific surface area of carbon favored the solid-gas reaction mechanism, whereas a small carbon nanoparticle size favored the solid-solid mechanism, along with the formation of carbides. Methanothermal reduction of WO3 started from 850 degrees C and yielded mainly W and WC at 1000 degrees C with WO3 conversion above 80%. This study thus indicates that carbo- and methanothermal reduction of tungsten trioxide can be used to produce metallic tungsten powder efficiently.