Solar hydrogen production via a two-step thermochemical process based on MgO/Mg redox reactions—Thermodynamic and kinetic analyses

Solar hydrogen production via a two-step water-splitting thermochemical cyclic process is considered via MgO/Mg redox reactions. The first endothermic step is the production of Mg by carbothermal or methanothermal reduction of MgO, using concentrated solar energy as the source of high-temperature process heat. The second exothermic step is the steam-hydrolysis of Mg for the production of H2 and MgO; the latter is recycled to the first step. Both reaction steps have been thermodynamically examined and experimentally investigated by means of thermogravimetric analysis. The carbothermal reduction of MgO was performed in the temperature range 1450–1550 °C using wood charcoal and petroleum coke as reducing agents. The steam-hydrolysis of Mg was studied in the temperature range 350–550 °C using various water vapor concentrations. Solid products were characterized via BET, XRD, and SEM. The rate laws of both reaction steps were determined by applying either a solid–solid diffusion kinetic model or the gas–solid shrinking core kinetic model.