Study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production

In this work, a complete thermodynamic study of the first step of the Mn2O3/MnO thermochemical cycle for solar hydrogen production has been performed. The thermal reduction of Mn2O3 takes place through a sequential mechanism of two reaction steps. The first step (reduction of Mn2O3 to Mn3O4) takes place at teomperatures above 700 °C, whereas the second reaction step (reduction of Mn3O4 to MnO) requires temperatures above 1350 °C to achieve satisfactory reaction rates and conversions. Equilibrium can be displaced to lower temperatures by increasing the inert gas/Mn2O3 ratio or decreasing the pressure. The thermodynamic calculations have been validated by thermogravimetric experiments carried out in a high temperature tubular furnace. Experimental results corroborate the theoretical predictions although a dramatically influence of chemical kinetics and diffusion process has been also demonstrated, displacing the reactions to higher temperatures than those predicted by thermodynamics. Finally, this work demonstrates that the first step of the manganese oxide thermochemical cycle for hydrogen production can be carried out with total conversion at temperatures compatible with solar energy concentration devices. The range of required temperatures is lower than those commonly reported in literature for the manganese oxide cycle obtained from theoretical and thermodynamic studies.

► Thermodynamic simulation of Mn2O3 thermal reduction validated by experimental data. ► Two steps sequential mechanism without coexistence of the three Mn2O3, Mn3O4 and MnO phases. ► Experimental temperatures higher than predicted due to reaction kinetics and O2 diffusion. ► Successful prediction of the effect of pressure, gas/solid ratio and presence of O2. ► Total conversion feasible at temperatures below 1400 °C, compatible with solar reactors.