Hydrogen production by three-step solar thermochemical cycles using hydroxides and metal oxide systems

This paper presents a thermodynamic and experimental study of three-step thermochemical cycles for hydrogen production involving hydroxides (NaOH and KOH). Solar concentrated energy was successfully used to reduce manganese, cobalt, and iron oxides into lower valence metal oxides, MnO, CoO, Fe3O4, and FeO, in the temperature range of 1300-1600 degrees C. In the reaction with NaOH and KOH, MnO and CoO were stable and did not produce hydrogen at 750 degrees C even in a strong oxidizing media, whereas the iron oxides FeO and Fe3O4 were able to generate hydrogen. For the NaOH activation reaction, the final chemical conversion rate was 28% at about 400 degrees C with FeO particles in the range of 30-50 μm, and a passivating layer was observed, which reduced the H2 production rate when the particle size increased. The reaction between Fe3O4 particles and NaOH reached a final conversion higher than 70% after 7 min of the reaction for particle sizes in the range of 30-125 μm. In addition, the reaction between Fe3O4 and KOH producing hydrogen was nearly complete. Although the three-step cycle based on FeO appears attractive in terms of theoretical productivity (156 mLH2g-1 of FeO assuming a complete reaction) and energy efficiency (41.3%), it requires a high temperature reduction reaction and a small particle size for the H2-production reaction. Finally, the comparison of iron oxide cycles highlights the high potential of the three-step cycle based on the Fe2O3/Fe3O4 pair, taking into account experimental chemical conversions (37 mLH2g-1 of Fe3O4 for a 75% chemical conversion). (authors)