Production of hydrogen from solar zinc in steam atmosphere

Production of hydrogen via hydrolysis of zinc with steam is an essential step in the Zn//Z nO thermochemical cycle for splitting of water. Recent studies on reducing ZnO to Zn metal with the aid of concentrated solar energy stimulated the interest in the hydrolysis of the zinc for hydrogen production. One of these studies was focusing on solar carbothermal reduction of ZnO to produce zinc powder (EC//FP5-SOLZINC project). The current paper deals with the hydrolysis process of this material which will be referred to, hereafter, as SOLZINC. Test results obtained during the hydrolysis of SOLZINC powder in batch experiments at atmospheric pressure demonstrate possibilities of fast and high conversion of SOLZINC powder with steam to ZnO powder and hydrogen without intermediate melting or evaporation of zinc and indicate that the reaction occurs in two different rates, depending on the preheating temperature. A slow reaction starts at about 250∘C and the hydrogen output increases with reactor temperature. The fast stage starts as the reactor temperature approaches 400∘C. Above this temperature, the reaction develops vigorously due to fast increase of the reaction rate with temperature resulting in releasing additional exothermic heat by the reacted powder. Increasing the preheating temperature (when the steam flow starts) from 200 to 550∘C can improve the SOLZINC conversion during the fast stage from 24% to 81% and increase the hydrogen yield. When the fast stage decays, slow reaction can be continued on for a long time until the hydrogen production is fully achieved.