The kinetics of hydrogen production in the oxidation of liquid zinc with water vapor

Thermochemical cycles where metal oxides are reduced and the elementary metal is reoxidized with water, thus generating hydrogen has been proposed for conversion of solar energy to chemical energy and fuels. The two steps of the cycle can be separated in time and place, thus providing also means for storage of solar energy in chemical form. The second step of these cycles involves the oxidation of the elementary metal in water vapor where hydrogen is generated and the metal oxide is recovered and recycled. Reduction of zinc oxide is a typical reaction used for such cycles. Therefore, the oxidation of liquid zinc with water, specifically, becomes an important part of such a cycle. The kinetics of oxidation of bulk of liquid zinc at 450–500°C, with water vapors bubbled through the liquid, was studied. The water vapor was introduced into the liquid using argon as a carrier gas. The water vapor partial pressure was in the range of 65–560 mb. The gas mixture was fed through a tube dipped in the liquid. The diameter of the tube and its depth were varied as parameters of the experiments. The results of the experiments show that the specific reaction rate (hydrogen production per cm2 of surface area of the bubble), Wsp, increases, as the water partial pressure is increased. The order of the reaction, with respect to water partial pressure, is 0 < n < 1. As a result of the experiments, the kinetic expression Wsp=kPH2O/(1+ b·PH2O) was found: k = 1.86×10−3·EXP(−40376/RT) mole·cm−2·s−1bar−1, b = 1.55×10−10·EXP(146330/RT) bar−1. The main stages of the reaction are interaction between elementary zinc and water vapor on the interface between the gas and the solid oxide layer, and the diffusion of zinc atoms through the film of solid zinc oxide.