Heterogeneous hydrolysis with zinc vapor under a temperature gradient for efficient solar hydrogen production

Abstract

Thermochemical cycles driven by concentrated sunlight are a promising method of producing hydrogen in a renewable manner. One such cycle that has received considerable attention is the two-step Zn/ZnO thermochemical cycle. The current work focuses on the hydrolysis step of the cycle wherein water vapor is reduced by Zn to produce hydrogen. A novel approach in which the oxidation reaction is conducted heterogeneously under an axial temperature gradient has been suggested as a promising method for quickly and reliably splitting water using a relatively low percentage of inert carrier gas in the mixture. Sim- ulations and experiments were conducted in order to gain insight into the oxidation of Zn with water vapor, to help identify optimal reactor design and operating parameters, and to demonstrate proof-of-concept for this approach. ☐ A thermodynamic model of the complete Zn/ZnO thermochemical cycle was used along with a simplified model of a non-isothermal heterogeneous hydrolysis reactor in order to highlight the effect of different hydrolysis conditions on the overall solar-to-chemical exergy efficiency, as well as the potential for heat recovery during hydrolysis. The effects of the reactor’s temperature profile and mass transfer were also investigated using the model to highlight the potential for Zn condensation during the reaction. ☐ A laboratory-scale reactor was also developed in order to demonstrate the heteroge- neous hydrolysis under cooling conditions, and to quantify the Zn to ZnO conversion under different temperature ranges, and combinations of inert gas, steam, and Zn flow rates. The reactor is a tube furnace with a series of interior quartz tubes that form separate Zn evap- oration and heterogenous reaction zones. A wet chemical method was used to determine the respective amounts of ZnO that was deposited by the reaction as well as any Zn that condensed in the reaction tube. ☐ A numerical model was also developed which accounts for the effects of incompressible fluid flow, mass transfer, reaction kinetics, as well as the accumulation of solid ZnO deposits in order to form a basis for predicting the transient behavior of a single reaction channel in heterogeneous hydrolysis reactor utilizing Zn vapor. The lattice Boltzmann method was used for this model because of its simplicity in handling of moving boundaries with complex shapes as well as the ease and efficiency of implementing the algorithm in a parallel computing environment. A parametric study was conducted in order to characterize the performance of the reactor channel for different non-dimensional parameters.