On the Effect of the Presence of Solid Diluents during Zn Oxidation by CO2

Abstract

We consider the solar thermochemical production of H2 and CO (syngas) from H2O and CO2 via a two-step ZnO/Zn redox cycle. The first step, driven by concentrated solar radiation, is the endothermic thermolysis of ZnO producing a gaseous mixture of O2 and Zn-vapor, which upon quenching precipitates a solid residue comprising Zn and ZnO in variable ratios. The second, nonsolar step is the exothermic oxidation of Zn by H2O and/or CO2 to form fuel (H2 and/or CO) and the solid product ZnO which is recycled to the solar reactor. It has been recognized that the presence of inert ZnO during the second step affects both the oxidation kinetics and the final asymptotic conversion of Zn. However, while the fraction of ZnO in a mixture with Zn leaving the thermolysis step generally varies with a solar reactor/quencher design and experimental conditions, all previously reported analyses have studied the oxidation kinetics of either pure Zn or of Zn particles blended with ZnO in a specific, preset mass ratio. This work examines the effect of dilution with inert particles on the Zn oxidation by CO2. Blends of commercially available Zn with ZnO or Al2O3 particles have been tested by thermogravimetry. The setup was carefully designed to ensure the absence of heat and mass-transfer intrusion on the oxidation kinetics while using a 15% CO2–Ar mixture or pure CO2 at 350 °C < T < 400 °C and ambient pressure. The effects of inert particle type, mass fraction, and blending method are reported and used to propose a simplified multipath reaction mechanism. The results are compared to the performance of a Zn/ZnO powder produced in a solar reactor.