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.

Mechanism of Zn Particle Oxidation by H2O and CO2 in the Presence of ZnO

“…Recent research has demonstrated that the ZnO contamination of Zn produced by solar thermal reduction of ZnO in fact facilitates the oxidation of the metallic Zn with either CO 2 or H 2 O. 21 , 22 It has been shown that the oxidation of solar Zn by CO 2 could be almost completed at temperatures as low as 350 °C in the matter of minutes (∼90% conversion in 180 s). 22 This reaction time is indeed 2-orders of magnitude longer than the residence time reported for the equivalent conversion of Zn vapor (∼1 s).…”

“… 21 , 22 It has been shown that the oxidation of solar Zn by CO 2 could be almost completed at temperatures as low as 350 °C in the matter of minutes (∼90% conversion in 180 s). 22 This reaction time is indeed 2-orders of magnitude longer than the residence time reported for the equivalent conversion of Zn vapor (∼1 s). 20 However, becaue of the difference between the feedstock densities (∼0.7 kg/m 3 for Zn-vapor at 1000 °C versus ∼440 kg/m 3 for solar Zn containing only 25 wt % Zn 23 ) the reactor volumes required for oxidizing solar Zn and Zn-vapor at the equivalent fuel productivity are about the same.…”

A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles

“…This step need not be solaraided and since the two steps are decoupled, the production of H 2 and/or CO can be carried out on demand and round-the-clock at convenient sites and independent of solar energy availability [66]. Non-solar exothermic oxidation of Zn by H 2 O and/or CO 2 to form fuel (H 2 and/or CO) has been investigated [67]. It was shown that the presence of inert ZnO affects both the oxidation kinetics and the final asymptotic conversion of Zn.…”

“…Such cast objects made of 1:3 Co 0. 67 were tested for cyclic TR/WS in a typical, laboratory-scale, non-solar-heated flow reactor between approximately 1673 K (TR) and 1273 K (WS). Structural integrity could be maintained over successive cycles, however only the first composition was proved capable of cyclic WS/TR operation.…”

Hydrogen production via thermochemical water splitting

“…Nonsolar exothermic oxidation of Zn by H 2 O and/or CO 2 to form fuel (H 2 and/or CO) has been investigated (Stamatiou, Steinfeld, & Jovanovic, 2013). The presence of inert ZnO affects both the oxidation kinetics and the final asymptotic conversion of Zn.…”