Date of Award

Spring 1-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Alan Weimer

Second Advisor

Hans Funke

Third Advisor

Hicham Idriss

Fourth Advisor

David Clouph

Fifth Advisor

Joel Kaar

Abstract

Solar thermal splitting of water or carbon dioxide is a promising technology for producing hydrogen and/or carbon monoxide. In a two-step cycle, a metal oxide is thermally reduced with concentrated solar radiation to release oxygen. The reduced metal oxide is then re-oxidized with steam or carbon dioxide to produce hydrogen or carbon monoxide. The two-step redox cycle can be operated either as a temperature swing where there is a temperature difference between the reduction and oxidation steps or isothermally. This work discusses various aspects of operating the redox cycle isothermally including redox cycle thermodynamics and overall system efficiency and describes solar reactor concepts based on isothermal operation.

This work reports the reduction kinetic study of the hercynite cycle (FeAl2O4) for high temperature solar thermochemical water splitting. The reaction kinetics has been evaluated using dynamic thermogravimetric and XRD analyses. Kinetic modeling results indicate that as-prepared hercynite materials undergoes reduction via two different reaction mechanisms. The reaction first proceeds by a nucleation and growth reaction mechanism, followed by a third-order kinetic model. XRD analyses show the occurrence of superstoichiometric oxygen in the spinel structure of FeAl2O4+δ in the second reaction mechanism, which indicates the formation of cationic vacancies. TGA and XRD analyses show that hercynite materials operates via a cation-vacancy mechanism when the materials are thermally reduced and oxidized with steam.

High-temperature thermochemical energy storage shows promise in aiding concentrating solar power plants in meeting variable, grid-scale electricity demand. In this work, manganese oxide-based mixed metal oxide particles have been designed and tested for high temperature solar thermochemical energy storage. We evaluate the effects of Al2O3, Fe2O3, and ZrO2 in Mn2O3-based spray-dried particles in a TGA between 650°C and 1,200°C over six consecutive redox cycles. Results are compared with thermodynamic predictions from 400–1,400°C under oxidizing and reducing atmospheres. A mixture of 2:1 Fe2O3:Mn2O3 formed iron manganese oxide spinel (MnFe2O4) on calcination, and demonstrated the highest thermochemical activity. Conversely, zirconia was an inert support that does not react with manganese oxide. The oxidation reaction kinetics of MnFe2O4 has been evaluated using solid-state kinetics theory and XRD analysis. A kinetics study indicates that the reaction proceeds by two different reaction mechanisms. The reaction first proceeds by a diffusion-controlled reaction mechanism with no phase change, followed by a nucleation-growth reaction mechanism.

Share

COinS