Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical & Biochemical Engineering

First Advisor

Alan Weimer

Second Advisor

Will Medlin

Abstract

The chemical looping hydrogen (CLH) process generates pure, separate streams of H2 and CO2 from synthesis gas without the use of expensive gas separation equipment. This technology is a potentially efficient method for future H2 production from coal or biomass with integrated CO2 capture. In the CLH process, a metal oxide material is reduced through contact with syngas at temperatures between 673 K and 1273 K, fully oxidizing the CO and H2 in the syngas to H2O and CO2. The reduced metal oxide is then contacted with steam to regenerate the metal oxide and produce H2. The mixed metal ferrites CoFe2O4 and NiFe2O4 are proposed as alternative metal oxides to the currently used Fe2O3. Thermodynamic analysis with the software package FactSageTM predicts high conversions of H2 and CO to H2 and CO2 during the CLH reduction step and complete ferrite regeneration during the H2O oxidation step. Laboratory experiments with mixed metal ferrites deposited on high surface area ZrO2 support structures indicate cyclability under CLH conditions, and post-cycling analysis shows complete regeneration of the mixed metal spinel with no detected metal oxide-support interactions. In a packed bed reactor, CoFe2O4 and NiFe2O4 show superior performance to Fe2O3, with over 99% conversion of CO and H2 to CO2 and H2O during reduction. Over 90% of the H2/CO used to reduce the mixed metal ferrites was recovered as H2 during H2O oxidation. For Fe2O3, the recovery was only 20%. A kinetic analysis of the oxidation step indicated a dual oxidation mechanism for mixed metal ferrites that involved an order of reaction model followed by a diffusion limited model at higher conversions. Diffusion limitations are attributed to the effect of incorporation of Co2+ and Ni2+ cations into the spinel lattice. The reduction reaction in a packed bed reactor is found to follow gas-solid equilibrium conversion values closely at low solid conversions. Analysis of the CLH system using an equilibrium limited model indicates these materials offer significant advantages in H2 output over Fe2O3 at lower reaction temperatures and with high CO2/CO and H2O/ H2 syngas.

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