Novel Mixed Metal Ferrites for Hydrogen Production Using Chemical Looping

Aston, Victoria J.

Graduate Thesis Or Dissertation

Novel Mixed Metal Ferrites for Hydrogen Production Using Chemical Looping Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/sn009x937

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 FactSage^TM 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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