On the Applicability of Iron-Based Oxygen Carriers and Biomass-Based Syngas for Chemical Looping Hydrogen Production

The chemical looping hydrogen (CLH) production process typically uses iron-based oxygen carrier materials and can provide hydrogen with high purity. Chemical looping is particularly attractive when renewable fuels like syngas from biomass gasifiers are used. This work provides a novel assessment of the possible thermodynamic and kinetic limitations for iron-based oxygen carriers in CLH fueled by biomass-based syngas, with a detailed study employing synthetic ilmenite (Fe2O3 + TiO2). Its phase diagram with H2/H2O- or CO/CO2-mixtures was compared to the typical Baur–Glaessner diagram for iron oxides. Thermogravimetric analyses underlined the necessity to consider TiO2 as a chemically active component for this material, in contrast to the common simplification of inert support materials. The validated phase diagram predicted stringent fuel limitations concerning H2O- or CO2-contents. This was confirmed by feeding a real biomass-based syngas, provided by a lab-scale gasifier, to a fixed bed CLH reactor. It was demonstrated for the H2/H2O-system that removing the oxidizing agent from the feed gas helps to overcome these limitations. Kinetic limitations within the thermodynamic boundaries were investigated using a recently published multiscale model for the H2/H2O-system. The influence of the fuel’s reduction potential on reaction rates was explored to formulate simple, kinetic design criteria. A significant retardation of conversion rate in the vicinity of the equilibrium was indicated, effectively narrowing the feasible composition range. Recommendations for the application of biomass-based syngas with iron-based oxygen carrier materials were provided.