Kinetic Investigations of a Two-Step Thermochemical Water-Splitting Cycle Using Mixed Iron Oxides Fixed on Ceramic Substrates

A two-step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. A reactor concept has been developed in which the metal oxide is fixed on multi-channelled honeycomb ceramic supports capable of adsorbing solar irradiation. In the solar furnace of DLR in Cologne coated honeycomb structures were tested in a solar receiver-reactor with respect to their water splitting capability and their long term stability. The concept of this new reactor design has proven feasible and constant hydrogen production during repeated cycles has been shown. For a further optimization of the process and in order to gain reliable performance predictions more information about the process especially concerning the kinetics of the oxidation and the reduction step are essential. To examine the kinetics of the water splitting and the regeneration step a test rig has been built up on a laboratory scale. In this test rig small coated honeycombs are heated by an electric furnace. The honeycomb is placed inside a tube reactor and can be flushed with water vapour or with an inert gas. A homogeneous temperature within the sample is reached and testing conditions are reproducible. Through analysis of the product gas the hydrogen production is monitored and a reaction rate describing the hydrogen production rate per gram ferrite can be formulated. Using this test set-up, SiC honeycombs coated with a zinc-ferrite have been tested. The influences of the water splitting temperature and the water concentration on the kinetics of the water splitting step have been investigated. A mathematical approach for the reaction rate was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ/mole was found.