Kinetic investigations of the hydrogen production step of a thermochemical cycle using mixed iron oxides coated on ceramic substrates

Abstract

A two-step thermochemical cycle for solar hydrogen production using mixed iron oxides as the metal oxide redox system has been investigated. The ferrite is coated on a honeycomb structure, which serves as the absorber for solar irradiation and provides the surface for the chemical reaction. Coated honeycomb structures have already been tested in a solar receiver reactor in the solar furnace of DLR in Cologne 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 hydrogen production during the water splitting 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 zinc ferrite have been tested. The influences of the temperature and the water concentration on the hydrogen production during the water splitting step have been investigated. An analysis of the ferrite conversion was performed using the Shrinking Core Model. A mathematical approach for the peak reaction rate at the beginning of the water splitting step was formulated and the activation energy was calculated from the experimental data. An activation energy of 110 kJ mol1 was found.