On the Kinetic Mechanisms of the Reduction and Oxidation Reactions of Iron Oxide/Iron Pellets for a Hydrogen Storage Process

Bernd Gamisch, Lea Huber, Matthias Gaderer and Belal Dawoud ()
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Bernd Gamisch: Laboratory of Sorption Processes, Technical University of Applied Sciences, OTH Regensburg, Galgenbergstraße 30, 93053 Regensburg, Germany
Lea Huber: Laboratory of Sorption Processes, Technical University of Applied Sciences, OTH Regensburg, Galgenbergstraße 30, 93053 Regensburg, Germany
Matthias Gaderer: Regenerative Energy Systems, TUM Campus Straubing for Biotechnology and Sustainability, Technical University of Munich, Schulgasse 16, 94315 Straubing, Germany
Belal Dawoud: Laboratory of Sorption Processes, Technical University of Applied Sciences, OTH Regensburg, Galgenbergstraße 30, 93053 Regensburg, Germany

Energies, 2022, vol. 15, issue 21, 1-29

Abstract: This work aims at investigating the kinetic mechanisms of the reduction/oxidation (redox) reactions of iron oxide/iron pellets under different operating conditions. The reaction principle is the basis of a thermochemical hydrogen storage system. To simulate the charging phase, a single pellet consisting of iron oxide (90% Fe 2 O 3 , 10% stabilising cement) is reduced with different hydrogen (H 2 ) concentrations at temperatures between 600 and 800 ° C . The discharge phase is initiated by the oxidation of the previously reduced pellet by water vapour (H 2 O) at different concentrations in the same temperature range. In both reactions, nitrogen (N 2 ) is used as a carrier gas. The redox reactions have been experimentally measured in a thermogravimetric analyser (TGA) at a flow rate of 250 m L / m i n . An extensive literature review has been conducted on the existing reactions’ kinetic mechanisms along with their applicability to describe the obtained results. It turned out that the measured kinetic results can be excellently described with the so-called shrinking core model. Using the geometrical contracting sphere reaction mechanism model, the concentration- and temperature-dependent reduction and oxidation rates can be reproduced with a maximum deviation of less than 5%. In contrast to the reduction process, the temperature has a smaller effect on the oxidation reaction kinetics, which is attributed to 71% less activation energy ( E a , Re = 56.9 kJ/mol versus E a , Ox = 16.0 kJ/mol ). The concentration of the reacting gas showed, however, an opposite trend: namely, to have an almost twofold impact on the oxidation reaction rate constant compared to the reduction rate constant.

Keywords: thermochemical hydrogen storage; iron/iron oxide redox reactions; reaction kinetics; shrinking-core model (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2022
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