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% Fe2O3, 10% stabilising cement) is reduced with different hydrogen (H2) concentrations at temperatures between 600 and 800∘C. The discharge phase is initiated by the oxidation of the previously reduced pellet by water vapour (H2O) at different concentrations in the same temperature range. In both reactions, nitrogen (N2) is used as a carrier gas. The redox reactions have been experimentally measured in a thermogravimetric analyser (TGA) at a flow rate of 250mL/min. 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 (Ea,Re=56.9kJ/mol versus Ea,Ox=16.0kJ/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.

This work aims at investigating the reduction/oxidation (redox) reaction kinetics on iron oxide pellets under different operating conditions of thermochemical hydrogen storage. In order to reduce the iron oxide pellets (90% Fe2O3, 10% stabilizing cement), hydrogen (H2) is applied in different concentrations with nitrogen (N2), as a carrier gas, at temperatures between between 700∘C and 900∘C, thus simulating the charging phase. The discharge phase is triggered by the flow of a mixture out of steam (H2O) and N2 at different concentrations in the same temperature range, resulting in the oxidizing of the previously reduced pellets. All investigations were carried out in a thermo-gravimetric analyzer (TGA) with a flow rate of 250mL/min. To describe the obtained kinetic results, a simplified analytical model, based on the linear driving force model, was developed. The investigated iron oxide pellets showed a stable redox performance of 23.8% weight reduction/gain, which corresponds to a volumetric storage density of 2.8kWh/(Lbulk), also after the 29 performed redox cycles. Recalling that there is no H2 stored during the storage phase but iron, the introduced hydrogen storage technology is deemed very promising for applications in urban areas as day-night or seasonal storage for green hydrogen.

A promising process to store hydrogen is the thermochemical storage based on the repeated reduction and oxidation (redox) of iron oxide or iron. This storage process is an intermittent, twophase reaction, which takes place under atmospheric pressure in a hydrogen atmosphere during reduction (charging) or in a steam atmosphere during oxidization (discharging). The investigations have been carried out at two constant temperatures, namely 700°C and 800°C. During the storage phase, only iron exists inside the storage reactor -a fact that makes the redox system much safer, compared to hydrogen storage under pressure in a tank.This work aims at studying the effect of adding different supporting materials upon producing the iron oxide storage composite samples on their thermochemical cycle stability. Furthermore, the influence of the temperature during the initial sintering process and the redox cycles on the reaction behavior of the iron oxide composites is investigated.It turned out that pure iron oxide pellets have lost about 65% of their redox potential after only three cycles. Applying 10 wt.% of calcium oxide has improved the cycle stability of the iron oxide pellets to over nine cycles. After nine cycles, a loss of redox performance by less than 5% was observed. This has been attributed to the densification of the sample's outer surface, which is associated with slowing down the gas diffusion rate into/out of the investigated sample. In addition, reducing the temperature during the cycling (800°C to 700°C) and the sintering from 1100°C to 950°C has shown a positive effect on enhancing the cycle stability.

No abstract

No abstract