Manganese‐iron oxide particles are a promising candidate for both chemical‐looping combustion (CLC) and thermochemical energy storage. In CLC, the ability of metal oxides to oxidize fuels in an oxygen‐free atmosphere and re‐oxidize in air is addressed. Whereas, reaction enthalpy is the main focus of thermochemical energy storage for, e. g. concentrated solar power or an industrial process that requires high temperature levels. Sufficient mechanical strength of the particles while they endure chemical, thermal, or mechanical stress is a crucial factor for both concepts. Particle stability is investigated here by adding 20 wt.% of TiO2, ZrO2, or CeO2 as a supportive material to (Mn0.7Fe0.3)2O3. Thermal cyclization and temperature shock tests are conducted in a packed bed reactor to identify chemical stability as well as the effect of chemical and thermal stress. A subsequent particle size distribution analysis is performed to determine the relevant breakage mechanism. Attrition resistance is tested with a customized attrition jet cup to estimate the mechanical strength of particles. It is found that the high tendency of unsupported manganese‐iron oxide particles towards agglomeration can be improved with any of the chosen additives. The particles with CeO2, and especially with ZrO2, as an additive indicate an increase in resistance towards attrition. However, adding TiO2 has a severe negative impact on the chemical reactivity of the manganese‐iron oxide.
High-temperature thermal energy storage enables concentrated solar power plants to provide base load. Thermochemical energy storage is based on reversible gas-solid reactions and brings along the advantage of potential loss-free energy storage in the form of separated reaction products and possible high energy densities. The redox reaction of metal oxides is able to store thermal energy at elevated temperatures with air providing the gaseous reaction partner. However, due to the high temperature level, it is crucial to extract both the inherent sensible and thermochemical energies of the metal-oxide particles for enhanced system efficiency. So far, experimental research in the field of thermochemical energy storage focused mainly on solar receivers for continuously charging metal oxides. A continuously operated system of energy storage and solar tower decouples the storage capacity from generated power with metal-oxide particles applied as heat transfer medium and energy storage material. Hence, a heat exchanger based on a countercurrent moving bed concept was developed in a kW-scale. The reactor addresses the combined utilization of the reaction enthalpy of the oxidation and the extraction of thermal energy of a manganese-iron-oxide particle flow. A stationary temperature profile of the bulk was achieved with two distinct temperature sections. The oxidation induced a nearly isothermal section with an overall stable off-gas temperature. The oxidation and heat extraction from the manganese-iron oxide resulted in a total energy density of 569 kJ/kg with a thermochemical share of 21.1%.
High temperature storage is a key factor for compensating the fluctuating energy supply of solar thermal power plants, and thus enables renewable base load power. In thermochemical energy storage, the thermal energy is stored as the reaction enthalpy of a chemically reversible gas-solid reaction. Metal oxides are suitable candidates for thermochemical energy storage for solar thermal power plants, due to their high reaction temperatures and use of oxygen as a gaseous reaction partner. However, it is crucial to extract both sensible and thermochemical energy at these elevated temperatures to boost the overall system efficiency. Therefore, this study focuses on the combined extraction of thermochemical and sensible energy from a metal oxide and its effects on thermal power and energy density during discharging. A counter-current moving bed, based on manganese-iron-oxide, was investigated with a transient, one-dimensional model using the finite element method. A nearly isothermal temperature distribution along the bed height was formed, as long as the gas flow did not exceed a tipping point. A maximal energy density of 933 kJ/kg was achieved, when ( Mn , Fe ) 3 O 4 was oxidized and cooled from 1050 ° C to 300 ° C . However, reaction kinetics can limit the thermal power and energy density. To avoid this drawback, a moving bed reactor based on the investigated manganese-iron oxide should combine direct and indirect heat transfer to overcome kinetic limitations.
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