Article pubs.acs.org/EF
Kinetics of Mn2O3−Mn3O4 and Mn3O4−MnO Redox Reactions Performed under Concentrated Thermal Radiative Flux Elisa Alonso,† Christian Hutter,‡ Manuel Romero,† Aldo Steinfeld,‡,§ and José Gonzalez-Aguilar*,† †
Institute IMDEA Energy, Avenida Ramón de la Sagra, 3, 28935 Móstoles, Spain Solar Technology Laboratory, Paul Scherrer Institute (PSI), 5232 Villigen PSI, Switzerland § Department of Mechanical and Process Engineering, Eidgenössische Technische Hochschule (ETH) Zürich, 8092 Zurich, Switzerland ‡
ABSTRACT: Manganese oxide based on the thermochemical cycle for splitting water is considered using concentrated solar energy. The high-temperature endothermic step is the thermal reduction of Mn2O3, which proceeds at above 1835 K via two sequential chemical reactions from Mn2O3 to Mn3O4 and then to MnO. The kinetic mechanisms of both reactions are investigated by a solar-driven thermogravimeter, with reactants directly exposed to high-flux thermal irradiation. With this arrangement, the overall kinetic rate laws are derived under similar heat- and mass-transfer characteristics existing in highly concentrating solar systems, such as solar towers or parabolic dishes. The experimental results suggest a nth-order rate for the conversion of Mn2O3 to Mn3O4 and a diffusion-controlled regime for the conversion of Mn3O4 to MnO. Activation energies for both reduction steps are determined and compared to previous reported data. It has been reported that the first chemical reaction (eq 4) proceeds at the temperature range between 800 and 1060 °C in nitrogen, air, or oxygen, while the second chemical reaction (eq 5) needed 1460 °C in nitrogen and does not progress when heating takes place in oxygen at 1460 °C.15 Solar reduction tests have been previously carried out using concentrated radiation in a directly irradiated reactor exposed to a solar furnace at above 1627 °C.16 These experiments yielded mixtures of MnO and Mn3O4, where the maximum MnO molar concentration found was 85%. Thermogravimetry (TG) and a non-solar aerosol flow reactor have been used by Francis17 and Francis et al.18 to analyze the temperature influence on the chemical kinetics of both chemical reactions. It was considered that the Avrami− Erofeev mechanism was the best at describing the kinetic model of the chemical reaction (eq 4). However, the adjustment was limited to a range of fractional extent of the reaction, α, between 0.15 and 0.85. For eq 5, Francis determined a dual mechanism composed of an Avrami−Erofeev mechanism up to an extent of reaction of approximately α = 0.60 and a nth-order model to higher values of α. An oxygen diffusion limitation was observed in thermogravimetric experiments, and it was particularly strong during the second chemical reaction. The kinetic parameters, e.g., rate constant and activation energy, appeared to be influenced by the size of the crucible, which would indicate mass-transfer resistance on the oxide surface.19,20 Similar diffusion effects on the reaction interface were found by Berg and Olsen,21 when manganese oxide decomposition was performed with reducing agents, such as H2, CO, CO2, and carbonaceous materials. The oxygen-releasing rate decreased with the diameter of the oxide particles. Recently, new thermodynamic and kinetics data obtained using thermogravimetric analysis have been reported.22,23
1. INTRODUCTION Water splitting by solar thermochemical cycles via metal oxide redox reactions potentially offers high energy conversion efficiencies.1,2 In particular, the two-step Fe3O4/FeO and ZnO/Zn cycles have been intensively investigated.3−11 At the end of the 1990s, Sturzenegger et al.12 proposed the three-step manganese oxide thermochemical cycle. It consists of three consecutive chemical reactions. Mn2O3 → 2MnO + 1/2O2
(
