Studies on the redox reaction kinetics of selected, naturally occurring oxygen carrier

Abstract

This paper presents the results of a chemical-looping combustion (CLC) study. The CLC technology is believed to be one of the most promising combustion technologies. The production of a concentrated CO2 stream that is obtained after the water condensation without any loss of energy in its separation is one of the most crucial advantages of this technology. The objective of this work was to study the kinetics of both the reduction and oxidation reactions for naturally occurring oxygen carriers that show promising reactivity in CLC reactions, and might therefore be utilized as oxygen carrier materials. Kryvbas, a Fe-based ore, was selected for this analysis because it possessed sufficient concentrations of the active metal oxides (Fe oxide above 80 % and traces of Mn oxide) and a high melting temperature that was above 1500 °C. Experiments were conducted under isothermal conditions within the temperature range of 750–950 °C with multiple redox cycles using a thermogravimetric analyzer (TG). For the reduction and oxidation reactions, CH4 (at different concentrations) and air were used, respectively. The sample showed promising results where a sufficient reactivity was observed with the fuel, and these results were reproducible. Both fresh material and samples that were used in multiple redox cycles were characterized by X–ray Diffraction (XRD) and Scanning Electron Microscopy combined with X–ray Microanalysis (SEM–EDS) in order to detect any structural or morphological changes as well as to determine the stability of the ore in repetitive CLC cycles. Kinetic parameters, such as the activation energy, the preexponential factor, and the reaction model, were determined for the redox reactions. Models of the redox reactions were selected by using a model fitting method. The F1 model (volumetric) was a suitable model for the modeling of the Kryvbas ore reduction reaction kinetics. The calculated E a was equal to 42.00 kJ mol−1, while the reaction order was determined to be equal to 1.98. The best fit for the oxidation reaction was obtained for the R3 model (shrinking core model). The oxidation (regeneration) reaction activation energy was equal to 16.70 kJ mol−1, and the reaction order was determined to be equal to approximately 0.49.