The redox reaction kinetics of Sinai ore for chemical looping combustion applications

Highlights: • Redox reaction kinetics of Fe-Mn-rich Sinai ore was determined by TGA. • The most suitable model for reduction was D3, while R3 for oxidation. • Activation energies 35.3 and 16.70 kJ/mole were determined for reduction and oxidation. • Repetitive redox reactions favor the formation of spinel phases in Sinai ore. • Multiple redox cycles induce formation of extensive porosity of the particles. - Abstract: The objective of this work was to study the use of Sinai ore, a Fe–Mn-based ore from Egypt, as a low-cost oxygen carrier (OC) in Chemical Looping Combustion (CLC). The Sinai ore was selected because it possesses relatively high amounts of iron and manganese oxides. Furthermore, those oxides have low cost, very favorable environmental and thermodynamic properties for the CLC process. The performance of the Sinai ore as an OC in CLC was compared to that of ilmenite (Norway Tellnes mine), the most extensively studied naturally occurring Fe-based mineral. The kinetics of the reduction and oxidation reactions with the two minerals were studied using a thermogravimetric analyzer (TGA). Experiments were conducted under isothermal conditions, with multiple redox cycles, at temperatures between 750 and 950 °C. For the reduction and oxidation reactions, different concentrations of CH₄ (10–25 vol.%) and O₂ (5–20 vol.%) were applied, respectively. The kinetic parameters, such as the activation energy (E_a), pre-exponential factor (A₀), and reaction order (n), were determined for the redox reactions. Furthermore, models of the redox reactions were selected by means of a model-fitting method. For the Sinai ore, the D3 model (3-dimensional diffusion) was suitable for modeling reduction reaction kinetics. The calculated E_a was 35.3 kJ/mole, and the reaction order was determined to be approximately 0.76. The best fit for the oxidation reaction was obtained for the R3 model (shrinking core). The oxidation (regeneration) reaction E_a was equal to 16.7 kJ/mole, and the determined reaction order was approximately 0.72. The crystalline phases present, as well as the morphology and inhomogeneities in elemental composition were studied for both materials, fresh as well as after multiple redox cycles, by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) combined with X-ray Microanalysis - Energy Dispersive Spectroscopy (EDS). Structural and morphological changes were detected and correlated to the reaction temperature as well as the reactant compositions and thus the stability of the ores in repetitive CLC cycles was determined.