Step-wise reduction kinetics of Fe2O3 by CO/CO2 mixtures for chemical looping hydrogen generation

摘要

The reduction kinetics of iron-based oxygen carriers significantly affect the efficiency of chemical looping hydrogen generation. Based on the thermodynamics properties of the Fe-CO-CO2 system, CO/CO2 mixtures in different volume ratios can decouple the reduction of Fe2O3 into three relatively independent steps. The step-wise reduction of Fe2O3 was conducted in a thermogravimetric analyzer at 800-900 degrees C. To investigate the reaction mechanisms, the Hancock-Sharp method and the nonlinear fitting approach were applied to select the kinetic models. The reaction model of step 1 and 2 are geometrical contracting model; the step 3 is controlled by nucleation and nuclei growth model and diffusion model. The activation energy of the three steps including Fe2O3 -> Fe3O4, Fe3O4 -> FeO and FeO -> Fe is estimated to be 34.92 +/- 1.24, 70.13 +/- 0.88 and 44.12 +/- 1.44 kJ/mol, respectively. The rate equations derived from the Arrhenius law were determined to predict the reaction rate at a specific CO concentration. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.