Chemical looping reforming of methane using magnetite as oxygen carrier: Structure evolution and reduction kinetics

One of the most important issues for chemical looping technology is to find low-cost oxygen carriers. This work presents the investigation on using a Panzhihua (China) magnetite as oxygen carrier for chemical looping reforming of methane (CLRM). The reactivity for coproduction of syngas and hydrogen was tested by an isothermal redox experiment using methane as a reducing fuel and steam as an oxidizing gas. The kinetics study was performed on both the fresh and recycled magnetite oxygen carriers. In the redox experiments, the produced hydrogen and syngas in a H-2/CO molar ratio of 2.0 can be stably obtained with high selectivity (ca. 95.1% for syngas and ca. 96.2% for H-2). The yields of hydrogen from the original and calcinated magnetite after successive cycling are 4.94 and 5.25 mmol/g, respectively. From the kinetic study via a thermogravimetric analyzer (TGA) method, it is found that the reduction of original magnetite to wilstite is well represented by the phase boundary controlled (contracting cylinder) mechanism, and the 1-D nuclei nucleation and growth integrated with diffusion mechanism can be successfully applied to describe the reduction of calcined magnetite. The activation energy for the reduction of original magnetite is 93.02 kJ/mol, which slightly decreases to 86.90 kJ/mol after successive cycling due to the formation of pores inside the oxygen carriers. This work gives full evidence to the feasibility of using magnetite concentrates as low-cost oxygen carrier for the CLRM system.