Non-isothermal carbothermic reduction kinetics of calcium ferrite and hematite as oxygen carriers for chemical looping gasification applications

Oxygen carriers play an important role in the excellent operation of the chemical looping gasification technology. The aim of this study is to perform a fundamental investigation on the reduction behavior and kinetics of CaO center dot Fe2O3 and Fe2O3 oxygen carriers. The oxygen carrier reaction performance was studied by a non-isothermal method using thermo-gravimetric analysis coupled with mass spectrometry. The application of model fitting method was adopted to describe the reduction process. X-ray diffraction measurements indicated that CaO center dot Fe2O3 was reduced to CaO and Fe following four stages (CaO center dot Fe2O3 -> CaO center dot FeO center dot Fe2O3 -> CaO center dot 3FeO center dot Fe2O3 -> 2CaO center dot Fe2O3 -> Fe). The predominance range of CO generation corresponded to the stage from CaO center dot 3FeO center dot Fe2O3 to Fe during CaO center dot Fe2O3 reduction, whereas during Fe2O3 reduction, it corresponded to the stage from FeO to Fe. The reduction of CaO center dot Fe2O3 can be described by a 2-D diffusion model during the entire reduction process, whereas the reduction of Fe2O3 can be initially described by a 3-D diffusion model when the reduction degree is within the range of 0-0.2, and subsequently by a reaction model when it is within the range of 0.2-1. The complex reduction intermediates of CaO center dot Fe2O3 resulted in the poor diffusion condition of lattice oxygen and gas products. CaO center dot Fe2O3 is a more promising oxygen carrier than Fe2O3 to generate CO in the chemical looping gasification technology.