Research articleReduction kinetics of hematite as oxygen carrier in chemical looping combustion

- Different reaction characteristics between TGA and FB reactor in CLC were analyzed.
- Reaction kinetics of hematite as OC reduced by CO in CLC was obtained.
- Utilizability difference of reduction stage of hematite in CLC reactor design was analyzed.

Chemical looping combustion (CLC), which has the characteristic of CO2 inherent separation, is a novel fossil fuel utilization technology for CO2 capture with low cost and low energy efficiency penalty. The reduction kinetics of oxygen carrier (OC) is one of the most important issues for CLC reactor design and computational fluid dynamics (CFD) simulation. In this work, well-organized isothermal experiments in a thermogravimetric analyzer (TGA) were first carried out to obtain the kinetics of a hematite as OC reduced by carbon monoxide. The reduction process was identified as two sequential stages, which correspond to the reduction of Fe2O3 to Fe3O4 and Fe3O4 to FeO, respectively. As obtained from the experimental result, the first stage is controlled by gas diffusion in the boundary layer on particle surface while the second stage is mainly controlled by heterogeneous chemical reaction. For the second stage, the activation energy, E, is 110.75 kJ/mol and the reaction order, n, is 1.5. Then, the same batch of hematite was tested by reacting with the reducing gas of 10 vol.% CO balanced by N2 in a fluidized bed reactor. The different reaction characteristics in TGA and fluidized bed reactor were analyzed. This work provides a deep understanding on the applicability of the OC reduction kinetics obtained by TGA to the reactor design and CFD simulation of a fluidized bed CLC rig.