The Ultra-Fast Oxidation Rate of a Synthetic Fe-Based Oxygen Carrier: Experiment and Simulation Study

The knowledge of the oxidation rate of the oxygen carrier (OC) is essential in designing and optimizing chemical looping processes. However, the oxidation is usually too rapid that the measured rate by TGA deviates from the intrinsic ones because of the transport limitations. In this work, a micro-fixed bed reactor was employed to investigate the oxidation rate of a synthetic Fe-based oxygen carrier (OC) via temperature-programmed oxidation, and step response experiments carried out at 300-450oC to lower the oxidation rate as much as possible. The synthetic Fe-based OC showed a very fast initial stage that consumed all the feed O2, and temperature-dependent Fe conversion after the initial stage. The experimental results were analyzed by simulations using a 1D heterogeneous reactor model accounting for potential interfacial mass transfer limitations. Even at a temperature as low as 300oC, O2 was fully consumed in the initial stage, so that only minimum oxidation rates could be estimated. The estimated minimum oxidation rate is, however, faster than those reported in the literature, and indicates that Fe in the OC is fully converted within 3s with 2% O2 at 450oC. The fast oxidation rate leads to a sharp reaction front propagating in the reactor at 450oC. Based on the estimated minimum oxidation rate, we suggest using the rate equation rO2=883*exp(-32000/RT)*(1-xs)2/3CsO2 to estimate the magnitude of oxidation rate of the Fe-based OC at higher temperature. This estimation indicates that the oxidation rate is faster than the typical mass transfer rate in a bubbling fluidized bed, and similar to that in a riser.