Spinel Ni(Al,Fe)2O4 Solid Solution as an Oxygen Carrier for Chemical Looping Combustion

Abstract

Our research staff has been able to perform long-term operation of a 200 kW gas-circulating combustion system successfully for over 100 h with NiO/Al2O3 oxygen carrier particles that were produced in large quantities using a pilot-plant spray dryer. After such a result, we are planning to scale-up the process to a 1–10 MW system. Iron oxide, which is relatively cheaper than expensive NiO, has been considered for this step, and nine different types of NiO/Fe2O3/Al2O3 mixed metal oxides were produced by adding iron oxide to NiO with a 22.5–47.5 wt % ratio, adding 30 wt % Al2O3 as support, and finally, forming material using the spray-drying method. Results from both physical property tests [tapping density, particle size analysis, attrition index (AI), porosity analysis, and Brunauer–Emmett–Teller (BET) surface area] and chemical property tests [scanning electron microscopy (SEM), electron probe X-ray microanalysis (EPMA), and X-ray diffraction (XRD)] suggest that a mixed metal oxide of 22.5 wt % NiO, 47.5 wt % Fe2O3, and 30 wt % Al2O3 calcined at 1100 °C is suitable for the fluidized-bed process with excellent oxygen-transfer capability. Oxygen-transfer capability was higher than the theoretical value of N1F2 particles, and it can be explained as spinel Ni(Al,Fe)2O4 solid solution through XRD analysis. It is believed that mixed metal oxides produced are good candidates for media-circulating combustion of the fluidized-bed process for the large capacity of a 1–10 MW system for the future. Because oxygen-transfer capability of the mixed metal oxides was maintained without degradation after multiple cycles and despite the oxidation rate being relatively slow, the reduction rate was very fast.