Use of an Fe-Based Residue from Alumina Production as an Oxygen Carrier in Chemical-Looping Combustion

Abstract

Chemical-looping combustion (CLC) has been emerging in recent years as a promising CO2 capture technology. In CLC, the oxygen needed in the combustion is supplied by an oxygen carrier (OC), normally a metal oxide, which circulates between fuel and air reactors. In the fuel reactor, the fuel is oxidized to CO2 and H2O, which facilitates CO2 capture once the water has been condensed. The reduced OC is then transported to the air reactor, where it is reoxidized before the start of a new cycle. Gaseous fuel combustion has been extensively studied to date, and now attention has been directed to solid fuels, such as coal. In this case, the OC should be inexpensive, as some losses are expected with the ashes. A bauxite waste from alumina production, containing Fe2O3, could fulfill the requirements for CLC of solid fuels. In this work, the reactivity of this waste material to different gases present in coal combustion was studied. Thermogravimetric analysis (TGA) together with experiments in a batch fluidized-bed reactor (FB) were performed in the 1100–1223 K temperature range using H2, CO, H2/CO mixtures, and CH4 as reducing gases. Similar to other natural minerals or waste materials, a gain in the bauxite residue reactivity with the number of cycles in TGA was observed. TGA experiments at 1173 K showed that the bauxite waste samples reacted more rapidly with H2 than with CO and CH4. The reactivity of this material was found to be comparable to those of other Fe-based synthetic materials and even higher than those of some minerals, such as ilmenite. In batch FB experiments, H2 and CO were completely converted. CH4 was partially converted at all temperatures and reducing times tested. With regard to the fluidization behavior of this waste, the material presented moderate attrition rates and no defluidization problems were observed during operation in the fluidized bed after >40 h. Therefore, this material has suitable properties to be considered as an OC for CLC with solid fuels.