Characterization of ilmenite used as oxygen carrier in a 100 kW chemical-looping combustor for solid fuels

Abstract

Chemical-looping combustion (CLC) is considered to be the most promising and economically viable process for carbon dioxide capture. The oxygen carrier has a central role in the chemical-looping combustion process. Ilmenite, a natural mineral composed of FeTiO3, has been extensively used as oxygen carrier in CLC systems due to its availability, relatively low cost and demonstrated reactivity. During the looping process ilmenite undergoes a series of chemical and mechanical transformations that eventually lead to the break-down of particles into smaller fragments, which are unfit for use in circulated fluidized bed (CFB) applications.In the present study a 100kW chemical-looping system was operated with ilmenite particles as oxygen carrier and with biomass char as fuel. Ilmenite particles were collected at the end of the run. The collected particles were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS). Thermodynamic predictions of the expected compounds at the given conditions were also used to compare with the experimentally obtained results.The aim of the study was to understand the change in morphology and chemical compositions of the collected ilmenite particles and to relate them to the exposure conditions in the CLC reactor.It was found that the aging of the ilmenite particles could be distinguished based on the particle morphology. Furthermore, a possible mechanism for the transformation of the ilmenite particles during the cyclic chemical-looping process was proposed.