Magnetic separation of ilmenite used as an oxygen carrier in fluidized bed combustion
Examensarbete på kandidatnivå
This bachelor thesis studies how oxidation and reduction cycles during oxygen carrier aided combustion (OCAC) operation, as well as ash interaction, affect the magnetic susceptibility of ilmenite particles used as oxygen carriers. Magnetically separated samples of bottom ash collected from Kraftringen’s circulating fluidized boiler located in Örtofta (Sweden) were analysed. The samples were part of an OCAC campaign from April 2018, in which waste wood and wood chips were used as fuel. Particles from the fresh material, the bottom ash, as well as the fractions separated by a magnet - the magnetic fraction (magnetic accept) and the non-magnetic fraction (magnetic reject) collected from different days were morphologically and chemically characterized using SEM-EDS. XRD was used for crystalline compounds identification and XRF was used for the analysis of the bulk composition of the samples. The aforementioned analysis were complemented with magnetic susceptibility measurements of the samples. The separation of the magnet was found not to be totally effective as artifacts (namely feldspar in the magnetic fraction and inactivated ilmenite in the magnetic reject) were detected in the samples. The results indicate that prolonged residence time of the bed material in the boiler increased the presence of CaTi0.8Fe0.2O2.9 enrichments in the ash layer of the particles from the magnetic accept fraction. It was observed that ash compounds do not interfere significantly in the bulk magnetic susceptibility. Magnetic susceptibility increased with longer residence time, where the suggested cause behind it was the possible Fe2+ migration towards the surface of ilmenite particles, and its oxidation to magnetic Fe oxides – titanomagnetite and hematite. The results from the work suggest that the magnetic susceptibility of bed material increases with residence time in the boiler, and so does the ash uptake. Early separation can result in fresh material ending in the magnetic reject fraction. Based on these results, an optimal time window for the extraction, separation, recirculation and/or renewal of the bed material can be defined that will likely result in an increase in separation efficiency of the bed material. Therewith, magnetic susceptibility measurements could be a useful tool in the optimization and decision making regarding this time window.