Magnetic Properties of Ilmenite used for Oxygen Carrier Aided Combustion

Other conference contribution, 2022

Oxygen carrier aided combustion (OCAC) is a combustion process that utilizes oxygen carrying particles in a fluidized bed to transport oxygen from oxygen-rich (near the air inlet) to oxygen-poor (near the fuel inlet) regions in the reactor. A commonly used oxygen-carrying material is ilmenite (FeTiO3) which is a naturally occurring mineral. At higher oxygen partial pressures (oxygen-rich locations) ilmenite can react to pseudobrookite (Fe2TiO5) and thereby take up oxygen. Upon reduction of pseudobrookite in oxygen-lean locations the oxygen is released, which enhances the distribution of oxygen through the reactor.
Ilmenite was used as bed material in an industrial 115 MWth circulating fluidized bed (CFB) boiler where recycled waste wood and wood chips were utilized as fuel. Bottom ash samples were extracted at different time intervals for a period of 14 days and subsequently separated into two fractions by a magnetic separator. The magnetic fraction was expected to be enriched in ilmenite, pseudobrookite and other iron-containing oxides and was therefore aimed to be recirculated into the boiler, while the non-magnetic fraction was considered as waste stream.
In this study, the bottom ash samples as well as the two different ash fractions (magnetic and non-magnetic) were analysed with a scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), an X-ray diffractometer (XRD), an X-ray fluorescence (XRF) spectrometer, and a magnetic susceptibility meter. The objective was to investigate the chemical and magnetic properties of the particles of each fraction.
The SEM-EDS analysis revealed that the non-magnetic fraction consists to the largest extent of feldspar (KAlSi3O8) particles which are a common constituent of soil and minerals. A significant amount of freshly introduced ilmenite particles was also separated as non-magnetic and is therefore lost from the process. Characteristic for these particles was a lack of ash layer, suggesting they had recently been added to the system. In the magnetic fraction, several feldspar particles were found which were covered by a layer rich in Ca, Fe, Ti, and Si. Comparing the XRD analysis of the ash prior to magnetic separation with its magnetic fraction revealed a decrease of the peaks corresponding to feldspar. The peaks corresponding to CaTiO3 increase at later times of the experimental campaign, and they have a similar intensity in both magnetic fraction and total ash. The removal of feldspar particles by magnetic separation was further corroborated by XRF analysis, where the concentration of K, Al and Si was significantly higher in the non-magnetic fraction. The concentration of Ca increased in the ash and the magnetic fraction at later stages of the experiment.
The present analyses provide valuable insight into the effect of wood ash on the chemical and magnetic properties of ilmenite during fluidized bed combustion. The efficiency of the magnetic separation is crucial for increasing the lifetime of the material and thereby decreasing cost and waste flows. Improvements in bed material handling could be further be applied for related processes such as chemical looping combustion.