Magnetic separation of ilmenite used as oxygen carrier during combustion of biomass and the effect of ash layer buildup on its activity and mechanical strength

Utilization of oxygen carriers in existing circulating fluidized bed (CFB) units is referred to as oxygen carrier aided combustion (OCAC) and is employed to achieve enhanced combustion of heterogeneous fuels, such as biomass and waste. Although oxygen carriers have successfully been implemented to industrial settings, the materials are more expensive than conventional alternatives. The oxygen carrier ilmenite has shown to be magnetically separable from the residual ash stream providing a potential to reuse the material and mitigate cost. For this purpose, it is imperative to understand how properties of the material such as its oxygen carrying capacity and mechanical strength are developed with increased residence time in the system. In this study, rock ilmenite was evaluated after being used during OCAC in two separate industrial facilities. In Chalmers semi-industrial (12 MWth) CFB boiler, a set of samples were extracted on daily basis. These provide insight to how the material changes over time. In the utility company Kraftringen's 115 MWth CFB boiler, one sample was retrieved and divided by a magnetic separator. To evaluate the oxygen transferring capacity, the materials were tested in a batch reactor with syngas as fuel. The present study shows that ash layers are not necessarily inhibitory to the oxygen transferring capacity, but long-term operation with thick layer buildup as a result causes reduction of the materials oxygen transferring capacity. Ash layer formation can instead improve mechanical strength of bed particles. Optimization measures are suggested where activated material is separated by magnet and reused in the system.