## 5 Conclusions  
This study has investigated and discussed the potentials for using products and by-products from metallurgical industries as oxygen carrying bed material in FBC. The study has been focusing on the situation in Sweden, which however is possible to extrapolate at least to neighboring countries Finland and Norway. The main findings are:  
* Previous research and commercial activities show that the efficiency and capacity of existing fluidized bed boilers can be increased by using an oxygen carrier as bed material, so called Oxygen Carrier Aided Combustion (OCAC). OCAC can be implemented on an industrial scale with no or little adaptation of the equipment. The improvements have often been significant e.g., up to 15% increase in thermal output, up to 30% reduction of excess air, and significant reduction in bed material consumption.
* At least 20% of all the silica sand mined in Sweden (160-180 kton/year), is used directly as bed material in FBC. Natural silica sand is a resource that plays an important role in groundwater production. Protecting natural deposits and replacing primary silica sand in industrial applications is in line with the Swedish environmental goals.
* Steel- and other metallurgical industries produce large flows of by-products that are suitable as oxygen-carrying bed material. These flows could completely replace silica sand consumption in existing FBC boilers, based on calculations done in this study. This could also be a way for metallurgical industries to monetize large amounts of by-products that currently is disposed of in landfill and be one step closer a circular use of materials.
* Some materials are produced directly as particles in existing processes and needs little in terms of additional processing. Notable such materials are granulated copper-smeter slag and iron ore fines.
* Other materials require significant mechanical processing (e.g., crushing and sieving) to produce particles that are suitable as bed material. This is likely to increases the price of the material, but also enables business opportunities. A notable such material is LD-slag.
* For researchers working with the development of CLC of biomass there might be things to learn from the commercialization of OCAC. While the strong focus on fuel conversion in CLC research is understandable, the OCAC experience shows that there is much more to a practically useful bed material than single parameters. Due to the influence of ash, the bed material consumption is likely to be larger than commonly anticipated. This makes factors such as good availability, low cost and acceptable end-of-life disposal very important.  
Conceptualization, F.S., M.R. and F.L.; methodology, writing--original draft preparation, F.S., M.R. and F.L.; writing--review and editing, F.S., M.R. and F.L.; visualization, F.S.; investigation, F.S.; supervision, M.R., F.L.; project administration, M.R.; funding acquisition, M.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Swedish Research Council, project number 2017-04553--Combustion of biomass by Oxygen Carrier Aided Combustion.  
[PERSON] at LKAB and [PERSON] at Boliden are acknowledged for providing valuable information about materials and production processes. [PERSON] and [PERSON] are acknowledged for providing data and knowledge about Swedish fluidized bed boilers.  
The authors declare no conflict of interest.