## 4 Discussion  
Research and large campaigns in commercial boilers show that using an oxygen carrier as bed material in FBC can increase efficiency and reduce emissions. The technology can  
Figure 8: The estimated potential for slag-bed material production compared to estimated current silica sand-bed material consumption. Blast furnace slag is included but is not an oxygen carrier. usually be implemented in existing industrial-scale boilers without any adaptation of the equipment. Comparing the data presented in Sections 3.2 and 3.3 it can further be concluded that, for the specific case Sweden, by-products from metallurgical industries could easily replace conventional bed material when it comes to quantity. A significant part of the by-products and intermediate products in metallurgical industries is furthermore produced as particles in a suitable size range for FBC. Those can potentially be utilized with only a few or no processing steps. Producers have expressed interest in increasing production of certain size-fractions if the market shows demand for it. It should also be mentioned that the bed material consumption in the boiler is potentially lower with novel bed materials than with silica sand. This is because they can be e.g., less prone to agglomerate. Out of the by-products with oxygen carrying properties, slags (LD-slag and copper smelter slag) show great potential due to large production and low utilization. Only about 1/3 of the copper slag is granulated and the rest is air-cooled and disposed of, meaning that there is potential to increase the production of granulated copper slag. Boliden is planning to expand granulation within the next 10 years and is actively looking for market opportunities. When it comes to the concept of using slag-based bed materials, GR Granule can be presented as an example. GR Granule is a bed material produced from blast furnace slag and sold by the Finnish company Fescon Oy. GR Granule is used commercially in around 13 boilers in Finland for combustion of fuels with high alkali content. It is not an oxygen carrier but has shown other advantageous properties in FBC when it comes to avoiding agglomeration and related problems [68,82], properties which have been connected to the high Mg-content. According to the producer, the bed material consumption in boilers was seen to decrease by up to 80% by switching from conventional bed material to GR-Granule. The material is produced from both water-granulated slag and air-cooled slag, making the process relevant to compare with both copper- and LD-slag. No available data on yield, cost, or production methods are disclosed by Fescon. Bottom ash handling is not different with GR-Granule than with silica sand [68], which is an important feature for boiler operators. Though there are seemingly many advantages, OCAC is to date only used in a few plants in Sweden. Power plant owners might argue that the concept is not yet well tested and that the risks connected to switching to a practically untested bed material outweigh the potential economic gain. A risk could be that the material causes corrosion or some other type of unexpected damage to the boiler. Another type of risk is that ash handling becomes significantly more expensive if the ash must be classified differently than with conventional bed material. Disposing of the bottom ashes already corresponds to a large part of the bed material cost. The boiler operators need to know how the material develops in the boiler to be able to plan all the aspects of material handling [54]. Gaining that kind of experience will take time. Another challenge can be the actual price per kg of a new bed material and how this is valued to the overall gains produced in a boiler when replacing the silica sand. That is, even though the cost per kg of bed material could be several times higher if using a mineral (e.g., ilmenite) than that for silica sand, it might still be economically favorable to change concerning the overall positive effects on boiler operation.