## 5 Conclusions  
Based on the experimental campaign with LD-slag, the following conclusions can be drawn:  
* From an operational point of view, LD-slag worked well as bed material in a CFB boiler. The material was easy to handle, to fill into and remove. No problems were encountered related to agglomeration.  
* The production of fly ash was three times as high as for similar operation with silica sand, but this did not result in any operational problems. The high production of fly ash is likely at least partly a result of the 10-15 wt% fine particles below 90 \(\upmu\)m present in the starting material.
* The temperature profile of the boiler suggests that the presence of LD-slag facilitates fuel conversion in the boiler, with the clearest sign being a much greater temperature drop in the cyclone in comparison to when only silica sand is used.
* The use of 100% LD-slag resulted in much higher emissions of CO than the sand reference. It is believed that this is connected to poor absorption of ash components, in particular potassium.
* During operation with 100% LD-slag, CO emissions could successfully be suppressed by addition of small amounts of ammonium sulphate to vortex finder of the primary cyclone.
* Operation with 10%, 30% and 50% LD-slag were feasible from a practical point of view and resulted in reasonably low CO emissions.
* Previous experiments with ilmenite [10] and manganese ore [14] in the same boiler both showed much better potential to achieve significantly reduced CO emissions at given air-to-fuel ratios. From this point of view LD-slag does not look promising.
* LD-slag did not show potential for reduction of NO emissions. For a given air-to-fuel ratio, NO emissions were always higher when LD-slag was included in the bed compared to the sand reference. This behavior was similar to previous experiments with ilmenite [10] and manganese ore [14] in the same boiler.
* In order to constitute a commercially attractive substitute for silica sand in fluidized bed combustion of biomass, the issue with CO emissions will need to be further examined and better understood. Also, the potential cost of LD-slag in the suitable size range and impact of changed ash properties needs to be carefully assessed.  
Conceptualization, M.R., M.H. and F.L.; methodology, M.R., M.H. and F.L.; formal analysis, M.R. and M.H.; investigation, M.R., M.H. and F.L.; writing--original draft preparation, M.R.; writing--review and editing, M.H. and F.L.; visualization, M.R.; project administration, M.R.; funding acquisition, M.R.  
This research was funded by the Swedish Energy Agency (grant number P43936-1) via the OxyCar-FBC project, which is a project performed within the framework of ERA-NET Bioenergy. The experimental campaign also received direct funding from SSAB Merox AB.  
Research engineers [PERSON] and [PERSON] are acknowledged for their assistance in the experimental work.  
The funders had no role in the collection, analyses, or interpretation of data, or in the writing of the manuscript. SSAB Merox AB contributed constructively to the design of the study with the aim of providing a thorough investigation of the potential use of one of their by-products for a new application. According to the contract between Chalmers and SSAB Merox AB the latter could have delayed publication of the results for up to four months, but choose to approve the decision to publish the results as soon as possible.