## 4 Discussion
### Commercial Outlook for LD-Slag as Bed Material  
In order to be an immediately attractive replacement to silica sand in existing fluidized bed boilers, LD-slag should exhibit similar or improved performance with respect to technical factors such as emission levels, sintering behaviour, flowability and attrition. It should also have the potential to reduce total costs of operation, which includes not only purchase cost per ton but also costs for handling and disposal of ash, as well as cost for downtime and maintenance. The campaign verified that LD-slag worked well as a bed material in a circulating fluidized bed boiler. At no occasion were there any problems related to material handling, agglomeration or sintering. The temperature profile of the boiler suggested that LD-slag did perform adequately as oxygen carrier, and it has been established in other studies that it has decent reactivity with fuel and air [23; 28]. However, the effect on CO emissions is a significant concern. With secondary measures in the form of ammonium sulphate added to the cyclone outlet and mixing the LD-slag with silica sand, the performance with respect to emissions was, at best, comparable to operation with silica sand. It is not immediately obvious that this will be the case in larger CFB boilers or in Bubbling Fluidized Bed (BFB) boilers. But it is clear that issues such as absorption of ash elements and reaction mechanisms should be studied further. The slight increase in NO emissions is probably not a big problem for commercial operators. In a modern utility boiler, NO emissions are typically controlled by addition of ammonia or urea,in order to reduce the final emissions so that they fulfil environmental criteria. As long as the amount saved on bed material is greater than the cost for NO reduction, the use of LD-slag will be attractive to plant operators from this perspective. Whether the use of LD-slag can reduce the total cost of operation for a fluidized bed boiler is not immediately clear. Pricing of industrial by-products such as LD-slag is not transparent. Even though there is limited demand and large stockpiles are available, the companies involved are keen on getting as good a price as possible out of these products. The sales price for slag-based products in Sweden are mostly in the range of 5-30 \(\up\)\(\up\)/ton (depending on the size fraction and quality). Most available products are based on blast furnace slag, but it seems reasonable to believe that LD-slag would be priced similarly. Onto this price the cost for transport as well as extra treatment such as crushing, drying, sieving and dedusting may need to be added. If elutriation is greater and more fly ash is generated, this would also add to the total costs. As comparison, the price for silica sand in Sweden is around 40-80 \(\up\)/ton and the price for ilmenite can be expected to be 6-8 times that of silica sand. It can be concluded that while it is not obvious that the final price for prepared LD-slag will be lower, it certainly is conceivable that it will. The use of LD-slag as a bed material in waste incineration in fluidized bed boilers is another option to consider. This is technically similar to biomass combustion but with significant differences in process economy. Waste incineration typically has very high consumption of bed material and very high air-to-fuel ratio. If LD-slag can be provided at a lower price than silica sand and provide improved combustion in the furnace, it could be attractive even if CO emissions have to be controlled with secondary measures such as ammonium sulphate injection. Also, the reduced tendency towards agglomeration of LD-slag could be a significant advantage for waste incineration. Finally, the bed material in coal power plants is mostly coal ash and limestone. The latter is added to capture sulphur as CaSO\({}_{4}\). If added to a coal power plant, LD-slag could potentially propagate combustion in the furnace by the mechanisms described in Section 1 above, while at the same time replace limestone for sulphur capture.