## 1 Introduction
### LD-Slag as Oxygen Carrier  
While it would be possible to produce synthetic oxygen carrier particles for OCAC, this would likely be a costly procedure. This is because ash elements present in biomass and waste fuels (K, Na, Ca, Si etc.) are expected to interact with the bed material in a potentially irreversible manner. For silica sand ash components are absorbed on the particle surface. For ilmenite it has been established that the ash component most central for operability during combustion of woody biomass, which is potassium, diffuses into the particle core resulting in formation of KTi\({}_{8}\)O\({}_{16}\)[11]. It is well established that alkali metals are central to several undesired phenomena such as sintering, agglomeration, corrosion and emissions [9]. Regardless of mechanism, perpetual enrichment of ash components in the bed material is not possible. Thus, the bed material will need to be replaced in regular intervals. In a biomass boiler it is common to replace one third of the sand bed each day, a procedure commonly referred to as regeneration. For waste incineration the whole sand inventory can very well need to be replaced on daily basis. An industrially accepted rule of thumb for the consumption of silica sand during biomass combustion is 3 kg/MWh\({}_{\text{th}}\). The corresponding number for waste incineration is 6 kg/MWh\({}_{\text{th}}\), but it can sometimes need to be considerably higher. While it is not obvious that regeneration will have to be done in precisely the same way for oxygen carrying materials as for sand, it seems reasonable to believe that the use of synthetic particles will be inconvenient from a cost and resource perspective. High-performing synthetic oxygen carriers are likely to be at least one order of magnitude more expensive compared to virgin minerals, which in turn very well could be one order of magnitude more expensive per ton than silica sand. Silica sand already represents a significant cost for biomass fired boilers. For a fluidized bed boiler at the scale of 100 MWh\({}_{\text{th}}\) (a common size in Swedish district heating system), the annual cost for silica sand is in the order of 1-5 MSEK annually, depending on fuel and annual hours of operation. Hence the use of considerably more expensive bed materials is not favored by plant operators, unless there are considerable practical gains. However, the use of bed materials with the potential to be even cheaper than silica sand and which also could provide certain technical advantages is met with considerable interest. Industrial by-products, for which there currently is limited demand, are likely the cheapest category of bed materials available. The theoretical performance of oxide materials can quite easily be determined by means of thermodynamic equilibrium calculations [16]. Iron oxides are viable oxygen carriers [1; 2; 3; 4; 16]. However, iron oxides are incapable of releasing significant amounts of gas phase O\({}_{2}\) via oxygen uncoupling [6]. Manganese oxides have higher oxygen transfer capacity by weight (7-10 wt% depending on temperature of operation) compared to iron oxide (3.3 wt%) [16]. This is because complete conversion of fuel is possible both for reduction of Mn\({}_{2}\)O\({}_{3}\)\(\rightarrow\) Mn\({}_{3}\)O\({}_{4}\) and for Mn\({}_{3}\)O\({}_{4}\)\(\rightarrow\) MnO, while iron is limited to the reaction Fe\({}_{2}\)O\({}_{3}\)\(\rightarrow\) Fe\({}_{5}\)O\({}_{4}\). Manganese oxides also have the ability to release oxygen directly in gaseous form in inert atmosphere at temperatures around 800 \({}^{\circ}\)C [6]. For mixed manganese oxides, which are not pure but also contain elements such as iron and silicon, other more favorable mechanisms for oxygen release which work at higher temperatures could also be available [17; 18]. LD-slag is the second largest by-product in steelmaking.