## 1 Introduction
### Oxygen Carriers as Bed Material in FBC  
Due to rapidly increasing CO\({}_{2}\)-concentration in the atmosphere, it has been suggested by the IPCC that carbon capture and storage (CCS), or some other form of carbon dioxide removal will be necessary to meet current climate targets [23]. CCS in combination with biomass combustion (Bio-Energy CCS; BECCS) could allow for net-negative emissions. Chemical-looping combustion (CLC) is an emerging combustion technology that could be very useful for CO\({}_{2}\) capture, since it has the advantage of inherent CO\({}_{2}\) separation [24]. In CLC, two interconnected reactors are used, one to which the air is supplied and one in which the fuel is converted. Instead of mixing the fuel and air, the conversion of fuel is made possible by providing oxygen via a solid oxygen carrier that circulates between the two reactors, as illustrated in Figures 1 and 2. Oxygen carriers are solid materials capable of providing oxygen for fuel conversion and becoming regenerated by oxidation under conditions typical for combustion. A set of desirable properties of the oxygen carrier have been established. Oxygen carriers should:  
* Have sufficiently high reactivity with oxygen and fuel at relevant conditions
* Have sufficiently high oxygen transport capacity to be viable
* Be environmentally sustainable
* Be economically feasible
* Have sufficiently good mechanical properties to allow for use in fluidized beds  
Failure to meet the minimum demands means that the material is unsuitable for large-scale application. There is, however, no fixed definition of the above properties, but rather they will vary for different applications. Oxides of Fe, Cu, Ni, and Mn, among other less studied materials, have been evaluated and a large number of oxygen carriers have been proposed for different applications [25; 26]. Experimental evaluations of oxygen carriers specifically for solid fuel CLC has been reviewed by several researchers: [27; 28; 29; 30]. Much of the early CLC research focuses on synthetic oxygen carriers. While synthetic materials potentially are well-performing and stable in sustained operation with clean fuels such as natural gas, they have yet to be proven to maintain their properties in the harsh chemical environment that is the reality for fuels with reactive ashes. In light of current knowledge about FBC, long oxygen carrier lifetime seems difficult to realize for such fuels. Thus, the high cost of synthetic materials makes them unlikely for applications where high regeneration rates are expected. For this reason, the interest in using mineral ores and similar affordable materials as oxygen carriers has increased lately [28; 30]. Figure 1: Schematic description of CLC. Figure 2: Schematic representation highlighting the main differences between conventional FBC (**left**), OCAC (**middle**) and CLC (**right**). CLC has yet to be developed into a practically viable technology. Instead, this study focuses on the use of oxygen carriers in the context of a related, and more readily available spinoff technology: Oxygen carrier aided combustion (OCAC). OCAC is a technology in which oxygen carrier particles are used as bed material in FBC. The oxygen carrier is used instead of, or in combination with, conventional bed material. Utilizing oxygen carriers in FBC changes the reaction paths in the bed which can provide many advantages. Increased efficiency and reduced emissions of CO and NO have been demonstrated. A discussion of the technology and practical aspects will be presented below. OCAC can be implemented in existing boilers with more or less no adaptation of the equipment, which was concluded in [31]. Further motivation for developing OCAC is that, even though it doesn't have the advantage of inherent CO\({}_{2}\)-separation that CLC possesses, it could become a bridging technology between current conventional, large-scale biomass combustion and future facilities for CO\({}_{2}\)-capture.