## 1 Introduction
### Oxygen Carrier Aided Combustion  
Oxygen Carrier Aided Combustion (OCAC) is a combustion concept related to CLC that can be implemented in existing fluidized bed boilers. It is realized by partial or complete substitution of the bed with an oxygen-carrying material, similar to the oxygen carrier used in CLC. The active bed material will be reduced in fuel rich parts of the combustion chamber, in accordance with Reaction (1). In oxygen rich parts it will be oxidized, in accordance with Reaction (2). Materials capable of releasing gas phase oxygen, such as described in Reaction (4), would actively release oxygen in parts of the combustion chamber where it is lacking, see Figure 1.  
Several interesting effects could potentially be achieved:
* Gas phase components can now be oxidized not only by homogenous reactions with oxygen but also by heterogeneous reactions with the solid oxygen carrier, as described in Reaction (1). * New ways for how oxygen is transported in the space dimension of the boiler will be introduced, thus minimizing the presence of reducing zones and reducing the emissions of CO and unburnt hydrocarbons, especially for combustion with low air-to-fuel-ratio. * The oxygen retained in the bed will act as an oxygen buffer, thwarting negative effects of uneven fuel feeding and load changes. * Enhanced fuel conversion in the bottom bed. In ordinary fluidized bed boilers, the more stable fuel components (such as for example CH4) do not burn in the bottom bed to any larger extent. The moderate temperature (\(\approx\)800-870 \({}^{\circ}\)C) and thermal inertia of the bed inhibits formation of sufficiently hot flames. However, it has been shown in CLC studies [2] that CH4 is readily oxidized by oxygen carrying solids. The apparent reason would be that the heterogeneous reaction between CH4 and oxygen carrier is not hampered by temperature to the same extent as the homogeneous reaction. Consequently, in OCAC the conversion of CH4 should proceed rapidly also inside the dense bottom bed, which also has been demonstrated experimentally [4]. * OCAC may allow for innovative boiler designs. The temperature profile of the combustion chamber changes due to improved conversion in the bottom bed and a more evenly distributed oxygen fugacity throughout the boiler. This could reduce hot spots. New possibilities with respect to boiler design may arise such as for example heat extraction in the bottom bed for reduced boiler height, enhanced temperature control, cheaper designs by omitting secondary air, etc. Unlocking these opportunities will require rethinking fluidized bed boilers as we know them. Figure 1: Schematic illustration of Oxygen Carrier Aided Combustion (OCAC) in Circulating Fluidized Bed (CFB) boiler. Simplified bulk reactions in dense bed and freeboard have been included, with new significant reaction pathways in italic. OC–O = oxidized oxygen carrier, OC = reduced oxygen carrier. * OCAC may offer opportunities to reduce traditional problems in biomass combustion. This includes sintering, agglomeration, fouling and corrosion issues connected to combustion of biomass in fluidized beds [5]. * OCAC may allow for a reduction in NO\({}_{x}\) emissions and dioxane emissions in waste incineration. Oxygen carriers typically have less oxidizing power compared to gaseous oxygen and there could also be heterogeneous effects. OCAC was first demonstrated in a campaign in Chalmers 12 MW\({}_{\text{th}}\) Circulating Fluidized Bed (CFB) Research Boiler in 2012 with promising results [6] (up to 80% reduction of CO emissions, up to 30% reduction of NO\({}_{x}\) emissions and significantly altered temperature profile with 40 wt % substitution of silica sand with oxygen carrier). In the campaign the mineral ilmenite (titanium-iron ore) was used as bed material. An analysis of bed samples extracted during the campaign has also been published, showing interesting phenomena such as diffusion of potassium into the core of ilmenite particles [7].