## 1 Introduction
### Oxygen Carrier Aided Combustion  
Oxygen Carrier Aided Combustion (OCAC) is a concept strongly related to Chemical Looping Combustion which can be directly implemented in existing fluidized bed boilers. It is realized by partial or complete substitution of the standard semi-inert bed material with an active oxygen carrier material. The active bed material will be reduced in fuel rich parts of the combustion chamber in accordance with reactions such as reaction (1), as well as by reaction with other products of pyrolysis such as CO and H\({}_{2}\). 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 release oxygen in parts of the combustion chamber where it is lacking, see Figure 1.  
In OCAC several interesting effects could potentially be achieved:  
* Gas phase fuel components can be oxidized not only by homogenous reactions with oxygen, but also by heterogeneous reactions with the oxygen carrier, as described in reaction (1). * New mechanisms for oxygen transport in the boiler will be introduced, thus minimizing the presence of reducing zones and potentially reducing the emissions of CO, H\({}_{2}\) and unburnt hydrocarbons. * The bed of oxygen carrier material will act as an oxygen buffer, possibly thwarting negative effects of uneven fuel feeding and load changes. * Enhanced fuel conversion in the dense bottom bed. In ordinary fluidized bed boilers the more stable fuel components (such as for example CH\({}_{4}\)) do not burn rapidly in the bottom bed since the moderate temperature (\(\approx\)800-850 \({}^{\circ}\)C) and thermal inertia of the bed inhibits formation of hot flames. However, it has been shown in CLC studies [3] that CH\({}_{4}\) is readily oxidized by oxygen carrying solids. The apparent reason would be that the heterogeneous reaction between CH\({}_{4}\) and oxygen carrier is not hampered by temperature to the same extent as the homogeneous reaction. Consequently, in OCAC the conversion of CH\({}_{4}\) should proceed more rapidly also inside the dense bottom bed, a phenomena that has also been demonstrated experimentally [8]. * 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 [9]. * OCAC may allow for the use of less excess air than what is needed in conventional boilers. 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. The last point is important. Mixing of oxygen and fuel is an important aspect affecting the performance of thermal power plants. Insufficient contact between fuel and oxygen results in emissions of carbon monoxide, unburnt hydrocarbons and char particles. For solid fuels, good mixing is sometimes difficult to achieve. In order to avoid poor fuel conversion and high emission levels, operation with a significant excess of air, compared to what is needed for stoichiometric combustion, is often necessary. This increase the total volumetric gas flow and results in: (i) increased boiler size, cost, and footprint; (ii) increased gas velocity and wear on heat transferring surfaces; (iii) increased heat losses associated with hot flue gas exiting the stack; and (iv) increased power consumption of support equipment, such as fans. OCAC was first demonstrated in a campaign in Chalmers 12 MW\({}_{\text{th}}\) Circulating Fluidized Bed (CFB) Research Boiler in 2012 with promising results [10] (up to 80% reduction of CO emissions and significantly altered temperature profile with 40 wt% substitution of silica sand with oxygen carrier). In that campaign the mineral ilmenite (titanium-iron ore) was used as the bed material.