## 2 Oxygen Carrier-Aided Combustion (OCAC)
### Review  
OCAC has been examined in large-scale experiments and operated in commercial settings in Sweden for the past few years. The concept was first demonstrated in 2012 in Chalmers University's 12 MW\({}_{\text{th}}\) CFB-type research boiler with the oxygen carrier ilmenite [34]. The concept has since been commercialized by the Swedish company Improbed AB, which is a subsidiary of the utility company E.ON. Table 1 shows an overview of activities performed in Sweden with oxygen carriers in industrial facilities. The methodology and results are described in detail in the respective reference, and a review of the findings are presented here. \begin{table}
\begin{tabular}{c c c c} \hline \hline  
showed that a 14% decrease in excess air was feasible with ilmenite. Similar observations were also made during operation with ilmenite in a BFB boiler. A six-week-long experiment (two weeks of reference operation using silica sand as bed material and four weeks of ilmenite operation, unpublished data) in a 19 MWth biomass-fired BFB boiler showed a drastic reduction of the CO concentration in the flue gases by more than 50% during the first day of ilmenite operation. Dedicated tests performed in Chalmers University's 12 MWth CFB boiler [36] confirmed that the oxygen buffering properties of ilmenite could compensate for insufficient mixing in the gaseous phase in space and time of the furnace. The effect of a more even oxygen distribution was further investigated by measuring the carbon monoxide concentration profile in three positions close to the furnace tube membrane wall in the Chalmers University boiler. The investigation [37] details that the concentration of CO near the wall was drastically reduced when ilmenite was used as bed material. The difference in CO emissions was reported the be most prominent as the excess oxygen (here defined as the concentration of oxygen measured in the flue gas) was decreased from the normal of 3.5 vol.-% down to 1.6 vol.-%. Several investigations further detail that higher thermal output was possible with ilmenite as bed material due to the improved oxygen distribution in the furnace. A reduced excess air flow results in a lower volume flow of flue gas, which in turn frees up space for increased fuel load without exceeding design values for flue gas velocities in the convection section of the boiler. Operating the boiler above design flue gas velocities increases the risk of erosion that leads to material losses on heat transferring equipment, unplanned stops, and higher maintenance costs. An increase in thermal output of 7% was reported in the tests at Ortofta [31] and as much as 13% in the tests at Handelo [44]. Previously unreported tests from the Orebro boiler and Eskilstuna boiler reveals an increase in thermal output of 3% and 8% respectively, both attributed to the effect of improved oxygen distribution in the furnace. Regeneration of the bed material is necessary to avoid an accumulation of ash species that can create boiler operating issues, such as sintering or agglomeration of the bed material. An industry-wide accepted rule of thumb is that waste combustion consumes around 6 kg fresh sand/MWh produced heat whereas the same metrics for biomass combustion is around 3 kg fresh sand/MWh produced heat [54]. This is detailed further in Section 3. In the case of ilmenite, it has been reported that less addition of fresh bed material is needed compared to sand. The consumption of fresh ilmenite was as low as 0.72 kg/MWh at Ortofta (wood and waste wood-fired) [31], 3 kg/MWh in P14 at Handelo (MSW-fired) [44]. Unpublished results reveal that as low as 0.8 kg/MWh was possible in P14 at Handelo when a system for magnetic separation [45] and recycling of the ilmenite was introduced. Unpublished results from the tests in Eskilstuna (biomass-fired) show that the consumption of fresh ilmenite can be kept at around 1 kg/MWh. The lower risk for agglomeration of ilmenite also allows for higher possible boiler temperatures [31].