## 2 Oxygen Carrier-Aided Combustion (OCAC)
### Review
An experimental campaign with LD-slag as bed material and wood chips as fuel has been conducted in Chalmers University's 12 MWth boiler [46]. Crushing and sieving was done by the steel producer SSAB to obtain suitable bed material particles. The resulting material was easy to handle and was pneumatically transported to the boiler and subsequently removed from the boiler by a bottom ash screw. It was more resistant towards sintering than silica sand. The mechanical integrity was decent even though the generation of fly ash increased significantly. The temperature profile of the furnace suggested that the fuel conversion was enhanced. However, the impact on emissions was not positive. When using 100% LD-slag the emissions of especially CO was much higher than what could have been expected with silica sand. The hypothesis presented was that the relatively poor ability of LD-slag to absorb alkali [47] could have contributed to inhibit full burnout in the cyclone. Recent studies have shown that potassium indeed can have this effect [54; 55]. The high CO emissions could be addressed by adding small amounts of ammonium sulphate to vortex finder of the primary cyclone to reject alkali as sulphate, or by using a bed with a mixture of LD-slag and silica sand [46]. The campaign in Chalmers University's 12 MWth boiler was followed by experiments in the commercial 95 MWth BFB plant at Savenas. That campaign was not successfully concluded according to the original plan [56] and was aborted having reached about 7 wt-% LD-slag in the bed. The reason was severe problems in the pneumatic transport of the LD-slag to the bed material silo. The bed material in this case was a commercial fraction of LD-slag normally used as feedstock for mineral wool production. It was not established why pneumatic transport did not work satisfactory, but it is believed to have been due to the high fraction of fine material in the batch. No significant effects were seen on the performance of the boiler with this low level of substitution. Experiments in a 13 MWth MSW-fired BFB showed that adding foundry slag into the bed lowered the CO-emissions by 80% and enabled approximately 15% increase in thermal output [53]. The slag content in the bed was continuously increased, reaching an estimated total amount of 18% foundry slag. The added foundry slag enabled as low as 3.8 vol-% excess oxygen without reaching unacceptable levels of CO-emissions, while the limit with silica sand was determined to be 4.6 vol-%. The risk for agglomeration was determined to be low and the experiment was carried out with no operational difficulties related to the slag [57]. The ash disposal could be handled in the same way as during normal operation with silica sand. For some of the campaigns outlined in Table 1, no reports are found in the open literature. These refer to activities conducted by companies at commercial facilities. Also, the campaign with Elwaleed C manganese ore is unpublished. This is because the material had too low mechanical integrity for operation in CFB, i.e., it was elutriated from the bed as dust in just a few hours. CLC has yet to be demonstrated and operated on a truly industrial scale. However, Chalmers University's 12 MWth semi-commercial research boiler is integrated with a BFB gasifier. When using oxygen-carrying bed material and the gasifier this allows for operation with continuous reduction and oxidation of the bed material in what could be described as CLC/CLG mode. However, the setup is not specifically adapted to achieve high fuel conversion--the bed is rather shallow, fuel is top-fed and the temperature is limited to around 830 \({}^{\circ}\)C. Nevertheless, fuel conversion of 55-60% is typically achieved.