## 4 Discussion  
### CO Emissions  
As has been explained above, our current hypothesis is that the results with respect to CO emissions are connected to poor absorption of ash components, especially potassium, by LD-slag. During normal biomass combustion with silica sand as the bed material, critical ash components are absorbed on the surface of the sand particles. This limits the effect of ash on the final combustion step. Since about one third of the bed material is replaced each day during normal operation, alkali metals and other ash components are prevented from accumulating in the bed. Among other oxygen carriers, ilmenite [11] has shown excellent ability to absorb ash components, while the situation for manganese ore is more complex [24]. As reported in a separate study [23], LD-slag from this campaign that had been in the boiler for 65 h had a K-content of 1.4 wt%. This can be compared to ilmenite from a similar campaign that had been in the boiler 44 h, which had a K-content of \(>\)4 wt% [11]. The actual average age of the samples will be slightly different from 65 h and 44 h due to elutriation and make-up flow, but it can safely be concluded that LD-slag absorbs much less potassium than ilmenite. \begin{table}
\begin{tabular}{l c c} \hline \hline
**Source of Ash** & **LD-Slag** & **Si-Sand** \\ \hline Secondary cyclone (kg/24 h) & 360 & 132 \\ Textile filter (kg/24 h) & 43 & 11 \\ Ash sweeping (kg/24 h) & 155 & 50 \\ \hline Total (kg/24 h) & 558 & 193 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of fly ash generation between LD-slag and silica sand. If LD-slag has poor capability to absorb the critical ash components, this mechanism for their removal will not work. Ash components will eventually saturate the boiler and be present during the final combustion stage in the cyclone, which can be associated with increased emissions. As explained above, the mechanism is not known, but char combustion via the shrinking sphere model with an ash layer that hinders oxygen diffusion has been suggested [9]. Another possibility is that the improved fuel conversion in the boiler vessel results in little fuel being left to burn in the cooled cyclone, resulting in such big drop in temperature that final combustion is prevented. A significant temperature drop is indeed seen in Figures 4 and 5, but it is difficult to believe that the measured temperature of 770-800 \({}^{\circ}\)C would be insufficient for oxidation of CO with O\({}_{2}\). Considering the reactive nature of alkali metals, heterogeneous effects cannot be entirely discarded either. The experiments presented here support the hypothesis that the effect on CO emissions in this case has to do with poor absorption of ash. With 10% substitution with LD-slag there is still 90% relatively fresh sand in the bed with proven capability to absorb ash. Consequently, the CO emissions are low. The increase to 30% and 50% LD-slag is achieved by removing bottom ash and adding fresh LD-slag, meaning that the amount of sand present in the bed is decreasing, while also becoming progressively more saturated with ash components as the hours of operation build up. Hence the ability of the bed to absorb ash should decrease greatly, and reduced performance with respect to CO emissions for the cases of 30% and 50% LD-slag seems reasonable. With 100% LD-slag, the mechanisms for removing alkali from the boiler could be expected to be poor, which should result in high emissions. Comparable CO emissions have previously been experienced with the Chalmers research boiler when bed materials for biomass gasification (e.g., olivine) have been tested, which by design should have poor ability to absorb alkali [25]. The experiments with ammonium sulphate added to the cyclone outlet were devised with the reasoning presented above in mind. Addition of sulphur to the vortex finder should result in sulphation of potassium, ultimately forming solid K\({}_{2}\)SO\({}_{4}\)[20], thus reducing the influence of gaseous alkali compounds on the final combustion stage.