## 2 Materials and Methods
### Methodology
The amounts removed and added were attuned to achieve the desired bed composition and a fixed pressure drop over the whole bed of 5.5 kPa. Two assumptions were required:  
* It was assumed that the composition of the bed material removed is the same as it would be if the whole bed were perfectly mixed. * It was assumed that there was no accumulation of ash or fuel in the bed. Both assumptions are likely to be imperfect, hence the bed composition reported in Table 3 should be seen as approximate. For the experiments with only manganese ore minor amounts of new material was added in the morning in order to obtain a pressure drop over the boiler of 5.5 kPa when necessary. However, in practice very little material was replaced during Days 8-15. A partial regeneration was done Day 16, during which 50% of the bed material was removed and replaced with fresh manganese ore. Aside from this, the bed was not regenerated over the course of the experimental campaign. A basic mass balance over the boiler has also been carried out. It was based on the fuel and ash flows during each day of the period of operation. The amount of wood chips and wood pellets (the latter was used as supplementary fuel to a connected gasification reactor during a period of time \begin{table}
\begin{tabular}{c c c c} \hline
**Test** & **Day** & **Sand (wt \%)** & **Mn Ore (wt \%)** & **Comment** \\ \hline I & 1 & 100 & 0 & Reference experiment \\ II & 2 & 90 & 10 & \\ III & 3 & 70 & 30 & \\  & 4 & 70 & 30 & \\ IV & 5 & 50 & 50 & \\ \hline \multicolumn{4}{c}{Boiler stop during weekend (Days 6–7) to remove material contaminated with sand.} \\ \hline  & 8 & 100 & Start-up with 100\% Mn ore \\ V-VI & 9 & 100 & Including operation at 800 \({}^{\circ}\)C \\  & 10 & 100 & \\  & 11 & 100 & \\  & 12 & 100 & \\ \hline \multicolumn{4}{c}{Operation with 100\% manganese ore during weekend (Days 13–14) by the plant operator.} \\ \hline VII & 15 & 100 & Experiments with sulphur feeding \\ VIII & 16 & 100 & Regenerated bed and sulphur feeding \\ \hline \end{tabular}
\end{table} 
Table 3: Overview of the experimental campaign in Chalmers Research Boiler. not covered in this article) added together with the fuel analysis were used in order to estimate the amount of ash added to the boiler:  
\[\dot{m}_{ash,in}=\dot{m}_{wood\text{ }chips}X_{ash,wood\text{ }chips}+\dot{m}_{wood\text{ }pellets}X_{ash,wood\text{ }pellets} \tag{8}\]  
where \(\dot{m}_{ash,in}\) = mass flow of ash added to the boiler (kg/h), \(\dot{m}_{wood\text{ }chips}\) = mass flow of wood chips added to the boiler (kg/h), \(x_{ash,wood\text{ }chips}\) = approximate ash content wood chips (-), \(\dot{m}_{wood\text{ }pellets}\) = mass flow of wood pellets added to the boiler (kg/h), and \(x_{ash,wood\text{ }pellets}\) = approximate ash content wood pellets (-). The amount of material elutriated from the boiler is accounted for at two collection points: the secondary cyclone (6) and the textile filter (7). The collection containers are each placed on a scale and their weight is logged during operation. The amount of material leaving the boiler was thus calculated as:  
\[\dot{m}_{material,out}=\dot{m}_{sec,cyclone}+\dot{m}_{text,filter} \tag{9}\]  
where \(\dot{m}_{ash,material,out}\) = mass flow material leaving the boiler (kg/h), \(\dot{m}_{sec,cyclone}\) = material collected in secondary cyclone (kg/h), and \(\dot{m}_{text,filter}\) = material collected in the textile filter (kg/h). It was assumed that the difference between the amount of ash added with the fuel and the amount of material elutriated from the boiler consisted of bed material. This assumption neglects the amount of ash which is accumulated within the boiler. This implies that the rate of attrition calculated from this mass balance could be underestimated, since ash remaining in the boiler would mean that more bed material has been elutriated.