## 2 Materials and Methods
### Methodology
The procedures during lambda experiments were as follows:  
* The air flow was kept constant at 2.15 kg/s. * The baseline fuel flow was 1800 kg/h, corresponding to roughly 5 MW\({}_{\rm th}\). The flow was adjusted depending on current moisture content in order to achieve an outlet oxygen concentration of 3.5 vol %. * The set point for the oxygen concentration was step-wise decreased. This resulted in the regulator system increasing the fuel flow. * The boiler is normally operating with an outlet oxygen concentration of 3.5 vol %, which was decreased in 0.5 vol % steps down to 1.5 vol %. Each operating point was kept until the oxygen concentration had stabilized for at least 10 min. * The target temperature was 870 \({}^{\circ}\)C in the dense bottom bed. Flue gas recirculation was increased only if necessary. Using this methodology, the measured emissions could then be expressed as function of air-to-fuel-ratio, \(\lambda\), which is defined in Equation (5). \[\lambda=\frac{l_{t}}{l_{0t}} \tag{5}\]  
\(l_{t}\) = dry air volume added to the combustion chamber (m\({}_{\rm n}\)\({}^{3}\)/m\({}_{\rm n}\)\({}^{3}\)\({}_{\rm fuel}\))  
\(l_{0t}\) = dry air volume needed for stoichiometric combustion of the fuel mix (m\({}_{\rm n}\)\({}^{3}\)/m\({}_{\rm n}\)\({}^{3}\)\({}_{\rm fuel}\))  
In practice the air-to-fuel ratio was calculated from the flue gas concentration by the following expressions, which are derived from mass balances and fuel data:  
\[\lambda=1+\frac{g_{0t}}{l_{0t}}\left[\frac{\left({\rm CO}_{2}\right)_{0t}}{ \left({\rm CO}_{2}\right)_{t}}-1\right] \tag{6}\]  
\[\lambda=1+\frac{g_{0t}}{l_{0t}}*\frac{\left({\rm O}_{2}\right)_{t}}{0.2095- \left({\rm O}_{2}\right)_{t}} \tag{7}\]  
where g\({}_{0t}\) = specific dry flue gas flow for stoichiometric combustion (m\({}_{\rm n}\)\({}^{3}\)/kg\({}_{\rm fuel}\)), \(l_{0t}\) = specific dry air flow for stoichiometric combustion (m\({}_{\rm n}\)\({}^{3}\)/kg\({}_{\rm fuel}\)), (CO\({}_{2}\))\({}_{0t}\) = CO\({}_{2}\) concentration for stoichiometric combustion of the fuel (-), (CO\({}_{2}\))\({}_{t}\) = Measured CO\({}_{2}\) concentration (-), and (O\({}_{2}\))\({}_{t}\) = Measured O\({}_{2}\) concentration (-). More than two weeks of operation of the research boiler was achieved, some of which used partial substitution of sand with manganese ore and some of which used 100% manganese ore. A summary of the experimental campaign can be found in Table 3.  
Except for the planned stop Days 6-7 the plant was in continuous operation over the whole time period. In Table 3, the column \"Test\" indicates periods during which lambda tests were performed and which will be referred to below. The experiments can be divided into four sub-campaigns. Day 1 is reference tests with sand, Days 2-5 the plant was operated with partial substitution of sand with manganese ore, Days 8-14 the plant was operated with 100% manganese ore and Days 15-16 it was operated with 100% manganese ore and sulphur feeding. Prior to Day 16, the bed was regenerated, with half of it being replaced with fresh manganese ore uncontaminated by ash. The reason for doing experiments with sulphur feeding was that the results obtained with 100% manganese ore were disappointing. One hypothesis was that the poor burnout could be associated with the presence of gas phase alkali metals during burnout in the cyclone. Thus a small trickle of elemental sulphur (1 kg/h) was fed via a separate feeding system into the bed on Days 15-16 of operation, in order to propagate sulphation of potassium to solid K\({}_{2}\)SO\({}_{4}\)[12], thus reducing the influence of gaseous alkali compounds on the combustion results. The effect of this will be further discussed below. For experiments with sand and ore mixtures, care was taken to adjust the amount of manganese in the bed so that real values as close to the reported ones as possible were achieved. Each morning some bed material mix was removed via the ash screw located in the bottom of the plant and replaced with fresh manganese ore.