## 2 Materials and Methods
### Methodology
In practice the air-to-fuel ratio was rather calculated directly from the measured composition by expression (6), which is derived from mass balances and fuel data:  
\[\lambda=\frac{\dot{v}_{air}}{\dot{v}_{air,0}} \tag{5}\]  
\[\lambda\approx 1+\frac{\dot{v}_{fg,0}}{\dot{v}_{air,0}}\Bigg{[}\frac{(CO_{2})_{ d,0}}{(CO_{2})_{d}}-1\Bigg{]}\approx 1+\frac{\dot{v}_{fg,0}}{\dot{v}_{air,0}}* \frac{(O_{2})_{d}}{0.2095-(O_{2})_{d}} \tag{6}\]  
\(\dot{v}_{air}\) = dry air volume added to the combustion chamber (m\({}_{n}\)\({}^{3}\)/m\({}_{n}\)\({}^{3}\),fuel) \(\dot{v}_{air,0}\) = dry air volume needed for stoichiometric combustion of the fuel mix (m\({}_{n}\)\({}^{3}\)/m\({}_{n}\)\({}^{3}\),fuel) \(\dot{v}_{fg,0}\) = dry flue gas volume for stoichiometric combustion of the fuel mix (m\({}_{n}\)\({}^{3}\)/m\({}_{n}\)\({}^{3}\),fuel) \((CO_{2})_{d,0}\) = CO\({}_{2}\) fraction in dry flue gas for stoichiometric combustion of the fuel mix (-) \((CO_{2})_{d}\) = Measured CO\({}_{2}\) fraction in dry flue gas (-) \((O_{2})_{d}\) = Measured O\({}_{2}\) fraction in dry flue gas (-)  
Samples of bed material were extracted from inside the boiler during operation. This was done using a special cooled collection probe. Some of these samples were subject to standardized bed agglomeration tests at the Research Institute of Sweden (RISE) in order to determine tendencies of the bed material to agglomerate at severe conditions. The methodology for agglomeration tests was as follows. A sample of 200 g was placed in a 140 cm long tube reactor with an inner diameter of 7 cm. The sample was located on a sintered metal plate located in the middle of the tube, which also acted as gas distributor. The sample was fluidized with 12 l\({}_{n}\)/min air, which corresponds to a superficial gas velocity of about 0.3 m/s at 750 \({}^{\circ}\)C. The pressure drop over the bed was measured with a pressure transducer and the temperature was monitored with a K-type thermocouple. The test was initiated when the bed temperature was stabilized at 750 \({}^{\circ}\)C. From this point the bed temperature was increased by 3.5 \({}^{\circ}\)C/min. The maximum temperature was 1100 \({}^{\circ}\)C. A reduction in pressure drop across the fluidized bed was interpreted to indicate that agglomeration had occurred (elutriation is theoretically possible but should not occur at the conditions used). Three temperatures were reported:  
* T\({}_{\rm A1}\) (first tendency towards agglomeration): This is the lowest temperature when a disturbance in the pressure drop over the bed can be verified. The point was chosen where a continuous reduction in pressure drop of about 1 Pa/min could be seen. * T\({}_{\rm A2}\) (apparent agglomeration): The lowest temperature when agglomeration clearly is taking place. The bed is considered to be agglomerating when the pressure drop is reduced continuously with 5 Pa/min or more. * T\({}_{\rm A3}\) (complete agglomeration): The temperature when the pressure drop is stabilized at its minimum value. The research boiler was operated for two weeks, basically one week with practically 100% LD-slag and one week with partial substitution of silica sand. The boiler was stopped and emptied prior to the campaign, but since it is difficult to clean parts of the system, traces of sand (<1 wt%) could possibly be present also during the period of operation referred to as 100% LD-slag. Further, experiments were performed with addition of ammonium sulphate to the vortex finder (i.e., primary cyclone outlet) and with sulphur granulates fed directly to the bed. Both measures will be discussed more in detail in Section 3 below. A summary of the experimental campaign can be found in Table 4.  
The experiments can be divided into a number of categories. Days 2-5 involved experiments with almost 100% LD-slag, with or without addition of ammonium sulphate or sulphur granulates.