## 2 Materials and Methods
### Methodology  
Operability and behaviour of the boiler when using LD-slag as the bed material was studied by observations and measurements of the following process parameters:  
* Temperature at different key locations. Can potentially indicate whether LD-slag facilitates fuel conversion in the dense bed and if less combustion takes place in the cyclone, compared to when sand is used. * Pressure drop over key components. Indicates where the bed material is located, whether the bed fluidizes, the materials tendencies to agglomerate and if there is excessive attrition. * Amount of elutriated ash and solids captured in the secondary cyclone and the subsequent textile filter. * Bed, ash and fuel samples were also taken each day for various analyses, some of which will be referred to below. The effect on emissions was studied primarily by performing so called lambda experiments, which essentially consisted of a step-wise reduction of the air-to-fuel ratio during operation, and measuring of the resulting emissions. It should be pointed out that since the research boiler is an industrial production unit operated on commercial basis, the possibilities to perform truly comparative experiments are somewhat limited. For example, the control system of the boiler regulates the rate of fuel feeding based on the oxygen concentration in the flue gases and any changes here affect bed temperature. Also fuel quality and heat demand can vary over the course of a day or a campaign. The general procedures during lambda experiments were as follows:  
* The baseline air flow was 2.15 kg/s. * The baseline fuel flow was set to 1800 kg/h, which corresponds to roughly 5 MWth. The flow was adjusted depending on moisture content in order to achieve an outlet oxygen concentration of 3.5 vol% on dry basis. * The set point for the oxygen concentration was step-wise decreased, which results in the regulator system increasing the fuel flow. * The outlet oxygen concentration was decreased in 0.5 vol% steps down to 1.5 vol%. Each operating point was kept until the oxygen concentration had stabilized, and remained stable for at least 10 min. * The target temperature was 840-870 \({}^{\circ}\)C in the dense bottom bed. Temperature could be regulated by exchanging a fraction of the fuel for wood pellets with lower moisture content than wood \begin{table}
\begin{tabular}{l c} \hline \hline
**Gas Component** & **Measuring Method** \\ \hline Methane (CH\({}_{4}\)) & NDIR, GC \\ Carbon monoxide (CO) & NDIR, GC \\ Carbon dioxide (CO\({}_{2}\)) & NDIR, GC \\ Oxygen (O\({}_{2}\)) & PMOD, GC \\ Hydrogen (H\({}_{2}\)) & GC \\ Nitrogen (N\({}_{2}\)) & GC \\ Nitric oxide (NO) & CL \\ Nitric dioxide (NO\({}_{2}\)) & CL \\ Dmitrogen oxide (N\({}_{2}\)O) & GC \\ Total hydrocarbons (THC) & FID \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Overview of measured gas components and instrumentation. NDIR = Non-Dispersive Infrared Sensor, GC = Gas Chromatography, PMOD = paramagnetic oxygen detector, CL = Chemiluminescence, FID = Flame Ionization Detector. chips. Flue gas recirculation could also be altered for cooling the bed, but this was done only if necessary, since it can be expected to influence the fluid dynamics of the bed. Using this methodology, the emissions measured could then be expressed as function of air-to-fuel-ratio (\(\lambda\)), which is defined in Equation (5).