## 4 Results
### Steady-State Model Tuning
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter/Attribute** & **Description** & **Selected Value** \\ \hline Number of calculation nodes & The number of calculation nodes inside the riser. The parameter impacts numerical stability and model resolution. & 20 [nodes] (Varied from 10–100) \\ \hline \hline \multirow{6}{*}{Height of bottom node} & The height of the bottommost node (high-density bed zone). No core-annulus approach is implemented for this node. This height must be higher than the calculated bed height for numerical stability. & 0.7 [m] (Varied from 0.2–0.8) \\ \cline{1-1} \cline{2-3}  & The mass of particles in the fluidized bed. The value is estimated from the pressure drop in the experiment. & \\ \hline \hline \multirow{6}{*}{Convection heat transfer correlation} & The calculation method for the convective heat transfer coefficient between particles, gas, and tubes/walls. The parameter influences the heat transfer to the cooling lanes/insulation. & \multirow{2}{*}{\
\begin{tabular}{c} Mattman-Molerus-Wirth correlation (recommended for CFB) \\ \end{tabular} } \\ \cline{1-1} \cline{2-3}  & The coefficient modifies the heat transfer coefficient calculated by the convection heat transfer correlation. & \\ \hline \hline \multirow{6}{*}{
\begin{tabular}{c} Global split coefficient from core to annulus/annulus to core \\ \end{tabular} } & The coefficient reduces the calculated particle mass flows from the core to the annulus and vice versa. The parameter impacts the pressure profile. & 0.03 [-] (Varied from 0–1) \\ \cline{1-1} \cline{2-3}  & Defines the solid density on the interface between the high-density bed and the freeboard. The interface density affects the entrainment/the pressure profile. & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Attributes/parameters of the CFB combustor model. temperature outside of the refractory lining. This temperature is set to 25 \({}^{\circ}\)C and is not adjusted during the simulations. Figure 5 shows the pressure profile along with the furnace height during the long-term steady-state test with 845 kW\({}_{\text{th}}\) load. The average velocity according to the description in Section 2.2 is calculated to 4.2 m/s. In the model, the pressure drop in the freeboard is overestimated indicating that the entrainment is higher in the simulation compared to the experiment. The hydrodynamic conditions influence the chemical reactions, the heat transfer, and the temperature development in the furnace. Further improvements of the model are therefore useful to achieve better agreement in future studies--e.g., the interface density parameter can be set to smaller values. In this work, the interface density is already set 2 times smaller than the density in the splash zone. Therefore, it was chosen not to make the parameter even smaller, to have an appropriate compromise between the accuracy of the model and the physical foundation of the interface density. The agreement between experimental data and simulation is good both in terms of the total pressure drop and the reproduction of the typical CFB pressure profile. The figure also shows the standard errors for the experimental data calculated from the standard deviations in the measurement signals and the uncertainty of the measurement devices. The temperature profile is shown on the right in Figure 5. The temperatures in the simulation agree very well with the experiment. In the bed zone, the temperatures are almost constant due to the high level of mixing in the bottom region. At 2.7 m, secondary air is injected. However, the measurement shows decreasing temperatures from 2.4 to 6.3 m potentially for two reasons. Firstly, the cooling lances and the non-preheated secondary air might compensate for the combustion reactions. Secondly, no measurement is installed between these two locations. Therefore, the temperature might rise above the secondary air injection and it is just not shown by the available experimental data. However, the simulation does not show a temperature increase above 2.7 m either.