## 3 Results and Discussion
### Study of DFB for Indirect Gasification
#### 3.2.3 Advanced Dynamic Analysis on the Cold Model  
In this section, more dynamic performance of the cold model was examined on the previous parameters to simulate the reliability of the units. Because the aeration velocity (\(u_{\text{v}}\)) of the lower loop seal changed, the variance of pressure drop is observed in Figure 14 for two fluidized-bed reactors (i.e., gasifier and combustor) and for loop seals (i.e., upper and lower loop seal). The pressure drops of the gasified bed (\(\Delta P_{\text{S}}\)) and of the lower loop seal (\(\Delta P_{\text{LLS}}\)) decreased as the flow rate of exposure air increased, regardless of the position at the inlet and the air positions of the lower loop seal; on the other hand, the pressure drops of the combustion bed (\(\Delta P_{\text{C}}\)) and of the upper loop seal (\(\Delta P_{\text{ULS}}\)) were increased as the flow rate of aeration air increased. The front relationship is still true when the combustion reaction bed was aerated with different air velocity. However, the pressure oscillation was more dominated under the lower aeration of the combustion bed.  
Finally, the void fraction (\(\varepsilon\)) of the bed material was calculated by the differential pressure in each measured points. Under different \(u_{\text{C}}\)/\(u_{\text{mf}}\) and \(u_{\text{v}}\)/\(u_{\text{mf}}\), Figure 15 shows the bed sand distribution ratio (i.e., \(1-\varepsilon\)) in the gasifier, and Figure 16 shows the bed sand distribution ratio in the combustor. Because the gasification reactor operated at bubbling mode, the sand distribution in the bed was between 0.5 and 0.6. Similarly, the combustion reactor was operated in fast fluidization regime, therefore the bed sand distribution in the lower dense zone of the reactor was only 0.1 to 0.2. Hence, based on the results of testing, the dual fluidization-bed reactor was in accordance with the fluidization principles. Based on the suggestion of [PERSON] et al. (2018), the tested results of the cold system demonstrated the applicability of hydrodynamic performance in DFB.  
The experimental results show that \(G_{\text{S}}\) was mainly affected by the lower loop seal. The effect of the velocity in the combustor was relatively minor, but it can handle the higher \(G_{\text{S}}\) provided by the lower loop seal. The V-type upper loop seal also showed the ability to transport the bed material in the design condition (full load) and lower loading ones (partial load). The effect of the gas velocity in the gasifier on \(G_{\text{S}}\) was minor. In other words, the major effect of the \(G_{\text{S}}\) on the gasifier was the bed height, but it still could work well as \(G_{\text{S}}\) varies. The partial-load operation of the DFB could be set by adjusting the gas velocity in the lower loop seal. For the case of the 50% operating load, the \(G_{\text{S}}\) was near the half of design value. The (dimensionless) aeration velocity was found to be around 1.1 by following the line of \(u_{\text{C}}\)/\(u_{\text{mf}}\) = 86.1, as shown in Figure 13. Finally, the gasifier can be operated under the condition by adjusting the bed height.  
Figure 13: Change of the circulating bed material when air velocity adjustment at (**a**) position 1 and (**b**) position 2.  
Figure 14: Changes of the pressure drop for the four units in cold module with the different aeration velocity at lower loop seal positions.  
Figure 15: Distribution ratio of bed sand in gasifier under different operating conditions (the values of \(u_{v}/u_{\rm mf}\) are shown as legend in the plots).