## 3 Results and Discussion
### Study of DFB for Indirect Gasification
#### 3.2.2 Operating the Cold Model by Loop Seals  
Loop seal is utilized to avoid the penetration of the gases between the combustion reactor and the gasification reactor, as well as to transport the bed material between the two reactors (Figure 2a). For the upper loop seal, the bed material was coming from the combustor and collected there. Then, steam was adopted as the fluidizing gas for driving the bed material to move out the upper loop seal and enter the gasification reactor.  
Three types of upper loop seal were tested in the present study to identify the proper one for further study. The configurations shown in Figure 9 are U-type with baffle, U-  
The experimental results of the upper loop seals are shown in Figure 10. When the mean velocity (\(U_{v,\text{ULS}}\)) in the upper loop seal increased, the circulation rate of the bed material increased in the all types of upper loop seals. The V-type loop seal shows the lowest gas volume flow rate needed to provide the same circulation rate of the bed material, compared to the other two U-type loop seals. Hence, it was beneficial to adopt the V-type loop seal in the study.  
The lower loop seal, called V-type, was made with reference to the shape of VUT. In addition to the location at outlet side (for \(u_{o}\)) of the loop seal, the aeration point at seal inlet side (for \(u_{i}\)) was designed, for which two positions can be selected (Figure 11). Firstly, the aeration flow rate at the outlet position was set (i.e., \(u_{o}=0.42\) cm s\({}^{-1}\)) to a value below \(u_{\text{mf}}\), so that the bed material would not be blown off; and then, the air flow rate at the inlet position 1 or position 2 was adjusted for individual testing. The gas velocity at the V-type loop seal was defined by combing the both aeration flow rates at the inlet and outlet, i.e., \(u_{v}=u_{i}+u_{o}\).  
Figure 10: Test data of the three types of upper loop seal.  
Figure 9: Three types of upper loop seal and aeration position.  
Figure 12 shows the pressure distribution under three different \(u_{i}\) (i.e., \(u_{\rm v}/u_{\rm mf}\) within 0.7-2.4 @ \(u_{\rm c}/u_{\rm mf}=103.3\)). Regardless of the inlet positions, the pressure drops of the upper loop seal (P12-P13) and of the combustor (P7-P11) were increased as increasing \(u_{\rm v}/u_{\rm mf}\).  
The flux changes of the circulating bed material (\(G_{\rm S}\)) under different gas velocity of the loop seal (\(u_{\rm v}\)) and the combustor (\(u_{\rm c}\)) are shown in Figure 13. Figure 13a shows the result of the velocity variance at the position 1, and Figure 13b shows the counterparts at the position 2. It is obvious that the circulating bed material increased with the increase of the total gas velocity (\(u_{\rm v}\)) for the loop seal. Moreover, when the ratio of \(u_{x}\) to \(u_{\rm mf}\) was lower (i.e., \(u_{\rm v}/u_{\rm mf}<1.55\)), different gas velocity of the combustor (\(u_{\rm c}\)) exhibited similar characteristics and very close value of \(G_{\rm S}\), regardless of the inlet air at position 1 or 2. However, when the \(u_{\rm v}/u_{\rm mf}\) ratio was greater than 1.55 the variation trend of \(G_{\rm S}\) became diversified. Although the \(G_{\rm S}\) still increased along with the \(u_{\rm c}\), it will eventually approach an asymptotic value when the inlet air was injected at position 2. The \(G_{\rm S}\) change for the inlet air at position 1 was larger than that at position 2, and kept increasing when \(u_{\rm v}/u_{\rm mf}\) increased. However, it won't influence the  
Figure 11: V-type lower loop seal and aeration position.  
Figure 12: Pressure distributions under different aeration conditions.  
selection of the aeration position for the standard operating area, which was set for \(G_{\text{S}}\) = 11-13 kg m\({}^{-2}\) s\({}^{-1}\) (hatched area in the Figure 13). Nevertheless, the outcome reveals that inlet air at position 2 would provide favorably stable flow of circulating bed material from operational viewpoints.