## 3 Results and Discussion
### Study of DFB for Indirect Gasification  
#### 3.2.1 Hydrodynamic Properties of the DFB Cold Model  
In examining the cold-flow model (i.e., Figure 2), the other operating parameters (i.e., \(u_{\rm g}\) at room temperature) were based on the results of the dimensionless analysis. The variation in the averaged (repeated three times) mass flux of the bed sand versus the flow rate of air is shown in Figure 5, from which it is seen that the mass circulation flux of glass beads increased with the superficial velocity. Compared with the original design value (11.85 kg m\({}^{-2}\) s\({}^{-1}\)) by the dimensionless analysis (red cross in Figure 5), the circulation rate of the bed material (\(G_{\rm S}\) ) in the cold model can achieve the designated level at a lower gas velocity (about 2.2 m s\({}^{-1}\)). In other words, only 75% of the originally estimated value for the flow rate of operating air was needed to achieve the expected \(G_{\rm S}\). This outcome provided a safety factor for the engineering design. In the DFB, the heat produced in the combustion reactor was transferred to the gasification reactor by the bed material, which then supplied the energy needed for gasifying the feedstock. The circulation rate of the bed material (\(G_{\rm S}\)) was the key parameter to maintain the temperature in the gasification reactor. Lower \(G_{\rm S}\) will result in decreased bed temperature of the gasifier, and incomplete reactions of volatile and hydrocarbons. The tar content in the syngas will be increased as well with lower gasification temperature [6]. Higher tar content is an issue for the downstream applications, since the loading of tar handling and the operating cost would be increased. On the other hand, lower gas velocity in the combustor is beneficial to the operating cost, due to the fact that lower air volume flow rate requires less power consumption and smaller size of blower. Based on the nominal operating range in the previous examining results, a fixed operating condition was selected for performing the long-term operation. The average pressures at the height of each measurement location are shown in Figure 6, when \(u_{\rm c}\) and \(u_{\rm g}\) were equal to 2.4 and 0.15 m s\({}^{-1}\), respectively. The pattern was also displayed in multiple tests. The average pressure distribution measured by the system-wide cyclic operation was similar to that of the published literature by  
Figure 5: The flux of the bed material in combustor under different superficial gas velocity. the Vienna University of Technology (VUT) [10]. The result also shows that the pressure drop was consistent with the flow direction of the bed sand. Furthermore, the rate variance of circulating bed material was observed when the gas velocities in the gasifier (\(u_{\rm g}\)) and combustor (\(u_{\rm c}\)) were changed, as shown in Figure 7. The normal velocity of the air was set on 80% of that estimated by the dimensionless analysis. The operating region was within \(\pm 15\%\). It is shown that the flux variation of the bed material ranged from 11.9 to 13.5 kg m\({}^{-2}\) s\({}^{-1}\) in Figure 7a. The change within the highest and the lowest flux approached 12.45%, which provided sufficient adjustment ability to the temperature gradient between the two reactors. For the contour map (Figure 7b), it shows that the circulation rate of bed material at the lower-right corner exhibited the largest value, and the smallest circulation rate occurred at the upper-left corner. Hence, the high circulation rate of bed material could be obtained by increasing the gas velocity of the combustor or decreasing the gasifier's gas flow rate; at that time, more heat was transported when the temperature of the combustion furnace was fixed. Conversely, reducing the air velocity of the combustion furnace or increasing the air velocity of the gasifier led to lower heat delivery, due to slower circulation rate of the bed material. Figure 6: Average pressure distribution on the DFB.