## 4 Results and Discussion
### Effects of Bed Material  
Using silica sand as bed material, the effects of gas extraction/addition through a porous-membrane on flow dynamics in the bed were also thoroughly investigated based on the differential pressure measurements. The results listed in Table 1 reveal that silica sand has a larger minimum fluidization velocity, of around 0.069 m/s, compared to calcined dolomite particles, mainly due to its larger density. A comparison of the evolution of the \(\sigma_{r}\) along the height of the FBMR between the normal operation and gas extraction conditions is shown in Figure 12. When silica sand was used as bed material, the evolution of the \(\sigma_{r}\) along the bed height during the gas addition tests was fairly consistent with the results obtained in experiments when dolomite particles are used, irrespective of whether operating with a single activated membrane or three activated membranes. Therefore, more attention had been paid to gas extraction conditions. As illustrated in Figure 12, whether at the inlet velocity of 0.09 m/s or 0.14 m/s, the \(\sigma_{r}\) of the differential pressure measured at DP1, DP2 and DP3 were increased by gas extraction, which was similar to observations made during tests using dolomite as bed material. However, at the bed sections DP4 and DP5, gas extraction led to a smaller \(\sigma_{r}\) than that in the normal operation, contrary to experimental findings when dolomite particles were used. The effects of gas extraction at specified locations on the hydrodynamics in different regions of the bed, using silica sand as bed material, were further investigated, as shown in Figure 13. With an extraction rate of 1.5 L/min, representing an extraction fraction of 15 and 10% for an inlet velocity of 0.09 m/s and 0.14 m/s accordingly, gas extraction would also cause an increase of the \(\sigma_{r}\) of the differential pressure measured at the local bed section. Similar to tests with dolomite as bed material, the incrementation of the \(\sigma_{r}\) also decreased as the distance of the measurement location from the extraction location increased. However, the decreasing rate was much faster than that of the tests using dolomite as bed material, which even caused the average bubble size to be smaller than that in normal operation at higher bed section, as indicated by green arrows in Figure 13. In addition, the divergence of the \(\sigma_{r}\) between gas extraction and normal operation seemed to increase at higher bed sections. [PERSON] et al. (2019) found that gas extraction through vertical membrane panels only caused larger equivalent bubble size when particles with a mean diameter of 196 \(\upmu\)m were used as a bed medium, while smaller average bubble size was observed when particles with a mean diameter of 500 \(\upmu\)m were used. They attributed the increase of the average bubble size, in cases when small particles were used,  
Figure 11: Effects of the gas addition location on the energy distribution profiles of multi-scale resolved differential pressure signals from DP1\(\sim\)DP4. The other operational conditions were: \(T=550\)\({}^{\circ}\)C, \(P=0.1\) MPa, \(H_{ini}=1.28\) m, \(U=0.09\) m/s, addition rate = 1.5 L/min, calcined dolomite. to the formation of densified zones, and suspected that the decrease in the average bubble size in the tests using larger particles was mainly caused by a decrease of the effective fluidizing velocity. The energy distribution profiles of the multi-scale resolved differential pressure signals measured in experiments using silica sand as bed material are shown in Figure 14. In the local bed section, gas extraction increased the energy fractions of the D7-D6 (0.39\(\sim\)1.56 Hz) level sub-signals, but decreased the energy fractions of the D4-D3 (3.125\(-\)12.5 Hz) level sub-signals of the differential pressure. This effect of gas extraction would also prolong into the upper bed sections, but decreases rapidly along the bed height.