# Experimental Study of Entrainment and Mixing of Renewable Active Particles in Fluidized Beds
## 3 Results and Discussion
### Mixing in Binary-Mixture Fluidized Beds  
The mixing of the active particles with the bed material has a significant effect on many other phenomena like spatial temperature distribution and emissions-control reactions. Because of this, the main question to be answered by the evaluated experiments of this part of the study was, whether where the renewable fuels are placed within the reactor. The goal of each fluidized bed reactor is, of course, to keep the active particles inside the bed, that is, vertically, in the central region of the bed, which is also called the _bed body_[16; 30]. For discussing this, the _relative mass fraction_ is the most appropriate measure, as it is defined as the mass ratio of the active particles within the selected region over the amount of active particles in the whole bed. In this study, the following regions were considered, the top, center, and bottom of the bed. This corresponds to the industrial need for keeping the active particles within the bed body because the particles floating above the bed or sinking to the bottom do practically not utilize the advantages of the fluidized bed itself [16]. Regarding another measure, the _mass ratio_ of the active particles over the bed material, the industrially relevant scale was considered as it was kept between 1.96 to 3.2%. Figure 9 shows the vertical arrangement of the investigated renewable active particles as functions of the fluidization velocity. As visible, SRF has relatively high mass fraction in the central region (in the bed body) and at the bottom of the fluidized bed at low superficial velocities, while at elevated velocities, the entire mass of this fuel is shifted upwards. The opposite tendency can be observed in cases of all other investigated biomass fuels. This dissimilar mixing behavior of the SRF can be attributed to its non-homogeneous composition (due to diverse materials) compared to the other investigated active particles. Another possible reason is the particle size, e.g., the SRF has the widest PSD and the biggest mean size (see Figure 5). When the fluidization velocity increases, the number and size of gas bubbles increase, and this causes the shifting of the big SRF particles upwards (the umbrella effect). It is to note that no correlation was observed between the mixing characteristics and the entrainment behavior of the investigated biomass fuels. Figure 9: Vertical distributions (_relative mass fractions_) of the investigated biomass fuels in binary-mixture beds at different fluidization velocities. On the other hand, it is also important to understand the difference in the mixing characteristic among these nonspherical active particles and a representative spherical tracer. Previously, [PERSON] [31] studied the segregation behavior of the active particles in a fluidized bed using a spherical tracer in electromagnetic field [32]. The spherical tracer had an average density of 720 kg/m\({}^{3}\) and a diameter of 60 mm. This tracer showed clear segregation with an inverted-C profile when fluidized in sand. Figure 10 presents a comparison among the vertical concentration of the SRF, bark, sunflower shell, wheat shell, and the spherical tracer of [PERSON] [31] in the bubbling fluidized bed. (Note that the _vertical concentration_ used here differs from the _mass fraction_ used in Figure 9 because the _concentration_ depends on the mass of the bed material and the volume.) Figure 10 shows relatively higher concentration of the nonspherical active particles in the bottom region of the bed, which is in good agreement with the literature data [28; 33; 34]. This comparison indicates that the nonspherical active particles tend to exist in the bottom region more than the spherical tracer, as indicated by its lower concentration (probability) values. While the spherical tracer has a higher concentration in an upper region of the bed in comparison with the nonspherical active particles (SRF, bark, sunflower shell, and wheat shell).