The setup used under this study consists of a cylindrical Perspex column of diameter 10.4 cm and height 1.4 m as shown in Figure 3.1. Two ECT sensors are positioned at a space of 13 cm for measurement of solids fraction distributions across the bed diameter and along the bed axis. The lower sensor is mounted 15.7 cm above the gas distributor plate made of a highly porous stainless steel material with effective flow area, 40%.
Detailed description of this setup is given in the articles [A1, A4, A5, A7]. ECT measures the relative permittivity between two non-conducting media. Different materials have different permittivity, making it possible to measure the distribution of different solid materials in a fluidized bed using the ECT system. Each plane of the ECT sensors is divided into 32x32 pixels of which 812 pixels lie within the bed. The pixels hold the normalized permittivity of the denser material relative to the lighter material in the scale of 0 – 1. A value of 0 indicates that the bed is filled with the light material (air) and 1 indicates that the bed contains only the denser material (bed particles). The setup was used to measure the minimum fluidization and slugging velocity [A1] as well as the bubble properties [A4, A5, A6] based on the different materials and properties listed in Table 3.1. The setup was also used to investigate the mixing and segregation behaviour of biomass in a binary mixture [A7] using the materials given in Table 3.2.
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(a) (b)
Figure. 3.1. (a) Schematic illustration of a cold fluidized bed equipped with ECT sensors for measurement of solids fraction distribution (b) bed cross-section divided into 812 pixels.
Table 3.1. Properties of particles used in the cold bed behaviour studies.
Materials Mean size [µm] Density [kg/m3] 𝜀𝑚𝑓 [-] 𝑈𝑚𝑓 [cm/s]
Glass 188 2500 0.430 3.80
Glass 261 2500 0.450 8.15
Limestone 293 2837 0.530 13.80
Sand 483 2650 0.460 16.50
Glass 624 2500 0.488 23.20
Limestone 697 2837 0.607 39.24
Molecular sieve 2170 1300 0.472 76.85
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Table 3.2. Properties of particles used in the study of biomass behaviour at cold flows.
Materials Shape 𝜌𝑝
[kg/m3]
𝑑𝑝,𝑠𝑝ℎ [mm]
𝜑𝑝 [-]
𝜀 [-]
𝜀𝑚𝑓 [-]
𝑈𝑚𝑓
[m/s]
Wood pellets Cylindrical 1139 8.96 0.82 0.43 0.46 1.99
Wood chips Rectangular 423 6.87 0.75 0.49 0.57 1.27
Sand Angular 2650 0.293 0.86 0.42 0.46 0.079
3.1.1 Identification of flow regime transition
Figure 3.2 shows the average solid fraction fluctuations measured as described in the article [A1] at different superficial air velocities and bed positions. The results show that the solids fluctuations in both planes begin to increase above 0 after a certain velocity. The increase in the solids fluctuation at a higher gas velocity is attributed to the flow of bubbles. By considering that bubbles begin to rise in a bed of Geldart B particles as soon as it is fluidized, the minimum fluidization velocity 𝑈𝑚𝑓 is measured at the point where the fluctuations begin to increase from 0.
The figure also shows that the difference in the solids fluctuations between the upper and lower planes attains a peak value as the gas velocity is increased. At the peak point, the rate of change in the solids fluctuation with gas velocity is the same in both planes. As clearly described in the article [A1], slugs flow across the two planes in the region where the difference in the fluctuation curves decreases with increasing velocity. Below the peak fluctuation point, both planes bubble freely at a lower gas velocity, but at a velocity closer to the peak fluctuation (where the slope of the curve is lower), slugs flow only in the upper plane. The mean minimum slugging velocity 𝑈𝑚𝑠 over the bed is therefore obtained at the point where the fluctuation between the two planes is maximum.
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Figure 3.2. Solids fraction fluctuation at different gas velocities and two positions in a bed, showing the procedure of determining the minimum fluidization and slugging velocities.
Particle: 188 µm glass particles and bed height, 58 cm.
3.1.2 Measurement of bubble properties
Figure 3.3(a) shows the distribution of solids fraction measured with the lower plane ECT sensor in a bed containing 188 µm glass beads. The number in the colorbar indicates the normalized relative permittivity (solids fraction) of the particles due to flow of air at a velocity of 0.137 m/s in the bed with initial height, 58 cm. From the figure, a bubble is identified as a region where the solids fraction is less than 0.2 [A3]. Figure 3.3(b) displays a typical time evolution of the bubble-projected area in the deep bed, where due to bubble coalescence only a single bubble is observed at the measurement planes. The figure also shows that the projected bubble area rises to a peak value and then falls to zero when the bubble has completely passed the plane. Between two successive bubble passages, the bed is idle, giving rise to periodic fluctuation of the bed.
(a) (b)
Figure. 3.3. (a) Contour showing the distribution of solids fraction at the lower plane for a bed of the 188 µm glass particles at 𝑈0= 0.137 m/s; bed height = 58 cm. Increasing colour scale from 0 to 0.6 increases the solids concentration; in the bubble region, the solids fraction is less
than 0.2 [69] (b) evolution of the bubble-projected area with time.
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The peak of the projected area represents the bubble cross-sectional area through its centre. Assuming a spherical bubble, the bubble diameter 𝑑𝑏 is measured from
𝑑𝑏= 1
𝑁∑ (√4𝐴𝑏,𝑖
𝜋 )
𝑖 (3.1)
where 𝑁 is the number of bubble passages observed over the measurement period and 𝐴𝑏,𝑖 is the maximum projected area recorded during each bubble passage.
The bubble frequency 𝑓𝑏 is obtained from Eq. (3.2), where 𝑇𝑏 is the mean bubble period measured as the time between two successive bubble passages.
𝑓𝑏 = 1
𝑇𝑏 (3.2)
The volumetric bubble flux 𝐺 defined as the volume of bubbles passing in a unit time across a unit cross-sectional area of the bed is therefore determined from
𝐺 = 𝜋𝑑𝑏
3
6𝐴𝑇𝑏𝑎 (3.3)
where 𝐴 is the bed cross-sectional area and 𝑇𝑏𝑎 is the average time required for a complete passage of the bubbles through the plane.
By measurement of the time taken for a single bubble to move from the lower to the upper plane, the average bubble velocity can be determined. However, due to the spacing between the two ECT planes, the bubble may coalesce with another bubble or split into smaller bubbles before reaching the upper plane. This makes it difficult to trace a single bubble between the two planes and then difficult to compute the bubble travel time by any statistical method. Since the void or solids fraction can be measured at a given plane in the bed, the local bubble velocity can be computed by applying the two-phase theory [A6].
𝑢𝑏 = 𝐺/𝛿𝑏 (3.4)
where 𝛿𝑏 = 𝜀𝑓−𝜀𝑚𝑓
1−𝜀𝑚𝑓 (3.5)
3.1.3 Biomass distribution in a binary mixture
Because different solid materials have different relative permittivity, the concentration of biomass in a bed of binary mixture with inert particles can be determined by comparing the solids fraction of the mixture with that of the pure bed material at the
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same measurement position and gas velocity. The detail of this procedure is given in the article [A7]. Assuming 𝛼𝑖,𝑗,𝑠 is the solids fraction of the pure inert material (sand for example) and 𝛼𝑖,𝑗,𝑚 is the solids fraction of the mixture containing biomass at a given pixel (i,j), Eq. (3.6) can be derived for computing the mass concentration 𝑋𝑖,𝑗,𝑏 of the biomass particles at the same pixel position.
𝑋𝑖,𝑗,𝑏 = (𝛼𝑖,𝑗,𝑠−𝛼𝑖,𝑗,𝑚)
2
𝛼𝑖,𝑗,𝑚+𝛼𝑖,𝑗,𝑠(𝛼𝑖,𝑗,𝑠−𝛼𝑖,𝑗,𝑚)(𝜌𝑠
𝜌𝑏−1) (3.6)
where 𝜌𝑠 and 𝜌𝑏 are the densities of the bed material and biomass particles, respectively. In terms of volume fraction, the concentration of biomass in the bed can be obtained from
𝑌𝑖,𝑗,𝑏 = 𝑋𝑖,𝑗,𝑏
𝑋𝑖,𝑗,𝑏+𝜌𝑏
𝜌𝑠(1−𝑋𝑖,𝑗,𝑏) (3.7)
A typical result of Eq. (3.7) is shown in Figure 3.4. The figure compares the distribution of wood pellets with that of wood chips of close volumetric equivalent spherical diameter but smaller density (about 2.5 times smaller) at different gas velocities. Each bed contains 20 vol.% biomass and 80 vol.% sand particles. As can be seen, the homogeneity of pellets across the bed height increases with increasing gas velocity as more of the biomass particles move upwards from the bottom of the bed. The sinking of the wood chips from the upper part of the bed increases with increasing gas velocity, but the axial dispersion is more pronounced than in the pellet bed. The results also show that the tendency of biomass to move towards the walls is higher with the wood chips than with the wood pellets.
(a) (b)
Figure 3.4. Radial distribution of biomass in a bed mixture of sand and 20 vol.% of (a) wood pellets (b) wood chips. Upper plane = star data points with solid lines; lower plane = circle data
points with broken lines. Particles, see Table 3.2; Initial bed height = 50 cm.
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