(380aw) Single Bubble Dynamics in a Dense Phase Fluidized Bed Gasification Environment in Presence of Biomass Pellets

Biomass gasification in fluidized
beds is a process of important commercial value [1]–[3].
Simulation of these fluidized bed units strongly depends on establishing bubble
dynamics in dense phase sand fluidized beds in the presence of biomass[4].
In the present study, a single bubble model is studied, and this to provide a
phenomenological based framework for fully fluidized beds sand beds with
several coexisting bubbles.

Studies have been made measuring
bubbles with the CREC Optiprobe system [5]
in the presence of biomass [6].
To improve bubble dynamics studies, a combination of CREC Optical Probe system,
and a video camera is employed. Bubble velocity, and bubble dimensions, both
vertical (bubble axial chord) and horizontal (bubble frontal radius) were
measured. The effects of biomass pellet concentration on bubble rise velocity,
bubble size and shape are evaluated at conditions close to minimum
fluidization.

On this basis, two theoretical
bubble models were considered. The first one is based on a spherical cap shaped
bubble with a flat bubble-wake interphase, being this consistent with classic
works in the matter [7]
The second bubble shape model considered, has an irregular bubble-wake
interphase, consistent with recently obtained bubble images [8].
These phenomenologically based models use the wake fraction to relate the
bubble and the wake sizes, while additionally, including a wake angle parameter.
These two models allow us to provide a theoretical based bubble axial cord and
bubble frontal radius predictions with air volume as only input. Finally, bubble
rising velocity as predicted in previous studies is compared with single bubble
rise velocity in the presence of biomass.

The experimental data was collected
using a 44 cm diameter and 40 cm of height fluidized bed, loaded with sand with
a particle size distribution from 200 µm to 900 µm centered at 580 µm, and a
biomass pellet with dimensions 2.7 cm in length and 0.8 cm in diameter. This is
an anticipated reactor geometry and biomass pellet geometry in biomass
gasifiers.

The measurements with out any
treatment show an increment on the noise and a divergence from the theoretical
values that grows with the injected volume. A data treatment had to be designed
to improve the experimental data and close the bubble volume balance.

Figure 1 shows a comparation
between the measured Bubble Axial Chord (BAC) and the theoretical calculated
from the model. Furthermore, a comparation between different models of bubble
rise velocity [4]
are plotted together for comparation next to the experimental results for the
experimental bubbles is shown in Figure 2 (a).

The effect of 5% vol. of biomass
can be seen in Figure 2 (b). It is shown that the biomass makes the bubbles
both smaller and slower. It is also shown that smaller bubbles are closer to
the lines of the theoretical models than the bigger bubbles that tend to be
more scatter and far from their predicted position in the plot.

Figure 1. Comparation between the two model theoretical results and
experimental measurements after data treatment.

Figure 2. Comparation between various available models for bubble
rise velocity (BRV) and bubble diameter, a) Bubbles with out biomass and b)
Bubbles with 5% vol. biomass.

References

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