(746b) Drying of Biomass Pellets: Experimental Study and Comparison of the Performance of a Conventional Fluidized Bed and a Rotating Fluidized Bed in a Static Geometry

The use of
dry woody biomass for combustion or gasification improves the process
efficiency and reduces net air emissions. Dry biomass is also required for
pelletization which often precedes these two processes. Finally, dry biomass
facilitates transport and storage because of its reduced biological activity
its reduced density.

In this work, the
intensification of the drying of biomass particles using rotating fluidized beds
in a static geometry [1] was experimentally studied. Experiments in both a 10 cm
diameter, 1.5 m tall conventional, that is, gravitational fluidized bed and in
a 43 cm diameter, 5 cm length rotating fluidized bed in a static geometry (RFB-SG)
were carried out in a range of operating conditions and resulting solids outlet
humidity. A schematic representation of the RFB-SG and the rotating bed of
biomass particles are shown in Figure 1.

Figure 1. (a) Schematic representation of a Rotating
Fluidized Bed in a Static Geometry;

(b) Rotating bed of
biomass particles during drying.

The comparison between
the specific drying rates (the quantity of
water transferred per second and per m³ drying chamber) in the two types of
reactor (Figure 2) shows the process intensification that can be achieved. In
the range of operating conditions studied, the process intensification factor was
between 10 and 13.

The process
intensification results from 3 factors: (i) the higher gas-solid slip
velocities resulting in improved mass and heat transfer, (ii) the higher
particle bed density and (iii) the higher particle bed uniformity. The relative
importance of the three contributions is analyzed. The efficiency of the air
utilization was also measured and is discussed. Finally, possible improvements
of the RFB-SG design to reduce the air consumption are presented.

Figure 2. Comparison of the specific drying rates
obtained in the conventional fluidized bed and in the rotating fluidized bed in
a static geometry for varying mean biomass outlet humidity. Beech wood
particles are shown in the picture.

[1] J. De Wilde and A.de Broqueville:
Rotating fluidized beds in a static geometry: Experimental proof of concept.
AIChE Journal, 53, p. 793?810, 2007.