Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 430

Cooling jacket

Screw drive

FIGURE 12.18 Double-screw feeders help uniform flow of biomass.

A wire screw is suitable for a highly fibrous biomass. It is made of a helical
spring like wire with no central shaft or blades. Because there is minimum metal-
feed contact, there is less chance of feed buildup even if the feed is cohesive.
Multiple screws are effective especially for large-biomass fuels.
Figure 12.18 shows a feeder with two screws. Some feed systems use three,
four, or more.
The hopper outlet, to which the inlet of a feeder is connected, needs care-
ful design. Figure 12.9 showed two designs. The first ( Figure 12.9A ) has a
tapered wall hopper. It develops a large stagnant layer on the hopper's down-
stream wall. The second ( Figure 12.9B ) is a vertical hopper wall toward the
discharge end. This is superior to the traditionally inclined wall because it
develops a smaller stagnant layer and thus avoids formation of rat holes.
A screw feeder typically serves 3 m 2 or less area of a bubbling fluidized
bed, so several feeders are needed for a large bed. A major and very com-
mon operational problem arises when the fuel contains high moisture. It has
to be dried first before it enters the screw conveyor to avoid plugging.
Dai and Grace (2008) developed a theoretical model to determine the
mechanism of solids flow through a screw feeder. They noted that the torque
required by the screw is proportional to the vertical stress exerted on the hop-
per outlet by the bulk material in the hopper; it also depends strongly on screw
diameter. The choke section (the part of the screw extending beyond the hop-
per exit) accounts for more than half of the total torque required to feed the
biomass, especially with compressible particles. The torque, T, required by a
screw of diameter, D 0 , rotating in a shaft of diameter, D c , is given as:
T 5 K i σ v D 0 (12.9)

σ v is the vertical stress for the flow and D 0 is the screw diameter. The
constant, K i , depends on the ratios P/D 0 and D c /D 0 (normal stress/axial
stress) and the wall friction, where D c is the shaft diameter and P is the pitch
of the screw.

where