A screw feeder is a positive-displacement device. Not only can it move solid particles from a low-pressure zone to a high-pressure zone with a pressure seal; it can also measure the amount of fuel fed into the bed. By varying the speed of its drive, a screw feeder can easily control the feed rate. As with a gravity chute, the fuel coming out of a screw does not have any means for dispersion. An air dispersion jet employed under the screw feeder can serve this purpose.

Plugging of the screw is a common problem. Solids in the screw flights are compressed as they move downstream; sometimes they are packed so hard that they do not fall off the screw. Compaction against the sealed end of the trough carrying the screw is even worse, often leading to jamming of the screw. Plug­ging and jamming can be avoided by one of the following:

• Variable-pitch screw (Figure 8.17a)

• Variable diameter to avoid compression of fuels toward the feeder’s dis­charge end (Figure 8.17b)

• Wire screw

• Multiple screws (Figure 8.18)

A wire screw is suitable for a highly fibrous biomass. It is made of a helical springlike 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 8.18 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 careful design. Figure 8.9 showed two designs. The first (refer to Figure 8.9a) has a tapered wall hopper. It develops a large stagnant layer on the hopper’s down­stream wall. The second (refer to Figure 8.9b) is a vertical hopper wall toward the discharge end. This is superior to the traditional inclined wall because it develops a smaller stagnant layer and thus avoids formation of rat holes.

A screw feeder typically serves 3 m2 or less area of a bubbling fluidized bed, so several feeders are needed for a large bed. A major and very common 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 model of the mechanism of solid flow through a screw feeder. They noted that the torque required by the screw is proportional to the vertical stress exerted on the hopper 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 hopper 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, D0, rotating in a shaft of diameter, Dc, is given as

T = KovD30 (8.9)

where av is the vertical stress for the flow and D0 is the screw diameter. The constant, K, depends on the ratios P/D0 and Dc/D0 (normal stress/axial stress) and the wall friction, where Dc is the shaft diameter and P is the pitch of the screw.