Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 429

A gravity feeder is not a metering device. It can neither control nor mea-
sure the feed rate of the fuel. For this, a separate metering device such as a
screw feeder is required upstream of the chute.

12.4.3.2 Screw Feeder
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, but 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 12.17A )
Variable diameter to avoid compression of fuels toward the feeder's dis-
charge end ( Figure 12.17B )
Wire screw
Multiple screws ( Figure 12.18 )

(A)

P 2 P 1

P 1

P 2

(B)

FIGURE 12.17 Two types of screw used for trouble-free feeding of biomass. Uniform flow by
(A) variable pitch screw and (B) variable diameter screw. Source: Photograph by the author.