Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 434

Gasification and torrefaction are relatively slow processes, compared to
combustion, so the rapid mixing of fuels is not as critical as it is in a com-
bustor. Table 12.1 compares the characteristics of the two types of feeder as
used in a combustor. Such a comparison may be valid for fluidized-bed gasi-
fiers but only on a qualitative basis.
Overbed feeders can handle coarse particles; underbed feeders need fine
sizes with less moisture. An underbed system consists of crushers, bunkers,
gravimetric feeders, air pumps, a splitter, and small fuel-transporting lines.
An overbed feed system, however, consists of crushers, bunkers, gravimetric
feeders, small storage bins, a belt conveyor, and spreaders.

12.4.4.1 Overbed System
The overbed system ( Figure 12.20 ) is simple, reliable, and economical, but it
causes a loss of fine biomass particles through entrainment. In this system,
the top size of the fuel particles is coarser than that in an underbed system,
making fuel preparation simpler and less expensive. However, the feed can
contain a large amount of fines with a terminal velocity that is higher than
the superficial velocity in the freeboard. When the terminal velocity is lower
than the superficial velocity of the fluidized bed, the particles are elutriated
before they completely gasify, resulting in a large carbon loss. This repre-
sents most of the carbon loss in a fluidized-bed gasifier.
In an overbed feed system, biomass particles are crushed to sizes less
than 20 mm, which is usually coarser than the particle size used in the
underbed system. In a typical setup, the fuel passes through bunkers, gravi-
metric feeders, and a belt conveyor, and is then dropped into a feed hopper.
Fewer feed points is an important characteristic of an overbed feed sys-
tem when used in a fluidized-bed unit. A typical unit will receive the bio-
mass from a metering feeder. The chute will need a knife or isolation valve
for safety. A seal-like rotary air lock could prevent hot gas from the combus-
tor or gasifier to be transmitted into the fuel chute. Thereafter, the fuel is
spread over the bed of the reactor. An air jet is often used to facilitate the
flow of biomass through the fuel chute.
A rotary spreader throws the fuel particles over the bed surface. The
coarser particles travel deeper into the gasifier while the finer particles drop
closer to the feeder. The bed thus receives particles of a nonuniform size dis-
tribution. The maximum throwing distance of a typical spreader is around
4

5 m. The location of the spreaders is dependent on the dimensions of the
bubbling bed. When the width is less than the depth, the spreaders are
located on the side walls; when the depth is less than the width, they are
located on the front wall. When both width and depth are greater than 4.5 m,
the spreaders can be located on both the side walls. Sometimes air is used to
assist the throw of fuel by spreaders. An air jet also helps gravity into a
CFB unit.