Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 436

12.4.4.2 Underbed System
In an underbed feed system ( Figure 12.20 ), the fuel particles are crushed into
sizes smaller than 8

10 mm. Introduced in Section 12.4.2 , as pneumatic
feeding, this system is relatively expensive, complicated, and less reliable
than the overbed system (especially with moist fuels), but it does achieve
high char conversion efficiency.
Fuel entering at a feed point disperses over a much smaller area, than it
does in overbed feeding, so the feed points are more numerous and more
closely spaced. Spacing greatly affects gasification. Because a deeper bed
allows wider dispersion of the fuel and hence works with wider feed-point
spacing; increased spacing with no sacrifice of char conversion efficiency can
be achieved, but only if it is compensated by a corresponding increase in bed
height. A decrease in bed height must be matched by increased feed-point
spacing; otherwise, the conversion efficiency can drop. Coarser particles take
longer to gasify and are less prone to entrainment. Therefore, wider spacing is
preferred for them; finer particles require closer spacing.
The freeboard can provide room for further reaction of particles entrained
from the bed. Freeboard design is important, especially when wide feed-
point spacing is used.

12.4.4.3 Feed-Point Allocation
The excellent solids solids mixing in a fluidized bed helps disperse the fuel

over the bed. A single underbed fuel injection point is adequate for a small
bed having a cross-sectional size of less than 2 m 2 . A much larger area is
served by one overbed fuel feeder. A circulating fluidized bed would require
even a smaller number of feeder per unit bed area because of its superior
mixing. Larger beds need multiple feeders. The number required for a given
bed depends on factors such as quality of fuel, type of feeding system,
amount of fuel input, and bed area. Table 12.1 shows feed-point allocation
for some overbed and underbed fluidized-bed combustors.
Highly reactive fuels with high volatiles need a larger number of feed
injection points because they react relatively fast; less reactive fuels on the
other hand require fewer feed points.
Industrial designs often call for redundancy. For example, if a reactor needs
two overbed feeders, designers will provide three, each with a capacity that is at
least 50% of the design feed rate. In this way, if one feeder is out of service, the
plant can still maintain full output on the other two. The number of redundant
feeders depends on the capacity and reliability required of the plant.

12.5 COST OF BIOMASS-HANDLING SYSTEM
Material handling is a major item in the capital cost of a biomass conversion
plant. So it

is worth discussing this aspect of biomass-handling system