Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 426

It allows a magnet hanging from the top to remove magnetic materials
and other devices to remove oversized solids and other scrap materials as the
biomass moves along the belt. The biomass that remains is fed into a silo for
temporary storage.
12.3.5 Feeding
Feeding is the last step in the feedstock-handling stream. A feed system
should include a weighing scale and a tramp metal magnet. A final scalping
screen at this point to remove oversize could provide additional reliability to
the system. Many types of feeder are used depending on biomass type and
other process parameters. This topic is discussed next.
12.4 BIOMASS FEEDERS
Based on the type of biomass, feeders can be divided into two broad groups:
(1) those for harvested biomass and (2) those for nonharvested biomass.
Harvested fuels include long and slender plants like straw, grass, and
bagasse, which carry considerable amounts of moisture. Examples of nonhar-
vested fuels are wood chips, rice husk, shells, barks, and pruning. These
fuels are not as long or as slender as harvested fuels, and some of them are
actually granular in shape.
12.4.1 Feeding Systems for Harvested Fuel
Harvested biomass, such as straw and nonharvested hay, is pressed into bales
in the field, and sometimes the bales are left in the field to dry
( Figure 12.14 ). Baling facilitates transportation and handling ( Figure 12.15 ).
Cranes are used to load the bales at a certain rate depending on the rate of
fuel consumption. The bales are brought to the boiler house from storage by
chain conveyors.
Whole bales are fed into a bale shredder and a rotary cutter chopper to
reduce the straw to sizes adequate for feeding into a fluidized-bed gasifier or
combustor. In the final leg, the chopped straw is fed into the furnace by one
of the several feeder types. Figure 12.15 shows a ram feeder which pushes
the straw into the furnace. In some cases, the straw falls into a double-screw
stoker, which presses it into the furnace through a water-cooled tunnel.
12.4.2 Feeding Systems for Nonharvested Fuels
Wood and by-products from food-processing industries are generally granu-
lar in shape. Wood chips and bark may not be of the right size when deliv-
ered to the plant, so they need to be shredded to the desired size in a