Biomass Handling - Biomass Gasification, Pyrolysis and Torrefaction - page 424

Ejection of
chips

Knife

Log feed

Rollers

FIGURE 12.12 Drum chipper.

entrained beds for gasification require the biomass to be ground extremely
fine so that the particles can be conveyed like pulverized coal.

12.3.4.2 Size Classification
Oversized materials often cause major problems in a biomass plant. They
jam belts, bunkers, and other components. Sometimes trummels are used in
the fuel yard to separate the oversize pieces before feeding to the plant. A
trummel ( Figure 12.13 ) is a rotating drum, with holes of various sizes, that
separates the smaller and larger feed.
A scalping screen is recommended for removal of large oversized solids.
Disc screens are good for removing stones larger than the screen opening.

12.3.4.3 Drying
Freshly cut biomass can contain 40 60% surface moisture when harvested,

but thermal gasification typically requires a moisture content of less than
10 15%—this moisture is inherent in the biomass. Furthermore, biomass is

hygroscopic, so even after drying it can still absorb moisture from the atmo-
sphere; only after torrefaction does the biomass stop absorbing moisture (see
Chapter 4). This could happen even when the dried biomass is stored in a
shed. Because biomass is bulky, with low energy density, a very large stor-
age space is necessary for the typical fuel inventories required in an energy
conversion (boiler or gasifier) plant. For this reason, the biomass is often
stored outdoors, though it could absorb additional moisture from rain and
snow. Leaving freshly harvested biomass outdoors can at times have some
positive effect. For example, straw is sometimes left in the field for a few
days or weeks to lose moisture and to leach away K and Cl before it is put
in bales (Van Loo and Koppejan, 2008). Leaving wood logs outside over the