Biomass Characteristics - Biomass Gasification, Pyrolysis and Torrefaction - page 55

3.3.1 Structure of Wood
Wood is typically made of hollow elongated and spindle-shaped cells
arranged parallel to each other. Figure 3.4 is a photograph of the cross-
section of a tree trunk showing the overall structure of a mature tree wood.
Bark is the outermost layer of a tree trunk or branch. It comprises an
outer dead portion and an inner live portion. The inner live layer carries food
from the leaves to the growing parts of the tree. It is made up of another
layer known as sapwood, which carries sap from the roots to the leaves.
Beyond this layer lies the inactive heartwood. In any cut wood, we can easily
note a large number of radial marks. These radial cells (wood rays) carry
food across the wood layers.
Wood cells that carry fluids are also known as fibers or tracheids.Theyare
hollow and contain extractives and air. These cells vary in shape but are gener-
ally short and pointed. The length of an average tracheid is about 1000 μ mfor
hardwood and typically 3000

m for softwood (Miller, 1999).
Tracheids are narrow. For example, the average diameter of the tracheid of
softwood is 33

8000 μ

m. These cells are the main conduits for the movement of sap
along the length of the tree trunk. They are mostly aligned longitudinally, but
there are some radial tracheids that carry sap across layers. Lateral channels,
called pith, transport water between adjacent cells across the cell layers.
Softwood (coniferous) can have cells or channels for carrying resins. A hardwood
(deciduous), on the other hand, contains large numbers of pores or open vessels.
The tracheids or cells typically form an outer primary and an inner sec-
ondary wall. A layer called the middle lamella joins or glues together
the adjacent cells. The middle lamella is predominantly made of lignin.

μ

Wood rays

Heartwood

Live bark

Dead bark

Sapwood

FIGURE 3.4 Cross-section of a tree trunk showing outer dead bark, inner live bark, sapwood,
heartwood, and wood rays. Source: Photograph by author P. Basu.