Biomass Characteristics - Biomass Gasification, Pyrolysis and Torrefaction - page 67

the total amount of above-ground living organic matter in trees expressed as
oven-dry tons per unit area (e.g., tons per hectare) and includes all organic
materials: leaves, twigs, branches, main bole, bark, and trees.

3.5.2 Thermodynamic Properties
Gasification is a thermochemical conversion process, so the thermodynamic
properties of a biomass heavily influence its gasification. This section
describes three important thermodynamic properties: thermal conductivity,
specific heat, and heat of formation of biomass.

3.5.2.1 Thermal Conductivity
Biomass particles are subject to heat conduction along and across their fiber,
which in turn influences their pyrolysis behavior. Thus, the thermal conduc-
tivity of the biomass is an important parameter in this context. It changes
with density and moisture. Based on a large number of samples, MacLean
(1941) developed the following correlations (from Kitani and Hall, 1989,
p. 877):

K eff ð W = mK Þ 5 sp : gr ð 0 : 2 0 : 004m d Þ 1 0 : 0238 for m d greater than 40%

1

sp : gr ð 0 : 2 0 : 0055m d Þ 1 0 : 0238 for m d less than 40%

(3.9)

5 1

where sp.gr is the specific gravity of the fuel and m d is the moisture percent-
age of the biomass on a dry basis (db).
Unlike metal and other solids, biomass is highly anisotropic. The thermal
conductivity along fibers of biomass is different from that across them.
Conductivity also depends on the biomass' moisture content, porosity, and tem-
perature. Some of these depend on the degree of conversion as the biomass
undergoes combustion or gasification. A typical wood, for example, is made of
fibers, the walls of which have channels carrying gas and moisture. Thunman
and Leckner (2002) wrote the effective thermal conductivity parallel to
the direction of wood fiber as a sum of contributions from fibers, moisture, and
gas in it.

K eff 5 G ð x Þ K s 1 F ð x Þ K w 1 H ð x Þ½ K g 1 K rad W = m K for parallel to fiber

(3.10)

where G(x), F(x), and H(x) are functions of the cell structure and its dimen-
sionless length; K s , K w , and K g are thermal conductivities of the dry solid
(fiber wall), moisture, and gas, respectively; and K rad represents the contribu-
tion of radiation to conductivity.