Biomass Characteristics - Biomass Gasification, Pyrolysis and Torrefaction - page 77

and weak function of air temperature. For example, the equilibrium moisture
of wood increases from 3% to 27% when the relative humidity increases
from 10% to 80% (Kitani and Hall, 1989, p. 864). The moisture content of
some biomass fuels is given in Table 3.12 .

Basis of Expressing Moisture
Biomass moisture is often expressed on a db. For example, if W wet kg of wet
biomass becomes W dry after drying, its db (M dry ) is expressed as:

W wet 2 W dry

W dry

M dry 5 (3.18)

This can give a moisture percentage greater than 100% for very wet bio-
mass, which might be confusing. For that reason,

the basis of moisture

should always be specified.
The wet-basis moisture is:

W wet 2 W dry
W wet

M wet 5 (3.19)

The wet basis (M wet ) and the db (M dry ) are related as:

M wet

M dry 5 (3.20)

1 M wet

2

Fixed Carbon
Fixed carbon (FC) in a fuel is determined from the following equation, where
M, VM, and ASH stand for moisture, volatile matter, and ash, respectively.
FC 1 M VM ASH (3.21)

5 2 2 2

This represents the solid carbon in the biomass that remains in the
char in the pyrolysis process after devolatilization. With coal, FC
includes elemental carbon in the original fuel plus any carbonaceous resi-
due formed while heating, in the determination of VM (standard D-

3175).
Biomass carbon comes from photosynthetic fixation of CO 2 and thus
all of it is organic. During the determination of VM, a part of the organic
carbon is transformed into a carbonaceous material called pyrolytic carbon.
Since FC depends on the amount of VM, it is not determined directly.
VM also varies with the rate of heating. In a real sense, then, fixed car-
bon is not a fixed quantity, but its value, measured under standard condi-
tions, gives a useful evaluation parameter of the fuel. For gasification
analysis, FC is an important parameter because in most gasifiers the con-
version of fixed carbon into gases determines the rate of gasification and