The principle of action of different constructions
of these generators depends also on
thermal degradation and partial oxidation of
woody material. However, the temperature
needed to initiate the process results from
friction of a log pressed at about 1 kg/cm 2
against a rotating drum or disk (Rasmussen
1956 ). Access of air is possible due to perforations in the rotor. The temperature in the
friction zone may be controlled by adjusting
the pressure applied to the log and the speed
of rotation of the rotor. It is usually 300 –
350 ° C. The smoke is blown into the smoke
duct by a fan that may be fi xed on the shaft
of the rotor. Because of comparatively limited
access of air to the friction zone and low
temperature, the smoke contains fewer products
of thermal degradation and oxidation of
lignin than that from the earlier described
generators. The assets of the friction - type
machine are low consumption of wood
and the fact that the smoke production can
be started and stopped instantaneously.
However, because it makes a lot of noise
during operation and needs electrical power
for driving the rotor, it is not very often used
in the industry.