Dissertations in Forestry and Natural Sciences

4 Thermal processing of
wood
Thermal modification of wood has been conducted on an
industrial scale since the beginning of the 20 th century to
improve the dimensional stability and decay resistance of wood.
Nowadays, there are several commercial thermal modification
processes, such as ThermoWood ® (Finland), Plato ® (Holland),
Perdure process, and Retification ® (France). In general, the
temperatures of the thermal modification processes vary
between 180 °C and 260 °C. (Hill 2006b, Esteves and Pereira 2008,
Navi and Sandberg 2011)
Thermal modification exerts multiple effects on the physical
and biological properties of wood (Hill 2006b); these are mainly
induced by the chemical changes in the macromolecular
constituents. In addition to the improved dimensional stability
and decay resistance, thermally modified wood absorbs less
water but has a tendency to form cracks and splits and has
reduced impact toughness, modulus of rupture, and work to
fracture. The process variables, such as time and temperature of
the treatment, wood species, sample dimensions, and the use of
catalysis, have considerable effects on the resulting changes.
When the temperature increases to over 300 °C, the
degradation behavior of wood changes, and it becomes severely
degraded (Hill 2006b). In slow pyrolysis, wood is slowly heated
in the absence of oxygen up to a final temperature of 400–500 °C.
The heating rate of the process is typically in the range
5–10 °C/min (Klass 1998, Mohan et al. 2006, Dahmen et al. 2010,
Li and Suzuki 2010). The primary product of this process is
charcoal, but it also produces condensable vapors and noncondensable
gases. Charcoal can be utilized as a smokeless solid
fuel or as pure carbon in chemistry. On the other hand, the
charcoal-rich biochar could be exploited to boost agricultural
Dissertations in Forestry and Natural Sciences No 222 61