There are six main Thermo-treatment processes in Europe:
- Plato-Process (The Netherlands)
- Retification process (France)
- Bois perdure (France)
- OHT – oil-heat treatment (Germany)
- Thermo Wood process (Finland)
- Westwood (USA, Canada, Russia)
Here is the brief Review, based on "Treatment Technologies in Europe: Scientific
Background and Technological State-of-Art" by
Holger Militz, Professor Dr., Institute for Wood Biology and Wood Technology, University
Göttingen, Germany. See details of the processes at the Downloads Section on left.
It is known since longer that different intrinsic wood properties are changed due to treatment
at elevated temperatures (Stamm et al. 1946, Burmester 1973, Giebeler 1983). However, only
recently in Europe several attempts were made to develop industrial applicable technology to
thermally modify wood.
Some of those techniques are in the meanwhile turned to industrial processes with a total
production of heat treated timber in year 2007 of approx. 250.000 m3 and grow 30% a year (below the production grow in Finland).
HEAT TREATMENT PROCESSES
All the European heat processes have in common the treatment of sawn wood at elevated
temperatures in the range between 160°C and 260°C. The main differences between the
processes are to be seen in the process conditions (process steps, oxygen or nitrogen,
steaming, wet or dry process, use of oils, steering schedules etc.) and published in several
Plato-Process (PLATO BV - The Netherlands)
The PLATO-process uses different steps of treatment and combines successively a
hydrothermolysis step with a dry curing step. The impact of the hydrothermolysis in the
PLATO-treatment results in the occurrence of different chemical transformations. One aim of
this 2-step process is the use of the presence of abundant moisture in the woody cell wall
during the hydrothermolysis. This provokes an increased reactivity of the cell wall
components under relatively low temperature. In order to reach a selective degree of
depolymerisation of the hemicellulose during the hydrothermolysis, relative mild conditions
can be applied to limit unwanted side reactions (Tjeerdsma et al 1998b).
The PLATO-process (Ruyter 1989, Boonstra et al. 1998) principally consists of two stages
with an intermediate drying operation. In the first step (hydrothermolysis) of the process,
green or air dried wood, is treated at temperatures typically between 160 °C - 190 °C under
increased pressure (superathmospheric pressure). A conventional wood drying process is used
to dry the treated wood to a low moisture content (ca. 10%). In the second step (curing) the
dry intermediate product is heated again to temperatures between 170 °C - 190 °C.
The process time is depending on the wood species used, the thickness, shape of wood etc.,
and uses a thermolysis step (4-5 hours ) followed by an intermediate drying step (3-5 days )
and a final curing step (14-16 hours). In some cases, a conditioning step (2-3 days) is needed.
Depending on wood species and thickness of the material, these times can be shorter as well.
The heating medium can be steam or heated air.
Retification process (NOW New Option Wood - France)
The process starts with relatively dry wood (approx. 12 %) and heats up the material up to
200 °C – 240 °C in a nitrogen atmosphere (below 2 % oxygen). There are different production
sites in France (Duchez 2002).
Bois perdure (BCI-MBS - France)
The process starts with fresh wood, subsequently a fast drying process and heating up to 200
°C – 240 °C under steam atmosphere. The first step of the process consists in an artificial drying in the oven. Then the wood is heated under steam atmosphere (steam generated from the water of the wood).
OHT – Process (oil-heat treatment) (Menz Holz - Germany)
The process is performed in a closed process vessel. After loading the process vessel with
wood, hot oil is pumped from the stock vessel into the process vessel where the hot oil is kept
at high temperatures circulating around the wood. Before unloading the process vessel the hot
oil is pumped back into the stock vessel.
For different degrees of upgrading, different temperatures are used. To obtain a maximum
durability and a minimum oil consumption the process is operated at 220°C. However, to
obtain a maximum durability at acceptable strength reductions, temperatures between 180°C
and 200°C are used. It proved to be necessary to keep the desired process temperature (for
example 220°C) for 2-4 hours in the middle of the wooden pieces to be treated. Additional
time for heating up and cooling down is necessary, depending on the dimension of the wood.
Typical process duration for a whole treatment cycle (including heating up and cooling down)
for logs with a cross section of 100 mm x 100 mm and length of 4 meters is 18 hours.
The heating medium is crude vegetable oil. The oil serves for a fast and equal transfer of heat
to the wood, providing the same heat conditions all over the vessel and furthermore to limit the oxygen in the vessel. Linseed oil proved to be a good medium though the smell that
develops during the heat treatment may be a drawback. The smoke point and the tendency to
polymerisation are also important for the drying of the oil in the wood and for the stability of
the respective oil batch. The ability of the oil to withstand heating to a minimum temperature
of 230°C is a prerequisite. The consistency and colour of the oil changes during heat
treatment. The oil becomes thicker because volatile components evaporate, the products
arising from decomposition of the wood accumulate in the oil and change its composition.
This leads to improved setting of the oils.
Thermo Wood process (Stora, Finnforest, Stellac, Jartek - Finland)
Since the 1990th comprehensive research on heat treatment had led to commercialised
processes in Finland. An industrial scale wood heat treatment process, under trade name of
ThermoWood, has been developed at the Finnish Research Center VTT together with the
Finnish industry. Today the process is licensed to the members of the Finnish ThermoWood
Association. The ThermoWood process can divided into three main phases:
– Phase 1. Temperature increase and high temperature kilning. The kiln temperature is raised
at a rapid speed using heat and steam to a level of around 100ºC. Thereafter the temperature is
increased steadily to 130ºC during which time the high temperature drying takes place and the
moisture content in the wood reduces to nearly zero.
– Phase 2. Intensive heat treatment. Once the high temperature kiln drying has taken place the
temperature inside the kiln is increased to a level between 185ºC and 230ºC, once the
target level has been reached the temperature remains constant for 2-3 hours depending on the
– Phase 3. Cooling and moisture conditioning. The final stage is to lower the temperature
down using water spray systems and then once the temperature has reached 80-90ºC re-
moisturising and conditioning takes place to bring the wood moisture content to a useable
level over 4%. When raising or decreasing the temperature a special adjustment system is
used in order to prevent surface and inside cracking. The wood’s inside temperature regulate
the temperature rise in the kiln. The difference between kiln and wood temperature is
dependent on the dimensions of the wood specimens.
Raw material can be green or kiln dried wood. If the process starts from green wood the wood
can be dried in a very fast steam drying process. Fast drying is possible because we do not
have to care for the colour changes and because resins will anyway flow from the wood in
heat treatment process. The method is suitable for soft- and hardwoods. The process must be
optimised for each wood species.
Westwood (Westwood - USA, Canada, Russia)
Computer driven technology with fitback control uses the principally new way to control reactions inside the wood during the thermo-treatment. The modern system monitors and manages over 20 parameters every 5 seconds to control 3-dimensional heat wave inside the Chamber. As a result, WESTWOOD technology cycle time is shorter and optimized by the system itself. During an intensive heat treatment phase the air and wood temperature is raised to a level of 400 - 450° F. When the target level is reached the temperature remains constant for 2-3 hours. Steam is used to prevent the wood from burning and cracking. The system automatically adjusts to the type, size, humidity, chemical composition and geometry of the primary wood in order to obtain predictable results. This is especially important for hardwood species, which are difficult to process in manual systems with single temperature control.
At present, WESTWOOD is the most state-of-the-art wood heat treatment technology incorporating all achievements and development experience of similar technological processes. By adding high-end sensors and control systems from Omron (Japan), WESTWOOD technology allows to control processes through Internet, even through cell phones. See details at Westwood USA.