Thermal modification

The thermal modification of wood has become a topic of great interest since the first heat-treatment plant was built in Finland in the early 1990s. The production volume of thermally modified wood in Europe increased from 

One consequence of increased market relevance is that in November 2007 the European Committee for Standardization (CEN) drew up the first technical specification (TS) about thermally treated wood (TMT). 

Without application of chemicals as wood preservative, the thermal treatment can be seen as a contribution to the increasing environmental protection, even though an energy input is necessary for the production of TMT. There are different types of thermal modifications (Table 9.4.9). 

Of greatest importance still is the classical thermal modification in an atmosphere of water vapour and volatiles from wood without additional pressure. Most of the existing plants are based on this process, which has been optimized in different ways regarding the chamber system or the process sequence. The technical details in which the processes and accordingly the plants differ are company secrets. 

The other processes apply partly different atmospheres. In the oil-heat-treatment (OHT) vegetable oil is used in the French systems e.g. nitrogen is used (Retified Wood [bois retifie], NOW [new option wood]). The multi-stage PLATO (Providing Lasting Advanced Timber Option) process uses in the first 

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The cracking and the low-temperature oxidation of crude oils have been studied previously in order to simulate the thermal transformations of oil to gas and coke during enhanced oil recovery (1-6). Other authors characterize the thermal modifications of oil in the presence of a vapor phase (7)-... 

Zweifel, C., Conde-Petit, B., and Escher, F. 2000. Thermal modifications of starch during high-temperature drying of pasta. Cereal Chem. 77, 645-651. 

The application of heat to wood results in degradation associated with chemical changes in the material. If carefully controlled, the property changes that are obtained due to thermal modification can be used for certain applications. There has been a great deal of commercial activity in this sector, mainly in Finland, France and The Netherlands. Thermal modification is discussed in Chapter 5. 

The thermal modification of wood is defined as the application of heat to wood in order to bring about a desired improvement in the performance of the material. 

The properties of thermally modified wood are highly dependent upon the thermal treatment employed, and it is very important to take these into account when comparing the various treatment methods employed. This chapter examines the effect of the process variables upon the properties of thermally modified wood, and then considers the chemistry of thermal modification. Studies of physical changes are discussed, followed by an overview of the biological properties of thermally modified wood. A short description of some recent literature on the use of thermal treatment combined with compression and on hot oil treatments is also included. 

There are a variety of thermal modification methods that can be applied to wood, and the exact method of treatment can have a significant effect upon the properties of the thermally modified wood. Important process variables include the following ... 

The presence of water, or water vapour, affects the chemistry of thermal modification and heat transfer within the wood (Burmester, 1981). Under dry treatment conditions, the wood is dried prior to thermal modification, or water is removed by the use of an open system, or a recirculating system equipped with a condenser. In closed systems, water evaporated from the wood remains as high-pressure steam during the process. Steam can also be injected into the reactor to act as a heat-transfer medium, and can additionally act as an inert blanket to limit oxidative processes. Such steam treatment processes are referred to as hygrothermal treatments. Where the wood is heated in water, this is known as a hydrothermal process. Hydrothermal treatments have been extensively studied as a... 

The inherent heterogeneity of the material leads to variations in the responses of wood to thermal modification. The rate of transfer of heat into the interior of the wood is of paramount importance in order to ensure that there is a constant temperature throughout the sample. The thermal conductivity of dry wood is low and the heating method employed must ensure that the treatment is as even as possible. Heat transfer into the interior may be improved by the use of steam-heating. Heat transfer is a very significant factor in the treatment of timber of larger dimensions. 

Physical Changes in Wood due to Thermal Modification 5.4.1 Colour... 

Bourgois etal. (1989) reported that there was no change in crystallinity following thermal modification of wood for 30 minutes at temperatures ranging from 240 °C to... 

Figure 8.2 A diagram of the ThermoWood thermal modification process stages.

The Plato (Proving Lasting Advanced Timber Option) process is derived from a technology that was originally developed by Royal Dutch Shell, and was a spin-off from research into the hydrothermal conversion of biomass into liquid fuels. The Plato thermal modification process involves four stages (Figure 8.3) ... 

The Perdure process has been commercialized by PCI Industries Inc., based in Quebec. The Perdure process involves the drying and then subsequent heating of wood at temperatures ranging from 200 °C to 230 °C in a steam atmosphere. The company produces two kilns for thermal modification, the PC5 (of 8.75 m capacity) and the PC6 (of 10.5 m capacity). Process costs are of the order of 100 Euros per m. ... 

Boonstra, M.J., Tjeerdsma, B.E. and Groeneveld, H.A.C. (1998). Thermal modification of nondurable wood species. 1. The PLATO technology thermal modification of wood. International Research Group on Wood Preservation, Doc. No. IRGAVP/98-40123. 

Nuopponen, M., Vuorinen, T., Jamsa, S. and Viitaniemi, P. (2004). Thermal modifications in softwood smdied by FT-IR and UV resonance Raman spectroscopies. Journal of Wood Chemistry and Technology, 24(1), 13-26. 

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