Wood drying

When drying wood is known method for removal of moisture from wood . One also speaks of kiln-dried , in contrast to air-dried wood, which slowly loses moisture simply by lying under cover.

target

The aim of controlled drying processes is to achieve the moisture content of the wood. This is usually between 8% and 16% wood moisture (based on the weight of the kiln-dried wood). In comparison: wood fresh from the forest contains around 40% water. Drying damage (tearing, cladding, discarding the wood) can only be avoided through controlled drying . When drying wood for combustion to generate energy (firewood drying), the focus is on rapid dehydration in order to improve the combustion properties and heating capacity of the fuel.

Procedure

The wood is dried during wood processing either by drying in the open air (especially with firewood) or by technical drying using thermal processes (evaporation, evaporation) in dryers . Further methods for the rapid drying of very permeable and therefore easy-to-dry woods are high-temperature drying and high-frequency drying.

Convection drying

The most common method is convection drying, usually in the form of fresh air / exhaust air drying, in which the drying gradient is regulated by controlling the temperature, the flow rate of the drying air and the relative humidity in the drying chamber. In the “warm-up phase”, the relative humidity is initially kept at a high level for better heat transfer, and in some cases increased by spraying water into the drying chamber. In the "drying phase", the drying gradient is then optimized depending on the type of wood. The “conditioning phase” at the end of the process serves to balance the moisture within the wood. A partial recovery of the energy used can be done by using the condensation heat from the exhaust air.

Open air drying

Open-air drying as the most original form of convection drying has lost its importance in the area of lumber drying. It is still the most widely used method for drying firewood. The wood to be dried is stored in the open air, protected against precipitation and with as little air circulation as possible. The drying takes place without the use of external energy through the evaporation of water from the wood and through air movement ( convection ). Outdoor drying is the simplest and most energy-saving drying method. The relatively long drying time can be a disadvantage ( four to twelve months for split logs ).

Wood vacuum drying

The short drying time of vacuum drying and the associated rapid availability of wood have opened up a fixed area of application for vacuum drying in the wood sector. The basic principle of this process, which is mainly used for valuable wood and wood that is difficult to dry, is the pressure-dependent boiling point of the water. If the pressure in a vacuum system is reduced below the pressure at which the water in the wood begins to boil and evaporate at a given wood temperature, this leads to a total pressure gradient across the board cross-section and thus to a uniform and rapid flow of steam in the wood towards the surface . This enables relatively quick drying at low temperatures. However, the specific properties of the wood with regard to its moisture conductivity must be taken into account and the parameters pressure and temperature controlled in a suitable manner. The drying takes place in a pressure-tight container in which a negative pressure of 95–150 hPa is generated.

Firewood drying

When drying firewood, the focus is on the cost-effective and energy-efficient removal of moisture from the fuel - the calorific value of firewood increases linearly with decreasing moisture content. In contrast, drying damage does not play a role. For cost reasons and in order to preserve the environmental advantage of the biogenic fuel wood, the energy gain achieved by drying when using firewood must be significantly higher than the energy expenditure for drying. In the case of outdoor drying, the energy consumption is minimal. With convection drying, the financial outlay and the use of energy can be optimized by using otherwise unusable waste heat to dry the wood. If waste heat from the generation of bioenergy (e.g. combined heat and power in biogas systems ) is used to dry wood, the operator of the bioenergy system receives additional remuneration for the electricity fed in (CHP bonus) - but only from the commercial one since 2009 Drying wood chips or drying sawdust for the production of fuel pellets . In addition to chamber drying, there are other methods of convection drying (e.g. drum dryer ) and contact drying (e.g. screw dryer for wood chips ) for firewood .

literature

Thomas Trübswetter: Wood drying. Hanser Fachbuchverlag, 2006, ISBN 3-446-40477-5 .
Ulf Lohmann: Wood manual. DRW-Verlag, 1999, ISBN 3-87181-348-6 .
R. Brunner: Drying lumber. 5th edition. 1987.

Web links

IBT-Krämer Institute for Firewood Technology (information on firewood drying)

Basics of drying wood (information on drying lumber )

Individual evidence

↑ ^a ^b Georg Krämer: Introduction. In: Conference proceedings for energy wood specialist conference: Quality assurance as a competitive factor in the log market. Bad Wildungen School of Wood, 2008.
↑ New heat bonus also allows wood to be dried. In: Top Agrar. October 16, 2008. (pdf)
↑ Florian Gerlach: No longer the Cinderella of the wood industry. In: planting energy. VI / 2006, pp. 38-42.

[Kräm-1] Georg Krämer: Introduction. In: Conference proceedings for energy wood specialist conference: Quality assurance as a competitive factor in the log market. Bad Wildungen School of Wood, 2008.

[2] New heat bonus also allows wood to be dried. In: Top Agrar. October 16, 2008. (pdf)

[3] Florian Gerlach: No longer the Cinderella of the wood industry. In: planting energy. VI / 2006, pp. 38-42.