Wood dryer is a technological device used to decrease internal moisture of wood. The wood can be of the form of wooden chips or saw dust. Wooden chips can differ in the size. The input for a dryer is wooden chips with moisture of 50-65%. The output moisture percentage is regulated by the dryer and can vary between 10-30%. The heating medium is hot air.
We concentrate on using of the wasted heat from power plants, heat station or biogas stations. A common characteristics of these factories is the effectivity up to 40%. The rest of the energy is simply cooled into the ambient air.

The scheme of a wood chips dryer is on the next figure.

 Wood chips dryer - scheme

Wooden chips - why drying it?

  • The moisture content of fresh wood can vary between different kinds of trees and also from the outside in. Total moisture content can vary between 55-75% of the total wood mass. The water inside the wood can have three different forms - free water, bound water and molecular water.
    The free water can be dried out the most easily. It can be even dried outside, which typically lasts several months. Bounded water can be dried out under higher temperature as the structure of the wood needs to be disturbed. The molecular water cannot be isolated unless the wood chemically reacts with other substances. This does not happen during regular drying.
  • Water is substance with very high heat capacity. That means, the more water is contained in the fuel, the more energy it will carry out in the form of vapor. For a brief comparison consider alcohol ethanol and its heat capacity of 896kJ/kg. On the other hand, the heat capacity of water is 2250kJ/kg! This is the energy needed to change water with boiling temperature into steam of the same temperature. It is also energy that is carried away by the water vaporized during combustion. Another negative aspect of high moisture inside the fuel is worse quality of combustion.
    During the combustion process a lot of tar and similar substances are produced which then may stick inside the chimney. The result is not only less effective combustion, but also higher expenses to keep the chimney clean. There are a lot of negative economical and environmental effects.
    The first motivation for drying wooden chips is a higher heat value and improved effectivity of combustion. How significant is the effect of moisture in the wooden chips on the heat value? Compare - wood with moisture 60% has the heat value around 6MJ/kg. On the other side, if the wood is dried to moisture content of 20%, the heat value increases to 14MJ/kg!
  • The second motivation for investment into a wood dryer is utilization of the waste heat. City heat stations, power plants and other factories operates with relatively low heat efficiency. The rest of energy is wasted in the form of heat cooled into the air or water. European Union is increasing its pressure for better efficiency in the industry. As an example, in the Slovak Republic, starting from 2015, the government donations for electricity production will decrease if the power plant does not somehow use the waste heat.
    Analogous to cars that need to satisfy strict EURO norms, the industrial machines, power plants, heat plants and others will be forced to increase their effectivity and optimize the usage of fossil fuels. Economical and environmental aspects are taken into consideration.

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