Environmental Engineering Reference
In-Depth Information
ten people and in the process absorbs large quantities of carbon dioxide. The 15 cubic
metres of wood of this beech have a dry mass of 12 tons, of which around six tons are
bound to pure carbon. This corresponds to the carbon content of 22 tons of carbon
dioxide.
Yet even this massive beech is dwarfed by some tree species. The majestic sequoia
reaches heights of up to 115 m (377 ft), diameters of 11 m (36 ft) with a circumference
of over 30 m (98 ft) and volumes of 1500 cubic metres. Not only do trees reach an
impressive size, but they can also live to a very great age. Some pine trees live for 5000
years. There is a special type of pine where the rootstock can even survive for more than
10 000 years. New shoots that sprout from them 'only' live to about 2000 years. As a
result, trees are presumed to be the oldest, highest and heaviest living things on earth.
If one were to compress wood shavings into a cube with sides each one metre long,
the mass and calorifi c value of this cube would be considerably higher than a cubic
metre of wood pellets. The reason is that a relatively large amount of air is trapped
within a pile of pellets.
In technical jargon, the measure of capacity for one cubic metre of solid wood mass
without any gaps is also called a solid cubic metre. When round wood or fi rewood
is stacked very neatly, air gaps occur. The measure of capacity in this case is called
stacked cubic metre. When fi rewood is thrown loosely on a pile, the air gaps
increase. In terms of measure of capacity, this is referred to as bulk stacked cubic
metre (BCM). The various measurements of capacity can be converted approxi-
mately into one another, with the exact factors depending on the type of wood and
the shape the wood takes:
■
1 solid cubic metre = 1.4 stacked cubic metre = 2.5 bulk stacked cubic metres
(BCM)
As we all know, wet wood burns poorly, because the calorifi c value of wood
depends critically on water content. Damp wood is also heavier than dry wood.
This means that not only the wood itself but also the water it contains has to be
transported. The water evaporates when the wood burns. However, to evaporate,
it needs energy - which in turn comes from the wood. The calorifi c value drops as
a consequence.
Either the wood moisture or the water content is used to indicate the drying level.
As both quantities have different parameters and their values therefore differ con-
siderably, this can lead to some confusion. The water content describes the propor-
tion of water in wet wood. On the other hand, the wood moisture indicates the mass
of water contained in ratio to the mass of totally dry wood. If exactly half the weight
of wood consists of water, the water content is 50% but the wood moisture level is
100%.
Completely dry wood with 0% water content is referred to as bone-dry wood. For
example, bone-dry beech trees have a calorifi c value of fi ve kilowatt hours per kilo-
gram (kWh/kg). When wood is dried outdoors, its water content reduces to between
12 and 20%. The wood should be chopped as early as possible into small pieces,
covered and then left to air dry for at least one year, but ideally two years. Wood
