Environmental Engineering Reference
In-Depth Information
that has been allowed to dry in covered areas can even end up with water content
of less than 10%. Even with 15% water content, the calorifi c value of beechwood
is still 4.15 kWh/kg. With freshly cut wood with a water content of 50% the calorifi c
value drops to 2.16 kWh/kg (Figure 12.6). Consequently, its calorifi c value is con-
siderably less than half that of bone-dry wood.
6
beech, birch
pine, spruce
5
4
3
2
1
wood moisture u in %
11
18
25
43
67
100
150
0
15
0
10
20
30
40
50
60
water content w in %
Figure 12.6 Calorifi c values of wood depending on wood moisture and water content.
This example shows that fi rewood should be well dried before it is burnt to extract
the optimal energy content. The mass- related calorifi c value per kilogram differs
minimally with different types of wood. On the other hand, the volume-related calo-
rifi c value, thus the calorifi c value of a solid cubic metre or a stacked cubic metre,
varies considerably (Table 12.2). Heavy wood like beech burns longer than light-
weight spruce.
Table 12.2 Characteristics of different types of fi rewood.
Calorifi c
value dried
Density dried
Calorifi c value H i with w = 15%
H i0 in
kWh/kg
In kg/solid
cubic metre
In kWh/kg
In kWh/solid
cubic metre
In kWh/stacked
cubic metre
Beech
5.0
558
4.15
2720
1910
Birch
5.0
526
4.15
2570
1800
Pine
5.2
431
4.32
2190
1530
Spruce
5.2
379
4.32
1930
1350
In addition to providing poor calorifi c values, too much moisture in wood also has
some other undesirable effects. High water content means that wood is being burnt
under less than optimal conditions. As a result, it releases a high amount of harmful
substances and produces unpleasant quantities of smoke and a pungent smell.
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