1.1 Principles of wood combustion for generating heat
General principles
Net calorific value of wood biomass as a function of water content
The energy content of wood is generally given as the net calorific value
H
u
. The net calorific value is the energy that can be obtained from
burning one kilogram of damp wood. The water contained in the flue
gas is present in the form of water vapour. The gross calorific value
H
o
additionally takes account of the latent heat of condensation in the
water vapour, which is released as it cools back down to the initial
temperature.
The water content of a fuel charge is the main factor that influences
combustion. It determines the energy content and therefore the energy
conversion that can be achieved during combustion. In practice, the
net calorific value of wood fuels is between 5.0 kWh/kg (18 MJ/kg) at
a water content of 5 % and 1.5 kWh/kg (5.4 MJ/kg) at a water content
of 60 %. This depends on the type of wood and, more importantly, on
the water content or moisture level in the wood. The water content is
crucial for the controllability at partial load and the emissions from the
system.
Determining the water content
The water content is generally determined with reference to the so-
called drying cabinet procedure, on which the CEN standard is also
based. As part of this, a fuel sample is weighed when wet, dried in a
drying cabinet at approx. 105 °C for several hours, and then weighed
again. The proportion of water in the fuel is given as the water content
(w).
Application range of fuel according to water content
The boiler can be operated with wood biomass with a water content
between 5 and 30 %.
0
10
20
30
40
50
60
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
0
11
25
43
67
100
150
0
2
4
6
8
10
12
14
16
18
20
Net cv (Hu) in MJ/kg
Water content (w) in %
Moisture level (u) in %
Net cv (Hu) in kWh/kg
A
B
A
Softwood
B
Hardwood
Water content in %
0
5
10
15
20
25
30
Type of tree
Unit of measurement
Spruce
kWh/kg
5.20
4.91
4.61
4.32
4.02
3.73
3.44
kWh/smt
1971
1957
1942
1925
1906
1885
1860
kWh/stcm
1380
1370
1360
1348
1334
1319
1302
kWh/tcm
788
783
777
770
763
754
744
Pine
kWh/kg
5.20
4.91
4.61
4.32
4.02
3.73
3.44
kWh/smt
2241
2226
2209
2189
2168
2144
2116
kWh/stcm
1569
1558
1546
1533
1518
1500
1481
kWh/tcm
896
890
883
876
867
867
816
Principles of wood combustion
4
VIESMANN
VITOLIGNO 300-H
1
5672 507 GB