Principles Of Wood Combustion; Principles Of Wood Combustion For Generating Heat; General Principles - Viessmann VITOFLEX 300-UF Technical Manual

Principles of wood combustion

1.1 Principles of wood combustion for generating heat

General principles

1
Net calorific value of wood biomass as a function of moisture
content
The energy content of wood is given as the net calorific value H
The net calorific value is the energy that can be obtained from burn-
ing 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
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 moisture content of a fuel charge is the main factor that influen-
ces combustion. It determines the energy content and therefore the
energy conversion that can be achieved during combustion. In prac-
tice, the net calorific value of fuels is between 5.0 kWh/kg (18 MJ/kg)
at a moisture content of 5 % and 1.5 kWh/kg (5.4 MJ/kg) at a mois-
ture content of 60 %. This net calorific value depends on the type of
wood and, more importantly, on the moisture content or moisture
level in the wood. The moisture content is crucial for controllability at
partial load and the emissions performance of the system.
Net calorific value of wood in relation to moisture content (M)
Moisture level (u) in %
0
20
18
16
14
12
10
8
6
4
2
0
0
Water content (w) in %
A Softwood
B Hardwood
Viesmann
6
11 25
10 20
Determining the moisture content
The moisture content is determined with the drying cabinet method
. on which the CEN (European Committee for Standardisation) stand-
u
ard is 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
o
the moisture content (M).
43 67
A
B
30 40
100 150
5.5
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
50 60
VITOFLEX 300-UF