Fuels - Viessmann VITOLIGNO 200-S Technical Manual

Principles of wood combustion for generating heat
Freshly cut timber has a moisture level of 100 %. During storage over
one summer, this moisture level reduces to approx. 40 %. During stor-
age over several years, this moisture level reduces to approx. 25 %.
The diagram shows the dependency of the net calorific value on the
water content, using spruce as an example. With a water content of
20 % (moisture level 25 %), the net calorific value is 4.0 kWh/kg.
The net calorific value of timber stored over several years is approx.
twice that of freshly cut timber.
Storage
Not only is the combustion of moist timber uneconomical, but it also
leads to low combustion temperatures and high emissions plus tar
deposits inside the chimney stack.

1.2 Fuels

This boiler is only suitable for the combustion of natural logs. Logs
should ideally be 45 to 50 cm in length. Do not use the appliance to
burn fuels such as sawdust, fines (wood or coal), coke, woodchips,
briquettes or logging residues. If shorter logs are used, they should be
layered without air gaps. 25 cm logs can be inserted in line one after
another. Logs of 33 cm in length cannot be layered without gaps
between them and should not be used as they will reduce the boiler
output and increase the risk of ineffective gasification.
The rated boiler heating output will only be achieved with dry wood
with maximum 20 % water content or maximum humidity of 25 % (air-
dried wood).
When using softwood, about 44 % more (volume) is needed to produce
the same amount of energy as when using hardwood.
Wood of poor quality and high moisture content reduces the rated
heating output and the length of combustion.
For combustion it is important to use split logs. Splitting the wood –
preferably straight after felling – makes a significant contribution to
improving the combustion process. Enlarging the surface area enables
the wood to be degassed more quickly and simply. In addition, split
logs dry more quickly.
VITOLIGNO 200-S
(cont.)
Information on storing wood:
■ Split round timbers that have a diameter of 10 cm or more.
■ Stack the logs in a ventilated and preferably sunny spot underneath
a rain canopy.
■ Stack the logs with generous air gaps to enable the flowing air to
carry off the dissipating moisture.
■ A hollow space should be created underneath the wood pile, e.g. in
the form of storage beams, to allow moist air to escape downwards.
■ Never store freshly cut wood in a cellar, as air and sunshine are
required for drying. However, dried wood can be stored in a well
ventilated cellar.
Recommended split log dimensions
1
VIESMANN
5