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Important information for system operators
CAUTION Material losses
Unsuitable quality classes B1, B2 = inadmissible fuels
Quality class
Water content
B1
≤ 35%
B2
≤ 35%
4.2
Additional information regarding fuel
a) The amount of ash depends on the fuel!
Wood chips with a low proportion of bark have an ash content of approx. 0.5%.
Wood chips from short-rotation coppice, such as miscanthus and poplar, have an ash content of 3–6%.
Bark, fine fraction, small branches and needles increase the ash content.
In addition, impurities (soil in the bark, sand, stones, etc.) in the fuel can reduce the ash melting point, resulting in the
ash sintering. This solid ash residue is referred to as slag.
A high ash content and the formation of slag prevent the system from operating efficiently, as they result in shorter
maintenance intervals for the boiler.
b) Wet wood chips!
Simple to identify using the "hand test"
•
If the wood chips are dry to touch, the water content will be below 25%
•
If the wood chips feel wet, the water content will be over 35%
A water content above 30% has a highly detrimental effect on the wood chips' suitability for storage. It results in increa-
sed self-heating (risk of fire) and the wood chips also start to mould (health hazard, allergies) and decompose (decay
= energy loss).
c) Dry wood chips!
When the tree is felled, it has a water content of approx. 50%.
With air-dried wood chips, a water content of 20–30% is achieved in practice.
If the wood is dried for a very long period of time, the lowest values are approx. 15%.
The combustion process has been designed for this range.
If the wood chips are also "industrially dried", values below 10% can also be achieved. In such cases, the boiler tends
to "pulsate" when combustion starts. The combustion state only normalises again in the "Modulation" operating phase.
d) Old wood!
In the case of old wood, it is important to consider the wood's current energy content and to ensure that it is free from
pollutants.
If the wood is already rotten or has been chemically treated, it must not be used. If the wood chips are dark brown, very
light and already brittle, they no longer have any energy and are deemed to be "compost".
Suitability for
Ash content
storage
in dry mass
≤ 3.0%
Limited storage
above 30%
≤ 3.0%
11
Material
•
Forest and plantation wood as well as other
unprocessed wood
•
Chemically untreated wood residues
•
Forest and plantation wood as well as other
unprocessed wood
•
Residual wood from industry
•
Used wood, chem. untreated
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