9.5 Judging the quality
The base for the following tables are wood chips
from spruce or alder, with a water content of 30%
and size P16S.
Judging the quality
Criterion Comments
Ash content Contaminated material burns poorly and is often a sign of decayed or dirty wood chips.
Large pieces Fuel may contain individual, large pieces up to 20 cm long.
They are cut by the blade in the rotary valve. The majority of wood chip pieces should not be
longer than 5 cm to reliably prevent blockages in the fuel conveying system.
Dirt Soil and sand cause slag formation on the grate and result in more effort spent on cleaning d
ue to a lower ash melting point.
Green leaves and
needles
A layer of chips from green branches with leaves or needles can cause a blockage in a pile of
chips on which rising moisture condenses, resulting in decay and mould.
Metal, stones Even though the boiler cannot be brought to a halt by nails and small stones, such foreign ma
terials should be avoided in wood chips because they cause increased wear in the fuel conve
ying system.
Judging quality when buying in loose cubic metres
Criterion Comments
Effect on energy content
per loose cubic metre
Water content The lower the water content, the higher the heating value. In
addition, under a water content of 25% the wood shrinks.
So a cubic metre with 20% water
content, contains approx. 3% more wood than a cubic metre wi
th 30% water content. The water content is given as a
percentage of the total weight.
20%
30%
35%
40%
+6%
0%
-2,5%
-4%
Chip size The more finely the material is chopped, the more material fits
in a cubic metre.
P16S
P31S
+0%
-16%
Kind of wood Hardwood is denser and heavier and thus has a higher heat co
ntent per cubic metre.
Hornbeam, black locu
st
+53%
Beech +44%
Oak, ash +40%
Birch, maple +25%
Pine, larch +19%
Spruce, alder 0%
Fir, willow -6%
Poplar -19%
Bark content The lighter in colour the wood chips are, the lower their bark co
ntent.
Wood chips from small branches or wood chips with bark from
sawmills have a high bark content with high ash content and u
sually also higher dirt content.
This results in more effort spent on cleaning.
without bark +5%
10% bark 0%
30% bark -10%
Small branches Wood chips from small branches usually have a high degree of
decay.
Small branches -25%
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