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Water content Information on fuel
Dry like hay on a hard surface
For your own use, spread out moist wood chips in a
layer 10 cm thick on an asphalt or concrete surface on
hot summer days. Good results can also be had on
sunny autumn days if you turn the chips over several
times. Two days are usually enough to attain a water
content below 30%, making the chips fit for storage
even under poor conditions.
Drying in a covered mesh enclosure
When building a new wood chip storage facility, roofed
storage containers with mesh walls should be
considered for air-drying of moist chips. A windy
location is important. Southward orientation can help
with drying even in winter. The height of the container
depends on the height of the front end loader needed
for filling. The lowest wall element must be removable
to allow removal of the wood chips. The depth can be
up to 2 m. Drying time is 4 to 8 months and a water
content below 20% is attainable.
Artificial ventilation
In spite of a few elaborate pilot projects with solar
energy, drying with heated air blown into the store via
channels in its floor has not proven itself very
economical thus far. The energy costs for drying often
exceed the achievable increase in heating value.
Dry wood before chopping, fell in winter and chop
in summer
It is much easier to dry the wood before chopping it.
With interim storage before chopping in summer, a
water content below 30% can be reached, ensuring
problem-free storage for the wood chips.
The decision whether to dry entire trees, or trunks and
branches separately depends largely on the accessi-
bility of the forest and the harvesting methods. Here
are a few tips for orientation:
• Well-ventilated piles, trunks with bark removed, or
entire trees dry faster and better. Sun helps, and
wind is indispensable for drying.
• Softwood should be felled no later than December
and stored at least 50 m from the forest due to the
risk of beetles.
• If the first regeneration felling is done for softwood
in September, the wood will no longer be infested
by beetles in the spring. It can be left in the forest
without delimbing and chopped as entire trees in
summer.
Leave green branches in the forest as nutrients
Leave green branches and treetops in the forest; as
fuel they are only "air and water". As valuable
nutrients, they should remain in the forest.
9.4 Water content
Using an oven to determine water content
A kilogram of wood chips is spread out on a baking
sheet and dried in an oven for 6 to 12 hours at 101 to
104°C. To be certain of a temperature over 100°C in a
typical electric oven in spite of the inexact thermostat,
you can set it to 110°C, but no higher as the wood will
begin outgassing even at slightly higher temperatures.
Fine and very moist pieces will need to be turned a few
times. The difference in weight between the moist test
material and the dry material corresponds to the water
content.
Longer interim storage of samples can falsify their
water content.
Take samples after transport
A pile of wood chips has 10 to 30% more water content
in the upper layers than in the middle. The material is
mixed by the loading and unloading process during
transport. By taking about a litre from 5 different places
at a depth of at least 20 cm (never from the surface) in
each load, you can get a good average with low error.
Taking the test quantity from the total sample
From several transport loads, you will get more than 1
kg of test material. To obtain a smaller quantity, mix the
material by rebuilding the pile, always emptying the
shovel over the top of the pile so that the material is
distributed over its entire surface. Then flatten the pile
and remove the two opposing quarters from it. Repeat
the mixing and removal process until you have two
batches of test material of 1 kg each, 1 kg for the buyer,
who usually determines the water content in the oven,
and 1 kg for the seller as a control sample. The volume
for one kilogram is around 3 litres for wet, heavy wood
chips and as much as 5 litres for very dry and light
material.
Water content and moisture
Water content has become the established
measurement for wood used for generating energy; in
the lumber business, the wood's moisture is usually
stated.
Water content (%) =
water in the wood (kg) x 100
total weight of wood (kg)
Moisture content (%)
=
water in the wood (kg) x 100
dry weight of the wood (kg)
Water content (%) =
moisture (%) x 100
100% + moisture (%)
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