PRODUCTION OF
WOOD CHIPS
Sources of wood chips
Wood fuel arises from multiple sources
including forests, forest plantations, other
wooded land and trees outside forests, byproducts from wood processing, demolition wood
and processed wood-based fuels. In forest the
major resources are:
firewood;
logging residues (branches, tops, stems)
from final fellings;
small-dimension trees from cleanings and
thinnings;
other wood not demanded by the industry;
stumps.
Usually logging residues
and stumps make up forest
fuels, called energy wood.
Most often forest fuels arise in clearcuts done
by harvester. Forest fuel as whole trees, as
roughly delimbed tree parts (energy wood) and in
combination with pulpwood can also be obtained
in thinning young, dense stands.
Small-scale use of stumps is tested by a
couple of companies, but there is still a need for
research and development to secure sustainable
forest management and reduce costs.
Production cycle
Sustainable forest management provides
different assortments of timber, as well as
resources for energy production. Forest energy
production comprises several steps: forest
management planning and sustainable forest
harvesting, fuel production, transporting and
heat production. Recently the importance of ash
recycling is especially increasing and today ash
is already used as fertilizer in forest.
Procurement systems
The production of wood chips can be divided
into several stages:
felling for chipping;
chipping;
off-road hauling;
storage of energy wood;
road transport.
The choice of logging residue procurement
system is largely determined by the customer’s
reception facilities and fuel requirements, as
well as from available equipment and services.
Five main systems with several alternatives for
logging residue procurement can be identified.
The two most common systems are chipping at
the landing and trucking of chips to the
customer and the deliveries of loose logging
residues.
Quality of wood fuel
It depends mainly on:
moisture content;
particle size distribution;
tree species;
bulk density;
amount of dust
and fungal spores in the fuel;
ash content.
Most important quality aspects can be taken
into account in the whole chain of custody from
forest to end user of heat or electricity:
supply (selection of raw material,
harvesting time, proportion of soiling
admixed);
processing (soiling, selection of
technologies);
storage & logistics (drying, ventilation,
transport system);
combustion (compliance to boiler
requirements, dimensions, thermal
efficiency).
The quality requirements for wood chips
depend on the size of the installation in which it
Production process
Wood chips production is complicated. It starts
from forest owner and forests management
specialist followed by the operator of harvester,
forwarder, chipper and truck driver who delivers
wood chips to a heating plant or other
customer. Heat or electricity are supplied to end
users.
During the whole process wood is converted
not only in shape, but also in measurement
units:
the boiler plants use MWh;
the hauling operators talk in terms of
tonnes;
wood merchants mainly use volumes,
such as solid or loose cubic meter of chip
wood;
forest owner and end users are talking
about the money they get or have to pay for
energy wood or wood energy.
Importance of drying
Freshly felled wood has a moisture content of
40-60%. Leaving logging residues in forest for
one summer can reduce the moisture content by
about 10 to 15%. An additional benefit is that
needles will drop off which reduces corrosion
risk in the boiler and retains nutrients in the
forest.
Moisture content of around 30% can be
achieved for wood stored as roundwood in a
covered pile at the roadside or in a yard.
Natural drying can reduce moisture content to
Wood chips
Wood chips are a medium-sized solid material
made by crushing or chipping, larger pieces of
stemwood and branches. There is a difference
between wood chips from fresh and dried logging
residues.
Forwarding fresh logging residues frees up the
Cutover at once, facilitates
planning, increases harvestand improves forwarding
productivity. Yet, it depletes nutrients from forest,
and increases the volumes
and costs of transportation.
Chipping of dried
logging residues
means longer time
from harvesting to
chipping, but these
chips have higher
quality and thermal
efficiency.
Use of wood chips
Traditional use of wood chips is as solid fuel
for heating buildings or in energy plants for
generating electric power from renewable
energy sources. In a number of cases coal power
plants have been converted to run on wood chips.
This is fairly straightforward to do, since they
both use an identical steam turbine heat engine,
and the costs of wood chips are comparable to
those of coal.
In Sweden the energy produced from logging
residues by district heating companies has
increased over the past years.