Properties
Origin
The quality of the wood chip that you produce will depend
firstly on the origin of the material - for example, woodfuel from
brash or arboricultural arising will contain more needles or leaves
and bark than small roundwood, which may affect the ash
content.
Size of chip
Many smaller wood chip boilers use a screw auger feed mechanisms
which requires a small (usually less than 5 cm), consistent
particle size to feed efficiently. Large-scale industrial
facilities will have more flexible feed mechanisms such as walking
floors.
Specification: Particle size
(P16/P45/P63/P100)
Moisture content
Wood chips, when produced from freshly felled trees, can be up
to 70% moisture. Larger boilers over 400kW output may be able to
burn wet woodchips. Smaller boilers need chips of a moisture
content of 30% or less.
Moisture content will vary depending on when material is felled,
how long it is left to dry before chipping, and whether additional
processing by forced air drying is used. It is essential that wood
chip is supplied at an appropriate moisture content for the
combustion equipment, and at a reasonably consistent content.
Specification: Moisture content
(M20/M30/M40/M55/M65)
Contamination
Wood is composed primarily of cellulose and lignin, along with
small quantities of other chemical constituents. There may also be
a proportion of other materials such as soil. Factors such as the
method of wood fuel harvesting and the care of handling of the
product will determine the proportion of contaminants present in
wood fuel. The ash produced from burning woodfuel in a modern
efficient systems is about 0.5 to 2.5% of the oven dry weight of
the wood fuel.
Earth, stones and other inert matter can seriously damage a wood
fuel system, particularly those with an automatic feed. So, keep
wood chip on a clean hard standing and ensure that equipment used
to move wood chip, such as front loaders, are clear of stones and
dirt.
Specification: Ash content (A0.7/A1.5/A3.0/A6.0/A10.0)
Chemically handled wood or used wood should also give: Nitrogen
(N0.5/N1.0/N3.0/N3.0+)
Other specifications
Informative specifications include:
- Net energy content (lower heating value) as MJ/kg or kWh/m3
loose
- Bulk density in kg/m3 loose
- Chlorine content (Cl0.03/Cl0.07/Cl0.10/Cl0.10+)
- Nitrogen (N0.5/N1.0/N3.0/N3.0+)
Many other properties may also be specified, including
concentrations of many other elements and volatile matter and ash
melting behaviour. Different specifications are required for
different fuels, and for pellets and briquettes these include
mechanical durability and particle density.
Sampling and testing
The CEN/TC 355 standards include detailed specifications for
sampling and testing biomass fuel.