Wood chips

Wood chips are small pieces of wood formed by the chipping of trees, branches, stumps and wood waste. Wood chips can form a much more uniform fuel that can flow and can be fed to a boiler, gasifier or other conversion system as a steady flow using an auger feed or a conveyor. With a large surface area to volume ratio they can also be burned very efficiently.

Wood chips for energy applications

Wood chips may have a bulk energy density of about 40-50% of that of the solid wood.

Wood chips for energy applications should meet an appropriate quality standard if they are to be used reliably in combustion equipment, especially small scale and domestic equipment. Physical parameters, such as maximum size and absence of slivers or fines (sawdust), and maximum moisture content are important to allow reliable operation and prevent feed blockages. Also levels of contaminants and ash content will have an impact on emissions and maintenance schedules.

Wood chips specifications

Standards for wood chip fuel are used to describe the particle size characteristics and the moisture content of the chips, but they can also describe variables such as ash content, limits for contamination, calorific value and bulk density.

Under this standard, particle size distribution is described with the ‘P’ prefix (P16b, P45 etc) and moisture content with the ‘M’ Prefix (M25, M30 etc).

Note that there is no difference in the standards for naturally or mechanically dried wood chip, it will just be easier for producers of mechanically dried chip to produce lower moisture chip and to ensure a consistent moisture content, as the finished product does not rely upon natural seasoning and the variability of weather and climate.

Particle Sizes

	Main Fraction (min 75% by weight)	Fines Fraction (% by weight smaller than 3.15 mm)	Coarse Fraction (% by weight)	Max particle length/ Max cross sectional area
P16A	3.15 mm – 16 mm	Less than 12%	Less than 3% over 16 mm	31.5 mm / 1.0 cm2
P16B	3.15 – 16 mm	<12	<3% over 45 mm	120 mm / 1.0 cm2
P45A	8 – 45 mm	<8	<6% over 63 mm	120 mm / 5.0 cm2
P45B	8 – 45 mm	<8	<6% over 63 mm	350 mm / 5.0 cm2
P63	8 – 63 mm	<6	<6% over 100 mm	350 mm / 10.0 cm2
P100	16 – 100 mm	<4	<6% over 200 mm	350 mm / 18.0 cm2

The sizes given relate to the sieve hole diameter.

Moisture Content

Moisture content – wet basis i.e.  mass of water as a percentage of the wet weight
M15	Less than or equal to10%
M20	<=20%
M25	<=25%
Then in 5% increments up to 55%

Ash

Ash content, weight as a percentage of the dry weight
A0.5	Less than or equal to 0.5%
A0.7	<=0.7
A1.0	<=1.0
A1.5	<=1.5
A2.0	<=2.0
A3.0	<=3.0
A5.0	<=5.0
A7.0	<=7.0
A10.0	<=10.0
A10.0+	Over 10%