ISO/TS 17595:2025

Technical
Specification
ISO/TS 17595
First edition
Solid biofuels — Characterization
2025-06
of wood chip fuels — Essential
information for producers,
suppliers and users
Biocombustibles solides — Caractérisation des combustibles à
base de plaquettes de bois — Informations essentielles pour les
producteurs, les fournisseurs et les utilisateurs
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Quality specifications for wood chips . 1
4.1 General .1
4.2 Classification of raw material .2
4.3 Normative and informative properties of graded wood chips .3
4.4 Classification of particle size classes and fines fractions.3
4.5 Specifications of graded wood chips in accordance with ISO 17225-4 .4
4.6 Typical ranges for key quality properties .5
4.7 Principles for assessing the quality of wood chip fuel .6
4.8 Procurement approaches .7
5 Standards for on-site characterisation of graded wood chips . 8
5.1 General .8
5.2 Sampling method .9
5.3 Sample preparation . 12
5.4 Bulk density .14
5.5 Moisture content . .17
5.6 Particle size distribution .18
5.7 Heavy extraneous materials .19
6 Additional standards for characterisation of graded wood chips by external laboratories .22
6.1 General . 22
6.2 Ash content . 22
6.3 Calorific value . 23
6.4 Elemental analysis: C, H, N, S, Cl and ash forming elements .24
Annex A (informative) Calculating the net calorific value based on an empirical formula .25
Annex B (informative) Model sampling plan and sampling report .26
Annex C (informative) Examples of Data Logging Tables to Record Test Results .27
Annex D (informative) Basic information included in an external laboratory test report .29
Bibliography .31

iii
Foreword
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in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
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This document was prepared by Technical Committee 238, Solid biofuels and pyrogenic biocarbon.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
For effective use of solid biofuels in heating and power facilities, it is crucial to assess fuel quality starting
from the planning stage through daily routine operation. It is equally critical to be familiar with the quality
specifications needed for a particular application and to determine key fuel properties using proven and
well-validated test methods.
Numerous International Standards exist to characterize various types of solid biofuels, including wood
chips, pellets, and briquettes for a variety of residential, commercial, and industrial applications. In addition
to providing detailed information on fuel specifications and classes and test methods, these International
Standards make it possible to draw up clear and unambiguous fuel supply contracts. They also support the
creation of quality assurance and certification systems.
Wood chips are among the most commonly used solid biofuels in space heating applications in commercial
and institutional buildings, district heating, light industry and greenhouses. Wood chips specifications and
test methods are described in a large number of International Standards, either as stand-alone, or as part of
International Standards addressing various types of solid biofuels.
This document is intended primarily for wood chip producers, traders, and owners and operators of energy
facilities in small to medium residential, commercial and public sectors. It is based on relevant International
Standards. The aim of this document is to provide practical guidance and examples on quality specifications
relevant to wood chips, presents guiding principles for assessing the quality of wood chip fuel and gives
common procurement approaches. Only those quality properties and their test methods that are critical
for both internal quality control purposes and smooth and efficient boiler operation are included in this
document.
Clause 4 describes general information about the quality of wood chips. Clause 5 provides guidance on test
methods for the determination of essential physical properties of wood chips. These tests can be performed
on a regular basis at the site of the wood chip fuel producer, fuel supplier or energy facility. Sampling and
sample preparation are also described in Clause 5. A calculation tool, covering the properties described in
1)
Clause 5, is available as an MS Excel document to assist users in recording, calculating and reporting test
results in a consistent manner. Clause 6 provides practical information on essential tests that are carried
out by external laboratories. Annex A gives an empirical formula to calculate energy content of wood chips.
Annex B gives an example of a sampling plan and sampling report. Annex C includes examples of data logging
tables that can be used in reporting test results. Annex D shows key information expected to be found in a
laboratory report of graded wood chips.
The sampling techniques and test methods described in this document are aligned with the methods given
in the corresponding International Standards, with minor modifications to some steps, such as sample size,
sampling frequency, number of replicates, or measurement time. This is done to make them more practical
for routine testing. As such, the methods described in this document can lead to minor differences in the
results when compared with the corresponding International Standards. These differences will not impact
the reliability of assessing changes to the properties of wood chips when measured on a relative, day-to-day
basis. Boiler operators, owners and fuel producers are encouraged to incorporate them into their regular
quality monitoring and control routines.
1) Accessible at: https://standards.iso.org/iso/ts/17595/ed-1/en

v
Technical Specification ISO/TS 17595:2025(en)
Solid biofuels — Characterization of wood chip fuels —
Essential information for producers, suppliers and users
1 Scope
This document provides guidance on the characterization of wood chips produced from raw materials, as
defined in ISO 17225-4, for the following aspects:
— quality classes and specifications;
— sampling, sample preparation and test methods for physical characteristics which can be conducted on site;
— practical information about testing to be carried out by external laboratories.
This document provides additional information about the type and frequency of testing at an energy plant
site, starting from the planning and start-up stages of a project and throughout its regular operation.
This document is applicable for assessing changes in properties on a relative basis when testing is done
routinely. This document is not applicable for demonstrating conformance with the referenced International
Standards.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
the requirements of this document. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 16559, Solid biofuels — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 16559 apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Quality specifications for wood chips
4.1 General
Successful operation of a wood chips based bioenergy facility strongly depends on ensuring a good match
between wood chips quality, fuel handling equipment and the conversion technology.
Before investing in a boiler or gasifier, it is essential to determine key characteristics of the fuel intended to
be used, such as its origin and sources, typical moisture content, particle size distribution, ash content and
bulk density. This information will help equipment suppliers make appropriate recommendations on the
type and design of boilers and of equipment for fuel handling and storage.
Table 1 summarizes the generalized relationship between wood chips quality, installation types and sizes.
The size ranges used for grouping the applications in Table 1 are meant for illustrative purposes only and in

practice some overlap between the applications and the scales is expected. Wood chips quality requirements
and tolerance to variance in fuel quality depend on the size and type of energy conversion technology and
the design specifications of the equipment as defined by Original Equipment Manufacturers. For example,
medium to large scale boilers and heaters can typically accept wood chips with higher moisture content and
chip size as well as higher variance in the wood chip fuel quality from minute to minute with almost no impact
on operation; whereas small-scale gasification combined heat and power (CHP) systems demand stricter fuel
quality requirements with low moisture content and narrow particle size distribution and cannot tolerate
almost any variance in the wood chip fuel quality from minute to minute without upsetting the process. It is
[2]
considered a best practice for each facility to implement a wood chip fuel quality management plan .
Similarly, the particle size distribution of wood chips has an impact on the selected type and sizing of feeding
systems. For example, fuel feeding systems such as augers are sensitive to oversized particles and need to be
sized properly.
Table 1 — Typical association between wood chips quality and applications
Installation size and type Range of Moisture Particle size (P) Production processes
content (M)
% in mass, as received
Boilers up to 100 kW 15 % – 35 % P16s, P16, P31s Sieving and drying are
typically needed
Maximum length of 120 mm
Boilers from 100 kW to 500 kW < 40 % P 31s, P45s Seasoning is pre-
ferred; sieving and
Maximum length of 120 mm
— firing system: underfed, depending on the conver-
drying are sometimes
or 200 mm, depending on the
grate etc. sion system
needed
feeding system
Gasification based CHP systems up P45s Drying and sieving
to 500 kW of fines and large
Sensitive to fines and long
particles are typically
particles
— updraft gasifier > 30 %
needed
< 15 %
— downdraft gasifier
Boilers from 500 kW to 1,5 MW < 45 % P45, P63
depending on the feeding
— firing system: underfed, depending on the con-
system
grate etc. version system
Boilers and CHP systems from 1,5 MW < 55 % P45, P63
to 5 MW depending on the con- depending on the feeding
version system system
— firing system: fluidized
bed, grate etc.
There are three fuel specification standards relevant to wood chips quality in the ISO 17225 series, namely
ISO 17225-1 which specifies general requirements, ISO 17225-4 on graded wood chips and ISO 17225-9 on
[3]
graded hog fuel and wood chips for industrial use. ISO 17225-1 describes the fuel quality classes for a broad
range of solid biofuels produced from raw and processed materials originating from forestry, agricultural
and aquaculture activities and forms the basis for subsequent parts in the ISO 17225 series. Quality
properties and classes for wood chips suitable for residential and commercial space heating applications are
described in ISO 17225-4. ISO 17225-9 defines the fuel quality classes and specifications of graded hog fuel
and wood chips for industrial use. Compared to ISO 17225-4, ISO 17225-9 encompasses a wider range of raw
materials and allows higher threshold values for various property classes.
4.2 Classification of raw material
The origin and source of raw materials for wood chips production, as per the classification in ISO 17225–4,
are summarized in Table 2. Those interested in the full list of all raw materials for solid biofuels and wood
chips can refer to Table 1 in ISO 17225-1.

Table 2 — Origin and source of raw materials for graded wood chips
(based on ISO 17225-4)
1.1.1 Whole trees without roots
1.1.3 Stemwood
1.1 Forest, plantation and other virgin
1.1.4 Logging residues
wood
1.1.7 Segregated wood from gardens, parks, roadside maintenance, vineyard,
fruit orchards and driftwood from freshwater
1.2 By-products and residues from 1.2.1 Chemically untreated wood by-products and residues
wood processing industry
1.2.2 Chemically treated by-products and residues, fibres and wood constituents
(excluding fibres and wood constituents)
1.3 Used wood 1.3.1 Chemically untreated used wood
Drying and/or screening of raw materials may be needed when producing wood chips from materials such
as logging residues, short rotation coppice and wood from gardens, parks, roadside maintenance, vineyards,
fruit orchards and driftwood from freshwater. These processes can reduce bark and fines contents and
remove heavy extraneous materials (such as stones, sand, solid etc) ensuring the intended quality class
according to ISO 17225-4 are achieved.
Bark and chemically treated used wood are not suitable as raw material for use in energy conversion systems
that are specifically designed for residential, commercial, and institutional applications.
4.3 Normative and informative properties of graded wood chips
For fuels classes defined in ISO 17225-4, the properties that are mandatory to determine are defined as
normative properties. Informative properties are those that are voluntary to determine, and they can be
a useful tool for effective communication between the seller and buyer. Table 3 describes normative and
informative parameters for wood chips according to ISO 17225-4. Even though the property of heavy
extraneous materials is neither normative nor informative, it is included in this document as it can be an
important quality parameter to take into consideration.
Table 3 — Normative and informative properties for graded wood chips
Properties Standard Graded wood chips
(ISO 17225-4)
Class A1/A2 Class B1/B2
Origin and source ISO 17225-4 normative normative
Particle size (P) ISO 17827-1 normative normative
Moisture content (M) ISO 18134-2 normative normative
Ash content (A) ISO 18122 normative normative
Content of nitrogen (N), sulfur (S) and chlorine (Cl) ISO 16948 --- normative
Minor elements (such as Arsenic (As), Cadmium (Cd), ISO 16968 --- normative
Chromium (Cr), Copper (Cu), Lead (Pb), Mercury (Hg),
Nickel (Ni) and Zinc (Zn))
Net calorific value (Q) ISO 18125 informative informative
Bulk density (BD) ISO 17828 informative informative
Heavy extraneous materials (EM) ISO19743 --- ---
4.4 Classification of particle size classes and fines fractions
Raw woody biomass can be comminuted to wood chips or hog fuel. Wood chips consist of sub-rectangular
shape pieces with a typical length of 5 to 50 mm and are produced by sharp tools such as knives. Graded
wood chips are suited for automatically fed energy conversion installations. Hog fuel, on the other hand,
is comprised of pieces with varying sizes and shapes as it is produced using blunt tools such as crushers/
[3]
grinders, which break the raw woody biomass. Hog fuel is much more fibrous and undefined in size;
therefore, it is better suited for large installations (> 5 MW) with an appropriate fuel handling system.

Particle size and fines classifications for wood chips according to ISO 17225-4 are shown in Tables 4 and 5,
respectively. A sample can only belong to one size class, which is always the lowest possible class based on
the main fraction. Ps-classes additionally limit fines content and maximum length of particles.
Wood chips belonging to one of the Ps-classes are intended to be used in residential and small-scale
commercial applications while the other P-classes are for larger scale applications.
Table 4 — Classification of particle size distribution of graded wood chips
(based on ISO 17225-4)
Coarse fraction,
Main fraction Fines fraction (F) Max. length of
% in mass
Particle size
(minimum 60 % in mass), (< 3,15 mm), particles (L),
class
(sieve aperture size or
mm % in mass mm
length of particle, mm)
P16s 3,15 mm ≤ m < 16 mm ≤ 6 % ≥ 31,5 mm ≤ 15 % 45 mm
P31s 3,15 mm ≤ m < 31,5 mm ≤ 6 % ≥ 45 mm ≤ 10 % 120 mm
P45s 3,15 mm ≤ m < 45 mm ≤ 10 % ≥ 63 mm ≤ 10 % 200 mm
P16 3,15 mm ≤ m < 16 mm ≤ 6 % ≥ 31,5 mm
values from
P31 3,15 mm ≤ m < 31,5 mm ≤ 6 % ≥ 45 mm
value
F-classes
to be reported
P45 3,15 mm ≤ m < 45 mm ≤ 10 % ≥ 63 mm
in Table 5
P63 3,15 mm ≤ m < 63 mm ≤ 10 % ≥ 100 mm
Table 5 — Classification of fines fraction for graded wood chips
(based on ISO 17225-4)
Fines fraction, F (< 3,15 mm, % in mass)
F02 ≤ 2 F20 ≤ 20
F05 ≤ 5 F25 ≤ 25
F10 ≤ 10 F30 ≤ 30
F15 ≤ 15 F30+ > 30 (maximum value to be reported)
4.5 Specifications of graded wood chips in accordance with ISO 17225-4
In the ISO 17225 series , graded means that solid biofuel used in a particular application (household,
commercial, public-sector buildings, or industrial) meets specified properties expressed by quality classes
like A1, A2, B1 or B2. Key specifications for graded wood chips, in accordance with
ISO 17225-4, are presented in Figure 1. Bulk density and calorific values, though important, are not captured
in Figure 1; their typical ranges can be found in Table 6.
Class A1 represents wood chips with lower ash content, indicating no or very little bark, and lower moisture
content. If the moisture value is below 10 % in mass, it needs to be reported explicitly. Some technologies,
such as small downdraft gasification-based CHP (up to 500 kW), often require Class A1 wood chips with
moisture level below 20 % in mass. Wood chips in Class A2 may have a higher moisture content. Threshold
values for N, S, Cl and minor elements are not required for Class A1 and A2 wood chips as these classes of
fuels are produced from virgin material and chemically untreated wood residues.
Class B1 extends the origin and source of class A to include other materials, such as short rotation coppice,
wood from gardens and plantation, and chemically untreated by-products and residues from the wood
processing industry. Class B2 further extends the raw material to include chemically treated wood from
the wood processing industry, by-products and residues from the aforementioned industry and chemically
untreated used wood. Class B2 does not allow chemically treated raw materials containing halogenated
organic compounds or heavy metals exceeding virgin wood levels.

Figure 1 — Key specifications of graded wood chips (based on ISO 17225-4)
4.6 Typical ranges for key quality properties
Typical values for a selected number of key properties of wood chips are shown in Table 6. The values
presented in Table 6 are for two sources of wood chips covering opposite ends of the spectrum, i.e. wood
with no or insignificant amounts of bark, needles and leaves vs logging residues. Additional values for a
broader range of solid biofuels can be found in ISO 17225-1:2021, Annex B.
Table 6 — Typical values of selected properties for various sources of wood chips
a
(partly based on ISO 17225-1:2021, Table B.1 and B.3)
Wood without or with insignificant Logging residues from
amounts of bark, needles/leaves from
Origin and source
Coniferous wood Broad-leaf wood Coniferous wood Broad-leaf wood
Parameter Unit, dry Mean Range Mean Range Mean Range Mean Range
basis
Ash content % in 0,3 0,1 to 1,0 0,3 0,2 to 1,0 3,0 < 1 to 10 5,0 2 to 10
mass
Gross calorific MJ/kg 20,5 20,0 to 20,8 20,1 19,4 to 20,4 20,5 19,5 to 21,5 19,7 19,5 to 20,0
value
Net calorific MJ/kg 19,1 18,5 to 19,8 18,9 18,4 to 19,2 19,2 18,5 to 20,5 18,7 18,3 to 18,5
value
Nitrogen (N) % in 0,1 < 0,1 to 0,5 0,1 < 0,1 to 0,5 0,5 0,3 to 0,8 0,5 0,3 to 0,8
mass
Sulfur (S) % in < 0,02 < 0,01 to 0,02 < 0,01 to < 0,02 < 0,02 to 0,04 0,01 to 0,08
mass 0,02 0,05 0,06
Chlorine (Cl) % in 0,01 < 0,01 to 0,01 < 0,01 to 0,01 < 0,01 to 0,01 < 0,01 to 0,04
mass 0,02 0,02 0,04
1 300 600 to 3 000 5 000 2 000 to 4 000 3 000 to
Calcium (Ca) mg/kg 900 500 to
1 000 8 000 5 000
a
Typical values for Ca and Cl were collected from European laboratories and provided by BEA (Austria).
Values marked by < are below determination limits.

TTabablele 6 6 ((ccoonnttiinnueuedd))
Wood without or with insignificant Logging residues from
amounts of bark, needles/leaves from
Origin and source
Coniferous wood Broad-leaf wood Coniferous wood Broad-leaf wood
Parameter Unit, dry Mean Range Mean Range Mean Range Mean Range
basis
Potassium (K) mg/kg 400 200 to 500 800 500 to 1 500 2 000 1 000 to 1 500 1 000 to
4 000 4 000
Magnesium mg/kg 150 100 to 200 200 100 to 400 800 400 to 250 100 to 400
(Mg) 2 000
Sodium (Na) mg/kg 20 10 to 50 50 10 to 200 200 75 to 300 100 20 to 200
a
Typical values for Ca and Cl were collected from European laboratories and provided by BEA (Austria).
Values marked by < are below determination limits.
The values in Table 6 do not take into account variations caused by the method of harvesting and/or storage.
For example, the ash content can be substantially higher for wood chips containing extraneous materials
such as soil, or the net calorific values can be lower for biologically degraded wood chips.
4.7 Principles for assessing the quality of wood chip fuel
The types and frequency of wood chips quality assessment vary depending on the stages of a project, i.e.,
planning, start-up and regular operation, as shown in Table 7.
At the planning stage, it is essential to define the origin and sources and key attributes of the wood chips
that will be used, including its typical range for moisture content, particle size distribution, ash content,
bulk density and heavy extraneous materials. These values, which must be determined by an external
laboratory, will form the baseline for later comparison, and will help equipment suppliers determine the
type and design of fuel handling, storage and boiler or gasification system.
At the start-up phase, it is not necessary to repeat the aforementioned analyses, except moisture content
determination, unless it is suspected on visual inspection that the wood chips quality differs from those
previously determined at the planning phase.
Table 7 — Typical frequency of analysis of graded wood chips for the end-user
Properties Frequency at
a b
Planning phase Start-up phase Regular operation
Moisture content 2-3 times Every lot Every lot
Bulk density Once If a change expected or Once a year
suspected
Particle size distribution Once If a change expected or Once a year
suspected
Ash content Once If a change expected or Once a year
suspected
Calorific value Once If a change expected or If a change is expected or
suspected suspected
Heavy extraneous materials Once If suspected If suspected
a
Fuel analysis during planning stage is typically performed by external laboratories
b
It is recommended to send samples to external laboratory to verify baseline and compare with daily results

TTabablele 7 7 ((ccoonnttiinnueuedd))
Properties Frequency at
a b
Planning phase Start-up phase Regular operation
N, S, Cl Once If a change expected or Once a year
suspected
Major elements (Ca, Na, K, Mg) Once If increased levels If a change is expected or
(not requested in ISO 17225-4) suspected suspected
Minor elements Once If increased levels If a change is expected or
(As, Cd, Cr, Cu, Hg, Pb, Ni, Zn) suspected suspected
(these are requested only for class B
wood chips in ISO 17225-4)
a
Fuel analysis during planning stage is typically performed by external laboratories
b
It is recommended to send samples to external laboratory to verify baseline and compare with daily results
If fuel suppliers or raw materials are changed, it is good practice for the owner or operator of the energy
facility to perform a full analysis of the normative parameters to form a new baseline.
It is reasonable and cost effective particularly for the owners and operators of systems smaller than 1 MW
to ask for a third-party declaration on fuel quality from their fuel supplier instead of them sending samples
for independent analysis.
During operations, it is good practice to do a visual inspection of each arriving load of fuel before unloading.
At a visual inspection, one can observe the presence of oversized material (such as branches), lumps of snow
or ice, and colour of the load (overly green in colour could mean large amount of leaves or needles). If any of
these or other items are not agreed upon as part of contract, consider unloading the load at an other location
for closer inspection, if possible together with the supplier. In case of a dispute between seller and buyer,
sampling should be done by an independent expert and analysis carried out by external laboratory(ies).
If the visual inspection confirms that wood chips meet the requirements, take representative samples and
determine moisture content of the load. It is not necessary to do other analyses on each load of fuel. If the
batch does not conform to requirements, it is strongly advisable to do additional analyses, such as size
distribution, bulk density, ash content, and if required heavy extraneous materials. If it is suspected that
the fuel is contaminated with paint, impregnation fluids, glues, or non-woody particles, a more complete
chemical analysis should be done.
Furthermore, in the event of operational problems, having appropriate analyses of fuel is beneficial to
accurately identify the cause of the problem.
If the moisture content value is to be used for a special purpose, e.g. for accurate control of the boiler
performance or efficiency testing, the sample for determining the moisture content should be taken at the
corresponding time.
4.8 Procurement approaches
For contracting purposes, the measurement, validation and verification of the key quality properties are also
important as they have an influence in determining price. There are three common procurement approaches
used in the establishment of supply contracts for wood chips: by energy content, by weight and by volume.
Types of analysis required by each approach are different, for details see Table 8. Moreover, each approach
produces varying degree of accuracy in determining the price of wood chips in relation to the energy.
a) Procurement on the basis of the energy content is the most reliable method for both supplier and buyer.
It requires a truck scale or weighbridge (either on-site or close-by) to determine the mass of the load
and the determination of moisture content. This method is most robust to variance in fuel quality,
especially moisture content or particle size distribution. The energy content given in Table 6 can be
used in calculations. In Annex A, a simple formula is given on how to calculate the net calorific value of a
[4]
cargo of wood chips based on its mass and moisture content .

b) Procurement on weight basis (as received) calls for a narrow variation in the specified range of fuel
moisture content and needs only a truck scale. The main parameter influencing the cost per unit is
moisture content.
c) Procurement on volume basis does not require a truck scale nor any analyses. As in the case of
procurement on weight basis, the fuel needs to have a moisture content within a narrow-specified
range. This option is inherently prone to large variances and therefore should be considered as the last
resource.
Parameters influencing the costs per unit are numerous and include moisture content, particle size and
particle size distribution, and the compaction of the wood chips during production and transportation
causing variability in bulk density. It is critical for both parties to agree on where and how to determine
the volume of the fuel. Using bulk density and moisture content values, the energy content of the fuel can be
estimated.
Table 8 — Procurement approaches for wood chips
Procurement based on
Energy content Weight basis Volume basis
Parameters to be determined Mass Mass Volume
Moisture content
Calorific value (can use liter-
ature value as an alternative
to testing, see also formula 5
in 6.3)
Preconditions for applicabili- ---- Moisture: specified (nar- Moisture: specified
ty of this method row) range (narrow) range
Necessary equipment at close Truck scale / weighbridge Truck scale /weighbridge ----
to the point of delivery
Drying oven
Cost per unit, as received Cost / MJ Cost / kg Cost / m loose vol-
ume
Accuracy Range High Medium Low
More information on different payment methods and other practical factors in a good delivery contract can
be found in Reference [4].
Readers are encouraged to check the Bibliography section of this document to learn more about related
[5-8]
processes and practices to wood chips production and storage .
5 Standards for on-site characterisation of graded wood chips
5.1 General
Quality of wood chips can vary depending on many parameters such as tree species, harvesting time and
method, handling, chipping and storage practices, all of these parameters have an impact on the boiler
operation. This clause describes methods for sampling, sample preparation, and test methods for essential
physical characteristics of wood chips, namely moisture content, bulk density, particle size distribution
[1]
and heavy extraneous materials. It also links to the MS Excel calculation tool, specifically developed
to accompany this clause and designed for generating results, from raw data, for moisture content, bulk
density, particle size distribution and heavy extraneous materials.
The test methods for the essential physical properties and sampling techniques described in Clause 5 are
aligned with the principles of the corresponding International Standards. Only the information relevant to
wood chips is provided with minor modifications to some of the steps in the procedures, such as sample size,
sampling frequency, number of replicates, or measurement time. As such, the methods can lead to minor
differences in the results compared to the corresponding International Standards. The differences will not
impact the reliability of assessing changes to the properties of wood chips when measured on a relative,

day-to-day basis. However, the methods and examples provided in this document should not be considered,
in whole or in part, as conforming with the International Standards referred to. In the case of a dispute, only
the corresponding ISO Standards are applicable, not the simplified methods described in this document.
5.2 Sampling method
The characteristics of wood chip fuel tend to vary within the same lot, e.g. a pile, or truck load, and from one
lot to another. It is therefore essential for producers and end-users to take representative samples from the
lot, as this forms the basis for determination of quality and for payment for wood chips of the whole lot.
The sampling method described in Table 9 is based on ISO 21945, which is applicable to a broad range of
solid biofuels to take samples in small scale applications and storages with a size of ≤ 100 t and includes
preparation of sampling plans and reports. The test method presented in Table 9 contains information
applicable to wood chips only.
Samples can be collected either from a stream of moving materials or from stationary material. It is
preferred to collect a sample from a stream of moving materials wherever possible. This can be done while
building-up or reclaiming of stockpiles, from falling streams, conveyor belts, or grab and wheel loader
buckets. If samples are collected during build-up of piles, increments can be taken from the working face of
the stockpile or from the bucket of a wheel loader or grab.
Sometimes, multiple increments need to be collected and aggregated to form one combined sample. When
collecting increments, sample in a uniform way (e.g. equal weight or volume). If a mass (volume) of an
increment or combined sample is too large to handle or is more than what is required for the subsequent
tests, the sample size can be reduced following the method described in 5.3.
It is recommended to keep the sampling equipment clean and dry, work quickly during sampling and pay
attention not to lose any material, moisture or introduce contamination.
Table 9 — Illustrative example of sampling method for graded wood chips from moving and
stationary materials
Apparatus Equipment for manual sampling. In general, the sampling device should be robust with opening wide
enough to capture oversized particles (see Table 4 and Figure 2).
Sampling bucket for falling stream, with a top opening (W) large enough so that the container cuts
the whole cross section of the stream. The sampling bucket should be large enough to ensure that the
container can hold the entire increment.
Scoop or shovel for a stationary pile, with a flat bottom, with edges raised high enough to prevent
particles from falling off.
a) Bucket b) Scoop c) Shovel
Figure 2 — Examples of sampling equipment (drawings are from ISO 21945:2020)
Procedure Step 1) Visual inspection.
To verify the classification of wood chips, selection of the sampling equipment and the sampling method.
NOTE 1 If particle size distribution will be determined, inspect the visible surface of the complete lot
for the longest particle and note its length in the sampling report.
NOTE 2 Taking photographs of wood chips in the delivery is a good practice; include a ruler or any other
scale into photographs to estimate the size of the particles.

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Step 2) Prepare a sampling plan.
An example of a sampling plan is given in Annex B. Once filled out, the sampling plan becomes a sam-
pling report.
Step 3) Choose sampling methods
— Sampling from moving material:
— Stockpiles during build-up or reclaiming;
— Falling streams;
— Conveyor belts;
— Grabs and wheel loader buckets.
— Sampling from stationary material:
— Transport containers and lorries;
— Small built-in storages;
— Stationary stockpiles.
Step 4) Determine the minimum number of increments per lot (taken from Clause 9 in ISO 21945:2020):
— Mass of sub-lot or lot, m m ≤ 30 t 30 t ˂ m ≤ 100 t
— Sampling from moving material 5 10
— Sampling from stationary material 10 15
Step 5) Determine the minimum size (volume) of increments
For small wood chips (P16s, P16, P31s and P31) the minimum size of increment is 2 l, and for all other
wood chip fuels the minimum increment size is 3 l.
Step 6) Determine the volume of combined sample.
Required combined sample volume will vary depending on the analysis to be performed.
Required minimum volume for single analysis
Moisture content Moisture content Moisture content (M), particle
(M) only (M) and particle size distribution (P), bulk den-
a
size (P) sity (BD)
Wood chips: P16s, P16, 2 l 10 l 67 l
P31s, P31, P45s
Other wood chips e.g. P45, 3 l 11 l 68 l
P63
a
Material used for the bulk density determination can be used for the particle size distribution
determination.
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Step 7) Take a sample
Manual sampling from falling Manual sampling is only suited for low mass flows. Choose a
streams bucket which is large enough to hold increments and fill it up to
the brim. Take samples either by:
— passing a sampling bucket through the stream of falling
material so it cuts the whole cross section of the falling stream;
or,
— taking increments from a variety of points representing
the whole cross section of the falling stream of material.
Manual sampling from conveyor Take samples with a scoop across the direction of the mass flow,
belts covering the whole width of the belt and through the full depth of
the material on the belt.
WARNING Stop the conveyor during this op
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