CEN ISO/TS 20049-2:2022

SLOVENSKI STANDARD
01-junij-2022
Trdna biogoriva - Določanje samosegrevanja peletiziranih biogoriv - 2. del:
Preskusi ogrevanja košare (ISO/TS 20049-2:2020)
Solid biofuels - Determination of self-heating of pelletized biofuels - Part 2: Basket
heating tests (ISO/TS 20049-2:2020)
Biogene Festbrennstoffe - Bestimmung der Selbsterhitzung von pelletierten biogenen
Brennstoffen - Teil 2: Warmlagerungsprüfungen im Drahtnetzkorb (ISO/TS 20049-
2:2020)
Biocombustibles solides - Détermination de l'auto-échauffement des granulés de
biocombustibles - Partie 2: Essais utilisant la méthode du point de croisement (ISO/TS
20049-2:2020)
Ta slovenski standard je istoveten z: CEN ISO/TS 20049-2:2022
ICS:
75.160.40 Biogoriva Biofuels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN ISO/TS 20049-2
TECHNICAL SPECIFICATION
SPÉCIFICATION TECHNIQUE
April 2022
TECHNISCHE SPEZIFIKATION
ICS 27.190; 75.160.40
English Version
Solid biofuels - Determination of self-heating of pelletized
biofuels - Part 2: Basket heating tests (ISO/TS 20049-
2:2020)
Biocombustibles solides - Détermination de l'auto- Biogene Festbrennstoffe - Bestimmung der
échauffement des granulés de biocombustibles - Partie Selbsterhitzung von pelletierten biogenen
2: Essais utilisant la méthode du point de croisement Brennstoffen - Teil 2: Warmlagerungsprüfungen im
(ISO/TS 20049-2:2020) Drahtnetzkorb (ISO/TS 20049-2:2020)
This Technical Specification (CEN/TS) was approved by CEN on 27 March 2022 for provisional application.

The period of validity of this CEN/TS is limited initially to three years. After two years the members of CEN will be requested to
submit their comments, particularly on the question whether the CEN/TS can be converted into a European Standard.

CEN members are required to announce the existence of this CEN/TS in the same way as for an EN and to make the CEN/TS
available promptly at national level in an appropriate form. It is permissible to keep conflicting national standards in force (in
parallel to the CEN/TS) until the final decision about the possible conversion of the CEN/TS into an EN is reached.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN ISO/TS 20049-2:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
The text of ISO/TS 20049-2:2020 has been prepared by Technical Committee ISO/TC 238 "Solid
biofuels” of the International Organization for Standardization (ISO) and has been taken over as
which is held by SIS.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to announce this Technical Specification: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO/TS 20049-2:2020 has been approved by CEN as CEN ISO/TS 20049-2:2022 without any
modification.
TECHNICAL ISO/TS
SPECIFICATION 20049-2
First edition
2020-12
Solid biofuels — Determination of
self-heating of pelletized biofuels —
Part 2:
Basket heating tests
Biocombustibles solides — Détermination de l'auto-échauffement des
granulés de biocombustibles —
Partie 2: Essais utilisant la méthode du point de croisement
Reference number
ISO/TS 20049-2:2020(E)
©
ISO 2020
ISO/TS 20049-2:2020(E)
© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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below or ISO’s member body in the country of the requester.
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Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved

ISO/TS 20049-2:2020(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols . 3
5 Basket heating tests . 3
6 Tests for product classification . 4
6.1 UN classification . 4
6.1.1 General. 4
6.1.2 Test method for self-heating substances — UN MTC Test N.4 . 4
6.1.3 Classification criteria — GHS . 5
6.2 Classification criteria — IMO . 5
6.3 Applicability of UN MTC Test N.4 for pelletized biofuels . 5
7 Tests for determination of reaction kinetics . 6
7.1 General . 6
7.2 Isoperibolic test methods . 6
7.2.1 General. 6
7.2.2 Test procedure . 7
7.2.3 Determination of reaction kinetics . 7
7.2.4 Applicability for pelletized biofuels . 7
7.3 Crossing-point method . 8
7.3.1 General. 8
7.3.2 Test procedure . 8
7.3.3 Determination of reaction kinetics . 9
7.3.4 Applicability for pelletized biofuels . 9
7.4 Adiabatic hot storage tests .10
7.4.1 General.10
7.4.2 Test procedure .10
7.4.3 Determination of reaction kinetics .11
7.4.4 Applicability for pelletized biofuels .12
8 Sample handling .12
8.1 General .12
8.2 Sampling .12
8.3 Sample transport and storage .13
8.4 Sample preparation .13
8.5 Sample disposal .13
9 Test report .13
Annex A (informative) Example of calculating kinetic parameters from crossing-point
method tests .15
Annex B (informative) Use of data for calculations of critical conditions in storages .18
Bibliography .23
ISO/TS 20049-2:2020(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO’s adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 238, Solid biofuels.
A list of all parts in the ISO 20049 series can be found on the ISO website.
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 © ISO 2020 – All rights reserved

ISO/TS 20049-2:2020(E)
Introduction
There is a continuous global growth in production, storage, handling, bulk transport and use of solid
biofuels especially in the form of pelletized biofuels.
The specific physical and chemical characteristics of solid biofuels, their handling and storage can lead
to a risk of fire and/or explosion, as well as health risks such as intoxication due to exposure to carbon-
monoxide, asphyxiation due to oxygen depletion or allergic reactions.
Heat can be generated in solid biofuel by exothermic biological, chemical and physical processes.
Biological processes include the metabolism of fungus and bacteria and occur at lower temperatures;
the oxidation of wood constituents increases with temperature and dominates at higher temperatures;
the heat production from biological and chemical processes leads to transport of moisture in the bulk
material, with associated sorption and condensation of water, which both are exothermic processes.
In, for example, a heap of stored forest fuel or a heap of moist wood chips, all of these processes can be
present and contribute to heat production.
[6]
Solid biofuels such as wood pellets, however, are intrinsically sterile due to the conditions during
manufacturing (exposure to severe heat during drying, fragmentation during hammermilling and
pressure during extrusion) but can attract microbes if becoming wet during handling and storage
resulting in metabolism and generation of heat. Leakage of water into a storage of wood pellets can
also lead to the physical processes mentioned above. Non-compressed wood like feedstock and chips
typically have a fauna of microbes which under certain circumstances will result in heating. All the
processes mentioned above contribute to what is called self-heating although oxidation is likely to be
one of the main contributing factors in the temperature range under which most biofuels are stored.
The heat build-up can be significant in large bulk stores as the heat conduction in the material is low.
Under certain conditions the heat generation can lead to thermal runaway and spontaneous ignition.
The potential for self-heating seems to vary considerably for different types of solid biofuel pellets. The
raw material used, and the properties of these raw materials have proven to influence the propensity
for self-heating of the produced wood pellets. However, the production process (e.g. the drying process)
also influences the potential for self-heating. It is therefore important to be able to identify solid biofuel
pellets with high heat generation potential to avoid fires in stored materials.
Two intrinsically different types of tests methods can be used to estimate the potential of self-heating:
a) in the isothermal calorimetry method described in ISO 20049-1, the heat flow generated from the
test portion is measured directly;
b) in the basket heating tests described in this document, the temperature of the test portion is being
monitored and the critical ambient temperature (CAT), where the temperature of the test portion
just does not increase significantly due to self-heating, is used for indirect assessment of self-
heating.
These two methods are applied at different analysis temperature regimes. The operating temperature
for an isothermal calorimeter is normally in the range 5 °C to 90 °C whereas basket heating tests are
conducted at higher analysis (oven) temperatures. For basket heating tests with wood pellets, the CAT
is found for a 1 l sample portion in the range 150 °C to 200 °C.
NOTE 1 The two types of test methods referred to above do not measure heat production from physical
processes such as transport of moisture.
NOTE 2 It is likely that oxidation reactions taking place in the low respective high temperature regimes for
solid biofuel pellets are of different character and thus have different reaction rates and heat production rates.
In such a case, extrapolation of the data from a high temperature test series can lead to non-conservative results
and might not be applicable without taking the low temperature reactions into account. In the general case of
two reactions with different activation energies, the high activation energy is “frozen out” at low temperatures
[7]
and the low activation energy reaction is “swamped” at higher temperatures .
ISO/TS 20049-2:2020(E)
NOTE 3 It has been shown for a limited number of different types of wood pellets that the reaction rates in the
lower temperature regime measured by isothermal calorimetry were higher compared to the reaction rate data
[8]
determined from basket heating tests in the higher temperature regime .
Basket heating tests have been used traditionally for characterization of the tendency for spontaneous
ignition of predominantly coals, but also for other reactive organic materials such as, for example,
[9]
cottonseed meal, bagasse and milk powder . The principle used in this type of tests is to find the CAT
for a self-heating sample material of specific size and geometry.
There are several different methods described in the literature with different degrees of sophistication.
The variations span from simple pass and fail tests to more advanced tests from which data on reaction
[10]
rates can be extracted .
Basket heating tests are useful for assessment of self-heating of solid biofuel pellets. The test method
selected can be evaluated for its applicability based on the information given in this document.
A compilation of available basket heating test methods is given in this document. Guidance on the
suitability for application of these methods for tests with pelletized biofuels is provided.
Basic theory of the use of basket heating test data for calculations of critical conditions in storages is
provided in Annex B.
vi © ISO 2020 – All rights reserved

TECHNICAL SPECIFICATION ISO/TS 20049-2:2020(E)
Solid biofuels — Determination of self-heating of pelletized
biofuels —
Part 2:
Basket heating tests
1 Scope
This document specifies basket heating tests for the characterization of self-heating properties of solid
biofuel pellets.
This document includes:
a) a compilation of basket heating test methods;
b) guidance on the applicability and use of basket heating tests for solid biofuel pellets;
c) information on the application of basket heating test data for calculations of critical conditions in
storages.
Data on spontaneous heat generation determined using this document is only associated with the
specific quality and age of the sample material.
The information derived using this document is for use in quality control and in hazard and risk
assessments related to the procedures given in ISO 20024.
The described methods can be used for other substances than solid biofuel pellets (e.g. wood chips).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes 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 14780, Solid biofuels — Sample preparation
ISO 16559, Solid biofuels — Terminology, definitions and descriptions
ISO 18135, Solid Biofuels — Sampling
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 16559 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
analysis temperature
temperature of the analysis environment, i.e. the oven temperature
ISO/TS 20049-2:2020(E)
3.2
Biot number
quotient of the convective heat transfer coefficient (between the sample boundary and the
surrounding air) and the conduction in the sample material normalized by the characteristic
dimension of the test basket
3.3
critical ambient temperature
CAT
ambient temperature [the analysis temperature (3.1) or the temperature of a storage] where the
internal temperature of the test portion (3.6) or the stored material increases significantly (due to self-
heating (3.4))
3.4
self-heating
rise in temperature in a material resulting from an exothermic reaction within the material
[SOURCE: ISO 13943:2017, 3.341, modified — “” has been deleted from the beginning of the
definition.]
3.5
spontaneous ignition
ignition caused by an internal exothermic reaction
Note 1 to entry: See the definitions of ignition in ISO 13943.
[SOURCE: ISO 13943:2017, 3.24, modified — “spontaneous ignition” has replaced auto-ignition” has the
preferred term and the other terms have been deleted. Notes 1 to 3 have been deleted and a new Note 1
to entry has been added.]
3.6
test portion
sub-sample either of a laboratory sample (3.8) or a test sample (3.7)
3.7
test sample
laboratory sample (3.8) after an appropriate preparation made by the laboratory
Note 1 to entry: In this document, the test sample is typically a representative sample from a batch of solid biofuel
pellets.
3.8
laboratory sample
combined sample or a sub-sample of a combined sample for use in a laboratory
[SOURCE: ISO 16559:2014, 4.124]
2 © ISO 2020 – All rights reserved

ISO/TS 20049-2:2020(E)
4 Symbols
Symbol Quantity Typical unit
−1
A pre-exponential factor in Arrhenius expression s
B dimensionless adiabatic temperature rise dimensionless
hL
Bi dimensionless
Biot number, (Bi= )
λ
c ambient oxygen concentration by volume fraction dimensionless
−1 −1
C specific heat capacity of the reaction products J kg K
−1 −1
C specific heat capacity of the bulk material J kg K
p
d diameter of body m
2 −1
D diffusion coefficient m s
−1
E activation energy J mol
a
−1
H gross calorific value J kg
−2 −1
h heat transfer coefficient W m K
−2 −1
h radiative amount on heat transfer coefficient W m K
r
−2 −1
h convective amount on heat transfer coefficient W m K
c
L characteristic length m
n order of reaction dimensionless
P constant, see Formulae (2) and (3) dimensionless
−3
heat generation term, see Formula (B.1) W m
′
q
−1
Q heat of reaction J kg
−3
Q heat of reaction by volume of oxygen J m
−1 −1
R universal gas constant J mol K
Ra Rayleigh number dimensionless
t time s
T temperature K
T ambient temperature K
T crossing point temperature K
p
x length coordinate m
δ Frank-Kamenetskii parameter, see Formula (B.4) dimensionless
δ critical value of δ dimensionless
c
RT
ε dimensionless
activation energy parameter, (ε = )
E
Ф oxygen diffusion parameter, see Formula (B.13) dimensionless
−1 −1
λ thermal conductivity of sample W m K
−1 −1
λ thermal conductivity of air W m K
air
−3
ρ bulk density kg m
−2 −4
σ Stefan-Boltzmann coefficient W m K
5 Basket heating tests
The detailed test procedure varies between different isoperibolic and adiabatic methods. Isoperibolic
methods include that the test portion is put in a wire-mesh basket, which is placed in an oven heated to
a fixed elevated temperature. The oven is equipped with a fan to keep the temperature uniform and to
[9][10]
give a relatively large convective heat transfer coefficient to the test specimen . For adiabatic tests,
[5]
the oven temperature is adjusted to the temperature at the centre of the sample .
ISO/TS 20049-2:2020(E)
Basket heating tests are based on the Frank-Kamenetskii theory of criticality of a self-heating isotropic
slab (see Annex B) and have been developed to determine the reaction kinetics of the global reaction
responsible for heat production in a self-heating material.
NOTE 1 The large gap volume of pelletized material can lead to convective heat transport in the bulk if the
furnace is equipped with a fan. In this case, it is recommended to keep the air flow in the vicinity of the sample at
a low level and to correct the critical Frank-Kamenetskii parameter (see B.1.3) or to prevent convective transport
within the sample by further measures (e.g. finer mesh wire of the basket).
NOTE 2 The CAT for the test portion in a basket heating tests is not equal to the CAT for spontaneous ignition in,
for example, large-scale storage. The critical size for spontaneous ignition (if only heat transfer is considered) is
directly related to the surface area-volume ratio of the self-heating specimen where heat is produced distributed
in the volume and heat is dissipated from the surface area only. The test sample in laboratory size basket heating
test has a very high surface area-volume ratio and has consequently a high CAT compared to a larger specimen.
6 Tests for product classification
6.1 UN classification
6.1.1 General
The United Nations (UN) Globally Harmonized System of Classification and Labelling of Chemicals
[11]
(GHS) is the international convention for hazard communication and labelling of gases, vapours,
solid and liquid substances, and mixtures. The GHS defines limit values, classes and categories and
related measures in relation to the level of hazards during transportation, handling and storage.
[12]
The UN Manual of Tests and Criteria (MTC) prescribes specific test procedures in support of the GHS.
6.1.2 Test method for self-heating substances — UN MTC Test N.4
[12]
Test N.4 is described in the UN MTC Part III, 33.3.1.6 , sometimes called the “basket test”.
This basket heating test determines the ability of a substance to undergo oxidative self-heating with
exposure of it to air at temperatures of 100 °C, 120 °C or 140 °C in a 25 mm or 100 mm wire mesh cube.
The Test N.4 basket heating test is not intended for determination of self-heating kinetics but rather
prescribed to classify a material (e.g. solid biofuels) as meeting the criteria for self-heating set out by
[11]
the GHS for hazard communication and labelling purposes.
The test set-up consists of a hot-air circulating oven, cubic sample containers of 25 mm and 100 mm
sides made of stainless-steel net with a mesh opening of 0,05 mm, and thermocouples of 0,3 mm
diameter for the measurement of the oven temperature and the temperature of the centre of the
sample. The sample container is housed in a cubic container cover made from stainless-steel net with
a mesh opening of 0,60 mm, and is slightly larger than the test container. To avoid the effect of air
circulation, this cover is installed in a second steel cage, made from a net with a mesh size of 0,595 mm
and 150 mm × 150 mm × 250 mm in size.
The normal procedure is to start with a test at 140 °C with a 100 mm cube sample. The container is
housed in the cover and hung at the centre of the oven. The oven temperature is raised to 140 °C and
kept there for 24 h. A positive result is obtained if spontaneous ignition occurs or if the temperature
of the sample exceeds the oven temperature by 60 °C. If a negative result is obtained, no further test is
necessary.
If a positive result is obtained at 140 °C with a 100 mm cube sample, the substance is classified as a self-
heating substance and further testing shall be made to find the correct classification (see 6.1.3).
NOTE The bulk density tested can influence the test results. prEN 15188 suggests adjusting the bulk density
of the sample to the respective practical conditions (if known) and recording the tested bulk density. The UN
MTC contains no information on the bulk density to be tested.
4 © ISO 2020 – All rights reserved

ISO/TS 20049-2:2020(E)
6.1.3 Classification criteria — GHS
[11]
The classification criteria are given in chapter 2.11.2 of the GHS . The criteria are summarized in
Table 1.
Table 1 — Criteria in the GHS for self-heating substances and mixtures
Category Criteria
1 A positive result is obtained in a test using 25 mm sample cube at 140 °C.
2 a) A positive result is obtained in a test using a 100 mm sample cube at 140 °C and a negative
result is obtained in a test using a 25 mm cube sample at 140 °C and the substance or mixture
is to be packed in packages with a volume of more than 3 m ; or
b) A positive result is obtained in a test using a 100 mm sample cube at 140 °C and a negative
result is obtained in a test using a 25 mm cube sample at 140 °C, a positive result is obtained
in a test using a 100 mm cube sample at 120 °C and the substance or mixture is to be packed
in packages with a volume of more than 450 litres; or
c) A positive result is obtained in a test using a 100 mm sample cube at 140 °C and a negative
result is obtained in a test using a 25 mm cube sample at 140 °C and a positive result is
obtained in a test using a 100 mm cube sample at 100 °C.
NOTE Hazard packing groups classification is prescribed depending on the flammability characteristics of
[11]
the material, see Table 32.1 of the GHS .
6.2 Classification criteria — IMO
Handling guidelines and hazard classifications for all cargoes, including solid biofuels, transported
onboard ocean vessels are specified by the International Maritime Organization (IMO) in the
[13]
International Maritime Solid Bulk Cargoes Code . The code stipulates UN MTC Test N.4 to be used for
testing but adds additional criteria for solid possessing hazards compared to the GHS criteria in Table 1,
as follows:
a) Does the material undergo dangerous self-heating when tested in accordance with Test N.4 in a
100 mm sample cube at 140 °C?
If yes, Class 4.2 applies. Materials in this class are materials, other than pyrophoric materials,
which, in contact with air without energy supply, are liable to self-heating.
b) Does the material show a temperature increase of 10 °C or more when tested in accordance with
Test N.4 in a 100 mm sample cube at 140 °C?
If yes, test in a 100 mm sample cube at 100 °C and check the temperature increase is 10 °C or more:
1) if yes, material hazardous in bulk (MHB) applies;
2) if no, neither Class 4.2 nor MHB applies.
NOTE Wood pellets containing no binder and additives are given the designation MHB (OH) as a result of
a high emission of carbon monoxide and not MHB (SH), since wood pellets are not classified as self-heating in
accordance with the criteria specified under the GHS and the MTC.
6.3 Applicability of UN MTC Test N.4 for pelletized biofuels
Experience from the testing of wood pellets indicates that the CAT for this type of material always is
higher than 140 °C in 1,0 l basket heating tests; see, for example, Reference [8]. The 140 °C criterion
seems thus not to be generally applicable for solid biofuel pellets.
ISO/TS 20049-2:2020(E)
The reasons that this test is unsuitable as a general test method for solid biofuel pellets are the
following:
a) the criteria in Test N.4 is based on fix reaction kinetics of coal, which is not directly transferable to
solid biofuel pellets;
b) experience shows that the UN criteria based on self-ignition of the analytical sample in a 100 mm
sample cube test at 140 °C is not valid for solid biofuel pellets since the CAT of 1000 cm wood
pellets is normally higher;
c) there is no published information on the selectivity and the correlation to large scale storage of this
tests for solid biofuel pellets;
d) the self-heating process of wood pellets can undergo multi-step reactions at different temperature
ranges. Low temperature reactions are not covered by tests in accordance with the Test N.4 method.
7 Tests for determination of reaction kinetics
7.1 General
There are different basket heating tests available for the determination of reaction kinetics for self-
heating of reactive materials. The most important of these methods are summarized in 7.2 to 7.4.
7.2 Isoperibolic test methods
7.2.1 General
The original basket heating test method was developed at the Fire Research Station in UK and is
sometimes referred to as the “FRS method”. This is a rather time-consuming method to use because of
the large number of experiments that is needed for each material studied. This method does not exist in
[14] [9]
the form of a test standard but has been described in detail by Bowes and Beever .
Several investigations and interlaboratory comparisons in the past have shown significant differences
[15][16]
between the results of hot storage tests determined by different laboratories . Laboratory-specific
differences have been identified as possible reasons for the deviations, for example:
a) oven ventilation (enforced, natural convection);
b) oven size;
c) sample baskets (shape, size, construction);
d) radiation effects;
e) measuring precision (temperature difference between tests with ignition and no ignition);
f) minimum sample size.
For that reason, the original FRS method was modified and further developed in the European standard
EN 15188. The main difference is the use of an additional mesh wire screen and special volumes of the
sample baskets (cubes) to normalize/harmonize the test conditions in the surrounding of the samples
independent from used oven type and size. This is, however, an important deviation from the Frank-
Kamenetskii theory (see Annex B), which relies on a high Biot number of the test specimen to keep
the boundary of the tests specimen at the analysis (oven) temperature. On the other hand, the air flow
velocity in the vicinity of the sample is reduced to prevent convective mass and heat transport in the
sample. For these reasons, the critical Frank-Kamenetskii parameter shall be corrected in accordance
with B.1.3 if this method is used.
6 © ISO 2020 – All rights reserved

ISO/TS 20049-2:2020(E)
7.2.2 Test procedure
The general test procedure is to conduct the tests using a pre-heated oven with the sample placed in
a wire-mesh container in the centre of the oven. These methods involve a number of separate, rather
time-consuming, heating tests with at least three to four different sizes of sample containers. Thin
thermocouples are used for measuring the temperature in the oven and the temperatures at the centre
and the periphery of the sample. The CAT for each size of sample is determined by repetitive tests at
oven temperatures successively closer to the critical temperature. In this way, the critical value of the
temperature can be bracketed in as closely as desired. It is usually found that ignition is very sharply
defined and a difference in oven temperature of only 0,5 °C will produce a sharp rise in the recorded
[9]
central temperature . The closeness with which the critical temperature is determined is reflected
in the precision of the calculation of the lumped kinetic parameters. A maximum error of ±0,5 K is
recommended by Reference [9] if data should be used for extrapolations over a wide range of sizes.
The recommendation in prEN 15188 is that the oven temperatures of the test just producing ignition
and that of the test not producing an ignition differ by not more than 2 K. prEN 15188 requires to test
four different volumes with a minimum sample size of 100 cm ; the largest sample volume shall not be
smaller than 1 000 cm .
7.2.3 Determination of reaction kinetics
The indirect evaluation of the Frank-Kamenetskii parameter (see also Annex B) is based on the
determination of the critical temperature for a known size of a material in small-scale oven tests as
described above.
The Frank-Kamenetskii parameter δ is defined by Formula (1):
E
−
ρQA EL
RT
δ =⋅ ⋅e (1)
λ
RT
With Formula (2), Formula (1) can be rewritten as Formula (3):
E QA
 
P=ln ρ (2)
 
R λ
 
δT 
E
ln =−P (3)
RT
L
 
 
If the critical value of δ is inserted, the ambient temperature is equal to the CAT. A plot of ln(δ T /L )
c 0
versus for a number of tests with varying sample sizes (L) would form a straight line with –E/R as
CAT
the slope and P as intercept. The critical Frank-Kamenetskii parameters (δ ) for the geometries tested
c
have to be calculated in accordance with the principles discussed in Annex B. Thus, E and QA could be
extracted from such measurements.
Once the material parameters are determined from the small-scale tests, it would be possible to predict
the critical size for any full-scale configuration, see Formula (B.5), or to calculate the Frank-Kamenetskii
parameter for any specific configuration and compare with the critical parameter to get an assessment
of the criticality of such a configuration.
7.2.4 Applicability for pelletized biofuels
Isoperibolic tests of different sample volumes in accordance with prEN 15188 should be the most
appropriate basket heating method for testing solid biofuel pellets if the critical Frank-Kamenetskii
parameter is corrected in accordance with B.1.3. This recommendation is based on the following
observations:
a) the CAT of each size of test portion is accurately determined;
ISO/TS 20049-2:2020(E)
b) the determination of the CAT is based on measurement of the centre temperature of the test portion,
which shall reach thermal runaway, which means that the exact position of the thermocouple in the
sample is less important (in comparison to the alternative crossing-point method);
c) convective heat and mass transport in the sample is reduced or prevented even in bulks with large
gap volumes;
d) the method in accordance with prEN 15188 provides reproducible results independent of the type
of furnace used.
7.3 Crossing-point method
7.3.1 General
An alternative method for determination of the kinetic parameters in a self-heating substance is the
[17]
method described by Chen and Chong , commonly referred to as the “crossing-point temperature
method”. This method involves the periphery heating of an initially “cold” exothermic material being
subjected to a hot environment with a constant temperature and is based on analysis of the non-steady
solution of the energy conservation formula.
Consider a symmetrical sample specimen of a reactive material where the heat wave propagates
towards the centre. Initially, the centre temperature is lower than the periphery temperature and a
temperature in the material a small distance from the centre. At a certain time, the centre temperature
exceeds (by self-heating) the temperature measured a small distance from the centre. At that point
where the centre temperature just exceeds the other temperatures in the sample specimen, the centre
temperature is defined as the “crossing-point temperature” (T ).
p
[17]
It has been shown that the observation of T can be used as a physic-chemical property to indicate
p
the propensity of a solid material to self-heat. If T is identified experimentally, and a temperature–
p
time profile is recorded to determine the time derivate of the temperature at T , the kinetic parameters
p
could be derived.
The main advantage of the crossing-point method is that instead of carrying out a series of time-
consuming experiments with several sample sizes, as in the isoperibolic methods, each of the transient
experiments with the crossing-point method where only one sample size is needed produces a data
point in a rather short time. In order to obtain
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