SIST EN ISO 18125:2017

SLOVENSKI STANDARD
01-julij-2017
1DGRPHãþD
SIST EN 14918:2010
7UGQDELRJRULYD'RORþHYDQMHNDORULþQHYUHGQRVWL,62
Solid biofuels - Determination of calorific value (ISO 18125:2017)
Biogene Festbrennstoffe - Bestimmung des Heizwertes (ISO 18125:2017)
Biocombustibles solides - Détermination du pouvoir calorifique (ISO 18125:2017)
Ta slovenski standard je istoveten z: EN ISO 18125:2017
ICS:
75.160.40 Biogoriva Biofuels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 18125
EUROPEAN STANDARD
NORME EUROPÉENNE
May 2017
EUROPÄISCHE NORM
ICS 75.160.40; 27.190 Supersedes EN 14918:2009
English Version
Solid biofuels - Determination of calorific value (ISO
18125:2017)
Biocombustibles solides - Détermination du pouvoir Biogene Festbrennstoffe - Bestimmung des Heizwertes
calorifique (ISO 18125:2017) (ISO 18125:2017)
This European Standard was approved by CEN on 6 April 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, 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: Avenue Marnix 17, B-1000 Brussels
© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 18125:2017 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 18125:2017) has been prepared by Technical Committee ISO/TC 238 "Solid
biofuels" in collaboration with Technical Committee CEN/TC 335 “Solid biofuels” the secretariat of
which is held by SIS.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by November 2017, and conflicting national standards
shall be withdrawn at the latest by November 2017.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent
rights.
This document supersedes EN 14918:2009.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO 18125:2017 has been approved by CEN as EN ISO 18125:2017 without any modification.

INTERNATIONAL ISO
STANDARD 18125
First edition
2017-04
Solid biofuels — Determination of
calorific value
Biocombustibles solides — Détermination du pouvoir calorifique
Reference number
ISO 18125:2017(E)
©
ISO 2017
ISO 18125:2017(E)
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

ISO 18125:2017(E)
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
4.1 Gross calorific value . 2
4.2 Net calorific value . 3
5 Reagents . 3
6 Apparatus . 4
7 Preparation of test sample . 7
8 Calorimetric procedure . 8
8.1 General . 8
8.2 Preparing the bomb for measurement .10
8.2.1 General procedure .10
8.2.2 Using combustion aid .10
8.3 Assembling the calorimeter .11
8.4 Combustion reaction and temperature measurements .11
8.5 Analysis of products of combustion .12
8.6 Corrected temperature rise θ . 12
8.6.1 Observed temperature rise .12
8.6.2 Isoperibol and static-jacket calorimeters .12
8.6.3 Adiabatic calorimeters .14
8.6.4 Thermometer corrections .14
8.7 Reference temperature .14
9 Calibration .14
9.1 Principle .14
9.2 Calibrant .15
9.2.1 Certification conditions.15
9.2.2 Calibration conditions .15
9.3 Valid working range of the effective heat capacity ε.15
9.4 Ancillary contributions .16
9.5 Calibration procedure .16
9.6 Calculation of effective heat capacity for the individual experiment .17
9.6.1 Constant mass-of-calorimeter-water basis .17
9.6.2 Constant total-calorimeter-mass basis .17
9.7 Precision of the mean value of the effective heat capacity ε . 18
9.7.1 Constant value of ε . 18
9.7.2 ε as a function of the observed temperature rise .19
9.8 Redetermination of the effective heat capacity .19
10 Gross calorific value .19
10.1 General .19
10.2 Combustion .20
10.3 Calculation of gross calorific value .20
10.3.1 General.20
10.3.2 Constant mass-of-calorimeter-water basis .20
10.3.3 Constant total-calorimeter-mass basis .22
10.3.4 ε as a function of the observed temperature rise .23
10.4 Expression of results .23
10.5 Calculation to other bases .23
11 Performance characteristics .24
ISO 18125:2017(E)
11.1 Repeatability limit .24
11.2 Reproducibility limit .24
12 Calculation of net calorific value at constant pressure .24
12.1 General .24
12.2 Calculations .24
13 Test report .25
Annex A (normative) Adiabatic bomb calorimeters .26
Annex B (normative) Isoperibol and static-jacket bomb calorimeters.30
Annex C (normative) Automated bomb calorimeters .36
Annex D (informative) Checklists for the design and procedures of combustion experiments .39
Annex E (informative) Examples to illustrate the main calculations used in this document
when an automated bomb calorimeter is used for determinations .44
Annex F (informative) List of symbols used in this document .48
Annex G (informative) Default values of most used solid biofuels for the calculations of
calorific values .51
Annex H (informative) Flow chart for a routine calorific value determination .52
Bibliography .53
Index . .54
iv © ISO 2017 – All rights reserved

ISO 18125:2017(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.
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Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following URL:
www . i so .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 238, Solid biofuels.
INTERNATIONAL STANDARD ISO 18125:2017(E)
Solid biofuels — Determination of calorific value
1 Scope
This document specifies a method for the determination of the gross calorific value of a solid biofuel at
constant volume and at the reference temperature 25 °C in a bomb calorimeter calibrated by combustion
of certified benzoic acid.
The result obtained is the gross calorific value of the analysis sample at constant volume with all
the water of the combustion products as liquid water. In practice, biofuels are burned at constant
(atmospheric) pressure and the water is either not condensed (removed as vapour with the flue gases)
or condensed. Under both conditions, the operative heat of combustion to be used is the net calorific
value of the fuel at constant pressure. The net calorific value at constant volume may also be used;
formulae are given for calculating both values.
General principles and procedures for the calibrations and the biofuel experiments are presented in
the main text, whereas those pertaining to the use of a particular type of calorimetric instrument
are described in Annexes A to C. Annex D contains checklists for performing calibration and fuel
experiments using specified types of calorimeters. Annex E gives examples to illustrate some of the
calculations.
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 651, Solid-stem calorimeter thermometers
ISO 652, Enclosed-scale calorimeter thermometers
ISO 1770, Solid-stem general purpose thermometers
ISO 1771, Enclosed-scale general purpose thermometers
ISO 14780, Solid biofuels — Sample preparation
ISO 16559, Solid biofuels — Terminology, definitions and descriptions
ISO 18134-3, Solid biofuels — Determination of moisture content — Oven dry method — Part 3: Moisture
in general analysis sample
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:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
ISO 18125:2017(E)
3.1
gross calorific value at constant volume
absolute value of the specific energy of combustion, in joules, for unit mass of a solid biofuel burned in
oxygen in a calorimetric bomb under the conditions specified
Note 1 to entry: The products of combustion are assumed to consist of gaseous oxygen, nitrogen, carbon dioxide
and sulphur dioxide, of liquid water (in equilibrium with its vapour) saturated with carbon dioxide under the
conditions of the bomb reaction, and of solid ash, all at the reference temperature (3.4).
3.2
net calorific value at constant volume
absolute value of the specific energy of combustion, in joules, for unit mass of the biofuel burned
in oxygen under conditions of constant volume and such that all the water of the reaction products
remains as water vapour (in a hypothetical state at 0,1 MPa), the other products being as for the gross
calorific value, all at the reference temperature (3.4)
3.3
net calorific value at constant pressure
absolute value of the specific heat (enthalpy) of combustion, in joules, for unit mass of the biofuel
burned in oxygen at constant pressure under such conditions that all the water of the reaction products
remains as water vapour (at 0,1 MPa), the other products being as for the gross calorific value, all at the
reference temperature (3.4)
3.4
reference temperature
international reference temperature for thermochemistry of 25 °C is adopted as the reference
temperature for calorific values
Note 1 to entry: See 8.7.
Note 2 to entry: The temperature dependence of the calorific value of biofuels is small [less than 1 J/(g × K)].
3.5
effective heat capacity of the calorimeter
amount of energy required to cause unit change in temperature of the calorimeter
3.6
corrected temperature rise
change in calorimeter temperature caused solely by the processes taking place within the
combustion bomb
Note 1 to entry: The corrected temperature rise is the total observed temperature rise corrected for heat
exchange, stirring power, etc. (8.6).
Note 2 to entry: The change in temperature may be expressed in terms of other units: resistance of a platinum or
thermistor thermometer, frequency of a quartz crystal resonator, etc., provided that a functional relationship is
established between this quantity and a change in temperature. The effective heat capacity of the calorimeter (3.5)
may be expressed in units of energy per such an arbitrary unit. Criteria for the required linearity and closeness
in conditions between calibrations and fuel experiments are given in 9.3.
Note 3 to entry: A list of the symbols used and their definitions is given in Annex F.
4 Principle
4.1 Gross calorific value
A weighed portion of the analysis sample of the solid biofuel is burned in high-pressure oxygen in a bomb
calorimeter under specified conditions. The effective heat capacity of the calorimeter is determined in
calibration experiments by combustion of certified benzoic acid under similar conditions, accounted
for in the certificate. The corrected temperature rise is established from observations of temperature
2 © ISO 2017 – All rights reserved

ISO 18125:2017(E)
before, during and after the combustion reaction takes place. The duration and frequency of the
temperature observations depend on the type of calorimeter used. Water is added to the bomb initially
to give a saturated vapour phase prior to combustion (see 8.2.1 and 9.2.2), thereby allowing all the
water formed, from the hydrogen and moisture in the sample, to be regarded as liquid water.
The gross calorific value is calculated from the corrected temperature rise and the effective heat
capacity of the calorimeter, with allowances made for contributions from ignition energy, combustion of
the fuse(s) and for thermal effects from side reactions such as the formation of nitric acid. Furthermore,
a correction is applied to account for the difference in energy between the aqueous sulphuric acid
formed in the bomb reaction and gaseous sulphur dioxide, i.e. the required reaction product of sulphur
in the biofuel. The corresponding energy effect between aqueous and gaseous hydrochloric acid can be
neglected due to the usually low value for the correction regarding solid biofuels.
4.2 Net calorific value
The net calorific value at constant volume and the net calorific value at constant pressure of the
biofuel are obtained by calculation from the gross calorific value at constant volume determined on
the analysis sample. The calculation of the net calorific value at constant volume requires information
about the moisture and hydrogen contents of the analysis sample. In principle, the calculation of the net
calorific value at constant pressure also requires information about the oxygen and nitrogen contents
of the analysis sample.
5 Reagents
5.1 Oxygen, at a pressure high enough to fill the bomb to 3 MPa, pure with an assay of at least a volume
fraction of 99,5 %, and free from combustible matter.
Oxygen made by the electrolytic process may contain up to a volume fraction of 4 % of hydrogen.
5.2 Fuse.
5.2.1 Ignition wire, of nickel-chromium 0,16 mm to 0,20 mm in diameter, platinum 0,05 mm to
0,10 mm in diameter, or another suitable conducting wire with well-characterized thermal behaviour
during combustion.
5.2.2 Cotton fuse, of white cellulose cotton, or equivalent, if required (see 8.2.1).
5.3 Combustion aids of known gross calorific value, composition and purity, like benzoic acid,
n-dodecane, paraffin oil, combustion bags or capsules may be used.
5.4 Standard volumetric solutions and indicators, only for use when analysis of final bomb solutions
is required.
5.4.1 Barium hydroxide solution, c[Ba(OH) ] = 0,05 mol/l.
5.4.2 Sodium carbonate solution, c(Na C0 ) = 0,05 mol/l.
2 3
5.4.3 Sodium hydroxide solution, c(NaOH) = 0,1 mol/l.
5.4.4 Hydrochloric acid solution, c(HCI) = 0,1 mol/l.
5.4.5 Screened methyl orange indicator, 1 g/l solution.
Dissolve 0,25 g of methyl orange and 0,15 g of xylene cyanol FF in 50 ml of a volume fraction of 95 %
ethanol and dilute to 250 ml with water.
ISO 18125:2017(E)
5.4.6 Phenolphthalein, 10 g/l solution.
Dissolve 2,5 g of phenolphthalein in 250 ml of a volume fraction of 95 % ethanol.
5.5 Benzoic acid, of calorimetric-standard quality, certified by (or with certification unambiguously
traceable to) a recognized standardizing authority.
Benzoic acid is the sole substance recommended for calibration of an oxygen-bomb calorimeter. For
the purpose of checking the overall reliability of the calorimetric measurements, test substances, e.g.
n-dodecane, are used. Test substances are mainly used to prove that certain characteristics of a sample,
e.g. burning rate or chemical composition, do not introduce bias in the results. A test substance shall
have a certified purity and a well-established energy of combustion.
The benzoic acid is burned in the form of pellets. It is normally used without drying or any treatment
other than pelletizing; consult the sample certificate. It does not absorb moisture from the atmosphere
at relative humidities below 90 %.
The benzoic acid shall be used as close to certification conditions as is feasible; significant departures
from these conditions shall be accounted for in accordance with the directions in the certificate. The
energy of combustion of the benzoic acid, as defined by the certificate for the conditions utilized, shall
be adopted in calculating the effective heat capacity of the calorimeter (see 9.2).
6 Apparatus
6.1 General
The calorimeter (see Figure 1) consists of the assembled combustion bomb, the calorimeter can
(with or without a lid), the calorimeter stirrer, water, temperature sensor, and leads with connectors
inside the calorimeter can required for ignition of the sample or as part of temperature measurement
or control circuits. During measurements, the calorimeter is enclosed in a thermostat. The manner in
which the thermostat temperature is controlled defines the working principle of the instrument and
hence the strategy for evaluation of the corrected temperature rise.
In aneroid systems (systems without a fluid), the calorimeter can, stirrer and water are replaced by a
metal block. The combustion bomb itself constitutes the calorimeter in some aneroid systems.
In combustion calorimetric instruments with a high degree of automation, especially in the evaluation
of the results, the calorimeter is in a few cases not as well-defined as the traditional, classical-type
calorimeter. Using such an automated calorimeter is, however, within the scope of this document as
long as the basic requirements are met with respect to calibration conditions, comparability between
calibration and fuel experiments, ratio of sample mass to bomb volume, oxygen pressure, bomb liquid,
reference temperature of the measurements and repeatability of the results. A print-out of some
specified parameters from the individual measurements is essential. Details are given in Annex C.
As the room conditions (temperature fluctuation, ventilation, etc.) may have an influence on the
precision of the determination, the manufacturer’s instructions for the placing of the instrument shall
always be followed.
Equipment, adequate for determinations of calorific value in accordance with this document, is
specified in 6.2 to 6.8.
6.2 Calorimeter with thermostat
6.2.1 Combustion bomb, capable of withstanding safely the pressures developed during combustion.
The design shall permit complete recovery of all liquid products. The material of construction shall resist
corrosion by the acids produced in the combustion of biofuels. A suitable internal volume of the bomb
would be from 250 ml to 350 ml.
4 © ISO 2017 – All rights reserved

ISO 18125:2017(E)
WARNING — Bomb parts shall be inspected regularly for wear and corrosion; particular
attention shall be paid to the condition of the threads of the main closure. Manufacturers’
instructions and any local regulations regarding the safe handling and use of the bomb shall
be observed. When more than one bomb of the same design is used, it is imperative to use each
bomb as a complete unit. Swapping of parts may lead to a serious accident.
Key
1 stirrer 4 thermometer
2 thermostat lid 5 calorimeter can
3 ignition leads 6 thermostat
Figure 1 — Classical-type bomb combustion calorimeter with thermostat
6.2.2 Calorimeter can, made of metal, highly polished on the outside and capable of holding an amount
of water sufficient to completely cover the flat upper surface of the bomb while the water is being stirred.
A lid generally helps reduce evaporation of calorimeter water, but unless it is in good thermal contact
with the can, it lags behind in temperature during combustion, giving rise to undefined heat exchange
with the thermostat and a prolonged main period.
6.2.3 Stirrer, working at constant speed. The stirrer shaft should have a low-heat conduction and/or
a low-mass section below the cover of the surrounding thermostat to minimize transmission of heat to
or from the system; this is of particular importance when the stirrer shaft is in direct contact with the
stirrer motor. When a lid is used for the calorimeter can, this section of the shaft should be above the lid.
The rate of stirring for a stirred-water type calorimeter should be large enough to make sure that hot
spots do not develop during the rapid part of the change in temperature of the calorimeter. A rate of
ISO 18125:2017(E)
stirring such that the length of the main period can be limited to 10 min or less is usually adequate (see
Annexes A and B).
6.2.4 Thermostat (water jacket), completely surrounding the calorimeter, with an air gap of
approximately 10 mm separating calorimeter and thermostat.
The mass of water of a thermostat intended for isothermal operation shall be sufficiently large to
outbalance thermal disturbances from the outside. The temperature should be controlled to within
±0,1 K or better throughout the experiment. A passive constant temperature (“static”) thermostat
shall have a heat capacity large enough to restrict the change in temperature of its water. Criteria for
satisfactory behaviour of this type of water jacket are given in Annex B.
NOTE 1 For an insulated metal static jacket, satisfactory properties are usually ensured by making a wide
annular jacket with a capacity for water of at least 12,5 l.
NOTE 2 Calorimeters surrounded by insulating material, creating a thermal barrier, are regarded as static-
jacket calorimeters.
When the thermostat (water jacket) is required to follow closely the temperature of the calorimeter,
it should be of low mass and preferably have immersion heaters. Energy shall be supplied at a rate
sufficient to maintain the temperature of the water in the thermostat to within 0,1 K of that of the
calorimeter water after the charge has been fired. When in a steady state at 25 °C, the calculated mean
drift in temperature of the calorimeter shall not exceed 0,000 5 K/min (see A.3.2).
6.2.5 Temperature measuring instrument, capable of indicating temperature with a resolution of at
least 0,001 K so that temperature intervals of 2 K to 3 K can be determined with a resolution of 0,002 K
or better. The absolute temperature shall be known to the nearest 0,1 K at the reference temperature of
the calorimetric measurements. The temperature measuring device should be linear, or linearized, in its
response to changes in temperature over the interval it is used.
As alternatives to the traditional mercury-in-glass thermometers, suitable temperature sensors are
platinum resistance thermometers, thermistors, quartz crystal resonators, etc. which together with a
suitable resistance bridge, null detector, frequency counter or other electronic equipment provide the
required resolution. The short-term repeatability of this type of device shall be 0,001 K or better. Long-
term drift shall not exceed the equivalent of 0,05 K for a period of 6 months. For sensors with linear
response (in terms of temperature), drift is less likely to cause bias in the calorimetric measurements
than are nonlinear sensors.
Mercury-in-glass thermometers shall conform to ISO 651, ISO 652, ISO 1770 or ISO 1771. A viewer with
magnification about 5× is needed for reading the temperature with the resolution required.
A mechanical vibrator to tap the thermometer is suitable for preventing the mercury column from
sticking (see 8.4). If this is not available, the thermometer shall be tapped manually before reading the
temperature.
6.2.6 Ignition circuit
The electrical supply shall be 6 V to 12 V alternating current from a step-down transformer or direct
current from batteries. It is desirable to include a pilot light in the circuit to indicate when current is
flowing.
Where the firing is done manually, the firing switch shall be of the spring-loaded, normally open type,
located in such a manner that any undue risk to the operator is avoided (see warning in 8.4).
6.3 Crucible, of silica, nickel-chromium, platinum or similar unreactive material.
The crucible should be 15 mm to 25 mm in diameter, flat based and about 20 mm deep. Silica crucibles
should be about 1,5 mm thick and metal crucibles about 0,5 mm thick.
6 © ISO 2017 – All rights reserved

ISO 18125:2017(E)
If smears of unburned carbon occur, a small low-mass platinum or nickel-chromium crucible, for example
0,25 mm thick, 15 mm in diameter and 7 mm deep, may be used.
6.4 Ancillary pressure equipment
6.4.1 Pressure regulator, to control the filling of the bomb with oxygen.
6.4.2 Pressure gauge (e.g. 0 MPa to 5 MPa), to indicate the pressure in the bomb with a resolution of
0,05 MPa.
6.4.3 Relief valve or bursting disk, operating at 3,5 MPa, and installed in the filling line, to prevent
overfilling the bomb.
CAUTION — Equipment for high-pressure oxygen shall be kept free from oil and grease (high
vacuum grease recommended by the manufacturer can be used according to the operating
manual of the instrument). Do not test or calibrate the pressure gauge with hydrocarbon fluid.
6.5 Timer, indicating minutes and seconds.
6.6 Balances
6.6.1 Balance for weighing the sample, fuse, etc., with a resolution of at least 0,1 mg; 0,01 mg is
preferable and is recommended when the sample mass is of the order of 0,5 g or less (see 8.2.1).
6.6.2 Balance for weighing the calorimeter water, with a resolution of 0,5 g (unless water can be
dispensed into the calorimeter by volume with the required accuracy; see 8.3).
6.7 Thermostat (optional), for equilibrating the calorimeter water before each experiment to a
predetermined initial temperature, within about ±0,3 K.
6.8 Pellet press, capable of applying a force of about 10 t, either hydraulically or mechanically, and
having a die suitable to press a pellet having a diameter about 13 mm and a mass of (1,0 ± 0,2) g.
7 Preparation of test sample
Samples for the determination of calorific value shall be sampled in accordance with ISO 18135 and
shall be received in the laboratory in sealed air-tight containers or packages. The biofuel sample used
for the determination of calorific value shall be the general analysis sample (ground to pass a test sieve
with an aperture of 1,0 mm) prepared according to the procedure given in ISO 14780. Sieve with an
aperture less than 1,0 mm (0,5 mm or 0,25 mm) might be necessary for some solid biofuels to ensure
the requisite repeatability and a complete combustion.
Due to the low density of solid biofuels, they shall be tested in a pellet form. As a test portion, a pellet
of mass (1,0 ± 0,2) g is pressed with a suitable force to produce a compact, unbreakable test piece.
Alternatively, the test may be carried out in powder form test portion closed in a combustion bag or
capsule.
The general analysis sample shall be well mixed and in reasonable moisture equilibrium with the
laboratory atmosphere. The moisture content shall either be determined simultaneously with the
weighing of the samples for the determination of calorific value, or the sample shall be kept in a small,
effectively closed container until moisture analyses are performed, to allow appropriate corrections
for moisture in the analysis sample.
Determination of the moisture content (M ) of the general analysis sample shall be carried out by the
ad
method specified in ISO 18134-3.
ISO 18125:2017(E)
A flow chart for a routine calorific value determination can be found in Annex H.
8 Calorimetric procedure
8.1 General
The calorimetric determination consists of two separate experiments, combustion of the calibrant
(benzoic acid) and combustion of the biofuel, both under same specified conditions. The calorimetric
procedure for the two types of experiment is essentially the same. In fact, the overall similarity is a
requirement for proper cancellation of systematic errors caused, for example, by uncontrolled heat
leaks not accounted for in the evaluation of the corrected temperature rise θ .
The experiment consists of carrying out quantitatively a combustion reaction (in high-pressure oxygen
in the bomb) to defined products of combustion and of measuring the change in temperature caused by
the total bomb process.
The temperature measurements required for the evaluation of the corrected temperature rise θ are
made during a fore period, a main (= reaction) period, and an after period as outlined in Figure 2. For
the adiabatic type calorimeter, the fore and after periods need, in principle, be only as long as required
to establish the initial (firing) and final temperatures, respectively (see Annex A). For the isoperibol
(isothermal jacket) and the static-jacket type calorimeters, the fore and after periods serve to establish
the heat exchange properties of the calorimeter required to allow proper correction for heat exchange
between calorimeter and thermostat during the main period when combustion takes place. The fore
and after periods then have to be longer; see Annex B.
The power of stirring shall be maintained constant throughout an experiment which calls for a constant
rate of stirring. An excessive rate of stirring results in an undesirable increase in the power of stirring
with ensuing difficulties in keeping it constant. A wobbling stirrer is likely to cause significant short-
term variations in stirring power.
8 © ISO 2017 – All rights reserved

ISO 18125:2017(E)
Key
X time 1 fore period
Y temperature 2 main period
t temperature at the end of main period 3 after period
f
t ignition temperature 4 ignition
i
t jacket temperature
j
Figure 2 — Time-temperature curve (isoperibol calorimeter)
During combustion, the bomb head will become appreciably hotter than other parts of the bomb, and
it is important to have enough well-stirred water above it to maintain reasonably small temperature
gradients in the calorimeter water during the rapid part of the rise in temperature. For aneroid systems,
the particular design determines to what extent hot spots may develop (see Annex C).
Certain biofuels may persistently burn incompletely, “exploding” and/or leaving residues that contain
significant
...