Solid biofuels — Bridging behaviour of bulk biofuels

This document summarizes current knowledge concerning a test method and its technical implementation, and existing knowledge about the bridging performance of biofuels. The document consists of three parts, as follows: — Part I: Method for direct determination of bridging behaviour, to make it available for research and development purposes (see Clause 4). — Part II: Implementing the measurement principle, to assist in the construction of test apparatus and to illustrate the performance of a bridging test (see Clause 5). — Part III: Experience and results from bridging tests, to provide typical results on bridging for a wide range of biofuels already tested (see Clause 6).

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Publication Date
27-Aug-2020
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6060 - International Standard published
Start Date
28-Aug-2020
Completion Date
28-Aug-2020
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TECHNICAL ISO/TR
REPORT 23437
First edition
2020-08
Solid biofuels — Bridging behaviour of
bulk biofuels
Reference number
ISO/TR 23437:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TR 23437:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 23437:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Part I: Proposed method for direct determination of bridging behaviour ..............................................3

4.1 Introduction to the method .......................................................................................................................................................... 3

4.2 Principle ........................................................................................................................................................................................................ 3

4.3 Test equipment ....................................................................................................................................................................................... 4

4.4 Sampling and sample preparation ......................................................................................................................................... 5

4.5 Procedure .................................................................................................................................................................................................... 5

4.6 Calculation .................................................................................................................................................................................................. 6

4.7 Precision and bias ................................................................................................................................................................................ 6

4.8 Test reporting ........................................................................................................................................................................................... 7

5 Part II: Implementing the measuring principle ................................................................................................................... 7

5.1 Review of apparatus construction ......................................................................................................................................... 7

5.2 Other equipment ................................................................................................................................................................................14

5.3 Measurement performance .......................................................................................................................................................15

6 Part III: Experience and results from bridging tests ....................................................................................................17

6.1 General ........................................................................................................................................................................................................17

6.2 Performance characteristics of bridging test method ........................................................................................18

6.2.1 General...................................................................................................................................................................................18

6.2.2 Sensitivity analysis on testing accuracy ....................................................................................................18

6.2.3 Reproducibility (interlaboratory test results) ....................................................................................18

6.2.4 Repeatability ....................................................................................................................................................................19

6.3 Characterization of selected biomass fuels .................................................................................................................19

6.4 Influencing factors on bridging..............................................................................................................................................20

6.5 Outlook .......................................................................................................................................................................................................23

Bibliography .............................................................................................................................................................................................................................24

© ISO 2020 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TR 23437:2020(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.

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
---------------------- Page: 4 ----------------------
ISO/TR 23437:2020(E)
Introduction

In all particulate matter that is flowing through an opening, the particles have the tendency to form a

solid bridge over that opening. This can cause interruptions or failures, particularly during a vertical

transport, with the consequence of clogging of silo outlets, hoppers, down pipes, funnels or screw

conveyors. To understand this phenomenon better, a determination test method was developed. The

results of these tests can be used to improve the design of handling systems in order to minimize the

risk of bridging.

Bridging is a phenomenon that can occur because of the inhomogeneous nature of the biofuel,

particularly the variation in particle size, moisture content and number of overlong particles. In

addition, biofuels are often not well understood by the designers of handling, storage and conversion

systems. Bridging phenomenon can lead to an alternating build-up and collapse of bridges or shafts,

often called ratholes (see also Figure 1).

Comprehensive studies referring to the bridging behaviour of solid biomass fuels were first performed

[1] [3]

by Mattsson and by Mattsson and Kofman in the early 1990s. They considered the basic handling

characteristics of solid biofuels, i.e. the angle of repose, the friction of solid biofuels against surfaces

and the tendency to build bridges over an opening. As these parameters had until then never been

investigated with solid biomass fuels, new measuring principles and devices had to be developed. For

determining the bridge building tendency, a test apparatus was constructed consisting of a movable

floor which could be gradually opened so that a bridge of fuel could form over the opening until it finally

[1]

collapsed . Various fuels were tested and the impact of key parameters such as bed depth, moisture

content of the fuels and size distribution of the particles were studied.
[15]

The test method was further developed as part of the European Project Bionorm 2 . The objective

was to develop a mechanically improved apparatus to overcome deficiencies related to the inclination

of the flexible floor and by assuring constant and reproducible low bending radiuses at the edges of

the slot opening. At the same time, a new drive system for a moving floor was also developed, which

[5]

allows for a more sensitive and dynamic adjustment of the opening speed during measurement . Best

[6]

practice guidelines for the revised method were also developed and tested, and an international

[7]
interlaboratory test was performed .

The Bionorm 2 project also had the objective of providing detailed descriptions and procedures based on

the applied measurement principle. The intention was to establish a useful starting point for any future

attempt to develop a harmonized standard method for direct determination of bridging behaviour. In

order to document the extensive research and experimental work conducted, this document describes

the main outcome.

Bridging behaviour cannot be defined as an absolute value for a particular biofuel since the propensity

for bridging varies with moisture content, particle size distribution and content of overlong particles.

In existing product specifications of biofuels, bridging characteristics are not normally provided for

trade purposes due to variability from sample to sample. However, susceptibility to bridging has been

identified as useful for the engineering design of handling and storage facilities, and their relationship

to effective transportation of biofuels and safety.
© ISO 2020 – All rights reserved v
---------------------- Page: 5 ----------------------
TECHNICAL REPORT ISO/TR 23437:2020(E)
Solid biofuels — Bridging behaviour of bulk biofuels
1 Scope

This document summarizes current knowledge concerning a test method and its technical

implementation, and existing knowledge about the bridging performance of biofuels.

The document consists of three parts, as follows:

— Part I: Method for direct determination of bridging behaviour, to make it available for research and

development purposes (see Clause 4).

— Part II: Implementing the measurement principle, to assist in the construction of test apparatus and

to illustrate the performance of a bridging test (see Clause 5).

— Part III: Experience and results from bridging tests, to provide typical results on bridging for a wide

range of biofuels already tested (see Clause 6).
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
bridging
tendency of particles to form a stable arch across an opening and to hinder flow
Note 1 to entry: Bridging is illustrated in Figure 1 (left).

Note 2 to entry: As a consequence of bridging, biofuel conveying can be inhibited or intermittent until the

bridge collapses. All particles regardless of size can potentially form an arch. Bridging is caused by a number

of phenomena, including mechanical interlocking and interacting adherence forces between particles.

Accumulation of material of various sizes and moisture content can create clusters, which causes incoherent

flow. Friction between the material and containing walls can cause asymmetrical flow pattern resulting in

bridging. The distribution of particles of various sizes when filling a silo tends to concentrate heavier particles

at the circumference (rolling down the slope) while finer particles accumulate in the centre of the pile. During

the draining of a silo, the material in the centre will have a different flow pattern than the material coming from

the circumference of the pile. This can in some cases result in shafts or channels or “ratholes” as illustrated in

Figure 1. The phenomena can be avoided by proper design of the handling system.
© ISO 2020 – All rights reserved 1
---------------------- Page: 6 ----------------------
ISO/TR 23437:2020(E)

Figure 1 — Unfavourable flowing conditions of bulk fuels can cause the building of a bridge

(left) or a channel flow (right)
3.2
angle of repose

steepest angle of descent of a stock pile measured in degrees of the slope of material relative to the

horizontal plane when granular material on the slope face is on the verge of sliding

3.3
particle shape factor
PSF

reciprocal of the sphericity, which characterizes the degree of a particle’s approximation of an ideal sphere

Note 1 to entry: When measured by image analysis, the PSF is the measured circumference of the projection

area of a particle divided by the circumference of a circle with the same area as the particle. In the case of a

perfect sphere shape (round projection area), the PSF of the particle is PSF = 1,0. A high PSF characterizes a high

[9]
deviation from a round shape .
3.4
length–diameter ratio

ratio of a particle calculated from the maximum length and the minimum Feret diameter (3.5)

Note 1 to entry: When measured by image analysis, the LD is calculated from the maximum length as given in

[14]
Figure 2 and the minimum Feret diameter .
3.5
Feret diameter
caliper diameter
distance between two parallel planes restricting a particle

Note 1 to entry: The minimum Feret diameter is the shortest of such distances (see Figure 2).

2 © ISO 2020 – All rights reserved
---------------------- Page: 7 ----------------------
ISO/TR 23437:2020(E)
Key
1 maximum length 3 minimum Feret diameter
2 maximum width 4 length (90° to minimum Feret diameter)
Figure 2 — Important size parameters of a particle determined by image analysis
3.6
mean particle size

size of a particle defined as the maximum length as measured of each particle in a sample

Note 1 to entry: In the calculation of MP, all particles in a sample are considered according to their relative

volumetric share in their respective size class; this is done by calculating the weighted average. Mathematically

MP is derived as the sum of all multiplications between the mean size class and the relative share of particles in

this particular size class. The mean size class is calculated from the defined class boundaries (e.g. the mean size

[9]
class of the fraction between 8 mm to 16 mm is 12 mm) .
Note 2 to entry: In this definition, MP is determined by image analysis.
4 Part I: Proposed method for direct determination of bridging behaviour
4.1 Introduction to the method

Based on prior knowledge, as described in Part II (see Clause 5), a practical research method for direct

determination of bridging behaviour was developed through the European Bionorm 2 project. This

[1]

clause describes the method, which is based on previous research performed in Sweden and Denmark

[2][3][4]

. The method is suitable for all compressed and uncompressed particulate biofuels that either

have been reduced in size (such as most wood biofuels, including cut straw) or have a particulate

physical form (e.g. olive stones, nut shells, grain).
4.2 Principle

A sample is subjected to a bridging test by placing it over an expandable slot in order to allow the

building of a bridge. The opening width of the slot (see slot opening width l in Figure 3) is recorded as

a measure of the bridge building behaviour of that sample. This requires a frictionless opening of the

bottom slot.
© ISO 2020 – All rights reserved 3
---------------------- Page: 8 ----------------------
ISO/TR 23437:2020(E)
4.3 Test equipment
4.3.1 Bridging test apparatus.

A box with a bottom area (inside dimensions) of 1,1 m × 2,0 m and a minimum filling height of 0,75 m

(±0,01 m) is used. These dimensions accommodate a required sample volume of 1,65 m . The sides of

the box are made of low friction coated plywood or similar. The two sections of the bottom of the box

are made of flexible mats with low friction surfaces.

An expandable slot is formed between the two bottom sections of the box. The mat on each of the

two bottom sections forms the slot in terms of a quarter of a circular arch with an effective radius of

(32 ± 5) mm. The slot is closed when the two bottom sections are pushed together and the two mats

meet in the centre of the length of the box. The mats are fully even and horizontal, except at the round

edges (see Figure 3). The slot is gradually expandable while the edges remain parallel and the bottom

is prevented from becoming inclined during any phase of the opening procedure. The expansion is

executed in a way that ensures the mats remain in place, except at the rounded edges where they can

slide over a plate and form the rounded edges (see Figure 3). Thus, any friction between the bottom

sections and the biofuel sample in the box is avoided when the slot is expanded.

NOTE Alternatively, the mats on the two bottom sections can wind onto rollers under each bottom section.

Consequently, the effective radius becomes variable during the opening procedure. In this case, the mat is made

of thin material.

The movements of the two bottom sections is synchronized (ganged) and simultaneous during the

opening of the slot. The maximum width of the slot is 1,5 m across the bottom of the box. The edges

of the slot remain parallel during the opening procedure. A tolerance of 10 mm is acceptable. This

tolerance is measured as the difference of the opening width at both ends of the slot and it applies for

the full range of the slot opening.

The opening speed is 180 mm/min (±50 mm/min) or lower. The drive mechanism for the movable floor

allows for vibration-free, frequent and smooth starts and stops by the operator.

The box is positioned firmly at a height that ensures all sample materials fall freely through the slot

without causing any blockages below on the floor (e.g. a height of 1,5 m of the box bottom above the floor).

Key
r effective radius of the round edges at the opening
l opening width of the slot at the expandable base when the bridge collapses

Figure 3 — Functional principle of a bridging test apparatus with expandable slot

4.3.2 Loading device.

For repeated loading and unloading of large sample quantities (>1,65 m ), a wheel loader or fork lift

with suitable bucket volume is required (see 5.2).
4 © ISO 2020 – All rights reserved
---------------------- Page: 9 ----------------------
ISO/TR 23437:2020(E)

4.3.3 Metric ruler or measuring tape capable of determining the opening width between the rollers

to the nearest millimetre.
4.3.4 Rake, to level out the sample.
4.4 Sampling and sample preparation

The minimum volume of the laboratory sample is 1,65 m loose volume and is sampled in accordance

with ISO 18135. If required, the laboratory sample is reduced to the actual test portion of 1,65 m in

accordance with ISO 14780. All bridging tests are carried out with this test portion.

4.5 Procedure

a) The box is filled by pouring the test portion from a height of maximum 500 mm above the rim of

the box without applying any compaction to the sample. The surface is levelled out with a rake (see

Figure 4).

b) A slot is generated under the sample by starting the slot opening procedure. Some particles will

immediately fall through the slot but soon a bridge will form over the slot.

NOTE 1 Fine and granulated biofuel samples such as pellets or kernels can require some time to percolate

through the slot opening before forming a bridge.

c) As soon as the bridge collapses, the slot opening motion is stopped and the slot width is measured

to the nearest mm at the minimum horizontal distance between the two slot edges, as indicated

in Figure 3 by the letter “l”. The measure is recorded. In the case that a single overlong particle

prevents the collapse of the entire bridge, the slot opening movement is not continued and a 100 %

collapse is recorded.

d) The sample material, which has fallen through the slot and emptied the box completely, is then

unified with the remaining sample.

e) The box is reloaded with the unified test portion and the procedure in a) to d) is repeated until ten

measurements have been performed per sample. For pellet or grain samples, the total number of

repetitions can be reduced to five.

f) Before the start of the bridge determination tests and immediately after completion, a sub-sample

of the sample mass is collected and a determination of moisture content is performed in accordance

with ISO 18134-2. The moisture content to be reported is the average of the two determinations.

NOTE 2 In many cases, it is useful to provide further information on the tested biofuel, including by:

— collecting a sub-sample of the laboratory sample and performing a particle size classification in accordance

with ISO 17827-1;
— performing a determination of the bulk density in accordance with ISO 17828.
© ISO 2020 – All rights reserved 5
---------------------- Page: 10 ----------------------
ISO/TR 23437:2020(E)
Key
1 filling 4 start of slot opening (at a filling depth of 75 cm)
2 filling completed 5 record slot opening width l at 100 % bridge collapse
3 levelling 6 remove sample completely before refilling
Figure 4 — Stepwise procedure of a bridging determination test
4.6 Calculation

The measured bridging behaviour for a sample is calculated as the arithmetic mean and standard

deviation from the total of ten repeated measurements of the same sample (five for pellets or seeds) of

the slot opening width “l” as determined in 4.5.

The above average is useful information in order to compare biofuels. For the design of installations,

the maximum value for the ten (or five) tests is of importance.
4.7 Precision and bias

Because of the varying nature of solid biofuels covered by this document, it is not possible at this time

to give a precision statement (repeatability or reproducibility) for this test method.

Precision of measurement was proven to be highly fuel dependent. This is also evidenced by the results

given in 6.1.
6 © ISO 2020 – All rights reserved
---------------------- Page: 11 ----------------------
ISO/TR 23437:2020(E)
4.8 Test reporting
The test report includes at least the following information:

a) an identification of the laboratory performing the test and the date of the test;

b) an identification of product (or sample) tested;

c) a fuel characterization in accordance with ISO 17225-1:— , Table 1 (origin and source), Table 2

(traded form) and the relevant table for that fuel (choose from Tables 3 to 15);

NOTE A detailed fuel description (e.g. type of raw material, method of comminution, storage history,

photos with metric ruler) is highly recommended to allow sound data evaluation and interpretation of

measured results.
d) a reference to this document, i.e. ISO/TR 23437;

e) the individual results of the bridging test(s) as well as the maximum value, average and standard

deviation of the calculated average;
f) the average result from the moisture content determinations;
g) the result of the particle size classification, if performed;
h) the result of the bulk density determination, if performed;

i) any unusual features noted during the determination that could affect the result;

j) any deviation from this document, or operations regarded as optional;
k) the date of the test.
5 Part II: Implementing the measuring principle
5.1 Review of apparatus construction
[1]

The idea for the principle was conceived during a research project conducted by Mattsson in the late

1980s. A test bench was designed where the bridging test apparatus had a hanging bottom, sustained

by steel cables, and the opening was performed via two hand-operated rollers on which the bottom

rubber mat was rolled. This is referred to as “Mark 1 apparatus”.

In a later version in the 1990s, the manual operation was replaced by two electrical step motors.

These allowed a more gradual opening of the slot as well as automated reading of the width of the slot

opening l. Another good feature of this version was that the slot could be opened very quickly after the

reading had been completed. This saved a lot of time during the measurement cycle. This machine is

referred to as “Mark 2 apparatus”.

In the method revision within the Bionorm 2 project, the overall dimensions of previous machine were

retained but several changes were introduced. The bottom was kept completely flat, a smooth opening

drive was applied and the previously large roller radius, which formed the opening slot, was replaced

by a slim deflection edge of only about 32 mm radius at the slot opening (see radius r in Figure 3). This

machine is referred to as “Mark 3 apparatus”.

The defined measuring principle as described in Part I was accomplished in an apparatus construction

as shown in Figure 5. This equipment implements the required split floor design, which is fully

simultaneously movable to both sides. Both halves of the floor are mounted on an undercarriage, which

travels on wheels. The opening mechanism ensures a fully parallel movement to both sides. The opening

process is propelled by a crank handle. An alternative apparatus version with an electric motor (with

1) Under preparation. Stage at the time of publication: ISO/DIS 17225-1:2020.
© ISO 2020 – All rights reserved 7
---------------------- Page: 12 ----------------------
ISO/TR 23437:2020(E)

adjustable speed) was also successfully tested. It accelerates testing, e.g. by enabling a quick floor closure

after testing. In Figure 6, further details of the construction are shown with view from the bottom.

The recommendation is that any future machines are equipped with electric drive. The additional cost

of the electrification is easily gained back by the reduced time during testing and a more homogeneous

movement between tests is guaranteed.
Figure 5 — Example of a bridging test apparatus in accordance with 4.3.1
Key
1 opening slot 6 ladder
2 bulk container 7 undercarriage
3 movable floor 8 rail
4 weights 9 reels
5 two parallel drive screws 10 walking platform

Figure 6 — Bridging test apparatus — View from below with further details of the construction

8 © ISO 2020 – All rights reserved
---------------------- Page: 13 ----------------------
ISO/TR 23437:2020(E)

The apparatus has a walking platform in order to allow for a comfortable filling and levelling of the

sample (see Figure 6). The recommendation for future machines is that the walking platform be on

three sides of the box to ease the levelling out of the sample after tipping it into the box.

The technical data of the bridging tester are given in Table 1.
Table 1 — Technical data of the bridging test apparatus
Element Measurement/description
Length Height Width
Dimensions
(mm) (mm) (mm)
Complete testing apparatus 4 155 3 015 1 870
Bulk container (inside) 2 000 1 000 1 100
Bulk container (outside) 2 158 1 000 1 144
...

TECHNICAL ISO/TR
REPORT 23437
First edition
Solid biofuels — Bridging behaviour of
bulk biofuels
PROOF/ÉPREUVE
Reference number
ISO/TR 23437:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO/TR 23437:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 23437:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Part I: Proposed method for direct determination of bridging behaviour ..............................................3

4.1 Introduction to the method .......................................................................................................................................................... 3

4.2 Principle ........................................................................................................................................................................................................ 3

4.3 Test equipment ....................................................................................................................................................................................... 4

4.4 Sampling and sample preparation ......................................................................................................................................... 5

4.5 Procedure .................................................................................................................................................................................................... 5

4.6 Calculation .................................................................................................................................................................................................. 6

4.7 Precision and bias ................................................................................................................................................................................ 6

4.8 Test reporting ........................................................................................................................................................................................... 7

5 Part II: Implementing the measuring principle ................................................................................................................... 7

5.1 Review of apparatus construction ......................................................................................................................................... 7

5.2 Other equipment ................................................................................................................................................................................14

5.3 Measurement performance .......................................................................................................................................................15

6 Part III: Experience and results from bridging tests ....................................................................................................17

6.1 General ........................................................................................................................................................................................................17

6.2 Performance characteristics of bridging test method ........................................................................................18

6.2.1 General...................................................................................................................................................................................18

6.2.2 Sensitivity analysis on testing accuracy ....................................................................................................18

6.2.3 Reproducibility (round robin test results) .............................................................................................18

6.2.4 Repeatability ....................................................................................................................................................................19

6.3 Characterization of selected biomass fuels .................................................................................................................19

6.4 Influencing factors on bridging..............................................................................................................................................20

6.5 Outlook .......................................................................................................................................................................................................23

Bibliography .............................................................................................................................................................................................................................24

© ISO 2020 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO/TR 23437:2020(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.

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 PROOF/ÉPREUVE © ISO 2020 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 23437:2020(E)
Introduction

In all particulate matter that is flowing through an opening, the particles have the tendency to form a

solid bridge over that opening. This can cause interruptions or failures, particularly during a vertical

transport, with the consequence of clogging of silo outlets, hoppers, down pipes, funnels or screw

conveyors. To understand this phenomenon better, a determination test method was developed. The

results of these tests can be used to improve the design of handling systems in order to minimize the

risk of bridging.

Bridging is a phenomenon that can occur because of the inhomogeneous nature of the biofuel,

particularly the variation in particle size, moisture content and number of overlong particles. In

addition, biofuels are often not well understood by the designers of handling, storage and conversion

systems. Bridging phenomenon can lead to an alternating build-up and collapse of bridges or shafts,

often called ratholes (see also Figure 1).

Comprehensive studies referring to the bridging behaviour of solid biomass fuels were first performed

[1] [3]

by Mattsson and by Mattsson and Kofman in the early 1990s. They considered the basic handling

characteristics of solid biofuels, i.e. the angle of repose, the friction of solid biofuels against surfaces

and the tendency to build bridges over an opening. As these parameters had until then never been

investigated with solid biomass fuels, new measuring principles and devices had to be developed. For

determining the bridge building tendency, a test apparatus was constructed consisting of a movable

floor which could be gradually opened so that a bridge of fuel could form over the opening until it finally

[1]

collapsed . Various fuels were tested and the impact of key parameters such as bed depth, moisture

content of the fuels and size distribution of the particles were studied.
[15]

The test method was further developed as part of the European Project Bionorm 2 . The objective

was to develop a mechanically improved apparatus to overcome deficiencies related to the inclination

of the flexible floor and by assuring constant and reproducible low bending radiuses at the edges of

the slot opening. At the same time, a new drive system for a moving floor was also developed, which

[5]

allows for a more sensitive and dynamic adjustment of the opening speed during measurement . Best

[6]

practice guidelines for the revised method were also developed and tested, and an international

[7]
round robin test was performed .

The Bionorm 2 project also had the objective of providing detailed descriptions and procedures based on

the applied measurement principle. The intention was to establish a useful starting point for any future

attempt to develop a harmonized standard method for direct determination of bridging behaviour. In

order to document the extensive research and experimental work conducted, this document describes

the main outcome.

Bridging behaviour cannot be defined as an absolute value for a particular biofuel since the propensity

for bridging varies with moisture content, particle size distribution and content of overlong particles.

In existing product specifications of biofuels, bridging characteristics are not normally provided for

trade purposes due to variability from sample to sample. However, susceptibility to bridging has been

identified as useful for the engineering design of handling and storage facilities, and their relationship

to effective transportation of biofuels and safety.
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TECHNICAL REPORT ISO/TR 23437:2020(E)
Solid biofuels — Bridging behaviour of bulk biofuels
1 Scope

This document summarizes current knowledge concerning a test method and its technical

implementation, and existing knowledge about the bridging performance of biofuels.

The document consists of three parts, as follows:

— Part I: Method for direct determination of bridging behaviour, to make it available for research and

development purposes (see Clause 4).

— Part II: Implementing the measurement principle, to assist in the construction of test apparatus and

to illustrate the performance of a bridging test (see Clause 5).

— Part III: Experience and results from bridging tests, to provide typical results on bridging for a wide

range of biofuels already tested (see Clause 6).
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
bridging
tendency of particles to form a stable arch across an opening and to hinder flow
Note 1 to entry: Bridging is illustrated in Figure 1 (left).

Note 2 to entry: As a consequence of bridging, biofuel conveying can be inhibited or intermittent until the

bridge collapses. All particles regardless of size can potentially form an arch. Bridging is caused by a number

of phenomena, including mechanical interlocking and interacting adherence forces between particles.

Accumulation of material of various sizes and moisture content can create clusters, which causes incoherent

flow. Friction between the material and containing walls can cause asymmetrical flow pattern resulting in

bridging. The distribution of particles of various sizes when filling a silo tends to concentrate heavier particles

at the circumference (rolling down the slope) while finer particles accumulate in the centre of the pile. During

the draining of a silo, the material in the centre will have a different flow pattern than the material coming from

the circumference of the pile. This can in some cases result in shafts or channels or “ratholes” as illustrated in

Figure 1. The phenomena can be avoided by proper design of the handling system.
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ISO/TR 23437:2020(E)

Figure 1 — Unfavourable flowing conditions of bulk fuels can cause the building of a bridge

(left) or a channel flow (right)
3.2
angle of repose

steepest angle of descent of a stock pile measured in degrees of the slope of material relative to the

horizontal plane when granular material on the slope face is on the verge of sliding

3.3
particle shape factor
PSF

reciprocal of the sphericity, which characterizes the degree of a particle’s approximation of an ideal sphere

Note 1 to entry: When measured by image analysis, the PSF is the measured circumference of the projection

area of a particle divided by the circumference of a circle with the same area as the particle. In the case of a

perfect sphere shape (round projection area), the PSF of the particle is PSF = 1,0. A high PSF characterizes a high

[9]
deviation from a round shape .
3.4
length–diameter ratio

ratio of a particle calculated from the maximum length and the minimum Feret diameter (3.5)

Note 1 to entry: When measured by image analysis, the LD is calculated from the maximum length as given in

[14]
Figure 2 and the minimum Feret diameter .
3.5
Feret diameter
caliper diameter
distance between two parallel planes restricting a particle

Note 1 to entry: The minimum Feret diameter is the shortest of such distances (see Figure 2).

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Key
1 maximum length 3 minimum Feret diameter
2 maximum width 4 length (90° to minimum Feret diameter)
Figure 2 — Important size parameters of a particle determined by image analysis
3.6
mean particle size

size of a particle defined as the maximum length as measured of each particle in a sample

Note 1 to entry: In the calculation of MP, all particles in a sample are considered according to their relative

volumetric share in their respective size class; this is done by calculating the weighted average. Mathematically

MP is derived as the sum of all multiplications between the mean size class and the relative share of particles in

this particular size class. The mean size class is calculated from the defined class boundaries (e.g. the mean size

[9]
class of the fraction between 8 mm to 16 mm is 12 mm) .
Note 2 to entry: In this definition, MP is determined by image analysis.
4 Part I: Proposed method for direct determination of bridging behaviour
4.1 Introduction to the method

Based on prior knowledge, as described in Part II (see Clause 5), a practical research method for direct

determination of bridging behaviour was developed through the European Bionorm 2 project. This

[1]

clause describes the method, which is based on previous research performed in Sweden and Denmark

[2][3][4]

. The method is suitable for all compressed and uncompressed particulate biofuels that either

have been reduced in size (such as most wood biofuels, including cut straw) or have a particulate

physical form (e.g. olive stones, nut shells, grain).
4.2 Principle

A sample is subjected to a bridging test by placing it over an expandable slot in order to allow the

building of a bridge. The opening width of the slot (see slot opening width l in Figure 3) is recorded as

a measure of the bridge building behaviour of that sample. This requires a frictionless opening of the

bottom slot.
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4.3 Test equipment
4.3.1 Bridging test apparatus.

A box with a bottom area (inside dimensions) of 1,1 m × 2,0 m and a minimum filling height of 0,75 m

(±0,01 m) is used. These dimensions accommodate a required sample volume of 1,65 m . The sides of

the box are made of low friction coated plywood or similar. The two sections of the bottom of the box

are made of flexible mats with low friction surfaces.

An expandable slot is formed between in the two bottom sections of the box. The mat on each of the

two bottom sections forms the slot in terms of a quarter of a circular arch with an effective radius of

(32 ± 5) mm. The slot is closed when the two bottom sections are pushed together and the two mats

meet in the centre of the length of the box. The mats are fully even and horizontal, except at the round

edges (see Figure 3). The slot is gradually expandable while the edges remain parallel and the bottom

is prevented from becoming inclined during any phase of the opening procedure. The expansion is

executed in a way that ensures the mats remain in place, except at the rounded edges where they can

slide over a plate and form the rounded edges (see Figure 3). Thus, any friction between the bottom

sections and the biofuel sample in the box is avoided when the slot is expanded.

NOTE Alternatively, the mats on the two bottom sections can wind onto rollers under each bottom section.

Consequently, the effective radius becomes variable during the opening procedure. In this case, the mat is made

of thin material.

The movements of the two bottom sections is synchronized (ganged) and simultaneous during the

opening of the slot. The maximum width of the slot is 1,5 m across the bottom of the box. The edges

of the slot remain parallel during the opening procedure. A tolerance of 10 mm is acceptable. This

tolerance is measured as the difference of the opening width at both ends of the slot and it applies for

the full range of the slot opening.

The opening speed is 180 mm/min (±50 mm/min) or lower. The drive mechanism for the movable floor

allows for vibration-free, frequent and smooth starts and stops by the operator.

The box is positioned firmly at a height that ensures all sample materials fall freely through the slot

without causing any blockages below on the floor (e.g. a height of 1,5 m of the box bottom above the floor).

Key
r effective radius of the round edges at the opening
l opening width of the slot at the expandable base when the bridge collapses

Figure 3 — Functional principle of a bridging test apparatus with expandable slot

4.3.2 Loading device.

For repeated loading and unloading of large sample quantities (> 1,65 m ), a wheel loader or fork lift

with suitable bucket volume is required (see 5.2).
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4.3.3 Metric ruler or measuring tape capable of determining the opening width between the rollers

to the nearest millimetre.
4.3.4 Rake, to level out the sample.
4.4 Sampling and sample preparation

The minimum volume of the laboratory sample is 1,65 m loose volume and is sampled in accordance

with ISO 18135. If required, the laboratory sample is reduced to the actual test portion of 1,65 m in

accordance with ISO 14780. All bridging tests are carried out with this test portion.

4.5 Procedure

a) The box is filled by pouring the test portion from a height of maximum 500 mm above the rim of

the box without applying any compaction to the sample. The surface is levelled out with a rake (see

Figure 4).

b) A slot is generated under the sample by starting the slot opening procedure. Some particles will

immediately fall through the slot but soon a bridge will form over the slot.

NOTE 1 Fine and granulated biofuel samples such as pellets or kernels can require some time to percolate

through the slot opening before forming a bridge.

c) As soon as the bridge collapses, the slot opening motion is stopped and the slot width is measured

to the nearest mm at the minimum horizontal distance between the two slot edges, as indicated

in Figure 3 by the letter “l”. The measure is recorded. In the case that a single overlong particle

prevents the collapse of the entire bridge, the slot opening movement is not continued and a 100 %

collapse is recorded.

d) The sample material, which has fallen through the slot and emptied the box completely, is then

unified with the remaining sample.

e) The box is reloaded with the unified test portion and the procedure in a) to d) is repeated until ten

measurements have been performed per sample. For pellet or grain samples, the total number of

repetitions can be reduced to five.

f) Before the start of the bridge determination tests and immediately after completion, a sub-sample

of the sample mass is collected and a determination of moisture content is performed in accordance

with ISO 18134-2. The moisture content to be reported is the average of the two determinations.

NOTE 2 In many cases, it is useful to provide further information on the tested biofuel, including by:

— collecting a sub-sample of the laboratory sample and performing a particle size classification in accordance

with ISO 17827-1;
— performing a determination of the bulk density in accordance with ISO 17828.
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ISO/TR 23437:2020(E)
Key
1 filling 4 start of slot opening (at a filling depth of 75 cm)
2 filling completed 5 record slot opening width l at 100 % bridge collapse
3 levelling 6 remove sample completely before refilling
Figure 4 — Stepwise procedure of a bridging determination test
4.6 Calculation

The measured bridging behaviour for a sample is calculated as the arithmetic mean and standard

deviation from the total of ten repeated measurements of the same sample (five for pellets or seeds) of

the slot opening width “l” as determined in 4.5.

The above average is useful information in order to compare biofuels. For the design of installations,

the maximum value for the ten (or five) tests is of importance.
4.7 Precision and bias

Because of the varying nature of solid biofuels covered by this document, it is not possible at this time

to give a precision statement (repeatability or reproducibility) for this test method.

Precision of measurement was proven to be highly fuel dependent. This is also evidenced by the results

given in 6.1.
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4.8 Test reporting
The test report includes at least the following information:

a) an identification of the laboratory performing the test and the date of the test;

b) an identification of product (or sample) tested;

c) a fuel characterization in accordance with ISO 17225-1:—, Table 1 (origin and source), Table 2

(traded form) and the relevant table for that fuel (choose from Tables 3 to 16);

NOTE A detailed fuel description (e.g. type of raw material, method of comminution, storage history,

photos with metric ruler) is highly recommended to allow sound data evaluation and interpretation of

measured results.
d) a reference to this document, i.e. ISO/TR 23437;

e) the individual results of the bridging test(s) as well as the maximum value, average and standard

deviation of the calculated average;
f) the average result from the moisture content determinations;
g) the result of the particle size classification;
h) the result of the bulk density determination, if performed;

i) any unusual features noted during the determination that could affect the result;

j) any deviation from this document, or operations regarded as optional;
k) the date of the test.
5 Part II: Implementing the measuring principle
5.1 Review of apparatus construction
[1]

The idea for the principle was conceived during a research project conducted by Mattsson in the late

1980s. A test bench was designed where the bridging test apparatus had a hanging bottom, sustained

by steel cables, and the opening was performed via two hand-operated rollers on which the bottom

rubber mat was rolled. This is referred to as “Mark 1 apparatus”.

In a later version in the 1990s, the manual operation was replaced by two electrical step motors.

These allowed a more gradual opening of the slot as well as automated reading of the width of the slot

opening l. Another good feature of this version was that the slot could be opened very quickly after the

reading had been completed. This saved a lot of time during the measurement cycle. This machine is

referred to as “Mark 2 apparatus”.

In the method revision within the Bionorm 2 project, the overall dimensions of previous machine were

retained but several changes were introduced. The bottom was kept completely flat, a smooth opening

drive was applied and the previously large roller radius, which formed the opening slot, was replaced

by a slim deflection edge of only about 32 mm radius at the slot opening (see radius r in Figure 3). This

machine is referred to as “Mark 3 apparatus”.

The defined measuring principle as described in Part I was accomplished in an apparatus construction

as shown in Figure 5. This equipment implements the required split floor design, which is fully

simultaneously movable to both sides. Both halves of the floor are mounted on an undercarriage, which

travels on wheels. The opening mechanism ensures a fully parallel movement to both sides. The opening

process is propelled by a crank handle. An alternative apparatus version with an electric motor (with

adjustable speed) was also successfully tested. It accelerates testing, e.g. by enabling a quick floor closure

after testing. In Figure 6, further details of the construction are shown with view from the bottom.

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The recommendation is that any future machines are equipped with electric drive. The additional cost

of the electrification is easily gained back by the reduced time during testing and a more homogenous

movement between tests is guaranteed.
Figure 5 — Example of a bridging test apparatus in accordance with 4.3.1
Key
1 opening slot 6 ladder
2 bulk container 7 undercarriage
3 movable floor 8 rail
4 weights 9 reels
5 two parallel drive screws 10 walking platform

Figure 6 — Bridging test apparatus — View from below with further details of the construction

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The apparatus has a walking platform in order to allow for a comfortable filling and levelling of the

sample (see Figure 6). The recommendation for future machines is that the walking platform be on

three sides of the box to ease the levelling out of the sample after tipping it into the box.

The technical data of the bridging tester are given in Table 1.
Table 1 — Technical data of the bridging test apparatus
Element Measurement/description
Length Height Width
Dimensions
(mm) (mm) (mm)
Complete testing apparatus 4 155 3 015 1 870
Bulk container (in
...

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