ISO/TR 23437:2020

TECHNICAL ISO/TR
REPORT 23437
First edition
2020-08
Solid biofuels — Bridging behaviour of
bulk biofuels
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

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
Foreword
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electrotechnical standardization.
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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
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This document was prepared by Technical Committee ISO/TC 238, Solid biofuels.
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iv © ISO 2020 – All rights reserved

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.
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. T
...