ISO/PRF TS 21596

TECHNICAL ISO/TS
SPECIFICATION 21596
First edition
Solid biofuels — Determination
of grindability — Hardgrove type
method for thermally treated biomass
fuels
PROOF/ÉPREUVE
Reference number
ISO/TS 21596:2021(E)
ISO 2021
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ISO/TS 21596:2021(E)
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© ISO 2021

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

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

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

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

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

4 Principle ........................................................................................................................................................................................................................ 2

5 Apparatus ..................................................................................................................................................................................................................... 2

5.1 Standard HGI mill ................................................................................................................................................................................. 2

5.2 Sieves ............................................................................................................................................................................................................... 2

5.3 Glass measuring cylinder ............................................................................................................................................................... 2

5.4 Sieving machine ..................................................................................................................................................................................... 3

5.5 Balance ........................................................................................................................................................................................................... 3

5.6 Cutting mill ................................................................................................................................................................................................. 3

6 Sampling and sample preparation .................................................................................................................................................... 3

6.1 General ........................................................................................................................................................................................................... 3

6.2 Moisture conditions ............................................................................................................................................................................ 3

6.3 Pre-grinding and sieving ................................................................................................................................................................ 3

6.4 Sample size ................................................................................................................................................................................................. 3

7 Procedure..................................................................................................................................................................................................................... 3

8 Calculation .................................................................................................................................................................................................................. 4

9 Reproducibility ...................................................................................................................................................................................................... 5

10 Test report ................................................................................................................................................................................................................... 5

Annex A (normative) Calibration curve and Thermally Treated Biomass Grindability Index

(TTBGI) values for the reference materials ............................................................................................................................. 6

Bibliography ................................................................................................................................................................................................................................ 9

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Grindability characteristics are of fundamental importance during solid biofuel preparation for energy

conversion processes, requiring a predictable particle size distribution. Particles that are too large or

too wide in their distribution may result in feeding problems as well as un-burnt fuel passing through

the conversion process. Also, in case of processing of biomass for pre-treatment, consistency in particle

size determines the yield of such pre-treatment as well as the quality of the final solid biofuel.

The effectiveness of combustion in a pulverized fuel furnace depends strongly on the reactivity and

particle size distribution of the pulverized fuel among other factors. For coal, the Hardgrove Grindability

Index (HGI) test was developed to characterize the relative grindability of a particular quality of coal

relative to a pre-determined standard quality of coal (ASTM D 409). The HGI test is an empirical batch

method that simulates the continuous grinding and crushing operation of a ball, table or tube type of

industrial coal pulverizer, herein also called coal mill. The HGI value is an indication of the degree of

grinding required to reach a particular particle size necessary for effective combustion.

Manufacturers of coal pulverizers for preparation of pulverized coal for combustion in coal burning

plants provide curves that show the relationships between the HGI for coal, coal mill capacity in terms

of tonne/h and coal mill power in kW. This is done as part of the boiler contract and guarantees. It

also serves as a first indication as to how to operate a mill when pulverizing different coals with

predetermined standard properties. A HGI of 50, which is standard industry convention, implies that

100 % capacity in tonnes per hour (as indicated) can be reached for the coal at a fineness of 70 % less

than 75 microns and 99,5 % less than 300 microns. So, if a coal has a HGI of less than 50 it implies

some level of pulverizer capacity loss while on the other hand, a HGI higher than 50 indicates some

level of pulverizer capacity gain. The HGI method is internationally accepted and quoted as part of the

Grindability may also be applied to thermally pre-treated compressed biomass materials, such as

pellets, for pulverization in coal mills. Pre-treatment methods for biomass fuels such as torrefaction,

steam treatment or hydro-thermal carbonization (HTC) upgrade the properties of biomass making it

more effective as a fuel. With increasing interest in the use of such pre-treated biomass fuels for direct

co-firing applications in conventional pulverized coal boilers, this method herein describes a laboratory

procedure for determination of the grindability of pre-treated biofuels for powder fuel preparation.

The HGI determination is subject to many limitations, including the fact that measurements can be

insensitive to the heterogeneous properties of coal that arise from different mineral contents, maceral

constituents and levels of maturity. Three relevant adaptations are applied to the standard HGI method

in order to extend its applicability to thermally pre-treated biomass fuels. The adaptations and their

Coal pulverizers are volumetric devices but with the densities of coals being fairly similar, the capacity

of these devices is expressed in mass units per hour. In contrast different biomass materials have

different densities. Therefore, the sample amount was changed to the volume (75 ± 0,5) cm of input

material, which approximates the bulk volume of the standard reference coal samples with a particle

The standard HGI test for coal uses the mass of particles passing a sieve with an aperture size of 75 μm

as criteria for determining the grindability and the resulting HGI value. The furnace volume is designed

from the knowledge of coals to be utilized so as to provide an adequate residence time for complete

combustion. Combustion efficiency depends in part on fuel particle size especially for pulverized coal

furnaces. As the combustion efficiency also depends on the reactivity, which is linked to volatile matter

content of fuels, the particle size for high-volatile fuels, such as thermally treated biomass, would not

necessarily need to meet the strict fineness requirements for coal powders. Therefore, a sieve with an

aperture size of 500 μm is used for determination of grindability of thermally treated biomass. This

adaptation also has positive implications on the repeatability of the determination.

In the standard HGI test for coal, four references coals are needed for calibrating a HGI mill. This enables

reproducibility of the results across different labs. For the thermally treated biomass grindability

index determination, this is the very first step. The thermally treated biomass grindability index is

an index for materials originating from biomass. The sieving behavior of biomass materials differs

fundamentally from that of coals due to the difference in the sphericity of the coal particles. Biomass

materials, including thermally treated biomass, are more fibrous and have lengthy dust particles

compared to coals. Thermal treatment processes, such as torrefaction, modifies the property of biomass

by making biomass more like charcoal. Woody charcoal is a product of slow pyrolysis, which degrades

the fibrous nature of the biomass. This consequently improves its grindability as well as the sphericity

The adapted grindability method uses wood pellets (ISO 17225-2:2021 A1 class pellets) and wood

based charcoal (ISO 17225-1:2021 Table 14) as calibration reference materials. These biomass reference

materials will be given assigned Thermally Treated Biomass Grindability Index (TTBGI) values as a

result of the procedures described in the Annex A. The materials are available commercially for the

This document describes a method for determination of grindability of graded thermally treated and

densified biomass fuels such as classified in ISO/TS 17225-8, for the purpose of preparing fuels with a