The extensive knowledge of the fuel properties is a fundamental prerequisite for the design and operational optimization of the plant components of a pulverized coal firing system, in particular the milling plants.
For the characterisation of the grindability of lignite the Hardgrove Index (HGI) is also frequently used. This index was developed for bituminous coals and serves first of all for the characterisation of the crushing properties for fine grinding under compressive and crushing stresses. Therefore, the index is only regarded among experts as an auxiliary variable for the characterisation of lignites, as it has only a very limited significance for the large-scale process of the grinding under impact stresses.
In order to replace the Hardgrove Index for the characterisation of lignites by a considerably more significant index, testing equipment was developed in which the sample material is crushed by impact grinding. Such equipment (spreader disk mill) was found in 2002 at the Institute for Solid and Boundary Surface Process Engineering of the Technical University Munich [1]. The principle of this equipment is based on the defined
conditions of impact crushing with the exclusion of grinding flow effects and the quantitative determination of the crushing properties of the material under stresses comparable with a large-scale plant.
When this process, which was further developed for the needs of lignite as a fuel, was presented in public for the first time at the VGB Conference Fuel Engineering and Firing Systems in Kassel in June 2006, the response was very positive [2].
The present instruction sheet represents the completion of a long development work with numerous series of investigations, which were carried out at RWE Power and Alstom Power Systems in the years 2004-2008. The aim was to show a way how the material property Grindability of lignites can be determined - under defined conditions,
- with maximum reproducibility and comparability,
- under stresses comparable to a large-scale plant and
- with justifiable expenditure.
For a simple classification of lignites with different grindability an impact grindability index is introduced in analogy to the Hardgrove Index (HGI). This impact grindability index is to be used by the manufacturers and operators of lignite-fired steam generators as an index for the grindability of lignite in beater wheel, beater or fan mills.
It is requested that the VGB Office should be informed about the experience and improvement proposals collected with the application of this instruction sheet. These can form the basis for supplements or changes. Any liability for the correctness of the presentation is excluded.
The extensive analysis required in the preliminary stages of the preparation of this instruction sheet were performed in the laboratory of Alstom Power Systems, Stuttgart, under the direction of Mr. Klaus Kapp. At RWE Power the test series were carried out in the PBD-CM Department at the Neurath Power Plant.

We would like to thank Dr. Thomas Hilber, Alstom Power Systems, and Bernhard Röper, RWE Power, as well as Dr. Müller of the VGB Office for the preparation of this directive.
Mr. Dore, Hitachi Power Europe, on the manufacturer's side as well as Rudolf Bönisch, Vattenfall Europe Mining AG, on the utility’s side participated in the coordination of this directive.
The authors also thank the VGB Working Group "Fuel Engineering and Firing Systems" for hints and suggestions as well as for the support in the publication of this instruction sheet.

*Dieses Produkt ist ebenfalls in deutscher Sprache erhältlich.

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