1. Introduction

Combustion File 177 introduces [GLOSS]Coal[/GLOSS] and the various [GLOSS]Coal rank[/GLOSS] in which it exists. Here a first microscopic description of coal is introduced, leading to the use of [GLOSS]Maceral Analysis[/GLOSS] within pulverised fuel characterisation for coal, as previewed in CF48.

Knowledge of the component materials that make up coals is necessary to understand those coal quality parameters that affect the development of a coal deposit; i.e. [GLOSS]Calorific value[/GLOSS], [GLOSS]Volatile matter[/GLOSS] and other features such as coking properties and grindability.

The microscopic description of coal is in the realm of the Petrographer, based on maceral analyses. In this combustion file the concept of [GLOSS]Macerals[/GLOSS] are introduced.

2. Macerals

The organic units or macerals that make up a coal mass can be identified in all ranks of coal.

In essence, macerals are divided into three main groups:

• [GLOSS]Vitrinite[/GLOSS] (also known as [GLOSS]Huminite[/GLOSS]), which consists of woody material;
• [GLOSS]Exinite[/GLOSS] (also known as [GLOSS]Liptinite[/GLOSS]), which consists of spores, resins and cuticles (leaves), and;
• [GLOSS]Inertinite[/GLOSS], which consists of oxidised plant material.

The original classification of maceral groups into macerals and sub-macerals, is referred to as the Stopes-Heerlen system, and is shown in Table 1 below [1]. The combinations of macerals are known as [GLOSS]Microlithotypes[/GLOSS].

Table 1 The Stopes-Herleen Classification of Maceral Groups

3. Microlithotypes

The combination of the three main maceral groups (which are found in greater or lesser proportions depending on the coal rank) are known as microlithotypes, see Table 2.

Table 2 Composition of Microlithotypes

The relationship between the maceral types and the original plant material is relatively well documented.

The original plant materials that make up coal have different chemical compositions, which in turn determine the type of group macerals. These chemical differences are reasonably clear in low rank coals but with increasing rank it becomes more difficult to distinguish petrographically between the various macerals. This has been illustrated by analysis of microspore floras and the respective petrographic types.

Relationships have been established based on the study of thin layers of coal representing a moment in time during which environmental changes were minimal. In low rank coals such as lignites and sub bituminous coals the Vitrinite maceral group is known as Huminite, and is the precursor of the vitrinite maceral group found in higher rank coals. With increasing coal rank, homogenisation of the huminite-vitrinite group occurs and the term collinite is used to describe the homogeneous structureless vitrite.

A microlithotype analysis (under a microscope) can give a fair indication of the texture of a coal. If two coals have equivalent contents of vitrinite and one has a higher vitrite content, this is probably due to different thicknesses in the bands of vitrinite. Also the distribution of the inertinite content has an important influence on the combustibility and burnout of coal in a combustion environment.

4. Petrographic Analysis

The use of [GLOSS]Petrographic Analysis[/GLOSS] and the interpretation of maceral analyses for determining coal rank and reactivity behaviour is developed in separate Combustion Files, CF181 and CF182 respectively.

Sources

[1] E. Stach et al., Stach’s Textbook of coal petrology, ISBN 3443390684, Borntraeger, Berlin (1975).
[2] D. W. Van Krevelen, Coal – Typology, Chemistry, Physics and Constitution. ISBN 044440600, Elsevier, Amsterdam, 1981.
[3] J. F. Unsworth, D. J. Barratt and P. T. Roberts. Coal Quality and Combustion Performance – An International Perspective. ISBN 0444887032, Elsiver – Amsterdam (1991).