1. Background

[GLOSS]Coal[/GLOSS] is introduced as an industrial fuel in Combustion File CF62 and described in detail in CF177. Coal characterisation is introduced in CF48 under the general question of “What is pulverised fuel characterisation?”.

In CF48, through an analysis on the effect of varying the coal blend composition, as fired to a boiler, by adding a new coal to an existing blend, the need for impact assessment is discussed and the possibility of coal [GLOSS]maceral analysis[/GLOSS] is introduced as one of several possible characterisation techniques. [GLOSS]Macerals[/GLOSS] are described and discussed in CF179.

The present CF explains the techniques and procedures involved for the analysis of coals for macerals.

2. Maceral Analysis

A twin eyepiece reflected light microscope, capable of using a variety of objectives and an automatic point counter, is required for the microscopic examination of coal for macerals.

Today, maceral analyses are carried out exclusively in reflected light. Beforehand, thin sections were examined in transmitted light. The reasons for the former being the norm today, are as follows:

·          The rank of a coal can only be determined by measuring the reflectance of [GLOSS]vitrinite[/GLOSS] from prepared polished surfaces of the coal (it has been shown that the reflectance of vitrinite increases uniformly with rank).

·          As the absorption index rises with increasing rank, coals of high rank require sections to be very thin. The preparation of sufficiently thin sections is very difficult when the volatile matter falls below 25%.

·          Quantitative maceral analysis cannot be carried out on thin sections because the proportion of [GLOSS]inertinite[/GLOSS] (which is opaque) is overestimated.

With regard to the importance of polished surfaces, the methods and techniques are discussed below.

3. Sampling of the coal

Coal samples may be taken in a variety of ways:

–          From core samples, taken from coal seams.

–          From mined coal.

–          From coal during or after its preparation for processes such as [GLOSS]coke[/GLOSS] and [GLOSS]briquette[/GLOSS] production.

Regardless of the source of the coal sample, a polished section must be prepared.

In general, the sample must be dried, finely ground (not to PF fineness) and mixed with a synthetic resin (such as [GLOSS]Araldite®[/GLOSS]).

Enough synthetic resin should be added to give sufficient strength to the sample for polishing.

If large size coal particle fractions are used large errors in the subsequent analysis will result.  An approximate idea of the number of samples required for a representative analysis is given in Table 1 below.

Table 1: Number of specimens required to obtain
a representative quantitative analysis

Once a sample is prepared, it is then subject to several stages of polishing on high speed polishing machines using chromium oxide or high-grade alumina.

After the sample has been prepared it goes under the microscope. Now we are in the realm of the petrographers – those who practice the science of [GLOSS]petrography[/GLOSS], particularly in relation to coal.

4. Instruments and Analysis

A twin eyepiece reflected light microscope is required with at least a 25x30mm oil immersed objective and 8-10x oculars.

One of the oculars must have an adjustable eyepiece in which a micrometer [GLOSS]graticule[/GLOSS] is placed in the microscope eyepiece.

The polished surface is placed in the slide of an automatic point counter. The point-to-point distance (s, [mm]) used is chosen according to the mean size of the largest grain (d_max, [mm]) according to:

s = 1/2d_max  (1)

This implies that a polished surface sample of 20x20mm will represent 1500-1600 points (the minimum number of points to be statistically significant is 500).

The result of the maceral count is expressed as % by volume. The admissible scatter for repeat and comparative analysis based on 1000 points ranges from 2-2.5%.

Point-to-point measurements are made with a point counter, which consists of a traversing carriage attached to the microscope stage and a counting unit.

The carriage displaces the sample in a series of equally spaced steps and after each step the particle in the centre of view is identified and recorded by pressing the corresponding button on the counting unit. The counting unit and carriage are connected so that each time a count is made the carriage advances one step.

5.    Treatment of the results

This technique is used to determine the volumetric maceral content of the coal sample in terms of the three main groups described in CF179:

• Vitrinite (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;
• Inertinite, 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, which is detailed in CF179.

The combinations of macerals known as [GLOSS]Microlithotypes[/GLOSS], also detailed in CF179, can also be determined by the maceral analysis technique described above.

The interpretation of Maceral Analysis in terms of coal rank and the prediction of coal combustion performance are discussed primarily in CF182 and CF198.

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).

[4] L. Thomas, Handbook of Practical Coal Geology, ISBN 0 471 93557 3, John Wiley and Sons, Chichester (1992).