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Origin of Gases in Coal Mines 219
13.3.1 Proximate Analysis
This analysis basically measures four constituents of coal as follows:
1. Moisture
2. Volatile matter
3. Fixed carbon
4. Ash
American Standards for Testing Material (ASTM) uses the ash-freeefixed carbon
and calorific value of coal to rank coal seams. Table 13.4 shows a complete list.
International coal classification standards were developed by the Coal Committee
of the Economic Commission for Europe. Table 13.5 shows both ASTM and European
classifications side by side for ease in comparison.
International system uses a three digit number to describe three different properties
of the coal. The first digit shows the rank as shown in Table 13.5. The second digit
shows the Roga index or free swelling index when coal is heated rapidly. The third
digit shows the coking properties of metallurgical coal when it is heated slowly and
is measured by Gray-King Coke index [5].
13.3.2 Ultimate Analysis
Ultimate analysis of coal breaks it into four elements: O 2 ,H 2 , C, and N 2 . Usually car-
bon content increases with the rank of coal but H 2 and O 2 contents go down. Oxygen
on dry, ash-free basis is used to determine the liability of coal to spontaneous combus-
tion (discussed in Chapter 21).
13.3.3 Rank of Coal Versus Vitrinite Reflectance, R o
Vitrinite reflectance is a measure of percentage of incident light reflected from the
surface of vitrinite particle/bands in coal seams. R o is usually an average of several
readings. A typical coal seam has four megascopic bands known as vitrain, clarain,
durain and fusain. Only vitrain is used for the ranking of coal (Table 13.6).
13.4 Characterization of Methane from Coal
Most coalbed methane (CBM) can be classified in two groups: (a) biogenic and
(b) thermogenic. Low-rank coal usually contains methane of biogenic origin, whereas
more mature, high-rank coals have thermogenic methane. Besides rank (or R o ), there
are other indices that characterize CBM as either biogenic or thermogenic. Most
important indices are listed below.
13.4.1 Hydrocarbon Index
It is defined as the ratio of CH 4 to (C 2 H 6 þ C 3 H 8 ).
