Rank of coal seam
Coalification or metamorphosis of coal is defined as gradual changes in
the physical and chemical properties of coal in response to temperature
and time. The coal changes from peat through lignite and bituminous coal
to anthracite. With extreme metamorphisom, anthracite can change to graphite.
The rank of coal is the stage the coal has reached on the coalification
path. The changes, with increasing rank, include an increase in carbon
content, and decreases in moisture content and volatile matter as shown
in the table below.
COAL RANK |
CARBON CONTENT ( %) |
VOLATILE MATTER ( %) |
CALORIFIC VALUE (kJ/kG) |
MOISTURE CONTENT% |
PEAT |
60 |
>53 |
16800 |
>75 |
BROWN COAL |
60 – 71 |
53 – 49 |
23000 |
35 |
SUBBITUMINOUS COAL |
71 – 77 |
49 – 42 |
29300 |
25 - 10 |
BITUMINOUS COAL |
77 – 87 |
42 – 29 |
36250 |
8 |
ANTHRACITE |
|
29 -8 |
>36250 |
<8 |
- |
On dry ash free basis |
On dry ash free basis |
Ash
Free Basis |
In-situ |
Coalification of coal is generally a consequence of thermal effects and
compaction.The coalification processes involved in coal formation are marked
by a well defined progression of increasing rank. Coal rank increases with
depth, and the combination of depth of burial and geothermal gradient essentially
determine the rank of coal. Water, carbon dioxide and methane are generated
during the progressive coalifciation. Water and carbon dioxide are produced
during coalification of lower ranks of coal.
Methane is the predominant gas generated in the bituminous coal and anthracite
stages of coalification, and the carbon dioxide produced at lower ranks
is typically flushed out of the coal by methane. The sorption capacity
of coal increases with rank. Usually high rank coal can absorb more gas
and the adsorptive capacity of coal for methane increases with coal rank
. The sorption capacity of coal can be influenced by different intrusions
and by the tectonic events such as folding and faulting. Coals near igneous
intrusions such as dykes, may contain calcites and pyrites which are likely
to influence the drainability of gases. |
