Rank

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
> 87
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.

 

Coal Rank