The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 544

FIGURE 13.10
Coalifi cation, the Biochemical and
Geochemical Processes that Transform
Organic Materials into a Combustible-
Carbonaceous Solid

Time A one metre thick coal seam may have

started out as a layer of plant material
about 120 metres thick and taken over
200 million years to form.
Source:
World Coal Institute 2005

Pressure

Heat

PEAT

LIGNITE COAL

CARBON/ENERGY CONTENT OF COAL HIGH

HIGH MOISTURE CONTENT OF COAL

Low Rank Coals
47%

Hard Coals
53%

Lignite
17%

Sub-Bituminous
30%

Bituminous
52%

Anthracite
1%

Thermal
Steam Coal

Metallurgical
Coking Coal

Largely power
generation

Power generation
Cement manufacture
Industrial uses

Power generation
Cement manufacture
Industrial uses

Manufacture of
iron and steel

Domestic/
Industrial
including
smokeless
fuel

Coal bed methane (CBM) is generated microbially and/or thermogenically during coal for-
mation. The amount of methane commonly varies between 100 and 300 cm 3 /g of coal (Rice
1993). Methane may also be stored under pressure in the rock above a coal bed. Methane
is adsorbed in the coal micro-pores as a function of reservoir pressure. Pressure reduc-
tions liberate these gases. Mining can liberate the gas from its pressurized environment by
accessing and excavation of coal as well as subsequent fracturing of the overlying strata.
While the amount of methane gas generated is enormous, the actual quantities found
today are a function of Earth movement and erosion over geological times. Coal beds and