The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 543

The combustion of coal leaves a solid, unburnable residue called bottom ash, the nature
and amount of which depends on the composition of the coal and the degree of prepara-
tion. To this is added the l y ash removed from l ue gases by ESP, bag i lters, and scrub-
bers. The proportion of l y ash to bottom ash is about 80 to 20% in pulverized coal boilers.
Pulverized ash is of commercial value and a signii cant proportion of it is used in cement
making, or other construction related applications. Unused ash is disposed of in landi lls.
There is some concern about the health effects of residual trace metals in l y and bottom
ash. This has been the subject of some controversy, and there is a marked divergence of
opinion on the environmental and health risks from hazardous components of ash. In
some jurisdictions such as Indonesia, waste management regulations impose regulatory
challenges to the use of l y and bottom ash, and impose stringent requirements on the
design of disposal sites for solid combustion wastes such as ash or scrubber wastes.

Coal Mining and the Release of Methane

In many cases, coal mining also results in emissions of methane (CH 4 ), a potent greenhouse
gas. There are many sources of methane, usually involving the degradation of organic
matter to a more thermodynamically stable form, namely methane: biological processes as
a result of microbial action, as in biomass or landi lls, and/or from thermal processes as a
result of pressure as in case of petroleum and coal. Methane trapped in coal deposits and in
the surrounding strata is released during normal mining operations in both underground
and surface mines, and in handling of the coal after mining, as well as during post-mining.
Methane remains in the atmosphere for approximately 9 to 15 years. Methane is more than
20 times more effective at trapping heat in the atmosphere than carbon dioxide over a 100-
year period. Methane is of course also a primary constituent of natural gas and, as such,
represents an important energy source.
The geological formation of coal, commonly called coalii cation, which involves an increase
in carbon content and density as it changes from peat via lignite (65 to 72% mass C) to hard
coal such as bituminous coal (76 to 90% mass C) and anthracite (93% mass C), results in meth-
ane formation together with carbon dioxide and nitrogen. The three basic stages in the for-
mation of coal are: (1) formation of peat in a swamp; (2) production of a gel-like material,
gytta, by the partial decay of the peat by bacteria and fungi (bacterial decay); and (3) thermal
alteration (bituminization) of the gel to the various ranks of coal after burial under hundreds
or thousands of metres of sediment. Both decay and thermal alteration increase the percent of
carbon present and reduce the amount of water and other volatile gases (e.g. carbon dioxide
or methane). These changes increase the heat content of the coal and hence its rank.
In summary the process of coalii cation converts the plant material - which consists
essentially of compounds of carbon, hydrogen and oxygen - to coal which in its purest
form, consists essentially of carbon. The different grades or types of coal vary between rel-
atively unchanged plant material and pure carbon.
There are different ways in which the various types of coal can be classii ed ( Figure 13.10 ) .
Some classii cations rely on the use of the coal - for example coking coal or steam coal. One
common classii cation system divides coals into various rankings based on a range of proper-
ties. The order of rank of the coals from lowest energy-value to highest energy-value is:
●

Peat

●

Lignite

●

Sub-bituminous

●

Bituminous

●

Anthracite.