Process of Coal Formation
Coal is a fossil fuel formed by the decomposition of
land plant remains that have accumulated in swampy areas. Land
plants first appeared in the Silurian Period 400 million years ago
but it was not until the Carboniferous Period of 300 million years
ago that these plants developed sufficiently to form the forests
which produced the major coal deposits of the Northern Hemisphere.
In Australia, which was then part of a great land mass called
Gondwana (picture, right), coal formation occurred much later, in
the Permian Period, 250 million years ago. This applies to the
large deposits in Queensland and New South Wales. In Victoria, the
coals are much younger; being deposited 15 to 50 million years ago
during the Tertiary Period.
In these waterlogged environments, plants and tree debris
accumulated. As the layer of debris increased in thickness, the
floors of these vast swamps subsided slowly and fungi and bacteria
decomposed the plant material. This is the first stage in the
"coalification" process and is characterised by extensive
biochemical reactions.
During degradation of dead plant material, proteins, starches and
cellulose undergo more rapid decomposition than the woody material
(lignin) and the waxy parts of the plants (the leaf cuticles and
the spore and pollen walls). Thus the remains of many types of
vegetation, including tree stumps, leaves, spores, seed pods, and
resin are found in Victoria's brown coal. Some of the material is
similar to existing vegetation but, in general, most of the plants
have not grown in Victoria for millions of years.
To varying degrees, and depending upon climatic conditions, plant
constituents are decomposed under aerobic conditions to carbon
dioxide, water and ammonia. This process is called "humification"
and results in the formation of peat. This peat becomes covered
with layers of sediment, which excludes air, and hence the second
stage of coalification occurs under anaerobic conditions. In this
second stage of the process the combined effects of time,
temperature and pressure convert the peat firstly into brown coal
(lignite) and then into sub-bituminous coal, bituminous coal and
finally to anthracite. These three latter coals are usually called
black coals.
Brown Coal
Brown coal, sometimes called lignite, is a
relatively soft material which has a heating value only about
one-quarter of that for black coal. It has a much lower carbon
content than black coal and a higher moisture content. Where found
near the surface in thick seams, it can be mined economically on a
large scale by open-cut methods.
Australian brown coal deposits are Tertiary in age and range
from about 15 million to about 50 million years old. The main
deposits are in Victoria, the only State that produces brown coal.
In the Latrobe Valley in Gippsland, thick seams, up to 165 m, form
part of a sequence of brown coal measures that is the basis of
Victoria's electric power industry. Smaller deposits occur in the
Bacchus Marsh, Altona and Anglesea areas of Victoria, in the St
Vincents and Murray Basins and around Pidinga in South Australia,
in the Murray Basin in New South Wales and Victoria, at Waterpark
Creek near Rockhampton in Queensland, and at Scaddan, O'Sullivans
and Balladonia in the south east of Western Australia.
In 2009 Australia's economically recoverable brown coal
resources were reported to be 37 billion tonnes, all of which is in
Victoria and with over 90% in the La Trobe Valley. Australia has
about 24% of the world's economic resources and is ranked
first.
The La Trobe Valley mines of Yallourn, Hazelwood and Loy Yang
extract brown coal from large open-cut mines utilising giant
bucket-wheel excavators, or dredgers, which may weigh several
thousand tonnes. The coal is loaded onto conveyor belts for
delivery to power stations. However, in a recent development the
dredgers at Yallourn have been replaced by four large dozers. At
Anglesea, Alcoa of Australia Ltd operates an open-cut mine to
provide brown coal for its power station. This power station
provides most of the electricity for the company's aluminium
smelter at Point Henry. The small Maddingley mine near Bacchus
Marsh produces a horticultural product. Annual brown coal
production is about 68 million tonnes, all from Victoria and with
over 98% from the La Trobe Valley. Australia produces about 7% of
the world's brown coal and is ranked fifth largest after Germany
(21%), Russia (10%), Turkey (9%) and USA (8%). In Victoria, almost
all of the brown coal extracted is burnt to heat steam-generating
boilers in electrical power stations located near the coal mines.
It is also made into briquettes, which are used for industrial and
domestic heating in Australia and are also exported. Brown coal can
also be used to produce water gas, which is used in the production
of ammonia, solvents, and liquid fuels, and can be a source of
industrial carbon, used to decolourise and purify solutions and (as
char) in iron, glass, and cement manufacture.
Logging Requirements
Email the answers to the following questions;
- What is the maximum thickness of the coal seam at Loy
Yang?
- How much coal can each bucket dredger move per day?
- How long did it take the coal deposit at Loy Yang to form?
- Give a brief definition of the term Over Burden?
Answers to all questions can be found in the nearby interpretive
display.
Once you have received confirmation of your answers your
log must be accompanied by a photo of you with your GPSr in front
of the pit.
Other Activities
Visit Power Works Energy Technology Centre, Ridge
Rd, Morwell, Open 09:00 to 17:00 Monday to Friday. Power Works has
interactive displays covering power and power production, and
offers tours of the nearby Open cut mine.
References
http://www.loyyangpower.com.au/
http://www.australianminesatlas.gov.au/education/fact_sheets/coal.jsp
http://www.hk-phy.org/energy/power/source_phy/flash/formation_e.html
http://en.wikipedia.org/wiki/Lignite
http://en.wikipedia.org/wiki/Coal