The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 537

exothermic reaction generating heat as a by-product. If the temperature reaches somewhere
between 80ºC and 120ºC (called the 'threshold' temperature) a steady reaction commences.
The temperature of the coal will almost certainly continue to increase until, some-
where between 230ºC and 280ºC, the reaction becomes rapid and strongly exothermic -
the coal reaches 'ignition' or a 'l ash' point and starts to burn.
Not all types of coal are equally susceptible to spontaneous combustion. High-ranking
coals (that is, having high carbon content) are more i re prone than lower-ranking coals.
Another important factor is the size of the particles; the larger the effective area of the coal
(i ne particles), the more rapidly the reaction can proceed. Also, external factors play a role
in the oxidation reaction. Oxidation requires an adequate supply of air; cracks, i ssures,
and the porosity of rock and soil over the coal seams may encourage underground coal
i res by allowing oxygen to reach the coal. The moisture content is also important: coal
with high moisture content tends to exhibit a low oxidation rate. However, the presence of
some moisture favours continued oxidation.
Micro Climate
As with urbanization, mining may change the local climate in many ways. The microcli-
mates of urban areas differ substantially from the microclimates of surrounding natural
areas due primarily to the removal of vegetation, enabling the Earth's surface to become
much warmer, with the release of artii cially created energy into the environment (the
urban heat island effect).
At a mine site, exposed rock, roads, and buildings often replace natural vegetation, and the
vertical surfaces of mine pits and buildings are added to the natural landscape. Exposed
CASE 13.8
Uncontrolled Forest, Peat,
and Coal Fires in Indonesia
The combination of forest destruction, land clearance, and
an exceptionally severe El Nino climatic event in 1997 led
to the most severe forest and peat fi res ever known in
Indonesia. Between half a million and three million
hectares of vegetation burned, much of it on peat and
coal. The fi res penetrated up to 1.5 m into the dried-out
peat. It also ignited coal seams along their outcrops.
At least one billion tonnes of carbon were released into
the atmosphere - more than that released by the fossil
fuels burnt by the European Union in a year. It undid an
estimated ten years of carbon fi xation by all of the world's
pristine peat bogs. By some estimates this sudden release
of carbon may have added about 0.5 parts per million
CO 2 to the atmosphere, a signifi cant addition to the global
greenhouse gas concentration.
Once started peat and coal fi res are diffi cult to
extinguish. They continue to burn underground during the
tropical wet season only to start new forest fi res in the
following dry season.