Environmental Engineering Reference
In-Depth Information
monly 2-8 kW/m
2
, with the most productive operations
having 10-30 kW/m
2
. During the early 2000s, Wyo-
ming's large surface mines averaged 8-10 kW/m
2
, Vic-
toria's Latrobe Valley extraction rated up to 28 kW/m
2
,
and Queensland's Blair Athol approached 33 kW/m
2
(Australian Government 2005).
Among the world's remaining large coal producers,
only China followed a singular path as the slow progress
of mechanization in its large, state-owned enterprise was
accompanied by two waves of indiscriminate openings of
nonmechanized small coal mines (Smil 2004a). The first
brief pulse began in 1958, a part of Mao Zedong's delu-
sionary Great Leap Forward, when some 20 million peas-
ants opened up more than 100,000 small mines to
produce coal mostly for local smelting of inferior pig
iron. The second expansion was a part of post-1980 eco-
nomic reforms. By 1997 half of China's 1.3 Gt of raw
coal was coming from some 82,000 small mines known
for their low productivity and dangerous working condi-
tions. Their number was cut to 36,000 by the year 2000.
China is also the only major producer that uses mostly
raw fuel. Elsewhere the fuel is processed before market-
ing by washing (based on the difference of specific grav-
ities between lighter coal and heavier incombustible
waste, including pyritic sulfur), screening, and crushing
to sort the fuel to uniform sizes. Coal preparation facili-
ties have power densities of 8-10 kW/m
2
.
Seaborne trade of hard coal—Australia, China, and In-
donesia are leading exporters—uses large bulk carriers
with capacities of up to 200,000 dwt. Long-distance
land transport of coal remains dominated by railways.
Unit trains are the best solution for moving bulky and
dirty solids. They are permanently coupled assemblies of
powerful diesel locomotives and about 100 lightweight
aluminum cars, each with capacity of up to 90 t, that
shuttle between a mine and an electricity-generating
plant or a port on runs of 10
2
-10
3
km (Glover, Hinkle,
and Riley 1970). At arrival, a rotary car dumper turns the
cars 140
-160
to unload coal, or mechanical trippers
open the hatches for bottom-dump unloading. Typical
corridors claimed by railways are 20-30 m wide, but
cuts and fills may easily double that width. Dedicated
unit train railroads that move coal 500-1500 km to large
power plants have annual throughput densities between
100 W/m
2
and 400 W/m
2
, depending on the length
of the run, width of the right-of-ways, and capacity.
Moving coal at the plant from bunkers, silos, or outdoor
stockpiles to coal mills or directly to boilers is done by
belt conveyors, stackers, reclaimers, in-ground hoppers,
or plow feeders (McGraw 1982).
All these advances have been essential in order to mul-
tiply the world's coal output (fig. 8.5). The 1900 total of
less than 800 Mt of hard coals and lignites was doubled
by 1949 to nearly 1.3 Gt of hard coals and about 350 Mt
of lignites. Yet another doubling of the total coal ton-
nage (in terms of hard coal equivalent) took place by
1988, and global extraction peaked the next year at
nearly 4.9 Gt, with about 3.6 Gt contributed by hard
coals and 1.3 Gt by lignites (UNO 1990). About 40% of
that year's lignite production was coming from the now
defunct Communist states of East Germany and the
USSR, whose lignites were of particularly low quality,
averaging just 8.8 and 14.7 GJ/t, respectively (UNO
2000). The 1988 total was matched in 2003 with 4.04
Gt of hard coals and 886 Mt of brown coals and lignites,
and by 2005 the output grew to 4.97 Gt of hard
coal and 905 Mt of brown coal and lignites (WCI
2006). Growing output has been accompanied by de-
clining relative importance, but there has been a recent
gain.
