Coal is delivered to the plants by mass
transport systems, truck, rail, barge or collier. A large
coal train called a "unit train" may be two
kilometers long, containing 100 cars with 100 tons of
coal in each one, for a total load of 10,000 tons. A large
plant under full load requires at least one coal delivery
this size every day. Plants may get as many as three to
five trains a day, especially in "peak season",
during the summer months when power consumption is high.
A large thermal power plant such as the one at Nanticoke
Ontario stores several million tons of coal for winter
use when the lakes are frozen.
Modern unloaders use rotary dump devices,
which eliminate problems with coal freezing in bottom
dump cars. The unloader includes a train positioner arm
that pulls the entire train to position each car over
a coal hopper. The dumper clamps an individual car against
a platform that swivels the car upside down to dump the
coal. Swiveling couplers enable the entire operation to
occur while the cars are still coupled together. Unloading
a unit train takes about three hours.
Shorter trains may use railcars with
an "air-dump", which relies on air pressure
from the engine plus a "hot shoe" on each car.
This "hot shoe" when it comes into contact with
a "hot rail" at the unloading trestle, shoots
an electric charge through the air dump apparatus and
causes the doors on the bottom of the car to open, dumping
the coal through the opening in the trestle. Unloading
one of these trains takes anywhere from an hour to an
hour and a half. Older unloaders may still use manually
operated bottom-dump rail cars and a "shaker"
attached to dump the coal. Generating stations adjacent
to a mine may receive coal by conveyor belt or massive
diesel-electric-drive trucks.
