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16 Coal
For comparison, gasoline has a heat content of 20,400 BTUs, diesel is nearer 19,300
BTUs, and seasoned wood has a value of 5,000–8,000 BTUs.
Of course, these are all averages. Each source of coal is tested and graded for its
mineral and microscopic structure, based on structures called
macerals
, its energy
yield, and for various contents such as sulfur, water, and traces of substances such as
mercury and arsenic.
Coal Times and Places
Much North American coal was formed during the Carboniferous period, but almost
every time span saw coal formation after the first small, moss-like plants invaded
the land. Some sub-bituminous coal in the western United States date to the Triassic
and Jurassic periods (252–201 and 201–145 million years ago, respectively). Some
Canadian coal formed more recently during the Cretaceous period (145–66 million
years ago). Large amounts of coal in Wyoming date to the Cretaceous and early
Paleogene periods, making it 130–50 million years old.
No matter when or where coal formed, it is sometimes called “buried sunshine,”
because it contains carbon-based substances that got their energy from sunlight.
When coal burns today, it is releasing that prehistoric light energy, partly in the form
of light again, and also as heat.
Macerals
A high-power microscope shows that most rocks are made of mineral crystals
and grains. In coal, similar tiny particles from the original living plant matter
are called macerals. There are several main kinds, and their numbers and
proportions give the coal its features.
• Vitrinite macerals
are broken-down remains of harder plant parts like
trunks, stems, and roots.
• Liptinite macerals
come from softer plant parts such as leaves, buds,
saps, and resins.
• Inertite
is the fossilization product of charcoal, or plants that had burned in
natural wildfires that swept through the ancient forests.