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Chapter Three: Using Coal
electricity to make the tiny particles clump into larger, heavier ones, which then fall
and can be collected. Another method is to pass the emissions through a filter that
traps the particles.
The gaseous emissions from burning coal, and other fossil fuels—in fact, from
any fuel that burns—are a significant problem. To cut down on these troubles, power
generators in North America must have filters and
scrubbers
to remove the pollutants
from the air. In a wet scrubber, fine droplets of a chemical are sprayed into the flue
gases. This may be calcium carbonate (as in limestone), calcium hydroxide (as in lime)
or a similar chemical. The droplets react with the gases to make solids that can be
collected. In the spray-dry method, by contrast, jets of heated air turn the polluting
chemicals into a dry, powdery substance.
Metallurgic Coal
The other main usage of coal is for making iron and steel. When coal is used in this
way, it’s referred to as
metallurgic
or
coking
coal—that is, coal converted into coke.
Coke is a hard black or grey substance created by heating bituminous coking coal
to extremely high temperatures in an anaerobic environment. Because there is no
oxygen, the coal can’t burn. Instead of burning, it is transformed by heat, in a process
called destructive distillation, into an array of
substances, including coal tar, ammonia liquor,
and coal gas. At the end of the process, all that
remains is coke. All of these products are useful
in industry.
Coke contains more carbon (up to 95
percent) than the original bituminous coal. It
burns more effectively and hotter, with a heat
content similar to anthracite, at 12,000–13,500
BTUs per pound. Also, in terms of its chemistry,
it acts as a reducing agent—a substance that
removes oxygen. So it is both a primary fuel
and reducing agent for smelting, which means
Leftovers
Treating of 1 ton (0.9 metric
tons) of coal by destructive
distillation can produce:
• 1,700 pounds (770
kilograms) of coke
• 30 US gallons (115
liters) of coal tar
• 15 US gallons (57 liters)
of ammonia liquor
• 14,000 cubic feet (400
cubic meters) of coal
gas