Solving a Problem with a Problem
Used tires can reduce mercury
emissions.
Excessive levels of mercury in some
water bodies have resulted at various times in the closure of
both sport and commercial fishing seasons. Sometimes the mercury
had a natural origin - but often it came from an industrial source,
often a pulp and paper mill or a power plant.
Disposing of old tires is another environmental
challenge - and a big one. Each year, more than 200 million tires
are disposed of in the United States alone. On several occasions,
large, difficult to extinguish fires have burned in piles of
old tires.
A recent study conducted by the University
of Illinois and the Illinois State Geological Survey has found
that some of those used tires could be put to use cleaning up
mercury emissions from coal fired power plants. The tires, when
combined with pistachio shells, were found to work as well as
or better than current commercial products and might be cheaper
to produce.
"Mercury is a toxic pollutant that
can enter rivers, lakes and the human food chain," said
researcher Massoud Rostam-Abadi. "Coal-fired power plants
are one of the largest sources of mercury emissions. We looked
for materials that could effectively remove mercury from the
combustion flue gases."
In the study, adsorbents were prepared
from a variety of materials. The adsorbents were then evaluated
for their effectiveness at removing two forms of mercury emissions
- elemental mercury and mercuric chloride - from several different
simulated combustion gas streams.
"We found that mercury removal
was affected by both the properties of the adsorbent and the
flue gas compositions," Rostam-Abadi said. "In one
flue gas, the adsorbents were equally effective in removing both
forms of mercury. In another flue gas, the tire and pistachio
carbons had nearly five times larger capacity for the adsorption
of mercuric chloride" than other substances tested. Because
utilities produce different amounts of the two mercury emissions,
some tailoring of the tire and pistachio substance will be required
to achieve effective mercury removal for individual power plants.
"We are currently examining the
mixing of adsorbents to more effectively remove both forms of
mercury from individual power plants," Rostam-Abadi said.
" Also, depending on plant location, certain adsorbents
may be more economical than others."
In their work, the researchers also
showed that activated carbons containing sulfur additives were
significantly more effective at removing mercury emissions from
the flue gases - but adding the sulfur usually requires additional
processing steps and production costs. Tire rubber already contains
sulfur - which makes the rubber more durable - so activated carbon
from tires might prove more cost-effective than existing products.
