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SO2 Recovery and Environmental Impact
Name: Sarah
Status: Student
Age: N/A
Location: OH
Country: United States
Date: N/A
Question:
What affect does recovering SO2(g) from smokestacks
and installing catalytic afterburners on cars decrease waste and
reduce environmental impact? How do these processes work?
Replies:
Sarah,
Let me start off with shortly explaining the combustion process
inside of a gasoline engine. Air is sucked (via vacuum) into a
pipe, which is then mixed with vaporized gasoline. This air/fuel
mixture is then forced into one of several cylinders and then closed
off. Once the cylinder is closed off (intake and exhaust sides are
sealed), the piston moves up and compresses the air and fuel, at
which time the spark plugs spark and ignite the air/fuel
mixture. At this point the fuel only has a very short time to
combust and in the end you get a mixture of gas that still has come
oxygen, some unspent fuel, some fully combusted fuel and some
partially combusted fuel. MUCH heat is also a byproduct this this!
Once the air/fuel mixture is ignited, it forces the piston down and
then as the piston moves back up, the exhaust valves open and let
all of the spent fuel mixture out the opposite side of the
engine. The mixture then goes through a pipe and ends up coming out
your muffler. Before it gets there, though, it passes through a
catalytic converter, which is basically many many fine screens (like
a bunch of screen doors stacked ontop of eachother very very
close). These screens are coated with a metal catalyst that helps
oxidize the fuel that did not get fully combusted as well as turns
gases like carbon monoxide into carbon dioxide and water. (This is
why when you first start a car, you might notice water trickling
from the tail pipe--this is because the car is not warm enough quite
yet to combust all of the fuel or hot enough to form steam before it
leaves your car.) The formost gas to reduce is carbon monoxide
because it can go up into the fair reaches of the atmosphere and
react with light to form a radical. Radicals are molecules that are
deficient by one electron. When this occurs, it starts a chain of
reactions that ends up in depleting ozone from our
atmosphere. Ozone is responsible for keeping out the really high
energy radiation from the sun!
As far as SO2 (sulfur dioxide) goes, to make a long story short, SO2
can be converted into sulfuric acid (H2SO4) and when this happens
acid rain will occur. Acid, will of course, eat the paint off your
car and cause a massive wide-spread amount of damage.
Matt Voss
The removal of SO2 gas from the exhaust of vehicles is not an issue
since there is virtually no SO2 in gasoline. Its removal from coal
fired power plants and municipal waste incinerators is another
issue. There are several routes to reducing/eliminating SO2 from
these point sources. The first involves "scrubbing" the gaseous
effluents with water to form various acids derived not only from
SO2 but Cl2, and NOx pollutants as well. The water is then
neutralized with some sort of base and disposed of in accordance
with regulations set by the Environmental Protection Agency. The
solid effluent from the incinerator/power plant is another issue
because the acidic residues are in solid form. Here the
non-combustible ash residue from the coal and municipal waste is
mixed with calcium oxide (CaO = lime) with rapidly reacts with
water to form calcium hydroxide (Ca(OH)2 = slaked lime). This
reacts with the acid residues to form calcium salts, which are then
disposed of in solid waste landfills along with the other
non-combustibles. This procedure is not without its own problems,
however, because there is usually an excess of CaO (or Ca(OH)2)
which raises the pH of the landfill and makes it very alkaline.
This in turn causes other problems.
Vince Calder
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Update: June 2012
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