Most people have heard of acid rain, widely discussed as a threat to forests
downwind from coal-fired power plants. Acid rain is primarily caused by
sulfur dioxide (SO2), a byproduct of burning coal, oil or gas that is tinged with
sulfur. Because sulfur is a commonly occurring element, it is virtually impossible
to find deposits of these fossil fuels that do not contain sulfur. When sulfur dioxide
is emitted as these fuels are burned, it enters the atmosphere and reacts with
water. The outcome of this reaction is sulfuric acid (H2SO4); it is this acid that
gives the rain its name.
Not a Recent Problem
Most nations have moved to burning low-sulfur fossil fuels such as low-sulfur coal.
Since acid rain hasn’t been in the news lately, it is assumed by the public to be a t
hreat that has passed and in fact the measures put in place in the 1980’s and 90’s
have made a significant impact on the problem. Upon closer examination though,
the rain downwind from fossil fuel power plants is still acidic. Figures 1 and 2
show Eastern North America during the time periods of 1980-1984 and 1996-2000, respectively.
As one can see, there was a significant reduction in the acidity of the rain,
especially in the Great Lakes region over the intervening years. That said,
the pH of ‘clean’, natural rain is about 5.6; as a reference, vinegar stands
around 3.0. Given this, the rain in the region is still quite acidic and capable
of causing significant environmental issues.
The Impact of Sulfur Dioxide
After it has reacted in the atmosphere, sulfur dioxide falls to the Earth as acid
rain. The most obvious impact of acid rain is one that we can visibly see. Plant
life suited to normal-pH rain does not thrive well and will die in acid rain regions.
The forests of Europe were devastated by acid rain. Figure 3 is an area of the
Black Forest in Germany where there was significant tree-die-off from acid rain.
Figure 4 shows how this death begins at the branch-level.
Effects that are not seen, however, may be even more significant. When acid
rain falls in lakes, rivers and streams, the pH of the water is altered. Lakes that
become acidified cannot support the variety of life that they once did. Crayfish,
freshwater clams and muscles are the first to disappear and as these creatures
are removed from the food chain, others begin to die, as well. Lakes in
limestone-rich areas are less prone to these die-offs as the limestone can
neutralize the acid; lakes in regions where granite is common do not have this
natural buffer and are the first to show such distress.
In addition, as the aquatic populations are reduced, the animals that rely upon
the lakes for food and shelter are also impacted. Fish-eating birds and land
mammals migrate to other areas and frogs, snails and other lake-dwellers die off
from one generation to the next.
Didn’t We Already Solve the Problem? What More Can Be Done?
One solution employed in the 1980’s and 90’s was to build higher stacks or
chimneys. This effectively put the sulfur higher into the atmosphere and the
acid rain moved further downwind. It quickly became obvious that this was just
pushing the problem – not solving it. In fact in 1988, Prince Charles of Britain
recognized this, saying: “Our responsibilities do lie in not exporting our problems
abroad.”
The ending to this story doesn’t need to be so gloomy, though. There are
new technologies that are being employed at power plants around the world.
One such technology is flue gas desulfurization or FSD, essentially removing the
sulfur dioxide from the combustion gases as they ascend the chimney flue.
The three main methods employed to accomplish FSD are wet-scrubbing,
dry-scrubbing, and injection. In wet and dry scrubbing, the two most commonly
used methods; a slurry of limestone or lime is sprayed through the chimney as
the gases rise. This lime reacts with the SO2 and the resulting compounds
‘rain’ down to be collected at the chimney’s base.
There are also emerging technologies that could surpass the efficacy of lime
scrubbing. The Chendu power plant in China and the Pomorzany power plant
in Poland have installed new technology in which the flue gases are blasted with
electrons and then exposed to ammonia. This reaction is said to leave little
un-reacted SO2 that will escape the chimney and additionally it shows a similar
reduction in nitrous oxide (NOx). Though still in the early stages of testing,
this may lead to very clean power plants in the developing world offering hope
that the same problems that plagued Europe and North America, such as acid rain,
might be avoided as these emerging economies expand and develop in the 21st century.