Environmental Engineering Reference
Climate changes may not be prevented, but it may be possible to
minimize any damages from an altered climate. Certain preventive
strategies could actively limit emissions of substances thought to be
harmful. One strategy designed to avoid damage to the ozone layer was
the reduction or banning of all uses of CFCs.
The Montreal Protocol of 1987 proposed a 50% cut in CFCs by the year
2000, but not all nations signed the treaty. Most scientific studies push for
at least a 90% ban if the ozone hole is to be reversed. This would not only
help protect the ozone layer but would cut emissions of a trace greenhouse
gas that could be responsible for up to 25% of global warming. The use
of artificial fertilizers in agriculture also generates atmospheric nitrogen
compounds that can reach the stratosphere and possible destroy ozone.
Present theories of the origin of acid rain indicate that we can limit
acid rain by reducing sulfur dioxide emissions and moving to low-sulfur
fuels; but, only about 20% of the world's petroleum reserves are low in
sulfur. Switching U.S. midwestern power plants to low-sulfur coal could
cause economic problems since much of the coal from the Midwest and
Appalachia has a high sulfur content. Most of the electric power generated
in the Midwest uses high-sulfur coal and it would cost tens of billions of
dollars to scrub the sulfur out of coal.
An energy cost would also be paid for the processes that remove
the sulfur along with environmental problems from disposing of it. About
5% more coal would be needed to keep electricity production from these
power plants at current levels if most of the sulfur is scrubbed out.
It is also possible to keep sulfur dioxide from reaching the atmosphere
by washing the coal or by removing the SO from the flue gas. Simple
washing removes about 50% of the sulfur. Additional removal of up to
90% requires high temperatures and high pressures and may cost ten times
as much as washing. Flue gas desulfurization (scrubbing) by reacting the
effluent gas with lime or limestone in water can remove 80-90% of the
sulfur but creates large amounts of solid waste.
Techniques for minimizing emission of SO 2 from burning power
plants has no effect on nitrogen oxide (NO x ) emissions. Oxides of nitrogen
result from the burning of nitrogen normally found in combustion air.
The percentage of NO x generated by the burning of air is about 80% in
conventional coal-fired boilers and depends mostly on the temperature of